Inquiry into life [Fifteenth edition] 9781259426162, 1259426165

Table of contents :
Cover......Page 1
Title Page......Page 2
Copyright Page......Page 3
About the Authors......Page 4
Preface......Page 5
Acknowledgments......Page 11
Brief Contents......Page 12
Contents......Page 13
chapter 1 The Study of Life......Page 18
1.1 The Characteristics of Life......Page 19
1.2 The Classification of Organisms......Page 23
1.3 The Process of Science......Page 25
1.4 Challenges Facing Science......Page 30
chapter 2 The Molecules of Cells......Page 34
2.1 Basic Chemistry......Page 35
2.2 Molecules and Compounds......Page 39
2.3 Chemistry of Water......Page 42
2.4 Organic Molecules......Page 45
2.5 Carbohydrates......Page 47
2.6 Lipids......Page 50
2.7 Proteins......Page 52
2.8 Nucleic Acids......Page 55
chapter 3 Cell Structure and Function......Page 60
3.1 The Cellular Level of Organization......Page 61
3.2 Prokaryotic Cells......Page 63
3.3 Eukaryotic Cells......Page 65
3.4 The Cytoskeleton......Page 73
3.5 Origin and Evolution of the Eukaryotic Cell......Page 75
chapter 4 Membrane Structure and Function......Page 80
4.1 Plasma Membrane Structure and Function......Page 81
4.2 The Permeability of the Plasma Membrane......Page 83
4.3 Modifications of Cell Surfaces......Page 91
chapter 5 Cell Division......Page 96
5.1 The Cell Cycle......Page 97
5.2 Control of the Cell Cycle......Page 98
5.3 Mitosis: Maintaining the Chromosome Number......Page 101
5.4 Meiosis: Reducing the Chromosome Number......Page 105
5.5 Comparison of Meiosis with Mitosis......Page 109
5.6 The Human Life Cycle......Page 111
chapter 6 Metabolism: Energy and Enzymes......Page 116
6.1 Life and the Flow of Energy......Page 117
6.2 Energy Transformations and Metabolism......Page 119
6.3 Enzymes and Metabolic Pathways......Page 120
6.4 Oxidation-Reduction Reactions and Metabolism......Page 125
chapter 7 Cellular Respiration......Page 130
7.1 Overview of Cellular Respiration......Page 131
7.2 Outside the Mitochondria: Glycolysis......Page 133
7.3 Outside the Mitochondria: Fermentation......Page 135
7.4 Inside the Mitochondria......Page 136
chapter 8 Photosynthesis......Page 144
8.1 Overview of Photosynthesis......Page 145
8.2 Plants as Solar Energy Converters......Page 148
8.3 Plants as Carbon Dioxide Fixers......Page 151
8.4 Alternative Pathways for Photosynthesis......Page 154
8.5 Photosynthesis Versus Cellular Respiration......Page 156
chapter 9 Plant Organization and Function......Page 160
9.1 Cells and Tissues of Plants......Page 161
9.2 Plants Organs and Systems......Page 164
9.3 Monocot Versus Eudicot Plants......Page 166
9.4 Organization of Roots......Page 167
9.5 Organization of Stems......Page 170
9.6 Organization of Leaves......Page 176
9.7 Uptake and Transport of Nutrients......Page 178
chapter 10 Plant Reproduction and Responses......Page 186
10.1 Sexual Reproduction in Flowering Plants......Page 187
10.2 Growth and Development......Page 192
10.3 Asexual Reproduction and Genetic Engineering in Plants......Page 195
10.4 Control of Growth and Responses......Page 200
chapter 11 Human Organization......Page 207
11.1 Types of Tissues......Page 208
11.2 Body Cavities and Body Membranes......Page 214
11.3 Organ Systems......Page 215
11.4 Integumentary System......Page 217
11.5 Homeostasis......Page 221
chapter 12 Cardiovascular System......Page 227
12.1 The Blood Vessels......Page 228
12.2 Blood......Page 229
12.3 The Human Heart......Page 235
12.4 Two Cardiovascular Pathways......Page 239
12.5 Cardiovascular Disorders......Page 242
chapter 13 Lymphatic and Immune Systems......Page 249
13.1 The Lymphatic System......Page 250
13.2 Innate Immunity......Page 252
13.3 Adaptive Immunity......Page 254
13.4 Active Versus Passive Immunity......Page 258
13.5 Adverse Effects of Immune Responses......Page 262
13.6 Disorders of the Immune System......Page 265
chapter 14 Digestive System and Nutrition......Page 270
14.1 The Digestive Tract......Page 271
14.2 Accessory Organs of Digestion......Page 277
14.3 Digestive Enzymes......Page 279
14.4 Human Nutrition......Page 280
14.5 Eating Disorders......Page 289
14.6 Disorders of the Digestive System......Page 290
chapter 15 Respiratory System......Page 296
15.1 The Respiratory System......Page 297
15.2 Mechanism of Breathing......Page 300
15.3 Gas Exchanges in the Body......Page 303
15.4 Disorders of the Respiratory System......Page 306
chapter 16 Urinary System and Excretion......Page 315
16.1 The Urinary System......Page 316
16.2 Anatomy of the Kidney and Excretion......Page 318
16.3 Regulatory Functions of the Kidneys......Page 321
16.4 Disorders of the Urinary System......Page 324
chapter 17 Nervous System......Page 330
17.1 Nervous Tissue......Page 331
17.2 Transmission of Nerve Impulses......Page 332
17.3 The Central Nervous System......Page 336
17.4 The Limbic System and Higher Mental Functions......Page 341
17.5 The Peripheral Nervous System......Page 344
17.6 Drug Abuse......Page 348
17.7 Disorders of the Nervous System......Page 351
chapter 18 Senses......Page 358
18.1 Sensory Receptors and Sensations......Page 359
18.2 Somatic Senses......Page 360
18.3 Senses of Taste and Smell......Page 362
18.4 Sense of Vision......Page 364
18.5 Sense of Hearing......Page 368
18.6 Sense of Equilibrium......Page 370
18.7 Disorders that Affect the Senses......Page 371
chapter 19 Musculoskeletal System......Page 379
19.1 Overview of Bone and Cartilage......Page 380
19.2 Bones of the Skeleton......Page 383
19.3 Skeletal Muscles......Page 392
19.4 Mechanism of Muscle Fiber Contraction......Page 394
19.5 Whole Muscle Contraction......Page 400
19.6 Disorders of the Musculoskeletal System......Page 402
chapter 20 Endocrine System......Page 408
20.1 Overview of the Endocrine System......Page 409
20.2 Hypothalamus and Pituitary Gland......Page 412
20.3 Thyroid and Parathyroid Glands......Page 415
20.4 Adrenal Glands......Page 416
20.5 Pancreas......Page 418
20.6 Other Endocrine Glands......Page 419
20.7 Disorders of the Endocrine System......Page 420
chapter 21 Reproductive System......Page 429
21.1 Male Reproductive System......Page 430
21.2 Female Reproductive System......Page 433
21.3 Ovarian and Uterine Cycles......Page 435
21.4 Control of Reproduction......Page 439
21.5 Sexually Transmitted Diseases......Page 441
21.6 Reproductive Disorders and Assisted Reproductive Technologies......Page 449
chapter 22 Development and Aging......Page 457
22.1 Fertilization and Early Stages of Development......Page 458
22.2 Processes of Development......Page 461
22.3 Human Embryonic and Fetal Development......Page 465
22.4 Human Pregnancy, Birth, and Lactation......Page 470
22.5 Aging......Page 474
chapter 23 Patterns of Gene Inheritance......Page 481
23.1 Mendel's Laws......Page 482
23.2 Pedigree Analysis and Genetic Disorders......Page 489
23.3 Beyond Simple Inheritance Patterns......Page 492
23.4 Environmental Influences......Page 495
chapter 24 Chromosomal Basis of Inheritance......Page 499
24.1 Gene Linkage......Page 500
24.2 Sex-Linked Inheritance......Page 501
24.3 Changes in Chromosome Number......Page 504
24.4 Changes in Chromosome Structure......Page 508
chapter 25 DNA Structure and Gene Expression......Page 513
25.1 DNA Structure......Page 514
25.2 DNA Replication......Page 518
25.3 Gene Expression......Page 519
25.4 Control of Gene Expression......Page 525
25.5 Gene Mutations and Cancer......Page 528
chapter 26 Biotechnology and Genomics......Page 536
26.1 DNA Technology......Page 537
26.2 Biotechnology Products......Page 541
26.3 Gene Therapy......Page 544
26.4 Genomics, Proteomics, and Bioinformatics......Page 547
chapter 27 Evolution of Life......Page 552
27.1 Theory of Evolution......Page 553
27.2 Evidence of Evolution......Page 556
27.3 Microevolution......Page 562
27.4 Processes of Evolution......Page 565
27.5 Macroevolution and Speciation......Page 572
27.6 Systematics......Page 574
chapter 28 Microbiology......Page 580
28.1 The Microbial World......Page 581
28.2 Origin of Microbial Life......Page 582
28.3 Archaea......Page 586
28.4 Bacteria......Page 588
28.5 Viruses, Viroids, and Prions......Page 594
chapter 29 Protists and Fungi......Page 601
29.1 Protists......Page 602
29.2 Fungi......Page 612
chapter 30 Plants......Page 623
30.1 Evolutionary History of Plants......Page 624
30.2 Nonvascular Plants......Page 627
30.3 Seedless Vascular Plants......Page 628
30.4 Seed Plants......Page 632
chapter 31 Animals: The Invertebrates......Page 641
31.1 Evolutionary Trends Among Animals......Page 642
31.2 The Simplest Invertebrates......Page 646
31.3 The Lophotrochozoa......Page 649
31.4 The Ecdysozoa......Page 657
31.5 Invertebrate Deuterostomes......Page 664
chapter 32 Animals: Chordates and Vertebrates......Page 669
32.1 Chordates......Page 670
32.2 Vertebrates: Fish and Amphibians......Page 672
32.3 Vertebrates: Reptiles and Mammals......Page 676
32.4 Evolution of the Hominins......Page 681
32.5 Evolution of Modern Humans......Page 685
chapter 33 Behavioral Ecology......Page 690
33.1 Nature Versus Nurture: Genetic Influences......Page 691
33.2 Nature Versus Nurture: Environmental Influences......Page 692
33.3 Animal Communication......Page 695
33.4 Behaviors That Affect Fitness......Page 699
chapter 34 Population and Community Ecology......Page 707
34.1 The Scope of Ecology......Page 708
34.2 Patterns of Population Growth......Page 709
34.3 Interactions Between Populations......Page 712
34.4 Ecological Succession......Page 719
chapter 35 Nature of Ecosystems......Page 723
35.1 The Biotic Components of Ecosystems......Page 724
35.2 Energy Flow......Page 725
35.3 Global Biogeochemical Cycles......Page 727
chapter 36 Major Ecosystems of the Biosphere......Page 739
36.1 Climate and the Biosphere......Page 740
36.2 Terrestrial Ecosystems......Page 741
36.3 Aquatic Ecosystems......Page 749
chapter 37 Conservation Biology......Page 760
37.1 Conservation Biology and Biodiversity......Page 761
37.2 Value of Biodiversity......Page 763
37.3 Threats to Biodiversity......Page 766
37.4 Habitat Conservation and Restoration......Page 771
37.5 Working Toward a Sustainable Society......Page 774
Appendix A Answer Key......Page 782
Appendix B Metric System and The Periodic Table of the Elements......Page 797
Glossary......Page 800
Index......Page 824

Citation preview

Sylvia S. Mader Michael Windelspecht

INQUIRY INTO LIFE, FIFTEENTH EDITION Published by McGraw-Hill Education, 2 Penn Plaza, New York, NY 10121. Copyright © 2017 by McGraw-Hill Education. All rights reserved. Printed in the United States of America. Previous editions © 2014, 2011, and 2008. No part of this publication may be reproduced or distributed in any form or by any means, or stored in a database or retrieval system, without the prior written consent of McGraw-Hill Education, including, but not limited to, in any network or other electronic storage or ­transmission, or broadcast for distance learning. Some ancillaries, including electronic and print components, may not be available to customers outside the United States. This book is printed on acid-free paper. 1 2 3 4 5 6 7 8 9 0 DOW/DOW 1 0 9 8 7 6 ISBN 978-1-259-42616-2 MHID 1-259-42616-5 Senior Vice President, Products & Markets: Kurt L. Strand Vice President, General Manager, Products & Markets: Marty Lange Vice President, Content Design & Delivery: Kimberly Meriwether David Managing Director: Michael Hackett Executive Brand Manager: Michelle Vogler Brand Manager: Chris Loewenberg Director, Product Development: Rose Koos Product Developer: Anne Winch Market Development Manager: Jenna Paleski Marketing Manager: Christine Ho Director of Digital Content: Michael G. Koot, PhD Digital Product Analyst: Christine Carlson Director, Content Design & Delivery: Linda Avenarius Program Manager: Angela R. FitzPatrick Content Project Managers: April R. Southwood/Christina Nelson Buyer: Sandy Ludovissy Design: David W. Hash Content Licensing Specialists: Lori Hancock/Lorraine Buczek Cover Image: hands holding globe © Leonello Calvetti/Getty Images/RF; Siberian tiger (Panthera tigris altaica) running © Tom Brakefield/Getty Images/RF; humpback whale (Megaptera novaeangliae), Hawaii, Maui © M Swiet Productions/Getty Images/ RF; lyretail anthias or goldies (Pseudanthias squamipinnis) over coral reef, Egypt, Red Sea © Georgette Douwma/The Image Bank/Getty Images; Ti plant (Cordyline fruticosa) Carambola Botanical Gardens, Roatan, Honduras © Danita Delimont/Gallo Images/Getty Images Compositor: Aptara®, Inc. Printer: R. R. Donnelley All credits appearing on page or at the end of the book are considered to be an extension of the copyright page. Library of Congress Cataloging-in-Publication Data Names: Mader, Sylvia S., author. | Windelspecht, Michael, 1963- , author. Title: Inquiry into life / Sylvia S. Mader, Michael Windelspecht, Appalachian   State University. Description: Fifteenth edition. | New York, NY : McGraw-Hill, 2015. Identifiers: LCCN 2015044767 | ISBN 9781259426162 (alk. paper) Subjects: LCSH: Biology—Textbooks. Classification: LCC QH308.2 .M363 2015 | DDC 570—dc23 LC record available at http://lccn.loc.gov/2015044767 The Internet addresses listed in the text were accurate at the time of publication. The inclusion of a website does not indicate an endorsement by the authors or McGraw-Hill Education, and McGraw-Hill Education does not guarantee the accuracy of the information presented at these sites.

mheducation.com/highered

About the Authors Sylvia S. Mader Sylvia Mader has authored several nationally recognized biology texts published by McGraw-Hill. Educated at Bryn Mawr College, Harvard University, Tufts University, and Nova Southeastern University, she holds degrees in both Biology and Education. Over the years she has taught at University of Massachusetts, Lowell; Massachusetts Bay Community College; Suffolk University; and Nathan Mayhew Seminars. Her ability to reach out to science-shy students led to the writing of her first text, Inquiry into Life, which is now in its fifteenth edition. Highly acclaimed for her crisp and entertaining writing style, her books have become models for others who write in the field of © Jacqueline Baer biology. Photography Dr. Mader enjoys taking time to visit and explore the various ecosystems of the biosphere. Her several trips to the Florida Everglades and Caribbean coral reefs resulted in talks she has given to various groups around the country. She has visited the tundra in Alaska, the taiga in the Canadian Rockies, the Sonoran Desert in Arizona, and tropical rain forests in South America and Australia. A photo safari to the Serengeti in Kenya resulted in a number of photographs for her texts. She was thrilled to think of walking in Darwin’s foot steps when she journeyed to the Galápagos Islands with a group of biology educators. Dr. Mader was also a member of a group of biology educators who traveled to China to meet with their Chinese counterparts and exchange ideas about the teaching of modern-day biology.

Michael Windelspecht As an educator, Dr. Windelspecht has taught introductory biology, genetics, and human genetics in the online, traditional, and hybrid environments at community colleges, comprehensive universities, and military institutions. For over a decade he served as the Introductory Biology Coordinator at Appalachian State University, where he directed a program that enrolled over 4,500 students annually.  He received degrees from Michigan State University (BS, zoology-genetics) and the University of South Florida (PhD, evolutionary genetics) and has published papers in areas as diverse as science education, water quality, and the © Ricochet Creative evolution of insecticide resistance. His current interests are in the analysis of data from digital learning platforms for the Productions LLC development of personalized microlearning assets and next generation publication platforms. He is currently a member of the National Association of Science Writers and several science education associations. He has served as the keynote speaker on the development of multimedia resources for online and hybrid science classrooms. As an author and editor, Dr. Windelspecht has over 20 reference textbooks and multiple print and online lab manuals. He has founded several science communication companies, including Ricochet Creative Productions, which actively develops and assesses new technologies for the science classroom. You can learn more about Dr. Windelspecht by visiting his website at www.michaelwindelspecht.com.

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Preface Goals of the Fifteenth Edition Dr. Sylvia Mader’s text, Inquiry into Life, was originally developed to reach out to science-shy students. The text now represents one of the cornerstones of introductory biology education. Inquiry into Life was founded on the belief that teaching science from a human perspective, coupled with human applications, would make the material more relevant to the student. Interestingly, even though it has been over forty years since the first edition was published, this style of relevancy-based education remains the focus of the national efforts to increase scientific literacy in the general public. Our modern society is based largely on advances in science and technology over the past few decades. As we present in this text, there are many challenges facing humans, and an understanding of how science can help analyze, and offer solutions to, these problems is critical to our species’ health and survival.  The front cover of this text was chosen to indicate not only that humans are the stewards of the planet, but also that we have interactions with almost all of the life in the biosphere. It is important that we know not only why we are different, but how we are the same as the species we share the planet with. Students in today’s world are being exposed, almost on a daily basis, to exciting new discoveries and insights that, in many cases, were beyond our predictions even a few short years ago. It is our task, as instructors, not only to make these findings available to our students, but to enlighten students as to why these discoveries are important to their lives and society. At the same time, we must provide students with a firm foundation in those core principles on which biology is founded, and in doing so, provide them with the background to keep up with the many discoveries still to come. In addition to the evolution of the introductory biology curriculum, students and instructors are increasingly requesting digital resources to utilize as learning resources. McGraw-Hill Education has long been an innovator in the development of digital resources, and this text, and its authors, are at the forefront of the integration of these technologies into the science classroom. The authors of the text identified several goals that guided them through the revision of Inquiry into Life, Fifteenth Edition: 1. updating of chapter openers and the Science in Your Life features to focus on issues and topics important in a nonscience majors classroom 2. utilization of the data from the LearnSmart adaptive learning platforms to identify content areas within the text that students demonstrated difficulty in mastering 3. refinement of digital assets to provide a more effective assessment of learning outcomes to enable instructors in the flipped, online, and hybrid teaching environments 4. development of a new series of videos and websites to introduce relevancy and engage students in the content

Relevancy The use of real world examples to demonstrate the importance of biology in the lives of students is widely recognized as an effective teaching strategy for the introductory biology classroom. Students want to learn about the topics they are interested in. The development of relevancy-based resources is a major focus for the authors of the Mader series of texts. Some examples of how we have increased the relevancy content of this edition include: ∙ A series of new chapter openers to introduce relevancy to the chapter. The authors chose topics that would be of interest to a nonscience major, and represent what would typically be found on a major news source. ∙ The development of new relevancy-based videos, BioNow, that offer relevant, applied classroom resources to allow students to feel that they can actually do and learn biology themselves. For more on these, see page v. ∙ A website, RicochetScience.com, managed by the author, that provides updates on news and stories that are interesting to nonscience majors. The Biology101 project links these resources to the major topics of the text. The site also features videos and tutorial animations to assist the students in recognizing the relevancy of what they are learning in the classroom.

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Engaging Students Today’s science classroom relies heavily on the use of digital assets, including animations and videos, to engage students and reinforce difficult concepts. Inquiry, 15e, includes two resources specifically designed for the introductory science class to help you achieve these goals.

BioNow Videos A relevant, applied approach allows your ­students to feel they can actually do and learn biology themselves. While tying directly to the content of your course, the videos help students relate their daily lives to the biology you teach and then connect what they learn back to their lives. Each video provides an engaging and entertaining story about applying the science of biology to a real situation or problem. Attention is taken to use tools and techniques that any regular ­person could perform, so your students see the science as something they could do and understand.

A video series narrated and produced by Mader-series author Jason Carlson

Tutorial Videos The author, Michael Windelspecht, has prepared a series of tutorial videos to help students understand some of the more difficult topics in each chapter. Each video explores a specific figure in the text. During the video, important terms and processes are called out, allowing you to focus on the key aspects of the figure. For students, these act as informal office hours, where they can review the most difficult concepts in the chapter at a pace which helps them learn. Instructors of hybrid and flipped courses will find these useful as online supplements.

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Detailed List of Content Changes in Inquiry into Life, Fifteenth Edition Every chapter now includes links in the end-of-chapter material to the new BioNow relevancy videos. All of these are available in the instructor and student resources section within Connect. Chapter 1: The Study of Life has been reorganized to provide a briefer overview of biology as a science. The content on the scientific process (section 1.3) has been reworked with new examples, and a new section (1.4) has been added that explores some of the major challenges facing science.

Unit 1: Cell Biology Chapter 2: The Molecules of Cells contains a new feature, “Japan’s Nuclear Crisis,” that explores the fate of isotopes. Chapter 3: Cell Structure and Function is an expanded discussion of the principles of microscopy. The chapter opener for  Chapter 5: Cell D ­ ivision now focuses on Angelina Jolie. The section on the control of the cell cycle (5.3) now includes an expanded discussion and new figures of the effects of mutation. The Bioethical feature, “Genetic Testing for Cancer Genes,” is now located in this chapter. Chapter 6: Metabolism: Energy and Enzymes  contains new content on the function of ATP (6.2).  Chapter 7: Cellular Respiration  now explains why ATP yields rarely reach theoretical values.

Unit 2: Plant Biology Chapter 8: Photosynthesis contains an expanded discussion of how algae may be used to produce biofuel. Chapter 9: Plant Organization and Function  has been reorganized (sections 9.1 and 9.2) so that it follows the biological levels of organization. The chapter opener now explores the importance of the neem tree and biodiversity. The content on monocots and dicots (section 9.3) now includes differences in pollen types. Chapter 10: Plant Reproduction and Responses has a revised feature on pollinators that discusses the different types of plant pollinators. A new feature on the safety of genetically-modified plants has been added to section 10.3.

Unit 3: Maintenance of the Human Body Chapter 11: Human Organization now features artist Taylor Swift in the opener. Chapter 12: Cardiovascular System  begins with a discussion of hypertension in young adults. Chapter 13: Lymphatic and Immune Systems contains a new diagram on B cell clonal selection (Fig. 13.5) Chapter 14: Digestive System and Nutrition now contains the formula for BMI calculations and a new figure (Fig. 14.16) on the lipid content of selected fats and oils. Chapter 15: Respiratory System includes a new figure on Boyle’s Law (Fig. 15.7) and a new Health feature on the safety of e-cigs. viii

Unit 4: Integration and Control of the Human Body Chapter 17: Nervous System contains an updated PET scan (Fig. 17.19) on the effects of cocaine on the brain. Chapter 18: Senses has a new opener on LASIK surgery. Chapter 19: Musculoskeletal System contains a new figure (Fig. 19.11) on the types of movement associated with synovial joints and a new feature on the discovery of Botox®. Chapter 20: Endocrine System begins with a new article on diabetes in young adults.

Unit 5: Continuance of the Species Chapter 21: Reproductive System starts with new content on IVF and the “three-parent” baby, and the content on emergency contraception (section 21.4) has been expanded. A new feature on the challenges of developing a HUV vaccine is included. Chapter 22: Development and Aging  now begins with a discussion of the FOX03A gene and aging. Chapter 23: Patterns of Gene Inheritance opens with an article on the genetics of phenylketonuria. The traits used in the discussion of monohydrid and dihybrid crosses have been changed. Chapter 24: Chromosomal Basis of Inheritance contains a new figure on karyotypes (Fig. 24.7). Chapter 25: DNA Structure and Gene Expression  has a new figure as an overview of DNA replication (Fig. 25.4). The content on mutations in cell cycle regulating proteins has been moved to Chapter 5. Chapter 26: Biotechnology and Genomics starts with a new essay on producing insulin using biotechnology, and includes a featured reading on reproductive and therapeutic cloning.

Unit 6: Evolution and Diversity The opener for Chapter 27: Evolution of Life now contains references to resistance in Shigella. The geologic time scale (Table 27.1) has been updated to make it more useful for students. Chapter 28: Microbiology now begins with a look at the 2014–2015 Ebola outbreak in Africa. A new feature on DIY Biology has been ­ added.  Chapter 29: Protists and Fungi has a new opener on ­Naegleria fowleri. The content on protists in section 29.1 is now arranged by supergroups. Section 29.2 now contains more information on the evolutionary relationships of the fungi Chapter 30: Plants begins with an essay on the source of coal. Table 30.1 has been updated with more differences between monocots and  eudicots. Chapter 31: Animals: The Invertebrates has a  new figure on the general features of animals (Fig. 31.1). Chapter 32: Animals: Chordates and Vertebrates begins with

a look at canine evolution. The content on differences in vertebrate circulatory pathways has been made into a separate figure (Fig. 32.8). The content on human evolution in section 32.4 has been reworked to reflect new discoveries of human ancestors (Fig. 32.17) and a new diagram of human migration from Africa (Fig. 32.19). Material on Denisovans has been added to section 32.5.

Unit 7: Behavior and Ecology Chapter 33: Behavioral Ecology has a new feature on epigenetics and twin behavior studies. Chapter 34: Population

and Community Ecology  has a new graph on predator-prey cycles (Fig. 34.10). Chapter 35: Nature of Ecosystems now starts with an essay on the wolves of Yellowstone park. The chapter has a new feature on the California droughts and a­quifers. In Chapter 36: Major Ecosystems of the Biosphere, the section on aquatic ecosystems (section 36.3) now contains a feature on the fate of prescription medicine. Chapter 37: C ­ onservation Biology  contains a new diagram on the number of  catalogued ­species (Fig. 37.1)

Identifying Content to be Revised: Heat Maps This edition of Inquiry is the fourth textbook in the Mader series which has identified content areas for revision by analyzing the data derived from the LearnSmart platform. The premise is very straightforward. Students don’t know what they don’t know— but LearnSmart does. By compiling data from all of the probes answered by all of the students, and then overlaying that data on the text, we are able to visualize areas of content where the students are having problems. The authors were able to use this information to not only identify areas of the text that the students were having problems with, but also areas that needed additional digital resources, such as tutorials and new Connect questions.

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Acknowledgments Dr. Sylvia Mader is one of the icons of science education. Her dedication to her students, coupled to her clear, concise writing style, has benefited the education of thousands of students over the past four decades. As an educator, it is an honor to continue her legacy and to bring her message to the next generation of students. As always, I had the privilege to work with a phenomenal group of people on this edition. I would especially like to thank you, the numerous instructors who have shared emails with me or have invited me into your classrooms, both physically and virtually, to discuss your needs as instructors and the needs of your students. You are all dedicated and talented teachers, and your energy and devotion to quality teaching is what drives a textbook revision. Many dedicated and talented individuals assisted in the development of Inquiry into Life Fifteenth Edition. I am very grateful for the help of so many professionals at McGraw-Hill who were involved in bringing this book to fruition. Therefore, I would like to thank the following: ∙ The product developer, Anne Winch, for her patience and impeccable ability to keep me focused. ∙ My brand manager, Chris Loewenberg, for his guidance and reminding me why what we do is important. ∙ My marketing manager, Chris Ho, and market development manager, Jenna Paleski, for placing me in contact with great instructors, on campus and virtually, throughout this process. ∙ The digital team of Eric Weber and Christine Carlson for helping me envision the possibilities in our new digital world. ∙ My content project manager, April Southwood, and program manager, Angie Fitzpatrick, for calmly steering this project throughout the publication process. ∙ Lori Hancock and Jo Johnson for the photos within this text. Biology is a visual science, and your contributions are evident on every page.

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∙ David Hash for the design elements in this text, including one of the most beautiful textbook covers in the business. ∙ Dawnelle Krouse and Debbie Budde-Bandy who acted as my proofreaders and copyeditors for this edition. ∙ Jane Peden for her behind the scenes work that keeps us all functioning. ∙ Inkling for finally giving me the authoring platform I have been asking for, and Aptara for all of their technical assistance. Who I am, as an educator and an author, is a direct reflection of what I have learned from my students. Education is a mutualistic relationship, and it is my honest opinion that while I am a teacher, both my professional and personal life have been enriched by interactions with my students. They have encouraged me to learn more, teach better, and never stop questioning the world around me. Last, but never least, I want to acknowledge my wife, Sandra. You have never wavered in your energy and support of my projects. To my children, Devin and Kayla, your natural curiosity of the world we live in gives me the energy to want to make the world a better place. Michael Windelspecht, Ph.D. Blowing Rock, NC

Ancillary Authors Appendix Answer Key: Betsy Harris, Appalachian State University; SmartBook: Krissy Johnson and Alex James, Ricochet Creative Productions; Patrick Galliart, North Iowa Area Community College; Connect Question Bank: Alex James, Ricochet Creative Productions;  Test Bank: Dave Cox, Lincoln Land Community College; Lecture Outlines and    Instructor’s Manual:  Dan Matusiak, St. Charles Community College.

Brief Contents 1. The Study of Life  1

Unit 1  Cell Biology

Unit 5  Continuance of the Species 21. Reproductive System  412 22. Development and Aging  440

2. The Molecules of Cells  17

23. Patterns of Gene Inheritance  464

3. Cell Structure and Function  43

24. Chromosomal Basis of Inheritance  482

4. Membrane Structure and Function  63

25. DNA Structure and Gene Expression  496

5. Cell Division  79

26. Biotechnology and Genomics  519

6. Metabolism: Energy and Enzymes  99 7. Cellular Respiration  113

Unit 2  Plant Biology

Unit 6  Evolution and Diversity 27. Evolution of Life  535 28. Microbiology 563

8. Photosynthesis 127

29. Protists and Fungi  584

9. Plant Organization and Function  143

30. Plants 606

10. Plant Reproduction and Responses  169

31. Animals: The Invertebrates  624 32. Animals: Chordates and Vertebrates  652

Unit 3 Maintenance of the Human Body

Unit 7  Behavior and Ecology

11. Human Organization  190

33. Behavioral Ecology  673

12. Cardiovascular System  210

34. Population and Community Ecology  690

13. Lymphatic and Immune Systems  232

35. Nature of Ecosystems  706

14. Digestive System and Nutrition  253

36. Major Ecosystems of the Biosphere  722

15. Respiratory System  279

37. Conservation Biology  743

16. Urinary System and Excretion  298

Unit 4 Integration and Control of the Human Body 17. Nervous System  313 18. Senses 341 19. Musculoskeletal System  362 20. Endocrine System  391

xi

Contents

chapter 

1

chapter 

4

The Study of Life  1

Membrane Structure and Function  63

1.1 The Characteristics of Life  2

4.1 Plasma Membrane Structure and Function  64

1.2 The Classification of Organisms  6

4.2 The Permeability of the Plasma Membrane  66

1.3 The Process of Science  8

4.3 Modifications of Cell Surfaces  74

1.4 Challenges Facing Science  13 chapter 

Unit 1 Cell Biology chapter 

2

The Molecules of Cells  17 2.1 Basic Chemistry  18 2.2 Molecules and Compounds  22 2.3 Chemistry of Water  25

5

Cell Division  79 5.1 The Cell Cycle  80 5.2 Control of the Cell Cycle  81 5.3 Mitosis: Maintaining the Chromosome Number 84 5.4 Meiosis: Reducing the Chromosome Number  88 5.5 Comparison of Meiosis with Mitosis  92 5.6 The Human Life Cycle  94

2.4 Organic Molecules  28

6

2.5 Carbohydrates 30

chapter 

2.6 Lipids 33

Metabolism: Energy and Enzymes  99

2.7 Proteins 35

6.1 Life and the Flow of Energy  100

2.8 Nucleic Acids  38 chapter 

3

Cell Structure and Function  43 3.1 The Cellular Level of Organization  44 3.2 Prokaryotic Cells  46

6.2 Energy Transformations and Metabolism  102 6.3 Enzymes and Metabolic Pathways  103 6.4 Oxidation-Reduction Reactions and Metabolism 108 chapter 

7

3.3 Eukaryotic Cells  48

Cellular Respiration  113

3.4 The Cytoskeleton  56

7.1 Overview of Cellular Respiration  114

3.5 Origin and Evolution of the Eukaryotic Cell  58

7.2 Outside the Mitochondria: Glycolysis  116 7.3 Outside the Mitochondria: Fermentation  118 7.4 Inside the Mitochondria  119

xii

Contents xiii

Unit 2  Plant Biology chapter 

8

Photosynthesis 127 8.1 Overview of Photosynthesis  128 8.2 Plants as Solar Energy Converters  131 8.3 Plants as Carbon Dioxide Fixers  134 8.4 Alternative Pathways for Photosynthesis  137 8.5 Photosynthesis Versus Cellular Respiration  139 chapter 

9

Plant Organization and Function  143 9.1 Cells and Tissues of Plants  144 9.2 Plants Organs and Systems  147 9.3 Monocot Versus Eudicot Plants  149 9.4 Organization of Roots  150 9.5 Organization of Stems  153 9.6 Organization of Leaves  159 9.7 Uptake and Transport of Nutrients  161 chapter 

10

Plant Reproduction and Responses  169 10.1 Sexual Reproduction in Flowering Plants  170 10.2 Growth and Development  175 10.3 Asexual Reproduction and Genetic Engineering in Plants  178 10.4 Control of Growth and Responses  183

chapter 

12

Cardiovascular System  210 12.1 The Blood Vessels  211 12.2 Blood 212 12.3 The Human Heart  218 12.4 Two Cardiovascular Pathways  222 12.5 Cardiovascular Disorders  225 chapter 

13

Lymphatic and Immune Systems  232 13.1 The Lymphatic System  233 13.2 Innate Immunity  235 13.3 Adaptive Immunity  237 13.4 Active Versus Passive Immunity  241 13.5 Adverse Effects of Immune Responses  245 13.6 Disorders of the Immune System  248 chapter 

14

Digestive System and Nutrition  253 14.1 The Digestive Tract  254 14.2 Accessory Organs of Digestion  260 14.3 Digestive Enzymes  262 14.4 Human Nutrition  263 14.5 Eating Disorders  272 14.6 Disorders of the Digestive System  273 chapter 

15

Respiratory System  279

Unit 3 Maintenance of the Human Body chapter 

11

Human Organization  190

15.1 The Respiratory System  280 15.2 Mechanism of Breathing  283 15.3 Gas Exchanges in the Body  286 15.4 Disorders of the Respiratory System  289 chapter 

16

11.1 Types of Tissues  191

Urinary System and Excretion  298

11.2 Body Cavities and Body Membranes  197

16.1 The Urinary System  299

11.3 Organ Systems  198

16.2 Anatomy of the Kidney and Excretion  301

11.4 Integumentary System  200

16.3 Regulatory Functions of the Kidneys  304

11.5 Homeostasis 204

16.4 Disorders of the Urinary System  307

xiv

Contents

Unit 4 Integration and Control of the Human Body chapter 

17

Nervous System  313 17.1 Nervous Tissue  314 17.2 Transmission of Nerve Impulses  315 17.3 The Central Nervous System  319 17.4 The Limbic System and Higher Mental Functions 324 17.5 The Peripheral Nervous System  327 17.6 Drug Abuse  331

chapter 

Endocrine System  391 20.1 Overview of the Endocrine System  392 20.2 Hypothalamus and Pituitary Gland  395 20.3 Thyroid and Parathyroid Glands  398 20.4 Adrenal Glands  399 20.5 Pancreas 401 20.6 Other Endocrine Glands  402 20.7 Disorders of the Endocrine System  403

Unit 5  Continuance of the Species

17.7 Disorders of the Nervous System  334 chapter 

18

Senses 341

18.1 Sensory Receptors and Sensations  342 18.2 Somatic Senses  343 18.3 Senses of Taste and Smell  345 18.4 Sense of Vision  347 18.5 Sense of Hearing  351 18.6 Sense of Equilibrium  353 18.7 Disorders that Affect the Senses  354 chapter 

19

Musculoskeletal System  362 19.1 Overview of Bone and Cartilage  363 19.2 Bones of the Skeleton  366 19.3 Skeletal Muscles  375 19.4 Mechanism of Muscle Fiber Contraction  377 19.5 Whole Muscle Contraction  383 19.6 Disorders of the Musculoskeletal System  385

20

chapter 

21

Reproductive System  412 21.1 Male Reproductive System  413 21.2 Female Reproductive System  416 21.3 Ovarian and Uterine Cycles  418 21.4 Control of Reproduction  422 21.5 Sexually Transmitted Diseases  424 21.6 Reproductive Disorders and Assisted Reproductive Technologies  432 chapter 

22

Development and Aging  440 22.1 Fertilization and Early Stages of Development  441 22.2 Processes of Development  444 22.3 Human Embryonic and Fetal Development  448 22.4 Human Pregnancy, Birth, and Lactation  453 22.5 Aging 457 chapter 

23

Patterns of Gene Inheritance  464 23.1 Mendel’s Laws  465 23.2 Pedigree Analysis and Genetic Disorders  472 23.3 Beyond Simple Inheritance Patterns  475 23.4 Environmental Influences  478

Contents xv

chapter 

24

chapter 

28

Chromosomal Basis of Inheritance  482

Microbiology 563

24.1 Gene Linkage  483

28.1 The Microbial World  564

24.2 Sex-Linked Inheritance  484

28.2 Origin of Microbial Life  565

24.3 Changes in Chromosome Number  487

28.3 Archaea 569

24.4 Changes in Chromosome Structure  491

28.4 Bacteria 571

chapter 

25

DNA Structure and Gene Expression  496

28.5 Viruses, Viroids, and Prions  577 chapter 

29

25.1 DNA Structure  497

Protists and Fungi  584

25.2 DNA Replication  501

29.1 Protists 585

25.3 Gene Expression  502

29.2 Fungi 595

25.4 Control of Gene Expression  508 25.5 Gene Mutations and Cancer  511 chapter 

26

Biotechnology and Genomics  519 26.1 DNA Technology  520 26.2 Biotechnology Products  524

chapter 

30.1 Evolutionary History of Plants  607 30.2 Nonvascular Plants  610 30.3 Seedless Vascular Plants  611 30.4 Seed Plants  615

26.3 Gene Therapy  527 26.4 Genomics, Proteomics, and Bioinformatics  530

30

Plants 606

chapter 

31

Animals: The Invertebrates  624

Unit 6  Evolution and Diversity chapter 

27

Evolution of Life  535 27.1 Theory of Evolution  536 27.2 Evidence of Evolution  539

31.1 Evolutionary Trends Among Animals  625 31.2 The Simplest Invertebrates  629 31.3 The Lophotrochozoa  632 31.4 The Ecdysozoa  640 31.5 Invertebrate Deuterostomes  647 chapter 

32

27.3 Microevolution 545

Animals: Chordates and Vertebrates  652

27.4 Processes of Evolution  548

32.1 Chordates 653

27.5 Macroevolution and Speciation  555

32.2 Vertebrates: Fish and Amphibians  655

27.6 Systematics 557

32.3 Vertebrates: Reptiles and Mammals  659 32.4 Evolution of the Hominins  664 32.5 Evolution of Modern Humans  668

xvi

Contents

Unit 7  Behavior and Ecology chapter 

33

Behavioral Ecology  673 33.1 Nature Versus Nurture: Genetic Influences  674

chapter 

36

Major Ecosystems of the Biosphere  722 36.1 Climate and the Biosphere  723 36.2 Terrestrial Ecosystems  724 36.3 Aquatic Ecosystems  732

37

33.2 Nature Versus Nurture: Environmental Influences 675

chapter 

33.3 Animal Communication  678

Conservation Biology  743

33.4 Behaviors That Affect Fitness  682

37.1 Conservation Biology and Biodiversity  744

chapter 

34

Population and Community Ecology  690 34.1 The Scope of Ecology  691 34.2 Patterns of Population Growth  692 34.3 Interactions Between Populations  695 34.4 Ecological Succession  702 chapter 

35

Nature of Ecosystems  706 35.1 The Biotic Components of Ecosystems  707 35.2 Energy Flow  708 35.3 Global Biogeochemical Cycles  710

37.2 Value of Biodiversity  746 37.3 Threats to Biodiversity  749 37.4 Habitat Conservation and Restoration  754 37.5 Working Toward a Sustainable Society  757 Appendix A  Answer Key  A-1 Appendix B  M  etric System and The Periodic Table of the Elements A-16 Glossary G-1 Index I-1

CASE STUDY The Search for Life You may never have heard of Enceladus or Europa, but they are both at the frontline of our species’ effort to understand the nature of life. Enceladus is one of Saturn’s moons, and Europa orbits Jupiter. Why are these moons so special? Because scientists believe that both of these moons contain water, and plenty of it. While both Enceladus and Europa are far from the sun, the gravitational pull of their parent planets means that beneath the frozen surface of both of these moons are oceans of liquid water. And as we will see, water has an important relationship with life. At other locations in our solar system, scientists are looking for evidence of the chemicals that act as the building blocks of life. For example on Titan, a moon of Saturn, NASA’s Cassini-Huygens space probe has detected the presence of these building blocks, including lakes of methane and ammonia, and vast deposits of hydrogen and carbon compounds called hydrocarbons.  Even more recently, the Rosetta space probe, launched by the ESA, completed its 10 year mission to land a probe on the surface of a comet. One of the most important aspects of this mission is to determine whether the chemical composition of comets includes the organic building blocks of life. NASA has recently announced missions to Europa and Mars that will continue the search for signs of life in our solar system. The information obtained from these missions will help us better understand how life originated on our planet. In this chapter, we are going to explore what it means to be alive, and some of the general characteristics that are shared by all living organisms on our planet.

The Study of Life CHAPTER OUTLINE 1.1 The Characteristics of Life 1.2 The Classification of Organisms 1.3 The Process of Science 1.4 Challenges Facing Science

1

As you read through the chapter, think about the following questions:

1. What are the basic characteristics that define life? 2. What evidence would you look for on one of these moons or Mars that would tell you that life may have existed on them in the past?

3. What does it tell us if we discover life on one of these moons and it has characteristics similar to those of life on Earth? What if it is very different?

1

2

Chapter 1  The Study of Life

1.1  The Characteristics of Life Learning Outcomes Upon completion of this section, you should be able to 1. Identify the basic characteristics of life. 2. Distinguish between the levels of biological organization. 3. Recognize the importance of adaptation and evolution to life.

Life. Everywhere we look, from the deepest trenches of the oceans to the geysers of Yellowstone, we find that planet Earth is teeming with life. Without life, our planet would be nothing but a barren rock hurtling through space. The variety of life on Earth is staggering, recent estimates suggest that there are around 8.7 million species on the planet, and humans are a part of it. The variety of living organisms ranges in size from bacteria, much too small to be seen

Masai giraffes

by the naked eye, all the way up to giant sequoia trees that can reach heights of 100 meters (m) or more (Fig. 1.1). The diversity of life seems overwhelming, and yet all living organisms have certain characteristics in common. Taken together, these characteristics give us insight into the nature of life and help us distinguish living organisms from nonliving things. All life ­generally shares the following characteristics (1) is organized, (2) requires materials and energy, (3) has the ability to reproduce and develop, (4) responds to stimuli, (5) is homeostatic, and (6) has the capacity to adapt to their environment. In the next sections we explore each of these characteristics in more detail.

Life is Organized Life can be organized in a hierarchy of levels (Fig. 1.2). In trees, humans, and all other organisms, atoms join together to form ­molecules, such as DNA molecules that occur within cells. A cell is

rod-shaped E. coli, 7,000×

giant sequoia

planet Earth

mushroom on northern forest floor

young adults

euglena

Figure 1.1  Life on planet Earth.  If aliens ever visit our corner of the universe, they will be amazed at the diversity of life on our planet. Yet despite its diversity, all life shares some common characteristics.



Chapter 1  The Study of Life

Biosphere Regions of the Earth’s crust, waters, and atmosphere inhabited by living organisms

Figure 1.2  Levels of biological organization.  Life is connected from the atomic level to the biosphere. While the cell is the basic unit of life, it comprises molecules and atoms. The sum of all life on the planet is called the biosphere.

Ecosystem A community plus the physical environment

Community Interacting populations in a particular area

Population Organisms of the same species in a particular area

human

tree

Organism An individual; complex individuals contain organ systems

Organ System Composed of several organs working together

Organ Composed of tissues functioning together for a specific task

nervous system

shoot system

the brain

leaves

Tissue A group of cells with a common structure and function nervous tissue Cell The structural and functional unit of all living organisms

Molecule Union of two or more atoms of the same or different elements

Atom Smallest unit of an element; composed of electrons, protons, and neutrons

3

leaf tissue

nerve cell

plant cell

methane

oxygen

Chapter 1  The Study of Life

Solar energy

Heat

Producers

Heat

Consumers

Chemicals

the smallest unit of life, and some organisms are single-celled. In multicellular organisms, a cell is the smallest structural and functional unit. For example, a human nerve cell is responsible for conducting electrical impulses to other nerve cells. A tissue is a group of similar cells that perform a particular function. Nervous tissue is composed of millions of nerve cells that transmit signals to all parts of the body. Several tissues then join together to form an organ. The main organ that receives signals from nerves is the brain. Organs then work together to form an organ system. In the nervous system, the brain sends messages to the spinal cord, which in turn sends them to body parts through spinal nerves. Complex organisms such as trees and humans are a collection of organ systems. The levels of biological organization extend beyond the individual. All the members of one species (a group of interbreeding organisms) in a particular area belong to a population. A tropical grassland may have a population of zebras, acacia trees, and humans, for example. The interacting populations of the grasslands make up a community. The community of populations interacts with the physical environment to form an ecosystem. Finally, all the Earth’s ecosystems collectively make up the biosphere.

Chemicals

4

Life Requires Materials and Energy Living organisms need an outside source of materials and energy to maintain their organization and carry on life’s other activities. Plants, such as trees, use carbon dioxide, water, and solar energy to make their own food. Humans and other animals acquire materials and energy by eating food. The food we eat provides nutrients, which cells use as building blocks or for energy—the capacity to do work. Cells use energy from nutrients to carry out everyday activities. Some nutrients are broken down completely by chemical reactions to provide the necessary energy to carry out other reactions, such as building proteins. The term metabolism is used to describe all of the chemical reactions that occur in a cell.  Cells need energy to perform their metabolic functions, and it takes work to maintain the organization of a cell as well as an organism. The ultimate source of energy for nearly all life on Earth is the sun. Plants and certain other organisms are able to capture solar energy and carry on photosynthesis, a process that transforms solar energy into the chemical energy of organic nutrient molecules. All life on Earth acquires energy by metabolizing nutrient molecules made by photosynthesizers. This applies even to plants themselves. The energy and chemical flow between organisms also defines how an ecosystem functions (Fig. 1.3). Within an ecosystem, chemical cycling and energy flow begin when producers, such as grasses, take in solar energy and inorganic nutrients to produce food (organic nutrients) by photosynthesis. Chemical cycling (aqua arrows in Fig. 1.3) occurs as chemicals move from one population to another in a food chain, until death and decomposition allow inorganic nutrients to be returned to the producers once again. Energy (red arrows), on the other hand, flows from the sun through plants and the other members of the food chain as they feed on one another. The energy gradually dissipates and returns to the atmosphere as heat. Because energy does not cycle, ecosystems could not stay in existence without solar energy and the ability of photosynthetic organisms to absorb it.

Decomposers

Heat

Figure 1.3  Chemical cycling and energy flow in an

ecosystem.  In an ecosystem, chemical cycling (aqua arrows) and energy flow (red arrows) begin when plants use solar energy and inorganic nutrients to produce their own food. Chemicals and energy are passed from one population to another in a food chain. Eventually, energy dissipates as heat. With the death and decomposition of organisms, chemicals are returned to living plants once more.

Energy flow and nutrient cycling in an ecosystem climate largely determine not only where different ecosystems are found in the biosphere but also what communities are found in the ­ecosystem. For example, deserts exist in areas of minimal rain, while forests require much rain. The two most biologically diverse ­ecosystems—tropical rain forests and coral reefs—occur where solar energy is most abundant. One example of an ­ecosystem in North America is the grasslands, which are inhabited by populations of rabbits, hawks, and various types of grasses, among many others. These populations interact with each other by forming food chains in which one population feeds on another. For example, rabbits feed on grasses, while hawks feed on rabbits and other organisms.

Living Organisms Reproduce and Develop Life comes only from life. All forms of life have the capability of reproduction, or to make another organism like itself. Bacteria, protists, and other single-celled organisms simply split in two. In most multicellular organisms, the reproductive process begins with the pairing of a sperm from one partner and an egg from the other partner. The union of sperm and egg (Fig. 1.4), followed by



Chapter 1  The Study of Life

5

The movement of an organism, whether self-directed or in response to a stimulus, constitutes a large part of its behavior. Behavior is largely directed toward minimizing injury, acquiring food, and reproducing.

Living Organisms Are Homeostatic

a.

480×

b.

Figure 1.4  Growth and development define life.  Following the (a) fertilization of an egg cell by a sperm cell (b) humans grow and develop. All life exhibits the characteristics of growth and development.

Homeostasis means “staying the same.” Actually, the internal environment of an organism stays relatively constant. For example, human body temperature will show only a slight fluctuation throughout the day. Also, the body’s ability to maintain a normal internal temperature is somewhat dependent on the external temperature— we will die if the external temperature becomes too hot or cold. Organisms have intricate feedback and control mechanisms that do not require any conscious activity. These mechanisms may be controlled by one or more tissues themselves or by the nervous system. When you are studying and forget to eat lunch, your liver releases stored sugar to keep blood sugar levels within normal limits. Many organisms depend on behavior to regulate their internal environment. In animals, these behaviors are controlled by the nervous system and are usually not consciously controlled. For example, a lizard may raise its internal temperature by basking in the sun, or cool down by moving into the shade.

Organisms Have the Capacity to Adapt

Throughout the nearly 4 billion years that life has been on Earth, the environment has constantly been changing. For example, glaciers that once covered much of the world’s surface 10,000–15,000 years many cell divisions, results in an immature stage, which proceeds ago have since receded, and many areas that were once covered by through stages of development, or change, to become an adult. ice are now habitable. On a smaller scale, a hurricane or fire could When living organisms reproduce, their genes, or genetic drastically change the landscape in an area quite rapidly. instructions, are passed on to the next generation. Random combiAs the environment changes, some individuals of a species nations of sperm and egg, each of which contains a unique collec(a group of organisms that can successfully interbreed and produce tion of genes, ensure that the offspring has new and different fertile offspring) may possess certain features that make them better characteristics. An embryo develops into a whale, a yellow daffosuited to the new environment. We call such features adaptations. dil, or a human because of the specific set of genes it inherits from For example, consider a hawk, which can catch and eat a rabbit. A its parents. In all organisms, the genes are made of long DNA hawk, like other birds, can fly because it has hollow bones, which (deoxyribonucleic acid) molecules. DNA provides the blueprint, is an adaptation. Similarly, its strong feet can take the shock of a or instructions, for the organization and metabolism of the particulanding after a hunting dive, and its sharp claws can grab and hold lar organism. All cells in a multicellular organism contain the same onto prey. As is presented in the Scientific Inquiry feature, “Adaptset of genes, but only certain genes are turned on in each type of ing to Life at High Elevations,” humans also exhibit adaptations to specialized cell. You may notice that not all members of a species, their environment. including humans, are exactly the same, and that there are obvious Individuals of a species that are better adapted to their envidifferences between species. These differences are the result of ronment tend to live longer and produce more offspring than other mutations, or inheritable changes in the genetic information. individuals. This differential reproductive success, called natural Mutation provides an important source of variation in the genetic selection, results in changes in the characteristics of a population information. However, not all mutations are bad—the observable (all the members of a species within a particular area) through differences in eye and hair color are examples of mutations.  time. That is, adaptations that result in higher reproductive success tend to increase in frequency in a population from one generation Living Organisms Respond to Stimuli to the next. This change in the frequency of traits in populations Organisms respond to external stimuli, often by moving toward or and species is called evolution. away from a stimulus, such as the smell of food. Right now, your Evolution explains both the unity and diversity of life. As eyes and ears are receiving stimuli from the external environment. stated at the beginning of this chapter, all organisms share the same Movement in animals, including humans, is dependent upon their basic characteristics of life because we all share a common nervous and musculoskeletal systems. Other living organisms use a ­ancestor—the first cell or cells—that arose nearly 4 billion years variety of mechanisms in order to move. The leaves of plants track ago. During the past 4 billion years, the Earth’s environment has the passage of the sun during the day, and when a houseplant is changed drastically, and the diversity of life has been shaped by the placed near a window, hormones help its stem bend to face the sun. evolutionary responses of organisms to these changes.

6

Chapter 1  The Study of Life

SCIENCE IN YOUR LIFE  ►

SCIENTIFIC INQUIRY

Adapting to Life at High Elevations Humans, like all other organisms, have an evolutionary history. This means not only that we share common ancestors with other animals but also that over time we demonstrate adaptations to changing environmental conditions. One study of populations living in the highelevation mountains of Tibet (Fig. 1A) demonstrates how the processes of evolution and adaptation influence humans. Normally, if a person moves to a higher altitude, his or her body responds by making more hemoglobin, the component of blood that carries oxygen, which thickens the blood. For minor elevation changes, this does not present

Figure 1A  Humans’ adaptations to

their environments.  Humans have adaptations that allow them to live at high altitudes, such as these individuals in Tibet.

much of a problem. But for people who live at extreme elevations (some people in the Himalayas can live at elevations of over 13,000 ft, or close to 4,000 m), this can present a number of health problems, including chronic mountain sickness, a disease that affects people who live at high altitudes for extended periods of time. The problem is that, as the amount of hemoglobin increases, the blood thickens and becomes more viscous. This can cause elevated blood pressure, or hypertension, and an increase in the formation of blood clots, both of which have negative physiological effects. Because high hemoglobin levels would be a detriment to people at high elevations, it makes sense that natural selection would favor individuals who produced less hemoglobin at high elevations. Such is the case with the Tibetans in this study. Researchers have identified an allele of a gene that reduces hemoglobin production at high elevations. Comparisons between Tibetans at both high and low elevations strongly suggest that selection has played a role in the prevalence of the high-elevation allele. The gene is EPSA1, located on chromosome 2 of humans. EPSA1 produces a transcription factor, which basically regulates which genes are turned on and off in the body, a process called gene expression. The transcription factor produced by EPSA1 has a number of functions in the body. For example, in addition to controlling the amount of hemoglobin in the blood, this transcription factor also regulates other genes that direct how the body uses oxygen.

Check Your Progress  1.1 1. List the common characteristics of all living organisms. 2. Trace the organization of life from the cell to the biosphere. 3. Explain how adaptations relate to evolutionary change.

1.2  The Classification of Organisms Learning Outcomes Upon completion of this section, you should be able to 1. Describe how living organisms are classified. 2. Distinguish between the three domains of life.

Because life is so diverse, it is helpful to group organisms into categories. Taxonomy is the discipline of identifying and grouping organisms according to certain rules. Taxonomy makes sense

When the researchers examined the variations in EPSA1 in the Tibetan population, they discovered that their version greatly reduces the  production of hemoglobin. Therefore, the Tibetan population has lower hemoglobin levels than people living at lower altitudes, allowing these individuals to escape the consequences of thick blood. How long did it take for the original population to adapt to living at higher elevations? Initially, the comparison of variations in these genes between high-elevation and low-elevation Tibetan populations suggested that the event may have occurred over a 3,000-year period. But researchers were skeptical of that data since it represented a relatively rapid rate of evolutionary change. Additional studies of genetic databases yielded an interesting finding—the EPSA1 gene in Tibetans was identical to a similar gene found in an ancient group of humans called the Denisovans (see chapter 32). Scientists now believe that the EPSA1 gene entered the Tibetan population around 40,000 years ago, either through interbreeding between early Tibetans and Denisovans, or from one of the immediate ancestors of this lost group of early humans.

Questions to Consider 1. What other environments do you think could be studied to look for examples of human adaptation? 2. In addition to hemoglobin levels, do you think that people at high elevations may exhibit other adaptations?

out of the bewildering variety of life on Earth and is meant to provide valuable insight into evolution. Systematics is the study of the evolutionary relationships between organisms. As systematists learn more about living organisms, the taxonomy often changes. DNA technology is now widely used by systematists to revise current information and to discover previously unknown relationships between organisms. Several of the basic classification categories, also called taxa, are: domain, kingdom, phylum, class, order, family, genus, and finally species. These are listed in order from the most inclusive (domains), to the least inclusive (species).

Domains Domains are the largest classification category. Based upon biochemical and genetic evidence scientists have identified three domains: domain Archaea, domain Bacteria, and domain Eukarya (Fig. 1.5). Both domain Archaea and domain Bacteria



7

Chapter 1  The Study of Life

DOMAIN ARCHAEA

DOMAIN BACTERIA

Sulfolobus sp a. Archaea are capable of living in extreme environments.

33,2003

Escherichia coli 6,6003

b. Bacteria are found nearly everywhere.

DOMAIN EUKARYA Kingdom

Organization

Protista

Complex single cell, some multicellular

Type of Nutrition

Representative Organisms

Absorb, photosynthesize, or ingest food

Protozoans, algae, water molds, and slime molds paramecium

Fungi

Some single-celled, most multicellular filamentous forms with specialized complex cells

euglenoid

Plantae

Molds, yeasts, and mushrooms

Animalia

yeast

mushroom

bracket fungus

Photosynthesize food moss

Multicellular form with specialized complex cells

dinoflagellate

Absorb food

black bread mold

Multicellular form with specialized complex cells

slime mold

fern

pine tree

nonwoody flowering plant

Mosses, ferns, nonwoody and woody flowering plants

Invertebrates, fishes, reptiles, amphibians, birds, and mammals

Ingest food

sea star

earthworm

finch

human

c. Eukaryotes are divided into four kingdoms.

Figure 1.5  The three domains of life.  Archaea (a) and bacteria (b) are both prokaryotes but are so biochemically different that they are not believed to be closely related. c. Eukaryotes are biochemically similar but structurally dissimilar. Therefore, they have been categorized into four kingdoms. Many protists are

single-celled, but the other three kingdoms are characterized by multicellular forms.

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Chapter 1  The Study of Life

contain single-celled  prokaryotes, which lack the membranebound nucleus found in the cells of eukaryotes in domain Eukarya. The genes of eukaryotes are found in the nucleus.  Prokaryotes are structurally simple (Fig. 1.5a, b) but metabolically complex. The Archaea live in aquatic environments that lack oxygen or are too salty, too hot, or too acidic for most other organisms. Since these environments are similar to those of the primitive Earth, archaea represent the first cells that evolved on the planet. Bacteria are found almost anywhere—in the water, soil, and atmosphere, as well as on our skin and in our digestive tracts. Although some bacteria cause diseases, others are beneficial, both environmentally and commercially. For example, bacteria can be used to develop new medicines, to clean up oil spills, or to help purify water in sewage treatment plants.

another example, all species in the genus Pisum (pea plants) look quite similar, while the species in the plant kingdom can be quite different, as is evident when we compare grasses to trees. Systematics helps biologists make sense out of the bewildering variety of life on Earth because organisms are classified according to their presumed evolutionary relationships. Organisms placed in the same genus are the most closely related, and those in separate domains are the most distantly related. Therefore, all eukaryotes are more closely related to one another than they are to bacteria or archaea. Similarly, all animals are more closely related to one another than they are to plants. As more is learned about evolutionary relationships among species, systematic relationships are changed. Systematists are even now making observations and performing experiments that will soon result in changes in the classification system adopted by this text.

Kingdoms Systematists are in the process of deciding how to categorize archaea and bacteria into kingdoms. The eukaryotes are currently classified into at least four kingdoms with which you may be familiar (Fig. 1.5c). Protists (kingdom Protista) are primarily singlecelled organisms, but there are a few multicellular species. Some can make their own food (photosynthesizers), while others must ingest their food. Because of the great diversity among protists, however, the Protista have been split into various supergroups to more accurately represent their evolutionary relationships. Among the fungi (kingdom Fungi) are the familiar molds and mushrooms that help decompose dead organisms. Plants (kingdom Plantae) are well known as multicellular photosynthesizers, while animals (kingdom Animalia) are multicellular and ingest their food.

Other Categories The other classification categories are phylum, class, order, family, genus, and species. Each classification category is more specific than the one preceding it. For example, the species within one genus share very similar characteristics, while those within the same kingdom share only general characteristics. Modern humans are the only living species in the genus Homo, but many different types of animals are in the animal kingdom (Table 1.1). To take

TABLE 1.1  Classification of Humans Classification Category

Characteristics

Domain Eukarya

Cells with nuclei

Kingdom Animalia

Multicellular, motile, ingestion of food

Phylum Chordata

Dorsal supporting rod and nerve cord

Class Mammalia

Hair, mammary glands

Order Primates

Adapted to climb trees

Family Hominidae

Adapted to walk erect

Genus Homo

Large brain, tool use

Species Homo sapiens*

Body characteristics similar to modern humans

* To specify an organism, you must use the full binomial name, such as Homo sapiens.

Scientific Names Taxonomists assign a binomial, or two-part name, to each species. For example, the scientific name for human beings is Homo ­sapiens, and for the garden pea, Pisum sativum. The first word is the genus to which the species belongs, and the second word is the specific epithet, or species name. Note that both words are in italics, but only the genus name is capitalized. The genus name can be used alone to refer to a group of related species. Also, a genus can be abbreviated to a single letter if used with the species name (e.g., P. sativum). Scientific names are in a common language—Latin—and biologists use them universally to avoid confusion. Common names, by contrast, tend to overlap across multiple species.

Check Your Progress  1.2 1. Recognize the main criteria for classification of organisms into domains and kingdoms.

2. List the levels of taxonomic classification from most

inclusive to least inclusive. 3. Explain why scientists assign species to a hierarchical classification system (e.g., kingdom, phylum, class).

1.3  The Process of Science Learning Outcomes Upon completion of this section, you should be able to 1. Identify the components of the scientific method.  2. Distinguish between a theory and a hypothesis. 3. Analyze a scientific experiment and identify the hypothesis, experiment, control groups, and conclusions.

The process of science pertains to the study of biology. As you can see from Figure 1.2, the multiple stages of biological organization mean that life can be studied at a variety of levels. Some biological disciplines are cytology, the study of cells; anatomy, the study of structure; physiology, the study of function; botany, the study of plants; zoology, the study of animals; genetics, the



Chapter 1  The Study of Life

study of heredity; and ecology, the study of the interrelationships between organisms and their environment.  Religion, aesthetics, ethics, and science are all ways in which humans seek order in the natural world. The nature of scientific inquiry differs from these other ways of knowing and learning, because the scientific process uses the scientific method, a standard series of steps used in gaining new knowledge that is widely accepted among scientists. The scientific method (Fig. 1.6) acts as a guideline for scientific studies. Scientists often modify or adapt the process to suit their particular field of study. 

Observation Scientists believe that nature is orderly and measurable—that natural laws, such as the law of gravity, do not change with time—and that a natural event, or phenomenon, can be understood more fully through observation—a formal way of watching the natural world.  Scientists use all of their senses in making observations. The behavior of chimpanzees can be observed through visual means, the disposition of a skunk can be observed through olfactory means, and the warning rattles of a rattlesnake provide auditory information of imminent danger. Scientists also extend the ability of their senses by using instruments; for example, the microscope enables us to see objects that could never be seen by the naked eye. Finally, scientists may expand their understanding even further by taking advantage of the knowledge and experiences of other scientists. For instance, they may look up past studies at the library or on the Internet, or they may write or speak to others who are researching similar topics. 

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Hypothesis After making observations and gathering knowledge about a phenomenon, a scientist uses inductive reasoning to formulate a possible explanation. Inductive reasoning occurs whenever a person uses creative thinking to combine isolated facts into a cohesive whole. In some cases, a chance observation alone may help a scientist arrive at an idea.  One famous case pertains to the antibiotic penicillin, which was discovered in 1928. While examining a petri dish of bacteria that had become contaminated with the mold Penicillium, A ­ lexander Flemming observed an area that was free of bacteria. Flemming, an early expert on antibacterial substances, reasoned that the mold might have been producing an antibacterial compound.  We call such a possible explanation for a natural event a hypothesis. A hypothesis is not merely a guess; rather, it is an informed statement that can be tested in a manner suited to the processes of science.  All of a scientist’s past experiences, no matter what they might be, have the potential to influence the formation of a hypothesis. But a scientist considers only hypotheses that can be tested. Moral and religious beliefs, while very important in the lives of many people, differ between cultures and through time and may not be scientifically testable.

Predictions and Experiments Scientists often perform an experiment, which is a series of ­procedures, to test a hypothesis. To determine how to test a hypo­ thesis, a scientist uses deductive reasoning. Deductive reasoning involves “if, then” logic. In designing the experiment, the scientist

Observation

Potential hypotheses

Hypothesis 1 Hypothesis 2 Hypothesis 3

Remaining possible hypotheses

Hypothesis 2 Hypothesis 3

Last remaining possible hypothesis

Hypothesis 3

Prediction

Experiment

Reject hypothesis 1

Prediction

Experiment

Reject hypothesis 2

Figure 1.6  Flow diagram for the scientific

Modify hypothesis Predictions

Experiment 1

Experiment 2

Experiment 3

Predictions confirmed

Experiment 4

Conclusion

method.  On the basis of new and/or previous observations, a scientist formulates a hypothesis. The hypothesis is used to develop predictions to be tested by further experiments and/or observations, and new data either support or do not support the hypothesis. Following an experiment, a scientist often chooses to retest the same hypothesis or to test a related hypothesis. Conclusions from many different but related experiments may lead to the development of a scientific theory. For example, studies pertaining to development, anatomy, and fossil remains all support the theory of evolution.



Chapter 1  The Study of Life

may make a prediction, or an expected outcome, based on knowledge of the factors in the experiment.  The manner in which a scientist intends to conduct an experiment is called the experimental design. A good experimental design ensures that scientists are examining the contribution of a specific variable, called the experimental variable, to the observation. The result is termed the responding variable, or dependent variable, because it is due to the experimental variable: Experimental Variable (Independent Variable) Factor of the experiment being tested

Response Variable (Dependent Variable) Result or change that occurs due to the experimental variable

To ensure that the results will be meaningful, an experiment contains both test groups and a control group. A test group is exposed to the experimental variable, but the control group is not. If the control group and test groups show the same results, the experimenter knows that the hypothesis predicting a difference between them is not supported. Scientists often use model organisms and model systems to test a hypothesis. Model organisms, such as the fruit fly Drosophila melanogaster or the mouse Mus musculus, are chosen because they allow the researcher to control aspects of the experiment, such as age and genetic background. Cell biologists may use mice for modeling the effects of a new drug. Like model organisms, model systems allow the scientist to control specific variables and environmental conditions in a way that may not be possible in the natural environment. For example, ecologists may use computer programs to model how human activities will affect the climate of a specific ecosystem. While models provide useful information, they do not always answer the original question completely. For example, medicine that is effective in mice should ideally be tested in humans, and ecological experiments that are conducted using computer simulations need to be verified by field experiments. Biologists, and all other scientists, continuously design and revise their experiments to better understand how different factors may influence their original observation.

Presenting and Analyzing the Data The data, or results, from scientific experiments may be presented in a variety of formats, including tables and graphs. A graph shows the relationship between two quantities. In many graphs, the experimental variable is plotted on the x-axis (horizontal), and the result is plotted along the y-axis (vertical). Graphs are useful tools to summarize data in a clear and simplified manner. For example, the line graph in Figure 1.7 shows the variation in the concentration of blood cholesterol over a four-week study. The bars above each data point represent the variation, or standard error, in the results. The title and labels can assist you in reading a graph; therefore, when looking at a graph, first check the two axes to determine what the graph pertains to. By looking at this graph, we know that the blood cholesterol levels were highest during week 2, and we can see to what degree the values varied over the course of the study.

Statistical Data Most scientists who publish research articles use statistics to help them evaluate their experimental data. In statistics, the standard

Variation in Blood Cholesterol Levels 225

y-axis

Blood Cholesterol (mg/dl)

10

standard error 200 Data

175

150

Week 1

Week 2

Week 3

Week 4

x-axis

Figure 1.7  Presentation of scientific data.  This line graph

shows the variation in the concentration of blood cholesterol over a fourweek study. The bars above each data point represent the variation, or standard error, in the results. 

error, or standard deviation, tells us how uncertain a particular value is. Suppose you predict how many hurricanes Florida will have next year by calculating the average number during the past 10 years. If the number of hurricanes per year varies widely, your standard error will be larger than if the number per year is usually about the same. In other words, the standard error tells you how far off the average could be. If the average number of hurricanes is four and the standard error is ± 2, then your prediction of four hurricanes is between two and six hurricanes. In Figure 1.7, the standard error is represented by the bars above and below each data point. This provides a visual indication of the statistical analysis of the data.

Statistical Significance When scientists conduct an experiment, there is always the possibility that the results are due to chance or to some factor other than the experimental variable. Investigators take into account several factors when they calculate the probability value (p) that their results were due to chance alone. If the probability value is low, researchers describe the results as statistically significant. A probability value of less than 5% (usually written as p < 0.05) is acceptable; even so, keep in mind that the lower the p value, the less likely it is that the results are due to chance. Therefore, the lower the p value, the greater the confidence the investigators and you can have in the results. Depending on the type of study, most scientists like to have a p value of < 0.05, but p values of < 0.001 are common in many studies. 

Scientific Publications Scientific studies are customarily published in scientific journals (Fig. 1.8), so that all aspects of a study are available to the scientific community. Before information is published in scientific journals, it is typically reviewed by experts, who ensure that the research is credible, accurate, unbiased, and well executed. Another scientist should be able to read about an experiment in a scientific journal, repeat the experiment in a different location,



Chapter 1  The Study of Life

11

Figure 1.8  Scientific publications.  Scientific journals, such as Science, are scholarly journals in which researchers share their findings with other scientists. Scientific magazines, such as Scientific American and New Scientist contain articles that are usually written by reporters for a broader audience. 

and get the same (or very similar) results. Some articles are rejected for publication by reviewers when they believe there is something questionable about the design of an experiment or the manner in which it was conducted. This process of rejection is important in science since it causes researchers to critically review their hypotheses, predictions, and experimental designs, so that their next attempt will more adequately address their hypothesis. Often, it takes several rounds of revision before research is accepted for publication in a scientific journal.  Scientific magazines (Fig. 1.8), such as Scientific American, differ from scientific journals in that they report scientific findings to the general public. The information in these articles is usually obtained from articles first published in scientific journals.

Scientific Theory

theory is supported by a broad range of observations, experiments, and data, often from a variety of disciplines. Some of the basic theories of biology are: Theory

Concept

Cell

All organisms are composed of cells, and new cells only come from preexisting cells.

Homeostasis

The internal environment of an organism stays relatively constant—within a range that is protective of life.

Evolution

A change in the frequency of traits that affect reproductive success in a population or species across generations.

As stated earlier, the theory of evolution is the unifying conThe ultimate goal of science is to understand the natural world in cept of biology because it pertains to many different aspects of life. terms of scientific theories, which are concepts that join together For example, the theory of evolution enables scientists to underwell-supported and related hypotheses. In ordinary speech, the stand the history of life, as well as the anatomy, physiology, the word theory refers to a speculative idea. In contrast, a scientific embryological development of organisms, to name a few. 

Chapter 1  The Study of Life

State Hypothesis: Antibiotic B is a better treatment for ulcers than antibiotic A.

The theory of evolution has been a fruitful scientific theory, meaning that it has helped scientists generate new hypotheses. Because this theory has been supported by so many observations and experiments for over 100 years, some biologists refer to the principle of evolution, a term sometimes used for theories that are generally accepted by an overwhelming number of scientists. The term law instead of principle is preferred by some. For instance, in our discussion of energy in Chapter 6, we will examine the laws of thermodynamics. 

An Example of the Scientific Method We now know that most stomach and intestinal ulcers (open sores) are caused by the bacterium Helicobacter pylori. Let’s say investigators want to determine which of two antibiotics is best for the treatment of an ulcer. When clinicians do an experiment, they try to vary just the experimental variables—in this case, the medications being tested. A control group is not given the medications, but one or more test groups receive them. If by chance the control group shows the same results as a test group, the investigators immediately know that the results of their study are invalid, because the medications may have had nothing to do with the results. The study depicted in Figure 1.9 shows how investigators may study this hypothesis: 

Perform Experiment: Groups were treated the same except as noted.

Hypothesis: Newly discovered antibiotic B is a better treatment for ulcers than antibiotic A, which is in current use.

Experimental Design Control group: received placebo

Test group 1: received antibiotic A

Test group 2: received antibiotic B

Collect Data: Each subject was examined for the presence of ulcers.

Next, the investigators might decide to use three experimental groups: one control group and two test groups. It is important to reduce the number of possible variables (differences), such as sex, weight, and other illnesses, among the groups. Therefore, the investigators randomly divide a very large group of volunteers equally into the three groups. The hope is that any differences will be distributed evenly among the three groups. This is possible only if the investigators have a large number of volunteers. The three groups are to be treated like this:  Control group:  Subjects with ulcers are not treated with either antibiotic.  Test group 1:  Subjects with ulcers are treated with antibiotic A.  Test group 2:  Subjects with ulcers are treated with antibiotic B.

100

Effectiveness of Treatment

80 % Treated

12

60 40

60

80

20 0

10 Control Group

Test Group 1

Test Group 2

After the investigators have determined that all volunteers do have ulcers, they will want the subjects to think they are all receiving the same treatment. This is an additional way to protect the results from any influence other than the medication. To achieve this end, the subjects in the control group can receive a placebo, a treatment that appears to be the same as that administered to the other two groups but actually contains no medication. In this study, the use of a placebo would help ensure the same dedication by all subjects to the study.

Figure 1.9  Example of a controlled study.  In this study, a large number of people were divided into three groups. The control group received a placebo and no medication. One of the test groups received medication A, and the other test group received medication B. The results are depicted in a graph, and it shows that medication B was a more effective treatment than medication A for the treatment of ulcers. 



Chapter 1  The Study of Life

Results and Conclusion After two weeks of administering the same amount of medication (or placebo) in the same way, the stomach and intestinal linings of each subject are examined to determine if ulcers are still present. Endoscopy is a procedure that involves inserting an endoscope (a small, flexible tube with a tiny camera on the end) down the throat and into the stomach and the upper part of the small intestine. Then, the doctor can see the lining of these organs and can check for ulcers. Tests performed during an endoscopy can also determine if Helicobacter pylori is present.  Because endoscopy is somewhat subjective, it is probably best if the examiner is not aware of which group the subject is in; otherwise, the examiner’s prejudice may influence the examination. When neither the patient nor the technician is aware of the specific treatment, it is called a double-blind study. In this study, the investigators may decide to determine the effectiveness of the medication by the percentage of people who no longer have ulcers. So, if 20 people out of 100 still have ulcers, the medication is 80% effective. The difference in effectiveness is easily read in the graph portion of Figure 1.9. Conclusion: On the basis of their data, the investigators conclude that their hypothesis has been supported. 

Check Your Progress  1.3 1. Identify the role of the experimental variable in an experiment. 

2. Distinguish between the roles of the test group and the control group in an experiment.

3. Describe the process by which a scientist may test a hypothesis about an observation. 

1.4  Challenges Facing Science Learning Outcomes Upon completion of this section, you should be able to 1. Distinguish between science and technology. 2. Summarize some of the major challenges facing science.

13

identified and named. Extinction is the death of a species or larger classification category. It is estimated that presently we are losing hundreds of species every year due to human activities and that as much as 38% of all species, including most primates, birds, and amphibians, may be in danger of extinction before the end of the century. Many biologists are alarmed about the present rate of extinction and hypothesize it may eventually rival the rates of the five mass extinctions that occurred during our planet’s history. The last mass extinction, about 65 million years ago, caused many plant and animal species, including the dinosaurs, to become extinct. The two most biologically diverse ecosystems—tropical rain forests and coral reefs—are home to many organisms. These ecosystems are also threatened by human activities. The canopy of the tropical rain forest alone supports a variety of organisms, including orchids, insects, and monkeys. Coral reefs, which are found just offshore of the continents and islands near the equator, are built up from calcium carbonate skeletons of sea animals called corals. Reefs provide a habitat for many animals, including jellyfish, sponges, snails, crabs, lobsters, sea turtles, moray eels, and some of the world’s most colorful fishes. Like tropical rain forests, coral reefs are severely threatened as the human population increases in size. Some reefs are 50 million years old, yet in just a few decades, human activities have destroyed an estimated 25% of all coral reefs and seriously degraded another 30%. At this rate, nearly threequarters could be destroyed within 40 years. Similar statistics are available for tropical rain forests.  The destruction of healthy ecosystems has many unintended effects. For example, we depend on them for food, medicines (Fig. 1.10), and various raw materials. Draining of the natural wetlands of the Mississippi and Ohio Rivers and the construction of levees have worsened flooding problems, making once fertile farmland undesirable. The destruction of South American rain forests has killed many species that may have yielded the next miracle drug and has decreased the availability of many types of lumber. We are only now beginning to realize that we depend on ecosystems even more for the services they provide. Just as chemical cycling occurs within a single ecosystem, all ecosystems keep chemicals cycling throughout the biosphere. The workings of ecosystems ensure that

As we have learned in this chapter, science is a systematic way of acquiring knowledge about the natural world. Science is a slightly different endeavor than technology. Technology is the application of scientific knowledge to the interests of humans. Scientific investigations are the basis for the majority of our technological advances. As is often the case, a new technology, such as your cell phone or a new drug, is based on years of scientific investigations. In this section, we are going to explore some of the challenges ­facing science, technology, and society.

Biodiversity and Habitat Loss Biodiversity is the total number and relative abundance of species, the variability of their genes, and the different ecosystems in which they live. The biodiversity of our planet has been estimated to be around 8.7 million species, and so far, less than 2 million have been

Figure 1.10  The importance of biodiversity.  Snails of the

genus Conus are known to produce powerful painkillers. Unfortunately, their habitat on coral reefs is threatened by human activity.



14

Chapter 1  The Study of Life

the environmental conditions of the biosphere are suitable for the continued existence of humans. And several studies show that ecosystems cannot function properly unless they remain biologically diverse. We will explore the concept of biodiversity in greater detail in Chapters 33 through 37 of the text.

Emerging and Reemerging Diseases Over the past decade, avian influenza (H5N1 and H7N9), swine flu (H1N1), severe acute respiratory syndrome (SARS), and Middle East respiratory syndrome (MERS) have been in the news. These are called emerging diseases, meaning that they are relatively new to humans. Where do emerging diseases come from? Some of them may result from new and/or increased exposure to animals or insect populations that act as vectors for disease. Changes in human behavior and use of technology can also result in new diseases. SARS is thought to have arisen in Guandong, China, due to the consumption of civets, a type of exotic cat considered a delicacy. The civets were possibly infected by exposure to horseshoe bats sold in open markets. Legionnaires’ disease emerged in 1976 due to bacterial contamination of a large air-conditioning system in a hotel. The bacteria thrived in the cooling tower used as the water source for the air-conditioning system. In addition, globalization results in the transport of diseases all over the world that were previously restricted to isolated communities. The first SARS cases were reported in southern China in November of 2002. By the end of February 2003, SARS had reached nine countries/provinces, mostly through airline travel.  Some pathogens mutate and change hosts, jumping from birds to humans, for example. Before 1997, avian flu was thought to affect only birds. A mutated strain jumped to humans in the 1997 outbreak. To control that epidemic, officials killed 1.5 million chickens to remove the source of the virus. New forms of avian influenza (bird flu) are being discovered every few years.  Reemerging diseases are also a concern. Unlike an emerging disease, a reemerging disease has been known to cause disease in humans for some time, but generally has not been considered a health risk due to a relatively low level of incidence in human populations. However, reemerging diseases can cause problems. An excellent example is the Ebola outbreak in West Africa of 2014– 2015. Ebola outbreaks have been known since 1976, but generally have only affected small groups of humans. The 2014–2015

outbreak was still a regional event, but it has the potential to affect millions of people before it was brought under control. Both emerging and reemerging diseases have the potential to cause health problems for humans across the globe. Scientists investigate not only the causes of these diseases (for example, the viruses) but also their effects on our bodies and the mechanisms by which they are transmitted. We will take a closer look at viruses in Chapter 28 of the text. 

Climate Change The term climate change refers to changes in the normal cycles of the Earth’s climate that may be attributed to human activity. Climate change is primarily due to an imbalance in the chemical cycling of the element carbon. Normally, carbon is cycled within an ecosystem. However, due to human activities, more carbon dioxide is being released into the atmosphere than is being removed. In 1850, atmospheric CO2 was at about 280 parts per million (ppm); today, it is over 400 ppm. This increase is largely due to the burning of fossil fuels and the destruction of forests to make way for farmland and pasture. Today, the amount of carbon dioxide released into the atmosphere is about twice the amount that remains in the atmosphere. It’s believed that most of this dissolves in the ocean. The increased amount of carbon dioxide (and other gases) in the atmosphere is causing a rise in temperature called global warming. These gases allow the sun’s rays to pass through, but they absorb and radiate heat back to Earth, a phenomenon called the greenhouse effect.  There is a consensus among scientists from around the globe that climate change and global warming is causing significant changes in many of the Earth’s ecosystems and represents one of the greatest challenges of our time. Throughout this text we will return to how climate change is changing ecosystems, reducing biodiversity, and contributing to human disease. 

Check Your Progress  1.4 1. Explain how a new technology differs from a scientific

discovery. 2. Explain why the conservation of biodiversity is important to human society.  3. Summarize how emerging diseases and climate change have the potential for influencing the entire human population.



Chapter 1  The Study of Life

15

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1.1  Life Characteristics

SUMMARIZE 1.1  The Characteristics of Life Evolution accounts for both the diversity and the unity of life we see about us. All organisms share the following characteristics of life: ■ Organization: the levels of biological organization extend as follows: atoms and molecules → cells → tissues → organs → organ systems → organisms → populations → communities → ecosystems → biosphere. All of the members of one species in a given location is called a population. ■ Require materials and energy: living organisms need an outside source of materials and energy. Metabolism is the term used to summarize these chemical reactions in the cell. Photosynthesis is an example of a metabolic process. ■ Reproduce and develop: Organisms reproduce to pass on their genetic information, included in the genes of their DNA, to the next generation. Mutations introduce variation into the DNA. Development is the series of steps that an organism proceeds through to become an adult.  ■ Respond to stimuli: they detect and react to internal and external events. ■ Maintain homeostasis: homeostasis is the ability to maintain a stable internal environment.  ■ Adaptations: Adaptations allow an organism to exist in a particular environment. Evolution is the accumulation of these changes over multiple generations and the resulting adaptation to the organism’s environment. 

1.2  The Classification of Organisms ■ In taxonomy, organisms are assigned an italicized binomial name that

consists of the genus and the specific epithet. From the least inclusive to the most inclusive category, each species belongs to a genus, family, order, class, phylum, kingdom, and finally domain. Systematics is the study of evolutionary relationships between species.  ■ The three domains of life are Archaea, Bacteria, and Eukarya. Domain Archaea and domain Bacteria contain prokaryotic organisms that are structurally simple but metabolically complex. Domain Eukarya contains the eukaryotic protists, fungi, plants, and animals. Protists range from single-celled to multicellular organisms and include the protozoans and most algae. Among the fungi are the familiar molds and mushrooms. Plants are well known as the

 

Animations

1.2  Three Domains

multicellular photosynthesizers of the world, while animals are ­ ulticellular and ingest their food.  m ■ Each species has a Latin scientific name that consists of the genus and species name. Both names are italicized, but only the genus is capitalized, as in Homo sapiens.

1.3  The Process of Science When studying the natural world, scientists use a process called the scientific method.  ■ Observations, along with previous data, are used to formulate a hypothesis. Inductive reasoning allows a scientist to combine facts into a hypothesis. ■ New observations and/or experiments are carried out in order to test the hypothesis. Deductive reasoning allows for the development of a prediction of what may occur as a result of the experiment. A good experimental design includes an experimental variable and a control group. Scientists may use models and model organisms in their experimental design. ■ The data from the experimental and observational results are analyzed, often using statistical methods. The results are often presented in tables or graphs for ease of interpretation. ■ A conclusion is made as to whether the results support the hypothesis or do not support the hypothesis. ■ The results may be submitted to a scientific publication for review by the scientific community. ■ Over time multiple conclusions in a particular area may allow scientists to arrive at a theory (or principle or law), such as the cell theory or the theory of evolution. The theory of evolution is a unifying concept of biology.

1.4  Challenges Facing Science While science investigates the principles of the natural world, technology applies this knowledge to the needs of society. Some challenges that ­scientists are investigating include:  ■ The loss of biodiversity and habitats such as coral reefs and rain ­forests. This often results in the extinction of species. ■ Emerging diseases, such as avian influenza and SARS, and reemerging diseases, such as Ebola. ■ The impact of climate change and global warming.



16

Chapter 1  The Study of Life

ASSESS Testing Yourself Choose the best answer for each question.

1.1  The Characteristics of Life 1. Which of these is not a property of all living organisms? a. organization b. acquisition of materials and energy c. care for their offspring d. reproduction e. responding to the environment  2. The level of organization that includes cells of similar structure and function is a. an organ. b. a tissue. c. an organ system. d. an organism.  3. The process that involves passing on genetic information between generations is called a. natural selection. b. reproduction. c. development. d. metabolism.

1.2  The Classification of Organisms 4. Which of the following includes prokaryotic organisms? a. protists b. fungi c. archaea d. plants  5. The most inclusive level of classification is a. species. b. kingdom. c. domain. d. phylum.  6. The process by which evolution occurs is called a. natural selection. b. development. c. reproduction. d. taxonomy.

1.3  The Process of Science 7. After formulating a hypothesis, a scientist a. proves the hypothesis to be true or false. b. tests the hypothesis. c. decides how to best avoid having a control. d. makes sure environmental conditions are just right. e. formulates a scientific theory. 

8. Experiments examine the contribution of the _________ to the observation. a. responding variable c. standard deviation b. control group d. experimental variable 9. Which of the following is not correctly linked? a. model: a representation of an object used in an experiment b. standard deviation: a form of statistical analysis c. principle: a theory that is not supported by experimental evidence d. data: the results of an experiment or observation 

1.4  Challenges Facing Science 10. Which of the following applies scientific knowledge to the needs of society? a. evolution c. systematics b. taxonomy d. technology 11. Which of the following represents the permanent loss of a species? a. natural selection c. extinction b. greenhouse effect d. climate change 12. H5N1 and SARS are examples of a. extinct species. c. endangered habitats. b. forms of greenhouse gases. d. emerging diseases.

ENGAGE BioNOW Want to know how this science is relevant to your life? Check out the BioNow videos below: ■ Biodiversity ■ Characteristics of Life

Thinking Critically 1. Explain how model organisms make the study of the dependent variable in an experiment easier. 2. Suppose that we find a form of life on another planet that has a different characteristic than those listed in this chapter. What would this tell us about life on our planet and the process of evolution? 3. You are a scientist working at a pharmaceutical company and have developed a new cancer medication that has the potential for use in humans. Outline a series of experiments, including the use of a model, to test whether the cancer medication works.

PHOTO CREDITS Openers(both): © Science Source; 1.1(giraffes): © Sylvia S. Mader; 1.1(bacteria): © Eric Erbe/Chris Pooley, USDA-ARS; 1.1(sequoia): © Robert Glusic/Getty RF; 1.1(Earth): © Ingram Publishing/Alamy RF; 1.1(mushroom): © IT Stock/Age Fotostock RF; 1.1(people): © Blend Images/Ariel Skelley/Getty RF; 1.1(Euglena): © blickwinkel/Alamy; 1.4a: © David M. Phillips/Science Source; 1.4b: © Brand X Pictures/PunchStock RF; 1A: © Michael Freeman/Corbis RF; 1.5a: © Eye of Science/Science Source; 1.5b: © A.B. Dowsett/SPL/ Science Source; 1.8: © Ricochet Creative Productions LLC; 1.9(students, both): © image100 Ltd RF; 1.9(surgeon): © Phanie/Science Source; 1.10: © FLPA/Alamy.

UNIT 1  Cell Biology

CASE STUDY Sports Drinks and Exercise Water is an essential molecule for life. In the human body (which is 60-70% water), water regulates temperature, as well as serving as a transport medium for nutrients and wastes. During exercise a person can lose up to 64 ounces (oz) of fluid per hour, even more during extremely hot and humid weather. Since a loss of even 10% can have profound influence on how our bodies function, it is essential that we maintain an adequate state of hydration. To maintain adequate hydration, people need to be aware of their own body chemistry and how they respond to exercise. In general, you should drink 8–10 ounces of water for every 10–15 minutes of exercise. Only when exercise lasts longer than 90 minutes should you consider consuming 8–10 oz of a sports drink for every 15–30 minutes of exercise. Exercise causes our bodies to not only lose water but also deplete our supply of electrolytes and carbohydrates. This results in a drop in blood volume, forcing the heart to work harder to circulate the blood. Muscle cramps, fatigue, dizziness, and an increase in core body temperature can also result. In an attempt to avoid these issues, many athletes will consume sports drinks during exercise. There are a variety of sports drinks, but most contain water, carbohydrates, and electrolytes (salts). Issues arise when people consume sports drinks during workouts that are less than 90 minutes. For short duration exercise there is no need to replace electrolytes or carbohydrates. Each serving of a sports drink may contain up to 200 calories, thus reducing the effectiveness of exercise for weight loss. In this chapter, we will learn about the importance of water to our lives, as well as how elements combine to form the basic organic molecules that all life is built from. As you read through the chapter, think about the following questions:

1. What properties of water make it so essential to our lives? 2. Which elements are most abundant in living organisms?

The Molecules of Cells

2

CHAPTER OUTLINE 2.1 Basic Chemistry 2.2 Molecules and Compounds 2.3 Chemistry of Water 2.4 Organic Molecules 2.5 Carbohydrates 2.6 Lipids 2.7 Proteins 2.8 Nucleic Acids BEFORE YOU BEGIN

Before beginning this chapter, take a few moments to review the following discussions: Section 1.1  Why must organisms acquire materials and energy? Section 1.1  How does energy flow through an ecosystem? Figure 1.2  Where are atoms and molecules located in the levels of biological organization?

17

18

UNIT 1  Cell Biology

The Earth’s crust, as well as all organisms, are composed of elements, but they differ as to which elements are predominant (Fig. 2.1). Only six elements—carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur—are basic to life and make up about 95% of the body weight of organisms. The properties of these ­elements are essential to the uniqueness of cells and organisms. Other elements, including sodium, potassium, calcium, iron, and magnesium are also important to all organisms.

2.1  Basic Chemistry Learning Outcomes Upon completion of this section, you should be able to 1. Describe how protons, neutrons, and electrons relate to atomic structure. 2. Understand how to interpret the periodic table of elements. 3. Describe how variations in an atomic nucleus account for its physical properties. 4. Identify the beneficial and harmful uses of radiation.

Atomic Structure In the early 1800s, the English scientist John Dalton championed the atomic theory, which says that elements consist of tiny particles called atoms. An atom is the smallest part of an element that displays the properties of the element. An element and its atoms share the same name.  Elements are identified by an atomic symbol that is composed of one or two letters. These letters are usually derived from either the English or Latin names of the element. For example, the symbol H means a hydrogen atom, the symbol Cl stands for chlorine, and the symbol Na (for natrium in Latin) is used for a sodium atom. Physicists have identified a number of subatomic particles that make up atoms. The three best-known subatomic particles are positively charged protons, uncharged neutrons, and negatively charged electrons. Protons and neutrons are located within the nucleus of an atom, and electrons move around the nucleus.

Everything—including your computer, the chair you’re sitting on, the water you drink, and the air you breathe—is composed of matter. Matter refers to anything that takes up space and has mass. Matter has many diverse forms, but it can exist only in four distinct states: solid, liquid, gas, or plasma. All matter, both nonliving and in living organisms, is composed of certain basic substances called elements. An element is a substance that cannot be broken down to simpler substances with different properties by ordinary chemical means. Each element has it own unique properties such as density, solubility, melting point, and chemical reactivity. Only 92 naturally occurring elements (see Appendix B) serve as the building blocks of all matter. Other elements have been made in research labs, but they are not biologically important.

Percent by Weight

60

Earth’s crust organisms

40

20

0 Fe

Ca

K

S

P

Si

Al

Mg Na

O

N

C

H

Element

Figure 2.1  Elements that make up Earth’s crust and its organisms.  Humans are just one of the many organisms that exist on Earth. The graph

inset shows that Earth’s crust primarily contains the elements silicon (Si), aluminum (Al), and oxygen (O). Organisms primarily contain the elements oxygen (O), nitrogen (N), carbon (C), and hydrogen (H). Biological molecules often also contain the elements  sulfur (S) and phosphorus (P).



19

Chapter 2  The Molecules of Cells

Whereas each atom of an element has the same atomic number (and thus the same number of protons), the number of neutrons may vary slightly. Isotopes (Gk. isos, “equal”) are atoms of the same element that differ in the number of neutrons. For example, the element carbon has three naturally occurring isotopes: 

= proton = neutron = electron a.

12 C 6

b.

14 C 6

The term atomic mass refers to the average mass for all the isotopes of that atom. Since the majority of carbon is carbon 12, the atomic mass of carbon is closer to 12 than to 13 or 14. 

Subatomic Particles Atomic Mass Unit (AMU)

Location

+1

1

Nucleus

6

atomic number

Neutron

0

1

Nucleus

C

atomic symbol

Electron

–1

0

Electron shell

Particle Proton

Electric Charge

13 C 6

12.01

To determine the number of neutrons from the atomic mass, subtract the number of protons from the atomic mass and take the closest whole number It is important to note that the term mass is used, not weight, because mass is constant, while weight changes according to the gravitational force of a body. The gravitational force of the Earth is greater than that of the moon; therefore, substances weigh less on the moon, even though their mass has not changed.

c.

Figure 2.2  Model of a helium (He) atom.  Atoms contain subatomic particles called protons, neutrons, and electrons. Protons and neutrons are within the nucleus, and electrons are outside the nucleus. a. The shading shows the probable location of the electrons in the helium atom. b. A circle, called an electron shell or orbital, represents the average location of an electron. c. The electric charge and the atomic mass units of the subatomic particles vary as shown.

mass number atomic number

12 6C

atomic symbol

The Periodic Table Once chemists discovered a number of the elements, they began to realize that even though each element consists of a different atom, certain chemical and physical characteristics are common between groups of elements. The periodic table was constructed as a way to group the elements, and therefore atoms, according to these characteristics. Notice in Figure 2.3 that the periodic I 1

2

3

4

VIII

1

atomic number

H

atomic symbol

1.008

Periods

­ igure 2.2 shows the arrangement of the subatomic particles in a F helium atom, which has only two electrons. In Figure 2.2a, the shading shows the probable location of electrons, and in Figure 2.2b, the circle represents an electron shell (or orbital), which indicates the average location of electrons. The concept of an atom has changed greatly since Dalton’s day. If an atom could be drawn the size of a football field, the nucleus would be like a gumball in the center of the field. The electrons would be tiny specks whirling about in the upper stands. Most of an atom is empty space. We should also realize that both of the models in Figure 2.2 indicate only where the electrons are expected to be most of the time. In our analogy, the electrons might very well stray outside the stadium at times. All atoms of an element have the same number of protons. This is called the atomic number. The number of protons in the nucleus makes each atom unique. Generally, atoms are assumed to be electrically neutral, meaning that the number of electrons is the same as the number of protons in the atom. The atomic number tells you not only the number of protons but also the number of electrons.  Each atom also has its own mass number, which is the sum of the protons and neutrons in the nucleus. Protons and neutrons are assigned one atomic mass unit (AMU) each. Electrons are so small that their AMU is considered zero in most calculations (Fig. 2.2c). By convention, when we are discussing an atom independently of the periodic table (see below), the atomic number is written as a subscript to the lower left of the atomic symbol. The mass number is written as a superscript to the upper left of the atomic symbol.

atomic mass

II

III

2 atomic mass IV

V

VI

VII

He 4.003

3

4

5

6

7

8

9

10

Li

Be

B

C

N

O

F

Ne

6.941

9.012

10.81

12.01

14.01

16.00

19.00

20.18

11

12

13

14

15

16

17

18

Na

Mg

Al

Si

P

S

Cl

Ar

22.99

24.31

26.98

28.09

30.97

32.07

35.45

39.95

19

20

31

32

33

34

35

36

K

Ca

Ga

Ge

As

Se

Br

Kr

39.10

40.08

69.72

72.59

74.92

78.96

79.90

83.60

Groups

Figure 2.3  A portion of the periodic table.  In the periodic table, the elements are arranged in the order of their atomic numbers and placed in groups (vertical columns) and periods (horizontal rows). All the atoms in a particular group have certain chemical characteristics in common. The first four periods contain the elements that are most important in biology. The complete periodic table is in Appendix B.



20

UNIT 1  Cell Biology

table is arranged according to increasing atomic number. The vertical columns in the table are groups. The horizontal rows are periods, which cause each atom to be in a particular group. For example, all the atoms in group VII react with one atom at a time, for reasons we will soon explore. The atoms in group VIII are called the noble gases because they are inert and rarely react with another atom. Helium and krypton are examples of noble gases.

Radioactive Isotopes Some isotopes of an element are unstable, or radioactive. For example, unlike the other two isotopes of carbon, carbon 14 changes over time into nitrogen 14, which is a stable isotope of the element nitrogen. As carbon 14 decays, it releases various types of energy in the form of rays and subatomic particles. The radiation given off by radioactive isotopes can be detected in various ways. The Geiger counter is an instrument that is commonly used to detect radiation. In 1896, the French physicist Antoine-Henri Becquerel discovered that a sample of uranium would produce a bright image on a photographic plate even in the dark, and a similar method of detecting radiation is still in use today. Marie Curie, who worked with Becquerel, coined the term radioactivity and contributed much to its study. Today, biologists use radiation to date objects from our distant past, to create images, and to trace the movement of substances in the body. 

larynx thyroid gland

Low Levels of Radiation The chemical behavior of a radioactive isotope is essentially the same as that of the stable isotopes of an element. This means that you can put a small amount of radioactive isotope in a sample and it becomes a tracer, or chemical tag, by which to detect molecular changes. The importance of chemistry to medicine is nowhere more evident than in the many medical uses of radioactive isotopes. Specific tracers are used in imaging the body’s organs and tissues. For example, after a patient drinks a solution containing a minute amount of radioactive iodine 131, it becomes concentrated in the thyroid—the only organ to take up iodine. A subsequent image of the thyroid indicates whether it is healthy in structure and function (Fig. 2.4a). Positron emission tomography (PET) is a way to determine the comparative activity of tissues. Radioactively labeled glucose, which emits a subatomic particle known as a positron, is injected into the body. The radiation given off is detected by sensors and analyzed by a computer. The result is a color image that shows which tissues took up glucose and are metabolically active (Fig. 2.4b). A PET scan of the brain can help diagnose a brain tumor, Alzheimer disease, epilepsy, or whether a stroke has occurred.

High Levels of Radiation Radioactive substances in the environment can harm cells, damage DNA, and cause cancer. When researchers, such as Marie Curie, began studying radiation in the nineteenth century, its harmful effects were not known, and many developed cancer. The release of radioactive particles following a nuclear power plant accident, such as occurred in Japan in 2011 following the tsunamis (see the Health feature, “Japan’s Nuclear Crisis”), can have far-reaching and long-lasting effects on human health.  However, the effects of radiation can also be put to good use (Fig. 2.5). Radiation from radioactive isotopes has been used for many years to sterilize medical and dental products. Radiation is now used to sterilize the U.S. mail and other packages to free them of possible pathogens, such as anthrax spores. The ability of

trachea a.

b.

Figure 2.4   Low levels of radiation.  a. The asymmetrical

appearance of the thyroid on the PET (positron emission tomography) scanindicates the presence of a tumor that does not take up the radioactive iodine. b. A PET scan reveals which portions of the brain are most active (yellow and red colors).

a.

b.

Figure 2.5  High levels of radiation.  a. Radiation kills bacteria

and fungi. Irradiated peaches (top) spoil less quickly (compared to bottom) and can be kept for a longer length of time. b. Physicians use radiation therapy to kill cancer cells.



Chapter 2  The Molecules of Cells

SCIENCE IN YOUR LIFE  ►

21

HEALTH

Japan’s Nuclear Crisis On March 11, 2011, a magnitude 8.9 earthquake struck the coast of Japan, causing the shutdown of 11 of Japan’s nuclear power reactors, including reactors 1, 2, and 3 at the Fukushima Daiichi power plant. Shortly after the shutdown, the pressure within reactor 1 built up to twice that of normal levels. In an attempt to relieve pressure, steam-containing radiation was vented from the reactors. Over the next several days, the plant continued to have problems and the levels of radiation in the surrounding areas continued to rise. Shortly thereafter, ­Japanese officials formed a 10-kilometer (km) evacuation zone, which, over the next few days, was increased to 20 km, resulting in the relocation of close to 390,000 people. The Fukushima disaster released a number of radioactive isotopes into the atmosphere and the surrounding ocean. These included isotopes of iodine, cesium, and xenon. Each of these isotopes are a by-product of the nuclear fission reactions that occur within a nuclear power plant. As the uranium in these plants breaks down, the uranium atoms release energy (such as gamma radiation) and particles (alpha and beta particles). This is called radioactive decay, and the loss of particles effectively transforms the original uranium atom into isotopes of another element. In many cases, the resulting isotopes are also radioactive, and then undergo additional radioactive decay.  

Three of the more common isotopes released by the Fukushima disaster are listed below. These isotopes differ considerably in the types of radiation that they release, their half-lives, and their potential adverse effects on living organisms, such as humans. 

Cesium-134 and Cesium-137 Perhaps the biggest concern following the Fukushima disaster was the release of radioactive cesium. Naturally, cesium (Cs) is a metal with an atomic number of 55 and an atomic mass of approximately 132.9.  However, within a nuclear reactor, two isotopes of cesium (134Cs and 137Cs) are formed. Cesium 134 and Cesium 137 have half-lives of 2 years and 30 years, respectively. Both of these isotopes release both beta particles and gamma radiation as they decay.  Both beta particles and gamma radiation have the ability to penetrate tissues and damage cells and their DNA. Exposure to beta particles and gamma radiation, either internally or externally, can cause burns and greatly increases the chance of cancer. Following the Fukushima disaster, there were significant concerns of 137Cs entering the atmosphere and the Pacific Ocean. Because of the winds at the time, most of the 137Cs was carried back over Japan, forcing evacuations up to 35 kilometers (22 miles) inland. Large regions of this evacuation zone still remain

uninhabitable due to soil contamination by 137 Cs. Small amounts of atmospheric  137Cs were detected on the west coast of the United States, and some 137Cs has been detected in the waters off the coast of California, but in neither case at levels that are considered hazardous to humans.

Iodine -131 Radioactive iodine, 131I, was also released by the reactors. Iodine is a water-soluble element that has an atomic number of 53 and an atomic mass of approximately 126.9. There are several forms of iodine isotopes, with 131I being formed mainly by nuclear fission reactions. Like 137Cs, 131 I emits beta particles which may damage tissues. It also releases small amounts of gamma radiation.  Radioactive iodine only has a half-life of about 8 days, so its long-term effect on the environment is minimal. In our bodies, iodine is mainly used by the thyroid gland to manufacture the thyroid hormones associated with metabolic functions, including overall metabolic rate. Intense exposure to 131I may cause thyroid problems, including thyroid cancer. Interestingly, isotopes of iodine at low doses are used by the medical profession to diagnose problems with the thyroid gland and treat some forms of thyroid cancer.

Xenon-133 Xenon (Xe) is a noble gas with an atomic number of 54 and an atomic weight of 131.2. One isotope of xenon, 133Xe, has a half-life of only 5 days. Like 131I and 137Cs, 133Xe emits beta particles which may damage tissue, but the short halflife of the isotope limits the chances of severe problems. Further reducing the risk is the fact that, as a noble gas, xenon does not react with other elements, and thus is not easily introduced into the chemical compounds within cells.  Like radioactive iodine, 133Xe is used by the medical profession to diagnose disease. Since it is a gas, it is frequently used in the diagnosis of lung disorders. Studies are now underway to use 133Xe as a form of treatment for certain types of lung cancer, since it easily enters the lungs and the emission of beta particles can help destroy lung cancer cells.

Questions to Consider

Figure 2A  The Fukushima Power Plant.  The explosion of the plant following the tsunamis released radioactive isotopes into the atmosphere and surrounding water.

1. Describe the differences between 137Cs, 131 I,  and 133Xe and their non-radioactive isotopes. 2. Explain why 137Cs is more dangerous than 131 I or 133Xe.



22

UNIT 1  Cell Biology

Figure 2.6  Bohr models of atoms.  Electrons orbit the nucleus at particular energy levels (electron shells or orbitals). The first shell contains up to two electrons, and each shell thereafter is most stable when it contains eight electrons. Atoms with an atomic number above 20 may have more electrons in their outer shells. The outermost, or valence, shell helps determine the atom’s chemical properties and how many other elements it can interact with.

H

electron electron orbital nucleus

hydrogen 1 1H

O

oxygen 16 8O

radiation to kill cells is often applied to cancer cells. Targeted radioisotopes can be introduced into the body so that the subatomic particles emitted destroy only cancer cells, with little risk to the rest of the body. X rays, another form of high-energy radiation, can be used for medical diagnosis.

Electrons In an electrically neutral atom, the positive charges of the protons in the nucleus are balanced by the negative charges of electrons moving about the nucleus. Various models may be used to illustrate the structure of at atom. One of the more common (Fig 2.6) is called the Bohr model, and is named after the physicist Niels Bohr. The Bohr model is useful, but today’s physicists tell us it is not possible to determine the exact location of individual electrons at any given moment. In the diagrams in Figure 2.6, the energy levels of the electrons, called electron shells or electron orbitals, are drawn as concentric rings about the nucleus. In all atoms, the first shell (closest to the nucleus) can contain two electrons. The next electron shell contains a maximum of eight electrons. In all atoms, the lower shells are filled with electrons before the next higher level contains any electrons.  The sulfur atom, with an atomic number of 16, has two electrons in the first shell, eight electrons in the second shell, and six electrons in the third, or outer, orbital. Revisit the periodic table (see Fig. 2.3), and note that sulfur is in the third period. In other words, the horizontal row tells you how many electron shells an atom has. Also note that sulfur is in group VI. The group tells you how many electrons an atom has in its outer shell. Regardless of how many shells an atom has, the outermost shell is called the valence shell. The valence shell is important, because it determines many of an atom’s chemical properties. If an atom has only one shell, the valence shell is complete when it has two electrons. In atoms with more than one shell, the valence shell is most stable when it has eight electrons. This is called the octet

C

N

carbon 12 6C

nitrogen 14 7N

P

S

phosphorus 31P 15

sulfur 32S 16

rule. Each atom in a group within the periodic table has the same number of electrons in its valence shell. As mentioned previously, all the atoms in group VIII of the periodic table have eight electrons in their valence shell. These elements are also called the noble gases, because they do not ordinarily react.  The electrons in the valence shells play an important role in determining how an element undergoes chemical reactions. Atoms with fewer than eight electrons in the outer shell react with other atoms in such a way that after the reaction each has a stable outer shell. As we will see, the number of electrons in an atom’s valence shell determines whether the atom gives up, accepts, or shares electrons to acquire eight electrons in the outer shell. 

Check Your Progress  2.1 1. Identify why the key elements in the Earth’s crust would differ from those present in living organisms.

2. Predict the number of electrons, protons, and neutrons in calcium (see Fig. 2.3).

3. Explain how radiation can be both beneficial and harmful to humans.

4. Explain the differences between oxygen 16 and oxygen 18.

2.2  Molecules and Compounds Learning Outcomes Upon completion of this section, you should be able to 1. Describe how elements are combined into molecules and compounds. 2. List the different types of bonds that occur between elements. 3. Compare the relative strengths of ionic, covalent, and hydrogen bonds.



Chapter 2  The Molecules of Cells

Atoms, except for noble gases, routinely bond with one another. A molecule is formed when two or more atoms bond together. For example, oxygen does not exist in nature as a single atom, O. Instead, two oxygen atoms are joined to form a molecule of oxygen, O2. When atoms of two or more different elements bond together, the product is called a compound. Water (H2O) is a compound that contains atoms of hydrogen and oxygen. We can also speak of molecules of water because a molecule is the smallest part of a compound that still has the properties of that compound. Electrons possess energy, and the bonds that exist between atoms also contain energy. Organisms are directly dependent on ­chemical-bond energy to maintain their organization. When a chemical reaction occurs, electrons shift in their relationship to one another, and energy may be given off or absorbed. This same energy is used to carry on our daily lives.

Ionic Bonding Ions form when electrons are transferred from one atom to another. For example, sodium (Na), with only one electron in its valence shell, tends to be an electron donor (Fig. 2.7a). Once it gives up this electron, the second shell, with eight electrons, becomes its outer shell. Chlorine (Cl), on the other hand, tends to be an electron acceptor. Its outer shell has seven electrons, so it requires one more electron to have a complete outer shell. When a sodium atom and a chlorine atom come together, an electron is transferred from the sodium atom to the chlorine atom. Now both atoms have eight electrons in their outer shells. This electron transfer, however, causes a charge imbalance in each atom. The sodium atom has one more proton than it has electrons. Therefore, it has a net charge of +1 (symbolized by Na+).

Na

Cl

sodium atom (Na)

chlorine atom (Cl)

a.

Na

Cl

sodium ion (Na+)

chloride ion (Cl– )

sodium chloride (NaCl)

The chlorine atom has one more electron than it has protons. Therefore, it has a net charge of –1 (symbolized by Cl–). Such charged particles are called ions. Sodium (Na+) and chloride (Cl–) are not the only biologically important ions. Some, such as potassium (K+), are formed by the transfer of a single electron to another atom. Others, such as calcium (Ca2+) and magnesium (Mg2+), are formed by the transfer of two electrons. Ionic compounds are held together by an attraction between negatively and positively charged ions called an ionic bond. When sodium reacts with chlorine, an ionic compound called sodium chloride (NaCl) results. Sodium chloride is a salt, commonly known as table salt because it is used to season our food (Fig. 2.7b). Salts can exist as a dry solid, but when salts are placed in water, they release ions as they dissolve. NaCl separates into Na+ and Cl–. In biological systems, because they are 70–90% water, ionic compounds tend to be dissociated (ionized) rather frequently.

Covalent Bonding A covalent bond results when two atoms share electrons in such a way that each atom has an octet of electrons in the outer shell. In a hydrogen atom, the outer shell is complete when it contains two electrons. If hydrogen is in the presence of a strong electron acceptor, it gives up its electron to become a hydrogen ion (H+). But if this is not possible, hydrogen can share with another atom and thereby have a completed outer shell. For example, one hydrogen atom will share with another hydrogen atom. Their two shells overlap, and the electrons are shared between them (Fig. 2.8a). Each atom has a completed outer shell due to the sharing of electrons.

Na+ Cl–

–

+

23

b.

Figure 2.7  Formation of sodium chloride (table salt).  a. To form sodium chloride, an electron is transferred from the sodium atom to the chlorine atom. At the completion of the reaction, each atom is an ion containing eight electrons in its outer shell. b. In a sodium chloride crystal, ionic bonding between Na+ and Cl– causes the atoms to assume a three-dimensional lattice in which each sodium ion is surrounded by six chloride ions, and each chloride ion is surrounded by six sodium ions. This forms crystals, such as in table salt.



24

UNIT 1  Cell Biology

Structural Formula

Electron Model

H

H

Molecular Formula

H

H

H2

O

O

O2

a. Hydrogen gas

O

O

A more common way to symbolize that atoms are sharing electrons is to draw a line between the two atoms, as in the structural formula H—H. In a molecular formula, the line is omitted and the molecule is simply written as H2. Sometimes, atoms share more than one pair of electrons to complete their octets. A double covalent bond occurs when two atoms share two pairs of electrons (Fig. 2.8b). To show that oxygen gas (O2) contains a double bond, the molecule can be written as O“O. It is also possible for atoms to form triple covalent bonds, as in nitrogen gas (N2), which can be written as N‚N. Single covalent bonds between atoms are quite strong, but double and triple bonds are even stronger.

Shape of Molecules b. Oxygen gas

H

H C

H

H

H

C

H

CH4

H H

c. Methane

Structural formulas make it seem as if molecules are one-­ dimensional, but actually molecules have a three-dimensional shape that often determines their biological function. Molecules consisting of only two atoms are always linear, but a molecule such as methane with five atoms (Fig. 2.8c) has a tetrahedral shape. Why? Because, as shown in the ball-and-stick model, each bond is pointing to the corners of a tetrahedron (Fig. 2.8d, left). The space-filling model often comes closest to the actual shape of the molecule (Fig. 2.8d, right). There is a saying in biology that “structure equals function.” Frequently, the shape of a molecule is important to the structural and functional roles it plays in an organism. For example, hormones have specific shapes that allow them to be recognized by the cells in the body. Antibodies combine with disease-causing agents, like a key fits a lock, to protect us. Similarly, homeostasis is maintained only when enzymes have the proper shape to carry out their particular reactions in cells.

Space-filling Model

Ball-and-stick Model

Nonpolar and Polar Covalent Bonds hydrogen

H

carbon

C

H

covalent bond H

109°

H

d. Methane–continued

Figure 2.8  Covalently bonded molecules.  In a covalent bond, atoms share electrons, allowing each atom to have a completed valence shell. a. A molecule of hydrogen (H2) contains two hydrogen atoms sharing a pair of electrons with a single covalent bond. b. A molecule of oxygen (O2) contains two oxygen atoms sharing two pairs of electrons. This results in a double covalent bond. c. A molecule of methane (CH4) contains one carbon atom bonded to four hydrogen atoms. d. When carbon binds to four other atoms, as in methane, each bond actually points to one corner of a tetrahedron. Ball-and-stick models and space-filling models are threedimensional representations of a molecule.

When the sharing of electrons between two atoms is fairly equal, the covalent bond is said to be a nonpolar covalent bond. All the molecules in Figure 2.8, including methane (CH4), are nonpolar. In the case of water (H2O), however, the sharing of electrons between oxygen and each hydrogen is not completely equal (Fig. 2.9a). This is because, in some cases, one atom is able to attract electrons to a greater degree than the other atom. In this case, we say that the atom that has a greater attraction for a shared pair of electrons has a greater electronegativity. In the case of water, the oxygen atom is more electronegative than the hydrogen atom and therefore the oxygen atom can attract the electron pair to a greater extent than each hydrogen atom can. It may help to think of electronegativity as where the electron pair chooses to “spend its time.” In a water molecule, the shared electron pair spends more time around the nucleus of the oxygen atom than around the nucleus of the hydrogen atom. This causes the oxygen atom to assume a slightly negative charge (δ–), and it causes the hydrogen atoms to assume slightly positive charges (δ+). The unequal sharing of electrons in a covalent bond creates a polar covalent bond. The bond is called polar because it differs in polarity (the electrical charge) across the molecule.



Chapter 2  The Molecules of Cells

25

unzip. On the other hand, the hydrogen bonds acting together add stability to the DNA molecule. As we shall see, many of the important properties of water are the result of hydrogen bonding.

Electron Model

Check Your Progress  2.2 1. Identify whether carbon dioxide (CO2) and nitrogen gas (N2)

O

are considered molecules, compounds, or both.

2. Describe why a carbon atom (C) is capable of forming four H

covalent bonds. 3. Explain the difference between a polar and nonpolar covalent bond.

H

a. Water (H2O)

2.3  Chemistry of Water Learning Outcomes

δ+ H O δ– hydrogen bond

H

δ+

b. Hydrogen bonding between water molecules

Figure 2.9  Water molecule.  a. The electron model of water. b. Hydrogen bonding between water molecules. A hydrogen bond is the attraction of a slightly positive hydrogen to a slightly negative atom in the vicinity. Each water molecule can hydrogen-bond to four other molecules in this manner. When water is in its liquid state, some hydrogen bonds are forming and others are breaking at all times.

Hydrogen Bonding Polarity within a water molecule causes the hydrogen atoms in one molecule to be attracted to the oxygen atoms in other water molecules (Fig. 2.9b). This attraction, although weaker than an ionic or covalent bond, is called a hydrogen bond. Because a hydrogen bond is easily broken, it is often represented by a dotted line. Hydrogen bonding is not unique to water. Many biological molecules have polar covalent bonds involving an electropositive hydrogen and usually an electronegative oxygen or nitrogen. In these instances, a hydrogen bond can occur within the same molecule or between different molecules. Although a hydrogen bond is more easily broken than a covalent bond, many hydrogen bonds taken together are quite strong. Hydrogen bonds between cellular molecules help maintain their proper structure and function. For example, hydrogen bonds hold the two strands of DNA together. When DNA makes a copy of itself, each hydrogen bond easily breaks, allowing the DNA to

Upon completion of this section, you should be able to 1. Evaluate which properties of water are important for biological life. 2. Identify common acidic and basic substances. 3. Describe how buffers are important to living organisms.

The first cell(s) evolved in water, and organisms are composed of 70–90% water. Water is a polar molecule, and water molecules are hydrogen-bonded to one another (see Fig. 2.9b). Due to hydrogen bonding, water molecules cling together. Without hydrogen bonding between molecules, water would change from a solid to liquid state at –100°C and from a liquid to gaseous state at –91°C. This would make most of the water on Earth steam, and life unlikely. But because of hydrogen bonding, water is a liquid at temperatures typically found on Earth’s surface. It melts at 0°C and boils at 100°C. These and other unique properties of water make it essential to the existence of life.

Properties of Water Water has a high heat capacity.  A calorie is the amount of heat energy needed to raise the temperature of 1 gram (g) of water by 1°C. In comparison, other covalently bonded liquids require input of only about half this amount of energy to rise 1°C in temperature. The many hydrogen bonds that link water molecules help water absorb heat without a great change in temperature. Converting 1 g of the coldest liquid water to ice requires the loss of 80 calories of heat energy. Water holds onto its heat, and its temperature falls more slowly than that of other liquids. This property of water is important not only for aquatic organisms, but for all organisms. Because the temperature of water rises and falls slowly, organisms are better able to maintain their normal internal temperatures and are protected from rapid temperature changes.

Water has a high heat of vaporization.  Converting 1 g of the hottest water to a gas requires an input of 540 calories of heat energy. Water has a high heat of vaporization because hydrogen bonds must be broken before water boils and



26

UNIT 1  Cell Biology

collide, allowing reactions to occur. Nonionized and nonpolar molecules, such as oil, that cannot attract water are said to be hydrophobic.

Water molecules are cohesive and adhesive. 

Figure 2.10  Water’s high heat of vaporization.  When body temperature increases, sweat is produced by glands in the dermal layer of the skin. The evaporation of the water off your skin aids in cooling your body. changes to a vaporized state. Water’s high heat of vaporization gives animals in a hot environment an efficient way to release excess body heat (Fig. 2.10). For example, when we sweat, or get water splashed on us, our body heat is used to vaporize the water, thus cooling us down. Temperatures along coasts are moderate due to water’s high heat capacity and high heat of vaporization. During the summer, the ocean absorbs and stores solar heat, and during the winter, the ocean slowly releases it. In contrast, the interior regions of continents can experience severe changes in temperature.

Water is a solvent.  Due to its polarity, water facilitates chemical reactions, inside and outside living organisms. It dissolves a great number of substances. A solution contains dissolved substances called solutes. When ionic salts—for example, sodium chloride (NaCl)—are put into water, the negative ends of the water molecules are attracted to the sodium ions, and the positive ends of the water molecules are attracted to the chloride ions. This causes the sodium ions and the chloride ions to separate, or dissociate, in water: δ+ H

O

H δ+ δ–

Na+ H

O H

An ionic salt dissolves in water. O H

H

H δ+

H O H

O

δ– H δ+

Cl– H H O

Water is also a solvent for larger molecules that either contain ionized atoms or are polar molecules. Molecules that can attract water are said to be hydrophilic. When ions and molecules disperse in water, they move about and

Cohesion refers to the ability of water molecules to cling to each other due to hydrogen bonding. At any moment in time, a water molecule can form hydrogen bonds with at most four other water molecules. Because of cohesion, water exists as a liquid under the conditions of temperature and pressure present at the Earth’s surface. The strong cohesion of water molecules is apparent because water flows freely, yet water molecules do not separate from each other.  Adhesion refers to the ability of water molecules to cling to other polar surfaces. This is a result of water’s polarity. Multicellular animals often contain internal vessels in which water assists the transport of nutrients and wastes, because the cohesion and adhesion of water allow blood to fill the tubular vessels of the cardiovascular system. For example, the liquid portion of our blood, which transports dissolved and suspended substances about the body, is 90% water.  Cohesion and adhesion also contribute to the transport of water in plants. The roots of plants absorb water while the leaves lose water through evaporation. A plant contains a system of vessels that reaches from the roots to the leaves. Water evaporating from the leaves is immediately replaced with water molecules from the vessels. Because water molecules are cohesive, a tension is created that pulls a water column up from the roots. Adhesion of water to the walls of the vessels also helps prevent the water column from breaking apart. Because water molecules are attracted to each other, they cling together where the liquid surface is exposed to air. The stronger the force between molecules in a liquid, the greater the surface tension. This surface tension acts as a barrier between the surface of the water and the atmosphere. Water striders, a common insect, use the surface tension of water to walk on the surface without breaking the surface. 

Frozen water (ice) is less dense than liquid water.  As liquid water cools, the molecules come closer together. They are densest at 4°C, but they are still moving about, bumping into each other (Fig. 2.11). At temperatures below 4°C, including at 0°C when water is frozen, the water forms a regular crystal lattice that is rigid and has more open space between the water molecules. For this reason water expands as it freezes, which is why cans of soda burst when placed in a freezer or why frost heaves make northern roads bumpy in the winter. It also means that ice is less dense than liquid water, and floats on liquid water. If ice did not float on water, it would sink, and ponds, lakes, and perhaps even regions of the ocean would freeze solid. This would make life impossible in the water and also on land. Instead, bodies of water always freeze from the top down. When a body of water freezes on the surface, the ice acts as an insulator to prevent



Chapter 2  The Molecules of Cells

27

Acids and Bases ice lattice

When water ionizes, it releases an equal number of hydrogen ions (H+; also called protons1) and hydroxide ions (OH–) into the solution:

liquid water

1.0

Density (g/cm3)

H

O H water

+ OHH+ hydrogen hydroxide ion ion

Only a few water molecules at a time dissociate, and the actual number of H+ and OH– is very small (1 × 10–7 moles/liter).2

Acidic Solutions (High H+ Concentrations) 0.9 0

4 Temperature (°C)

100

a.

Lemon juice, vinegar, tomatoes, and coffee are all acidic solutions. Acids are substances that release hydrogen ions (H+) when they dissociate in water. Therefore, they contain a higher concentration of H+ than OH–. For example, hydrochloric acid (HCl) is an important acid that dissociates in this manner: HCl ⟶ H+ + Cl– Because dissociation is almost complete, HCl is called a strong acid. If hydrochloric acid is added to a beaker of water, the number of hydrogen ions (H+) increases greatly.

Basic Solutions (Low H+ Concentrations) Baking soda and antacids are common basic solutions familiar to most people. Bases are substances that either take up hydrogen ions (H+) or release hydroxide ions (OH–). They contain a higher concentration of OH– than H+. For example, sodium hydroxide (NaOH) is an important base that dissociates in this manner: NaOH ⟶ Na+ + OH– Because dissociation is almost complete, sodium hydroxide is called a strong base. If sodium hydroxide is added to a beaker of water, the number of hydroxide ions increases.

pH Scale

b.

Figure 2.11  Ice is less dense than water.  a. Water is more dense at 4°C than at 0°C. While most substances contract when they solidify, water expands when it freezes because the water molecules in ice form a lattice in which the hydrogen bonds are farther apart than in liquid water. b. This allows ice to float on the surface of liquid water.

the water below it from freezing. This protects aquatic organisms so that they can survive the winter. As ice melts in the spring, it draws heat from the environment, helping to prevent a sudden change in temperature that might be harmful to life.

The pH scale is used to indicate the acidity or basicity (alkalinity) of solutions.3 The pH scale (Fig. 2.12) ranges from 0 to 14. A pH of 7 represents a neutral state in which the hydrogen ion and hydroxide ion concentrations are equal. A pH below 7 is an acidic solution because the hydrogen ion concentration [H+] is greater than the hydroxide concentration [OH–]. A pH above 7 is basic because [OH–] is greater than [H+]. As we move down the pH scale from pH 14 to pH 0, each unit has ten times the H+ concentration of the previous unit. As we move up the scale from 0 to 14, each unit has ten times the OH– concentration of the previous unit. The pH scale was devised to eliminate the use of cumbersome numbers. For example, the possible hydrogen ion concentrations A hydrogen atom contains one electron and one proton. A hydrogen ion has only one proton, so it is often simply called a proton.

1

In chemistry, a mole is defined as the amount of matter that contains as many objects (atoms, molecules, ions) as the number of atoms in exactly 12 g of 12C. 2

3 pH is defined as the negative log of the hydrogen ion concentration [H+]. A log is the power to which ten must be raised to produce a given number.



28

UNIT 1  Cell Biology

H+

4 3 2 1 0 Acidic

Neutral pH

uice

hydrochloric acid (HCl)

human blood egg w hites, se

ea Gr

7 8 5 6 9

on j

stomach acid

b sto akin ma g s ch oda an tac , ids

s oe at

lem

r

milk

k lac ,b

m to s er, oda vin eg a

pure water, tears

ad

e , urin beer er root wat rain ffee mal co

nor

bre

be

a wate

r

Blood always contains a combination of carbonic acid and bicarbonate ions. When hydrogen ions (H+) are added to blood, the following reaction occurs:

10 11 12 13 14 Basic

OH-

H+ + HCO3– ⟶ H2CO3

alt tS

e ak

L

old eh us onia o h m am e nat rbo bica soda of oven cleaner

sodium hydroxide (NaOH)

When hydroxide ions (OH–) are added to blood, this reaction occurs: OH– + H2CO3 ⟶ HCO3– + H2O These reactions prevent any significant change in blood pH.

Check Your Progress  2.3 1. Compare the difference between water’s high heat capacity and high heat of vaporization.

2. Explain the difference between cohesion and adhesion. 3. Explain why a solution with a pH of 6 contains more H+ than a solution with a pH of 8.

Figure 2.12  The pH scale.  The dial of this pH meter indicates that pH

ranges from 0 to 14, with 0 the most acidic and 14 the most basic. pH 7 (neutral pH) has equal amounts of hydrogen ions (H+) and hydroxide ions (OH–). An acidic pH has more H+ than OH–, and a basic pH has more OH– than H+.

of a solution are on the left in the following listing, and the pH is on the right: [H+] 

(moles per liter)

0.000001 = 1 × 10−6 0.0000001 = 1 × 10−7 0.00000001 = 1 × 10−8

pH

6 7 8

To further illustrate the relationship between hydrogen ion concentration and pH, consider the following question: Which of the pH values listed indicates a higher hydrogen ion concentration [H+] than pH 7, and therefore would be an acidic solution? A number with a smaller negative exponent indicates a greater quantity of hydrogen ions than one with a larger negative exponent. Therefore, pH 6 is an acidic solution.

Buffers and pH A buffer is a substance that keeps pH within normal limits. Many commercial products, such as aspirin, shampoos, or deodorants, are buffered as an added incentive for us to buy them. Buffers resist pH changes because they can take up excess hydrogen ions (H+) or hydroxide ions (OH–). In animals, the pH of body fluids is maintained within a narrow range, or else health suffers. The pH of our blood when we are healthy is always about 7.4—that is, just slightly basic (alkaline). If the blood pH drops to about 7, acidosis results. If the blood pH rises to about 7.8, alkalosis results. Both conditions can be life threatening. Normally, pH stability is possible because the body has built-in mechanisms to prevent pH changes. Buffers are the most important of these mechanisms. For example, carbonic acid (H2CO3) is a weak acid that minimally dissociates and then re-forms in the following manner: dissociates ⟶ H+ + H2CO3 HCO3– ⟵ re-forms bicarbonate ion carbonic acid

2.4  Organic Molecules Learning Outcomes Upon completion of this section, you should be able to 1. Compare inorganic molecules to organic molecules. 2. Identify the role of a functional group. 3. Recognize how monomers are joined to form polymers.

Inorganic molecules constitute nonliving matter, but even so, inorganic molecules such as salts (e.g., NaCl) and water play important roles in living organisms. The molecules of life, however, are organic molecules. Organic molecules always contain carbon (C) and hydrogen (H). The chemistry of carbon accounts for the formation of the very large variety of organic molecules found in living organisms. Carbon atoms contain four electrons in their outer shell. In order to achieve eight electrons in the outer shell, a carbon atom shares electrons covalently with as many as four other atoms, as in methane (CH4). Carbon atoms often share electrons with other carbon atoms, forming long hydrocarbon chains.

H

H

H

H

H

H

H

H

H

C

C

C

C

C

C

C

C

H

H

H

H

H

H

H

H

H

Under certain conditions, a hydrocarbon chain can turn back on itself to form a ring compound. The carbon chain of an organic molecule is often called its skeleton, or backbone. Just as a skeleton accounts for your body’s shape, so does the carbon skeleton of an organic molecule account for its shape. It is possible for organic molecules to have the same molecular formula, but different shapes. These molecules are called isomers. What gives an organic molecule its unique chemical characteristics are the combinations of atoms, called functional groups, that are attached to the carbon skeleton. 



Chapter 2  The Molecules of Cells

Functional Groups

monomer

The chemical reactivity of an organic molecule is determined by the types and locations of functional groups on the organic molecule. A functional group is a specific combination of bonded atoms that always has the same chemical properties and therefore always reacts in the same way.  Typically, the carbon skeleton acts as a framework for the positioning of the functional groups. It is important to recognize that how the functional groups are positioned in the organic molecule determines the chemical properties of the molecule. For example, the addition of an —OH (hydroxyl group) to a carbon skeleton gives that molecule the chemical characteristics of an alcohol. Table 2.1 lists some of the more common functional groups. The R indicates the “remainder” of the molecule. This is the place on the functional group that attaches to the carbon skeleton. 

OH

HH

dehydration reaction

monomer

29

monomer

H2O

monomer

a. Synthesis of a biomolecule

From Monomers to Polymers Many of the organic molecules that you are familiar with, such as carbohydrates, lipids, proteins, and nucleic acids, are macromolecules (also called biomolecules), meaning that they contain smaller subunits joined together. The carbohydrates, proteins, and nucleic acids are referred to as polymers, since they are constructed by linking together a large number of the same type of subunit, called a monomer. Lipids are not polymers  because they contain two

monomer

OH

hydrolysis reaction

H

monomer

H2O

TABLE 2.1  Functional Groups Functional Groups Group

Structure

Hydroxyl

R

Carbonyl

R

OH

O

C

H

O R

Carboxyl (acidic)

Compound

Significance

Alcohol as in ethanol

Polar, forms hydrogen bond Present in sugars, some amino acids

Aldehyde as in formaldehyde Ketone as in acetone

Polar Present in sugars Polar Present in sugars

Carboxylic acid as in acetic acid

Polar, acidic Present in fatty acids, amino acids

Amine as in tryptophan

Polar, basic, forms hydrogen bonds Present in amino acids

Thiol as in ethanethiol

Forms disulfide bonds Present in some amino acids

R

C

O

R

C

R

N

Sulfhydryl

R

SH

Phosphate

Organic phosphate as in phosphoryR O P OH lated molecules

Amino

OH H H

O

OH R = remainder of molecule

Polar, acidic Present in nucleotides, phospholipids

monomer

monomer

b. Degradation of a biomolecule

Figure 2.13  Synthesis and degradation of polymers.  a. In

cells, synthesis often occurs when monomers join (bond) during a dehydration reaction (removal of H2O). b. Degradation occurs when the monomers in a polymer separate during a hydrolysis reaction (addition of H2O).

different types of subunits (glycerol and fatty acids). Polymers may vary considerably in length. Just as a train increases in length when boxcars are hitched together one by one, so a polymer gets longer as monomers bond to one another. Cells have a common way of joining monomers to build polymers. During a dehydration reaction, an —OH (hydroxyl group) and an —H (hydrogen atom), the equivalent of a water molecule, are removed as the reaction proceeds (Fig. 2.13a). To degrade polymers, the cell uses a hydrolysis reaction, in which the components of water are added (Fig. 2.13b).

Check Your Progress  2.4 1. Describe why organic molecules are considered the molecules of life.

2. Compare and contrast dehydration and hydrolysis reactions.



30

UNIT 1  Cell Biology

2.5  Carbohydrates Learning Outcomes Upon completion of this section, you should be able to 1. Identify the structural components of a carbohydrate. 2. List several examples of important monosaccharides and polysaccharides.

Carbohydrates are almost universally used as an energy source for living organisms, including humans. Carbohydrates also play a structural role in woody plants, bacteria, and animals such as insects. In addition, carbohydrates on cell surfaces are involved in cell-to-cell recognition, as we will learn in Chapter 4. Carbohydrate molecules are characterized by the presence of the atomic grouping H—C—OH, in which the ratio of hydrogen atoms (H) to oxygen atoms (O) is approximately 2:1. The term carbohydrate (literally, carbon-water) includes single sugar molecules and chains of sugars. Chain length varies from a few sugars to hundreds of sugars. The monomer subunits, called monosaccharides, are assembled into long polymer chains called polysaccharides. 

Monosaccharides–Simple Sugars Monosaccharides (mono,  one; saccharide, sugar) consist of only a single sugar molecule and are commonly called simple sugars. A monosaccharide can have a carbon backbone of three to seven carbons.  For example, pentoses are monosacharides with five carbons, and hexoses are monosaccharides with six carbons. Monosaccharides have a large number of hydroxyl

H

6 CH2OH O 5C

H C 4 OH HO C 3

H

CH2OH H

O

H

C1

H

C OH

2

OH

H OH

HO

OH

H

a.

O

H

OH

b.

c.

C6H12O6

Figure 2.14  Three ways to represent the structure of glucose. 

C6H12O6 is the molecular formula for glucose. a. This structure shows all the carbon atoms. b. This structure only shows the carbon atoms outside the ring. c.  This is the simplest way to represent glucose. Note that in a and b, each carbon has an attached H and OH group. Those groups are assumed in c. CH2OH

O

CH2OH

H

H

O

+ HO

OH glucose C6H12O6 monosaccharide

+

groups (—OH), and the presence of this polar functional group makes them soluble in water.  Glucose is a hexose sugar found in our blood (Fig. 2.14). Our bodies use glucose as an immediate source of energy. Other monosaccharides include fructose, found in fruits, and galactose, a constituent of milk. All of these are isomers with the molecular formula C6H12O6. Each also forms a ring structure when placed in a water environment. The exact shape of the ring differs, as does the arrangement of the hydrogen (—H) and hydroxyl (—OH) groups attached to the ring.

Dissaccharides A disaccharide (di, two) contains two monosaccharides that have joined during a dehydration reaction. Figure 2.15 shows how the disaccharide maltose forms when two glucose molecules bond together. Note the position of this bond. Our hydrolytic digestive juices can break this bond, and the result is two glucose molecules. When glucose and fructose join, the disaccharide sucrose forms. Sucrose is another disaccharide of special interest because we use it at the table to sweeten our food. We acquire sucrose from plants such as sugarcane and sugar beets. You may also have  heard of lactose, a disaccharide found in milk. Lactose is glucose combined with galactose. Some people are lactose intolerant because they cannot break down lactose. This leads to unpleasant gastrointestinal symptoms when they consume dairy products.

Polysaccharides–Complex Carbohydrates Long polymers such as starch, glycogen, and cellulose are ­polysaccharides (poly, many) that contain long chains of glucose subunits.  Due to their length, they are sometimes referred to as complex carbohydrates.

Energy Storage Polysaccharides Starch and glycogen are large storage forms of glucose found in plants and animals, respectively. Some of the polymers in starch are long chains of up to several thousand glucose units. Starch has fewer side branches, or chains of glucose that branch off from the main chain, than does glycogen, as shown in Figures 2.16 and 2.17. Flour, which we use for baking and usually acquire by grinding wheat, is high in starch, and so are potatoes. After we eat starchy foods such as potatoes and bread, starch is hydrolyzed into glucose, which will then enter the bloodstream. The liver stores glucose as glycogen. In between meals, the liver releases glucose so that the blood glucose concentration is always about 0.1%. CH2OH

dehydration reaction hydrolysis reaction

CH2OH

O

O

glucose C6H12O6

maltose C12H22O11

monosaccharide

disaccharide

O

+

H2O

water

+

water

Figure 2.15  Synthesis and degradation of maltose, a disaccharide.  Synthesis of maltose occurs following a dehydration reaction when a bond forms between two glucose molecules and water is removed. Degradation of maltose occurs following a hydrolysis reaction when this bond is broken by the addition of water.



Chapter 2  The Molecules of Cells CH2OH

O

CH2OH O

H H OH

H

H

O OH

H O

CH2OH O

H H OH

H

H

OH O H

H O

CH2OH O

H H OH

H

H

OH OH

H O

CH2OH

O

H H OH

H

H

OH

O

O

CH2OH O

H

H

H

H OH

H

H

OH

CH2OH O

H O

H

H OH

H

H

OH

O

CH2OH O

H

31

H

H OH

H

H

OH

O

H O

H OH

H

H

OH

H O

branched nonbranched

starch granule

glycogen granule

cell wall

liver cells

potato cells

Figure 2.16  Starch structure and function.  Starch is composed of chains of glucose molecules. Some chains are branched, as indicated. Starch is the storage form of glucose in plants. The electron micrograph shows starch granules in potato cells.

Figure 2.17  Glycogen structure and function.  Glycogen is a highly branched polymer of glucose molecules that serves as the storage form of glucose in animals. The electron micrograph shows glycogen granules in liver cells.

Structural Polysaccharides

cellulose fibers

O

•

H

•• ••

••

•• ••

O

CH2OH

O O

H OH

O

glucose molecules

••

O

OH

••

OH H

H H

H

••

H

O

••

•• ••

CH2OH

H

H

OH

•

•

••

OH

••

OH H H

CH2OH

OH

••

H H

••

O H

OH

••

••

H OH

H

O

H

H

••

••

O

H

O

H

••

••

CH2OH

OH H

H

••

••

OH

O O

H

O O

H OH ••

••

H

H

H

••

••

H

O

CH2OH

OH

••

•• •• ••

••

•• ••

H

CH2OH

OH H

OH H

H

H

H

••

••

H

H

OH

OH H

••

••

O

H

O O

H OH

H

••

••

CH2OH

OH

O

H

OH

••

H

O O

H OH ••

••

CH2OH H

CH2OH H

•

H

••

••

H

OH

OH H

H

H H

H

••

O

O O

H OH

••

CH2OH H

H

1. Identify the structural element that all carbohydrates have

943

microfibrils

CH2OH

Check Your Progress  2.5

cellulose fiber

plant cell wall

••

Some types of polysaccharides function as structural components of cells. The polysaccharide cellulose is found in plant cell walls, which helps account for the strong nature of these walls. In cellulose (Fig. 2.18), the glucose units are joined by a slightly different type of linkage than that found in starch or glycogen. Notice the alternating up/down position of the oxygen atoms in the linked glucose units in Figure 2.18. This small difference is significant because it prevents us from digesting foods containing this type of linkage. Therefore, cellulose largely passes through our digestive tract as fiber, or roughage. Most doctors now recognize that fiber in the diet is necessary to good health, and some studies have suggested it may even help prevent colon cancer. Chitin, which is found in the exoskeleton (shell) of crabs and related animals, is another structural polysaccharide. Scientists have discovered that chitin can be made into a thread and used as a suture material.

H CH2OH

H

O

O

Figure 2.18  Cellulose structure and function.  In cellulose, the linkage between glucose molecules is slightly different from that in starch or glycogen. Plant cell walls contain cellulose, and the rigidity of these cell walls permits nonwoody plants to stand upright as long as they receive an adequate supply of water. in common. 2. Explain why starch in plants is a source of glucose for our bodies but cellulose in plants is not.

32

UNIT 1  Cell Biology

H H C

H C

O OH

HO

O

+

OH

HO

O H C

OH

HO

C

C

C

H

H

H

H

C

C

C

C

H

H

H

H

H

H

H

H

H

H

C

C

C

C

C

C

H

H

H

H

H

H

H

H

H

C

C

C

C

C

H

H

glycerol

H C

H

H

H

H

H O

dehydration reaction H C

H

O

O

H

H

H

H

C

C

C

C

C

H

H

H

H

O

H

H

H

H

H

H

C

C

C

C

C

C

C

H

H

H

H

H

H

O

H

H

H

H

H

C

C

C

C

C

C

H

H

hydrolysis reaction

H

H

H C H

3 fatty acids

O

H fat molecule

H

H

+

3 H2O

H

3 water molecules

Figure 2.19  Synthesis and degradation of a triglyceride.  When a fat molecule (triglyceride) forms, three fatty acids combine with glycerol, and three water molecules are produced.

milk

O

C

HO

butter H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

saturated fatty acid with no double bonds

saturated fat

a.

corn

O

C

HO

corn oil

H

H

H

H

H

H

H

H

H

H

H

C

C

C

C

C

C

C

C

C

C

C

C

C

C

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

C

C

C

H

H

H

unsaturated fatty acid with double bonds (yellow) and a trans bond (red) b.

unsaturated fat

Figure 2.20  Saturated fats, unsaturated fats, and trans-fats.  Saturated fats have no double bonds between the carbon atoms, whereas unsaturated fats possess one or more double bonds. In a trans-fat, the hydrogen atoms are on opposite sides of the double bond. A fatty acid has a carboxyl group attached to a long hydrocarbon chain. a. If there are no double bonds between the carbons in the chain, the fatty acid is saturated. b. If there are double bonds between some of the carbons, the fatty acid is unsaturated and a kink occurs in the chain. A trans-fat is an unsaturated fat with the type of double bond shown in red.



Chapter 2  The Molecules of Cells

2.6  Lipids Learning Outcomes Upon completion of this section, you should be able to 1. Compare the structures of fats, phospholipids, and steroids. 2. Identify the functions lipids play in our bodies.

Trans-fats are often produced by hydrogenation, or the chemical addition of hydrogen to vegetable oils. This is done to convert the fat into a solid and is often found in processed foods.

Phospholipids Phospholipids, as their name implies, contain a phosphate group. Essentially they are constructed like fats, except that in place of the third fatty acid, there is a polar phosphate group or a grouping that contains both phosphate and nitrogen. The phosphate group forms the polar (hydrophilic) head of the molecule, while the rest of the molecule becomes the nonpolar (hydrophobic) tails (Fig. 2.21).

Polar Head

-O

CH2 CH2 CH2 CH2 CH2 CH2

outside cell a. Plasma membrane of a cell

C CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2

CH

CH2

CH2 CH2

CH2

O

CH

CH2

CH2

O

O

CH2

inside cell

glycerol

2

O C

phosphate 1C H

2C H

CH2

Saturated, Unsaturated, and Trans–Fatty Acids

3

The most familiar lipids are those found in fats and oils. Fats tend to be of animal origin (e.g., lard and butter), and are solid at room temperature. Oils, which are usually of plant origin (e.g., corn oil and soybean oil), are liquid at room temperature. Fat has several functions in the body: it is used for long-term energy storage, it insulates against heat loss, and it forms a protective cushion around major organs. Fats and oils form when one glycerol molecule reacts with three fatty acid molecules (Fig. 2.19). A fat molecule is sometimes called a triglyceride because of its three-part structure. While fats and oils are hydrophobic molecules, the addition of emulsifiers can allow them to mix with water. Emulsifiers contain molecules with a nonpolar end and a polar end. The molecules position themselves about an oil droplet so that their nonpolar ends project inward and their polar ends project outward. As a result, the fat or oil disperses in the water. This process is called emulsification. Emulsification takes place when dirty clothes are washed with soaps or detergents. It explains why some salad dressings are uniform in consistency (emulsified), while others separate into two layers. Also, prior to the digestion of fatty foods, fats are emulsified by bile in the intestines. The liver produces bile, which is then stored in the gallbladder. Individuals who have had their gallbladder (an organ where bile is stored) removed may have trouble digesting fatty foods.

O

Fats and Oils

R O P O

Lipids contain more energy per gram than other biological molecules while fats and oils function as energy storage molecules in organisms. Phospholipids form a membrane that separates the cell from its environment and forms its inner compartments as well. The steroids are a large class of lipids that includes, among others, the sex hormones. Lipids are diverse in structure and function, but they have a common characteristic: they do not dissolve in water. Lipids are hydrophobic.

33

CH2 CH2 CH2

fatty acids

CH2

CH2 A fatty acid is a hydrocarbon chain that ends with the acidic group CH2 —COOH (Fig. 2.19). Most of the fatty acids in cells contain 16 or CH2 CH2 18 carbon atoms per molecule, although smaller ones with fewer CH2 CH2 Nonpolar Tails carbons are also known. CH3 Fatty acids are either saturated or unsaturated (Fig. 2.20). CH2 ­Saturated fatty acids have no double bonds between carbon atoms. CH3 The carbon chain is saturated, so to speak, with all the hydrogens it can hold. Saturated fatty acids account for the solid nature at room b. Phospholipid temperature of fats such as lard and butter. Unsaturated fatty acids structure have double bonds between carbon atoms wherever the number of Figure 2.21  Phospholipids form membranes.  a. Phospholipids hydrogens is less than two per carbon atom. This produces a bend, arrange themselves as a bilayer in the plasma membrane that surrounds cells. or kink, in the fatty acid chain. Unsaturated fatty acids account for b. Phospholipids are constructed like fats, except that in place of the third the liquid nature of vegetable oils at room temperature.  fatty acid, they have a polar phosphate group. The bilayer structure forms Unsaturated fats are also often referred to as being “cis” or because the polar (hydrophilic) head is soluble in water, whereas the two “trans.” This terminology refers to the configuration of the hydrononpolar (hydrophobic) tails are not. The polar heads interact with the inside gen atoms in the double bond of an unsaturated fat (Fig. 2.20b). and outside of the cell while the nonpolar tails interact with each other.

34

UNIT 1  Cell Biology

SCIENCE IN YOUR LIFE  ►

HEALTH

A Balanced Diet Everyone agrees that we should eat a balanced diet, but just what is a balanced diet? The U.S. Department of Agriculture (USDA) released a new Food Plate in June 2011 (Fig. 2B). The new food plate contains a very colorful plate that is divided into the four basic food groups with a side circle representing a dairy component. The general guidelines are encouraging people to consume fewer calories by avoiding oversized portions. Half the plate should include fruits and vegetables, while making half of the grains consumed being whole grains. Reduction of foods that contain high levels of sodium is also recommended, as is an increase in the amount of water consumed.

Carbohydrates

make them semisolid. Trans-fats are found in shortenings, solid margarines, and some processed foods.

The new food plate advocates an intake of low-fat types of foods like 1% milk and low-fat yogurt instead of whole milk or regular yogurt. Polyunsaturated and monosaturated oils are still recommended in the Food Plate. These oils have been found to be protective against the development of cardiovascular disease.

Other nutrients

Figure 2B  MyPlate food guide.  The U.S. Department of Agriculture (USDA) developed this Food Plate as a guide to better health. The different widths of the food groups suggest what portion of your meal should consist of each category. The five different colors illustrate that foods, in correct proportion from all groups, are needed each day for good health. The orange represents grains; the green represents vegetables; the red represents fruits; the blue represents dairy; and the purple represents protein. For more details visit www.ChooseMyPlate.gov.

Carbohydrates include fruits, vegetables, and grains. They are the quickest, most readily available source of energy for the body. Complex carbohydrates, such as those in whole-grain breads and cereals, are preferable to simple carbohydrates, such as candy and ice cream, because they contain dietary fiber (nondigestible plant material), plus vitamins and minerals. Insoluble fiber has a laxative effect, and soluble fiber combines with the cholesterol in food and prevents cholesterol from exiting the intestinal tract and entering the bloodstream. Researchers have found that the starch in potatoes and processed foods, such as white bread and white rice, leads to a high blood glucose level just as  simple carbohydrates do. Research is

underway to determine if this is why many adults are developing type 2 diabetes.

Fats We have known for many years that saturated fats in animal products contribute to the formation of deposits called plaque, which clog arteries and lead to high blood pressure and heart attacks. Even more harmful than naturally occurring saturated fats are the so-called trans-fats, created artificially using vegetable oils. Trans-fats are partially hydrogenated to

Phospholipids illustrate that the chemistry of a molecule helps determine its function. Phospholipids are the primary components of cellular membranes. They spontaneously form a bilayer in which the hydrophilic heads face outward toward watery solutions and the tails form the hydrophobic interior. Plasma membranes separate extracellular from intracellular environments and are absolutely vital to the form and function of a cell.

Red meat is rich in protein, but it is usually also high in saturated fat; therefore, lean meats, fish, and chicken are preferred sources of protein. Also, a combination of rice and legumes (a group of plants that includes peas and beans) can provide all of the amino acids the body needs to build cellular proteins.

Questions to Consider 1. List the proportion of your plate that should consist of fruits and vegetables. Are you getting the recommended amounts of these important foods? Do you have variety in the types of fruits and vegetables you consume? 2. Why are some fats “good” for you and some “bad” for you? What are ideal sources for these fats? 3. Chemically, what is the difference between a “whole-grain” carbohydrate and a simple carbohydrate?

Steroids Steroids have a backbone of four fused carbon rings. Each one differs primarily by the arrangement of the atoms in the rings and the type of functional groups attached to them. Cholesterol is a steroid formed by the body that also enters the body as part of our diet. Cholesterol has several important functions. It is a component of an animal cell’s plasma membrane and is the precursor of several



35

Chapter 2  The Molecules of Cells OH CH3 CH3

O b. Testosterone CH3 HC

CH3

(CH2)3 HC CH3

OH CH3

CH3

CH3

HO

c. Estrogen

HO a. Cholesterol

Figure 2.22  Steroids.  a. Built like cholesterol, (b) testosterone and (c) estrogen have different effects on the body due to different functional groups attached to the same carbon skeleton. Testosterone is the male sex hormone (left), and estrogen is the female sex hormone (right). 

other steroids, such as bile salts and the sex hormones testosterone and estrogen (Fig. 2.22). We now know that a diet high in saturated fats, trans-fats, and cholesterol can cause fatty material to accumulate inside the lining of blood vessels, thereby reducing blood flow. The Health feature, “A Balanced Diet,” ­discusses which sources of carbohydrates, fats, and proteins are recommended for inclusion in the diet.

Check Your Progress  2.6 1. List the two types of lipid molecules found in the plasma membranes of animal cells.

2. Explain how the presence of a double bond in an unsaturated fatty acid affects whether that substance is a solid or liquid.

3. Distinguish between the roles of triglycerides and steroids in humans.

2.7  Proteins Learning Outcomes Upon completion of this section, you should be able to 1. Describe the functions of proteins in cells. 2. Explain how a polypeptide is constructed from amino acids. 3. Compare the four levels of protein structure.

Proteins are polymers composed of amino acid monomers. An amino acid has a central carbon atom bonded to a hydrogen atom

and three functional groups. The name of the molecule is appropriate because one of these groups is an amino group (—NH2) and another is an acidic group (—COOH). The third group is called an R group and determines the uniqueness of each amino acid. The R group varies from having a single carbon to being a complicated ring structure (Fig. 2.23). Proteins perform many functions. Proteins such as keratin, which makes up hair and nails, and collagen, which lends support to ligaments, tendons, and skin, are structural proteins. Some proteins are enzymes. Enzymes are necessary contributors to the chemical workings of the cell, and the body. Enzymes speed chemical reactions. They work so quickly that a reaction that normally takes several hours or days without an enzyme takes only a fraction of a second with an enzyme. Many hormones, messengers that influence cellular metabolism, are also proteins. The proteins actin and myosin account for the movement of cells and the ability of our muscles to contract. Some proteins transport molecules in the blood. Hemoglobin is a complex protein in our blood that transports oxygen. Antibodies in blood and other body fluids are proteins that combine with foreign substances, preventing them from destroying cells and upsetting homeostasis. Proteins in the plasma membrane of cells have various functions: some form channels that allow substances to enter and exit cells; some are carriers that transport molecules into and out of the cell; and some are enzymes.

Peptides Figure 2.24 shows that a synthesis reaction between two amino acids results in a dipeptide and a molecule of water. A polypeptide



36

UNIT 1  Cell Biology

Name

Structural Formula H H3N+

alanine (Ala)

H3N+

valine (Val)

C

O O-

H

O

H3C

H3

cysteine (Cys)

R group has a branched carbon chain

OCH3

H C

O C

R group contains sulfur

O-

CH2 SH H H3

N+

C

O C R group has a ring structure

O-

CH2

phenylalanine (Phe)

Figure 2.23  Representative amino acids.  Amino acids differ

from one another by their R group. The simplest R group is a single hydrogen atom (H). R groups (blue) containing carbon vary as shown. These are just 4 of the 20 amino acids found in living organisms.

is a chain of amino acids that are joined to one another by a ­peptide bond. The atoms associated with a peptide bond unevenly because oxygen is more electronegative than nitrogen. Therefore, the hydrogen attached to the nitrogen has a slightly positive charge, while the oxygen has a slightly negative charge: -

Oδ

peptide bond

C N Hδ

+

amino group

H

The polarity of the peptide bond means that hydrogen bonding is possible between the C“O of one amino acid and the N—H of another amino acid in a polypeptide. This hydrogen bonding influences the structure, or shape, of a protein.

Levels of Protein Organization

C

CH

N+

R group has a single carbon atom

C

CH3

C

R Group

Proteins can have up to four levels of structural organization (Fig. 2.25). The first level, called the primary structure, is the linear sequence of the amino acids joined by peptide bonds. Polypeptides can be quite different from one another. A polypeptide chain may be made from any combination of 20 different amino acids. Each particular polypeptide has its own sequence of amino acids and its own sequence of R groups. The secondary structure of a protein comes about when the polypeptide takes on a certain orientation in space. A coiling of the chain results in an α (alpha) helix, or a right-handed spiral, similar to a spiral staircase. Or a folding of the chain results in a β (beta) pleated sheet similar to a hand-held fan. Hydrogen bonding between peptide bonds holds the shape in place. The tertiary structure of a protein is its final three-­dimensional shape. In muscles, myosin molecules have a rod shape ending in globular (globe-shaped) heads. In enzymes, the polypeptide bends and twists in different ways. Invariably, the hydrophobic portions are packed mostly on the inside, and the hydrophilic portions are on the outside where they can make contact with water. The tertiary shape of a polypeptide is maintained by various types of bonding between the R groups; covalent, ionic, and hydrogen bonding all occur. One common form of covalent bonding between R groups is a disulfide (S—S) linkage between two cysteine amino acids. Some proteins have only one polypeptide, and others have more than one polypeptide, each with its own primary, secondary, and tertiary structures. In proteins with multiple polypeptide chains, these separate polypeptides are arranged to give such proteins a fourth level of structure, termed the quaternary structure. Hemoglobin is a complex protein having a quaternary structure. Most enzymes also have a quaternary structure. Thus, proteins can differ in many ways, such as in length, sequence, and structure. Each individual protein is chemically unique as well. The final shape of a protein is very important to its function. As we will discuss in Chapter 6, enzymes cannot function unless they have their normal shape. When proteins are exposed to extremes in heat and pH, they undergo an irreversible change

peptide bond

acidic group

H

H

N

C

O C

R amino acid

OH

+ H

H

R

N

C

OH C

H

O

dehydration reaction hydrolysis reaction

amino acid

H

H

H

O

N

C

C

R

R N

C

H

H

dipeptide

OH C

+

H2O

O water

Figure 2.24  Synthesis and degradation of a dipeptide.  Following a dehydration reaction, a peptide bond joins two amino acids, and a water molecule is released. Following a hydrolysis reaction, the bond is broken with the addition of water.



37

Chapter 2  The Molecules of Cells

H3N+ Primary Structure

COO−

amino acid

This level of structure is determined by the sequence of amino acids that join to form a polypeptide.

peptide bond

C

O C

O

CH

C Secondary Structure Hydrogen bonding between amino acids causes the polypeptide to form an alpha helix or a beta pleated sheet.

N

CH

H

R

CH N H

CH

CH N

R

N O

C

N H O

O C

C

R

CH N

C C

N H

R C C O H H N N

H

R

O C

O C

CH N

hydrogen bond

hydrogen bond C

C

O

R

O C

H

R

N H

R

O C

CH

C

C

R

R

C

N H

C C O H N H N

R

R α (alpha) helix

N H

O C R

C O

C

C

C O H N

C O C R

R

N H O

O C

C

C

R

N H

C O H N

β (beta) pleated sheet

Tertiary Structure Due in part to covalent bonding between R groups the polypeptide folds and twists, giving it a characteristic globular shape.

disulfide bond

Quaternary Structure This level of structure occurs when two or more polypeptides join to form a single protein.

Figure 2.25  Levels of protein organization.  All proteins have a primary structure. Both fibrous and globular proteins have a secondary structure. They are either α (alpha) helices or β (beta) pleated sheets. Globular proteins always have a tertiary structure, and most have a quaternary structure.

in shape. This is referred to as being denatured. For example, we are all aware that adding acid to milk causes curdling and that heating causes egg white, which contains a protein called albumin, to coagulate. Denaturation occurs because the normal bonding between the R groups has been disturbed. Once a ­protein loses its normal shape, it is no longer able to function normally. For example researchers hypothesize that an ­alteration in protein organization, forming structures called prions, is related to the development of Alzheimer disease and

Creutzfeldt-Jakob disease (the human form of “mad cow” disease).

Check Your Progress  2.7 1. List some of the functions of proteins. 2. Describe how amino acids are formed. 3. Compare and contrast the four levels of protein structure.



38

UNIT 1  Cell Biology

2.8  Nucleic Acids Learning Outcomes Upon completion of this section, you should be able to 1. Compare the structure and function of DNA and RNA. 2. Explain the role of ATP in the cell.

The two types of nucleic acids are DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). DNA stores genetic information in the cell and in the organism. Further, the cell replicates and transmits this information when the cell copies itself as well as when the organism reproduces. The discovery of the structure of DNA has had an enormous influence on biology and on society in general. The science of biotechnology (Chapter 26) is largely devoted to altering the genes in living organisms. We have discovered how many of our genes work and have learned how to manipulate them.

T

A C

G T

A

G

C

Structure of DNA and RNA Both DNA and RNA are polymers of nucleotides. Every nucleotide is a molecular complex of three subunits (Fig.  2.26)—­phosphate (phosphoric acid), a pentose sugar, and a nitrogen-containing base. The nucleotides in DNA contain the sugar deoxyribose and the nucleotides in RNA contain the sugar ribose. This difference accounts for their respective names (Table 2.2). There are four different types of bases in DNA: adenine (A), thymine (T), guanine (G), and cytosine (C). The base can have two rings (adenine or guanine) or one ring (thymine or cytosine). In O –O

P

phosphate

O

O–

C

P 55'

nitrogencontaining base

S

P

a. Space-filling model

Cytosine Guanine Phosphate

S A T

Sugar Adenine Thymine

b. Double helix H N

N

O

H

N

H

N

O

H

N

N

N

N

sugar

O 4'

C G

sugar

H 1'

cytosine (C)

2' 3' pentose sugar

H N

Nucleotide structure

Figure 2.26  Structure of a nucleotide.  Each nucleotide consists of a pentose sugar, a nitrogen-containing base, and a phosphate functional group.

TABLE 2.2  DNA Structure Compared With RNA Structure

guanine (G)

N

H

CH3

O C

N sugar

N

H

N N

N O

adenine (A)

sugar

thymine (T)

c. Complementary base pairing

DNA

RNA

Sugar

Deoxyribose

Ribose

Bases

Adenine, guanine, thymine, cytosine

Adenine, guanine, uracil, cytosine

Strands

Double stranded with base pairing

Single stranded

Helix

Yes

No

Figure 2.27  Overview of DNA structure.  The structure of DNA

is absolutely essential to its ability to replicate and to serve as the genetic material. a. Space-filling model of DNA’s double helix. b. Complementary base pairing between strands. c. Ladder configuration. Notice that the uprights are composed of alternating phosphate and sugar molecules and that the rungs are complementary nitrogen-containing paired bases. The heredity information stored by DNA is the sequence of its bases, which determines the primary structure of the cell’s proteins.



Chapter 2  The Molecules of Cells

39

H2O P

adenosine

P

P

triphosphate

P

adenosine

ATP

P

+

diphosphate ADP

P

phosphate energy

Figure 2.28  ATP reaction.  ATP, the universal energy “currency” of cells, is composed of adenosine and three phosphate groups (called a triphosphate). When cells require energy, ATP undergoes hydrolysis, producing ADP + P , with the release of energy.

RNA, the base uracil (U) replaces the base thymine. These structures are called bases because their presence raises the pH of a solution. The nucleotides form a linear molecule called a strand, which has a backbone made up of alternating phosphates and sugars, with the bases projecting to one side of the backbone. The nucleotides and their bases occur in a specific order. After many years of work, researchers now know the sequence of the bases in human DNA— the human genome. This breakthrough is expected to lead to improved genetic counseling, gene therapy, and medicines to treat the causes of many human illnesses. DNA is double stranded, with the two strands twisted about each other in the form of a double helix (Fig. 2.27a, b). In DNA, the two strands are held together by hydrogen bonds between the bases. When unwound, DNA resembles a ladder. The uprights (sides) of the ladder are made entirely of the alternating phosphate and sugar molecules, and the rungs of the ladder are made only of complementary paired bases. Thymine (T) always pairs with adenine (A), and guanine (G) always pairs with cytosine (C). Complementary bases have shapes that fit together (Fig. 2.27c). Complementary base pairing allows DNA to replicate in a way that ensures the sequence of bases will remain the same. The base sequence of specific sections of DNA contain a code that specifies the sequence of amino acids in the proteins of the cell. RNA is single stranded and is formed by complementary base pairing with one DNA strand. There are several types of RNA. One type of RNA, mRNA, or messenger RNA, carries the information from the DNA strand to the ribosome where it is translated into the sequence of amino acids specified by the DNA.

ATP (Adenosine Triphosphate) In addition to being the monomers of nucleic acids, nucleotides have other metabolic functions in cells. When adenosine (adenine plus ribose) is modified by the addition of three phosphate groups instead of one, it becomes ATP (adenosine triphosphate), an energy carrier in cells. Glucose is broken down in a stepwise fashion so that the energy of glucose is converted to that

of  ATP molecules. ATP molecules serve as small “energy ­packets” suitable for supplying energy to a wide variety of a cell’s chemical reactions. ATP can be said to be the energy “currency” of the cell. Reactions in the cell that need energy require ATP. ATP is a high-energy molecule because the last two phosphate bonds are unstable and easily broken. In cells, the terminal phosphate bond usually is hydrolyzed, leaving the molecule ADP (adenosine diphosphate) and a molecule of inorganic phosphate, P (Fig. 2.28). The cell uses the energy released by ATP breakdown to synthesize macromolecules such as carbohydrates and proteins. ­Muscle cells use the energy for muscle contraction, while nerve cells use it for the conduction of nerve impulses. After ATP breaks down, it is rebuilt by the addition of P to ADP. Notice in Figure 2.28 that an input of energy is required to re-form ATP.

Check Your Progress  2.8 1. Identify how information is stored in DNA. 2. Describe how energy is stored in ATP.

Conclusion Most people realize that during and after exercising they need to replace their lost fluids and electrolytes. Each of us will be unique in how much fluid we lose, based upon our individual body chemistry and the nature of our activity. What people are starting to realize is that it isn’t necessary to replace lost fluids with sports drinks if the duration of our activity level is relatively short. While sports drinks are tasty and appear to be a growing fad, people need to pay more attention to the calories that they contain. The excessive consumption of sports drinks, when not necessary, can deter a person’s ability to lose weight, which can lead to health problems. Our best bet to maintain hydration levels is to rely on water, the essential element of life.



40

UNIT 1  Cell Biology

MEDIA STUDY TOOLS http://connect.mheducation.com Maximize your study time with McGraw-Hill SmartBook®, the first adaptive textbook.

 

MP3 Files

2.2  Chemical Bonding 2.3  Water and pH 2.4  Organic Molecules 2.5 Carbohydrates 2.6 Lipids 2.7 Protein 2.8 Nucleic Acids • ATP

 

Animations

  Tutorial

2.2  Ionic versus Covalent Bonding • Electronegativity 2.7  Protein Denaturation • How Prions Arise 2.8  DNA Structure

SUMMARIZE 2.1  Basic Chemistry ■ Both living organisms and nonliving things are composed of matter

consisting of elements.  The most significant elements found in all organisms are carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur. ■ Elements contain atoms, and atoms are composed of subatomic particles. Protons and neutrons in the nucleus determine the atomic mass of an atom. The atomic number indicates the number of protons and the number of electrons in electrically neutral atoms. Protons have positive charges, neutrons are uncharged, and electrons have negative charges. Isotopes are atoms of a single element that differ in their numbers of neutrons. Radioactive isotopes have many uses, including serving as tracers in biological experiments and medical procedures. ■ Electrons occupy energy levels (electron shells) at discrete distances from the nucleus. The number of electrons in the valence shell determines the reactivity of an atom. The first shell is complete when it is occupied by two electrons. Shells beyond the first shell are stable when eight electrons are present. The octet rule states that atoms react with one another in order to have a stable valence shell. Most atoms do not have filled outer shells, and this causes them to react with one another to satisfy the octet rule. In the process, they form compounds and/or molecules.

2.2  Molecules and Compounds ■ Compounds and molecules are formed when elements associate with

each other. 

■ Ions form when atoms lose or gain one or more electrons to achieve a

completed outer orbital. An ionic bond is an attraction between oppositely charged ions. A covalent bond is one or more shared pairs of electrons. ■ In nonpolar covalent bonds the sharing of electrons between atoms is fairly equal. In polar covalent bonds, the sharing of electrons is not equal. One of the atoms exerts greater attraction for the shared electrons than the other, resulting in a difference in electronegativity. This creates a slight charge on each atom. A hydrogen bond is a weak

2.2  Hydrogen Bonds 2.7  Levels of Protein Organization

attraction between a slightly positive hydrogen atom and a slightly negative oxygen or nitrogen atom within the same or a different molecule. Hydrogen bonds help maintain the structure and function of cellular molecules.

2.3  Chemistry of Water ■ Water is a polar molecule. Its polarity allows hydrogen bonding to

occur between water molecules. Water’s polarity and hydrogen bonding account for its unique properties. These properties include: ∙ Water has a high heat capacity. Water can absorb heat (measured in calories) without a great change in temperature. ∙ Water has a high heat of evaporation. A large amount of heat is required to cause liquid water to change to a gas. ∙ Water is a solvent. Water allows the formation of solutions with many different solutes. Molecules that attract water are hydrophilic, whereas molecules that repel water are hydrophobic. ∙ Water is cohesive and adhesive. Cohesion allows water molecules to cling together and accounts for the surface tension of water. Adhesion allows water to cling to surfaces, such as internal transport vessels. ∙ Frozen water (ice) is less dense than liquid water. This allows ice to float on liquid water. ■ A small fraction of water molecules dissociate to produce an equal number of hydrogen ions and hydroxide ions. Solutions with equal numbers of H+ and OH– are termed neutral. In acidic solutions, there are more hydrogen ions than hydroxide ions. These solutions have a pH less than 7. In basic solutions, there are more hydroxide ions than hydrogen ions. These solutions have a pH greater than 7. Cells are sensitive to pH changes. Biological systems often contain buffers that help keep the pH within a normal range.

2.4  Organic Molecules ■ The chemistry of carbon accounts for the chemistry of organic

­molecules. Carbohydrates, lipids, proteins, and nucleic acids are macromolecules with specific functions in cells.  ■ Isomers are organic molecules with the same molecular formula, but different structures.



Chapter 2  The Molecules of Cells

■ Polymers are formed by the joining together of monomers. Long-

chain carbohydrates, proteins, and nucleic acids are all polymers. For each bond formed during a dehydration reaction, a molecule of water is removed. For each bond broken during a hydrolysis reaction, a molecule of water is added.

2.5  Carbohydrates ■ Monosaccharides, disaccharides, and polysaccharides are all carbohy-

drates. Therefore, the term carbohydrate includes both the monomers (e.g., glucose) and the polymers (e.g., starch, glycogen, and cellulose).  ■ Monosaccharides include hexose sugar and pentose sugars. Glucose is a common hexose sugar used for energy.  ■ Dissacharides consist of two monosaccharides.  ■ Polysaccharides may serve as energy-storage molecules, such as starch (in plants)  and glycogen (in animals), or as structural molecules, such as cellulose and chitin. 

2.6  Lipids ■ Lipids include a wide variety of compounds that are insoluble in

water. The majority of lipids are triglycerides, including the fats and oils. These are involved in long-term energy storage and contain one glycerol and three fatty acids. Saturated fatty acids do not have carbon– carbon double bonds, but unsaturated fatty acids do have double bonds in their hydrocarbon chain. The double bond causes a kink in the molecule that accounts for the liquid nature of oils at room temperature. Trans-fats are a form of unsaturated fatty acid.  ■ A phospholipid replaces one of the fatty acids with a phosphate group. In water, phospholipids form a bilayer, because the head of each molecule is hydrophilic and the tails are hydrophobic. Steroids, such as cholesterol, have the same four-ring structure as cholesterol, but each differs by the attached functional groups. Steroids may be used to form hormones, such as testosterone and estrogen.

2.7  Proteins ■ Proteins have numerous functions in cells. Some are enzymes that

speed chemical reactions. The shape of a protein determines its structure. ■ The primary structure of a polypeptide is its own particular sequence of the possible 20 types of amino acids. The secondary structure is often an alpha (α) helix or a beta (β) pleated sheet. Tertiary structure occurs when a polypeptide bends and twists into a three-dimensional shape. A protein can contain several polypeptides, and this accounts for a possible quaternary structure. When proteins lose their shape they are said to be denatured.

2.8  Nucleic Acids ■ Nucleic acids are polymers of nucleotides. Each nucleotide has three

components: a sugar, a base, and phosphate (phosphoric acid). DNA (deoxyribonucleic acid), which contains the sugar deoxyribose, is the genetic material that stores information for its own replication and for the sequence of amino acids in proteins. DNA, with the help of RNA (ribonucleic acid), specifies protein synthesis.  ■ There are four different types of bases in DNA: adenine (A), thymine (T), guanine (G), and cytosine (C). In RNA, the base uracil (U) replaces the base thymine. ■ DNA molecules consist of two strands arranged as a double-helix. RNA is usually single-stranded. ■ ATP (adenosine triphosphate), with its unstable phosphate bonds, is the energy currency of cells. Hydrolysis of ATP to ADP (adenosine diphosphate) + P releases energy that the cell uses to do metabolic work.

41

ASSESS Testing Yourself Choose the best answer for each question.

2.1  Basic Chemistry 1. The atomic number tells you the a. number of neutrons in the nucleus. b. number of protons in the atom. c. atomic mass of the atom. d. number of its electrons if the atom is neutral. e. Both b and d are correct.  2. Isotopes differ in their a. number of protons. c. number of neutrons. b. atomic number. d. number of electrons.  3. The periodic table provides us with what information? a. the atomic number, symbol, and mass b. how many shells an atom has c. how many electrons are in the outer shell d. All of these are correct. 

2.2  Molecules and Compounds 4. An atom that has two electrons in the valence shell, such as magnesium, would most likely a. share to acquire a completed outer shell. b. lose these two electrons and become a negatively charged ion. c. lose these two electrons and become a positively charged ion. d. bind with carbon by way of hydrogen bonds. e. bind with another magnesium atom to satisfy its energy needs.  5. When an atom gains electrons, it a. forms a negatively charged ion. b. forms a positively charged ion. c. forms covalent bonds. d. forms ionic bonds. e. gains atomic mass.  6. An unequal sharing of electrons is a characteristic of a/an a. ionic bond. b. polar covalent bond. c. nonpolar covalent bond. d. All of these are correct.

2.3  Chemistry of Water 7. Hydrogen bonds are formed as a result of which of the following? a. ionic bonds b. nonpolar covalent bonds c. polar covalent bonds d. None of these are correct.  8. Which of these properties of water can be attributed to hydrogen bonding between water molecules? a. Water stabilizes temperature inside and outside the cell. b. Water molecules are cohesive. c. Water is a solvent for many molecules. d. Ice floats on liquid water. e. All of these are correct.  9. The fact that water molecules cling not only to themselves, but also the interior surfaces of vessels, can be attributed to the fact that: a. water is cohesive and adhesive b. water acts as a solvent c. water has a high heat of vaporization d. All of these are correct.



42

UNIT 1  Cell Biology

2.4  Organic Molecules 10. Which of these is not a characteristic of carbon? a. forms four covalent bonds b. bonds with other carbon atoms c. is sometimes ionic d. can form long chains e. sometimes shares two pairs of electrons with another atom  11. Which of the following reactions combines two monomers to produce a polymer? a. dehydration b. hydrolysis c. phosphorylation d. None of these are correct.

2.5  Carbohydrates 12. The monomers of the carbohydrates are the  a. polysaccharides.  b. disaccharides. c. monosaccharides. d. waxes.  13. Which of the following polysaccharides is used as an energy-storage molecule in plants? a. glycogen  c. starch b. chitin  d. cellulose 14. Fructose and galactose are both isomers of a. glycogen.  c. starch. b. glucose.  d. maltose.

20. Which of the following is formed by the linking of two amino acids? a. a peptide bond  b. a functional group  c. quaternary structure d. an ionic bond

2.8  Nucleic Acids 21. Which of the following is incorrect regarding nucleotides? a. They contain a sugar, a nitrogen-containing base, and a phosphate group. b. They are the monomers of fats and polysaccharides. c. They join together by alternating covalent bonds between the sugars and phosphate groups. d. They are present in both DNA and RNA.  22. Which of the following is correct regarding ATP? a. It is an amino acid. b. It has a helical structure. c. It is a high-energy molecule that can break down to ADP and phosphate. d. It is a nucleotide component of DNA and RNA. 

ENGAGE BioNOW Want to know  how this science is relevant to your life? Check out the BioNow videos below: ■ Saltwater Filter

2.6  Lipids

■ Properties of Water

15. A fatty acid is unsaturated if it a. contains hydrogen. b. contains carbon–carbon double bonds. c. contains a carboxyl (acidic) group. d. is bound to a glycerol.  16. Which of the following is incorrect regarding phospholipids? a. The heads are polar. b. The tails are nonpolar. c. They contain a phosphate group in place of one fatty acid. d. They are energy-storage molecules in the cell.  17. A lipid that contains four fused carbon rings is a  a. triglyceride.  c. phospholipid. b. wax.  d. steroid.

Thinking Critically

2.7  Proteins

PHOTO CREDITS

18. The chemical differences between one amino acid and another is due to which of the following? a. amino group  d. peptide bond b. carboxyl group  e. carbon atoms c. R group 19. Which of the following levels of protein structure is determined by interactions of more than one polypeptide chain? a. primary  c. tertiary b. secondary  d. quaternary

1. On a hot summer day, you decide to dive into a swimming pool. Before you begin your dive, you notice that the surface of the water is smooth and continuous. After the dive, you discover that some water droplets are clinging to your skin and that your skin temperature feels cooler. Explain these observations based on the properties of water.  2. Because proteins are composed of the same limited number of amino acids, why are they all so different? 3. Explain why a radioactive isotope of an element reacts chemically the same as a non-radioactive isotope of the same element.

Opener: © Tobias Titz/Getty RF; 2.1: © Tom Mareschal/Alamy RF; 2.4a: © Biomed Commun./Custom Medical Stock Photo; 2.4b(left): © Mazzlota et al./Science Source; 2.4b(right): © National Institutes of Health; 2.5a: © Kim Scott/Ricochet Creative Productions LLC; 2.5b: © Mark Kostich/Getty RF; 2A: © TEPCO/AFP/Getty Images/Newscom; 2.7b(crystals): © Evelyn Jo Johnson; 2.7b(salting food): © PM Images/Getty RF; 2.10: © PNC/Getty RF; 2.11b: © Gonzalo Azumendi/Getty Images; 2.16: © Jeremy Burgess/SPL/ Science Source; 2.17: © Don W. Fawcett/Science Source; 2.18: © Scimat/Science Source; 2.22: © Ingram Publishing RF; 2B: © USDA, ChooseMyPlate.gov website; 2.27a: © Molekuul/SPL/Age Fotostock RF.

CASE STUDY

 ay-Sachs: When Lysosomes Fail T to Function

Tay-Sachs is a recessive neurological disease that is typically caused by the inheritance of a faulty gene from each of your parents. It is more common in individuals who are of eastern and central European Jewish heritage. TaySachs disease is caused by the buildup of harmful quantities of a fatty substance called ganglioside GM2. This substance normally exists in the tissues and nerve cells in the brain. However, in Tay-Sachs patients, GM2 accumulates and the nerve cells begin to become malformed, resulting in a deterioration of mental and physical abilities. This deterioration leads to deafness, blindness, and atrophy of the muscles. Dementia and seizures often develop as well. A child who inherits Tay-Sachs will most likely die by the age of 4 due to severe neurological deterioration and recurring infections. The root cause of Tay-Sachs disease is due to a mutation in the HEXA gene, which provides instructions for the production of an enzyme called beta-hexosaminidase A. This enzyme is produced in the lysosomes and is responsible for breaking down toxic substances and fatty acids that accumulate within the cell. The lysosome also acts as a recycling center within the cell. When the production of beta-hexosaminidase A is interrupted, the lysosome cannot perform its normal duties, leading to Tay-Sachs disease. Because Tay-Sachs disease prevents the normal functioning of the lysosomal enzymes, it is often referred to as a lysosomal storage disorder. Currently, there is no treatment or cure for Tay-Sachs disease in humans. Clinical trials in gene therapy have been successful in curing Tay-Sachs in some lab animals. A variety of other clinical trials are under way with hopes of finding a cure in the near future.

Cell Structure and Function

3

CHAPTER OUTLINE 3.1 The Cellular Level of Organization 3.2 Prokaryotic Cells 3.3 Eukaryotic Cells 3.4 The Cytoskeleton 3.5 Origin and Evolution of the Eukaryotic Cell

BEFORE YOU BEGIN Before beginning this chapter, take a few moments to review the following discussions: Section 1.2  What are the main differences between prokaryotic and eukaryotic cells? Section 2.3  Why is water so important to cells? Section 2.6  What role do phospholipids play in forming the cell membrane?

As you read through the chapter, think about the following questions:

1. What other cellular organelles have a similar function to the lysosome? 2. Why doesn’t the cell “clean up” the faulty lysosomes?

43

44

UNIT 1  Cell Biology

3.1  The Cellular Level of Organization Learning Outcomes Upon completion of this section, you should be able to 1. Explain why cells are the basic unit of life. 2. List the basic principles of the cell theory. 3. Explain the difference between the surface-area-tovolume ratios for large and small cells.

The cell marks the boundary between the nonliving and the living. The molecules that serve as food for a cell and the macromolecules that make up a cell are not alive, and yet the cell is alive. The cell is the structural and functional unit of an organism. It is the smallest structure capable of performing all the functions necessary for life. Thus, the answer to what life is must lie within the cell. The smallest living organisms are single-celled, while larger organisms are multicellular—that is, composed of many cells. There are basically two different types of cells. Prokaryotic cells lack a membrane-bound nucleus. Two of the three domains of life (see section 1.2) contain prokaryotic cells—the bacteria and the archaeans. While the bacteria and archaeans were once thought to be closely related because of their similar size and shape, comparisons of DNA and RNA sequences now show these groups to be biochemically distinct from each other. The second type of cell possesses a membrane-bound nucleus, which houses the genetic material. These are called eukaryotic cells, and they belong to the domain Eukarya, which consists of plants, fungi, animals, and protists. In this chapter we will explore the basic structure of the prokaryotic bacteria and eukaryotic cells. We will visit the archaeans again in Chapter 28.

The Cell Theory Today, we are accustomed to thinking of living organisms as being constructed of cells. But the word “cell” didn’t enter biology until 0.1 nm

1 nm

10 nm

100 nm

protein

amino acids

1 μm

10 μm

100 μm

the seventeenth century. The Dutch scientist Anton van Leeuwenhoek is recognized for making some of the earliest microscopes and observing things that no one had seen before. Robert Hooke, an English scientist, confirmed van Leeuwenhoek’s observations and was the first to use the term “cell.” The tiny chambers he observed in the honeycomb structure of cork reminded him of the rooms, or cells, in a monastery. Over 150 years later—in the 1830s—the German microscopist Matthias Schleiden stated that plants are composed of cells. His counterpart, Theodor Schwann, stated that animals are also made up of living units called cells. This was quite a feat, because aside from their own exhausting examination of tissues, both had to take into consideration the studies of many other microscopists. Rudolf Virchow, another German microscopist, later came to the conclusion that cells come from preexisting cells. Sperm fertilizing an egg and a human developing from the zygote is just one example. These observations became the basis of the cell theory, which states that all organisms are made up of basic living units called cells, and that all cells come only from previously existing cells. Today, the cell theory is a basic theory of biology. Microscopes have changed significantly since van Leeuwenhoek’s time. Advances in the magnification power of the microscope have allowed scientists to study not only the tremendous variety of cell types, but how these cells function internally. The Scientific Inquiry feature, “Modern Microscopy,” explores some of the different types of microscopes that scientists use to study cells.

Cell Size Cells are quite small. A frog’s egg, at about 1 millimeter (mm) in diameter, is large enough to be seen by the human eye. But most cells are far smaller than 1 mm. Some are even as small as 1  micrometer (µm)—one-thousandth of a millimeter. Cell inclusions and macromolecules are smaller than a micrometer and are measured in terms of nanometers (nm). Figure 3.1 outlines the

1 mm

1 cm

chloroplast plant and animal cells

human egg

atom

ant

1m

10 m

100 m 1 km

rose

mouse

frog egg

virus most bacteria

0.1 m

ostrich egg human

electron microscope

blue whale

light microscope human eye

Figure 3.1  The sizes of various objects.  It takes a microscope to see most cells and lower levels of biological organization.

Cells are visible with the light microscope, but not in much detail. An electron microscope is necessary to see organelles in detail and to observe viruses and molecules. In the metric system (see Appendix B), each higher unit is ten times greater than the preceding unit. (1 meter = 102 cm = 103 mm = 106 µm = 109 nm.)

SCIENCE IN YOUR LIFE  ►

SCIENTIFIC INQUIRY

Modern Microscopy Microscopes have given scientists a deeper look into how life works than is possible with the naked eye. Today, there are many types of microscopes. A compound light microscope uses a set of glass lenses to focus light rays passing through a specimen to produce an image that is viewed by the human eye. A transmission electron microscope (TEM) uses a set of electromagnetic lenses to focus electrons passing through a specimen to produce an image, which is projected onto a fluorescent screen or photographic film. A scanning electron microscope (SEM) uses a narrow beam of electrons to scan over the surface of a specimen that is coated with a thin metal layer. Secondary electrons given off by the metal are detected and used to produce a three-dimensional image on a television screen. Figure 3A shows these three types of microscopes and their images.

Magnification, Resolution, and Contrast Magnification is the ratio between the size of an image and its actual size. Electron microscopes magnify to a greater extent than do

compound light microscopes. A light microscope can magnify objects about a thousand times, but an electron microscope can magnify them hundreds of thousands of times. The difference lies in the means of illumination. The path of light rays and electrons moving through space is wavelike, but the wavelength of electrons is much shorter than the wavelength of light. This difference in wavelength accounts for the electron microscope’s greater magnifying capability and its greater ability to distinguish between two points (resolving power). Resolution is the minimum distance between two objects that allows them to be seen as two separate objects. A microscope with poor resolution might enable a student to see only one cellular granule, while the microscope with the better resolution would show two granules next to each other. The greater the resolving power, the greater the detail seen. If oil is placed between the sample and the objective lens of the compound light microscope, the resolving power is increased, and if ultraviolet light is used instead of visible light, it is also increased. But typically, a light

microscope can resolve down to 0.2 μm, while the transmission electron microscope can resolve down to 0.0002 μm. If the resolving power of the average human eye is set at 1.0, then the typical compound light microscope is about 500, and the transmission electron microscope is 500,000. The ability to make out, or resolve, a particular object can depend on contrast, a difference in the shading of an object compared to its background. Higher contrast is often achieved by staining cells with colored dyes (light microscopy) or with electron-dense metals (electron microscopy), which make them easier to see. Optical methods such as phase contrast and the use of fluorescently tagged antibodies can also help us visualize subcellular components such as specific proteins.

Questions to Consider 1. What happens to an image if the magnification is increased without  increasing resolution? 2. Why is it necessary to artificially color electron micrographs?

red blood cells

red blood cell

red blood cell

blood vessel wall blood vessel wall Light micrograph

250×

eye ocular lens light rays

objective lens specimen condenser lens

10,000× Transmission electron micrograph electron source electron beam electromagnetic condenser lens specimen electromagnetic objective lens electromagnetic projector lens

light source

observation screen or photographic plate

a. Compound light microscope

b. Transmission electron microscope

blood vessel wall Scanning electron micrograph electron gun electron beam electromagnetic condenser lenses

scanning coil final (condenser) lens secondary electrons specimen

electron detector TV viewing screen

c. Scanning electron microscope

Figure 3A  Diagram of microscopes with accompanying micrographs.  The compound light microscope and the transmission electron microscope provide an internal view of an organism. The scanning electron microscope provides an external view of an organism.



46

UNIT 1  Cell Biology

One 4-cm cube

Eight 2-cm cubes

Sixty-four 1-cm cubes

Figure 3.2  Surface-area-to-volume relationships.  All three have the same volume, but the group on the right has four times the surface area.

visual range of the eye, light microscope, and electron microscope. The discussion of microscopy in the Scientific Inquiry feature, “Modern Microscopy,” explains why the electron microscope allows us to see so much more detail than the light microscope. The fact that cells are so small is a great advantage for multicellular organisms. Nutrients, such as glucose and oxygen, enter a cell, and wastes, such as carbon dioxide, exit a cell at its surface. Therefore, the amount of surface area affects the ability to get material into and out of the cell. A large cell requires more nutrients and produces more wastes than a small cell. But, as cells get larger in volume, the proportionate amount of surface area actually decreases. For example, for a cube-shaped cell, the volume increases by the cube of the sides (height × width × depth), while the surface area increases by the square of the sides and number of sides (height × width × 6). If a cell doubles in size, its surface area increases only fourfold, while its volume increases eightfold. Therefore, small cells, not large cells, are likely to have a more adequate surface area for exchanging nutrients and wastes. As Figure 3.2 demonstrates, cutting a large cube into smaller cubes provides a lot more surface area per volume. Because of this, smaller cells tend to have higher metabolic rates. Most active cells are small.  Initially, a frog’s egg is not active (and thus fairly large), but once the egg is fertilized and cellular activities begin, the egg divides repeatedly without growth. These cell divisions restore the amount of surface area needed for adequate exchange of materials. Further, cells that specialize in absorption have modifications that greatly increase their surface-area-to-­ volume ratio. For example, the columnar epithelial cells along the surface of the intestinal wall have surface foldings called microvilli (sing., microvillus) that increase their surface area.

Check Your Progress  3.1 1. Identify why humans are made up of trillions of cells instead of just one.

2. Explain why the cell is the basic unit of life. 3. Describe the metabolic challenges of a large cell compared to a cell smaller in size.

3.2  Prokaryotic Cells Learning Outcomes Upon completion of this section, you should be able to 1. Describe the fundamental components of a bacterial cell. 2. Identify the key differences between the archaea and bacteria.

As mentioned in Section 3.1, cells can be categorized by the presence or absence of a nucleus. The subject of this section are the prokaryotic cells, or those cells that lack a membrane-bound nucleus. The domains Archaea and Eubacteria consist of prokaryotic cells. Prokaryotes generally exist as single-celled organisms or as simple strings and clusters.  The term bacteria is often used to refer to prokaryotic cells that are members of the domain Eubacteria. Many people think of germs when they hear the word bacteria, but not all bacteria cause disease. In fact, most bacteria are beneficial and are essential for other living organisms’ survival. 

Plasma Membrane and Cytoplasm All cells are surrounded by a plasma membrane consisting of a phospholipid bilayer embedded with protein molecules. The plasma membrane is a boundary that separates the living contents of the cell from the surrounding environment. Inside the cell is a semifluid medium called the cytoplasm. The cytoplasm is composed of water, salts, and dissolved organic molecules. The plasma membrane regulates the entrance and exit of molecules into and out of the cytoplasm. protein molecules

phospholipid bilayer

Structure of a Bacteria Figure 3.3 illustrates the main features of a bacterial cell’s anatomy. The cell wall, located outside of the plasma membrane, contains peptidoglycan, a complex molecule that is unique to bacteria and composed of chains of disaccharides joined together by peptide chains. The cell wall acts as a form of protection for bacteria. Some classes of antibiotics interfere with the synthesis of the peptidoglycans. In some bacteria, the cell wall is further surrounded by a gelatinous sheath called a capsule. Some bacteria have long, thin appendages called flagella (sing., flagellum), which are composed of subunits of the protein flagellin. The flagella rotate like propellers, allowing the bacterium to move rapidly in a fluid medium. Some bacteria also have fimbriae, which are short appendages that help them attach to an appropriate surface. The capsule and fimbriae often increase the ability of pathogenic bacteria to cause disease. Prokaryotes have a single chromosome (loop of DNA and associated proteins) located within a region of the cytoplasm called the nucleoid. In prokaryotes the nucleoid is not surrounded by a membrane. Many prokaryotes also have small accessory rings of



47

Chapter 3  Cell Structure and Function

Ribosome: site of protein synthesis Fimbriae: hairlike bristles that allow adhesion to surfaces Nucleoid: location of the bacterial chromosome Plasma membrane: sheath around cytoplasm that regulates entrance and exit of molecules Cell wall: covering that supports, shapes, and protects cell

Flagellum: rotating filament present in some bacteria that pushes the cell forward

Capsule: gel-like coating outside cell wall Escherichia coli

32,000×

Figure 3.3   Prokaryotic cell.  Prokaryotic cells lack a nucleus and other membrane-bound organelles, but they possess a nucleoid region where the DNA is located.

DNA called plasmids that are located within the cytoplasm and contain small amounts of genetic information. They can be passed from one cell to another.  Within the cytoplasm are thousands of ribosomes for the synthesis of proteins. The ribosomes of prokaryotic organisms are smaller and structurally different from those of eukaryotic cells, which makes ribosomes a good target for antibacterial drugs. In addition, the photosynthetic cyanobacteria have light-sensitive pigments, usually within a series of internal membranes called thylakoids. Although prokaryotes are structurally simple, they are much more metabolically diverse than eukaryotes. Many of them can synthesize all their structural components from very simple, even inorganic, molecules. Indeed, humans exploit the metabolic capability of bacteria by using them to produce a wide variety of chemicals and products. Prokaryotes also have adapted to living in almost every environment on Earth. In particular, archaeans have been found living under conditions that would not support any other form of life. Archaeal membranes have unique membrane-spanning lipids that help them survive in extremes of heat, pH, and salinity. Table 3.1 compares the

major structures of prokaryotes (archaea and bacteria) with those of eukaryotes.

TABLE 3.1  Comparison of Major Structural Features of Archaea, Bacteria, and Eukaryotes Archaea

Bacteria

Eukaryotes

Cell wall

Usually present, no peptidoglycan

Usually present, with peptidoglycan

Sometimes present, no peptidoglycan

Plasma membrane

Yes

Yes

Yes

Nucleus

No

No

Yes

Membranebound organelles

No

No*

Yes

Ribosomes

Yes

Yes

Yes, larger than prokaryotic

* Some possess internal membranes where chemical reactions may occur.



48

UNIT 1  Cell Biology

Check Your Progress  3.2

TABLE 3.2  Structures of Eukaryotic Cells

1. Explain the function of the plasma membrane. 2. Identify the key bacterial structures and their function. 3. Explain the general differences between a bacteria, an

Structure

archaean, and a eukaryotic cell.

Composition

Cell wall (plants, fungi, and some Contains polysacharrides protists)

Upon completion of this section, you should be able to 1. Recognize the structure and function of the organelles within eukaryotic cells. 2. Identify the cellular structures unique to both plant and animal cells. 3. Distinguish between the roles of the chloroplast and mitochondria in a cell.

Eukaryotic cells are structurally very complex. The principal distinguishing feature of eukaryotic cells is the presence of a nucleus, which separates the genetic material from the cytoplasm of the cell. In addition, eukaryotic cells possess a variety of other organelles, many of which are surrounded by membranes. Animals, plants, fungi, and protists are all composed of eukaryotic cells.

Cell Walls The majority of the eukaryotic cells (with the exception of animals and some protists) have a permeable but protective cell wall that surrounds the plasma membrane. We will take a more indepth look at the plasma membrane in Chapter 4. Many plant cells have both a primary and a secondary cell wall. A main constituent of a primary cell wall is cellulose. Cellulose forms fibrils that lie at right angles to one another for added strength. The secondary cell wall, if present, forms inside the primary cell wall. Such secondary cell walls contain lignin, a substance that makes them even stronger than primary cell walls. The cell walls of some fungi are composed of cellulose and chitin, the same type of polysaccharide found in the exoskeleton of insects. Algae, members of the kingdom Protista, contain cell walls composed of cellulose.

Organelles of Eukaryotic Cells Originally the term organelle referred to only membrane-bound structures within a cell. However, it has come to mean any welldefined subcellular structure that performs a particular function (Table 3.2).  We can think of a eukaryotic cell as being a cellular factory. Just as all the assembly lines of a factory operate at the same time, so do all the organelles of a cell. Raw materials enter a factory where different departments turn them into various products. In the same way, the cell takes in chemicals, and then the organelles process them. The factory must also be supplied with energy, and get rid of waste, and the cell performs that function as well.

Support and protection

Plasma membrane

Phospholipid bilayer with embedded proteins

Defines cell boundary; regulates molecule passage into and out of cells

Nucleus

Nuclear envelope, nucleoplasm, chromatin, and nucleoli

Storage of genetic information; synthesis of DNA and RNA

Nucleoli

Concentrated area of chromatin, RNA, and proteins

Ribosomal subunit formation

Ribosomes

Protein and RNA in two subunits

Protein synthesis

Rough endoplasmic reticulum (RER)

Network of folded membranes studded with ribosomes

Folding, modification, and transport of proteins for export or associated with membranes

Smooth endoplasmic reticulum (SER)

Network of folded membranes having no ribosomes

Lipid and carbohydrate synthesis in some cells; detoxification of chemicals

Golgi apparatus

Stack of small membranous sacs

Processing, packaging, and distribution of proteins and lipids

Lysosomes (animal cells only)

Membranous vesicle containing digestive enzymes

Intracellular digestion; recycling of cellular components

Vacuoles and vesicles

Membranous sacs of various sizes

Storage of substances

Peroxisomes

Membranous vesicle containing specific enzymes

Breakdown of fatty acids and other metabolic tasks

Mitochondria

Inner membrane (cristae) bounded by an outer membrane

Cellular respiration

Chloroplasts (plant cells and some protists)

Membranous grana bounded by two membranes

Photosynthesis

Cytoskeleton

Microtubules, intermediate filaments, actin filaments

Shape of cell and movement of its parts

3.3  Eukaryotic Cells Learning Outcomes

Function

Cilia and flagella 9 + 2 pattern of microtubules (cilia are rare in plant cells)

Movement of cell

Centriole (animal 9 + 0 pattern of cells only) microtubules

Formation of basal bodies

In this chapter, we will focus primarily on animal and plant cells (Figs. 3.4 and 3.5). While there are many similarities in these cell types, there are some important differences. For example, both contain mitochondria, but chloroplasts are primarily found in the cells of plants, while centrioles are found in the cells



Chapter 3  Cell Structure and Function

49

of animals. In the illustrations throughout this text, note that each of the organelles has an assigned color.

The Nucleus The nucleus, which has a diameter of about 5 µm, is a prominent structure in the eukaryotic cell. In photographs or slides it often looks like a dark golf ball within the cell. The nucleus is of primary importance because it stores the genetic material, DNA. In the factory analogy, the nucleus is the head office. DNA governs the characteristics of the cell and its metabolic functioning. Every cell in an individual contains the same DNA, but which genes are turned on and which are turned off will differ among cells. When you look at the nucleus, even in an electron micrograph, you cannot see a DNA molecule

nuclear envelope endoplasmic reticulum nucleolus chromatin

10,000×

Plasma membrane protein phospholipid

Nucleus: nuclear envelope chromatin nucleolus Cytoskeleton: microtubules actin filaments intermediate filaments

Endoplasmic Reticulum: rough ER smooth ER

vesicle centrioles**

ribosomes

centrosome

cytoplasm

peroxisome mitochondrion

lysosome* Golgi apparatus polyribosome

* not commonly found in plant cells ** not found in plant cells

Figure 3.4  Animal cell anatomy.  Micrograph of an insect cell and drawing of a generalized animal cell. See Table 3.2 for a description of these structures, along with a listing of their functions.



50

UNIT 1  Cell Biology

(Fig. 3.6). Instead, what you see is chromatin, which consists of DNA and associated proteins. Chromatin in most eukaryotic cells is not one continuous strand. During most of the cell’s lifetime, chromatin is present, but when the cell is ready to undergo cell division, it will undergo coiling and become highly condensed

structures called chromosomes. Human cells contain 46 chromosomes, which are immersed in a semifluid medium called the nucleoplasm. A difference in pH between the nucleoplasm and the  cytoplasm suggests that the nucleoplasm has a different composition. When you look at an electron micrograph of a nucleus, you may see one or more regions that look darker than the rest of the chromatin. These are nucleoli (sing., nucleolus), where a type of RNA, called ribosomal RNA mitochondrion (rRNA), is produced. It is here that the rRNA joins with proteins to form the subunits of ribosomes nucleus (described below). The nucleus is separated from the cytoplasm peroxisome by a double membrane known as the nuclear ribosomes envelope, which is continuous with the endoplasmic reticulum (discussed later in this section). The nuclear envelope has nuclear pores of sufficient central vacuole plasma membrane cell wall chloroplast

Figure 3.5  Plant cell anatomy.  Micrograph of

a plant cell and drawing of a generalized plant cell. See Table 3.2 for a description of these structures, along with a listing of their functions.

12,300× Nucleus: command center of cell • Nuclear envelope: double membrane with nuclear pores that encloses nucleus

Central vacuole*: large, fluid-filled sac that stores metabolites and helps maintain turgor pressure

• Nucleolus: produces subunits of ribosomes

Cell wall of adjacent cell

• Chromatin: diffuse threads containing DNA and protein • Nuclear pore: permits passage of proteins into nucleus and ribosomal subunits out of nucleus Ribosomes: carry out protein synthesis

Chloroplast*: carries out photosynthesis, producing sugars

Centrosome: microtubule organizing center (lacks centrioles) Endoplasmic reticulum: protein and lipid metabolism • Rough ER: studded with ribosomes that synthesize proteins

Mitochondrion: organelle that carries out cellular respiration, producing ATP molecules

• Smooth ER: lacks ribosomes, synthesizes lipid molecules Peroxisome: vesicle that is involved in fatty acid metabolism Golgi apparatus: processes, packages, and secretes modified proteins Cytoplasm: semifluid matrix outside nucleus that contains organelles

Microtubules: protein cylinders that aid movement of organelles Actin filaments: protein fibers that play a role in cell division and shape Plasma membrane: surrounds cytoplasm, and regulates entrance and exit of molecules Cell wall*: outer surface that shapes, supports, and protects cell *not found in animal cells



Chapter 3  Cell Structure and Function

51

nuclear envelope nucleolus

Nuclear envelope: inner membrane outer membrane nuclear pore

nuclear pore chromatin nucleoplasm

phospholipid

Figure 3.6  Anatomy of the nucleus.  The nucleus contains chromatin. The nucleolus is where rRNA is produced and ribosomal subunits are

assembled. The nuclear envelope contains pores, as shown in the larger micrograph of a freeze-fractured nuclear envelope. Nuclear pores serve as passageways for substances to pass into and out of the nucleus.

size (100 nm) to permit the bidirectional transport of proteins and ribosomal subunits. Interestingly, during cell division, the nuclear envelope completely disappears and the contents of the nucleus are mixed with the cytoplasm (see Chapter 5). Following cell division, the nuclear envelope re-forms around the chromosomes, and the other contents of the nucleus are transported into the nucleus. Regulation of transport through the nuclear pores can control events within the nucleus or the entire cell.

Ribosomes Ribosomes are responsible for the synthesis of proteins using messenger RNA (mRNA) as a template. This process is discussed in more detail in Chapter 25. Ribosomes are composed of two subunits, called “large” and “small” because of their relative sizes. Each subunit is a complex of unique ribosomal RNA (rRNA) and

protein molecules. Ribosomes can be found individually in the cytoplasm, as well as in groups called polyribosomes (several ribosomes associated simultaneously with a single mRNA molecule). Ribosomes can also be found attached to the endoplasmic reticulum, a membranous system of sacs and channels discussed in section 3.4. Proteins synthesized at ribosomes attached to the endoplasmic reticulum have a different destination from that of proteins synthesized at ribosomes free in the cytoplasm.

The Endomembrane System The endomembrane system consists of the nuclear envelope, the endoplasmic reticulum, the Golgi apparatus, and several vesicles (tiny membranous sacs). Continuing the factory analogy, the endomembrane system is essentially the transportation and product-processing section of the cell. This system compartmentalizes the cell so that particular enzymatic reactions are



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UNIT 1  Cell Biology

ribosomes

nuclear envelope rough endoplasmic reticulum

smooth endoplasmic reticulum

52,500×

Figure 3.7  Endoplasmic reticulum (ER).  Ribosomes are present on rough ER, which consists of flattened sacs, but not on smooth ER, which is more tubular. Proteins are synthesized and modified by rough ER, whereas smooth ER is involved in lipid synthesis, detoxification reactions, and several other possible functions. restricted to specific regions. Organelles that make up the endomembrane system are connected either directly or by transport vesicles.

specialized function, smooth ER also forms vesicles in which products are transported to the Golgi apparatus.

The Endoplasmic Reticulum

The Golgi apparatus is named for Camillo Golgi, who discovered its presence in cells in 1898. The Golgi apparatus consists of a stack of three to twenty slightly curved sacs whose appearance can be compared to a stack of pancakes (Fig. 3.8). The Golgi apparatus is referred to as the shipping center of the cell because it collects, sorts, packages, and distributes materials such as proteins and lipids. In animal cells, one side of the stack (the inner face) is directed toward the ER, and the other side of the stack (the outer face) is directed toward the plasma membrane. Vesicles can frequently be seen at the edges of the sacs. The Golgi apparatus receives proteins and also lipid-filled vesicles that bud from the ER. These molecules then move through the Golgi from the inner face to the outer face. During their passage through the Golgi apparatus, proteins and lipids can be modified before they are repackaged in secretory vesicles. Secretory vesicles proceed to the plasma membrane, where they discharge their contents. This action is termed secretion or exocytosis. The Golgi apparatus is also involved in the formation of lysosomes, vesicles that contain enzymes that remain within the cell. It appears that proteins made at the rough ER have specific molecular tags that serve as “zip codes” to tell the Golgi apparatus whether they belong inside the cell in some membrane-bound organelle or in a secretory vesicle.

The endoplasmic reticulum (ER) is an interconnected system of membranous channels and sacs (flattened vesicles) that is physically continuous with the outer membrane of the nuclear envelope.  Rough endoplasmic reticulum (RER) is studded with ribosomes on the side of the membrane that faces the cytoplasm (Fig. 3.7). As proteins are synthesized on these ribosomes they pass into the interior of the ER, where processing and modification begin. Proteins synthesized here are destined for the membrane of the cell or to be secreted from the cell. Proper folding, processing, and transport of proteins are critical to the functioning of the cell. For example, in cystic fibrosis a mutated plasma membrane channel protein is retained in the endoplasmic reticulum because it is folded incorrectly. Without this protein in its correct location, the cell is unable to regulate the transport of the chloride ion, resulting in an inability to regulate water levels in some cells. Smooth endoplasmic reticulum (SER) is continuous with rough ER but does not have attached ribosomes. Smooth ER synthesizes the phospholipids found in cell membranes as well as those that perform various other functions. In the testes, it produces testosterone, and in the liver, it helps detoxify drugs. In muscle cells, the smooth ER stores calcium ions. Regardless of any

The Golgi Apparatus



Chapter 3  Cell Structure and Function

53

secretion plasma membrane

Incoming vesicle brings substances into the cell that are digested when the vesicle fuses with a lysosome

Secretory vesicle fuses with the plasma membrane as secretion occurs

enzyme

Golgi apparatus modifies lipids and proteins from the ER; sorts them and packages them in vesicles

Lysosome contains digestive enzymes that break down worn-out cell parts or substances entering the cell at the plasma membrane

protein Transport vesicle shuttles proteins to various locations such as the Golgi apparatus

Transport vesicle shuttles lipids to various locations such as the Golgi apparatus lipid

Rough endoplasmic reticulum folds and processes proteins and packages them in vesicles; vesicles commonly go to the Golgi apparatus

Smooth endoplasmic reticulum synthesizes lipids and also performs various other functions ribosome

Nucleus

Figure 3.8  Endomembrane system.  The organelles in the endomembrane system work together to carry out the functions noted.

Lysosomes Lysosomes are membrane-bound vesicles produced by the Golgi apparatus. Lysosomes contain hydrolytic digestive enzymes. Lysosomes are the garbage disposals of the factory. Sometimes macromolecules are brought into a cell by vesicle formation at the plasma membrane (Fig. 3.8). When a lysosome fuses with such a vesicle, its contents are digested by lysosomal enzymes into simpler subunits that then enter the cytoplasm. Some white blood cells defend the body by engulfing pathogens via vesicle formation. When lysosomes fuse with these vesicles, the bacteria are digested. Even parts of a cell are digested by its own lysosomes (called autodigestion). For example, the finger webbing

found in the human embryo is later dissolved by lysosomes so that the fingers are separated. Lysosomes contain many enzymes for digesting all sorts of molecules. Occasionally, a child inherits the inability to make a lysosomal enzyme, and therefore has a lysosomal storage disease. For example, in Tay-Sachs disease (see chapter opener), the cells that surround nerve cells cannot break down the lipid ganglioside GM2, which then accumulates inside lysosomes and affects the nervous system. At about six months, the infant can no longer see and, then, gradually loses hearing and even the ability to move. Death follows at about three years of age.



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UNIT 1  Cell Biology

cholesterol. In a disease called adrenoleukodystrophy (ALD), cells lack a carrier protein to transport an enzyme into peroxisomes. As a result, long-chain fatty acids accumulate in the brain, resulting in neurological damage. Plant cells also have peroxisomes. In germinating seeds, they oxidize fatty acids into molecules that can be converted to sugars needed by the growing plant. In leaves, peroxisomes can carry out a reaction that is opposite to photosynthesis—the reaction uses up oxygen and releases carbon dioxide.

peroxisome

Energy-Related Organelles

34,000×

Figure 3.9  Peroxisomes.  Peroxisomes contain one or more enzymes that can oxidize various organic substances. Peroxisomes also contain the enzyme catalase, which breaks down the hydrogen peroxide (H2O2) that builds up after organic substances are oxidized.

Vacuoles A vacuole is a large membranous sac. A vacuole is larger than a vesicle. Although animal cells have vacuoles, they are much more prominent in plant cells. Typically, plant cells have a large central vacuole filled with a watery fluid that provides added support to the cell (see Fig. 3.5). Vacuoles store substances. Plant vacuoles contain not only water, sugars, and salts, but also pigments and toxic molecules. The pigments are responsible for many of the red, blue, or purple colors of flowers and some leaves. The toxic substances help protect a plant from herbivorous animals. The vacuoles present in single-celled protozoans are quite specialized. They include contractile vacuoles for ridding the cell of excess water and digestive vacuoles for breaking down nutrients.

Peroxisomes Peroxisomes, similar to lysosomes, are membrane-bound vesicles that enclose enzymes (Fig. 3.9). These enzymes break down fatty acids, and in the process produce hydrogen peroxide (H2O2), a toxic molecule. However, peroxisomes also contain an enzyme called catalase which breaks down the H2O2 to water.  The enzymes present in a peroxisome depend on the function of the cell. Peroxisomes are especially prevalent in cells that are synthesizing and breaking down fats. In the liver, some peroxisomes break down fats and others produce bile salts from

Life is possible only because of a constant input of energy. Organisms use this energy for maintenance and growth. Chloroplasts and mitochondria are the two eukaryotic membranous organelles that specialize in converting energy to a form the cell can use. Chloroplasts use solar energy to synthesize carbohydrates, which are broken down by the mitochondria (sing., mitochondrion) to produce ATP molecules, as shown in Figure 3.10. This diagram shows that chemicals recycle between chloroplasts and mitochondria in the presence of solar energy. When cells use ATP as an energy source, energy dissipates as heat. Most life cannot exist without a constant input of solar energy. Plants, algae, and cyanobacteria are all capable of carrying on photosynthesis in this manner: solar energy + carbon dioxide + water

carbohydrate + oxygen

Plants and algae have chloroplasts, while cyanobacteria carry on photosynthesis within independent thylakoids. Solar energy is the ultimate source of energy for most cells because nearly all organisms,

solar energy

carbohydrate (high chemical energy)

chloroplast

mitochondrion

ATP CO2 + H2O (low chemical energy)

usable energy for cells

Figure 3.10  Energy-producing organelles in eukaryotic cells. 

Chloroplasts use sunlight to produce carbohydrates, which in turn are used by mitochondria. The mitochondria then produce carbon dioxide and water, which in turn is used by the chloroplasts.



Chapter 3  Cell Structure and Function

either directly or indirectly, use the carbohydrates produced by photosynthesizers as an energy source. We will take a closer look at photosynthesis in Chapter 8. Many organisms carry on cellular respiration, the process by which the chemical energy of carbohydrates is converted to that of ATP (adenosine triphosphate), the common energy carrier in cells. Eukaryotic organisms, complete the process of cellular respiration in mitochondria. Cellular respiration can be represented by this equation: carbohydrate + oxygen

carbon dioxide + water + energy

Here, energy is in the form of ATP molecules. When a cell needs energy, ATP supplies it. The energy of ATP is used for all energy-requiring processes in cells. Chapter 7 explores how cellular respiration occurs within cells.

The chloroplast is a tiny solar panel for collecting sunlight and a factory for making carbohydrates.

Mitochondria The majority of eukaryotic cells, with the exception of a few protists, contain mitochondria. Photosynthetic eukaryotes, such as plants and algae, contain  both chloroplasts and mitochondria. Mitochondria are usually 0.5–1.0 µm in diameter and 2–5 µm in length. Mitochondria are the power plants of the cell. Substrates broken down in the cytoplasm are transported into the mitochondria and converted into ATP to be used by the cell for its various needs. Mitochondria are also involved in cellular differentiation and cell death. It is now recognized that mitochondria play a role in the aging process.

Figure 3.11  Chloroplast structure. 

Chloroplasts Plant and algal cells contain chloroplasts (Fig. 3.11), the organelles that allow them to use solar energy to produce organic molecules. Chloroplasts are about 4–6 µm in diameter and 1–5 µm in length. They belong to a group of organelles known as plastids. Among the plastids are also the amyloplasts, common in roots, which store starch, and the chromoplasts, common in leaves, which contain red and orange pigments. A chloroplast is green because it contains the green pigment ­chlorophyll. A typical plant cell in a leaf may contain approximately 50 chloroplasts. Chloroplasts divide by splitting in two in a manner similar to how bacteria divide. A chloroplast is bounded by two membranes that enclose a fluid-filled space called the stroma (Fig. 3.11). The stroma contains a single circular DNA molecule as well as ribosomes. The chloroplast contains its own genetic material and makes most of its own proteins. The others are encoded by nuclear a. genes and imported from the cytoplasm. A membrane system within the stroma is organized into interconnected flattened sacs called thylakoids. In certain regions, the thylakoids are stacked up in double membrane structures called grana (sing., granum). There can be hundreds of grana within a single chloroplast (Fig. 3.11).  Chlorophyll, which is located within the thylakoid membranes of grana, captures the solar energy needed to enable chloroplasts to produce carbohydrates. The solar energy excites an electron within the chlorophyll molecule. The energy from that excited electron is used to make highenergy compounds. The stroma contains the enzymes that synthesize carbohydrates from carbon dioxide and water using these high-energy compounds.

55

Chloroplasts carry out photosynthesis. a. Electron micrograph of a longitudinal section of a chloroplast. b. Generalized drawing of a chloroplast in which the outer and inner membranes have been cut away to reveal the grana, each of which is a stack of membranous sacs called thylakoids. In some grana, but not all, it is obvious that thylakoid spaces are interconnected.

23,000× outer membrane

grana

thylakoid space stroma

thylakoid

inner membrane

b.



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UNIT 1  Cell Biology

demand. Mutations in the mitochondrial DNA usually affect high-energy-demand tissues such as the eye, central nervous system, and muscles. In humans, the mother’s egg supplies the mitochondria. The father’s sperm typically does not contribute any mitochondria to the offspring. This has made mitochondrial DNA studies useful for population genetic studies. An interesting example of this is the search for “mitochondrial Eve,” which examines different models on how early humans migrated from Africa (see section 32.5).

Figure 3.12  Mitochondrion structure.  Mitochondria are

involved in cellular respiration. a. Electron micrograph of a longitudinal section of a mitochondrion. b. Generalized drawing in which the outer membrane and portions of the inner membrane have been cut away to reveal the cristae.

Check Your Progress  3.3 1. Explain the function of the cell wall in eukaryotes. 2. Describe how the endomembrane system acts as a transport system.

3. Explain why plant cells need both chloroplasts and mitochondria.

3.4  The Cytoskeleton Learning Outcomes 85,000×

a. double membrane

outer membrane

cristae

matrix

inner membrane

b.

Mitochondria, like chloroplasts, divide by splitting in two. Mitochondria are also bounded by a double membrane (Fig. 3.12). In mitochondria, the inner fluid-filled space is called the matrix. As with chloroplasts, mitochondria contain their own circular DNA chromosome and encode some, but not all, of their own proteins. The matrix contains ribosomes, and enzymes that break down carbohydrate products, releasing energy to be used for ATP production. The inner membrane of a mitochondrion invaginates to form cristae (sing., crista). Cristae provide a much greater surface area to accommodate the protein complexes and other participants that produce ATP. The number of mitochondria per cell can vary considerably. Some cells have only one mitochondrion, while other cells may have thousands. Tissues that need large amounts of energy have more mitochondria per cell than tissues with lower energy

Upon completion of this section, you should be able to 1. Compare and contrast the structural differences between actin filaments, intermediate filaments, and microtubules. 2. Identify the cellular structures that are composed of the different cytoskeleton components.

The protein components of the cytoskeleton interconnect and extend from the nucleus to the plasma membrane in eukaryotic cells. Prior to the 1970s, scientists believed that the cytoplasm was an unorganized mixture of organic molecules. Then, high-voltage electron microscopes, which can penetrate thicker specimens, showed that the cytoplasm is highly organized. The technique of immunofluorescence microscopy identified the makeup of the protein components within the cytoskeletal network (Fig. 3.13). The cytoskeleton contains actin filaments, intermediate filaments, and microtubules, which maintain cell shape and allow the cell and its organelles to move. Therefore, the cytoskeleton is often compared with the bones and muscles of an animal. However, the cytoskeleton is dynamic, especially because its protein components can assemble and disassemble as needed.

Actin Filaments Actin filaments (formerly called microfilaments) are long, extremely thin, flexible fibers (about 7 nm in diameter) that occur in bundles or meshlike networks. Each actin filament contains two chains of globular actin monomers twisted about one another in a helical manner. Actin filaments play a structural role when they form a dense, complex web just under the plasma membrane, to which they are anchored by special proteins. They are also seen in the microvilli that project from intestinal cells, and their presence accounts for the formation of pseudopods (false feet), extensions that allow certain cells to move in an amoeboid fashion.



57

Chapter 3  Cell Structure and Function

actin subunit

Chara a. Actin filaments

850×

fibrous subunits

human b. Intermediate filaments

1,000×

tubulin dimer

chameleon c. Microtubules

Figure 3.13  The cytoskeleton.  The cytoskeleton maintains the shape of the cell and allows its parts to move. Three types of protein components make up

the cytoskeleton. a. Left to right: Fibroblasts in animal tissue contain actin filaments. The drawing shows that actin filaments are composed of a twisted double chain of actin subunits. The giant cells of the green alga Chara rely on actin filaments to move organelles from one end of the cell to another. b. Left to right: Fibroblasts in an animal tissue contain intermediate filaments. The drawing shows that fibrous proteins account for the ropelike structure of intermediate filaments. Human hair is strengthened by the presence of intermediate filaments. c. Left to right: Fibroblasts in an animal tissue contain microtubules. The drawing shows that microtubules are hollow tubes composed of tubulin subunits. The skin cells of a chameleon rely on microtubules to move pigment granules around so that they can take on the color of their environment.

To produce movement, actin filaments interact with motor molecules, which are proteins that can attach, detach, and reattach farther along the actin filament. For example, in muscle cells the motor molecule myosin pulls actin filaments along in this way using the energy of ATP. Myosin has both a head and a tail. The tails of several muscle myosin molecules are joined to form a thick filament, while the heads interact with ATP and the actin filament.

actin filament myosin molecules

ADP + P

ATP tail

head

membrane

During animal cell division, the two new cells form when actin, in conjunction with myosin, pinches off the cells from one another.



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UNIT 1  Cell Biology

Intermediate Filaments Intermediate filaments (8–11 nm in diameter) are intermediate in size between actin filaments and microtubules that will perform a structural role in the cell. They are a ropelike assembly of fibrous polypeptides that vary according to the type of tissue. Some intermediate filaments support the nuclear envelope, whereas others support the plasma membrane and take part in the formation of cell-to-cell junctions. In skin cells, intermediate filaments, made of the protein keratin, provide mechanical strength. They, too, are dynamic structures.

Microtubules Microtubules are small, hollow cylinders about 25 nm in diameter and 0.2–25 µm in length. Microtubules are made of the globular protein tubulin, which is of two types, called α and β. Microtubules have 13 rows of tubulin dimers, surrounding what is an empty central core. The regulation of microtubule assembly is controlled by a microtubule organizing center. In most eukaryotic cells, the main microtubule organizing center is in the centrosome, which lies near the nucleus. Microtubules radiate from the centrosome, helping to maintain the shape of the cell and acting as tracks along which organelles can move. Whereas the motor molecule myosin is associated with actin filaments, the motor molecules kinesin and dynein are associated with microtubules. Before a cell divides, microtubules disassemble and then reassemble into a structure called a spindle. The spindle apparatus attaches to the chromosomes and ensures that they are distributed in an orderly manner. It also participates in dividing the cell in half. At the end of cell division, the spindle disassembles, and microtubules reassemble once again into their former array.

Centrioles  Both plant and animal cells contain centrosomes, the major microtubule organizing center for the cell. But in animal cells, a centrosome contains two centrioles lying at right angles to each other. The centrioles may be involved in the process of microtubule assembly and disassembly. Centrioles are short ­cylinders of microtubules with a 9 + 0 pattern of microtubule triplets—that is, a ring having nine sets of microtubule triplets, with none in the middle (Fig. 3.14). Before an animal cell divides, the centrioles replicate such that the members of each pair are again at right angles to one another (Fig. 3.14). Then, each pair becomes part of a separate centrosome. During cell division, the centrosomes move apart and may function to organize the mitotic spindle (see Chapter 5). Cilia and Flagella  Cilia (sing., cilium) and flagella (sing., flagellum) are hairlike projections that can move either in an undulating fashion, like a whip, or stiffly, like an oar. Some cells that have these organelles are capable of movement. For example, single-celled organisms called paramecia move by means of cilia, whereas sperm cells move by means of flagella. In the human body, the cells that line our upper respiratory tract have cilia that sweep debris trapped within mucus back up into the throat, where it can be swallowed or ejected. This action helps keep the lungs clean.

one microtubule triplet

Figure 3.14  Centrioles.  In a nondividing animal cell, a single pair of centrioles lies in the centrosome located just outside the nucleus. Just before a cell divides, the centrioles replicate, producing two pairs of centrioles. During cell division, centrioles in their respective centrosomes separate so that each new cell has one centrosome containing one pair of centrioles. In eukaryotic cells, cilia are much shorter than flagella, but they have a similar construction. Both are membrane-bound ­cylinders. The cylinders are composed of nine microtubule ­doublets arranged in a circle around two central microtubules. Therefore, they have a 9 + 2 pattern of microtubules. Cilia and flagella move when the microtubule doublets slide past one another (Fig. 3.15).

Check Your Progress  3.4 1. Identify the structural makeup of actin filaments, intermediate filaments, and microtubules.

2. Contrast the structure of cilia and flagella to that of centrioles.

3. Explain how cilia and flagella move.

3.5  Origin and Evolution of the Eukaryotic Cell Learning Outcomes Upon completion of this section, you should be able to 1. Define endosymbiosis. 2. Describe how the endosymbiotic theory explains eukaryotic cell evolution.

The fossil record, which is based on the remains of ancient life, suggests that the first cells were prokaryotes. Therefore, scientists believe that eukaryotic cells evolved from prokaryotic cells. Biochemical data suggest that eukaryotes are more closely related to the archaea than the bacteria. The eukaryotic cell probably evolved from a prokaryotic cell in stages. Invagination of the plasma membrane might explain the origin of the nuclear envelope and such



Chapter 3  Cell Structure and Function

59

outer microtubule doublet

flagellum

central microtubules

The shaft of the flagellum has a ring of nine microtubule doublets anchored to a central pair of microtubules.

dynein side arm

Flagellum cross section

Sperm

275X

20,000X

The side arms of each doublet are composed of dynein, a motor molecule.

Flagellum

shaft

dynein side arms

ATP

plasma membrane

In the presence of ATP, the dynein side arms reach out to their neighbors, and bending occurs.

Cilia

Figure 3.15  Structure of a flagellum or cilium.  The shaft of a flagellum (or cilium) contains microtubule doublets whose side arms are motor

molecules that cause the projection to move. Sperm have flagella. Without the ability of sperm to move to the egg, human reproduction would not be possible. Cilia cover the surface of the cells of the respiratory system where they beat upward to remove foreign matter.

organelles as the endoplasmic reticulum and the Golgi apparatus. Some believe that the other organelles could also have arisen in a similar fashion. There is evidence that a similar process was involved in the origin of the energy organelles. Observations in the laboratory indicate that an amoeba infected with bacteria can become dependent upon them. Some investigators believe mitochondria and chloroplasts are derived from prokaryotes that were taken up by larger cells (Fig. 3.16). Perhaps mitochondria were originally aerobic heterotrophic bacteria, and chloroplasts were originally cyanobacteria. The eukaryotic host cell would have benefited from an ability to utilize oxygen or synthesize organic food when, by chance, the prokaryote was taken up and not destroyed. After the prokaryote entered the host cell, the two would have begun living together cooperatively. This proposal is known as  the endosymbiotic theory (endo-, in; symbiosis,

living together). Some of the evidence supporting this hypothesis is as follows: 1. Mitochondria and chloroplasts are similar to bacteria in size and in structure. 2. Both organelles are bounded by a double membrane—the outer membrane may be derived from the engulfing vesicle, and the inner one may be derived from the plasma membrane of the original prokaryote. 3. Mitochondria and chloroplasts contain a limited amount of genetic material and divide by splitting. Their DNA (deoxyribonucleic acid) is a circular loop like that of prokaryotes. 4. Although most of the proteins within mitochondria and chloroplasts are now produced by the eukaryotic host, they do have their own ribosomes and they do produce some proteins. Their ribosomes resemble those of prokaryotes.



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Figure 3.16  Origin of organelles.  Invagination of the plasma membrane could have created the nuclear envelope and an endomembrane system. The endosymbiotic theory suggests that mitochondria and chloroplasts were once independent prokaryotes that took up residence in a eukaryotic cell.

Original prokaryotic cell DNA

1. Cell gains a nucleus by the plasma membrane invaginating and surrounding the DNA with a double membrane. Nucleus allows specific functions to be assigned, freeing up cellular resources for other work. 2. Cell gains an endomembrane system by proliferation of membrane. Increased surface area allows higher rate of transport of materials within a cell.

5. The RNA (ribonucleic acid) base sequence of the ribosomes in chloroplasts and mitochondria also suggests a prokaryotic origin of these organelles. It is also possible that the flagella of eukaryotes are derived from an elongated bacterium with a flagellum that became attached to a host cell. However, the flagella of eukaryotes are constructed differently from those of modern bacteria.

Check Your Progress  3.5 1. Recognize the evidence that suggests chloroplasts and

mitochondria were once independently living prokaryotes.

2. Identify the evolutionary steps required for an animal or plant cell to contain mitochondria and chloroplasts.

3. Cell gains mitochondria. Ability to metabolize sugars in the presence of oxygen enables greater function and success.

aerobic bacterium mitochondrion

4. Cell gains chloroplasts. Ability to produce sugars from sunlight enables greater function and success.

Animal cell has mitochondria, but not chloroplasts.

photosynthetic bacterium

chloroplast

Plant cell has both mitochondria and chloroplasts.

Conclusion Tay-Sachs disease is a recessive neurological disorder that is more common in individuals of eastern and central European Jewish heritage than the rest of the general population. A child who inherits Tay-Sachs will most likely die by the age of 4 due to neurological breakdown and recurring infections as the result of faulty lysosomes. The lysosomes do not produce the enzyme necessary to digest the fatty substance ganglioside GM2. Instead, GM2 accumulates within the lysosome, causing it to distend—which in turn impacts the function of neurological cells in the brain. Deafness, blindness, and seizures are some of the physical complications associated with the disease. Unfortunately, there is no cure or treatment available for individuals affected by Tay-Sachs disease. Clinical trials are under way with hopes of a cure in the near future.

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3.2  Cell Wall Antibiotics 3.3 Lysosomes 3.5 Endosymbiosis

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Chapter 3  Cell Structure and Function

61

SUMMARIZE

■ Peroxisomes contain enzymes that oxidize molecules by producing

3.1  The Cellular Level of Organization

■ Cells require a constant input of energy to maintain their structure.

■ The cell theory states that all organisms are composed of cells, the

smallest units of living matter and that new cells come only from preexisting cells. ■ Cells must remain small in order to have an adequate ratio of surfacearea-to-volume for exchange of molecules with the environment.

3.2  Prokaryotic Cells ■ All cells have a plasma membrane consisting of a phospholipid

■

■

■

■

bilayer with embedded proteins. The membrane regulates the movement of molecules into and out of the cell. The inside of the cell is filled with a fluid called cytoplasm.  Prokaryotic cells do not have a nucleus, but they do have a nucleoid that is not bounded by a nuclear envelope. They also lack most of the other organelles that compartmentalize eukaryotic cells, although some have internal membranes, such as the thylakoids of photosynthetic prokaryotes.  Some prokaryotic cells possess a gel-like sheath called a capsule and most possess a cell wall for protection. Prokaryotic cells may possess fimbriae that attach them to surfaces. The DNA of a prokaryote is found primarily in a single, circular ­chromosome. Plasmids in the cytoplasm may contain small amounts of DNA. Some prokaryotic cells move by the use of flagella (sing., flagellum).

3.3  Eukaryotic Cells ■ Eukaryotic cells are very complex. Animals, plants, fungi, and pro-

■ ■ ■

■

■

■

■

■

tists are all examples of eukaryotes. The common feature is the presence of a nuclear envelope that contains the genetic material. Some eukaryotic cells contain cell walls for protection. Eukaryotic cells contain organelles, which are subcellular structures that perform a particular function. The nucleus of eukaryotic cells is bounded by a nuclear envelope containing nuclear pores. These pores serve as passageways between the cytoplasm and the nucleoplasm. Within the nucleus, the chromatin is a complex of DNA and protein. Chromatin is divided into separate structures called chromosomes. The nucleolus is a special region of the chromatin where rRNA is produced and where proteins from the cytoplasm gather to form ribosomal subunits. Ribosomes are organelles that function in protein synthesis. They can be bound to the endoplasmic reticulum (ER) or can exist within the cytoplasm singly or in groups called polyribosomes. The endomembrane system includes the ER, the Golgi apparatus, the lysosomes, and other types of vesicles and vacuoles. The endomembrane system compartmentalizes the cell. The rough endoplasmic reticulum (RER) is covered with ribosomes and is involved in the folding, modification, and transport of proteins. The smooth endoplasmic reticulum (SER) has various metabolic functions depending on the cell type, but it also forms vesicles that carry products to the Golgi apparatus. The Golgi apparatus processes proteins and repackages them into lysosomes, which carry out intracellular digestion, or into vesicles for transport to the plasma membrane or other organelles. Vacuoles are large storage sacs, and vesicles are smaller ones. The large single plant cell vacuole not only stores substances but also lends support to the plant cell.

hydrogen peroxide, which is subsequently broken down.

Chloroplasts capture solar energy to carry on photosynthesis, which produces carbohydrates. The mitochondria undergo cellular respiration  to produce ATP. The mitochondria are considered the energy powerhouse of the cell. ■ Chloroplasts contain internal membranes than enclose the stroma. These membranes, called thylakoids, are organized into stacks called grana (sing. granum). ■ The mitochondria contain internal membranes, called cristae, that enclose a space called the matrix.

3.4  The Cytoskeleton ■ The cytoskeleton contains actin filaments, intermediate filaments,

and microtubules. These maintain cell shape and allow the cell and its organelles to move. Actin filaments interact with motor molecules such as myosin in muscle cells. Microtubules are present in centrioles, cilia, and flagella. In the cytoplasm they serve as tracks along which vesicles and other organelles move due to the action of specific motor molecules.

3.5  Origin and Evolution of the Eukaryotic Cell ■ The first cells were probably prokaryotic cells. Eukaryotic cells most

likely arose from prokaryotic cells by a process called the endosymbiotic theory. ■ Biochemical data suggest that eukaryotic cells are closer evolutionarily to the archaea than to the bacteria. ■ The nuclear envelope most likely evolved through invagination of the plasma membrane, but mitochondria and chloroplasts may have arisen through endosymbiotic events.

ASSESS Testing Yourself Choose the best answer for each question.

3.1  The Cellular Level of Organization 1. The cell theory states: a. Cells form as organelles and molecules become grouped together in an organized manner. b. The normal functioning of an organism does not depend on its individual cells. c. The cell is the basic unit of life. d. Only eukaryotic organisms are made of cells. 2. As the size of a cell decreases, the ratio of its surface area to volume: a. increases b. decreases c. stays the same

3.2  Prokaryotic Cells 3. Which of the following would not be found in a prokaryotic cell? a. organelles b. DNA c. nucleoid d. nucleus



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4. The ________ is involved in the movement of a prokaryotic cell. a. fimbriae b. capsule c. nucleoid d. flagella 5. Small circular pieces of DNA that are found in the cytoplasm of a prokaryotic cell are called: a. capsules b. nucleoids c. plasmids d. ribosomes

3.3  Eukaryotic Cells 6. Eukaryotic cells contain a. a nucleus b. DNA c. plasma membrane d. mitochondria e. All of these are correct. 7. The combination of DNA and protein in the nucleus of a eukaryotic cell is called: a. nucleoplasm b. plasmids c. nucleoids d. chromatin 8. This organelle acts as a processing center for vesicles leaving the endoplasmic reticulum. a. peroxisomes b. ribosomes c. Golgi apparatus d. nucleolus 9. This organelle acts as a digestive organelle and may also be used to recycle the internal components of a cell. a. ribosome b. lysosome c. peroxisome d. Golgi apparatus 10. The mitochondria of a eukaryotic cell is the site of a. photosynthesis b. fatty acid metabolism c. cellular respiration d. protein synthesis

3.4  The Cytoskeleton 11. This component of the cytoskeleton is located just under the plasma membrane of a eukaryotic cell. a. actin filaments b. microtubules c. centrosomes d. intermediate filaments

12. Centrosomes and centrioles act as organizing centers for what component of the cytoskeleton? a. actin filaments b. microtubules c. intermediate filaments d. All of these are correct.

3.5  Origin and Evolution of the Eukaryotic Cell 13. The endosymbiotic theory explains the origins of which of the following components of a eukaryotic cell? a. mitochondria b. chloroplasts c. internal membranes d. All of these are correct. 14. According to the endosymbiotic theory, the _____  were originally photosynthetic bacteria. a. ribosomes c. mitochondria b. chloroplasts d. nucleus

ENGAGE BioNOW Want to know  how this science is relevant to your life? Check out the BioNow videos below: ■ Cell Size ■ Cell Division

Thinking Critically 1. One aspect of the new science of synthetic biology involves the laboratory design of cells to perform specific functions. Suppose that you wanted to make a protein for use in a drug trial. Design a cell that would build the protein and export it from the cell. 2. Why does the science of synthetic biology still need to adhere to the principles of the surface-area-to-volume ratio? 3. How does an understanding of basic cell biology become important as we search for life on other planets, including the moons of our solar system?

PHOTO CREDITS Opener: © Bruce Dale/National Geographic/Getty Images; 3Aa: © Ed Reschke/Getty Images; 3Ab: © Alfred Pasieka/Science Source; 3Ac © Science Photo Library RF/Getty RF; 3.3: © Sercomi/Science Source; 3.4: © Alfred Pasieka/Science Source; 3.5: © Biophoto Associates/ Science Source; 3.6(top right): © E.G. Pollock; 3.6(bottom blow up): © Ron Milligan/Scripps Research Institute; 3.7: © Martin M. Rotker/Science Source; 3.9: © EM Research Services, Newcastle University; 3.11a: © Science Source; 3.12a: © Keith R. Porter/Science Source; 3.13a(actin): © Thomas Deerinck/Science Source; 3.13a(Chara): © Bob Gibbons/Alamy; 3.13b(intermediate): © Cultura Science/Alvin Telser, PhD/Getty Images; 3.13b(girls): © Amos Morgan/Getty RF; 3.13c(microtubules): © Dr. Gopal Murti/Science Source; 3.13c(chameleon): © hlansdown/Getty RF; 3.15(sperm): © David M. Phillips/Science Source; 3.15(flagellum): © Steve Gschmeissner/Science Source; 3.15(cilia): © Dr. G. Moscoso/Science Source.

CASE STUDY Red Hot Chili Peppers Have you ever bitten into a hot pepper and had the sensation that your mouth is on fire? Your eyes water and you are in real pain! The feelings of heat and pain are due to a membrane protein in your sensory nerves. Capsaicin is the chemical in chili peppers that binds to a channel protein in specialized sensory nerve cell endings called nociceptors (noci- means hurtful). One of the important functions of a membrane is to control what molecules move into and out of the cell and when they move. This particular channel protein, when activated, allows calcium ions to flow into the cell. In addition to capsaicin, other factors such as an acidic pH, heat, electrostatic charges, and a variety of chemical agents can activate this channel protein. Once activated by any of these signals, the response is the same. The channel opens, calcium ions flow into the cell, and the nociceptor sends a signal to the brain. The brain then interprets this signal as pain. As long as the capsaicin is present, this pathway will continue to send signals to the brain. So the quickest way to alleviate the pain is to remove the capsaicin and close the channel protein. While some people drink cold water, this does very little other than cool down their mouth because capsaicin is lipid-soluble and does not dissolve in water. However, drinking milk, or eating rice or bread, usually helps. If you are a true “chili head,” you know that if you survive the first bite, the next bite is easier. That is because within minutes, the pathway becomes desensitized, or fails to respond, to the pain. However, other pathways, such as those in the eyes, may become activated if exposed to the capsaicin! In this chapter, we will discuss the various functions of proteins embedded in the membranes of your cells and how the membranes control what enters and leaves the cells. We will also describe how cells communicate with each other through signals sent to receptor proteins in the cell membrane.

Membrane Structure and Function

4

CHAPTER OUTLINE 4.1 Plasma Membrane Structure and Function

4.2 The Permeability of the Plasma Membrane 4.3 Modifications of Cell Surfaces

BEFORE YOU BEGIN Before beginning this chapter, take a few moments to review the following discussions: Section 2.6  How does the structure of a phospholipid make it an ideal molecule for the plasma membrane? Section 2.7  How does a protein’s shape relate to its function? Figures 3.4 and 3.5  What are the key features of animal and plant cells?

As you read through this chapter, think about the following questions:

1. What are the roles of the proteins in the plasma membrane of cells? 2. What type of transport is the calcium channel in this story exhibiting?

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4.1  Plasma Membrane Structure and Function Learning Outcomes Upon completion of this section, you should be able to 1. Describe the fluid-mosaic model of membrane structure. 2. Describe the diverse roles of proteins in membranes.

As we introduced in Chapter 3, the plasma membrane separates the internal environment of the cell from the external environment. In doing so, it regulates the entrance and exit of molecules from the cell. In this way, it helps the cell and the organism maintain a steady internal environment, a process called homeostasis. The plasma membrane is made primarily of phospholipids, a type of lipid with both hydrophobic and hydrophilic properties. The phospholipids of the membrane form a bilayer, with the hydrophilic (water-loving) polar heads of the phospholipid molecules facing the outside and

plasma membrane

inside of the cell (where water is found), and the hydrophobic (water-fearing) nonpolar tails facing each other (Fig. 4.1). The phospholipid bilayer has a fluid consistency, comparable to that of light oil. The fluidity of the membrane is regulated by steroids such as cholesterol, which serve to stiffen and strengthen the membrane. Throughout the membrane are numerous proteins, in which protein molecules are either partially or wholly embedded. These proteins are scattered either just outside or within the membrane, and may be either partially or wholly embedded in the phospholipid bilayer. Peripheral proteins are associated with only one side of the plasma membrane. Peripheral proteins on the inside of the membrane are often held in place by cytoskeletal filaments. In contrast, integral proteins span the membrane, and can protrude from one or both sides. They are embedded in the membrane, but they can move laterally, changing their position in the membrane. The proteins in the membrane form a mosaic pattern, and this combination of proteins, steroids, and phospholipids is called the fluid-mosaic model of membrane structure (Fig. 4.1).

Figure 4.1  Fluid-mosaic model of plasma membrane structure.  The membrane is

composed of a phospholipid bilayer in which proteins are embedded (integral proteins) or associated with the cytoplasmic side (peripheral proteins). Steroids (cholesterol) help regulate the fluidity of the membrane. Cytoskeleton filaments are attached to the inside surface by membrane proteins. carbohydrate chain

extracellular matrix (ECM)

glycoprotein glycolipid

phospholipid

hydrophobic hydrophilic tails heads phospholipid bilayer

filaments of cytoskeleton

peripheral protein

integral protein cholesterol

Outside cell

Inside cell



Chapter 4  Membrane Structure and Function

SCIENCE IN YOUR LIFE  ►

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HEALTH

How Cells Talk to One Another All organisms are able to sense and respond to specific signals in their environment. A bacterium that has taken up residence in your body is responding to signaling molecules when it finds food and escapes immune cells in order to stay alive. Signaling helps bread mold on stale bread in your refrigerator detect the presence of an opposite mating strain and begin its sexual life cycle. Similarly, the cells of an embryo are responding to signaling molecules when they move to specific locations and assume the shape and perform the functions of specific tissues (Fig. 4A). In plants, external signals, such as a change in the amount of light, tells them when it is time to resume growth or flower. Internal signaling molecules enable plants to coordinate the activities of roots, stems, and leaves.

Cell Signaling The cells of a multicellular organism “talk” to one another by using signaling molecules, called chemical messengers. Some messengers are produced at a distance from a target tissue and, in animals, are carried by the circulatory system to various sites around the body. For example, the pancreas releases a hormone called insulin, which is transported in blood vessels to the liver, and thereafter, the liver stores glucose as glycogen. Failure of the liver to respond appropriately results in a medical condition called diabetes. Growth factors act locally as signaling molecules and cause cells to divide. Overreacting to growth factors can result in a tumor characterized by unlimited cell division. We have learned that cells respond to only certain signaling molecules. Why? Because they must bind to a receptor protein, and cells have receptors for only certain signaling molecules. Each cell has receptors for numerous signaling molecules and often the final response is due to a summing up of all the various signals received. These molecules tell a cell what it should be doing at the moment, and without any signals, the cell dies. Signaling not only involves a receptor protein, it also involves a pathway called a transduction pathway and a response. To understand the process, consider an analogy. When a TV camera (the receptor) is shooting a scene, the

picture is converted to electrical signals (transduction pathway) that are understood by the TV in your house and are converted to a picture on your screen (the response). The process in cells is more complicated because each member of the pathway can turn on the activity of a number of other proteins. As shown in Figure 4A, the cell response to a transduction pathway can be a change in the shape or movement of a cell, the activation of a particular enzyme, or the activation of a specific gene.

is an aggressive form of cancer, in which the cells possess an enhanced ability to move (metastasis) by entering neighboring blood vessels. By identifying the signals generated by the melanoma cells, and their target receptors on the blood vessel cells, researchers have been able to develop chemicals that prevent the movement of the melanoma cells. These chemicals are being used to develop drugs that prevent the spread of melanoma in the body.

Advances in Understanding Cell Communication

Questions to Consider

The importance of cell signaling causes much research to be directed toward understanding the intricacies of the process, and this research is starting to yield some significant results. For example, researchers have discovered the basis of communication between melanoma cells and neighboring cells of the body. Melanoma

1. What happens if a cell is missing a receptor for a particular signaling molecule? 2. How does the binding of one type of signaling molecule sometimes result in multiple cellular responses? 3. As a cancer researcher, which segment of the signal transduction pathway would you target to prevent the spread of cancer and why?

1. Receptor: Binds to a signaling molecule, becomes activated and initiates a transduction pathway.

3. Response: Targeted protein(s) bring about the response(s) noted. plasma membrane

signaling molecule

receptor activation

2. Transduction pathway: Series of relay proteins that ends when a protein is activated. unactivated receptor protein

Cytoplasm

Targeted protein:

Cellular response:

structural protein

Altered shape or movement of cell

enzyme

nuclear envelope

Nucleus

gene regulatory protein

Altered metabolism or a function of cell

Altered gene expression and the amount of a cell protein

Figure 4A  Cell signaling and the transduction pathway.  The process of signaling

involves three steps: binding of the signaling molecule, transduction of the signal, and response of the cell depending on what type protein is targeted.

Both phospholipids and proteins can have attached carbohydrate Functions of the Membrane Proteins (sugar) chains. If so, these molecules are called glycolipids and The plasma membranes of various cells and the membranes of vari­glycoproteins, respectively. Because the carbohydrate chains occur ous organelles each have their own unique collections of proteins. only on the outside surface and peripheral proteins occur asymmetriThe peripheral proteins often have a structural role in that they cally on one surface or the other, the two halves of the membrane are help  stabilize and shape the plasma membrane (see section 3.4). not identical. These molecules play an important role in cellular They may also function in signaling pathways. The integral identification.

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UNIT 1  Cell Biology

Allows a particular molecule or ion to cross the plasma membrane freely. Cystic fibrosis, an inherited disorder, is caused by a faulty chloride (Cl–) channel; a thick mucus collects in airways and in pancreatic and liver ducts. a. Channel protein

Selectively interacts with a specific molecule or ion so that it can cross the plasma membrane. The family of GLUT carriers transfers glucose in and out of the various cell types of the body. Different carriers respond differently to blood levels of glucose. b. Carrier protein

Shaped in such a way that a specific molecule can bind to it. Some types of dwarfism result not because the body does not produce enough growth hormone, but because the plasma membrane growth hormone receptors are faulty and cannot interact with growth hormone. d. Receptor protein

The MHC (major histocompatibility complex) glycoproteins are different for each person, so organ transplants are difficult to achieve. Cells with foreign MHC glycoproteins are attacked by white blood cells responsible for immunity.

c. Cell recognition protein Catalyzes a specific reaction. The membrane protein, adenylate cyclase, is involved in ATP metabolism. Cholera bacteria release a toxin that interferes with the proper functioning of adenylate cyclase, which eventually leads to severe diarrhea.

Figure 4.2  Examples of membrane protein e. Enzymatic protein

proteins largely determine a membrane’s specific functions. Integral proteins can be of the following types: Channel proteins are involved in the passage of molecules through the membrane. They have a channel that allows a substance to simply move across the membrane (Fig. 4.2a). For example, a channel protein allows hydrogen ions to flow across the inner mitochondrial membrane. Without this movement of hydrogen ions, ATP would never be produced. Channel proteins may contain a gate that must be opened by the binding of a specific molecule to the channel. Carrier proteins are also involved in the passage of molecules through the membrane. They combine with a substance and help it move across the membrane (Fig. 4.2b). For example, a carrier protein transports sodium and potassium ions across a nerve cell membrane. Without this carrier protein, nerve conduction would be impossible. Cell recognition proteins are glycoproteins (Fig. 4.2c). Among other functions, these proteins help the body recognize when it is being invaded by pathogens so that an immune reaction can occur. Receptor proteins have a shape that allows a specific molecule to bind to it (Fig. 4.2d). The binding of this molecule causes the protein to change its shape and thereby bring about a cellular response. The coordination of the body’s organs is totally dependent on such signal molecules. For example, the liver stores glucose after it is signaled to do so by insulin. The Health feature “How Cells Talk to One Another” explores the function of receptor proteins. 

diversity.  These are some of the functions performed by integral proteins found in the plasma membrane.

Enzymatic proteins carry out metabolic reactions directly (Fig. 4.2e). The integral membrane proteins of the electron transport chain carry out the final steps of aerobic respiration. Without the presence of enzymes, some of which are attached to the various membranes of the cell, a cell would never be able to perform the metabolic reactions necessary to its proper function.

Check Your Progress  4.1 1. Describe the role of proteins, steroids, and phospholipids in the fluid-mosaic model.

2. Distinguish between the roles of the various integral proteins in the plasma membrane.

4.2  The Permeability of the Plasma Membrane Learning Outcomes Upon completion of this section, you should be able to 1. Explain why a membrane is selectively permeable. 2. Predict the movement of molecules in diffusion and osmosis. 3. Describe the role of proteins in the movement of molecules across a membrane.



Chapter 4  Membrane Structure and Function

67

TABLE 4.1  Passage of Molecules Into and Out of the Cell Energy Not Required

Energy Required

Name

Direction

Requirement

Examples

Diffusion

Toward lower concentration

Concentration gradient

Lipid-soluble molecules and gases

Facilitated transport

Toward lower concentration

Channels or carrier and concentration gradient

Some sugars and some amino acids

Active transport

Toward higher concentration

Carrier plus energy

Sugars, amino acids, and ions

Exocytosis

Toward outside

Vesicle fuses with plasma membrane

Macromolecules

Endocytosis

Toward inside

Vesicle formation

Macromolecules

The plasma membrane regulates the passage of molecules into ­Carrier proteins are specific for the substances they transport and out of the cell. This function is critical because the life of the across the plasma membrane—for example, sodium ions, amino cell depends on maintenance of its normal composition. The plasma acids, or glucose. membrane can carry out this function because it is selectively Vesicle formation is another way a molecule can exit a cell by ­permeable, meaning that certain substances can move across the exocytosis or enter a cell by endocytosis. This method of crossing membrane while others cannot. a plasma membrane is reserved for macromolecules or even larger Table 4.1 lists, and Figure 4.3 illustrates, which types of molmaterials, such as a virus. ecules can freely (i.e., passively) cross a membrane and which may require transport by a carrier protein and/or an expenditure of energy. In general, small, noncharged molecules, such as carbon dioxide, oxygen, glycerol, and alcohol, can freely cross the membrane. They charged molecules are able to slip between the hydroand ions + – philic heads of the phospholipids and water outside cell pass through the hydrophobic tails of the membrane. These molecules are H2O r, re said to go “down” their concentration ola np bic co o noncharged n gradient as they move from an area o ph molecules dro where their concentration is high to an hy area where their concentration is low. Some molecules are able to go “up” their concentration gradient, or move + from an area where their concentration macromolecule – is low to an area where their concentration is high, but this requires energy. Water, a polar molecule, would not be expected to readily cross the primarily nonpolar membrane. While the small size of the water molecule may allow some water to diffuse across the phospholipid water inside cell plasma membrane, the majority of molecule cells have special channel proteins called aquaporins that allow water to quickly cross the membrane. protein Large molecules and some ions and charged molecules are unable to freely cross the membrane. They can cross the plasma membrane through channel proteins, with the assistance of carrier proteins, or in vesicles. A channel protein forms a pore through the Figure 4.3  How molecules cross the plasma membrane.  Molecules that can diffuse across the membrane that allows molecules of a plasma membrane are shown with long back-and-forth arrows. Substances that cannot diffuse across the membrane are indicated by the curved arrows. certain size and/or charge to pass.



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UNIT 1  Cell Biology

time

time

crystal dye

a. Crystal of dye is placed in the water

b. Diffusion of water and dye molecules

c. Equal distribution of molecules results

Figure 4.4  Process of diffusion.  Diffusion is spontaneous, and no chemical energy is required to bring it about. a. When a dye crystal is placed in

water, it is concentrated in one area. b. The dye dissolves in the water, and there is a net movement of dye molecules from a higher to a lower concentration. There is also a net movement of water molecules from a higher to a lower concentration. c. Eventually, the water and the dye molecules are equally distributed throughout the container.

Diffusion and Osmosis

Osmosis

Diffusion is the movement of molecules from a higher to a lower concentration—that is, down their concentration gradient—until equilibrium is achieved and they are distributed equally. Diffusion is a physical process that can be observed with any type of molecule. For example, when a crystal of dye is placed in water (Fig. 4.4), the dye and water molecules move in various directions, but their net movement, which is the sum of their motion, is toward the region of lower concentration. Eventually, the dye is dissolved in the water, resulting in equilibrium and a colored solution. A solution contains both a solute, usually a solid, and a solvent, usually a liquid. In Figure 4.4, the solute is the dye and the solvent is the water molecules. Once the solute and solvent are evenly distributed, their molecules continue to move about, but there is no net movement of either one in any direction. The chemical and physical properties of the plasma membrane allow only a few types of molecules to enter and exit a cell simply by diffusion. Gases can diffuse through the lipid bilayer. This is the mechanism by which oxygen enters cells and carbon dioxide exits cells. Also, consider the movement of oxygen from the alveoli (air sacs) of the lungs to the blood in the lung capillaries (Fig. 4.5). After inhalation (breathing in), the concentration of oxygen in the alveoli is higher than that in the blood. Therefore, oxygen diffuses into the blood. Diffusion also plays an important role in maintaining the resting potential of neurons using gradients of potassium and sodium ions (see ­section 17.1) Several factors influence the rate of diffusion. Among these factors are temperature, pressure, electrical currents, and molecular size. For example, as temperature increases, the movement of molecules increases, which in turn increases the rate of diffusion.

The diffusion of water across a selectively permeable membrane due to concentration differences is called osmosis. To illustrate

O2 O2

O2 O2

O2

O2

O2

O2

oxygen O2

O2

O2 O2

alveolus

bronchiole capillary

Figure 4.5  Gas exchange in lungs. 

Oxygen (O2) diffuses into the capillaries of the lungs because there is a higher concentration of oxygen in the alveoli (air sacs) than in the capillaries.



Chapter 4  Membrane Structure and Function

69

over time

water

solute

less water (higher percentage of solute)

more water (lower percentage of solute)

more water (lower percentage of solute) 10%

thistle tube

5%

selectively permeable membrane

a.

beaker

< 10%

> 5% c.

less water (higher percentage of solute)

b.

Figure 4.6  Osmosis demonstration.  a. A thistle tube, covered at the broad end by a differentially permeable membrane, contains a 10% solute

solution. The beaker contains a 5% solute solution. b. The solute (green circles) is unable to pass through the membrane, but the water (blue circles) passes through in both directions. There is a net movement of water toward the inside of the thistle tube, where the percentage of water molecules is lower. c. Due to the incoming water molecules, the level of the solution rises in the thistle tube.

osmosis, a thistle tube containing a 10% solute solution1 is covered at one end by a selectively permeable membrane and then placed in a beaker containing a 5% solute solution (Fig. 4.6). The beaker has a higher concentration of water molecules (lower percentage of solute), and the thistle tube has a lower concentration of water molecules (higher percentage of solute). Diffusion always occurs from higher to lower concentration. Therefore, a net movement of water takes place across the membrane from the beaker to the inside of the thistle tube. The solute does not diffuse out of the thistle tube. Why not? Because the membrane is not permeable to the solute. As water enters and the solute does not exit, the level of the solution within the thistle tube rises (Fig. 4.6c). In the end, the concentration of solute in the thistle tube is less than 10%. Why? Because there is now less solute per unit volume of solution. Furthermore, the concentration of solute in the beaker is greater than 5% because there is now more solute per unit volume. Water enters the thistle tube due to the osmotic pressure of the solution within the thistle tube. Osmotic pressure is the pressure that develops in a system due to osmosis.2 In other words, the greater the possible osmotic pressure, the more likely it is that water will diffuse in that direction. Due to osmotic pressure, water is absorbed by the kidneys and taken up by capillaries in the tissues. Osmosis also occurs across the plasma membrane.

Isotonic Solution  In the laboratory, cells are normally placed in isotonic solutions. The prefix iso- means “the same as,” and the term tonicity refers to the osmotic pressure or tension of the Percent solutions are grams of solute per 100 ml of solvent. Therefore, a 10% solution is 10g of sugar with water added to make 100 ml of solution.

1

2 Osmotic pressure is measured by placing a solution in an osmometer and then immersing the osmometer in pure water. The pressure that develops is the osmotic pressure of a solution.

solution. In an isotonic solution, the solute concentration and the water concentration both inside and outside the cell are equal, and therefore there is no net gain or loss of water (Fig. 4.7). For example, a 0.9% solution of the salt sodium chloride (NaCl) is known to be isotonic to red blood cells. Therefore, intravenous solutions medically administered usually have this tonicity. Terrestrial animals can usually take in either water or salt as needed to maintain the tonicity of their internal environment. Many animals living in an estuary, such as oysters, blue crabs, and some fishes, are able to cope with changes in the salinity (salt concentrations) of their environment using specialized kidneys, gills, and other structures.

Hypotonic Solution  Solutions that cause cells to swell, or even to burst, due to an intake of water are said to be hypotonic solutions. The prefix hypo- means “less than” and refers to a solution with a lower concentration of solute (higher concentration of water) than inside the cell. If a cell is placed in a hypotonic solution, water enters the cell. The net movement of water is from the outside to the inside of the cell. Any concentration of a salt solution lower than 0.9% is hypotonic to red blood cells. Animal cells placed in such a solution expand and sometimes burst or lyse due to the buildup of pressure. The term cytolysis is used to refer to disrupted cells. However, if the cell is a red blood cell, the term hemolysis is used. The swelling of a plant cell in a hypotonic solution creates turgor pressure. When a plant cell is placed in a hypotonic solution, the cytoplasm expands because the large central vacuole gains water and the plasma membrane pushes against the rigid cell wall. The plant cell does not burst because the cell wall does not give way. Turgor pressure in plant cells is extremely important to the maintenance of the plant’s erect position. If you forget to water your plants, they wilt due to decreased turgor pressure.



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UNIT 1  Cell Biology

Animal cells

15,000× plasma membrane

nucleus

15,000×

In an isotonic solution, there is no net movement of water.

In a hypotonic solution, water enters the cell, which may burst (lysis).

15,000×

In a hypertonic solution, water leaves the cell, which shrivels (crenation).

Plant cells

cell wall

nucleus central vacuole

plasma membrane

chloroplast 400× In an isotonic solution, there is no net movement of water.

400× In a hypotonic solution, the central vacuole fills with water, turgor pressure develops, and chloroplasts are seen next to the cell wall.

In a hypertonic solution, the central vacuole loses water, the cytoplasm shrinks (plasmolysis), and chloroplasts are seen in the center of the cell.

Figure 4.7  Osmosis in animal and plant cells.  The arrows indicate the movement of water molecules. To determine the net movement of water, compare the number of arrows that are taking water molecules into the cell with the number that are taking water out of the cell. In an isotonic solution, a cell neither gains nor loses water; in a hypotonic solution, a cell gains water; and in a hypertonic solution, a cell loses water.

Organisms that live in fresh water have to prevent their internal environment from becoming hypotonic. Many protozoans, such as paramecia, have contractile vacuoles that rid the body of excess water. Freshwater fishes have well-developed kidneys that excrete a large volume of dilute urine. Even so, they have to take in salts at their gills. Even though freshwater fishes are good osmoregulators, they would not be able to survive in either distilled water or a marine environment.

Hypertonic Solution  Solutions that cause cells to shrink or shrivel due to loss of water are said to be hypertonic solutions. The prefix hyper- means “more than” and refers to a solution with a higher percentage of solute (lower concentration of water) than the cell. If a cell is placed in a hypertonic solution, water leaves the cell. The net movement of water is from the inside to the outside of the cell. Any concentration of a salt solution higher than 0.9% is hypertonic to red blood cells. If animal cells are placed in this solution, they shrink. The term crenation refers to the shriveling of a cell in a hypertonic solution. Meats are sometimes preserved by salting them. The salt kills any bacteria present because it makes the meat a hypertonic environment. When a plant cell is placed in a hypertonic solution, the plasma membrane pulls away from the cell wall as the large central vacuole loses water. This is an example of plasmolysis, shrinking

of the cytoplasm due to osmosis. The dead plants you may see along a salted roadside died because they were exposed to a hypertonic solution during the winter. Also, when salt water invades coastal marshes due to storms or human activities, coastal plants die. Without roots to hold the soil, it washes into the sea, thereby losing many acres of valuable wetlands. Marine animals cope with their hypertonic environment in  various ways that prevent them from losing water. Sharks increase  or decrease the urea in their blood until their blood is isotonic with the environment. Marine fishes and other types of animals excrete salts across their gills. Have you ever seen a marine turtle cry? It is ridding its body of salt by means of glands near the eye.

Transport by Carrier Proteins The plasma membrane impedes the passage of all but a few substances. Yet, biologically useful molecules are able to enter and exit the cell at a rapid rate because of carrier proteins in the membrane. Carrier proteins are specific. Each can combine with only a certain type of molecule or ion, which is then transported through the membrane. The precise mechanism by which a carrier protein functions varies, but generally after a carrier combines with a molecule, the carrier undergoes a change in shape that moves the molecule across the membrane. Carrier proteins are



Chapter 4  Membrane Structure and Function

required for both facilitated transport and active transport (see Table 4.1).

Facilitated Transport Facilitated transport explains the passage of such molecules as glucose and amino acids across the plasma membrane even though they are not lipid-soluble. The passage of glucose and amino acids is facilitated by their reversible combination with carrier proteins, which in some manner transport them through the plasma membrane. These carrier proteins are specific. For example, various sugar molecules of identical size might be present inside or outside the cell, but glucose can cross the membrane hundreds of times faster than the other sugars. Another example is provided by the aquaporins, which allow water to rapidly move across the plasma membrane of the cell. A model for facilitated transport (Fig. 4.8) shows that after a carrier has assisted the movement of a molecule to the other side of the membrane, it is free to assist the passage of other similar molecules. Neither diffusion nor facilitated transport requires an expenditure of energy (use of ATP) because the molecules are moving down their concentration gradient. 

Active Transport During active transport, molecules or ions move through the plasma membrane, accumulating either inside or outside the cell. For example, iodine collects in the cells of the thyroid gland; glucose is completely absorbed from the gut by the cells lining the digestive tract; and sodium can be almost completely withdrawn from urine by cells lining the kidney tubules. In these instances, molecules have moved to the region of higher concentration, exactly opposite to the process of diffusion. Both carrier proteins and an expenditure of energy are needed to transport molecules against their concentration

71

gradient. In this case, chemical energy, usually in the form of ATP, is required for the carrier to combine with the substance to be transported. Therefore, it is not surprising that cells involved primarily in active transport, such as kidney cells, have a large number of mitochondria near membranes where active transport is occurring. Proteins involved in active transport are often called pumps because, just as a water pump uses energy to move water against the force of gravity, proteins use energy to move a substance against its concentration gradient. One type of pump that is active in all animal cells, but is especially associated with nerve and muscle cells, moves sodium ions (Na+) to the outside of the cell and potassium ions (K+) to the inside of the cell. These two events are linked, and the carrier protein is called a sodium-­potassium pump. A change in carrier shape after the attachment of a phosphate group, and again after its detachment, allows the carrier to combine alternately with sodium ions and potassium ions (Fig. 4.9). The phosphate group is donated by ATP when it is broken down enzymatically by the carrier. The sodium-potassium pump results in both a solute concentration gradient and an ­electrical gradient for these ions across the plasma membrane. The passage of salt (NaCl) across a plasma membrane is of primary importance to most cells. The chloride ion (Cl–) usually crosses the plasma membrane because it is attracted by positively charged sodium ions (Na+). First sodium ions are pumped across a membrane, and then chloride ions simply diffuse through channels that allow their passage. As noted in Figure 4.2a, the genetic disorder cystic fibrosis results from a faulty chloride channel. In cystic fibrosis, Cl– transport is reduced, and so is the flow of Na+ and water. Research has shown that the lack of water causes the mucus in the bronchial tubes and pancreatic ducts to be abnormally thick, thus interfering with the function of the lungs and pancreas.

Inside plasma membrane

carrier protein

solute Outside

Figure 4.8  Facilitated transport.  During facilitated transport, a carrier protein speeds the rate at which the solute crosses the plasma membrane toward a lower concentration. Note that the carrier protein undergoes a change in shape as it moves a solute across the membrane.



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UNIT 1  Cell Biology

carrier protein

+

Na

Na+

+

Na+

K+ K+

K+

+ Na

Na +

Na +

Na+

K+

Na +

K+

Inside

Na +

1. Carrier has a shape that allows it to take up 3 Na+. K+

Na +

+

Na

Na +

N

+

Na +

K+

K

K+

+

ADP

ATP

Na+

K+

P

+

a

Na +

Na

+

K+

K+

Na +

Na +

K+

a

Na +

K+

Na +

K+

N

Na

+

Na

Na +

K+

K+

K+

Outside

Na +

K+

+

Na

K+

Na +

K+

2. ATP is split, and phosphate group attaches to carrier.

6. Change in shape results and causes carrier to release 2 K+ inside the cell.

+

K+ + Na

Na +

Na

N

a+

K+

+

Na

K+

Na +

K+

K+

+

Na

+

Na

Na +

K+ K+

K+ K+

K+

Na +

+

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K+

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Na +

Na +

+

Na

K+ K+

+

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K+

+

Na

K+

K+ N a+

Na +

K+

K+

K+

P

+

Na Na +

Na +

+

Na

K+

3. Change in shape results and causes carrier to release 3 Na+ outside the cell.

5. Phosphate group is released from carrier.

K+ Na +

K+

P

+

Na

K

+

+

Na

Na +

K+

4. Carrier has a shape that allows it to take up 2 K+.

Figure 4.9  The sodium-potassium pump.  The same carrier protein transports sodium ions (Na+) to the outside of the cell and potassium ions (K+) to the inside of the cell because it undergoes an ATP-dependent change in shape. Three sodium ions are carried outward for every two potassium ions carried inward. Therefore, the inside of the cell is negatively charged compared to the outside.

Bulk Transport How do macromolecules such as polypeptides, polysaccharides, or polynucleotides enter and exit a cell? Because they are too large to be transported by carrier proteins, macromolecules are  transported into and out of the cell by vesicle formation. Vesicle formation is called membrane-assisted transport because membrane is needed to form the vesicle. Vesicle formation requires an expenditure of cellular energy, but an added benefit

is that the vesicle membrane keeps the contained macromolecules from mixing with molecules within the cytoplasm. Exocytosis is a way substances can exit a cell, and endocytosis is a way substances can enter a cell.

Exocytosis During exocytosis, a vesicle fuses with the plasma membrane as secretion occurs (Fig. 4.10). Hormones, neurotransmitters, and



Chapter 4  Membrane Structure and Function

Outside

plasma membrane

secretory vesicle

Inside

73

Phagocytosis  When the material taken in by endocytosis is large, such as a food particle or another cell, the process is called phagocytosis. Phagocytosis is common in single-celled organisms such as amoebas (Fig. 4.11a). It also occurs in humans. Certain types of human white blood cells are amoeboid—that is, they are mobile like an amoeba, and they are able to engulf debris such as worn-out red blood cells or viruses. When an endocytic vesicle fuses with a lysosome, digestion occurs. We will see that this process is a necessary and preliminary step toward the development of immunity to bacterial diseases.

Pinocytosis  Pinocytosis occurs when vesicles form around a liquid or around very small particles (Fig. 4.11b). Blood cells, cells that line the kidney tubules or the intestinal wall, and plant root cells all use pinocytosis to ingest substances. Whereas phagocytosis can be seen with the light microscope, an electron microscope is required to observe pinocytic vesicles, which are Figure 4.10  Exocytosis.  Exocytosis deposits substances on the outside of the cell and no larger than 0.1–0.2 µm. Still, pinocytosis allows secretion to occur. involves a significant amount of the plasma membrane because it occurs continuously. However, the loss of plasma membrane due to pinocytosis is balanced digestive enzymes are secreted from cells in this manner. The by the occurrence of exocytosis. Golgi apparatus often produces the vesicles that carry these cell products to the membrane. Notice that during exocytosis, the Receptor-Mediated Endocytosis Receptor-mediated membrane of the vesicle becomes a part of the plasma membrane, endocytosis is a form of pinocytosis that is quite specific which is thereby enlarged. For this reason, exocytosis can be a because it uses a receptor protein shaped so that a specific molnormal part of cell growth. The proteins released from the vesicle ecule, such as a vitamin, peptide hormone, or lipoprotein, can adhere to the cell surface or become incorporated in an extracelbind to it (Fig. 4.11c). The receptors for these substances are lular matrix. found at one location in the plasma membrane. This location is Cells of particular organs are specialized to produce and called a coated pit because there is a layer of protein on the cytoexport molecules. For example, pancreatic cells produce digestive plasmic side of the pit. Once formed, the vesicle becomes enzymes or insulin, and anterior pituitary cells produce growth uncoated and may fuse with a lysosome. When empty, used veshormone, among other hormones. In these cells, secretory vesicles icles fuse with the plasma membrane, and the receptors return to accumulate near the plasma membrane, and the vesicles release their former location. their contents only when the cell is stimulated by a signal received Receptor-mediated endocytosis is selective and much more at the plasma membrane. A rise in blood sugar, for example, sigefficient than ordinary pinocytosis. It is involved in uptake and nals pancreatic cells to release the hormone insulin. This is called also in the transfer and exchange of substances between cells. Such regulated secretion, because vesicles fuse with the plasma memexchanges take place when substances move from maternal blood brane only when it is appropriate to the needs of the body. into fetal blood at the placenta, for example. The importance of receptor-mediated endocytosis is demonEndocytosis strated by a genetic disorder called familial hyper-cholesterolemia. Cholesterol is transported in the blood by a complex of lipids and During endocytosis, cells take in substances by vesicle formaproteins called low-density lipoprotein (LDL). Ordinarily, body tion. A portion of the plasma membrane invaginates to envelop cells take up LDL when LDL receptors gather in a coated pit. But the substance, and then the membrane pinches off to form an in some individuals, the LDL receptor is unable to properly bind to intracellular vesicle. Endocytosis occurs in one of three ways, as the coated pit, and the cells are unable to take up cholesterol. illustrated in Figure 4.11. Phagocytosis transports large subInstead, cholesterol accumulates in the walls of arterial blood vesstances, such as viruses, and pinocytosis transports small subsels, leading to high blood pressure, occluded (blocked) arteries, stances, such as macromolecules, into cells. Receptor-mediated and heart attacks. endocytosis is a special form of pinocytosis.



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UNIT 1  Cell Biology

plasma membrane paramecium pseudopod of amoeba

vesicle forming

vesicle 43×

a. Phagocytosis

vesicles forming solute

vesicle

b. Pinocytosis

receptor protein

coated pit

solute

coated vesicle

coated vesicle coated pit c. Receptor-mediated endocytosis

Figure 4.11  Three methods of endocytosis.  a. Phagocytosis occurs when the substance to be transported into the cell is large. Amoebas ingest by phagocytosis. Digestion occurs when the resulting vesicle fuses with a lysosome. b. Pinocytosis occurs when a macromolecule such as a polypeptide is transported into the cell. The result is a vesicle. c. Receptor-mediated endocytosis is a form of pinocytosis. Molecules first bind to specific receptor proteins, which migrate to, or are already in, a coated pit. The vesicle that forms contains the molecules and their receptors.

Check Your Progress  4.2 1. Contrast diffusion with facilitated transport. 2. Explain the movement of water between hypotonic and hypertonic environments. 3. Describe the differences between facilitated and active transport. 4. Discuss the potential benefits of receptor-mediated endocytosis.

4.3  Modifications of Cell Surfaces Learning Outcomes Upon completion of this section, you should be able to 1. Explain the role of the extracellular matrix in animal cells. 2. Compare the structure and function of adhesion, tight, and gap junctions.



Chapter 4  Membrane Structure and Function

Most cells do not live isolated from other cells. Rather, they live and interact within an external environment that can dramatically affect cell structure and function. This extracellular environment is made of large molecules produced by nearby cells and secreted from their membranes. In plants, prokaryotes, fungi, and most algae, the extracellular environment is a fairly rigid cell wall, which is consistent with a somewhat sedentary lifestyle. Animals, which tend to be more active, have a more varied extracellular environment that can change, depending on nature of the organism or the tissue type.

Cell Surfaces in Animals In this section, we will focus on two different types of animal cell surface features: (1) the extracellular matrix (ECM) that is observed outside cells, and (2) junctions that occur between some types of cells. Both of these can connect to the cytoskeleton and contribute to communication between cells, and therefore tissue formation.

Extracellular Matrix A protective extracellular matrix (ECM) is a meshwork of proteins and polysaccharides in close association with the cell that

Inside (cytoplasm) actin filament

75

produced them (Fig. 4.12). Collagen and elastin fibers are two well-known structural proteins in the ECM; collagen resists stretching and elastin gives the ECM resilience. Fibronectin is an adhesive protein (colored green in Fig. 4.12) that binds to a protein in the plasma membrane called integrin. Integrins are integral membrane proteins that connect to fibronectin externally and to the actin cytoskeleton internally. Through its connections with both the ECM and the cytoskeleton, integrin plays a role in cell signaling, permitting the ECM to influence the activities of the cytoskeleton and, therefore, the shape and activities of the cell. Amino sugars in the ECM form multiple polysaccharides that attach to a protein and are, therefore, called proteoglycans. Proteoglycans, in turn, attach to a very long, centrally placed polysaccharide. The entire structure, which looks like an enormous bottle brush, resists compression of the extracellular matrix. Proteoglycans assist cell signaling when they regulate the passage of molecules through the ECM to the plasma membrane, where receptors are located. Thus, the ECM has a dynamic role in all aspects of a cell’s behavior. In Chapter 11, during the discussion of tissues, you’ll see that the extracellular matrix varies in quantity and in consistency from being quite flexible, as in loose connective tissue; semiflexible, as in cartilage; and rock solid, as in bone. The extracellular matrix of bone is hard because, in addition to the components mentioned, mineral salts, notably calcium salts, are deposited outside the cell.

Junctions Between Cells

integrin elastin

fibronectin collagen

proteoglycan

Outside (extracellular matrix)

Figure 4.12  Extracellular matrix of an animal cell.  In the extracellular matrix (ECM), collagen and elastin have a support function, while fibronectins bind to integrin, thus assisting communication between the ECM and the cytoskeleton.

Certain tissues of vertebrate animals are known to have junctions between their cells that allow them to behave in a coordinated manner. Three types of junctions are shown in Figure 4.13. Adhesion junctions (Fig. 4.13a) serve to mechanically attach adjacent cells. Desmosomes are one form of adhesion junction. In a desmosome, internal cytoplasmic plaques, firmly attached to the intermediate filament cytoskeleton within each cell, are joined by integral membrane proteins called cadherins between cells. The result is a sturdy but flexible sheet of cells. In some organs—such



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UNIT 1  Cell Biology

cytoplasmic plaque

plasma membranes

filaments of cytoskeleton adhesion proteins 30,000×

intercellular space

Plant Cell Walls

a. Adhesion junction

plasma membranes tight junction proteins

81,000×

in the intestine, the digestive juices stay out of the rest of the body, and in the kidneys, the urine stays within kidney tubules, because the cells are joined by tight junctions. A gap junction (Fig. 4.13c) allows cells to communicate. A gap junction is formed when two identical plasma membrane channels join. The channel of each cell is lined by six plasma membrane proteins. A gap junction lends strength to the cells, but it also allows small molecules and ions to pass between them. Gap junctions are important in heart muscle and smooth muscle because they permit a flow of ions that is required for the cells to contract as a unit.

intercellular space

b. Tight junction

plasma membranes membrane channels

In addition to a plasma membrane, plant cells are surrounded by a porous cell wall that varies in thickness, depending on the function of the cell. All plant cells have a cell wall. The primary cell wall contains cellulose fibrils (very fine fibers) in which microfibrils are held together by noncellulose substances. Pectins allow the wall to stretch when the cell is growing, and noncellulose polysaccharides harden the wall when the cell is mature. Pectins are especially abundant in the middle lamella, which is a layer of adhesive substances that holds the cells together. In a plant, the cytoplasm of living cells is connected by ­plasmodesmata (sing., plasmodesma), numerous narrow, membrane-lined channels that pass through the cell wall. Cytoplasmic strands within these channels allow direct exchange of some materials between adjacent plant cells and eventually connect all the cells within a plant. The plasmodesmata allow only water and small solutes to pass freely from cell to cell.

Check Your Progress  4.3 1. Describe the molecule composition of the extracellular matrix of an animal cell.

2. Explain the difference between the function of an adhesion, 96,000×

intercellular space

gap, and tight junction. 3. Contrast the extracellular matrix of an animal cell with the cell wall of a plant cell.

c. Gap junction

Figure 4.13  Examples of cell junctions.  a. In adhesion junctions,

such as the desmosome, adhesive proteins connect two cells. b. Tight junctions between cells have joined their adjacent plasma membranes, forming an impermeable layer. c. Gap junctions allow communication between two cells by joining plasma membrane channels between the cells.

a. © Kelly, 1966. Originally published in The Journal of Cell Biology, 28:51–72.

as the heart, stomach, and bladder, where tissues get stretched— desmosomes hold the cells together. Adhesion junctions are the most common type of intercellular junction between skin cells. Another type of junction between adjacent cells are tight junctions (Fig. 4.13b), which bring cells even closer than desmosomes. Tight junction proteins actually connect plasma membranes between adjacent cells together, producing a zipperlike fastening. Tissues that serve as barriers are held together by tight junctions;

Conclusion In the case of the particular channel described in the opening of this chapter, exposure to the capsaicin molecules in a chili pepper opens a channel protein in the membrane of the cells. This allows calcium ions to enter the cell and initiate a cascade of events that results in the sensation of pain. As we have seen, this is a type of facilitated diffusion, because the calcium ions are moving down their concentration gradient with the assistance of a channel protein. It is interesting to note that later in this text (section 19.4) you will learn about a different type of voltage-gated ion channel that also allows calcium ions to enter the cell. In cells with this type of channel, muscular contraction or neuron excitation occurs.



Chapter 4  Membrane Structure and Function

77

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4.1  Membrane Structure 4.2  Diffusion • Osmosis

 

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4.2  How Diffusion Works • How Osmosis Works • Hemolysis and Crenation • Effect of Tonicity on Cells • How Facilitated Diffusion Works • How the Sodium-Potassium Pump Works • Cotransport • Endocytosis and Exocytosis

SUMMARIZE 4.1  Plasma Membrane Structure and Function ■ The plasma membrane plays an important role in isolating the cell

from the external environment and in maintaining homeostasis within the cell. According to the fluid-mosaic model of the plasma membrane, a lipid bilayer is fluid and has the consistency of light oil. The hydrophilic heads of phospholipids form the inner and outer surfaces, and the hydrophobic tails form the interior. ■ Proteins within the membrane are the mosaic portion. The peripheral proteins often have a structural role in that they help stabilize and shape the plasma membrane. They may also function in signaling pathways. The integral proteins have a variety of functions, including acting as channel proteins, carrier proteins, cell recognition proteins, receptor proteins, and enzymatic proteins. ■ Carbohydrate chains are attached to some of the lipids and proteins in the membrane. These are glycolipids and glycoproteins.

4.2  The Permeability of the Plasma Membrane ■ The plasma membrane is selectively permeable, meaning that some

■

■

■

substances, such as gases, freely cross a plasma membrane, while ­others—particularly ions, charged molecules, and macromolecules— have to be assisted across. Passive ways of crossing a plasma membrane (diffusion and facilitated transport) do not require an expenditure of chemical energy (ATP). Active ways of crossing a plasma membrane (active transport and vesicle formation) do require an expenditure of chemical energy. Lipid-soluble compounds, water, and gases simply diffuse across the plasma membrane by moving down their concentration gradient (high to low concentration). The diffusion of water across a membrane is called osmosis. Water (a solvent) moves across the membrane into the area of lower water (or higher solute) content. When cells are in an isotonic solution, they neither gain nor lose water; when they are in a hypotonic solution, they gain water; and when they are in a hypertonic solution, they lose water. Osmotic pressure occurs as a result of differences in tonicity. Some molecules are transported across the membrane by carrier proteins that span the membrane. During facilitated transport, a carrier

 

3D Animations

4.1  Membrane Transport: Lipid Bilayer 4.2  Membrane Transport: Diffusion • Membrane Transport: Osmosis • Membrane Transport: Active Transport

  Tutorials 4.2  Osmosis and Tonicity • Sodium-Potassium Pump

protein assists the movement of a molecule down its concentration gradient. No energy is required. ■ During active transport, a carrier protein acts as a pump that causes a substance to move against its concentration gradient. Energy in the form of ATP molecules is required for active transport to occur. The sodium-potassium pump is one example of active transport. ■ Larger substances can exit and enter a membrane by exocytosis and endocytosis. Exocytosis involves secretion. Endocytosis includes phagocytosis and pinocytosis. Receptor-mediated endocytosis, a type of pinocytosis, makes use of receptor molecules in the plasma membrane and a coated pit, which pinches off to form a vesicle.

4.3  Modifications of Cell Surfaces ■ Animal cells have an extracellular matrix (ECM) that influences

their shape and behavior. The amount and character of the ECM varies by tissue type. Some animal cells have junction proteins that join them to other cells of the same tissue. Adhesion junctions and tight junctions help hold cells together; gap junctions allow passage of small molecules between cells. ■ Plant cells have a freely permeable cell wall, with cellulose as its main component. Also, plant cells are joined by narrow, membrane-lined channels called plasmodesmata that span the cell wall and contain strands of cytoplasm that allow materials to pass from one cell to another.

ASSESS Testing Yourself Choose the best answer for each question.

4.1:  Plasma Membrane Structure and Function

1. In the fluid-mosaic model, the fluid properties are associated with the nature of the ______ and the mosaic pattern is established by the _______. a. nucleic acids; phospholipids b. phospholipids; embedded proteins ■ c. embedded proteins; cholesterol d. phospholipids; nucleic acids 

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UNIT 1  Cell Biology

2. Which of the following is not a function of proteins present in the plasma membrane? a. Proteins assist the passage of materials into the cell. b. Proteins interact with and recognize other cells. c. Proteins bind with specific hormones. d. Proteins carry out specific metabolic reactions. e. Proteins produce lipid molecules.  3. The carbohydrate chains projecting from the plasma membrane are involved in a. adhesion between cells. c. cell-to-cell recognition. b. reception of molecules. d. All of these are correct.

4.2  The Permeability of the Plasma Membrane 4. When a cell is placed in a hypotonic solution, a. solute exits the cell to equalize the concentration on both sides of the membrane. b. water exits the cell toward the area of lower solute concentration. c. water enters the cell toward the area of higher solute concentration. d. there is no net movement of water or solute.  5. When a cell is placed in a hypertonic solution, a. solute exits the cell to equalize the concentration on both sides of the membrane. b. water exits the cell toward the area of lower solute concentration. c. water exits the cell toward the area of higher solute concentration. d. there is no net movement of water or solute.  6. Which of the following is incorrect regarding facilitated diffusion? a. It is a passive process. b. It allows the movement of molecules from areas of low concentration to areas of high concentration. c. It may use either channel or carrier proteins. d. It allows the rapid transport of glucose across the membrane. 7. The sodium-potassium pump a. helps establish an electrochemical gradient across the membrane. b. concentrates sodium on the outside of the membrane. c. uses a carrier protein and chemical energy. d. is present in the plasma membrane.  e. All of these are correct.  8. Which of the following processes is involved in the bulk transport of molecules out of the cell? a. phagocytosis b. pinocytosis c. receptor-mediated endocytosis d. exocytosis e. None of these are correct.  9. Which process uses special proteins on the surface of the membrane to identify specific molecules for transport into the cell? a. phagocytosis b. pinocytosis c. receptor-mediated endocytosis d. exocytosis

4.3  Modifications of Cell Surfaces 10. The extracellular matrix a. assists in the movement of substances across the plasma membrane. b. prevents the loss of water when cells are placed in a hypertonic solution. c. has numerous functions that affect the shape and activities of the cell that produced it. d. contains the junctions that sometimes occur between cells. e. All of these are correct.  11. Which of the following junctions allows for cytoplasm-to-cytoplasm communication between cells? a. adhesion junctions b. tight junctions c. gap junctions d. None of these are correct.

ENGAGE BioNOW Want to know  how this science is relevant to your life? Check out the BioNow video below: ■ Saltwater Filter

Thinking Critically 1. When a signal molecule such as a growth hormone binds to a receptor protein in the plasma membrane, it stays on the outside of the cell. How might the inside of the cell know that the signal has bound? 2. As mentioned, cystic fibrosis is a genetic disorder caused by a defective membrane transport protein. The defective protein closes chloride channels in membranes, preventing chloride from being exported out of cells. This results in the development of a thick mucus on the outer surfaces of cells. This mucus clogs the ducts that carry digestive enzymes from the pancreas to the small intestine, clogs the airways in the lungs, and promotes lung infections. Why do you think the defective protein results in a thick, sticky mucus outside the cells, instead of a loose, fluid covering? 

PHOTO CREDITS Opener: © Tooga Productions, Inc./Getty Images; 4.7(top left, center, right): © David M. Phillips/Science Source; 4.7(bottom left, center): © Dwight Kuhn; 4.7(bottom right): © Ed Reschke/Getty Images; 4.11a: © Eric Grave/Phototake; 4.11b: © Don W. Fawcett/Science Source; 4.11c(both): © Mark Bretscher; 4.13a: © SPL/Science Source; 4.13b–c: © David M. Phillips/Science Source.

CASE STUDY Genetics of Breast Cancer In 2013, actress Angelina Jolie announced to her shocked fans that she was going to undergo a double mastectomy (removal of breast tissue) to prevent breast cancer. Every year almost 233,000 American women are diagnosed with breast cancer. While Angelina did not have breast cancer, she has a history of this cancer in her family, and she had tested positive for a BRCA1 (breast cancer susceptibility gene 1) gene mutation, linked to both breast and ovarian cancer. Cancer results from a failure to control the cell cycle, a series of steps that all cells go through prior to initiating cell division. BRCA1, and a similar gene, BRCA2, are important components of that control mechanism. Both of these genes are tumor suppressor genes. At specific checkpoints in the cell cycle, the proteins encoded by these genes check the DNA for damage. BRCA1 acts as a gatekeeper, preventing cells from dividing continuously. Each cell normally has two copies of BRCA1, one inherited from each parent. In Angelina’s case, the mutated version of BRCA1 indicated that each of her cells only had a single functioning copy of the gene, and therefore she was at a higher risk of developing breast cancer in the future.  It is estimated that 1 in 833 people possess the BRCA1 mutation associated with breast cancer. Although it is not the only genetic contribution to breast and ovarian cancer, it does play a major role. In this chapter we will examine not only how cells divide, but how the process of cell division is controlled. As you read through this chapter, think about the following questions:

1. What are the roles of the checkpoints in the cell cycle? 2. At what checkpoint would you think that BRCA1 would normally be active?

3. Why would a failure of the checkpoints in the cell cycle result in cancer?

Cell Division

5

CHAPTER OUTLINE 5.1 The Cell Cycle 5.2 Control of the Cell Cycle 5.3 Mitosis: Maintaining the Chromosome Number

5.4 Meiosis: Reducing the Chromosome Number 5.5 Comparison of Meiosis with Mitosis 5.6 The Human Life Cycle

BEFORE YOU BEGIN Before beginning this chapter, take a few moments to review the following discussions: Section 2.8  What is the role of DNA in a cell? Section 3.1  What does the cell theory tell us about the need for cell division? Section 3.4  What are microtubules and actin filaments?

79

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UNIT 1  Cell Biology

5.1  The Cell Cycle

The Cell Cycle Cell division is a part of the cell cycle. The cell cycle is an orderly set of stages that take place between the time a cell divides and the time the resulting cells also divide.

Learning Outcomes Upon completion of this section, you should be able to 1. Distinguish between the two processes that change the number of cells in the body. 2. Describe the stages of the cell cycle and what occurs in each stage.

The Stages of Interphase As Figure 5.1 shows, most of the cell cycle is spent in interphase. This is the time when a cell carries on its usual functions, which are dependent on its location in the body. It also gets ready to  divide: it grows larger, the number of organelles doubles, and  the amount of DNA doubles. For mammalian cells, ­interphase lasts for about 20 hours, which is 90% of the cell cycle. Interphase is divided into three stages: the G1 stage occurs before DNA synthesis, the S stage includes DNA synthesis, and the G2 stage occurs after DNA synthesis. Originally G stood for the “gaps” that occur before and after DNA synthesis during interphase. But now that we know growth occurs during these stages, the G can be thought of as standing for “growth.” During the G1 stage, a cell doubles its organelles (such as mitochondria and ribosomes), and it accumulates the materials needed for DNA synthesis. Some cells, such as nerve and muscle cells, typically pause during the cell cycle. These cells are said to have entered a G0 stage. Cells in G0 phase continue to perform their normal functions, but no longer prepare for cell division. Some cells may enter G0 phase if their DNA is damaged. If the repair of the DNA can not be completed, then the cell may undergo apoptosis (see below).

We start life as a single-celled fertilized egg. By the process of cell division, we grow into organisms that contain trillions of cells. The process of cell division serves to increase the number of somatic cells, or body cells. However, cell division does not stop when you become an adult. Every day your body produces thousands of new red blood cells, skin cells, and cells that line your respiratory and digestive tracts. In addition to growth, cell division is involved in the repair of tissues after an injury. Apoptosis, or programmed cell death, decreases the number of cells. Apoptosis occurs during development to remove unwanted tissue—for example, the tail of a tadpole disappears as it matures into a frog. In humans, the fingers and toes of the embryo are initially webbed, but are normally freed from one another later in development as a result of apoptosis. Apoptosis also plays an important role in preventing cancer. An abnormal cell that could become cancerous will often die via apoptosis, thus preventing a  tumor from developing. Both cell division and apoptosis are ­normal parts of growth and development.

Interphase S (growth and DNA replication)

e as ph a t e om Pr

se

Metapha

e se

Anapha

ph as

Te lo

M Mitosis

e

kin

to Cy

sis

G2 (growth and final preparations for G2 division) se

G0

G1 (growth)

ha

G1

Pr op

G1 checkpoint Cell cycle main checkpoint. If DNA is damaged, apoptosis will occur. Otherwise, the cell is committed to divide when growth signals are present and nutrients are available.

G2 checkpoint Mitosis checkpoint. Mitosis will occur if DNA has replicated properly. Apoptosis will occur if the DNA is damaged and cannot be repaired.

M M checkpoint Spindle assembly checkpoint. Mitosis will not continue if chromosomes are not properly aligned.

Figure 5.1  The cell cycle.  Cells normally go through a cycle that consists of four stages: G1, S, G2, and M. The G0 stage represents a holding stage for cells that are not undergoing division.Checkpoints regulate the speed at which a cell moves through the cell cycle.



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Chapter 5  Cell Division normal cells

apoptotic cell blebs

Cell rounds up, and nucleus collapses.

Chromatin condenses, and nucleus fragments.

Plasma membrane blisters, and blebs form.

cell fragment

Cell fragments contain DNA fragments.

Figure 5.2  Apoptosis.  Apoptosis is a sequence of events that results in a fragmented cell. The fragments are engulfed by white blood cells

2,500×

and neighboring tissue cells.

During the S stage, DNA replication occurs. At the beginning of the S stage, each chromosome is composed of one DNA molecule, which is called a chromatid. At the end of this stage, each chromosome consists of two sister chromatids that have identical DNA sequences. Another way of expressing this is to say that DNA replication has resulted in duplicated chromosomes. The sister chromatids remain attached until they are separated during mitosis. sister chromatids

replication chromosome consisting of one chromatid

division

centromere duplicated chromosome

During the G2 stage, the cell synthesizes the proteins needed for cell division, such as the proteins that make up the microtubules found in the spindle apparatus (see section 5.3). Some cells, such as nerve and muscle cells, typically do not complete the cell cycle and are permanently arrested. These cells exit interphase and enter a stage called G0. While in the G0 stage, the cells continue to perform normal, everyday processes, but no preparations are being made for cell division. Cells may not leave the G0 stage without proper signals from other cells and other parts of the body. 

The Mitotic Stage Following interphase, the cell enters the M (for mitotic) stage. This stage not only includes mitosis, the division of the nucleus and genetic material, but also cytokinesis, the division of the cytoplasm (which may not occur in all cells). During mitosis (covered in section 5.3), the sister chromatids of each chromosome separate, becoming daughter chromosomes that are distributed to two daughter nuclei. When cytokinesis is complete, two daughter cells are present, each identical to the original mother

cell. Mammalian cells usually require only about four hours to complete the mitotic stage.

Apoptosis During apoptosis, the cell progresses through a typical series of events that bring about its destruction (Fig. 5.2). The cell rounds up and loses contact with its neighbors. The nucleus fragments, and the plasma membrane develops blisters. Finally, the cell fragments, and its bits and pieces are engulfed by white blood cells. The majority of cells contain enzymes, called caspases, that bring about apoptosis. The enzymes are ordinarily held in check by inhibitors, but they can be unleashed either by internal or external signals. There are two sets of caspases. The first set, the “initiators,” receive the signal to activate the second set, the “executioners,” which then activate the enzymes that dismantle the cell. For example, executioners turn on enzymes that tear apart the cytoskeleton and enzymes that chop up DNA.

Check Your Progress  5.1 1. Illustrate the cell cycle, and explain the major events that occur during each stage.

2. Explain why apoptosis is a beneficial process.

5.2  Control of the Cell Cycle  Learning Outcomes Upon completion of this section, you should be able to 1. Distinguish between internal and external controls of the cell cycle. 2. Describe the checkpoints for the cell cycle. 3. Differentiate between the role of proto-oncogenes and tumor suppressor genes in regulating the cell cycle.

Eukaryotic cells have evolved a complex system for regulation of the cell cycle. The cell cycle is controlled by both internal and external signals. The internal signals ensure that the stages follow



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one another in the normal sequence and that each stage is properly completed before the next stage begins. The external signals tell the cell whether or not to divide. The events of the cell cycle must occur in the correct order, even if the steps take longer than normal. The red stop signs in Figure 5.1 represent three checkpoints when the cell cycle possibly stops. Researchers have identified proteins called cyclins that increase and decrease as the cell cycle continues. The appropriate cyclin has to be present for the cell to proceed from the G1 stage to the S stage and from the G2 stage to the M stage. The first checkpoint during G1 allows the cell to determine whether conditions are favorable to begin the cell cycle. The cell needs to assess whether there are building blocks available for duplication of the DNA and if the DNA is intact. DNA damage can stop the cell cycle at the G1 checkpoint. In mammalian cells, the p53 protein stops the cycle at the G1 checkpoint when DNA is damaged. First, the p53 protein attempts to initiate DNA repair, but if that is not possible, the cell enters G0 phase and undergoes apoptosis. The cell cycle stops at the G2 checkpoint if DNA has not finished replicating. This prevents the initiation of the M stage before completion of the S stage. Also, if DNA is damaged, stopping the cell cycle at this checkpoint allows time for the damage to be repaired. If repair is not possible, apoptosis occurs. Another cell cycle checkpoint occurs during the mitotic (M) stage. The cycle stops if the chromosomes are not going to be distributed accurately to the daughter cells.

proto-oncogene Codes for a growth factor, a receptor protein, or a signaling protein in a stimulatory pathway. If a proto-oncogene becomes an oncogene, the end result can be unregulated cell division.

These checkpoints are critical for preventing cancer development (see section 25.5). A damaged cell should not complete mitosis, but instead should undergo apoptosis. Mammalian cells tend to enter the cell cycle only when stimulated by an external factor. Growth factors are hormones that are received at the plasma membrane. These signals set into motion the events that result in the cell entering the cell cycle. For example, when blood platelets release a growth factor, skin fibroblasts in the vicinity are stimulated to finish the cell cycle so an injury can be repaired.

Proto-oncogenes and Tumor Suppressor Genes Two types of genes control the movement of a cell through the  cell cycle: proto-oncogenes and tumor suppressor genes. Proto-oncogenes encode proteins that promote the cell cycle and prevent apoptosis. They are often likened to the gas pedal of a car because they cause cells to continue through the cell cycle. Tumor suppressor genes encode proteins that stop the cell cycle and promote apoptosis. They are often likened to the brakes of a car because they inhibit cells from progressing through the cell cycle (Fig. 5.3a). Cancer is unregulated cell growth. Carcinogenesis, or the development of cancer, is a multistage process involving disruption of normal cell division and behavior. This typically occurs due to mutations in either proto-oncogenes or tumor suppressor genes. These genetic changes may be inherited from a parent, the result of

growth factor receptor

growth factor Activates signaling proteins in a stimulatory pathway that extends to the nucleus.

Activated Ras protein

Mutated Ras protein

Activation Signaling pathway

Stimulatory pathway

gene product promotes cell cycle Inhibitory pathway

gene product inhibits cell cycle

Proteins for cell division

Signaling pathway Proteins for cell division

DNA

DNA

b. Effect of growth factor

Figure 5.3  Role of proto-oncogenes and tumor suppressor genes.  a. Mutated proto-oncogenes and

a. Stimulatory pathway and inhibitory pathway

tumor suppressor gene Codes for a signaling protein in an inhibitory pathway. If a tumor suppressor gene mutates, the end result can be unregulated cell division.

tumor suppressor genes may cause cancer by changing stimulatory and inhibitory pathways. b. RAS is a proto-oncogene that promotes cell division. When a growth factor binds to the Ras receptor on the cell surface, a signal transduction pathway is activated and relays a signal to the nucleus to express genes whose products promote cell division. Mutations in the RAS proto-oncogene cause the Ras protein to be always activated— thus sending a signal for cell division without the presence of a growth factor.



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inhibiting growth factor stimulating growth factor receptor

plasma membrane

signaling pathway

cytoplasm transcription factor

cytoplasm protein that overstimulates the cell cycle

nucleus

receptor

plasma membrane

signaling pathway

transcription factor nucleus

oncogene

mutated tumor suppressor gene

protein that is unable to inhibit the cell cycle or promote apoptosis

Figure 5.4  Proto-oncogenes mutate to form oncogenes 

Figure 5.5  Mutations in tumor suppressor genes  A mutated

An oncogene codes for a protein that either directly or indirectly overstimulates the cell cycle.

tumor suppressor genes codes for a product that either directly or indirectly fails to inhibit the cell cycle.

an error in replication, or induced by an environmental agent. Later, the altered cell grows and divides to become a population of cells, also called a tumor. The characteristics of cancer cells are discussed more fully in Chapter 25.  When proto-oncogenes mutate (Fig. 5.4), they become cancercausing genes, called oncogenes. For example, the protein product of the RAS proto-oncogene is part of a signal transduction pathway, one of a series of relay proteins (see Fig. 5.3b). When a signaling molecule, such as a growth factor, binds to a receptor on the cell surface, the Ras protein is activated. Activated Ras protein

conveys a message to the nucleus, resulting in gene expression that brings about cell division. If the RAS proto-oncogene is mutated to become an oncogene in such a way that the Ras protein is always activated, no growth factor needs to bind to the cell for the altered Ras protein to send the signal to divide. An altered Ras protein is found in approximately 25% of all tumors. When tumor suppressor genes mutate, their products no longer inhibit the cell cycle (Fig. 5.5). p53 and BRCA1 are both examples of tumor suppressor genes. The p53 protein is such an important protein for regulating the cell cycle that almost half of

SCIENCE IN YOUR LIFE  ►

BIOETHICAL

Genetic Testing for Cancer Genes Genetic tests have become increasingly available for certain cancer genes. If women test positive for defective BRCA1 and BRCA2 genes, they have an increased risk for earlyonset breast and ovarian cancer. If individuals test positive for the APC gene, they are at greater risk for the development of colon cancer. Other genetic tests exist for rare cancers, including retinoblastoma and Wilms tumor. Advocates of genetic testing say that it can alert those who test positive for these mutated genes to undergo more frequent mammograms or colonoscopies. Early detection of cancer clearly offers the best chance for successful treatment. Others feel that genetic testing is unnecessary because nothing can presently be done to prevent the disease. Perhaps it is enough for those who have a family history of cancer to

schedule more frequent checkups, beginning at a younger age. Although the U.S. Equal Employment Opportunity Commission (EEOC) prohibits discrimination based on genetic information, recent surveys suggest that the majority of people remain concerned that being predisposed to cancer might threaten one’s job or health insurance. Individuals opposed to genetic testing suggest that genetic testing be confined to a research setting, especially because it is not known which particular mutations in the genes predispose a person to cancer. They are afraid, for example, that a woman with a defective BRCA1 or BRCA2 gene might make the unnecessary decision to have a bilateral mastectomy. The lack of proper counseling also concerns many. In a study of 177 patients who underwent APC gene testing for susceptibility to colon cancer, less than 20%

received counseling before the test. Moreover, physicians misinterpreted the test results in nearly one-third of the cases. Another concern is that testing negative for a particular genetic mutation may give people the false impression that they are not at risk for cancer. Such a false sense of security can prevent them from having routine cancer screening. Regular testing and avoiding known causes of cancer—such as smoking, a high-fat diet, or too much sunlight (see Chapter 25)— are important for everyone.

Questions to Consider 1. Should genetic testing for cancer be available for everyone, or should genetic testing be confined to a research setting?  2. If genetic testing for cancer were offered to you, would you take advantage of it? Why or why not?



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all human cancers have a mutation in the p53 gene. Mutations in BRCA1 are often associated with breast and ovarian cancer. The Bioethical feature, “Genetic Testing for Cancer Genes,” discusses some of the challenges facing testing for these genes.

Check Your Progress  5.2 1. Explain what factors would cause a cell to stop at each of the three cell cycle checkpoints.

2. Distinguish between the action of oncogenes and mutated tumor suppressor genes.

5.3  Mitosis: Maintaining the Chromosome Number Learning Outcomes Upon completion of this section, you should be able to 1. Explain the role of mitosis and how it maintains the chromosome number of a cell. 2. Summarize the major events that occur during mitosis and cytokinesis. 3. Compare and contrast mitosis and cytokinesis in plant and animal cells.

Eukaryotic chromosomes are composed of chromatin, a combination of both DNA and protein. Some of these proteins are concerned with DNA and RNA synthesis, but a large proportion, termed histones, have an important role in chromosome structure. On average, a human cell contains at least 2 m of DNA. Yet all of this DNA is packed into a nucleus that is about 5 μm in diameter.

centriole

The histones are responsible for packaging the DNA so that it can fit into such a small space. When a eukaryotic cell is not undergoing division, the chromatin is dispersed or extended. This makes the DNA available for gene expression (see Chapter 25). At the time of cell division, chromatin coils, loops, and condenses into a highly compacted form. Each species has a characteristic chromosome number. For instance, human cells contain 46 chromosomes, corn has 20 chromosomes, and the crayfish has 200! This number is called the diploid (2n) number because it contains two (a pair) of each type of chromosome. Humans have 23 pairs of chromosomes. One member of each pair originates from the mother, and the other member originates from the father.  Half the diploid number, called the haploid (n) number of chromosomes, contains only one of each kind of chromosome. In the life cycle of humans, only sperm and eggs have the haploid number of chromosomes.

Overview of Mitosis Mitosis is nuclear division in which the chromosome number stays constant. A 2n nucleus divides to produce daughter nuclei that are also 2n. It would be possible to diagram this as 2n → 2n. Figure 5.6 gives an overview of mitosis; for simplicity, only four chromosomes are depicted. Before nuclear division takes place, DNA replication occurs, duplicating the chromosomes. Each replicated chromosome is composed of two sister chromatids held together in a region called the centromere. Sister chromatids are genetically identical—they contain the same DNA sequences. At the completion of mitosis, each of the chromosomes in the daughter cells consist of a single chromatid, sometimes referred to as a daughter chromosome. Notice that

chromosome duplicated chromosome consisting of two sister chromatids

centromere

DNA REPLICATION

MITOSIS

DURING INTERPHASE

2n = 4

2n = 4

2n = 4

2n = 4

Figure 5.6  Mitosis overview.  Following DNA replication during interphase, each chromosome in the parental nucleus is duplicated and consists of two

sister chromatids. During mitosis, the centromeres divide and the sister chromatids separate, becoming daughter chromosomes that move into the daughter nuclei. Therefore, daughter cells have the same number and kinds of chromosomes as the parental cell. (The blue chromosomes were inherited from one parent, and the red chromosomes were inherited from the other.)



each daughter nucleus gets a complete set of chromosomes and has the same number of chromosomes as the parental cell. This makes the daughter cells genetically identical to each other and to the parental cell. Mitosis is the type of nuclear division that occurs when tissues grow or when repair occurs. Following fertilization, the zygote begins to divide mitotically, and mitosis continues during development and the life span of the individual.

Mitosis in Detail Mitosis is nuclear division that produces two daughter nuclei, each with the same number and kinds of chromosomes as the parental nucleus. During mitosis, a spindle brings about an orderly distribution of chromosomes to the daughter cell nuclei. The spindle contains many fibers, each composed of a bundle of microtubules. Microtubules are able to disassemble and assemble. The centrosome, which is the main microtubule organizing center of the cell, divides during late interphase. It is believed that centrosomes are responsible for organizing the spindle. In animal cells, each centrosome contains a pair of barrel-shaped organelles called centrioles and an aster, which is an array of short microtubules that radiate from the centrosome. The fact that plant cells lack centrioles suggests that centrioles are not required for spindle formation.

Mitosis in Animal Cells Mitosis is a continuous process that is arbitrarily divided into several phases for convenience of description: prophase, prometaphase, metaphase, anaphase, and telophase (Fig. 5.7).

Prophase  It is apparent during early prophase that cell division is about to occur. The centrosomes begin moving away from each other toward opposite ends of the nucleus. Spindle fibers appear between the separating centrosomes as the nuclear envelope begins to fragment, and the nucleolus begins to disappear. The chromatin condenses and the chromosomes are now visible. Each is duplicated and composed of sister chromatids held together at a centromere. The spindle begins forming during late prophase. Prometaphase  During prometaphase, preparations for sister chromatid separation are evident. Kinetochores appear on each side of the centromere, and these attach sister chromatids to the kinetochore spindle fibers. These fibers extend from the poles to the chromosomes, which will soon be located at the center of the spindle.  The kinetochore fibers attach the sister chromatids to opposite poles of the spindle, and the chromosomes are pulled first toward one pole and then toward the other before the chromosomes come into alignment. Notice that even though the chromosomes are attached to the spindle fibers in prometaphase, they are still not in alignment. Metaphase  By the time of metaphase, the fully formed spindle consists of poles, asters, and fibers. The metaphase plate is a plane perpendicular to the axis of the spindle and equidistant from the

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poles. The chromosomes attached to centromeric spindle fibers line up at the metaphase plate during metaphase. Polar spindle fibers reach beyond the metaphase plate and overlap.

Anaphase  At the beginning of anaphase, the centromeres uniting the sister chromatids divide. Then the sister chromatids separate, becoming daughter chromosomes that move toward the opposite poles of the spindle. Daughter chromosomes have a centromere and a single chromatid. What accounts for the movement of the daughter chromosomes? First, the kinetochore spindle fibers shorten, pulling the daughter chromosomes toward the poles. Second, the polar spindle fibers push the poles apart as they lengthen and slide past one another. Telophase  During telophase, the spindle disappears, and nuclear envelope components reassemble around the daughter chromosomes. Each daughter nucleus contains the same number and kinds of chromosomes as the original parental cell. Remnants of the polar spindle fibers are still visible between the two nuclei. The chromosomes become more diffuse once again, and a nucleolus appears in each daughter nucleus. Cytokinesis is under way, and soon there will be two individual daughter cells, each with a nucleus that contains the diploid number of chromosomes.

Mitosis in Plant Cells As with animal cells, mitosis in plant cells permits growth and repair. A particular plant tissue called meristematic tissue retains the ability to divide throughout the life of a plant. Meristematic tissue is found at the root tip and also at the shoot tip of stems. Lateral meristematic tissue accounts for the ability of trees to increase their girth each growing season. Figure 5.7 also illustrates mitosis in plant cells. Exactly the same phases occur in plant cells as in animal cells. Although plant cells have a centrosome and spindle, there are no centrioles or asters during cell division. The spindle still brings about the distribution of the chromosomes to each daughter cell.

Cytokinesis in Animal and Plant Cells Cytokinesis, or cytoplasmic cleavage, usually accompanies mitosis, but they are separate processes. Division of the cytoplasm begins in anaphase and continues in telophase but does not reach completion until just before the next interphase. By that time, the newly forming cells have received a share of the cytoplasmic organelles that duplicated during the previous interphase.

Cytokinesis in Animal Cells As anaphase draws to a close in animal cells, a cleavage furrow, which is an indentation of the membrane between the two daughter nuclei, begins to form. The cleavage furrow deepens when a band of actin filaments, called the contractile ring, slowly forms a constriction between the two daughter cells. The action of the contractile ring can be likened to pulling a drawstring ever tighter about the middle of a balloon, causing the balloon to constrict in the middle.



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centrosome has centrioles

MITOSIS

Animal Cell at Interphase

250×

250×

450×

duplicated chromosome

aster nuclear envelope fragments

spindle pole

kinetochore

centromere

chromatin condenses

nucleolus disappears

kinetochore spindle fiber

spindle fibers forming

Early Prophase Centrosomes have duplicated. Chromatin is condensing into chromosomes, and the nuclear envelope is fragmenting.

polar spindle fiber

Prophase Nucleolus has disappeared, and duplicated chromosomes are visible. Centrosomes begin moving apart, and spindle is in process of forming.

Prometaphase The kinetochore of each chromatid is attached to a kinetochore spindle fiber. Polar spindle fibers stretch from each spindle pole and overlap.

centrosome lacks centrioles

Plant Cell at Interphase

400×

cell wall

chromosomes

900×

spindle pole lacks centrioles and aster

500×

Figure 5.7  Phases of mitosis in animal and plant cells.  Following interphase, the steps of mitosis include prophase; prometaphase, metaphase, anaphase, and telophase. The blue chromosomes were inherited from one parent, and the red from the other parent.



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250×

250×

chromosomes at metaphase plate

250×

daughter chromosome

cleavage furrow

nucleolus

kinetochore spindle fiber

Metaphase Centromeres of duplicated chromosomes are aligned at the metaphase plate (center of fully formed spindle). Kinetochore spindle fibers attached to the sister chromatids come from opposite spindle poles.

spindle fibers

900×

Anaphase Sister chromatids part and become daughter chromosomes that move toward the spindle poles. In this way, each pole receives the same number and kinds of chromosomes as the parent cell.

900×

Telophase Daughter cells are forming as nuclear envelopes and nucleoli reappear. Chromosomes will become indistinct chromatin.

cell plate

900×



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UNIT 1  Cell Biology

A narrow bridge between the two cells can be seen during telophase, and then the contractile ring continues to separate the cytoplasm until there are two daughter cells (Fig. 5.8).

Cytokinesis in Plant Cells Cytokinesis in plant cells occurs by a process different from that seen in animal cells (Fig. 5.9). The rigid cell wall that surrounds plant cells does not permit cytokinesis by furrowing. Instead, cytokinesis in plant cells involves building new cell walls between the daughter cells. Cytokinesis is apparent when a small, flattened disk appears between the two daughter plant cells. Electron micrographs reveal that the disk is at right angles to a set of microtubules. The Golgi apparatus produces vesicles, which move along the microtubules to the region of the disk. As more vesicles arrive and fuse, a cell plate can be seen. The membrane of the vesicles completes the plasma

cytoplasm

cell plate cell membrane

24,000×

Figure 5.9  Cytokinesis in plant cells.  During cytokinesis in a plant cell, a cell plate forms midway between two daughter nuclei and extends to the plasma membrane.

4,000× cleavage furrow

membrane for both cells, and they release molecules that form the new plant cell walls. The cell walls are later strengthened by the addition of cellulose fibrils.

Check Your Progress  5.3 1. Distinguish between the number of chromatids each

contractile ring

chromosome contains before and after replication.

2. Summarize the events that are occurring in each stage of mitosis.

3. Compare and contrast cytokinesis in plant and animal cells.

5.4  Meiosis: Reducing the Chromosome Number Learning Outcomes 4,000×

Figure 5.8  Cytokinesis in animal cells.  A single cell becomes two cells by a furrowing process. A contractile ring composed of actin filaments gradually gets smaller, and the cleavage furrow pinches the cell into two cells.

Upon completion of this section, you should be able to 1. Summarize the purpose of meiosis. 2. Explain what is meant by the term homologous chromosomes. 3. Describe the events of meiosis.

Meiosis occurs in any life cycle that involves sexual reproduction. Meiosis reduces the chromosome number in such a way that the daughter nuclei receive only one of each kind of chromosome. In other words, meiosis reduces the chromosome number of a cell from diploid (2n) to haploid (n). The process of meiosis ensures



Chapter 5  Cell Division

that as a result of sexual reproduction the next generation of individuals will have the diploid number of chromosomes and a combination of traits different from that of either parent.

Overview of Meiosis At the start of meiosis, the parental cell has the diploid number of chromosomes. In Figure 5.10, the diploid number is 4, which you can verify by counting the number of centromeres. Notice that meiosis requires two cell divisions, meiosis I and meiosis II, and that the four daughter cells have the haploid number of chromosomes. DNA replication occurs prior to meiosis I.

centromere

homologous chromosome pair

nucleolus centrioles homologous chromosome pair

2n = 4

DNA REPLICATION synapsis

2n = 4 sister chromatids MEIOSIS I Homologues synapse and then separate.

89

Recall that when a cell is 2n, or diploid, the chromosomes occur in pairs. For example, the 46 chromosomes of humans occur in 23 pairs. In Figure 5.10, there are two pairs of chromosomes. The members of a pair are called homologous chromosomes (or homologues). In the diagrams in this text, homologues have the same size but are indicated by d­ ifferent colors—the blue chromosomes are inherited from one parent, and the red are inherited from the other.

Meiosis I During meiosis I, the homologous chromosomes come together and line up side by side. This process is called synapsis and it results in an association of four chromatids that stay in close proximity during the first two phases of meiosis I. Because of synapsis, there are pairs of homologous chromosomes at the metaphase plate during meiosis I. Notice that only during meiosis I is it possible to observe paired chromosomes at the metaphase plate. When the members of these pairs separate, each daughter nucleus receives one member of each pair. Therefore, each daughter cell now has the haploid number of chromosomes, as you can verify by counting its centromeres.

Meiosis II and Fertilization Replication of DNA does not occur between meiosis I and meiosis II because the chromosomes are still duplicated (composed of two sister chromatids). During meiosis II, the centromeres divide and the sister chromatids separate, becoming daughter chromosomes that are distributed to daughter nuclei. In the end, each of four daughter cells has the haploid number of chromosomes, and each chromosome consists of one chromatid. In some life cycles, such as that of humans (see Fig. 5.16), the daughter cells mature into gametes (sex cells—sperm and egg) that fuse during fertilization. Fertilization restores the diploid number of chromosomes in a cell that will develop into a new individual. If the gametes carried the diploid instead of the haploid number of chromosomes, the chromosome number would double with each fertilization.

Meiosis in Detail Meiosis, which requires two nuclear divisions, results in four daughter nuclei, each having one of each kind of chromosome and therefore half the number of chromosomes as the parental cell.

MEIOSIS II Sister chromatids separate, becoming daughter chromosomes.

First Division

n=2

n=2

Figure 5.10  Overview of meiosis.  Following DNA replication, each chromosome is duplicated. During meiosis I, the homologous chromosomes pair during synapsis and then separate. During meiosis II, the centromeres divide and the sister chromatids separate, becoming daughter chromosomes that move into the daughter nuclei.

The phases of meiosis I for an animal cell are diagrammed in Figure 5.11. Meiosis helps ensure that genetic variation of the parental genes occurs through two key events: crossing-over and independent assortment of chromosome pairs. Because the members of a homologous pair can carry slightly different instructions for the same genetic trait (see Chapter 23), these events are significant. For example, one homologue may carry instructions for brown eyes, while the corresponding homologue may carry instructions for blue eyes. Fertilization ensures the offspring will have different combinations of genes from either parent.



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Figure 5.11  Meiosis I in an animal cell.  Meiosis I introduces genetic variation by the process of crossing over (indicated by the exchange of color between the nonsister chromatids). Note that the daughter cells only possess one copy of each chromosome.

2n = 4

DNA REPLICATION MEIOSIS I Prophase I Homologous chromosomes pair during synapsis.

Metaphase I Homologous chromosome pairs align at the metaphase plate.

Anaphase I Homologous chromosomes separate, pulled to opposite poles by centromeric spindle fibers.

Telophase I Daughter cells have one chromosome from each homologous pair.

Prophase I  In prophase I, synapsis occurs, and then the spindle appears while the nuclear envelope fragments and the nucleolus disappear. Figure 5.12 shows that during synapsis, the homologous chromosomes come together and line up side by side. Now an exchange of genetic material may occur between the nonsister chromatids of the homologues. This exchange is called crossing-over. Crossing-over means that the chromatids held together by a centromere are no longer identical. As a result of crossing-over, the daughter cells receive chromosomes with recombined genetic material. In Figures 5.11 and 5.12, crossingover is represented by an exchange of color between the chromosomes. Because of crossing-over, the chromosomes in the daughter cells have a different combination of genetic material than the parental cell. Metaphase I and Anaphase I  During metaphase I, the homologues align at the metaphase plate. Depending on how they align, the maternal or paternal member of each pair may be oriented toward either pole. Independent assortment occurs when these homologous pairs separate from each other during anaphase I, generating cells with different combinations of maternal and paternal chromosomes. Notice that each chromosome still consists of two sister chromatids. Figure 5.13 shows two possible orientations for a cell that contains only two pairs of chromosomes (2n = 4). Gametes from the first cell, but not the other, will have two paternal or two maternal chromosomes. The gametes from the other cell will have different combinations of maternal and paternal chromosomes. When all possible orientations are

sister chromatids

n=2 Interkinesis Chromosomes still consist of two chromatids.

crossing-over synapsis between nonsister of homologues chromatids

chromatids after exchange

recombinant daughter chromosomes

Figure 5.12  Synapsis and crossing-over.  During prophase I,

from left to right, duplicated homologous chromosomes undergo synapsis when they line up with each other. During crossing-over, nonsister chromatids break and then rejoin. The two resulting daughter chromosomes will have a different combination of genetic material than they had before.



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91

Figure 5.13  Independent assortment.  Two possible orientations of homologous chromosome pairs at the metaphase plate are shown for metaphase I. Each of these will result in daughter nuclei with a different combination of parental chromosomes (independent assortment). In a cell with two pairs of homologous chromosomes, there are 22 possible combinations of parental chromosomes in the daughter nuclei.

considered for a cell containing two pairs of chromosomes, the result will be 22, or four, possible combinations of maternal and paternal chromosomes in the resulting gametes from this cell. In humans, where there are 23 pairs of chromosomes, the number of possible chromosomal combinations in the gametes is a staggering 223, or 8,388,608. And this does not even consider the genetic recombinations that are introduced due to crossing-over. Together, prophase I, metaphase I, and anaphase I introduce the genetic variation that helps ensure that no two offspring have the same combination of genes as the parents.

Telophase I  In some species, including humans, telophase I occurs at the end of meiosis I. If so, the nuclear envelopes re-form, and nucleoli reappear. This phase may or may not be accompanied by cytokinesis, which is separation of the cytoplasm. Interkinesis  The period of time between meiosis I and meiosis II is called interkinesis.  Because the chromosomes are already duplicated, there is no replication of DNA during interkinesis. The daughter cells of meiosis I then proceed directly into meiosis II.

Second Division Phases of meiosis II for an animal cell are diagrammed in Figure 5.14. At the beginning of prophase II, a spindle appears while the nuclear envelope disassembles and the nucleolus disappears. Each duplicated chromosome attaches to the spindle, and then they align at the metaphase plate during metaphase II. During anaphase II, sister chromatids separate, becoming daughter chromosomes that move into the daughter nuclei. In telophase II, the spindle disappears as the nuclear envelope re-forms.

During the cytokinesis that can follow meiosis II, the plasma membrane furrows to produce two complete cells, each of which has the haploid number of chromosomes. Because each cell from meiosis I undergoes meiosis II, there are four daughter cells altogether.

The Importance of Meiosis Meiosis produces haploid cells that are genetically different than the original diploid parent cell. The introduction of this variation occurs in two ways. First, during prophase I, crossingover between nonsister chromatids of the paired homologous chromosomes rearranges genes, with the result that the sister chromatids of each homologue may no longer be identical. ­Second, the independent assortment of chromosomes during anaphase I means that the gametes produced by meiosis II may  have different combinations of chromosomes than the parental cell. Upon fertilization, the combining of chromosomes from genetically different gametes, even if the gametes are from a single individual (as in many plants), helps ensure that offspring are not identical to their parents. This genetic variability is the main advantage of sexual reproduction. The staggering amount of genetic variation achieved through meiosis is particularly important to the long-term survival of a species because it increases genetic variation within a population. If the environment changes, genetic variability among offspring introduced by sexual reproduction may be advantageous. Under the new conditions, some offspring may have a better chance of survival and reproductive success than others in a population.



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MEIOSIS II

n=2

n=2

Prophase II Cells have one chromosome from each homologous pair.

Metaphase II Chromosomes align at the metaphase plate.

Anaphase II Daughter chromosomes move toward the poles.

Telophase II Spindle disappears, nuclei form, and cytokinesis takes place.

n=2

n=2 Daughter Cells Meiosis results in four haploid daughter cells.

Figure 5.14  Meiosis II in an animal cell.  During meiosis II, sister chromatids separate, becoming daughter chromosomes that are distributed to the

daughter nuclei. Following meiosis II, there are four haploid daughter cells. Comparing the number of centromeres in each daughter cell with the number in the parental cell at the start of meiosis I (see Fig. 5.12) verifies that each daughter cell is haploid.

Check Your Progress  5.4 1. Explain why there are two divisions in meiosis and why

the DNA is not replicated between meiosis I and meiosis II. 2. Outline the major events that occur during meiosis I and II, focusing on the activities of the chromosomes. 3. Explain how crossing-over and independent assortment introduce genetic variation.

5.5  Comparison of Meiosis with Mitosis Learning Outcomes Upon completion of this section, you should be able to 1. Compare and contrast the processes of meiosis and mitosis. 2. Identify the differences in the behavior of homologous chromosomes in meiosis and mitosis.



Chapter 5  Cell Division

Meiosis I

MEIOSIS

MITOSIS Prophase I Synapsis and crossing-over occur.

Prophase No synapsis.

Metaphase I Homologues align independently.

Metaphase Chromosomes align at the metaphase plate.

Anaphase I Homologues separate.

Anaphase Sister chromatids separate.

Telophase Daughter cells form.

Telophase I Daughter cells form.

Meiosis II

93

Sister chromatids separate.

Daughter cells are not genetically identical to parental cell.

Daughter cells are genetically identical to parental cell.

Figure 5.15  Meiosis I compared to mitosis.  In meiosis metaphase I, homologues are paired at the metaphase plate, but in mitosis metaphase, all chromosomes align individually at the metaphase plate. Individual homologues separate during meiosis anaphase I, so that the daughter cells are haploid.

Figure 5.15 compares meiosis to mitosis. Notice the following differences: ■■

■■ ■■

DNA replication takes place only once prior to either meiosis or mitosis. However, meiosis requires two nuclear divisions, whereas mitosis requires only one nuclear division. Meiosis followed by cytokinesis produces four daughter cells. Mitosis followed by cytokinesis results in two daughter cells. The four daughter cells following meiosis are haploid and have half the chromosome number as the parental cell. The daughter cells following mitosis have the same chromosome number as the parental cell.

■■

The daughter cells resulting from meiosis are not genetically identical to each other or to the parental cell. The daughter cells resulting from mitosis are genetically identical to each other and to the parental cell.

The specific differences between these nuclear divisions can be categorized according to when they occur and the processes involved.

Occurrence Meiosis occurs only at certain times in the life cycle of sexually reproducing organisms. In humans, meiosis occurs only in the



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UNIT 1  Cell Biology

TABLE 5.1  Comparison of Meiosis I with Mitosis

TABLE 5.2  Comparison of Meiosis II with Mitosis

Meiosis I

Mitosis

Meiosis II

Mitosis

Prophase I Pairing of homologous chromosomes

Prophase No pairing of chromosomes

Prophase II No pairing of chromosomes

Prophase No pairing of chromosomes

Metaphase I Homologous duplicated chromosomes at metaphase plate

Metaphase Duplicated chromosomes at metaphase plate

Metaphase II Haploid number of duplicated chromosomes at metaphase plate

Metaphase Duplicated chromosomes at metaphase plate

Anaphase I Homologous chromosomes separate.

Anaphase Sister chromatids separate, becoming daughter chromosomes that move to the poles.

Anaphase II Sister chromatids separate, becoming daughter chromosomes that move to the poles.

Anaphase Sister chromatids separate, becoming daughter chromosomes that move to the poles.

Telophase I Two haploid daughter cells

Telophase/Cytokinesis Two daughter cells, identical to the parental cell

Telophase II Four haploid daughter cells, not identical to parental cell

Telophase/Cytokinesis Two daughter cells, identical to the parental cell

reproductive organs and produces the gametes. Mitosis occurs almost continuously in all tissues during growth and repair.

Processes To summarize the differences in processes, Tables 5.1 and 5.2 separately compare meiosis I and meiosis II to mitosis.

5.6  The Human Life Cycle Learning Outcomes Upon completion of this section, you should be able to 1. Describe the human life cycle in terms of haploid and diploid cells. 2. Explain the process of gamete production in both males and females.

Comparison of Meiosis I to Mitosis Notice that these events distinguish meiosis I from mitosis: ■■ ■■

■■

Homologous chromosomes pair and undergo crossing-over during prophase I of meiosis, but not during mitosis. Paired homologous chromosomes align at the metaphase plate during metaphase I in meiosis. These paired chromosomes have four chromatids altogether. Individual chromosomes align at the metaphase plate during metaphase in mitosis. They each have two chromatids. Homologous chromosomes (with centromeres intact) separate and move to opposite poles during anaphase I in meiosis. Centromeres split, and sister chromatids, now called daughter chromosomes, move to opposite poles during anaphase in mitosis.

Comparison of Meiosis II to Mitosis The events of meiosis II are just like those of mitosis, except that  in meiosis II, the nuclei contain the haploid number of chromosomes.

Check Your Progress  5.5 1. Describe the differences in the activity of the homologous

chromosomes in prophase and metaphase of meiosis I and mitosis. 2. Compare and contrast the outputs of meiosis and mitosis with regards to the number of cells and the chromosome complement of each cell.

The human life cycle requires both meiosis and mitosis (Fig.  5.16). A haploid sperm (n) and a haploid egg (n) join at fertilization, and the resulting zygote has the full, or diploid (2n), number of chromosomes. During development of the fetus before birth, mitosis keeps the chromosome number constant in all the cells of the body. After birth, mitosis is involved in the continued growth of the child and repair of tissues at any time. As a result of mitosis, each somatic cell in the body has the same number of chromosomes.

Spermatogenesis and Oogenesis in Humans In human males, meiosis is a part of spermatogenesis, which occurs in the testes and produces sperm. In human females, meiosis is a part of oogenesis, which occurs in the ovaries and produces eggs (Fig. 5.17). Further description of the human reproductive system can be found in Chapter 21.

Spermatogenesis Following puberty, the time of life when the sex organs mature, spermatogenesis is continual in the testes of human males. As many as 300,000 sperm are produced per minute, or 400 million per day. During spermatogenesis, primary spermatocytes, which are diploid, divide during the first meiotic division to form two secondary spermatocytes, which are haploid. Secondary spermatocytes divide during the second meiotic division to produce four



95

Chapter 5  Cell Division

MITOSIS

SPERMATOGENESIS

2n

primary spermatocyte

2n 2n MITOSIS

2n

Meiosis I

2n

secondary spermatocytes n

Meiosis II

spermatids n zygote Metamorphosis and maturation

2n = 46 diploid (2n) MEIOSIS

haploid (n) n = 23

FERTILIZATION

sperm n

n

n egg

OOGENESIS primary oocyte

sperm

2n Meiosis I

Figure 5.16   Life cycle of humans.  Meiosis in males is a part of

sperm production, and meiosis in females is a part of egg production. When a haploid sperm fertilizes a haploid egg, the zygote is diploid. The zygote undergoes mitosis as it develops into a newborn child. Mitosis continues throughout life during growth and repair.

first polar body n Fertilization

secondary oocyte n

Meiosis II

spermatids, which are also haploid. What is the difference between the chromosomes in haploid secondary spermatocytes and those in haploid spermatids? The chromosomes in secondary spermatocytes are duplicated and consist of two chromatids, whereas those in spermatids consist of only one chromatid. Spermatids then mature into sperm (spermatozoa).

Oogenesis Meiosis in human females begins in the fetus. In the ovaries of the fetus, all of the primary oocytes (diploid cells) begin meiosis but become arrested in prophase I. Following puberty and the initiation of the menstrual cycle, one primary oocyte begins to complete meiosis. It finishes the first meiotic division as two cells, each of which is haploid, although the chromosomes are still duplicated. One of these cells, termed the secondary oocyte, receives almost all of the cytoplasm. The other is the first polar body, a nonfunctioning cell. The polar body contains duplicated chromosomes but very little cytoplasm and may or may not divide again. Eventually

second polar body

Meiosis II is completed after entry of sperm

n egg sperm nucleus

n

n

zygote 2n

Figure 5.17  Spermatogenesis and oogenesis in mammals. 

Spermatogenesis produces four viable sperm, whereas oogenesis produces one egg and at least two polar bodies. In humans, both the sperm and the egg have 23 chromosomes each. Therefore, following fertilization, the zygote has 46 chromosomes.



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UNIT 1  Cell Biology

it disintegrates. If the secondary oocyte is fertilized by a sperm, it completes the second meiotic division, in which it again divides unequally, forming an egg and a second polar body. The chromosomes of the egg and sperm nuclei then join to form the 2n zygote. If the secondary oocyte is not fertilized by a sperm, it disintegrates and passes out of the body with the menstrual flow.

Check Your Progress  5.6 1. Summarize the cells in the human body that are haploid and the role these cells play in the life cycle.

2. Contrast the number of sperm that are produced from one primary spermatocyte and the number of eggs produced from one oocyte.

Conclusion Approximately 10–15% of the women who are diagnosed with breast cancer have a hereditary form of the disease. This means that they inherited a genetic mutation that increases their risk of developing cancer. A genetic mutation does not guarantee that they will develop cancer, nor does it determine when or where they may develop cancer, if they do. Many of these mutations occur in proto-oncogenes or tumor suppressor genes. In Angelina Jolie’s case, she inherited a mutated BRCA1 gene. The BRCA1 gene is a tumor suppressor gene whose protein product is involved in DNA repair. Tumor suppressor genes act as gatekeepers for the cell cycle and thus control the rate at which cells divide.

MEDIA STUDY TOOLS http://connect.mheducation.com Maximize your study time with McGraw-Hill SmartBook®, the first adaptive textbook.

 

MP3 Files

5.3 Mitosis 5.4 Meiosis

 

Animations

5.1  How the Cell Cycle Works 5.2  Cell Proliferation Signaling Pathway • Control of the Cell Cycle • How Tumor Suppressor Genes Block Cell Division 5.3  Mitosis • Cytokinesis 5.4  How Meiosis Works • Meiosis I • Meiosis with Crossing-Over • Random Orientation of Chromosomes During Meiosis • Meiosis II 5.5  Comparison of Meiosis and Mitosis

SUMMARIZE 5.1  The Cell Cycle ■ Cell division increases the number of somatic cells in the body, and

apoptosis reduces this number when appropriate. Cells go through a cell cycle that includes (1) interphase and (2) cell division, consisting of mitosis and cytokinesis. Interphase, in turn, includes G1 (growth as certain organelles double), S (DNA synthesis), and G2 (growth as the cell prepares to divide). Cell division occurs during the mitotic stage (M) when daughter cells receive a full complement of chromosomes. ■ During S phase, DNA replication occurs. Each chromatid is copied, forming sister chromatids.

 

3D Animations

5.1  Cell Cycle and Mitosis: Interphase 5.2  Cell Cycle and Mitosis: Checkpoints 5.3  Cell Cycle and Mitosis: Mitosis • Cell Cycle and Mitosis: Cytokinesis 5.4  Meiosis: Interphase and Meiosis I • Meiosis: Meiosis II • Meiosis: Genetic Diversity

  Tutorials 5.2  Proto-oncogenes • Tumor Suppressor Genes 5.3 Mitosis 5.4 Meiosis

5.2  Control of the Cell Cycle ■ Internal and external signals control the cell cycle, which can stop at

any of three checkpoints: in G1 prior to the S stage, in G2 prior to the M stage, and near the end of mitosis. DNA damage is one reason the cell cycle stops. The p53 protein is active at the G1 checkpoint, and if DNA is damaged and can’t be repaired, this protein initiates apoptosis. ■ Both proto-oncogenes and tumor suppressor genes produce proteins that control the cell cycle. Proto-oncogenes encode proteins that promote the cell cycle and prevent apoptosis. RAS is an example of a proto-oncogene. Mutations in proto-oncogenes create oncogenes, which may cause cancer. When tumor suppressor genes mutate, their products no longer inhibit the cell cycle. p53 and BRCA1 are tumor suppressor genes.



Chapter 5  Cell Division

5.3  Mitosis: Maintaining the Chromosome Number ■ Each species has a characteristic number of chromosomes. The total

number is the diploid (2n) number, and half this number is the haploid (n) number. Among eukaryotes, cell division involves nuclear division and division of the cytoplasm (cytokinesis). ■ Replication of the chromatids precedes cell division. The duplicated chromosome is composed of two sister chromatids held together at a centromere. During mitosis, the centromeres divide, and daughter chromosomes go into each new nucleus. ■ Mitosis has the following phases: prophase,  when chromosomes condense and the nuclear envelope dissolves; prometaphase, when the chromosomes are attached to spindle fibers; metaphase, when the chromosomes are aligned at the metaphase plate; anaphase, when the chromatids separate, becoming daughter chromosomes that move toward the poles; and telophase, when new nuclear envelopes form around the daughter chromosomes and cytokinesis is well under way. ■ Cytokinesis differs between plant and animal cells. Plant cells form a cell plate between the daughter cells, while animal cells develop a cleavage furrow that separates the daughter cells.

5.4  Meiosis: Reducing the Chromosome Number ■ Meiosis occurs in any life cycle that involves sexual reproduction.

The end result of meiosis is daughter cells with the haploid number of homologous chromosomes (homologues). In some life cycles, the daughter cells become gametes, and upon fertilization, the offspring have the diploid number of chromosomes, the same as their parents. Crossing-over and independent assortment of chromosomes during meiosis I ensure genetic variation in daughter cells. ■ Meiosis utilizes two nuclear divisions. During meiosis I, homologous chromosomes undergo synapsis, and crossing-over between nonsister chromatids occurs. When the homologous chromosomes separate during meiosis I, each daughter nucleus receives one member from each pair of chromosomes. Therefore, the daughter cells are haploid. Interkinesis during meiosis I and meiosis II does not include DNA replication. Distribution of daughter chromosomes derived from sister chromatids during meiosis II then leads to a total of four new cells, each with the haploid number of chromosomes.

5.5  Comparison of Meiosis with Mitosis ■ While mitosis and meiosis differ in the number of cells produced, and

the chromosome complement of each cell, there are many similarities in the processes. The primary difference is the behavior of the homologous chromosomes.

5.6  The Human Life Cycle ■ The human life cycle involves both mitosis and meiosis. Mitosis

ensures that each somatic cell has the diploid number of chromosomes. In humans, meiosis is a part of spermatogenesis and oogenesis. Spermatogenesis in males produces four viable sperm, whereas oogenesis in females produces one egg and polar bodies. Oogenesis does not go on to completion unless a sperm fertilizes the secondary oocyte. Fertilization produces a zygote, which grows by mitosis.

97

ASSESS Testing Yourself Choose the best answer for each question.

5.1  The Cell Cycle 1. Label this drawing of the cell cycle and briefly summarize what is happening at each stage. Interphase c. a. b.

e.

d.

f.

2. Apoptosis: a. decreases the number of cells in the body b. is programmed cell death c. is a natural process d. All of these are correct.

5.2  Control of the Cell Cycle 3. At which of the following checkpoints is the DNA checked for damage and, if damage is present, the cell is placed in G0 phase? a. M c. G2 b. G1 d.  None of these are correct. 4. Where is the checkpoint that assesses the DNA for damage? a. G1 c. G2 b. S d. M 5. Which of the following act as the brakes of the cell cycle and prevent cells from dividing too quickly? a. oncogenes b. proto-oncogenes c. cyclins d. tumor suppressor genes

5.3  Mitosis: Maintaining the Chromosome Number 6. Which of these is paired incorrectly? a. prometaphase—the kinetochores become attached to spindle fibers b. anaphase—daughter chromosomes migrate toward spindle poles c. prophase—the chromosomes condense and the nuclear envelope disintegrates d. metaphase—the chromosomes are aligned at the metaphase plate e. telophase—a resting phase between cell division cycles 7. If a parent cell has a diploid number of 18 chromosomes before mitosis, how many chromosomes will the daughter cells have? a. 9 d. 36 b. 18  e. 64 c. 27



98

UNIT 1  Cell Biology

8. Cytokinesis in animal cells involves the formation of a/an: a. oncogene b. cell plate c. cleavage furrow d. sister chromatid

5.4  Meiosis: Reducing the Chromosome Number 9. If a parent cell has 22 chromosomes, the daughter cells following meiosis II will have a. 22 chromosomes. b. 44 chromosomes. c. 11 chromosomes. d. All of these are correct. 10. Crossing-over occurs between a. sister chromatids of the same chromosome. b. chromatids of nonhomologous chromosomes. c. nonsister chromatids of a homologous pair. d. None of these are correct. 11. Which of these helps provide genetic diversity? a. independent alignment during metaphase I b. crossing-over during prophase I c. random fusion of sperm and egg nuclei during fertilization d. All of these are correct.

5.5  Comparison of Meiosis with Mitosis 12. The pairing of homologous chromosomes occurs during which of the following? a. mitosis  b. meiosis I c. meiosis II  d. All of these are correct. 13. Sister chromatids separate during anaphase of which of the following? a. mitosis  b. meiosis I c. meiosis II  d. Both a and c are correct.

5.6  The Human Life Cycle 14. Polar bodies are produced during a. DNA replication. d. oogenesis. b. mitosis. e. None of these are correct. c. spermatogenesis. 15. Following fertilization, the somatic cells of humans divide by: a. apoptosis c. mitosis b. meiosis d. DNA replication

ENGAGE BioNOW Want to know  how this science is relevant to your life? Check out the BioNow video below: ■ Cell Division

Thinking Critically 1. BPA is a chemical compound that has historically been used in the manufacture of plastic products. However, cells often mistake BPA compounds for hormones that accelerate the cell cycle. Because of this, BPA is associated with an increased risk of certain cancers.  a. How might BPA interact with the cell cycle and its checkpoints?  b. Why do you think that very small concentrations of BPA might have a large effect on the cell?  2. How would a nonfunctional cell cycle checkpoint lead to carcinogenesis? 3. From an evolutionary standpoint, why is sexual reproduction advantageous for the continuation of a species?

PHOTO CREDITS Opener: © MRP/Alamy; 5.2: © Steve Gschmeissner/Science Source; 5.7 animal cells(early prophase, prophase, metaphase, anaphase, telophase): © Ed Reschke; 5.7(prometaphase): © Michael Abbey/Science Source; 5.7 plant cells(early prophase, prometaphase): © Ed Reschke; 5.7(prophase, metaphase, anaphase, telophase): © Kent Wood/Science Source; 5.8(top): © National Institutes of Health (NIH)/USHHS; 5.8(bottom): © Steve Gschmeissner/ SPL/Getty RF; 5.9: © Biophoto Associates/Science Source.

CASE STUDY Vampire Bats Vampire bats were inspiration for the many legends about vampires. Vampire bats do exist, but they are not as frightening as legends would have you believe. Vampire bats are not large animals and they do not normally attack humans. The body of the common vampire bat, Desmodus rotundus, is about the size of a human thumb with a wingspan of only eight inches. These bats normally feed on cattle, horses, and pigs. Also, rather than “suck” the blood of their animal host the bats make small cuts in the skin of the animal and lap up the blood that flows from the injury. Scientists have long known that vampire bat saliva has an amazing ability to dissolve blood clots, allowing the blood to continue to flow from the wound while the bat feeds. Normally, blood clots due to the action of fibrin, an insoluble protein in the blood plasma. Fibrin is dissolved by the enzyme plasmin, which circulates in the blood in an inactive form called plasminogen. Another enzyme activates plasminogen, which is converted to plasmin, and then dissolves the fibrin of the clot. Researchers have discovered an enzyme in the saliva of the vampire bat that is 150 times more potent at activating plasminogen—and thus dissolving clots—than any known drug. This enzyme may one day be used to treat victims of stroke, caused when a clot blocks blood supply to the brain. This chapter describes the general characteristics and functions of enzymes, and how enzymes function in the flow of energy and metabolism. Each enzyme in our body is responsible for a unique metabolic reaction, as illustrated by the vampire bat enzyme, which is responsible for converting plasminogen into plasmin. As you read through the chapter, think about the following questions:

1. What is the role of an enzyme in a metabolic pathway? 2. What environmental factors influence the rate of enzyme activity?

6

Metabolism: Energy and Enzymes CHAPTER OUTLINE 6.1  Life and the Flow of Energy 6.2 Energy Transformations

and Metabolism 6.3 Enzymes and Metabolic Pathways 6.4 Oxidation-Reduction Reactions and Metabolism

BEFORE YOU BEGIN Before beginning this chapter, take a few moments to review the following discussions: Section 1.1  What is the relationship between energy and the basic characteristics of life? Section 2.7  How does the shape of a protein relate to its function? Section 4.2  In what ways does energy play a role in the movement of molecules across a plasma membrane?

99

100

UNIT 1  Cell Biology

6.1  Life and the Flow of Energy

Two Laws of Thermodynamics

Learning Outcomes Upon completion of this section, you should be able to 1. Describe the different forms of energy. 2. Summarize the two laws of thermodynamics and how these laws apply to cells.

Energy is the ability to do work or bring about a change. In order to maintain their organization and carry out metabolic activities, cells as well as organisms need a constant supply of energy. This energy allows living organisms to carry on the processes of life, including growth, development, response to stimuli, metabolism, and reproduction. The majority of organisms get their energy from organic nutrients produced by photosynthesizers (algae, plants, and some bacteria). Therefore, life on Earth is ultimately dependent on solar energy.

Figure 6.1 illustrates the flow of energy in a terrestrial ecosystem. Through photosynthesis, plants capture a small portion of the incoming solar energy. This is stored in the chemical bonds of organic molecules, such as carbohydrates. When plants use their carbohydrates (through cellular respiration), some of the energy dissipates as heat. In an ecosystem, plants serve as the food source for other organisms, such as moose. The cellular respiration pathways in the moose use these carbohydrates as energy, once again releasing energy as heat. All living organisms metabolize organic nutrient molecules, and eventually, all the captured solar energy dissipates as heat. Therefore, energy flows through an ecosystem. It does not recycle.  Two laws of thermodynamics explain why energy flows in ecosystems and in cells. The first law of thermodynamics explains the ability of organisms to convert chemical energy to mechanical energy:

Forms of Energy Energy occurs in two forms: kinetic and potential. Kinetic energy is the energy of motion, as when a ball rolls down a hill or a moose walks through grass. Potential energy is stored energy—its capacity to accomplish work is not being used at the moment. The food we eat has potential energy because it can be converted into various types of kinetic energy. Food is specifically called chemical energy because it contains energy in the chemical bonds of Solar energy organic molecules. When a moose walks, it has converted chemical energy into a type of kinetic energy called mechanical heat energy (Fig. 6.1).

During photosynthesis, plant cells use solar energy to convert energy-poor molecules, such as carbon dioxide and water, to energy-rich molecules, such as carbohydrates. As we have seen, not all of the captured solar energy is used to form carbohydrates; some becomes heat:

sun H 2O solar energy

heat

carbohydrate synthesis

Figure 6.1  Flow of energy.  The plant converts solar energy to the chemical energy of organic molecules through photosynthesis. Both the moose and the plant convert a portion of this chemical energy to kinetic energy to do work by cellular respiration. Eventually, all solar energy absorbed by the plant dissipates as heat. heat

Chemical energy

heat

CO2

heat

Mechanical energy



101

Chapter 6  Metabolism: Energy and Enzymes

Obviously, plant cells do not create the energy they use to produce carbohydrate molecules. That energy comes from the sun. The energy from the sun is not destroyed in the process because heat is also a form of energy. Similarly, a moose uses the energy derived from carbohydrates to power its muscles. And as its cells use this energy, none is destroyed, but some becomes heat, which dissipates into the environment:

H2 O

C6H12O6

CO2

Glucose • more organized • more potential energy • less stable (entropy)

heat

kinetic energy

Carbon dioxide and water • less organized • less potential energy • more stable (entropy)

a. carbohydrate

muscle contraction

The second law of thermodynamics, therefore, applies to living systems.

The heat released during photosynthesis (from the plant) and cellular respiration (from both the plant and moose) dissipates into the environment. This energy is no longer usable— that is, it is not available to do work. With transformation upon transformation, eventually all usable forms of energy become heat that is lost to the environment. Heat that dissipates into the environment cannot be captured and converted to one of the other forms of energy. As a result of the second law of thermodynamics, no process requiring a conversion of energy is ever 100% efficient. Much of the energy is lost in the form of heat. In automobiles, the gasoline engine is between 20% and 30% efficient in converting chemical energy into mechanical energy. The majority of energy is obviously lost as heat. Cells are capable of about 40% efficiency, with the remaining energy given off to their surroundings as heat.

H+

channel protein H+

H+

H+ H+

H+

H+ H+

H+

H+ H+

H+

H+

Unequal distribution of hydrogen ions

H+

H+

H+

H+ H+

Equal distribution of hydrogen ions

• more organized • more potential energy • less stable (entropy)

kinetic energy

• less organized • less potential energy • more stable (entropy)

• more organized • more potential energy • less stable (entropy)

kinetic energy

• less organized • less potential energy • more stable (entropy)

b.

Cells and Entropy The second law of thermodynamics can be stated another way: Every energy transformation makes the universe less organized and more disordered. The term entropy is used to indicate the relative amount of disorganization. Because the processes that occur in cells are energy transformations, the second law means that every process that occurs in cells always does so in a way that increases the total entropy of the universe. Then, too, any one of these processes makes less energy available to do useful work in the future. Figure 6.2 illustrates the second law of thermodynamics. The second law of thermodynamics tells us that glucose tends to break apart into carbon dioxide and water (Fig. 6.2a). Why? Because glucose is more organized, and therefore less stable, than its breakdown products. Also, hydrogen ions on one side of a membrane tend to move to the other side unless they are prevented from doing so (Fig. 6.2b). Why? Because when they are distributed randomly, entropy has increased. As an analogy, you know from experience that a neat room (Fig. 6.2c) is more organized but less stable than a messy room, which is disorganized but more stable. How do you

c.

Figure 6.2  Cells and entropy.  The second law of thermodynamics

tells us that (a) glucose, which is more organized, tends to break down to carbon dioxide and water, which are less organized. b. Similarly, hydrogen ions (H+) on one side of a membrane tend to move to the other side so that the ions are randomly distributed. c. Entropy also affects nonliving structures, such as this room. All of these are examples of a loss of potential energy and an increase in entropy.



102

UNIT 1  Cell Biology

know a neat room is less stable than a messy room? Consider that a neat room always tends to become more messy. Cellular processes, such as the synthesis of glucose and ion transport across a membrane, are possible because cells have the ability to obtain an input of energy from an outside source. This energy ultimately comes from the sun. Life depends on a constant supply of energy from the sun because the ultimate fate of all solar energy in the biosphere is to become randomized in the universe as heat. A living cell remains organized because it functions to maintain a constant flow of energy.

Check Your Progress  6.1 1. Explain why energy is not recycled in an ecosystem. 2. Summarize how the two laws of thermodynamics are involved as you consume your daily meals.

6.2  Energy Transformations and Metabolism Learning Outcomes Upon completion of this section, you should be able to 1. Identify how the terms anabolic, catabolic, endergonic, and exergonic relate to metabolic reactions. 2. Summarize the ATP cycle and the role of ATP in the cell.

Cellular metabolism is the sum of all the chemical reactions that occur in a cell. A significant part of cellular metabolism involves the breaking down and the building up of molecules. The term ­catabolism is used to refer to the breaking down of molecules, and the term anabolism is used to refer to the building up (synthesis) of molecules. In a chemical reaction, reactants are substances that participate in a reaction, while products are substances that form as a result of a reaction. For example, in the reaction A + B → C + D, A and B are the reactants while C and D are the products. How do you know whether this reaction will proceed in the indicated direction? Using the concept of entropy, it is possible to state that a reaction will occur if it increases the entropy of the universe. But in cell biology, we do not usually wish to consider the entire universe; we simply want to consider a particular reaction. In such instances, cell biologists use the concept of free energy instead of entropy. Free energy (also called ΔG) is the amount of energy available— that is, energy that is still “free” to do work—after a chemical reaction has occurred. The change in free energy after a reaction occurs is calculated by subtracting the free energy content of the reactants from that of the products. A negative result (negative ΔG) means that the products have less free energy than the reactants, and the reaction will go forward. In our reaction, if C and D have less free energy than A and B, the reaction will occur. Exergonic reactions are spontaneous and release energy, while endergonic reactions require an input of energy to occur. In the body, many reactions, such as protein synthesis, nerve impulse conduction, or muscle contraction, are endergonic, and they are driven by the energy released by exergonic reactions. ATP is a carrier of energy between exergonic and endergonic reactions.

ATP: Energy for Cells ATP (adenosine triphosphate) is the common energy currency of cells. When cells require energy, they “spend” ATP. The more active the organism, the greater the demand for ATP. However, the amount on hand at any one moment is minimal because ATP is constantly being generated from ADP (adenosine diphosphate) and a molecule of inorganic phosphate, P (Fig. 6.3). A cell is assured of a supply of ATP, because glucose breakdown during cellular respiration provides the energy for the buildup of ATP in mitochondria. Only 39% of the free energy of glucose is transformed to ATP; the rest is lost as heat. There are many biological advantages to the use of ATP as an energy carrier in living systems. ATP is a common and universal energy currency because it can be used in many different types of reactions. Also, when ATP is converted to energy, ADP and P , the amount of energy released is sufficient for a particular biological function, and little energy is wasted. In addition, ATP breakdown can be coupled to endergonic reactions in such a way that it minimizes energy loss.

Structure of ATP ATP is a nucleotide composed of the nitrogen-­containing base adenine and the 5-carbon sugar ribose (together called adenosine) and three phosphate groups (see Fig. 6.3). ATP is called a “high-energy” compound because of the energy stored in the chemical bonds of the phosphates. Under cellular conditions, the amount of energy released when ATP is hydrolyzed to ADP + P is about 7.3 kcal per mole.1

Function of ATP In living systems, ATP can be used for the following: Chemical work. ATP supplies the energy needed to synthesize macromolecules (anabolism) that make up the cell, and therefore the organism. Transport work. ATP supplies the energy needed to pump substances across the plasma membrane. Mechanical work. ATP supplies the energy needed to permit muscles to contract, cilia and flagella to beat, chromosomes to move, and so forth.

Coupled Reactions In coupled reactions, the energy released by an exergonic reaction is used to drive an endergonic reaction. ATP breakdown is often coupled to cellular reactions that require an input of energy. Coupling, which requires that the exergonic reaction and the endergonic reaction be closely tied, can be symbolized like this: ATP

A+B 1

ADP +

coupling

P

C+D

A mole is the number of molecules present in the molecular weight of a substance (in grams).



Chapter 6  Metabolism: Energy and Enzymes

103

adenosine triphosphate ATP is unstable and has a high potential energy. P

P

P

ATP

Exergonic Reaction: • The hydrolysis of ATP releases previously stored energy, allowing the change in free energy to do work and drive other processes.

Endergonic Reaction: • Creation of ATP from ADP and P requires input of energy from other sources. ADP +

P

P

Figure 6.3  The ATP cycle.  In

cells, ATP carries energy between exergonic reactions and endergonic reactions. When a phosphate group is removed by hydrolysis, ATP releases the appropriate amount of energy for most metabolic reactions.

P

+

P

+ adenosine diphosphate phosphate ADP is more stable and has lower potential energy than ATP.

This reaction tells you that coupling occurs, but it does not show how coupling is achieved. A cell has two main ways to couple ATP hydrolysis to an energy-requiring reaction: ATP is used to energize a reactant, or ATP is used to change the shape of a reactant. Both can be achieved by transferring a phosphate group to the reactant, so that the product is phosphorylated. An example of a coupled reaction is when a polypeptide is synthesized at a ribosome. There, an enzyme transfers a phosphate group from ATP to each amino acid in turn, and this transfer supplies the energy needed to overcome the energy cost associated with bonding one amino acid to another. Through coupled reactions, ATP allows chemical reactions that may not be favorable from an energy perspective for the cell. However, even though they are unfavorable, these processes must occur to create the high degree of order and structure essential for life. Macromolecules must be made and organized to form cells and tissues; the internal composition of the cell and the organism must be maintained; and movement of cellular organelles and the organism must occur if life is to continue.  Figure 6.4 shows that ATP breakdown provides the energy necessary for muscular contraction to occur. During muscle contraction, myosin filaments pull actin filaments to the center of the cell, and the muscle shortens. First, the myosin head combines with ATP (three connected green triangles) and takes on its resting shape. Next, ATP breaks down to ADP (two ­connected green triangles) plus P (one green triangle). A resulting change in shape allows myosin to attach to actin. Finally, the release of ADP and P from the myosin head causes it to

change its shape again and pull on the actin filament. The cycle then repeats. During this cycle, chemical energy has been transformed to mechanical energy, heat has been released, and entropy has increased.

Check Your Progress  6.2 1. Determine whether an anabolic reaction is more likely to be exergonic or endergonic.

2. Explain how the ATP pathway is similar to a rechargeable battery.

6.3  Enzymes and Metabolic Pathways Learning Outcomes Upon completion of this section, you should be able to 1. Explain the purpose of a metabolic pathway and how enzymes help regulate it. 2. Recognize how enzymes influence the activation energy rates of a chemical reaction. 3. Identify how environmental conditions influence the activity of an enzyme.

Reactions do not occur haphazardly in cells. They are usually part of a metabolic pathway, a series of linked reactions. Metabolic pathways begin with a particular reactant and terminate with an end



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1 Myosin assumes its resting shape when it combines with ATP.

2 ATP TP splits split into ADP and nd P , causing myosin yosin to change its shape and allowing it to attach to actin.

3 Release of ADP and P cause myosin to again change shape and pull against actin, generating force and motion.

actin

myosin

ATP

P

ADP

Figure 6.4  Coupled reactions.  Muscle contraction occurs only when it is coupled to ATP breakdown. product. Some metabolic pathways are cyclical, regenerating the starting material. While it is possible to write an overall equation for a pathway as if the beginning reactant went to the end product in one step, actually many specific steps occur in between. In the pathway, one reaction leads to the next reaction, which leads to the next reaction, and so forth in an organized, highly structured manner. This arrangement makes it possible for one pathway to lead to several others because various pathways have several molecules in common. Also, metabolic energy is captured and utilized more easily if it is released in small increments rather than all at once. A metabolic pathway can be represented by the following diagram: E1 E2 E3 E4 E5 E6 A B C D E F G In this diagram, the letters A–F are reactants, and the letters B–G are products in the various reactions. In other words, the products from the previous reaction become the reactants of the next reaction. The letters E1–E6 are enzymes. Enzymes are typically proteins that function as catalysts to speed a chemical reaction. Some forms of RNA molecules, called ribozymes, can act as catalysts. Catalysts participate in chemical reactions, but are not used up by the reaction. Note that the enzyme does not determine whether the reaction goes forward; that is determined by the free energy of the reaction. Enzymes simply increase the rate of the reaction. The reactants in an enzymatic reaction are called the substrates for that enzyme. In the first reaction, A is the substrate for E1,

and B is the product. Now B becomes the substrate for E2, and C is the product. This process continues until the final product (G) forms. Any one of the molecules (A–G) in this linear pathway could also be a substrate for an enzyme in another pathway. The presence or absence of an active enzyme determines which reaction takes place. A diagram showing all the possibilities would be highly branched.

Energy of Activation Molecules frequently do not react with one another unless they are activated in some way. In the lab, for example, in the absence of an enzyme, activation is very often achieved by heating a reaction flask to increase the number of effective collisions between molecules. The energy that must be added to cause molecules to react with one another is called the energy of activation (Ea) (Figure 6.5). Even though the reaction will proceed (ΔG is negative), the energy of activation must be overcome. The burning of firewood is a very exergonic reaction, but firewood in a pile does not spontaneously combust. The input of some energy, perhaps a lit match, is required to overcome the energy of activation. Figure 6.5 shows Ea when an enzyme is not present compared to when an enzyme is present, illustrating that enzymes lower the amount of energy required for activation to occur. Nevertheless, the addition of the enzyme does not change the end result of the reaction. Notice that the energy of the products is less than the energy of the reactants. This results in a negative ΔG, so the reaction will proceed. But the reaction will not go at all unless the energy of activation is overcome. Without the enzyme, the reaction rate will be very slow. By lowering the energy of activation, the enzyme increases the rate of the reaction.



Chapter 6  Metabolism: Energy and Enzymes Degradation The substrate is broken down to smaller products.

products enzyme

energy of activation (Ea) energy of reactant

energy of activation (Ea)

Free Energy

105

substrate

enzyme-substrate complex active site

energy of product enzyme not present enzyme present

a.

enzyme

Synthesis The substrates are combined to produce a larger product.

product

Progress of the Reaction

enzyme

Figure 6.5  Energy of activation (Ea).  Enzymes speed the rate of reactions because they lower the amount of energy required for the reactants to react.

Sugar in your kitchen cupboard will break down to carbon dioxide and water because the energy of the products (carbon dioxide and water) is much less than the free energy of the reactant (sugar). However, the rate of this reaction is so slow that you never see it. If you eat the sugar, the enzymes in your digestive system greatly increase the speed at which the sugar is broken down. However, the end result (carbon dioxide and water) is still the same.

How Enzymes Function The following equation, which is illustrated in Figure 6.6, is often used to indicate that an enzyme forms a complex with its substrate:

substrates

enzyme-substrate complex

active site

b.

enzyme

Figure 6.6  Enzymatic action.  An enzyme has an active site where

the substrates and enzyme fit together in such a way that the substrates react. Following the reaction, the products are released, and the enzyme is free to act again. a. The enzymatic reaction can result in the degradation of a substrate into multiple products (catabolism) or, (b) the synthesis of a product from multiple substrates (anabolism).

S  +  E     ES    E +  P substrate enzyme enzyme-substrate product complex In most instances, only one small part of the enzyme, called the active site, complexes with the substrate(s). It is here that the enzyme and substrate fit together, seemingly like a key fits a lock. However, cell biologists now know that the active site undergoes a slight change in shape in order to accommodate the substrate(s). This is called the induced fit model because the enzyme is induced to undergo a slight alteration to achieve optimum fit (Fig. 6.7). The change in shape of the active site facilitates the reaction that now occurs. After the reaction has been completed, the product(s) is released, and the active site returns to its original state, ready to bind to another substrate molecule. Only a small amount of enzyme is actually needed in a cell because enzymes are not used up by the reaction.

active site

a.

substrate

b.

Figure 6.7  Induced fit model.  These computer-generated images show an enzyme called lysozyme that hydrolyzes its substrate, a polysaccharide that makes up bacterial cell walls. a. Shape of enzyme when no substrate is bound to it. b. After the substrate binds, the shape of the enzyme changes so that hydrolysis can better proceed.



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Some enzymes do more than simply complex with their substrate(s); they participate in the reaction. For example, trypsin digests protein by breaking peptide bonds. The active site of trypsin contains three amino acids with R groups that actually interact with members of the peptide bond—first to break the bond and then to introduce the components of water. This illustrates that the formation of the enzyme-substrate complex is very important in speeding up the reaction. Every reaction in a cell requires that its specific enzyme be present. Because enzymes complex only with their substrates, they are often named for their substrates with the suffix -ase, as in the following examples:

Factors Affecting Enzymatic Speed

Generally, enzyme activity increases as substrate concentration increases because there are more collisions between substrate molecules and the enzyme. As more substrate molecules fill active sites, more product results per unit time. But when the enzyme’s active sites are filled almost continuously with substrate, the enzyme’s rate of activity cannot increase any more. The maximum rate has been reached.

Temperature and pH As the temperature rises, enzyme activity increases (Fig. 6.8). This occurs because higher temperatures cause more effective collisions between enzyme and substrate. However, if the temperature rises beyond a certain point, enzyme activity eventually levels out and then declines rapidly because the enzyme is denatured. Most enzymes are proteins, and the shape of the protein responds to temperature. Temperature often influences the protein's secondary and tertiary structure (see Fig. 2.25), preventing it from binding its substrate(s) efficiently. Therefore, as the structure of the enzyme changes, its activity decreases. Each enzyme also has a preferred pH at which the rate of the reaction is highest. Figure 6.9 shows the preferred pH for the enzymes pepsin and trypsin. At this pH value, these enzymes have their normal configurations. The globular structure of an enzyme is dependent on interactions, such as hydrogen bonding, between R groups (see Fig. 2.25). A change in pH can alter the ionization of R groups and disrupt normal interactions, and under extreme pH conditions, denaturation eventually occurs. If the enzyme’s shape is altered, it is then unable to combine efficiently with its substrate.

Enzyme Activation Not all enzymes are needed by the cell all the time. Genes can be turned on to increase the concentration of an enzyme in a cell or

Rate of Reaction (product per unit of time)

Enzymatic reactions proceed quite rapidly. Consider, for example, the breakdown of hydrogen peroxide (H2O2) as catalyzed by the enzyme catalase: 2 H2O2 → 2 H2O + O2. The breakdown of hydrogen peroxide can occur 600,000 times a second when catalase is present! In order to achieve maximum product per unit time, there should be enough substrate to fill the enzyme’s active sites most of the time. In addition to substrate concentration, the rate of an enzymatic reaction can also be affected by environmental factors (temperature and pH) or by cellular mechanisms, such as enzyme activation, enzyme inhibition, and cofactors.

Substrate Concentration

0

10

20

30

40

50

60

Temperature °C a. Rate of reaction as a function of temperature.

b. Body temperature of ectothermic animals often limits rates of reactions.

c. Body temperature of endothermic animals promotes rates of reactions.

Figure 6.8  The effect of temperature on rate of reaction.  a. Usually, the rate of an enzymatic reaction doubles with every 10°C rise in temperature. This enzymatic reaction is maximum at about 40°C. Then it decreases until the reaction stops altogether, because the enzyme has become denatured. b. The body temperature of ectothermic animals, which require an environmental source of heat, often limits rates of reactions. c. The body temperature of endothermic animals, which generate heat through their own metabolism, promotes rates of reaction.



Chapter 6  Metabolism: Energy and Enzymes

Rate of Reaction (product per unit of time)

pepsin

first reactant A

trypsin

A 0

1

2

3

4

5

6

7

8

9

10

11

12

107

site of enzyme where end product F can bind

E1

E2 B

E3 C

E4 D

E5 E

end product F

a. Active pathway

pH

Figure 6.9  The effect of pH on rate of reaction.  The preferred pH for pepsin, an enzyme that acts in the stomach, is about 2, while the preferred pH for trypsin, an enzyme that acts in the small intestine, is about 8. At the preferred pH, an enzyme maintains its shape so that it can bind with its substrates.

turned off to decrease the concentration. But enzymes can also be present in the cell in an inactive form. Activation of enzymes occurs in many different ways. Some enzymes are covalently modified by the addition or removal of phosphate groups. An enzyme called a kinase adds phosphates to proteins, as shown below, and an enzyme called a phosphatase removes them. In some proteins, adding phosphates activates them; in others, removing phosphates activates them. Enzymes can also be activated by cleaving or removing part of the protein, or by associating with another protein or cofactor. P

P

altered site of enzyme due to binding of F

first reactant A

E1

end product F

E1

end product F

Reactant A cannot bind, and no product results.

b. Inactive pathway

Figure 6.10  Feedback inhibition.  a. In an active pathway, the

first reactant (A) is able to bind to the active site of enzyme E1. b. Feedback inhibition occurs when the end product (F) of the metabolic pathway binds to the first enzyme of the pathway—at a site other than the active site. This binding causes the active site to change its shape. Now reactant A is unable to bind to the enzyme’s active site, and the whole pathway shuts down.

kinase inactive protein

active protein

Enzyme Inhibition Enzyme inhibition occurs when the substrate is unable to bind to the active site of an enzyme. The activity of almost every enzyme in a cell is regulated by feedback inhibition. In the simplest case, when there is plenty of product, it binds to the enzyme’s active site, and then the substrate is unable to bind. As the product is used up, inhibition is reduced, and more product can be produced. In this way, the concentration of the product is always kept within a certain range. Most metabolic pathways in cells are regulated by a more complicated type of feedback inhibition (Fig. 6.10). In these instances, the end product of an active pathway binds to a site other than the active site of the first enzyme. The binding changes the shape of the active site so that the substrate is unable to bind to the enzyme, and the pathway shuts down (inactive). Therefore, no more product is produced. As discussed in the Health feature, “Enzyme Inhibitors Can Spell Death,” many poisons, such as cyanide and sarin, are enzyme inhibitors. Penicillin is an antimicrobial agent that blocks the active site of an enzyme unique to bacteria. Therefore, penicillin is a poison for bacteria.

Enzyme Cofactors

as copper, zinc, or iron. These helpers are called cofactors. The organic, nonprotein molecules are called coenzymes. These cofactors assist the enzyme and may even accept or contribute atoms to the reactions. Examples of these are NAD+ (nicotinamide adenine dinucleotide), FAD (flavin adenine dinucleotide), and NADP+ (nicotinamide adenine dinucleotide phosphate), each of which plays a significant role in either cellular respiration or photosynthesis  Vitamins are often components of coenzymes. Vitamins are relatively small organic molecules that are required in trace amounts in our diet and in the diets of other animals for synthesis of coenzymes that affect health and physical fitness. The vitamin becomes a part of the coenzyme’s molecular structure. For example, the vitamin niacin is part of the coenzyme NAD, and riboflavin (B2) is part of the coenzyme FAD. A deficiency of any one of these vitamins results in a lack of the coenzyme and therefore a lack of certain enzymatic actions. In humans, this eventually results in vitamin-deficiency symptoms. For example, niacin deficiency results in a skin disease called pellagra, and riboflavin deficiency results in cracks at the corners of the mouth.

Check Your Progress  6.3 1. Summarize why enzymes are needed in biochemical pathways and how cells may regulate their activity.

2. Describe how enzymes accelerate chemical reactions. 3. Discuss why the three-dimensional shape of an enzyme is

important to its function. Many enzymes require an inorganic ion or an organic, but nonprotein, helper to function properly. The inorganic ions are metals such

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SCIENCE IN YOUR LIFE  ►

HEALTH

Enzyme Inhibitors Can Spell Death Cyanide gas was formerly used to execute people. How did it work? Cyanide can be fatal because it binds to a mitochondrial enzyme necessary for the production of ATP. MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) is another enzyme inhibitor that stops mitochondria from producing ATP. The toxic nature of MPTP was discovered in the early 1980s, when a group of intravenous drug users in California suddenly developed symptoms of Parkinson disease, including uncontrollable tremors and rigidity. All of the drug users had injected a synthetic form of heroin that was contaminated with MPTP. Parkinson disease is characterized by the death of brain cells, the very ones that are also destroyed by MPTP. Sarin is a chemical that inhibits an enzyme at neuromuscular junctions, where nerves stimulate muscles. When the enzyme is inhibited, the signal for muscle contraction cannot be turned off, so the muscles are unable to relax and become paralyzed. Sarin can be fatal if the muscles needed for breathing become paralyzed. In 1995, terrorists released sarin gas on a subway in Japan (Fig. 6A). Although many people developed symptoms, only 17 died. A fungus that contaminates and causes spoilage of sweet clover produces a chemical called warfarin. Cattle that eat the spoiled feed die from internal bleeding because warfarin inhibits a crucial enzyme for blood clotting. Today, warfarin is widely used as a rat poison. Unfortunately, it is not uncommon for warfarin to be mistakenly eaten by pets and even very small children, with tragic results.

Many people are prescribed a medicine called warfarin (Coumadin), which acts as an anticoagulant to prevent inappropriate blood clotting. It is often prescribed for people who have received an artificial heart valve or have irregular heartbeats. These examples all show how our understanding of science can have positive or negative consequences, and emphasize the role of ethics in scientific investigation.

Questions to Consider 1. Do you feel it is ethical to use dangerous chemicals to treat diseases? 2. Many drug companies are actively looking for species that might contain chemical compounds that are new to humans. How might this have both beneficial and harmful side effects?

Figure 6A  Sarin gas.  The aftermath when sarin, a nerve gas that results in the inability to breathe, was released by terrorists in a Japanese subway in 1995.

6.4  Oxidation-Reduction Reactions and Metabolism Learning Outcomes Upon completion of this section, you should be able to 1. Explain how equations for photosynthesis and cellular respiration represent oxidation-reduction reactions. 2. Summarize the relationship between the metabolic reactions of photosynthesis and cellular respiration.

In the next two chapters, you will explore two important metabolic pathways: cellular respiration (see Chapter 7) and photosynthesis (see Chapter 8). Both of these pathways are based on the use of special enzymes to facilitate the movement of electrons. The movement of these electrons plays a major role in the energyrelated reactions associated with these pathways.

Oxidation-Reduction Reactions When oxygen (O) combines with a metal such as iron or magnesium (Mg), oxygen receives electrons and becomes an ion that is negatively charged. The metal loses electrons and becomes an ion that is positively charged. When magnesium oxide (MgO) forms, it is appropriate to say that magnesium has been oxidized. On the other hand, oxygen has been reduced because it has gained negative charges (i.e., electrons). Reactions that involve the gain and loss of electrons are called oxidation-reduction reactions. Sometimes, the terms oxidation and reduction are applied to other reactions, whether or not oxygen is involved. When discussing metabolic reactions, oxidation represents the loss of electrons, and reduction is the gain of electrons. In the reaction Na + Cl → NaCl, sodium has been oxidized (loss of electron), and chlorine has been reduced (gain of electrons). Because oxidation and reduction go hand-in-hand, the entire reaction is called a redox reaction.



Chapter 6  Metabolism: Energy and Enzymes

Photosynthesis

109

Cellular respiration

carbohydrate

sun

O2

chloroplast

mitochondrion heat

heat

CO2 + H2O ATP

for synthetic reactions, active transport, muscle contraction, nerve impulse

Figure 6.11  Relationship of chloroplasts to mitochondria.  Chloroplasts produce energyrich carbohydrate. Carbohydrate is broken down in mitochondria, and the energy released is used for the buildup of ATP. Mitochondria can also respire molecules derived from fats and amino acids for the buildup of ATP. Usable energy is lost as heat due to the energy conversions of photosynthesis, cellular respiration, and the use of ATP in the body.

OIL RIG Oxidation Is Loss

Reduction Is Gain

The terms oxidation and reduction also apply to covalent reactions in cells. In this case, however, oxidation is the loss of hydrogen atoms (e– + H+), and reduction is the gain of hydrogen atoms. Notice that when a molecule loses a hydrogen atom, it has lost an electron, and when a molecule gains a hydrogen atom, it has gained an electron. This form of oxidation-reduction is exemplified in the overall equations for photosynthesis and cellular respiration.

Chloroplasts and Photosynthesis The chloroplasts in plants capture solar energy and use it to convert water and carbon dioxide into a carbohydrate. Oxygen is a by-product that is released. The overall equation for photosynthesis can be written like this: energy + 6 CO2 + 6 H2O  carbon water dioxide

 C6H12O6 + 6 O2 glucose oxygen

This equation shows that during photosynthesis, hydrogen atoms are transferred from water to carbon dioxide as glucose forms. In this reaction, therefore, carbon dioxide has been reduced and water

heat

has been oxidized. It takes energy to reduce carbon dioxide to glucose, and this energy is supplied by solar energy. Chloroplasts are able to capture solar energy and convert it to the chemical energy of ATP, which is used along with hydrogen atoms to reduce carbon dioxide. The reduction of carbon dioxide to form a mole of glucose stores 686 kcal in the chemical bonds of glucose. This is the energy that living organisms utilize to support themselves only because carbohydrates (and other nutrients) can be oxidized in mitochondria.

Mitochondria and Cellular Respiration Mitochondria, present in both plants and animals, oxidize ­carbohydrates and use the released energy to build ATP molecules (Fig. 6.11 right). Cellular respiration therefore consumes oxygen and produces carbon dioxide and water, the very molecules taken up by chloroplasts. The overall equation for cellular respiration is the opposite of the one we used to represent photosynthesis: C6H12O6 + 6 O2  glucose oxygen

  6 CO2 + 6 H2O + energy carbon water dioxide

In this reaction, glucose has lost hydrogen atoms (been oxidized), and oxygen has gained hydrogen atoms (been reduced). When oxygen gains electrons, it becomes water. The complete oxidation of a mole of glucose releases 686 kcal of energy, and some of this energy is used to synthesize ATP molecules. If the energy within



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UNIT 1  Cell Biology

O2

glucose were released all at once, most of it would dissipate as heat instead of some of it being used to produce ATP. Instead, cells oxidize glucose step by step. The energy is gradually stored and then converted to that of ATP molecules, which is used by organisms in a variety of ways.  Figure 6.11 shows us very well that chloroplasts and mitochondria are involved in a cycle. Carbohydrate produced within chloroplasts becomes a fuel for cellular respiration in mitochondria, while carbon dioxide released by mitochondria becomes a substrate during photosynthesis in chloroplasts. These organelles are involved in a redox cycle because carbon dioxide is reduced during photosynthesis and carbohydrate is oxidized during cellular respiration. Note that energy does not cycle between the two organelles; instead, it flows from the sun through each step of photosynthesis and cellular respiration until it eventually becomes unusable heat as ATP is used by the cell.

CO2

Breathing

Cellular Respiration and Humans

Eating

O2

CO2

Humans, like all eukaryotic organisms, are involved in the cycling of molecules between chloroplasts and mitochondria. Our food is derived from plants or we eat other animals that have eaten plants. Also, we take in oxygen released by plants. Nutrients from our food and oxygen enter our mitochondria, which produce ATP (Fig. 6.12). Without a supply of energy-rich molecules, ultimately derived from plants, we could not produce the ATP molecules needed to maintain our bodies. On the other hand, our mitochondria release carbon dioxide and water. The carbon dioxide is exhaled and it enters the atmosphere where it is accessible to plants once again. In Chapter 7 we will explore how our bodies use energy sources other than carbohydrates for fuel.

Check Your Progress  6.4 1. Explain why overall equations for photosynthesis and cellular respiration represent redox reactions. 2. Summarize how the inputs and outputs of cellular respiration and photosynthesis are related. Nutrients

Conclusion ATP Cellular respiration

Figure 6.12  Relationship between breathing, eating, and

cellular respiration.  The O2 we inhale and the nutrients resulting from the digestion of our food are carried by the bloodstream to our cells, where they enter mitochondria and undergo cellular respiration. Following cellular respiration, the ATP stays in the cell, but the CO2 is carried to the lungs for exhalation.

The enzymes in the saliva of the vampire bat are interfering with a complicated biochemical pathway in the blood. The clotting cascade involves several enzymes that circulate in the blood. When a blood vessel is damaged, platelets release prothrombin activator, an enzyme that converts the plasma protein prothrombin into thrombin. Thrombin, in turn, is an enzyme that converts fibrinogen into fibrin, forming the basis of the clot. When the damage has been repaired, the plasma protein plasminogen is converted into plasmin, which destroys the fibrin. By releasing a potent plasminogen enzyme in their saliva, vampire bats are able to manipulate a biochemical pathway for their own needs.



Chapter 6  Metabolism: Energy and Enzymes

111

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6.1  Laws of Thermodynamics 6.2 ATP 6.3 Enzymes

 

Animations

  Tutorials

6.2  Breakdown of ATP and Cross-Bridge Movement During Muscle Contraction 6.3  Biochemical Pathways • How Enzymes Work • Feedback Inhibition of Biochemical Pathways • B Vitamins

SUMMARIZE 6.1  Life and the Flow of Energy ■ Energy is the ability to do work. Energy may exist in several forms,

including potential energy (chemical energy) and kinetic energy (mechanical energy).  ■ Two laws of thermodynamics are basic to understanding how living organisms relate to energy. The first law of thermodynamics states that energy cannot be created or destroyed but can only be changed from one form to another. The second law of thermodynamics states that one usable form of energy cannot be converted into another form without loss of usable energy. Therefore, every energy transformation makes the universe less organized. As a result of these laws, we know that the entropy of the universe is increasing and that only a constant input of energy maintains life’s organization.

6.2  Energy Transformations and Metabolism ■ The term metabolism encompasses all the chemical reactions occur-

ring in a cell. Catabolism involves breaking down reactants, whereas anabolism involves building up products. Free energy is the amount of energy that is actually available to do work. ■ Considering individual reactions, only those that result in products that have less usable energy than the reactants go forward. Such reactions, called exergonic reactions, release energy. Endergonic reactions, which require an input of energy, occur in cells as coupled reactions with an exergonic process. For example, glucose breakdown is an exergonic metabolic pathway that drives the buildup of many ATP (adenosine ­triphosphate) molecules. These ATP molecules then supply energy for cellular work. ATP goes through a cycle of constantly being built up from, and then broken down to, ADP (adenosine diphosphate) + P .

6.3  Enzymes and Metabolic Pathways ■ A metabolic pathway is a series of reactions that proceed in an

6.3  Energy of Activation

such as temperature or pH, affects the shape of a protein. A denatured enzyme loses its ability to do its job. ■ Cellular mechanisms regulate enzyme quantity and activity. The activity of most metabolic pathways is regulated by enzyme inhibition, usually associated with feedback. Many enzymes have cofactors or coenzymes, such as vitamins, that help them carry out a reaction.

6.4  Oxidation-Reduction Reactions and Metabolism ■ The overall equation for photosynthesis is the opposite of that for cellu-

lar respiration. Both processes involve oxidation-reduction reactions. Redox reactions are a major way in which energy is transformed in cells. ■ During photosynthesis, carbon dioxide is reduced to glucose, and water is oxidized. Glucose formation requires energy, and this energy comes from the sun. Chloroplasts capture solar energy and convert it to the chemical energy of ATP molecules, which are used along with hydrogen atoms to reduce carbon dioxide to glucose. During cellular respiration, glucose is oxidized to carbon dioxide, and oxygen is reduced to water. This reaction releases energy, which is used to synthesize ATP molecules in all types of cells. ■ Energy flows through all living organisms. Photosynthesis is a metabolic pathway in chloroplasts that transforms solar energy to the chemical energy within carbohydrates, and cellular respiration is a metabolic pathway completed in mitochondria that transforms this energy into that of ATP molecules. Eventually, the energy within ATP molecules becomes heat. ■ Cellular respiration is an aerobic process that requires oxygen and gives off carbon dioxide. Cellular respiration involves oxidation. The air we inhale contains the oxygen, and the food we digest after eating contains the carbohydrate glucose needed for cellular respiration.

ASSESS Testing Yourself

Choose the best answer for each question. orderly, step-by-step manner. Each reaction requires a specific enzyme. Enzymes function by reducing the energy of activation (Ea) needed 6.1  Life and the Flow of Energy for the reaction to proceed. 1. The fact that energy transformations increase the amount of entropy ■ Reaction rates increase when enzymes form a complex with their is the basis of which of the following? ­substrates, called an induced fit model. Generally, enzyme activity a. cell theory c. second law of thermodynamics increases as substrate concentration increases. Once all active sites are b. first law of thermodynamics d. oxidation-reduction reactions  filled, the maximum rate has been achieved. Any environmental factor,

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2. The energy stored in the carbon–carbon bonds of glucose is an example of _____ energy. a. kinetic b. potential c. chemical d. mechanical e. Both b and c are correct.  3. During energy transformations, the majority of energy is converted to a. chemical bonds. b. heat. c. ATP. d. glucose molecules.

6.2  Energy Transformations and Metabolism 4. Exergonic reactions a. are spontaneous. b. have a negative ΔG value. c. release energy. d. All of these are correct.  5. Which of the following is incorrect regarding ATP? a. It is the energy currency of the cell. b. It is stable. c. It is recycled using ADP and inorganic phosphate. d. Cells keep only small amounts of ATP on hand.  6. The sum of all the chemical reactions in a cell is called a. free energy. b. entropy. c. metabolism. d. oxidation-reduction reactions.

6.3  Enzymes and Metabolic Pathways 7. Which of the following is incorrect regarding the active site of an enzyme? a. is unique to that enzyme b. is the part of the enzyme where its substrate can fit c. can be used over and over again d. is not affected by environmental factors, such as pH and temperature  8. Which of the following environmental conditions may have an influence on enzyme activity? a. substrate concentration b. temperature c. pH d. All of these are correct.  9. In which of the following does an inhibitor bind to a site other than the active site of the enzyme? a. competitive inhibition b. noncompetitive inhibition c. redox reactions d. None of these are correct. 

10. Enzymes catalyze chemical reactions by which of the following? a. lowering the energy of activation in the reaction b. raising the energy of activation in the reaction c. increasing entropy d. increasing the free energy of the products

6.4  Oxidation-Reduction Reactions and Metabolism 11. The gain of electrons by a molecule is called a. inhibition. b. entropy. c. oxidation. d. reduction.  12. In which of the following processes is carbon dioxide reduced to form carbohydrate? a. cellular respiration  b. noncompetitive inhibition  c. photosynthesis d. induced fit model

ENGAGE BioNOW Want to know how this science is relevant to your life? Check out the BioNow videos below: ■ Nut Fungus ■ Energy Part I : Energy Transfers ■ Energy Part II : Photosynthesis ■ Energy Part III : Cellular Respiration

Thinking Critically 1. Entropy is increased when nutrients break down, so why are enzymatic metabolic pathways required for cellular respiration? (Hint: see Fig. 6.5.) 2. Why would you expect glucose storage as glycogen to be an energyrequiring process? 3. If photosynthesis and cellular respiration are reverse equations, then why can’t mitochondria carry on photosynthesis?

PHOTO CREDITS Opener: © Nick Hawkins/NHPA/Photoshot; 6.2c(both): © Keith Eng, 2008; 6.8b: © Brand X Pictures/PunchStock RF; 6.8c: © Joel Simon/Getty RF; 6A: © AP Photo/Chikumo Chiaki; 6.11(leaves): © Comstock/PunchStock RF; 6.11(runner): © Photodisc/Getty RF; 6.12: © Brand X Pictures/Getty Images RF.

CASE STUDY Metabolic Demands on Athletes During a typical basketball game, such as the 2015 NCAA championship game between Duke and Wisconsin, the starting players run an average of 4–5 kilometers (around 3 miles) during a 40-minute game. However, unlike the endurance running experienced by marathoners, basketball players experience periods of intense activity (sprinting), followed by brief periods of rest. This start-and-stop nature of the game means that the muscles of the athlete are constantly switching between aerobic and anaerobic metabolism. During aerobic metabolism, the muscle cells use oxygen in order to completely break down glucose, producing more ATP, a high-energy molecule used for muscle contraction, than otherwise. The breakdown of glucose with oxygen to produce carbon dioxide and water in the cytoplasm and mitochondria of the cell is called cellular respiration. However, running short, fast sprints quickly depletes oxygen levels and drives the muscles into anaerobic metabolism. Without oxygen, glucose cannot be broken down completely. It is changed into lactate, which is responsible for that muscle burn we sometimes feel after strenuous exercise. Once oxygen is restored to the muscles, the body is able to return to aerobic metabolism and dispose of the lactate. In this chapter, we will discuss the metabolic pathways of cellular respiration that allow the energy within a glucose molecule, and other organic nutrients, to be converted into ATP. As you read through the chapter, think about the following questions:

1. What are the differences between the aerobic and anaerobic pathways? 2. How is the energy of a glucose molecule harvested by a cell? 3. How are other organic nutrients, such as proteins and fats, used as energy?

Cellular Respiration

7

CHAPTER OUTLINE 7.1  Overview of Cellular Respiration 7.2 Outside the Mitochondria: Glycolysis 7.3 Outside the Mitochondria: Fermentation

7.4 Inside the Mitochondria

BEFORE YOU BEGIN Before beginning this chapter, take a few moments to review the following discussions: Section 2.5  What is the role of carbohydrates in the body? Section 3.3  What is the structure and function of the mitochondria in a cell? Figure 6.3  How does the ATP cycle resemble a rechargeable battery?

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7.1  Overview of Cellular Respiration

NADH + H+

Learning Outcomes Upon completion of this section, you should be able to 1. Describe the overall equation for cellular respiration. 2. Explain the role of electron carriers in respiration. 3. Summarize the phases of cellular respiration and indicate where they occur in a cell.

Cellular respiration is the release of energy from molecules such as glucose accompanied by the use of this energy to synthesize ATP molecules. Cellular respiration is an aerobic process that requires oxygen (O2) and gives off carbon dioxide (CO2). It usually involves the complete breakdown of glucose as shown here: ADP + P C6H12O6 + 6O2 glucose

oxygen

36-38

ATP 6CO2 + 6H2O carbon water dioxide

Glucose is a high-energy molecule, and its breakdown products, CO2 and H2O, are low-energy molecules. As glucose is broken down, energy is released. This energy is used to produce ATP molecules. The breakdown of one glucose molecule results in the production of between 36 to 38 ATP molecules. This represents about 39% of the potential energy within a glucose molecule. The rest of the energy dissipates. However, this conversion is more efficient than many others. For example, only between 14 and 30% of the energy within gasoline is converted to the motion of a car. The pathways of cellular respiration allow the energy within a glucose molecule to be released slowly so that the ATP can be produced gradually. The energy in the ATP can be used for most cellular reactions, such as joining one amino acid to another during protein synthesis or joining actin to myosin during muscle contraction. This energy is released by a relatively simple procedure: the removal of a single phosphate group. The cell uses the reverse reaction for building up ATP again.

NAD+ and FAD Cellular respiration involves many individual reactions, each one catalyzed by its own enzyme. Some of these enzymes utilize the coenzyme NAD+ (nicotinamide adenine dinucleotide) as an electron carrier. Coenzymes help an enzyme do its job and may even participate in the reaction. In this instance, NAD+ receives two electrons (is reduced) as the substrate glucose is oxidized. Recall from Chapter 6 that each electron is received by NAD+ as part of a hydrogen atom. A hydrogen atom consists of a hydrogen ion (H+) and an electron (e–). As shown in Figure 7.1, NAD+ receives two e– and two H+ to give NADH + H+. FAD (flavin adenine dinucleotide) is another coenzyme frequently used as an electron carrier. When FAD accepts two e– and two H+, FADH2 results. NAD+ and FAD are analogous to electron shuttle buses. They pick up electrons at specific enzymatic

reduction

oxidation

2H

2H

2e– + 2H+

2e– + 2H+

NAD+

Figure 7.1  The NAD+ cycle.  The coenzyme NAD+ accepts two

hydrogen atoms (H+ + e–), and NADH + H+ results. When NADH passes on electrons, NAD+ results. Only a small amount of NAD+ need be present in a cell, because each NAD+ molecule is recycled.

reactions in either the cytoplasm or the matrix of the mitochondria and carry these high-energy electrons to an electron transport chain in the cristae of the mitochondria, where they drop them off. The empty NAD+ or FAD is then free to go back and pick up more electrons.

Phases of Cellular Respiration The metabolic pathways of cellular respiration couple the release of energy within a glucose molecule to the production of ATP. The coupling of these reactions reduces the amount of energy lost as heat, which would be significant if glucose breakdown occurred all at once. Cellular respiration involves four phases (Fig. 7.2). The first phase, glycolysis, takes place outside the mitochondria and does not utilize oxygen. Therefore, glycolysis is anaerobic. The other phases take place inside the mitochondria, where oxygen is the final acceptor of electrons. These phases are aerobic. ■■

■■

■■

■■

Glycolysis is the breakdown of glucose (C6H12O6) to two molecules of pyruvate, a C3 molecule. Oxidation by removal of electrons (e–) and hydrogen ions (H+) provides enough energy for the immediate buildup of two ATP. During the preparatory (prep) reaction, pyruvate is oxidized to a C2 acetyl group carried by CoA (coenzyme A), and CO2 is removed. Because glycolysis ends with two molecules of pyruvate, the prep reaction occurs twice per glucose molecule. The citric acid cycle is a cyclical series of oxidation reactions that give off CO2 and produce one ATP. The citric acid cycle used to be called the Krebs cycle in honor of the researcher who worked out most of the steps. The cycle is now named for citric acid (or citrate), the first molecule in the cycle. The citric acid cycle turns twice because two acetyl CoA molecules enter the cycle per glucose molecule. Altogether, the citric acid cycle accounts for two immediate ATP molecules per glucose molecule. The electron transport chain is a series of membrane-bound carriers that pass electrons from one carrier to another. Highenergy electrons are delivered to the chain, and low-energy electrons leave it (Fig. 7.3). The electron transport chain is like a flight of stairs. As something bounces down the stairs it loses potential energy. Similarly, as electrons pass through the carriers from a higher-energy to a lower-energy state, energy



Chapter 7  Cellular Respiration

115

Figure 7.2  Overview of Cellular Respiration.  The complete breakdown of glucose during cellular respiration consists of four phases. Glycolysis in the cytoplasm produces pyruvate, which enters mitochondria if oxygen is available. The preparatory reaction and the citric acid cycle that follow occur inside the mitochondria. Also inside mitochondria, the electron transport chain receives the electrons that were removed from glucose breakdown products. The result of glucose breakdown is a maximum of 36 to 38 ATP, depending on the particular cell. e–

NADH

e–

NADH

e–

e– Cytoplasm

e–

NADH and FADH2

Mitochondrion

–

e Glycolysis glucose

e– Citric acid cycle

Preparatory reaction

pyruvate

Electron transport chain and chemiosmosis

2 ATP 2 ADP 4 ADP

4 ATP total 2

ATP

net gain

2 ADP

e– high-energy electrons

energy for synthesis of

ATP

electron transport chain

2

ATP

32 or 34 ADP

32 or 34

ATP

is released and used for ATP synthesis. The electrons from one glucose molecule passing down the electron transport chain result in a maximum of 32 or 34 ATP, depending on the type of cell. In cellular respiration, the low-energy electrons are finally received by O2, which then combines with H+ and becomes water. Pyruvate is a pivotal metabolite in cellular respiration. If oxygen is not available to the cell, fermentation occurs in the cytoplasm instead of continued aerobic (with oxygen) cellular respiration. During fermentation, pyruvate is reduced to lactate or to carbon dioxide and alcohol, depending on the organism. As we shall see in section 7.3, fermentation results in a net gain of only two ATP per glucose molecule.

Check Your Progress  7.1 e–

low-energy electrons

1. Explain the role of NAD+ and FAD in cellular respiration. 2. Distinguish between the aerobic and anaerobic phases of cellular respiration.

Figure 7.3  Electron transport chain.  High-energy electrons are

delivered to the chain. As they pass from carrier to carrier, energy is released and used for ATP production.

3. Summarize the location and function of each phase of cellular respiration.



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synthesize four ATP. This is substrate-level ATP synthesis, in which an enzyme passes a high-energy phosphate to ADP, and ATP results:

7.2  Outside the Mitochondria: Glycolysis Learning Outcomes Upon completion of this section, you should be able to 1. Describe the location and inputs and outputs of glycolysis. 2. Explain why ATP is both an input and output of glycolysis.

Glycolysis, which takes place within the cytoplasm, is the breakdown of glucose to two pyruvate molecules. Because glycolysis occurs universally in all organisms, it most likely evolved before the citric acid cycle and the electron transport chain. Glycolysis most likely evolved when environmental conditions were anaerobic and before cells had mitochondria. This may be why glycolysis does not require oxygen and occurs in the cytoplasm.

NADH +

H+

NADH + H+

ADP

P

ATP P

Subtracting the two ATP that were used to get started, glycolysis yields a net gain of two ATP (Fig. 7.4).

Inputs and Outputs of Glycolysis

e– NADH + H+ e– and FADH2

e–

Glycolysis pyruvate

Glycolysis

e–

inputs

Citric acid cycle

Preparatory reaction

outputs

6C glucose 2 NAD+

e– Electron transport chain and chemiosmosis

2

2 ATP

2 (3C) pyruvate 2 NADH

ATP

2 ADP

4 ADP + 4 P

2 ADP

2

4 ADP 4 ATP total ATP

me

e–

e–

2

zy

P

All together, the inputs and outputs of glycolysis are as follows:

Cytoplasm

glucose

en

net

2 ADP

2

ATP

32 ADP 32 or 34 or 34

4 ATP

ATP

total

net gain

ATP

Energy-Investment Steps As glycolysis begins, two ATP are used to activate glucose, and the molecule that results splits into two C3 molecules (G3P, glyceraldehyde 3-phosphate), each of which has an attached phosphate group. From this point on, each C3 molecule undergoes the same series of reactions (Fig. 7.4).

Energy-Harvesting Steps Oxidation of G3P occurs by the removal of hydrogen atoms (H+ + e–). The hydrogen atoms are picked up by NAD+, and NADH + H+ results. Later, NADH will pass electrons on to the electron transport chain. Oxidation of G3P and subsequent substrates results in four high-energy phosphate groups, which are used to

Notice that, so far, we have accounted for only two of the 36 to 38 ATP molecules that are theoretically possible when glucose is completely broken down. When oxygen is available, the end product, pyruvate, enters the mitochondria, where it undergoes further breakdown. If oxygen is not available, pyruvate can be used for fermentation as described in section 7.3. For each glucose that enters glycolysis, two ATP, two NADH + H+, and two pyruvate are formed.

Check Your Progress  7.2 1. Explain why there is an energy-investment phase and energy-harvesting phase to glycolysis.

2. Summarize the inputs and outputs of glycolysis and state the net number of ATP that are produced.



Chapter 7  Cellular Respiration

117

Glycolysis Energy-investment Step

C

C

C

C

C

C

glucose –2

ATP

ATP

ATP

ADP

P

C

C

C

C

C

C

C

C

C

G3P Energy-harvesting Steps

P

C

C

NADH P

C

P

P

C

BPG

C

C

P

ADP

E2

Substrate-level ATP synthesis.

ATP

ATP P

C

C

C

C

3PG

C

C

P

3PG E3

H2O P

H2O C

C

C

C

PEP

(net gain)

P

ADP

ATP

ATP

ATP

C

E4

Substrate-level ATP synthesis.

ATP C

2

C

Oxidation of 3PG occurs by removal of water.

PEP

ADP +2

Oxidation of G3P occurs as NAD+ receives high-energy electrons.

BPG

ADP ATP

3-phosphoglycerate

NAD+

P

+2

3PG

Splitting produces two 3-carbon molecules.

P

NADH

C

1,3-bisphosphoglycerate

Two ATP are used to get started.

E1

C

BPG

G3P

NAD+

P

glyceraldehyde-3-phosphate

ADP C

P

G3P

C

C

pyruvate

C

C

C

pyruvate

Two molecules of pyruvate are the end products of glycolysis.

Figure 7.4  Glycolysis.  Glycolysis begins with glucose and ends with two pyruvate molecules. There is a gain of two NADH + H+ and a net gain of two ATP from glycolysis.

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UNIT 1  Cell Biology

7.3  Outside the Mitochondria: Fermentation

glucose –2

2

ATP

Learning Outcomes

ATP 2 ADP

Upon completion of this section, you should be able to 1. Explain how ATP can continue to be produced in the absence of oxygen. 2. Describe the advantages and disadvantages of fermentation.

The complete breakdown of glucose requires an input of oxygen to keep the electron transport chain working. If oxygen is limited, pyruvate molecules accumulate in the cell, and intermediates, such as NAD+ and FAD, cannot be recycled. To correct for this, cells may utilize anaerobic pathways, such as fermentation. There are two basic forms of fermentation—lactic acid and alcohol.

2 C C C

P

G3P 2 NAD+

2 P

2 NADH C C C P

2 P

BPG 4 ADP +4

ATP

4

ATP 2 C C C

Lactic Acid Fermentation In human cells, like other animal cells, in a low oxygen environment the pyruvate formed by glycolysis is reduced to form lactate, which is converted to lactic acid in the water environment of the cell.  The electrons needed to reduce pyruvate to lactic acid are supplied by the NADH molecules from glycolysis (Fig. 7.5). Normally, NADH contributes its electrons to the electron transport chain and is then recycled as NAD+ for use in glycolysis. But in the absence of oxygen, the NAD+ is regenerated when pyruvate is reduced to lactic acid. Therefore, the lactic acid pathway helps to recycle NAD+ molecules and allow glycolysis to proceed with ATP production. Notice that the fermentation pathway only yields 2 ATP per glucose molecule. Despite the low amounts of ATP produced, fermentation is essential to humans. It is commonly used as a pathway in muscle cells. When our muscles are working vigorously over a short period of time, as when we run, fermentation is a way to produce ATP even though oxygen is temporarily in limited supply. Lactate, however, is toxic to cells. At first, blood carries away all the lactate formed in muscles. But eventually lactate begins to build up, lowering the pH and causing the muscles to “burn.” When we stop running, our bodies are in oxygen debt, as signified by the fact that we continue to breathe very heavily for a time. Recovery is complete when the lactate is transported to the liver, where it is reconverted to pyruvate. Some of the pyruvate reenters the cellular respiration pathways, and the rest is converted back to glucose.

Alcohol Fermentation In other organisms, the pyruvate is reduced to produce alcohol. As was the case with lactic acid fermentation, the electrons needed to reduce the pyruvate are supplied by NADH molecules. In the process, NAD+ molecules are regenerated for use in glycolysis

pyruvate

C C

or 2

ATP

2 CO2

(net gain) C C C

C C

2 lactate or 2 alcohol Animals and bacteria

Plants and yeast

Figure 7.5  Fermentation.  Fermentation consists of glycolysis followed by a reduction of pyruvate by NADH + H+. The resulting NAD+ returns to the glycolytic pathway to pick up more hydrogen atoms. (Fig.  7.5). However, unlike lactic acid fermentation, alcohol fermentation releases small amounts of CO2. Yeasts are good examples of organisms that use alcohol fermentation. Yeasts are eukaryotic organisms, and thus can carry out cellular respiration. But when they are in an anaerobic environment, they can use the fermentation process to produce small amounts of ATP. In the process they generate ethyl alcohol and CO2 as a result of fermentation. When yeast is used to leaven bread, the CO2 produced makes bread rise. Fermentation produces the ethanol that is found in many alcoholic beverages, from beer to wine. However, it is important to note that the alcohol that is produced is a waste product for the yeast, it is not used as an energy source. Eventually, yeasts are killed by the very alcohol they produce. Other organisms, such as the bacteria, vary as to whether they produce an organic acid, such as lactate, or an alcohol and CO2.

Energy Yield of Fermentation The ATP produced during fermentation are actually the products of glycolysis. This is because fermentation follows glycolysis



Chapter 7  Cellular Respiration

(Fig.  7.5). Therefore, the anaerobic pathways produce only two ATP by substrate-level ATP synthesis. These two ATP represent only a small fraction of the potential energy stored in a glucose molecule. As noted earlier, complete glucose breakdown during cellular respiration results in a maximum of 36 to 38 ATP. Therefore, following fermentation, most of the potential energy a cell can capture from the respiration of a glucose molecule is still waiting to be released.

119

The preparatory (prep) reaction is named because it produces the molecule that can enter the citric acid cycle. In this reaction, pyruvate is converted to a two carbon (C2) acetyl group attached to coenzyme A (CoA), and CO2 is given off. This is an oxidation reaction in which hydrogen atoms (H+ + e–) are removed from pyruvate by NAD+ and NADH + H+ results. This reaction occurs twice per glucose molecule:

2 NAD +

2 NADH + H+

Fermentation inputs

outputs

glucose 2

2 lactate or 2 alcohol and 2 CO2 2 ADP

ATP

4 ADP + 2

4

P

2

ATP

ATP

net gain

Check Your Progress  7.3 1. Describe the environmental conditions that would cause a muscle cell to undergo fermentation.

2. Explain how fermentation acts as a NAD+ recycling system.

7.4  Inside the Mitochondria Learning Outcomes Upon completion of this section, you should be able to 1. Recognize the role of the mitochondria in cellular respiration. 2. Summarize the inputs and outputs of the preparatory reaction, the citric acid cycle, and the electron transport chain. 3. Identify how each stage of the aerobic pathway contributes to the generation of ATP in a cell.

The final reactions of cellular respiration, the preparatory reaction, the citric acid cycle, and the electron transport chain, all occur within the mitochondria.

Preparatory Reaction As stated, the preparatory reaction occurs inside the mitochondria. Where specifically does it occur? As you know, the cristae of a mitochondrion are folds of inner membrane that jut out into the matrix, an innermost compartment filled with a gel-like fluid. The preparatory reaction and the citric acid cycle are located in the matrix (see Fig. 7.2).

2 pyruvate + 2 CoA

2 acetyl CoA + 2 carbon dioxide

Citric Acid Cycle The citric acid cycle (also called the Krebs cycle) is a cyclical metabolic pathway located in the matrix of mitochondria (Fig. 7.6). At the start of the citric acid cycle, the C2 acetyl group carried by CoA joins with a C4 molecule, and a C6 citrate molecule results. Note that because there are two acetyl CoA m ­ olecules entering the cycle for each glucose, the cycle will turn twice. The CoA returns to the preparatory reaction to pick up another acetyl group. During the citric acid cycle, each acetyl group received from the preparatory reaction is oxidized to two CO2 molecules. As the reactions of the cycle occur, oxidation is carried out by the removal of hydrogen atoms (H+ + e–). In three instances, NADH + H+ results, and in one instance, FADH2 is formed. Substrate-level ATP synthesis is also an important event of the citric acid cycle. In substrate-level ATP synthesis, an enzyme passes a high-energy phosphate to ADP, and ATP results. Because the citric acid cycle turns twice for each original glucose molecule, the inputs and outputs of the citric acid cycle per glucose molecule are as follows:

inputs

Citric acid cycle

2 ADP + 2 P

outputs 4 CO2 6 NADH + H+ 2 FADH2

2 acetyl groups 6 NAD+ 2 FAD 2

ATP

The six carbon atoms originally located in the glucose molecule have now become CO2. The preparatory reaction produces two CO2, and the citric acid cycle produces four CO2 per glucose molecule.

Electron Transport Chain The electron transport chain located in the cristae of the mitochondria, is a series of carriers that pass electrons from one to the other. Some of the electron carriers of the system are called cytochrome molecules. Cytochromes are a class of iron-containing proteins important in redox reactions.



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NADH

NADH

–

e

e–

e–

e–

Glycolysis pyruvate

e–

NADH and FADH2

e–

glucose

Figure 7.6  Citric acid cycle.  The net result of this cycle of reactions is the oxidation of an acetyl group to two molecules of CO2, along with a transfer of electrons to NAD+ and FAD and a gain of one ATP. The citric acid cycle turns twice per glucose molecule.

e–

Electron transport chain and chemiosmosis

Citric acid cycle

Preparatory reaction Matrix

2 ATP 2 ADP

4 ADP

4 ATP total 2

ATP

net

2 ADP

2

ATP

32 ADP or 34

32 or 34

ATP

NAD+

1. The C2 acetyl group combines with a C4 molecule to produce citrate, a C6 molecule.

Preparatory reaction

NADH + H+ CO2

citrate

C5

CoA acetyl CoA

NAD+

Citric acid cycle

C4 NADH + H+

5. Additional oxidation reactions produce an FADH2 and another NADH + H+ and regenerate original C4 molecule.

2. Oxidation reactions produce two NADH + H+.

CoA

NADH + H+

C4 NAD+

CO2

3. The loss of two CO2 results in a new C4 molecule.

C4 FAD ATP FADH2

Figure 7.7 is arranged to show that high-energy electrons enter the system and low-energy electrons leave the system. Notice that when NADH gives up its two electrons to the chain, it becomes NAD+ and two H+ remain. Similarly, when FADH2 gives up two electrons to the chain, it becomes FAD and two H+ remain. The next carrier gains the electrons and is reduced. Then each of the carriers, in turn, becomes reduced and then oxidized as the electrons move down the system. As the electrons pass from one carrier to the next, energy that will be used to produce ATP molecules is captured and stored as a hydrogen ion gradient (Fig. 7.8). Oxygen receives the energy-spent electrons from the last of the carriers. After receiving electrons, oxygen combines with hydrogen ions and forms water. Once NADH has delivered electrons to the electron transport chain, the NAD+ that results can pick up more hydrogen atoms. In like manner, once FADH2 gives up electrons to the chain, the FAD that results can pick up more hydrogen atoms. The recycling of coenzymes and ADP increases cellular efficiency because it does away with the necessity to synthesize NAD+ and FAD every time.

4. One ATP is produced by substrate-level ATP synthesis.

Generating ATP The electron transport chain is located within the cristae of the mitochondria. The cristae increase the internal surface area of a mitochondrion, thereby increasing the area devoted to ATP formation. Figure 7.7 is a simplified overview of the electron transport chain. Figure 7.8 shows how the components of the electron transport chain are arranged in the cristae. We have been stressing that the carriers of the electron transport chain accept electrons, which they pass from one to the other. What happens to the hydrogen ions from NADH + H+? The complexes in the cristae use the energy released by electrons as they move down the electron transport chain to pump H+ from the mitochondrial matrix into the space between the outer and inner membrane of a mitochondrion. This space is called the intermembrane space. The pumping of H+ into the intermembrane space establishes an unequal distribution of H+ ions; there are many H+ in the intermembrane space and few in the matrix of a mitochondrion. This means that the intermembrane space is positively charged in relation to the matrix as well as more acidic.



Chapter 7  Cellular Respiration

NADH

e–

NADH

e–

e–

e–

e–

Glycolysis glucose

pyruvate

e–

NADH and FADH2

e–

Electron transport chain and chemiosmosis

Citric acid cycle

Preparatory reaction

2 ATP 2 ADP

4 ADP

4 ATP total 2

net

ATP

2 ADP

2

ATP

32 or ADP 32 or 34 34

ATP

NADH + H+ e– d re

NAD+ + 2H+

ion uc t electron carrier

o

xid

atio n

ADP +

P

2e–

ATP

made by chemiosmosis

e–

electron carrier

The cristae also contain an ATP synthase complex. The H+ ions flow through an ATP synthase complex from the intermembrane space into the matrix. The flow of H+ through an ATP synthase complex brings about a change in shape, which causes the enzyme ATP synthase to synthesize ATP from ADP + P . Mitochondria produce ATP by chemiosmosis, which indicates that ATP production is tied to an electrochemical gradient, namely the unequal distribution of H+ across the cristae. Once formed, ATP molecules pass into the cytoplasm. Most ATP is produced by the electron transport chain and chemiosmosis. Per glucose molecule, ten NADH and two FADH2 take electrons to the electron transport chain. For each NADH formed inside the mitochondria by the citric acid cycle, three ATP result, but for each FADH2, only two ATP are produced. Figure 7.8 explains the reason for this difference: FADH2 delivers its electrons to the transport chain after NADH, and therefore these electrons do not participate in as many redox reactions and don’t pump as many H+ as NADH. Therefore, FADH2 cannot account for as much ATP production.  Chemiosmosis is similar to using water behind a dam to generate electricity. The pumping of H+ out of the matrix into the intermembrane space is like pumping water behind the dam. The floodgates of the dam are like the ATP synthase complex. When the floodgates are open, the water rushes through, generating electricity. In the same way, H+ rushing through the ATP synthase complex is used to produce ATP.

FADH2

Energy Yield from Cellular Respiration

FAD + 2H+

2e–

Figure 7.9 provides the theoretical ATP for each stage of cellular respiration. However, we know now that cells rarely ever achieve these theoretical values. Several factors can lower the ATP yield for each molecule of glucose entering the pathway: 

electron carrier ADP +

P

■■

2e– ATP

made by chemiosmosis

electron carrier

2e–

■■

electron carrier ADP + P 2e–

ATP

2H+ 1 2

O2

121

made by chemiosmosis

H2O

Figure 7.7  The electron transport chain.  NADH and FADH2

In some cells, NADH cannot cross mitochondrial membranes, but a “shuttle” mechanism allows its electrons to be delivered to the electron transport chain inside the mitochondria. The cost to the cell is one ATP for each NADH that is shuttled to the ETC. This reduces the overall count of ATP produced as a result of glycolysis, in some cells, to four instead of six ATP.  At times, cells need to expend energy to move ADP molecules and pyruvate into the cell and to establish protein gradients in the mitochondria. 

There is still considerable research into the precise ATP yield per glucose molecule. However, most estimates place the actual yield at around 30 ATP per glucose. Using this number we can calculate that only between 32 and 39 percent of the available energy is usually transferred from glucose to ATP. The rest of the energy is lost in the form of heat.  It is also important to note that while glucose is the preferred energy source for the cellular respiration pathways, our diets do not consist entirely of carbohydrates. The Health feature, “Metabolic Fate of Pizza,” explores how proteins and lipids (fats) may be used to generate energy by cellular respiration.

bring electrons to the electron transport chain. As the electrons move down the chain, energy is captured and used to form ATP. For every two electrons that enter by way of NADH, two to three ATP result. For every two electrons that enter by way of FADH2, one to two ATP result. Oxygen, the final acceptor of the electrons, becomes a part of water.

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UNIT 1  Cell Biology

Figure 7.8  Cellular organization of the electron

cristae

transport chain.  The electron transport chain is located in the cristae of the mitochondria. As electrons move from one protein complex to the other, hydrogen ions (H+) are pumped from the mitochondrial matrix into the intermembrane space. As hydrogen ions flow down a concentration gradient from the intermembrane space into the matrix, ATP is synthesized by the enzyme ATP synthase. ATP leaves the matrix by way of a channel protein.

intermembrane space

H+

H+

H+

matrix

Electron transport chain

NADH-Q reductase

H+

H+

H+

cytochrome reductase

H+

H+

cytochrome c

H+

coenzyme Q

H+

cytochrome oxidase

H+ H+

e-

H+

eFADH2 high energy electron

H+

NADH

NAD+

FAD + 2 H+

H+

ATP

low energy electron

eH+

2

H2O

ADP + P

H+

H+

H+

1 2

H+

H+

O2

H+ H+

Matrix

H+

H+ H+

ATP channel protein

H+

+

H

ATP synthase complex

Chemiosmosis ATP

H+ H+

H+

H+

H+

Intermembrane space H+

H+



Chapter 7  Cellular Respiration

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HEALTH

Metabolic Fate of Pizza Obviously our diets do not solely consist of carbohydrates. Because fats and proteins are also organic nutrients, it makes sense that our bodies can utilize the energy found in the bonds of these molecules. In fact, the metabolic pathways we have discussed in this chapter are more than capable of accessing the energy of fats and proteins. For example, lets trace the fate of a pepperoni pizza, which contains carbohydrates (crust), fats (cheese), and protein (pepperoni). We already know that the glucose in the carbohydrate crust is broken down during cellular respiration. When the cheese in the pizza (a fat) is used as an energy source, it breaks down to glycerol and three fatty acids. As Figure 7A indicates, glycerol can be converted to pyruvate and enter glycolysis. The fatty acids are converted to 2-carbon acetyl CoA that enters the citric acid cycle. An 18-carbon fatty acid results in nine acetyl CoA molecules. Calculation shows that respiration of these can produce a total of 108 ATP molecules. This is why fats are an efficient form of stored energy—the three long fatty acid chains per fat molecule can produce considerable ATP when needed. Proteins are less frequently used as an energy source, but are available as necessary. The carbon skeleton of amino acids can enter glycolysis, be converted to acetyl groups, or enter the citric acid cycle at some other juncture. The carbon skeleton is produced in the liver when an amino acid undergoes deamination, or the removal of the amino group. The amino group becomes ammonia (NH3), which enters the urea cycle and becomes part of urea, the primary excretory product of humans. Just where the carbon skeleton begins degradation depends on the length of the R group, because this determines the number of carbons left after deamination. In Chapter 14, “Digestive System and Nutrition,” we will take a more detailed look at the nutritional needs of humans, including discussions on how vitamins and minerals interact with metabolic pathways, and the dietary guidelines for proteins, fats, and carbohydrates.

proteins

carbohydrates

amino acids

glucose

Glycolysis

fats

glycerol

fatty acids

ATP

pyruvate

acetyl CoA

Citric acid cycle

ATP

Electron transport chain

ATP

Figure 7A  The metabolic pool concept.  Carbohydrates, fats, and

proteins can be used as energy sources, and their monomers (carbohydrates and proteins) or subunits (fats) enter degradative pathways at specific points.

Questions to Consider 1. How might a meal of a cheeseburger and fries be processed by the cellular respiration pathways?

2. While Figure 7A does not indicate the need for water, it is an important component of our diet. Where would water interact with these pathways?



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UNIT 1  Cell Biology

Figure 7.9  The energy

Cytoplasm

2 net

glycolysis

ATP

4 or 6

ATP

6

ATP

18

ATP

4

ATP

subtotal 32 or 34

ATP

2 NADH + H+ 2 pyruvate 2 NADH + H+

2 acetyl CoA

2 CO2 6 NADH + H+

2

ATP

Citric acid cycle

Electron transport chain

illustrates the energy yield per glucose molecule. Substrate-level ATP synthesis during glycolysis and the citric acid cycle accounts for four ATP. Chemiosmosis accounts for a maximum of 32 or 34 ATP, depending on the shuttle mechanism involved for transporting cytoplasmic NADH + H+ into the mitochondrion. The theoretical maximum total of ATP is therefore 36 to 38 ATP, although most cells produce fewer ATP.

glucose

Mitochondrion

yield of cellular respiration.  This diagram

FADH2

2 4 CO2

6 O2 subtotal 4

ATP 36 to 38 total

Check Your Progress  7.4 1. Explain the relationship between the metabolic pathways

within the mitochondria and glycolysis. 2. Calculate the number of NADH, FADH2, and ATP molecules produced by each stage of cellular respiration per glucose molecule. 3. Discuss why there is variation in the number of ATP molecules produced per glucose.

6 H2O

ATP

Conclusion Whether you are playing basketball, studying for an exam, or even sleeping, your metabolic pathways have the ability to break down energy nutrients, primarily carbohydrates, and produce ATP. The specific pathway that is followed is dependent on the availability of oxygen. During aerobic exercise, oxygen is available for the muscles to completely break down glucose to carbon dioxide and water. If oxygen supplies are low, then muscle cells may switch to fermentation pathways to produce small amounts of ATP to sustain muscle function. 

MEDIA STUDY TOOLS http://connect.mheducation.com Maximize your study time with McGraw-Hill SmartBook®, the first adaptive textbook.

 

MP3 Files

7.1  Cellular Respiration

 

Animations

7.1  How the NAD+ Works 7.2  How Glycolysis Works 7.4  How the Krebs Cycle Works • Electron Transport System and ATP Synthesis • Proton Pump

 

3D Animation

7.2  Cellular Respiration: Glycolysis 7.4  Cellular Respiration: Citric Acid Cycle • Cellular Respiration: Electron Transport Chain • Cellular Respiration: Summary

  Tutorial 7.4  Electron Transport Chain



Chapter 7  Cellular Respiration

SUMMARIZE 7.1  Overview of Cellular Respiration ■ The metabolic pathways that produce ATP may be either aerobic or

anaerobic, depending on the availability of oxygen. 

■ During aerobic cellular respiration, glucose is oxidized to CO2 and

H2O. Four phases are required for glucose to be metabolized to carbon dioxide and water. These include glycolysis, the preparatory reactions, the citric acid cycle, and the electron transport chain. During oxidation of substrates, electrons are removed along with hydrogen ions (H+). NAD+ (nicotinamide adenine dinucleotide) and FAD (flavin adenine dinucleotide) act as electron carriers. In the absence of oxygen, fermentation reactions occur.

7.2  Outside the Mitochondria: Glycolysis ■ Glycolysis, the breakdown of glucose to two pyruvates, is a series of

anaerobic enzymatic reactions that occur in the cytoplasm. ■ Oxidation by NAD+ releases enough energy immediately to give a net gain of two ATP by substrate-level ATP synthesis. Two NADH + H+ are formed.

7.3  Outside the Mitochondria: Fermentation ■ Fermentation involves glycolysis, followed by the reduction of pyru-

vate by NADH + H+ to either lactate or alcohol and CO2. The reduction process regenerates NAD+ so that it can accept more electrons during glycolysis. ■ Although fermentation results in only two ATP, it still serves a purpose: In humans, it provides ATP energy for short-term, strenuous muscular activity. The accumulation of lactate puts the individual in oxygen debt because oxygen is needed to completely metabolize lactate to CO2 and H2O.

125

mitochondrial membrane, the ATP counts are reduced by two because the electrons from NADH generated in the cytoplasm during glycolysis must be transferred from outside to inside the mitochondria, using two ATP. ■ Of the maximum number of 36 or 38 ATP formed by cellular respiration, four are produced outside the electron transport chain: two by glycolysis and two by the citric acid cycle. The rest are produced by the electron transport chain.

ASSESS Testing Yourself Choose the best answer for each question.

7.1  Overview of Cellular Respiration For questions 1–5, match the letters in the diagram to the appropriate statement. 1. Preparatory reaction 2. Electron transport chain 3. Glycolysis 4. Citric acid cycle 5. NADH + H+ and FADH2

NADH + H+

NADH + H+

e. a.

b.

c.

d.

7.4  Inside the Mitochondria ■ Pyruvate from glycolysis enters a mitochondrion, where the prepara-

■

■

■

■

tory (prep) reaction takes place. During this reaction, oxidation occurs as CO2 is removed. NAD+ and FAD are reduced, and CoA receives the C2 acetyl group that remains. Because the reaction must take place twice per glucose, two NADH + H+ result. The acetyl group enters the citric acid cycle, a series of reactions located in the mitochondrial matrix. Complete oxidation follows, as two CO2, three NADH + H+, and one FADH2 are formed. The cycle also produces one ATP. The entire cycle must turn twice per glucose molecule. The final stage of glucose breakdown involves the electron transport chain located in the cristae of the mitochondria. The electrons received from NADH and FADH2 are passed down a chain of electron carriers until they are finally received by O2, which combines with H+ to produce H2O. As the electrons pass down the chain, ATP is produced. The carriers of the electron transport chain are located in protein complexes on the cristae of the mitochondria. Each protein complex receives electrons and pumps H+ into the intermembrane space, setting up an electrochemical gradient. When H+ ions flow down this gradient through the ATP synthase complex, energy is released and used to form ATP molecules from ADP and P . This is ATP synthesis by chemiosmosis. To calculate the total number of ATP produced per glucose molecule, consider that for each NADH + H+ formed inside the mitochondrion, three ATP are produced. Each molecule of FADH2 results in the formation of only two ATP because the electrons enter the electron transport chain at a lower energy level than the electrons delivered by NADH. In cells that cannot transfer NADH across the

ATP

ATP

ATP

7.2  Outside the Mitochondria: Glycolysis 6. During glycolysis, what is the net production of ATP per glucose molecule? a. 0 b. 1 c. 2 d. 8 e. 32–34 7. Which of the following is not a product or reactant of glycolysis? a. NADH b. ATP c. pyruvate d. oxygen

7.3  Outside the Mitochondria: Fermentation 8. Which of these is incorrect regarding fermentation? a. there is a net gain of only 2 ATP per glucose b. it occurs in the cytoplasm c. the process starts with glucose entering glycolysis d. NADH donates electrons to the electron transport chain e. it is anaerobic



126

UNIT 1  Cell Biology

9. Fermentation is primarily involved in the recycling of a. ADP. b. oxygen. c. pyruvate. d. NAD+.

7.4  Inside the Mitochondria 10. The greatest contributor of electrons to the electron transport chain is a. oxygen.  b. the prep reaction. c. glycolysis.  d. fermentation. e. the citric acid cycle.  11. Which of these is not true of the citric acid cycle? a. The citric acid cycle includes the prep reaction.  b. The citric acid cycle produces ATP by substrate-level ATP synthesis. c. The citric acid cycle occurs in the mitochondria. d. The citric acid cycle produces two ATP per glucose molecule.  12. Which of these is not true of the electron transport chain? a. The electron transport chain is located on the cristae of the mitochindria.  b. The electron transport chain produces more NADH than any metabolic pathway. c. The electron transport chain contains cytochrome molecules. d. The electron transport chain ends when oxygen accepts electrons. 

13. The oxygen required by cellular respiration is reduced and becomes part of which molecule? a. ATP  c. H2O b. pyruvate d. CO2

ENGAGE BioNOW Want to know  how this science is relevant to your life? Check out the BioNow videos below: ■ Energy Part I : Energy Transfers ■ Energy Part II: Photosynthesis ■ Energy Part III: Cellular Respiration

Thinking Critically 1. Bacteria do not have mitochondria, and yet they contain an electron transport chain. On what membrane could this be located? 2. Rotenone is a broad-spectrum insecticide that inhibits the electron transport chain. Why might it be toxic to humans? 3. Some fat-burning compounds accelerate the movement of fatty acids into the cellular respiration pathways. Explain how these compounds may work.

PHOTO CREDITS Opener: © Getty Images Sport/Getty Images; 7A: © C Squared Studios/Getty RF.

UNIT 2  Plant Biology

CASE STUDY Colors of Fall Taking a walk in the woods in the fall when the leaves are turning colors can be an enjoyable and relaxing form of exercise. Interestingly, the same process that causes leaves to change colors in the fall is also involved in the ripening of fruits such as apples and pears—and the end product of the process may have health benefits as well! Leaves contain several types of pigments, including the green chlorophylls and the yellow to red carotenoids. These pigments absorb the solar energy that the plant utilizes to carry out photosynthesis. In the fall when lower temperatures signal a change in the seasons, the supply of water and nutrients to the leaves declines and chlorophyll begins to degrade. Now, the carotenoids, which were formerly masked by chlorophyll, become visible allowing us to enjoy the change of color. The breakdown of chlorophyll provides us with colorful woodlands and can also give us a healthy dose of antioxidants when we eat fruits.  Researchers have discovered that when a fruit ripens and its skin changes color, chlorophyll is degraded in the same manner as in leaves. This produces antioxidants that become concentrated in the skin. Antioxidants stabilize free radicals, dangerous molecules that otherwise damage the DNA and proteins of a cell. Several health problems, including cancer and heart disease, are thought to be promoted by the accumulation of free radicals. In this chapter, we will see how pigments are involved in the process of photosynthesis. The solar energy that is captured by the pigments is used by the plant to make its food. It is this food that will be used to sustain the plant and the organisms that feed on the plant. As you read through the chapter, think about the following questions:

1. Which pigments provide the maximum efficiency for a plant as it conducts photosynthesis?

2. Why do leaves appear green in the spring and summer and then turn

Photosynthesis

8

CHAPTER OUTLINE 8.1  Overview of Photosynthesis 8.2 Plants as Solar Energy Converters 8.3 Plants as Carbon Dioxide Fixers 8.4 Alternative Pathways for Photosynthesis 8.5 Photosynthesis versus Cellular Respiration

BEFORE YOU BEGIN Before beginning this chapter, take a few moments to review the following discussions: Section 3.3  Which cellular structures are necessary for photosynthesis? Figure 6.1  How does energy flow in biological systems? Section 6.3  What role do enzymes play in regulating metabolic processes?

to red or yellow in the fall?

127

128

UNIT 2  Plant Biology

8.1  Overview of Photosynthesis Learning Outcomes Upon completion of this section, you should be able to 1. Compare and contrast autotrophs and heterotrophs. 2. Explain the role of photosynthesis for all organisms on Earth. 3. Recognize the overall chemical equation for photosynthesis. 4. Describe the process of photosynthesis in terms of two sets of reactions that take place in a chloroplast.

Photosynthesis converts solar energy into the chemical energy of a carbohydrate. Photosynthetic organisms, including plants, algae, and cyanobacteria, are called autotrophs because they produce their own food (Fig. 8.1). Each year, photosynthesizing organisms produce between 100 and 200 billion metric tons of carbohydrates. No wonder photosynthetic organisms are able to sustain themselves and all other organisms on Earth. With some exceptions, it is possible to trace the majority of food chains back to plants and algae. In other words, producers,

which have the ability to synthesize carbohydrates, feed not only  themselves but also consumers, which must take in preformed  organic molecules. Collectively, consumers are called heterotrophs. Both autotrophs and heterotrophs use organic molecules produced by photosynthesis as a source of building blocks for growth and repair and as a source of chemical energy for cellular work. Pigments allow photosynthetic organisms to capture solar energy, which acts as the “fuel” that makes photosynthesis possible. Most photosynthetic organisms contain chlorophyll, the pigment that gives them a green color. However, the green of chlorophyll can be masked by other pigments. The carotenoids give photosynthesizing cells a yellow to red color. In addition, the phycobilins give red algae their red color and cyanobacteria a ­bluish color.

Flowering Plants as Photosynthesizers Although many different organisms can photosynthesize, we will focus our discussion on the flowering plants. Portions of the plant, particularly the leaves, contain chlorophyll and other pigments that enable the plant to carry on photosynthesis. The leaf of a flowering

Figure 8.1 

Photosynthetic organisms.  Photosynthetic organisms include (a) cyanobacteria such as Oscillatoria, which are a type of bacterium; (b) algae such as kelp, which typically live in water and can range in size from microscopic to macroscopic; and (c) plants such as the sequoia, which typically live on land.

a. Oscillatoria

40×

b. Kelp

c. Sequoias



Chapter 8  Photosynthesis

129

cuticle a. Leaf cross section upper epidermis

mesophyll

lower epidermis

CO2 O2

leaf vein inner membrane outer membrane

stomata

stroma

stroma granum

b. Chloroplast

d. Chloroplast, micrograph

23,000×

Figure 8.2  Leaves and photosynthesis.  a. The thylakoid space thylakoid membrane

c. Grana

channel between thylakoids

plant contains mesophyll tissue, which contains cells that are ­specialized for photosynthesis (Fig. 8.2a). The raw materials for photosynthesis are water and carbon dioxide. The roots of a plant absorb water, which then moves through vascular tissue up the stem to a leaf by way of the leaf veins. Carbon dioxide in the air enters a leaf through small openings called stomata (sing., stoma) that are surrounded by guard cells. After entering a leaf, carbon dioxide and water diffuse into the cells and enter the chloroplasts (Fig. 8.2b, d), the organelles that carry on photosynthesis. A double membrane surrounds a chloroplast and its fluidfilled interior, which is called the stroma. A different membrane system within the stroma forms flattened sacs called thylakoids (Fig. 8.2c). In some places, thylakoids are stacked to form grana (sing., granum), which were named because they looked like piles

raw materials for photosynthesis are carbon dioxide and water. Water, which enters a leaf by way of leaf veins, and carbon dioxide, which enters by way of the stomata, diffuse into the cells and enter the chloroplasts. b. Chloroplasts have two major parts. c. The grana are made up of thylakoids, membranous disks that contain photosynthetic pigments such as chlorophylls a and b. These pigments absorb solar energy. d. The stroma, as indicated in this micrograph, is a fluid-filled space where carbon dioxide is enzymatically reduced to carbohydrate.

of seeds to early microscopists. The space of each thylakoid is connected to the space of every other thylakoid within a chloroplast, thereby forming a continuous inner compartment within the ­chloroplast called the thylakoid space. Chlorophyll and other pigments that are part of a thylakoid membrane are capable of absorbing solar energy. This is the energy that drives photosynthesis. The stroma, filled with enzymes, is where carbon dioxide is first attached to an organic compound and is then reduced to a carbohydrate. Therefore, it is proper to associate the absorption of solar energy with the thylakoid membranes making up the grana, and to associate the reduction of carbon ­dioxide to a carbohydrate with the stroma of a chloroplast. Humans, and indeed nearly all organisms, release carbon dioxide into the air as a waste product from metabolic processes



130

UNIT 2  Plant Biology

such as cellular respiration. This is some of the same carbon ­dioxide that enters a leaf through the stomata and is converted to ­carbohydrate. Carbohydrate, in the form of glucose, is the chief energy source for most organisms.

The overall chemical equation for photosynthesis tells us why plants are so important to our lives. Plants provide us with food in the form of carbohydrates, and much of the oxygen that we breathe and use for cellular respiration.

Photosynthetic Reaction

Two Sets of Reactions

For convenience, the overall equation for photosynthesis is sometimes simplified in this manner:

The word photosynthesis suggests that the process requires two sets of reactions: photo, which means light, refers to the reactions that capture solar energy, and synthesis refers to the reactions that produce carbohydrate. The two sets of reactions are called the light reactions (lightdependent reactions) and the Calvin cycle reactions (light-­ independent reactions) (Fig. 8.3). We will see that the light reactions release oxygen and provide the molecules that allow the Calvin cycle reactions to reduce carbon dioxide to a carbohydrate. The coenzyme NADP+ (nicotinamide adenine dinucleotide phosphate) carries hydrogen atoms from the light reactions to the Calvin cycle reactions. When NADP+ accepts hydrogen atoms, it becomes NADPH. Also, ATP carries energy from the light ­reactions to the Calvin cycle reactions. The light reactions are sometimes called the light-­dependent reactions because they cannot occur unless light is present. The  Calvin cycle reactions are sometimes called the light-­ independent reactions because they will occur whether light is present or not.

6 CO2 + 6 H2O

solar energy pigments

C6H12O6 + 6 O2

This equation shows glucose and oxygen as the products of photosynthesis. The oxygen given off by photosynthesis comes from water. This was proven experimentally by exposing plants first to CO2 and then to H2O that contained an isotope of oxygen called heavy oxygen (18O). Only when heavy oxygen was a part of water did this isotope appear in O2 given off by the plant. Therefore, O2 released by chloroplasts comes from H2O, not from CO2. The overall equation tells us that during photosynthesis, H2O is oxidized and CO2 is reduced to form a carbohydrate. Reduction of any molecule requires energy. During photosynthesis this energy is ultimately provided by the sun.

solar energy

H2O

CO2

ADP + P NADP+ Light reactions

Calvin cycle reactions

NADPH ATP

stroma

thylakoid membrane O2

CH2O

Figure 8.3  Overview of photosynthesis.  The process of photosynthesis consists of the light reactions and the Calvin cycle reactions. The light reactions, which produce ATP and NADPH, occur in the thylakoid membrane. These molecules are used in the Calvin cycle reactions in the stroma to reduce carbon dioxide to a carbohydrate.



Chapter 8  Photosynthesis

131

Visible Light

Check Your Progress  8.1 1. Recognize the primary source of energy for carbohydrate production in plants.

2. Identify the major components in the structure of a chloroplast.

3. Identify the overall equation for photosynthesis. 4. Describe the two sets of reactions involved in photosynthesis.

8.2  Plants as Solar Energy Converters Learning Outcomes Upon completion of this section, you should be able to 1. Identify the photosynthetic pigments required to absorb the various wavelengths of light necessary for photosynthesis. 2. Explain the role of the noncyclic electron pathway and the cyclic electron pathway. 3. Describe the organization of the thylakoid and how this organization is critical to the production of ATP during photosynthesis.

During the light reactions, the various pigments within the thylakoid membranes absorb solar energy. Solar energy (radiant energy from the sun) can be described in terms of its wavelength and its energy content. Figure 8.4 lists the different types of radiant energy, from the shortest wavelength, gamma rays, to the longest, radio waves. White, or visible, light is only a small portion of this spectrum.

Visible light itself contains various wavelengths of light. When it is passed through a prism, we see all the different colors that make up visible light. The sensory areas of our   brains identify these wavelengths as distinct colors. The colors in visible light range from violet (the shortest wavelength) to indigo, blue, green, y­ ellow, orange, and red (the longest wavelength). The energy content is highest for violet light and lowest for red light. Only about 42% of the solar radiation that hits Earth’s atmosphere ever reaches the surface of Earth, and most of this radiation is within the visible-light range. Higher-energy wavelengths are screened out by the ozone layer in the atmosphere, and lowerenergy wavelengths are screened out by water vapor and carbon dioxide (CO2) before they reach the Earth’s surface. Both the organic molecules within organisms and certain life processes, such as vision and photosynthesis, are adapted to the solar radiation that is most prevalent in the environment. The pigments found within most types of photosynthesizing cells, the chlorophylls a and b and the carotenoids, are capable of absorbing various portions of visible light. The absorption spectrum for these pigments is shown in Figure 8.5. Both chlorophyll a and chlorophyll b absorb violet, indigo, blue, and red light better than the light of other colors. Because green light is reflected and only minimally absorbed, leaves appear green to us. The yellow or orange carotenoids are able to absorb light in the violet-blue-green range. These pigments and others become noticeable in the fall when chlorophyll breaks down and the other pigments are uncovered. Photosynthesis begins when the pigments within thylakoid membranes absorb solar energy.

Increasing wavelength chlorophyll a chlorophyll b carotenoids

Gamma rays

X rays

UV

MicroInfrared waves

Radio waves

visible light

Relative Absorption

Increasing energy

380 380

500

600

750

500

600

750

Wavelengths (nm)

Wavelengths (nm)

Figure 8.4  The electromagnetic spectrum.  The electromagnetic

spectrum extends from the very short wavelengths of gamma rays through the very long wavelengths of radio waves. Visible light, which drives photosynthesis, is expanded to show its component colors. Each color varies in its wavelength and energy content.

Figure 8.5  Photosynthetic pigments and photosynthesis. 

The photosynthetic pigments in chlorophylls a and b and the carotenoids absorb certain wavelengths within visible light. This is their absorption spectrum. Notice that they do not absorb green light. That is why the leaves of plants appear green to us. You observe the color that is not absorbed by the leaf.



132

UNIT 2  Plant Biology

H2O

CO2

solar energy

ADP + P NADP+

sun

Light reactions

Calvin cycle

sun

NADPH ATP

thylakoid membrane

energy level

electron acceptor

electron acceptor O2

CH2O

e– e–

e–

ele

e–

e–

ctro

e–

e– n tr ans por t ch

ATP

e–

ain

NADP+ H+

e–

(ET

e–

C)

e– e–

NADPH

e– reaction center

reaction center

pigment complex

pigment complex

Photosystem I e–

Photosystem II CO2

H2O

CH2O

Calvin cycle reactions

2H+

1 2

O22

Figure 8.6  Noncyclic electron pathway: Electrons move from water to NADP+.  Energized electrons (taken from water, which splits,

releasing oxygen) leave photosystem II and pass down an electron transport chain, leading to the formation of ATP. Energized electrons (replaced by photosystem II) leave photosystem I and pass to NADP+, which then combines with H+, becoming NADPH.

Light Reactions The light reactions that occur in the thylakoid membrane consist of two electron pathways called the noncyclic electron pathway and the cyclic electron pathway. Both electron pathways produce ATP, but only the noncyclic pathway also produces NADPH.

Noncyclic Electron Pathway The noncyclic electron pathway is so named because the electron flow can be traced from water to a molecule of NADP+ (Fig. 8.6). This pathway uses two photosystems, called photosystem I (PS I) and photosystem II (PS II). The photosystems are named for the order in which they were discovered, not for the order in which they participate in the photosynthetic process. A photosystem consists of a pigment complex (molecules of chlorophyll a, chlorophyll b, and the carotenoids) and an electron acceptor within the thylakoid membrane. The pigment complex serves as an “antenna” for gathering solar energy.

The noncyclic pathway begins with photosystem II. The pigment complex absorbs solar energy, which is then passed from one pigment molecule to another until it is concentrated in a particular pair of chlorophyll a molecules called the reaction center. Electrons (e–) in the reaction center chlorophyll become so energized that they escape from the reaction center and move to a nearby electron acceptor. Photosystem II would disintegrate without replacement electrons. These replacement electrons are provided by water, which splits, releasing oxygen to the atmosphere. Many organisms, including plants and even ourselves, use this oxygen. The hy­drogen ions (H+) stay in the thylakoid space and contribute to the formation of a hydrogen ion gradient. As shown in the overall chemical reaction for photosynthesis, water is used up and oxygen is produced. The electron acceptor that received the energized electrons from the reaction center will send the electrons down a series of carriers that pass electrons from one to the other, known as the electron transport chain. As the electrons pass from one carrier to



133

Chapter 8  Photosynthesis

the next, the energy that is released is used to move hydrogen ions (H+) from the stroma into the thylakoid space, forming a hydrogen ion gradient. When these hydrogen ions flow down their electrochemical gradient through ATP synthase complexes, ATP production occurs, as will be described shortly. Notice that this ATP will be used in the Calvin cycle reactions in the stroma to reduce carbon dioxide to a carbohydrate. Similarly, when the photosystem I pigment complex absorbs solar energy, energized electrons leave its reaction center and are captured by a different electron acceptor. After going through the electron transport chain, the electrons from photosystem II are now low-energy electrons. These electrons are used to replace those lost by photosystem I. The electron acceptor in photosystem I passes its electrons to NADP+ molecules. Each NADP+ accepts two electrons H2O

The following molecular complexes are present in the thylakoid membrane and are involved in the noncyclic electron pathway (Fig. 8.7): ■■

■■

Photosystem II, which consists of a pigment complex and an electron acceptor molecule, receives electrons from water, which splits, releasing oxygen. The electron transport chain carries electrons from photosystem II to photosystem I, and pumps H+ from the stroma into the thylakoid space.

produces NADPH and ATP. Electrons move through photosystem II, photosystem I, and electron transport chains within the thylakoid membrane. Electrons pass to NADP+, after which it becomes NADPH. A carrier at the start of the electron transport chain pumps hydrogen ions from the stroma into the thylakoid space. When hydrogen ions flow back out of the space into the stroma through the ATP synthase complex, ATP is produced from ADP + P  .

ADP + P

Light reactions

The Organization of the Thylakoid Membrane

Figure 8.7  Organization of a thylakoid.  Each thylakoid membrane within a granum

CO2

solar energy

NADP+

and an H+ to become a reduced form of the m ­ olecule—that is, NADPH. This NADPH will also be used by the Calvin cycle reactions in the stroma to reduce carbon dioxide to a carbohydrate.

Calvin cycle reactions

NADPH ATP

thylakoid membrane

thylakoid membrane O2

thylakoid space

CH2O

thylakoid

granum

photosystem II H+

electron transport chain

stroma photosystem I

Pq

NADP reductase

e– e–

H+

e–

NADP+

NADPH

e–

e– H+ H2O

2

H+

+

H+

H+

H+

H+

H+ H+

H+

H+

1 2 O2

ATP synthase

H+

H+

H+ H+ Thylakoid space

H+

H+

ATP

H+ H+

chemiosmosis P

+ ADP

Stroma



134 ■■

■■

UNIT 2  Plant Biology

Photosystem I, which also consists of a pigment complex and an electron acceptor molecule, is adjacent to NADP reductase, which reduces NADP+ to NADPH. The ATP synthase complex crosses the thylakoid membrane and contains an interior channel and a protruding ATP synthase, an enzyme that joins ADP + P .

Cyclic Electron Pathway Under some environmental conditions, such as high oxygen levels, NADPH levels may accumulate in the cell, and therefore photosynthetic cells may enter the cyclic electron pathway (Fig. 8.8). This pathway is also found in many prokaryotic cells. It differs from the noncyclic reactions in that the electrons are recycled back to photosystem I. The pathway begins when the photosystem I

H2O

solar energy

CO2

ATP Production The thylakoid space acts as a reservoir for hydrogen ions (H+). Each time oxygen is removed from water, two H+ remain in the thylakoid space. As the electrons move from carrier to carrier along the electron transport chain, the electrons give up energy, which is used to pump H+ from the stroma into the thylakoid space. This is like pumping water into a reservoir. The end result is that there are more H+ in the thylakoid space than in the stroma. Because H+ is charged, this is an electrochemical gradient. Not only are there more hydrogen ions, but there are more positive charges within the thylakoid space than the stroma. When a channel is opened in the ATP synthase complex, H+ flows from the thylakoid space into the stroma, just like water flowing out of the reservoir. The flow of H+ from high to low concentration across the thylakoid membrane provides the energy that allows the ATP synthase enzyme to enzymatically produce ATP from ADP + P . This method of producing ATP is called chemiosmosis because ATP production is tied to the establishment of an H+ gradient.

ADP + P NADP+

Calvin cycle reactions

Light reactions NADPH + H+ ATP

thylakoid membrane O2

CH2O

Check Your Progress  8.2

electron acceptor

e– e–

elec tro n

tr

1. Explain why leaves appear green. 2. Compare the production of NADPH to ATP in noncyclic

ATP

e–

photosynthesis.

e–

3. Identify which part of a thylakoid will contain the

an

energy level

sun

e–

Photosystem I

spor t chain

reaction center

pigment complex absorbs solar energy that is passed from one pigment to the next until it becomes concentrated in a reaction center. As with photosystem II, electrons (e–) become so energized that they escape from the reaction center and move to nearby electron acceptor molecules. This time, instead of the electrons moving on to NADP+, energized electrons (e–) taken up by an electron acceptor are sent down an electron transport chain. As the electrons pass from one carrier to the next, their energy becomes stored as a hydrogen (H+) gradient. Again, the flow of hydrogen ions down their electrochemical gradient through ATP synthase complexes produces ATP. The spent electrons return to photosystem I after the electron transport chain. This is how photosystem I receives replacement electrons and why this electron pathway is called cyclic. It is also why the cyclic pathway produces ATP but does not produce NADPH.

e– e–

CO2

CH2O

Calvin cycle reactions and other enzymatic reactions

pigment complex

Figure 8.8  Cyclic electron pathway: Electrons leave and

return to photosystem I.  Energized electrons leave the photosystem I reaction center and are taken up by an electron acceptor, which passes them down an electron transport chain before they return to photosystem I. Only ATP production occurs as a result of this pathway.

photosystems, electron transport chain, and the ATP synthase complex. 4. Describe why the H+ gradient across a thylakoid membrane is referred to as a storage of energy.

8.3  Plants as Carbon Dioxide Fixers Learning Outcomes Upon completion of this section, you should be able to 1. Describe the three phases of the Calvin cycle. 2. Explain how the products of the Calvin cycle are used to form the other molecules found in plants.



Chapter 8  Photosynthesis

The Calvin cycle reactions take place after the light reactions. This is a series of reactions that produce a carbohydrate before returning to the starting point (Fig. 8.9). The cycle is named for Melvin Calvin, who, with colleagues, used the radioactive isotope 14C as a tracer to discover the reactions making up the cycle. This series of reactions uses atmospheric carbon dioxide to produce a carbohydrate. Humans and most other organisms take in oxygen from the atmosphere and release carbon dioxide. The ­Calvin cycle includes (1) carbon dioxide fixation; (2) carbon

135

dioxide reduction; and (3) regeneration of RuBP (ribulose-1, 5-bisphosphate), the starting material of the cycle.

Fixation of Carbon Dioxide Carbon dioxide fixation is the first step of the Calvin cycle. During this reaction, three molecules of carbon dioxide from the atmosphere are attached to three molecules of RuBP, a 5-carbon molecule. The result is three 6-carbon molecules. Each 6-carbon

H2O

CO2

solar energy

ADP + P NADP+ Light reactions

Calvin cycle

NADPH ATP

Metabolites of the Calvin Cycle

stroma O2

CH2O

3 CO2 intermediate

RuBP

ribulose-1,5-bisphosphate

3PG

3-phosphoglycerate

BPG

1,3-bisphosphoglycerate

G3P

glyceraldehyde-3-phosphate

3 C6

3 RuBP C5

6 3PG C3

CO2 fixation

CO2 reduction

Calvin cycle

3 ADP + 3 P

regeneration of RuBP These ATP molecules were produced by the light reactions.

3 ATP

5 G3P C3

6 ATP

6 ADP + 6 P

These ATP and NADPH molecules were produced by the light reactions.

6 BPG C3

6 NADPH 6 G3P C3

net gain of one G3P Other organic molecules

6 NADP+

×2 Glucose

Figure 8.9  The Calvin cycle reactions.  The Calvin cycle is divided into three portions: CO2 fixation, CO2 reduction, and regeneration of RuBP. Because five G3P are needed to re-form three RuBP, it takes three turns of the cycle to have a net gain of one G3P. Two G3P molecules are needed to form glucose.



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UNIT 2  Plant Biology

molecule then splits in half, forming a total of six 3-carbon ­molecules. This first 3-carbon molecule is called 3-­phosphoglycerate or 3PG. The enzyme that speeds this reaction, called RuBP carboxylase, is a protein that makes up about 20% to 50% of the protein content in chloroplasts. The reason for its abundance may be that it is unusually slow (it processes only a few molecules of substrate per second compared to thousands per second for a typical enzyme), and so there has to be a lot of it to keep the Calvin cycle going.

Reduction of Carbon Dioxide Each of two 3PG molecules undergoes reduction to G3P in two steps. The first step converts 3PG into 1,3-bisphosphoglycerate (BPG) using ATP, and the second reaction converts BPG into glyceraldehyde-3-phosphate (G3P) using NADPH.

SCIENCE IN YOUR LIFE   ►

ATP

ADP +

3PG

BPG

NADPH

P

G3P

NADP+

As 3PG becomes G3P, ATP becomes ADP + P , and NADPH becomes NADP+.

This sequence of reactions uses ATP and NADPH from the light reactions. These reactions also result in the reduction of

ECOLOGY

Diesel Power from Algae Algae are at the base of many food chains. But researchers are now using algae to produce a variety of different fuel types. Solazyme, a San Francisco–based company, has discovered the methods necessary to produce crude oil from various plant-based sugars with the help of microalgae (Fig. 8A). The process they have developed does not depend on the photosynthetic ability of the algae. It uses genetically modified algae grown in large vats. Instead of being exposed to sunlight, the algae are fed sugar that they convert into various types of oil. Different types of algae produce different types of oil. Because the algae are grown in the dark, the genes for photosynthesis have been switched off. With these genes turned off, the algae actually make more oil. Algae-based biofuels are considered “green” fuels. These fuels can be used for a variety of purposes, but one of the more common is the production of biodiesel that can be used in cars and mass transportation systems. Some of the benefits of Solazyme-based fuels over traditional petroleum-based fuels are that they meet industry standards and can be used with factory-standard diesel engines without any type of modifications. Not just automobiles may be powered by these biofuels. In 2010, Solazyme delivered 80,000 liters of marine diesel and jet fuel to the U.S. Navy that was derived from algae-based production. In 2011, a United Airways jet conducted the first commercial flight powered by biofuels. The oils produced by the algae are not only used for the production of fuel, but may

Figure 8A  Production of biodiesel from algae.  The algae in these containers have the

ability to produce a form of oil that can be used as a fuel.

also be used to manufacture cosmetics and food additives. With the increasing concern over the effects of global climate change on the world’s populations and ecosystems, many governments (both local and national) and private industries are looking to use biofuels as an alternative “green-energy” source. Currently, demand still outpaces the production abilities of the companies, but advances in technology

and techniques will increasingly make biofuels an attractive alternate energy source. 

Questions to Consider 1. Why would growing algae help reduce the carbon footprint of a city or town? 2. Fossil fuels are still relatively cheap to extract and produce. What technology advances do you think are needed to make biodiesel cheaper to produce?



Chapter 8  Photosynthesis

carbon dioxide to a carbohydrate because R—CO2 has become R—CH2O. Energy and electrons are needed for this reduction reaction, and they are supplied by ATP and NADPH.

Regeneration of RuBP Notice that the Calvin cycle reactions in Figure 8.9 are multiplied by three because it takes three turns of the Calvin cycle to allow one G3P to exit. For every three turns of the Calvin cycle, five molecules of G3P are used to re-form three molecules of RuBP and the cycle continues. Notice that 5 × 3 (carbons in G3P) = 3 × 5 (carbons in RuBP):

5 G3P 3 ATP

3 RuBP

Glucose phosphate can be combined with fructose (and the phosphate removed) to form sucrose, the molecule plants use to transport carbohydrates from one part of the plant to the other. Glucose phosphate is also the starting point for the synthesis of starch and cellulose. Starch is the storage form of glucose. Some starch is stored in chloroplasts, but most starch is stored in roots. Cellulose is a structural component of plant cell walls and becomes fiber in our diet because we are unable to digest it. A plant can use the hydrocarbon skeleton of G3P to form fatty acids and glycerol, which are combined in plant oils, such as corn oil, sunflower oil, or olive oil—commonly used in cooking. As described in the Ecology feature, “Diesel Power from Algae,” these oils are being used as a new source of fuel called biodiesel. Also, when nitrogen is added to the hydrocarbon skeleton derived from G3P, amino acids are formed.

Check Your Progress  8.3

3 ADP + P

1. Describe how carbon dioxide is fixed and then reduced to a carbohydrate.

As five molecules of G3P become three molecules of RuBP, three molecules of ATP become three molecules of ADP + P .

2. Illustrate why it takes three turns of the Calvin cycle to produce one glucose molecule.

3. Explain why G3P is an important molecule in plant metabolism.

This reaction also uses some of the ATP produced by the light reactions.

The Importance of the Calvin Cycle G3P (glyceraldehyde-3-phosphate) is the product of the Calvin cycle that can be converted to other molecules a plant needs. Compared to animal cells, algae and plants have enormous biochemical capabilities. They use G3P for glucose, sucrose, starch, cellulose, fatty acid, and amino acid synthesis. Glucose phosphate is one of the organic molecules that results from G3P metabolism. Glucose is the molecule that plants and animals most often metabolize to produce the ATP molecules they require for their energy needs. CO2

8.4  Alternative Pathways for Photosynthesis Learning Outcomes Upon completion of this section, you should be able to 1. Contrast C 3, C4, and CAM photosynthesis. 2. Explain how different photosynthetic modes allow plants to adapt to a particular environment.

Plants are able to live under all sorts of environmental conditions, and one reason is that various modes of photosynthesis have evolved (Fig. 8.10).

CO2

night

mesophyll C4 cell bundle sheath cell

RuBP Calvin cycle 3PG (C3)

137

CO2

CO2 C4

day

CO2

Calvin cycle

Calvin cycle

G3P

G3P

G3P mesophyll cell a. CO2 fixation in a C3 plant, tulip

b. CO2 fixation in a C4 plant, corn

c. CO2 fixation in a CAM plant, pineapple

Figure 8.10  Alternative pathways for photosynthesis.  Photosynthesis can be categorized according to how, where, and when CO2 fixation

occurs. a. In C3 plants, CO2 is taken up by the Calvin cycle directly in mesophyll cells. b. C4 plants form a C4 molecule in mesophyll cells prior to releasing CO2 to the Calvin cycle in bundle sheath cells. c. CAM plants fix CO2 at night, forming a C4 molecule.



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UNIT 2  Plant Biology

C3 Photosynthesis The leaves of C3 plants have a particular structure and a different means of fixing CO2 compared with C4 plants. In C3 plants, such as wheat, rice, and oats, mesophyll cells are in parallel layers. The bundle sheath cells around the plant veins do not contain chloroplasts (Fig. 8.11a). This structure exposes the cells containing the Calvin cycle to the incoming CO2. CO2 is fixed by RuBP carboxylase of the Calvin cycle, and the first detectable molecule following fixation is a 3-carbon molecule (see Fig. 8.9). Unfortunately, RuBP carboxylase can not only bind CO2, it can also bind with O2. When the enzyme binds oxygen, it undergoes a nonproductive, wasteful reaction called photorespiration because it uses oxygen and releases carbon dioxide. Photorespiration makes C3 photosynthesis an inefficient way to produce carbohydrate when oxygen concentrations rise in the leaf space. This can happen when drought conditions occur because this type of weather leads to the closing of stomata in order to conserve water.

C4 Photosynthesis In C4 plants, such as sugarcane and corn, the mesophyll cells are arranged in concentric rings around the bundle sheath cells, which also contain chloroplasts (Fig. 8.11b). In the mesophyll cells, CO2 is initially fixed by forming a 4-carbon molecule. The SCIENCE IN YOUR LIFE  ►

mesophyll cells

bundle sheath cell a. C3 Plant

vein stoma

bundle sheath cell

vein stoma

b. C4 Plant

Figure 8.11  C3 and C4 plant leaf cell arrangement.  a. C3

plants contain mesophyll cells in parallel layers. The bundle sheath cells do not contain chloroplasts. b. C4 plants contain mesophyll cells arranged in concentric rings around chlorophyll containing bundle sheath cells.

formation of this molecule accounts for the terms C4 plant and C4 photosynthesis. The 4-carbon molecule releases CO2 to the Calvin cycle and carbohydrate production follows. Because of the need to transport molecules from where CO2 is fixed to where the Calvin cycle is located, it would appear that the

ECOLOGY

The New Rice According to international government leaders who met in Japan in July of 2008, a global food crisis is developing. The world’s population continues to grow, and grain is being diverted for biofuels and to feed livestock. The world’s farmers are not keeping up with the demand for wheat, sorghum, maize, and rice. But according to the International Rice Research Institute (IRRI) in Los Baños, Philippines, two-thirds of the world’s poorest people subsist primarily on rice (Fig. 8B). So a shortage of rice affects world hunger more than the other grains. More rice is being consumed than grown, and world rice stockpiles have been declining since 2001. In order to keep up with demand, the world must grow 50 million tons more rice per year than it did in 2005. This requires an increase globally of 1.2% per year. Several environmental factors are causing these declines, including global climate change and regional flooding. For example, most varieties of rice will die if they are submerged for three or more days. However, one variety, called flood resistant (FR), could survive even if submerged for three weeks. A single gene, called the Sub1A gene, appeared to confer a significant degree of submergence tolerance to the FR variety. IRRI scientists were able to

Figure 8B  Rice to feed the world. 

Rice provides food for two-thirds of the world’s poorest people. New technologies may increase the production of rice.

developed by the Chinese are best suited for growth in the tropics and are not effective in temperate zones. In addition, the rice lacks in flavor compared to the inbred varieties. One of the more long-term research programs with rice is trying to convert rice from a C3 into a C4 plant! C4 plants are 50% more efficient at turning sunlight into food than C3 plants. There are two aspects of a C4 plant that would need to be introduced into rice. The first would be the actual enzymes involved in fixing carbon dioxide. The second would be the leaf anatomy of the mesophyll and bundle sheath cells. Researchers have succeeded in introducing the appropriate enzymes from maize, a C4 plant, into rice plants. Without the proper leaf anatomy, however, there is no guarantee the cloned enzymes will increase the efficiency of photosynthesis.

Questions to Consider introduce the gene into commercial rice strains via hybridization, and four varieties are currently in field trials. China plants approximately 57% of its rice crop in hybrid varieties. Hybrid rice produces approximately 20% more per acre than the traditional inbred varieties. The hybrids

1. If the poorest people in the world go hungry due to the lack of grain, how much difference would it make to solving world hunger if you reduced your meat and fuel consumption? 2. Are genetically modified organisms the only way to feed the world’s growing population?



Chapter 8  Photosynthesis

C4 pathway would be little utilized among plants. However, when the weather is hot and dry, C4 plants have an advantage. This is because when the stomata close in order to conserve water and oxygen increases in leaf air space, RuBP carboxylase is not exposed to this oxygen in C4 plants and photorespiration does not occur. Instead, in C4 plants, the carbon dioxide is delivered to the Calvin cycle, which is located in bundle sheath cells that are sheltered from the leaf air spaces. When the weather is moderate, C3 plants ordinarily have the advantage, but when the weather becomes hot and dry, C4 plants have the advantage, and we can expect them to predominate. In the early summer, C3 plants such as Kentucky bluegrass and creeping bent grass predominate in lawns in the cooler parts of the United States, but by mid-summer, crabgrass, a C4 plant, begins to take over.

CAM Photosynthesis Another alternative pathway, called the CAM pathway (Fig. 8.10c), has also evolved because of environmental pressures. CAM stands for crassulacean-acid metabolism. Crassulaceae is a family of flowering succulent plants that live in warm, arid regions of the world. CAM photosynthesis is prevalent among most succulent plants that grow in desert environments, including the cacti, but it is now known to occur in a variety of other plants. While the C4 pathway separates components of photosynthesis by location, the CAM pathway separates them in time. CAM plants fix CO2 into a 4-carbon molecule at night, when they can keep their stomata open without losing much water. The 4-carbon molecule is stored in large vacuoles in their mesophyll cells until the next day. During the day, the 4-carbon molecule releases the CO2 to the Calvin cycle within the same cell. Plants are capable of fixing carbon by more than one pathway. These pathways appear to be the result of adaptation to different climates.

139

respiration is the mitochondrion, whereas the organelle for photosynthesis is the chloroplast. Note that in some bacteria, such as the cyanobacteria, photosynthesis occurs without chloroplasts—this is because these bacteria do have thylakoid-like structures in the cell. Photosynthesis is the formation of glucose, whereas cellular respiration is the breaking down of glucose. Figure 8.12 compares the two processes. The following overall chemical equation for photosynthesis is the opposite of that for cellular respiration. The reaction in the forward direction represents photosynthesis, and the word energy stands for solar energy. The reaction in the opposite direction represents cellular respiration, and the word energy then stands for ATP:

energy + 6 CO2 + 6 H2O

photosynthesis

C6H12O6 + 6 O2

cellular respiration

Both photosynthesis and cellular respiration are metabolic pathways within cells, and therefore consist of a series of reactions that the overall equation does not indicate. Both pathways, which use an electron transport chain located in membranes, produce ATP by chemiosmosis. Both also use an electron carrier; photosynthesis uses NADP+, and cellular respiration uses NAD+. Both pathways utilize the following reaction, but in opposite directions. For photosynthesis, read the reaction from left to right; for cellular respiration, read the reaction from right to left:

ATP

ADP +

P

Check Your Progress  8.4 1. Identify various plants that use a method of photosynthesis other than C3 photosynthesis. 2. Explain why C4 photosynthesis is advantageous in hot, dry conditions.

8.5  Photosynthesis Versus Cellular Respiration Learning Outcomes Upon completion of this section, you should be able to 1. Compare the overall chemical equation for photosynthesis and cellular respiration. 2. Describe the similarities and differences between cellular respiration and photosynthesis.

Both plant and animal cells carry on cellular respiration, but animal cells cannot photosynthesize. Plants, algae, and cyanobacteria are capable of photosynthesis. The organelle for cellular

3PG

NAD(P)H + H+

G3P

NAD(P)+

Both photosynthesis and cellular respiration occur in plant cells. In plants, both processes occur during the daylight hours, whereas only cellular respiration occurs at night. During daylight hours, the rate of photosynthesis exceeds the rate of cellular respiration, resulting in a net increase and storage of glucose. The stored glucose is used for cellular metabolism, which continues during the night.

Check Your Progress  8.5 1. Explain the similarities, and differences, between cellular

respiration and photosynthesis. Use the chemical equation for these reactions in your answer. 2. Explain why a plant cell must contain both chloroplasts and mitochondria.



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UNIT 2  Plant Biology

ADP

Thylakoid membrane

ADP

ATP

H2O

Cristae

O2

H2O

ATP

O2

H2O

CO2

solar energy

NADH + H+

e–

NADH + H+

e–

e– ADP + P NADP+ Light reactions

e– NADH + H+ e– and FADH2

e–

e–

Glycolysis

Calvin cycle reactions

glucose

Citric acid cycle

Preparatory reaction

pyruvate

Electron transport chain

NADPH

ATP

stroma thylakoid membrane

Stroma

4 ADP 4 ATP total

CH2O

O2

NADPH CO2

2 ATP 2 ADP 2 ATP net gain

NADP

+

CH2O

Photosynthesis

Matrix

2 ADP

2 ATP

NAD+ CH2 O

32 ADP 32 ATP or 34 or 34

NADH CO2

Cellular Respiration

Figure 8.12  Photosynthesis versus cellular respiration.  Both photosynthesis and cellular respiration have an electron transport chain located within membranes, where ATP is produced. Both processes have enzyme-catalyzed reactions located within the fluid interior of respective organelles. In photosynthesis, hydrogen atoms are donated by NADPH + H+ when CO2 is reduced in the stroma of a chloroplast. During cellular respiration, NADH forms when glucose is oxidized in the cytoplasm and glucose breakdown products are oxidized in the matrix of a mitochondrion.

Conclusion Throughout the summer and fall every year, plants undergo a wide variety of color changes. During the summer, leaves and fruits are typically green due to the large amounts of chlorophyll present in them. As the season progresses, the chlorophyll degrades and the carotenoids, yellow to red pigments, become more visible. It is this wide range of pigments that enables the plants of the world to absorb the necessary solar energy to run photosynthesis. Once the solar energy is absorbed, a series of chemical reactions are set in place that will ultimately produce

oxygen and glucose. The consumers of the world require this oxygen and glucose to run cellular respiration. The additional benefit that scientists have discovered is that the degradation of chlorophyll in the skin of fruits results in the production of antioxidants. It is thought that antioxidants help stabilize free radicals, the molecules that can damage DNA and lead to cancer. Photosynthesis not only supplies the oxygen and glucose necessary for consumers, but it also has health benefits for us as well.



Chapter 8  Photosynthesis

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Animations

8.2  Electron Transport System and ATP Synthesis • Proton Pump

 

  Tutorials

3D Animations

8.1  Photosynthesis: Structure of a Chloroplast 8.2  Photosynthesis: Properties of Light • Photosynthesis: Light-Dependent Reactions 8.3  Photosynthesis: Calvin Cycle

SUMMARIZE 8.1  Overview of Photosynthesis ■ Photosynthesis is absolutely essential for the continuance of life

because by this process autotrophs, such as plants, manufacture the food needed for all life, including the heterotrophs, to survive. ■ Chloroplasts carry on photosynthesis. During photosynthesis, solar energy is converted to chemical energy within carbohydrates. A chloroplast contains two main portions: the stroma and membranous grana made up of thylakoid sacs. Chlorophyll and other pigments like the carotenoids that are located within the thylakoid membrane absorb solar energy, and enzymes in the fluid-filled stroma reduce CO2. CO2 enters the leaf through small openings called stomata. ■ The overall equation for photosynthesis is 6 CO2 + 6 H2O → C6H12O6 + 6 O2. Photosynthesis consists of two reactions: the light reactions and the Calvin cycle reactions. The Calvin cycle reactions, located in the stroma, use NADP+  (nicotinamide adenine dinucleotide phosphate) and ATP to reduce carbon dioxide. These molecules are produced by the light reactions located in the thylakoid membranes of the grana, after chlorophyll captures the energy of sunlight.

8.2  Plants as Solar Energy Converters ■ Photosynthesis uses solar energy in the visible-light range. Chloro-

phylls a and b and the carotenoids largely absorb violet, indigo, blue, and red wavelengths and reflect green wavelengths. This causes leaves to appear green to us. ■ Photosynthesis begins when pigment complexes within photosystem I and photosystem II absorb radiant energy. In the noncyclic electron pathway, electrons are energized in photosystem II before they enter an electron transport chain. Electrons from H2O replace those lost in photosystem II. As the electrons pass through the electron transport chain, they help establish a hydrogen ion gradient. The electrons energized in photosystem I pass to NADP+, which becomes NADPH. Electrons from the electron transport chain replace those lost by photosystem I. ■ In the cyclic electron pathway of the light reactions, electrons energized by the sun leave photosystem I and enter an electron transport chain that produces a hydrogen ion gradient. Then the energy-spent electrons return to photosystem I.

8.2  Noncyclic Photosynthesis 8.3  Calvin Cycle

■ The hydrogen ion gradient across the thylakoid membrane is used to

synthesize ATP using an ATP synthase enzyme complex.

8.3  Plants as Carbon Dioxide Fixers ■ The ATP and NADPH made in thylakoid membranes pass into the

stroma, where carbon dioxide is reduced during the Calvin cycle reactions. ■ Carbon dioxide fixation attaches CO2 to a 5-carbon molecule named RuBP by the enzyme RuBP carboxylase. The resulting 6-carbon molecule splits into two molecules of 3-carbons, each called 3PG. ATP and NADPH from the light reactions are then used to reduce 3PG to G3P. ■ G3P is used to synthesize various molecules, including carbohydrates such as glucose.

8.4  Alternative Pathways for Photosynthesis ■ In C3 plants, the first molecule observed following carbon dioxide

fixation is a 3-carbon molecule. The structure of these plants allows RuBP carboxylase to bind with oxygen during photorespiration. ■ Plants utilizing C4 photosynthesis have a different construction than plants using C3 photosynthesis. C4 plants fix CO2 in mesophyll cells and then deliver the CO2 to the Calvin cycle in bundle sheath cells. Now, O2 cannot compete for the active site of RuBP carboxylase when stomata are closed due to hot and dry weather. This represents a partitioning of pathways in space: carbon dioxide fixation occurs in mesophyll cells, and the Calvin cycle occurs in bundle sheath cells. ■ CAM plants fix carbon dioxide at night when their stomata remain open. This represents a partitioning of pathways in time: carbon ­dioxide fixation occurs at night, and the Calvin cycle occurs during the day.

8.5  Photosynthesis versus Cellular Respiration ■ Both photosynthesis and cellular respiration utilize an electron trans-

port chain and ATP synthesis. However, photosynthesis reduces CO2 to a carbohydrate. The oxidation of H2O releases O2. Cellular respiration oxidizes carbohydrate, and CO2 is given off. Oxygen is reduced to water.



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UNIT 2  Plant Biology

8. In the absence of sunlight, plants are not able to engage in the Calvin cycle due to a lack of a. ATP. c. NADPH. b. oxygen. d. Both a and c are correct.

ASSESS Testing Yourself Choose the best answer for each question.

8.4  Alternative Pathways for Photosynthesis

8.1  Overview of Photosynthesis

9. CAM photosynthesis a. is the same as C4 photosynthesis. b. is an adaptation to cold environments in the Southern Hemisphere. c. is prevalent in desert plants that close their stomata during the day. d. occurs in plants that live in marshy areas. e. stands for “chloroplasts and mitochondria.” 10. C4 photosynthesis a. is the same as C3 photosynthesis, because it takes place in chloroplasts. b. occurs in plants whose bundle sheath cells contain chloroplasts. c. takes place in plants such as wheat, rice, and oats. d. is an advantage when the weather is hot and dry. e. Both b and d are correct.

1. Label each of the items in the diagram below:

CO2

H2O

solar energy

ADP + P NADP+ Light reactions

e.

d.

NADPH ATP

c.

a. b.

CH2O

2. The function of light reactions is to a. obtain CO2. b. make carbohydrate. c. convert light energy into a usable form of chemical energy. d. regenerate RuBP. 3. The Calvin cycle reactions a. produce carbohydrate. b. convert one form of chemical energy into a different form of chemical energy. c. regenerate more RuBP. d. use the products of the light reactions. e. All of these are correct.

8.2  Plants as Solar Energy Converters 4. The final acceptor of electrons during the light reactions of the noncyclic electron pathway is a. PS I. d. NADP+. b. PS II. e. water. c. ATP. 5. The oxygen given off by photosynthesis comes from a. H2O. c. glucose. b. CO2. d. RuBP. 6. The noncyclic electron pathway, but not the cyclic pathway, generates a. 3PG. c. ATP. b. chlorophyll. d. NADPH.

8.5  Photosynthesis versus Cellular Respiration 11. Cellular respiration and photosynthesis both a. use oxygen. b. produce carbon dioxide. c. contain an electron transport chain. d. occur in the chloroplast. 12. Which of the following does not occur in photosynthesis? a. reduction of CO2 c. reduction of oxygen b. oxidation of water d. All of these are correct.

ENGAGE BioNOW Want to know  how this science is relevant to your life? Check out the BioNow videos below: ■ Energy Part I: Energy Transfers ■ Energy Part II: Photosynthesis ■ Energy Part III: Cellular Respiration

Thinking Critically 1. How do broad, thin leaves provide an advantage for photosynthesis? 2. Artificial leaves are beginning to be developed by biotechnology companies. Outline the basic characteristics that an artificial leaf would need to have to manufacture a sugar. 3. In the 17th century a Belgian doctor, Jan Baptista van Helmont, planted a small willow tree in a pot of soil. He weighed the tree and the soil. The tree was watered for 5 years and weighed 74.4 kg more than when he began, but the soil had only lost 57 g of mass. Explain what accounts for the plant’s increase in biomass.

8.3  Plants as Carbon Dioxide Fixers 7. The ATP and NADPH from the light reactions are used to a. split water. b. cause RuBP carboxylase to fix CO2. c. re-form the photosystems. d. cause electrons to move along their pathways. e. convert 3PG to G3P.

PHOTO CREDITS Opener(trees): © Don Johnston/Getty Images; opener(fruit): © Evelyn Jo Johnson; 8.1a: © Sinclair Stammers/Getty Images; 8.1b: © Chuck Davis/Stone/Getty Images; 8.1c: © Dynamic Graphics Group/Creatas/Alamy RF; 8.2d: © Science Source; 8A: © Agencja Fotograficzna; 8.10a: © Evelyn Jo Johnson; 8.10b: © David Frazier/Corbis RF; 8.10c: © Pixtal/Age fotostock RF; 8B: © Doable/A.collection/Getty Images.

CASE STUDY The Amazing Neem Tree If you were to walk into a health-food store looking for neem products, you would find oils, toothpaste, soap, and facial washes. At a plant nursery, you would find neem insecticide and fungicide. But what exactly is neem? The neem tree (Azadirachta indica) is found in India and Pakistan and is one of the oldest and widely used plants in the world. It is locally known as the “village pharmacy,” and the United Nations declared neem the “tree of the 21st century” due to its many uses in health and agriculture.  Neem’s nontraditional medical uses are many. Bark and roots act as an analgesic and diuretic, and provide flea and tick protection to dogs. The sap effectively treats skin diseases, such as psoriasis. Gum, a sticky substance exuded by stems, is used to combat scabies (an itch mite) and surface wounds. And in some countries, the tree’s woody twigs aid dental hygiene, in the form of homemade toothbrushes.  The neem tree also plays an important role in agriculture and pest control. In India and Pakistan, for example, neem leaves have traditionally been mixed with stored grains or placed in drawers with clothing, keeping insects at bay. Not surprisingly, the U.S. Department of Agriculture funds a significant amount of research on neem, due to its potential as an all-natural insecticide and fungicide.  In this chapter, we consider the structure of roots, stems, and leaves and discuss the specialized cells and tissues that make up these organs.  As you read through the chapter, think about the following questions:

1. How do each of the vegetative plant organs contribute to the success

9

Plant Organization and Function

CHAPTER OUTLINE 9.1  Cells and Tissues of Plants 9.2 Plant Organs and Systems 9.3 Monocot Versus Eudicot Plants 9.4 Organization of Roots 9.5 Organization of Stems 9.6 Organization of Leaves 9.7 Uptake and Transport of Nutrients BEFORE YOU BEGIN Before beginning this chapter, take a few moments to review the following discussions:

Section 2.3  How do the properties of water make it possible for the movement of nutrients in plants? Section 3.5  Which cell structures are unique to plants? Section 8.1  Which parts of the plant are responsible for conducting photosynthesis?

of flowering plants? 

2. How do modifications of vegetative organs increase fitness?

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UNIT 2  Plant Biology

9.1  Cells and Tissues of Plants Learning Outcomes Upon completion of this section, you should be able to 1. Explain the function of meristematic tissue. 2. Describe the types of tissues found in plants.

Plants are an exceptionally diverse kingdom of eukaryotes (see Fig. 1.5). Scientists have currently identified over 346,000 species of plants, the majority of which are the flowering plants, or angiosperms. We will explore the diversity of plants again in Chapter 30. In this chapter we will use the angiosperms as our example as we examine the structure and function of a typical plant.

Types of Tissue in Plants Meristematic tissue is present in the shoot tip and root tip, where it is called the apical meristem. Apical meristem continually produces three types of primary meristem that develop into the three types of specialized primary tissues in the body of a plant: protoderm gives rise to epidermal tissue; ground meristem produces ground tissue; and procambium produces vascular tissue. The functions of these three specialized tissues include the following: 1. Epidermal tissue forms the outer protective covering of a plant. 2. Ground tissue fills the interior of a plant. 3. Vascular tissue transports water and nutrients in a plant and provides support. A summary of each of these tissue types is presented in Table 9.1.

Meristematic Tissue Plants have levels of biological organization similar to animals (see Fig. 1.2). As in animals, a tissue is composed of specialized cells that perform a particular function, and an organ is a structure made up of multiple tissues. In plants, simple tissues are made up of a single cell type, whereas complex tissues contain a variety of cell types. All the tissue types in a plant arise from meristematic tissue (Fig. 9.1). Meristematic tissue allows a plant to grow its entire life because it retains cells that forever have the ability to divide and produce more tissues. Plants can grow their entire lives. Even a 5,000-year-old tree is still growing!

Epidermal Tissue The entire body of both nonwoody (herbaceous) and young woody plants is covered by a layer of epidermis. The walls of epidermal cells that are exposed to air are covered with a waxy cuticle to minimize water loss. The cuticle also protects against bacteria and other organisms that might cause disease. In roots, certain epidermal cells have long, slender projections called root hairs (Fig. 9.2a). The hairs increase the surface area of the root for absorption of water and minerals.

TABLE 9.1  Specialized Tissues in Plants Tissue Meristem cell

Cell Types

Description

Function

Tissue that covers roots, leaves, and stems

Protection, prevention of water loss

Periderm (older woody plants)

Composed of waterproof cork cells

Protection against attacks by fungi, bacteria, and animals

Parenchyma

Most abundant, corresponds to the typical plant cell

Gives rise to specialized, photosynthesis, and storage cells

Colenchyma

Contains thick primary walls

Gives flexible support to immature regions of plant body

Epidermal Epidermis tissue

Cell division

Ground tissue Meristem cell

Differentiated cell

Cell division

Meristem cell

Differentiated cell

Cell division

Meristem cell

Vascular: xylem

Differentiated cell

Figure 9.1  Role of meristematic tissue.  Meristematic tissue gives rise to a variety of different tissue types, including epidermal, ground, and vascular tissue.

Vascular: phloem

Sclerenchyma Contains thick secondary walls, contains lignin making the wall tough and hard

Supports the mature regions of the plant

Tracheids

Elongated with tapered ends

Transport water and minerals from roots to leaves

Vessel elements

Short and wide, contain perforated plates

Transport water and minerals from roots to leaves

Sieve-tube members

Lack nuclei, associated with a companion cell, sieve plate

Transport sugar and organic compounds throughout the plant



Chapter 9  Plant Organization and Function

Figure 9.2  Modifications of epidermal tissue.  a. Root epidermis has root hairs to

145

periderm

absorb water. b. Leaf epidermis contains stomata (sing., stoma) for gas exchange. c. Periderm includes cork and cork cambium. Lenticels in cork are important in gas exchange.

lenticel

cork cambium cork

chloroplasts

cabbage seedling

stoma root hairs

guard cell

epidermal cells

elongating tip of root a. Root hairs

b. Stoma of leaf

On stems, leaves, and reproductive organs, epidermal cells produce small hairs that have two important functions: protecting the plant from too much sun and conserving moisture. Sometimes these hairs help protect a plant from herbivores by producing toxic substances. Under the slightest pressure, the stiff hairs of the stinging nettle lose their tips, forming “hypodermic needles” that inject an intruder with a stinging secretion. A waxy cuticle reduces the gas exchange in leaves. Leaves will often contain specialized cells called guard cells and microscopic pores called stomata (sing., stoma) that assist in gas exchange. Guard cells, which are epidermal cells with chloroplasts, surround stomata (Fig. 9.2b). When the stomata are open, gas exchange is possible but water loss also occurs. In older woody plants, the epidermis of the stem is replaced by boxlike cork cells. At maturity, cork cells can be sloughed off, and new cork cells are made by a meristem called cork cambium (Fig. 9.2c). As the new cork cells mature, they increase slightly in volume, and their walls become encrusted with a lipid material that makes them waterproof and chemically inert. These nonliving cells protect the plant and make it resistant to attack by fungi,

100× a. Parenchyma cells

c. Cork of older stem

bacteria, and animals. Some cork tissues are commercially used for bottle corks and other products. The cork cambium overproduces cork in certain areas of the stem surface, causing ridges and cracks to appear. The areas where the cork layer is thin and the cells are loosely packed are called lenticels. Lenticels are important in gas exchange between the interior of a stem and the air.

Ground Tissue Ground tissues form the bulk of a plant and fill the space between the epidermal and the vascular tissue. Most of the photosynthesis and carbohydrate storage takes place in ground tissue. Ground tissue is also responsible for producing hormones, toxins, pigments, and other specialized chemicals. The cell types present in ground tissue are parenchyma, collenchyma, and sclerenchyma cells. Parenchyma cells are the most abundant and correspond best to the typical plant cell (Fig. 9.3a). These are the least specialized of the cell types and are found in all the organs of a plant.

255× b. Collenchyma cells

340× c. Sclerenchyma cells

Figure 9.3  Ground tissue cells.  a. Parenchyma cells are the least specialized of the plant cells. b. The walls of collenchyma cells are much thicker than those of parenchyma cells. c. Sclerenchyma cells have very thick walls and are nonliving at maturity—their primary function is to give strong support.



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UNIT 2  Plant Biology

Parenchyma cells can divide and give rise to more specialized cells such as roots. They have relatively thin walls, are alive at maturity, and may contain chloroplasts and carry on photosynthesis, or they may contain colorless plastids that store the products of photosynthesis. The fleshy part of an apple is mostly storage parenchyma cells. Collenchyma cells are like parenchyma cells except they have thicker primary walls (Fig. 9.3b). The thickness is irregular with the corners of the cell being thicker than other areas. Collenchyma cells often form bundles just beneath the epidermis and give flexible support to immature regions of a plant body. The familiar strands in celery stalks (leaf petioles) are composed mostly of collenchyma cells. Sclerenchyma cells have a thick secondary cell wall that forms between the primary cell wall and the plasma membrane. The secondary cell wall is impregnated with lignin, a highly resistant organic substance that makes the walls tough and hard (Fig. 9.3c). If we compare a cell wall to reinforced concrete, cellulose fibrils would play the role of steel rods, and lignin would be analogous to the cement. Most sclerenchyma cells are dead at maturity. Their primary function is to support the mature regions of a plant. Tracheids and vessel elements are types of sclerenchyma cells that not only function in support but also transport water. Two other types of sclerenchyma cells are fibers and sclereids. Fibers are long and slender, and may be grouped in bundles that can be used commercially. Hemp fibers can be used to make rope, and flax fibers can be woven into linen. Flax fibers, however, are not lignified, which is why linen is soft. Sclereids, which are shorter than fibers and have variable shapes, are found in seed coats and nutshells. Sclereids, or “stone cells,” are responsible for the gritty texture of pears as well as the hardness of nuts and peach pits.

tracheid

vessel

water flow pits vessel element

perforation plate

pits

a. Xylem

sieve tube water and nutrient flow plasmodesma cell membrane sieve-tube member nucleus sieve plate companion cell

Vascular Tissue There are two types of vascular (transport) tissue. Xylem transports water and minerals from the roots to the leaves, and phloem transports sugar and other organic compounds, including hormones, throughout the plant. Both xylem and phloem are considered complex tissues because they are composed of two or more kinds of cells. Xylem contains two types of conducting cells: tracheids and vessel elements (Fig. 9.4a). Both types of conducting cells are hollow and nonliving, but the vessel elements are shorter and wider. Vessel elements have plates with perforations in their end walls and are arranged to form a continuous vessel for water and mineral transport. The elongated tracheids, with tapered ends, form a less efficient means of transport, but water can move across the end walls and side walls because there are pits, or depressions, where the secondary wall does not form. In addition to vessel elements and tracheids, xylem contains parenchyma cells that store various substances and fibers that lend support. The conducting cells of phloem are sieve-tube members arranged to form a continuous sieve tube (Fig. 9.4b). Sieve-tube members contain cytoplasm but no nuclei. The term sieve refers to a cluster of pores in the end walls, collectively called a sieve plate. Each sieve-tube member has a companion cell, which contains

b. Phloem

4003

Figure 9.4  Structure of xylem and phloem.  a. Photomicrograph of xylem vascular tissue and drawing showing tracheids and vessel elements. b. Photomicrograph of phloem vascular tissue and drawing showing sieve tubes and companion cells.

a nucleus. The two are connected by numerous plasmodesmata, strands of cytoplasm extending from one sieve-tube member to another, through the sieve plate. The nucleus of the companion  cell controls and maintains the life of both cells. The com­ panion cells are also believed to be involved in phloem’s transport function. It is important to realize that vascular tissue (xylem and phloem) extends from the root through stems to the leaves and vice versa (see Fig. 9.5). In the roots, the vascular tissue is located in the vascular cylinder. In the stem, it forms vascular bundles, and in the leaves, it is found in leaf veins.



Chapter 9  Plant Organization and Function terminal bud

Check Your Progress  9.1 1. Describe the type of tissue in a plant that gives rise to all

the other types and allows plants to grow their entire lives. 2. Identify the functions of epidermal tissue, ground tissue, and vascular tissue in plants. 3. Recognize the cell types found in ground tissue and vascular tissue and indicate their function.

blade leaf

vein petiole

9.2  Plant Organs and Systems

axillary bud

Learning Outcomes

stem

Upon completion of this section, you should be able to 1. Describe the vegetative and reproductive organs of plants and indicate their major functions. 2. Compare the structure and function of roots, stems, and leaves.

The cells and tissues of plants are organized into organs and organ systems in the same manner as animals (see Fig. 1.2). Recall that an organ is a structure that contains different tissues and the ability to perform a specific function. In plants, the roots, stems, and leaves are called the vegetative organs. Vegetative organs are involved in growth and nutrition, but not reproduction. Flowers, seeds, and fruit are examples of reproductive organs. The organs of flowering plants are organized into organ systems called the root system and the shoot system (Fig. 9.5). The root system simply consists of the roots, whereas the shoot system consists of the stem and leaves. 

147

node

internode

node vascular tissues shoot system root system

Root System Roots have various adaptations and associations to perform the functions of anchorage, absorption of water and minerals, and storage of carbohydrates. The root system in the majority of plants is located underground. The depth and distribution of plant roots depends on the type of plant, the timing and amount of rainfall, and the soil composition. For example, plants growing in deserts tend to have deeper roots than those growing in temperate grasslands. A common misconception is that the size and distribution of tree roots reflect the aboveground trunk and branches of the tree. In reality, 90% of a tree’s roots are located within 1 meter (m) of the surface. However, a tree’s roots extend out much farther than the crown of the tree. On average, a tree’s roots will extend two to four times the diameter of the aboveground portion of the tree. Extensive root systems of plants help anchor them in the soil and provide support (Fig. 9.6a). The root system absorbs water and minerals from the soil for the entire plant. The cylindrical shape of a root allows it to penetrate the soil as it grows and to absorb water from all sides. The absorptive capacity of a root is also increased by its many branch (lateral) roots and root hairs located in a special zone near a root tip. Root hairs, which are projections from epidermal root-hair cells, are so numerous that they increase the absorptive surface of a root tremendously. In a classic study completed in 1937, H. J. Dittmer counted 13,800,000 root hairs in a single, 20-inch-high rye plant. Since root-hair cells are constantly being

branch root root hairs

primary root

Figure 9.5  Organization of a plant body.  The body of a plant

consists of a root system and a shoot system. The shoot system contains the vegetative organs of the stem and leaves. Axillary buds can develop into branches or flowers, the reproductive organs of a plant. The root system and shoot system are connected by vascular tissue that extends from the roots to the leaves.

replaced, a typical rye plant may form about 100 million new roothair cells every day. Roughly pulling a plant out of the soil damages the small lateral roots and root hairs, and these plants do not fare well when transplanted. Transplantation is usually more ­successful if part of the surrounding soil is moved along with



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UNIT 2  Plant Biology

blade

stems

petiole

stem roots lateral root

a. Root system, dandelion

b. Shoot system, bean seedling

c. Leaves, pumpkin seedling

Figure 9.6  Vegetative organs of a eudicot.  a. The root system anchors the plant and absorbs water and minerals. b. The shoot system, the stem and branches, support the leaves and transport water and organic nutrients. c. The leaves carry out photosynthesis. the plant, since this leaves many of the branch roots and the root hairs intact. Roots also have other functions. In certain plants, the roots are modified for food storage. For example, branch roots form storage organs for carbohydrates in yams and sweet potatoes. Roots can also store water. Some plants in the pumpkin family store large amounts of water in their roots. Roots also produce hormones that stimulate the growth of stems and coordinate their size with the size of the root.

Shoot System: Stems The shoot system of a plant is composed of the stem, the branches, and the leaves. A stem, the main axis of a plant, terminates in tissue that allows the stem to elongate and produce leaves (Fig. 9.6b). A stem can sometimes expand in girth as well as length. As trees grow taller each year, they accumulate woody tissue that adds to their girth. The increase in girth strengthens the stem of the plant. Most stems are upright and support leaves in such a way that each leaf is exposed to as much sunlight as possible. A node occurs where leaves are attached to the stem, and an internode is the region between the nodes (see Fig. 9.5). The presence of nodes and internodes is the characteristic used to identify a stem, even if it happens to be an underground stem. Some stems are horizontal, and in these cases the nodes may give rise to new plants. In addition to supporting the leaves, a stem has vascular tissue that transports water and minerals from the roots through the stem to the leaves. Vascular tissue also transports the organic products of photosynthesis, usually in the opposite direction. Xylem cells consist of nonliving cells that form a continuous pipeline for water and mineral transport, whereas phloem cells consist of living cells that join end to end for organic nutrient transport. Stems may have functions other than transport. In some plants (e.g., cactus), the stem is the primary photosynthetic organ. In succulent plants, the

stem is a water reservoir. Tubers are underground horizontal stems that store nutrients.

Shoot System: Leaves Leaves are the major component of the plant that require H2O, CO2, and sunlight to carry on photosynthesis. With few exceptions, their cells are living, and the bulk of a leaf contains tissue specialized to carry on photosynthesis. Leaves receive water from the root system by way of the stem. The position of the leaves on a plant maximizes the plant’s photosynthetic efforts. The shape of the leaf also influences its function. For example, leaves that are broad and flat have the maximum surface area for the absorption of carbon dioxide and the collection of solar energy needed for photosynthesis. Also, unlike stems, leaves are almost never woody. When describing a leaf, the wide portion of a leaf is called the blade, and the stalk that attaches the blade to the stem is the petiole (Fig. 9.6c). The size, shape, color, and texture of leaves can be highly variable. These characteristics are fundamental in plant identification. The leaves of some aquatic duckweeds may be less than 1 millimeter (mm) in diameter, while some palms may have leaves that exceed 6 m in length. The shapes of leaves can vary from cactus spines to deeply lobed oak leaves. Leaves can exhibit a variety of colors, from many shades of green to deep purple. The texture of leaves varies from smooth and waxy like a magnolia to coarse like a sycamore. Not all leaves are foliage leaves. Some are specialized to protect buds, attach to objects (tendrils), store food (bulbs), or even capture insects. The upper acute angle between the petiole and stem is the leaf axil, where an axillary bud (or lateral bud) originates. This bud may become a branch or a flower. Plants that lose their leaves every year are called deciduous. This is in contrast to most of the gymnosperm plants (conifers), which usually retain their leaves for two to seven years. These plants are called evergreens.



Chapter 9  Plant Organization and Function

Check Your Progress  9.2 1. List the organs found in the root system and shoot system of a plant.

149

that major veins originate from points along the centrally placed main vein, and palmate venation means that the major veins all originate at the point of attachment of the blade to the petiole:

2. Compare the structure and function of roots, stems, and leaves.

9.3  Monocot versus Eudicot Plants Netted venation:

Learning Outcome Upon completion of this section, you should be able to 1. List and describe the key features of monocots and eudicots.

Flowering plants are divided into two groups, depending on the number of cotyledons, or seed leaves, in the embryonic plant (Fig. 9.7). Most cotyledons emerge, grow larger, and become green when the seed germinates. Some plants have one cotyledon, and are known as monocotyledons, or monocots. Other embryos have two cotyledons, and are known as eudicotyledons, or eudicots. The vascular (transport) tissue is organized differently in monocots and eudicots. In the monocot root, vascular tissue occurs in a ring. In the eudicot root, phloem, which transports organic nutrients, is located between the arms of xylem, which transports water and minerals, and has a star shape. In the monocot stem, the vascular bundles, which contain vascular tissue surrounded by a bundle sheath, are scattered. In a eudicot stem, the vascular bundles occur in a ring. Leaf veins are vascular bundles within a leaf. Monocots exhibit parallel venation, and eudicots exhibit netted venation, which may be either pinnate or palmate. Pinnate venation means

Root

palmately veined

Adult monocots and eudicots have other structural differences, such as the number of flower parts and the number of apertures (pores or slits) of pollen grains. The flower parts of monocots are arranged in multiples of three, and the flower parts of eudicots are arranged in multiples of four or five. Eudicot pollen grains usually have three apertures, and monocot pollen grains usually have one aperture. Although the division between monocots and eudicots may seem to be of limited importance, it does in fact affect many aspects of their structure. The eudicots are the larger group and include some of our most familiar flowering plants—from dandelions to oak trees. The monocots include grasses, lilies, orchids, and palm trees, among others. Some of our most significant food sources are monocots, such as rice, wheat, and corn.

Check Your Progress  9.3 1. Compare the following in monocots and eudicots: number of cotyledons, leaf venation, and flower parts.

2. List examples of plants that are monocots and eudicots.

Stem

Leaf

Flower

Pollen

Monocots

Seed

pinnately veined

Root xylem and phloem in a ring

Vascular bundles scattered in stem

Leaf veins form a parallel pattern

Flower parts in threes and multiples of three

One pore or slit

Two cotyledons in seed

Root phloem between arms of xylem

Vascular bundles in a distinct ring

Leaf veins form a net pattern

Flower parts in fours or fives and their multiples

Three pores or slits

Eudicots

One cotyledon in seed

Figure 9.7  Flowering plants are either monocots or eudicots.  The features that are used to distinguish monocots from eudicots: the number of cotyledons in the seed; the arrangement of vascular tissue in roots, stems, and leaves; the number of flower parts; and the number of pores in the pollen grain.



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UNIT 2  Plant Biology

9.4  Organization of Roots Learning Outcomes Upon completion of this section, you should be able to 1. List the zones in the root involved in primary growth. 2. Describe the anatomy of eudicot and monocot roots. 3. Identify different types of roots and root specializations.

Primary growth, which causes a plant to grow lengthwise, is centered in the apex (tip) of the shoot and the root systems. The growth of many roots is continuous, pausing only when temperatures become too cold or when water becomes too scarce. Figure 9.8a, a longitudinal section of a eudicot root, reveals zones where cells are in various stages of differentiation as primary growth occurs. These zones are called the zone of cell division, the zone of elongation, and the zone of maturation.

Vascular cylinder endodermis pericycle phloem xylem cortex

epidermis root hair 300× b. Vascular cylinder phloem

Zone of maturation

endodermis

water and minerals

Casparian strip xylem of vascular cylinder pericycle Zone of elongation

c. Casparian strip

procambium Zone of cell division

ground meristem protoderm root apical meristem protected by root cap root cap

a. Root tip

Figure 9.8  Eudicot root tip.  a. The root tip is divided into three zones. b. The vascular cylinder of a eudicot root contains the vascular tissue. Xylem is typically star-shaped, and phloem lies between the points of the star. c. Water and minerals either pass between cells, or they enter root cells directly. In any case, because of the Casparian strip, water and minerals must pass through the cytoplasm of endodermal cells in order to enter the xylem. In this way, endodermal cells regulate the passage of minerals into the vascular cylinder.



Chapter 9  Plant Organization and Function

cortex

branch root

pericycle endodermis epidermis vascular tissue in core

Figure 9.9  Branching of eudicot root.  This cross section shows the origination and growth of a branch root from the pericycle.

The zone of cell division is protected by the root cap, which is composed of parenchyma cells and protected by a slimy sheath. As the root grows, root cap cells are constantly removed by rough soil particles and replaced by new cells. The zone of cell division contains the root apical meristem, where mitosis produces relatively small, many-sided cells having dense cytoplasm and large nuclei. These meristematic cells give rise to the primary meristems, called protoderm, ground meristem, and procambium. Eventually, the primary meristems develop, respectively, into the three mature tissue types discussed previously: epidermis, ground tissue (i.e., cortex), and vascular tissue.

Anatomy of a Eudicot Root Figure 9.8a, b also shows a cross section of a root at the zone of maturation. These specialized tissues are identifiable: Epidermis.  The epidermis, which forms the outer layer of the root, consists of only a single layer of cells. The majority of epidermal cells are thin-walled and rectangular. The epidermal cells in the zone of maturation have root hairs that project as far as 5–8 mm into the soil. Cortex.  Underneath the epidermis is a layer of large, thin-walled parenchyma cells that make up the cortex, a type of ground tissue. These irregularly shaped cells are loosely packed, so that water and minerals can move through the cortex without entering the cells. The cells contain starch granules, which function in food storage. Endodermis. The endodermis is a single layer of rectangular cells that forms a boundary between the cortex and the inner vascular cylinder. The endodermal cells fit snugly together and are bordered on four sides by a layer of impermeable lignin and

151

suberin known as the Casparian strip (Fig. 9.8c). This strip prevents the passage of water and mineral ions between adjacent cell walls. The two sides of each cell that contact the cortex and the vascular cylinder respectively remain permeable. Therefore, the only access to the vascular cylinder is through the endodermal cells, as shown by the arrows in ­Figure 9.8c. Vascular tissue.  The first layer of cells within the vascular cylinder is the pericycle. This tissue can become meristematic and start the development of branch roots (Fig. 9.9). The main portion of the vascular cylinder contains xylem and phloem. The xylem appears star-shaped in eudicots because several arms of tissue radiate from a common center (see Fig. 9.8). The phloem is found in separate regions between the arms of the xylem.

Anatomy of Monocot Roots Monocot roots (Fig. 9.10) have the same growth zones as eudicot roots, but they do not undergo secondary growth as many eudicot roots do. A monocot root contains pith, a type of ground tissue, which is centrally located. The pith is surrounded by a vascular ring composed of alternating xylem and phloem bundles.

Root Diversity vascular cylinder

a.

Roots tend to be of two varieties. In eudicots, the first (primary) root grows straight down and is called a taproot. Monocots have a fibrous root system,  which may have large numbers of fine roots of similar diameter.

60×

pith phloem pericycle endodermis with Casparian strip xylem cortex

epidermis b.

Figure 9.10  Monocot root.  a. This overall cross section of a monocot root shows that a vascular ring surrounds a central pith. b. The enlargement shows the placement of various tissues.



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UNIT 2  Plant Biology

Many mature plants have a combination of taproot and fibrous root systems. 

Taproot

a.

b.

c.

d.

Fibrous root

Some plants have roots that possess a variety of adaptations to better perform their functions. Root adaptations can improve anchorage to the ground or the storage of carbohydrates. Other root systems need to increase absorption of water, minerals, oxygen, or sunlight. Most roots store some food, but in certain plants, such as the sweet potato or carrot, the roots are enlarged and store large quantities of starch. Perennials rely on this stored carbohydrate to grow year after year (Fig. 9.11a). When roots develop from organs of the shoot system instead of the root system, they are known as adventitious roots. One style of adventitious root is typically found in corn plants. These are called prop roots because they emerge above the soil and act as anchors for the plant. Specialized corn prop roots, produced toward the base of stems, support these plants in high wind (9.11b).  Roots, like all organs, need oxygen to perform cellular respiration. Roots normally get their oxygen from the air pockets in the soil, but oxygen becomes scarce when the plant grows exclusively in water. Large trees growing in water, such as mangrove and bald cypress, have evolved spongy roots called pneumatophores, which extend above the water’s surface and enhance gas exchange (Fig. 9.11c). Many epiphytes (plants that live in or on trees) have aerial roots for a variety of reasons. English ivy uses aerial roots to climb the bark of trees. Orchids use their roots to capture moisture in the air and support their weight, and in some instances, green aerial roots perform photosynthesis (Fig. 9.11d).  Plants such as dodders and broomrapes are parasitic on other plants. Their stems have rootlike projections called haustoria (sing., haustorium) that grow into the host plant and make contact with vascular tissue from which they extract water and nutrients.

Symbiotic Relationships in Roots Two symbiotic relationships assist roots in taking up mineral nutrients. In the first type, legumes (soybeans and alfalfa) have roots infected by nitrogen-fixing Rhizobium bacteria. These bacteria can

Figure 9.11  Root diversity and specialization.  a. Sweet potato

plants have food storage roots. b. Prop roots are specialized for support. c. The pneumatophores of this tree allow it to acquire oxygen even though it lives in water. d. The aerial roots of orchids offer physical support, water and nutrient uptake, and in some cases, even photosynthesis. 

fix atmospheric nitrogen (N2) by breaking the N ≡ N bond and reducing nitrogen to NH4+ for incorporation into organic compounds. The bacteria live in root nodules and are supplied with carbohydrates by the host plant (Fig. 9.12a). The bacteria, in turn, furnish their host with nitrogen compounds. The second type of symbiotic relationship, called a mycorrhizal association, involves fungi and almost all plant roots (Fig. 9.12b). Only a small minority of plants do not have mycorrhizae (sing., mycorrhiza). Ectomycorrhizae form a mantle that is exterior to the root that grows between the cell walls. Endomycorrhizae can penetrate cell walls. In any case, the fungus increases the surface area available for mineral and water uptake and breaks down organic matter, releasing nutrients for the plant to use. In return, the root furnishes the fungus with sugars and amino acids. Plants are extremely dependent on mycorrhizae for maximum growth (Fig. 9.12c). Orchid seeds, which are quite small and contain limited nutrients, do not germinate until a mycorrhizal fungus

e



plant without mycorrhizae nodule

root

Chapter 9  Plant Organization and Function plant with mycorrhizae

153

has invaded their cells. Nonphotosynthetic plants, such as Indian pipe, use their mycorrhizae to extract nutrients from nearby trees. Plants without mycorrhizae are usually limited as to the environment in which they can grow.

Check Your Progress  9.4

mycorrhizae in root cells

1. Explain how a plant controls what enters the vascular bundle.

2. Contrast eudicot and monocot roots. 3. List several specializations of roots and identify which of the three main functions of a root is involved in each specialization.

a. Root nodule

100×

b. Mycorrhizae

9.5  Organization of Stems Learning Outcomes Upon completion of this section, you should be able to 1. Identify the structures of a woody twig. 2. Describe the anatomy of eudicot and monocot stems. 3. Explain the secondary growth of stems. 4. Identify the adaptations that lead to stem diversity.

plant without mycorrhizae

plant with mycorrhizae

root

mycorrhizae in root cells

The anatomy of a woody twig helps us review the organization of a stem (Fig. 9.13). The terminal bud contains the shoot tip protected by bud scales, which are modified leaves. Leaf scars and bundle scars mark the location of leaves that have dropped. Dormant axillary buds that can give rise to branches or flowers are also found here. Each spring when growth resumes, bud scales fall off and leave a scar. You can tell the age of a stem by counting these groups of bud scale scars because there is one for each year’s growth. The terminal bud includes the shoot apical meristem and also leaf primordia (young leaves) that differentiate from cells produced by the shoot apical meristem (Fig. 9.14a). The activity of

Figure 9.13  Woody twig.  The major parts of a stem are illustrated by a woody twig collected in winter.

bud scale

b. Mycorrhizae

100×

one year’s growth

internode node node

terminal bud

Figure 9.12  Root nodules and mycorrhizae. 

a. Nitrogen-fixing bacteria live in nodules on the roots of plants, particularly legumes. Plants grown with mycorrhizae (plant on right) grow much larger than a plant (left) grown without mycorrhizae. b. Ectomycorrhizae on red pine roots. 

lenticel

terminal bud scale scar

axillary bud

leaf scar

stem

bundle scars



154

UNIT 2  Plant Biology Three Primary Meristems: protoderm

leaf primordium

ground meristem

shoot apical meristem protoderm

procambium

ground meristem procambium

Primary Tissues: epidermis pith

axillary bud

cortex

vascular cambium vascular bundles pith primary xylem

primary xylem

vascular cambium primary phloem cortex

primary phloem a. Shoot tip

b. Fate of primary meristems

Figure 9.14  Shoot tip and primary meristems.  a. The shoot apical meristem within a terminal bud is surrounded by leaf primordia. b. The shoot apical meristem produces the primary meristems. Protoderm gives rise to epidermis; ground meristem gives rise to pith and cortex; and procambium gives rise to vascular tissue, including primary xylem, primary phloem, and vascular cambium. the terminal bud would be hindered by a protective covering comparable to the root cap. Instead, the leaf primordia fold over the apical meristem, providing protection. At the start of the season, the leaf primordia are, in turn, covered by terminal bud scales (scalelike leaves), but these drop off, or abscise, as growth continues. The shoot apical meristem produces everything in a shoot: leaves, axillary buds, additional stem, and sometimes flowers. In the process, it gives rise to the same primary meristems as in the root. These primary meristems, in turn, develop into the differentiated tissues of a shoot system. The protoderm becomes the epidermis of the stem and leaves. Ground meristem produces parenchyma cells that become the cortex and pith in the stem and mesophyll in the leaves. Procambium differentiates into the xylem and phloem of a vascular bundle. Certain cells become tracheids and others become vessel elements. The first sieve-tube members of a vascular bundle do not have companion cells and are short-lived (some live only a day before being replaced). Mature vascular bundles contain fully differentiated xylem and phloem, and a lateral meristem called vascular cambium, which is responsible for secondary growth. Vascular cambium is discussed more fully later in this section.

Herbaceous Stems Mature nonwoody stems, called herbaceous stems, exhibit only primary growth. The outermost tissue of herbaceous stems is the epidermis, which is covered by a waxy cuticle to prevent water loss. In the vascular bundle, xylem is typically found toward the inside of the stem, and phloem is found toward the outside. In a herbaceous eudicot stem, such as a sunflower, the vascular bundles are arranged in a distinct ring that separates the cortex from the central pith, where water and the products of photosynthesis are stored (Fig. 9.15). The cortex is sometimes green and carries on photosynthesis. In a monocot stem such as corn, the vascular bundles are scattered throughout the stem, and often there is no well-defined cortex or well-defined pith (Fig. 9.16).

Woody Stems A woody plant, such as an oak tree, has both primary and secondary tissues. Primary tissues are those new tissues formed each year from primary meristems right behind the shoot apical meristem. Secondary tissues develop during the first and subsequent years of  growth from lateral meristems: vascular cambium and cork cambium. Primary growth occurs in all plants. Secondary growth,



155

Chapter 9  Plant Organization and Function epidermis

epidermis

ground tissue

cortex vascular bundle

pith

vascular bundle

253 103 fibers xylem

ground tissue (parenchyma)

phloem

xylem

phloem

bundle sheath cells

epidermis

pith

vascular cambium

parenchyma collenchyma

vessel element air space

sieve-tube member

companion cell

Figure 9.15  Herbaceous eudicot stem.  In eudicot stems, the vascular bundles are arranged in a ring around the pith.

Figure 9.16  Monocot stem.  In the monocot stem, the vascular bundles are scattered throughout the stem.

which occurs only in conifers and woody eudicots, increases the girth of trunks, stems, branches, and roots. Trees and shrubs undergo secondary growth due to activities of the vascular cambium (Fig. 9.17). In herbaceous plants, vascular cambium is present between the xylem and phloem of each vascular bundle. In woody plants, the vascular cambium develops to form a ring of meristem that divides parallel to the surface of the plant, and produces new xylem and phloem each year. Eventually, a woody

eudicot stem has an entirely different organization from that of a herbaceous eudicot stem. A woody stem has no distinct vascular bundles and instead has three distinct areas: the bark, the wood, and the pith. The pith is a collection of parenchyma cells located at the center of the stem. Vascular cambium lies between the bark and the wood, which are discussed next. You will also notice in Figure 9.17c the xylem rays and phloem rays that are visible in the cross section of a woody stem.



156

a.

UNIT 2  Plant Biology

pith primary xylem primary phloem cortex epidermis

Vascular cambium: Lateral meristem that will produce secondary xylem and secondary phloem in each succeeding year. Periderm: As a stem becomes woody, epidermis is replaced by the periderm.

The bark of a tree can be removed. However, doing so is very ­harmful because without phloem organic nutrients cannot be transported. Cork cambium develops beneath the epidermis. When cork cambium first begins to divide, it produces tissue that disrupts the epidermis and replaces it with cork cells. Cork cells are impregnated with suberin, a waxy layer that makes them waterproof but also causes them to die. This is protective because now the stem is less edible. But an impervious barrier means that gas exchange is impeded except at lenticels, which are pockets of loosely arranged cork cells not impregnated with suberin.

Wood b.

pith primary xylem secondary xylem vascular cambium secondary phloem primary phloem cortex cork cambium cork

lenticel

Bark: Includes periderm and also living secondary phloem. Wood: Increases each year; includes annual rings of xylem.

c.

Wood is secondary xylem that builds up year after year, thereby increasing the girth of trees. In trees that have a growing season, vascular cambium is dormant during the winter. In the spring, when moisture is plentiful and leaves require much water for growth, the secondary xylem contains wide vessel elements with thin walls. In this so-called spring wood, wide vessels transport sufficient water to the growing leaves. Later in the season, moisture is scarce, and the wood at this time, called summer wood, has a lower proportion of vessels (Fig. 9.18). Strength is required because the tree is

cork cork cambium cortex Bark phloem ray

xylem ray phloem ray

phloem

secondary xylem vascular cambium secondary phloem primary phloem

Vascular Cambium

summer wood spring wood

cork cambium cork

Figure 9.17  Secondary growth of stems.  a. Diagram showing a

eudicot herbaceous stem just before secondary growth begins. b. Secondary growth has begun, and periderm has replaced the epidermis. Vascular cambium produces secondary xylem and secondary phloem each year. c. In a two-year-old stem, the primary phloem and cortex have disappeared, and only the secondary phloem (within the bark) produced by vascular cambium will be active that year. Secondary xylem builds up to become the annual rings of a woody stem.

secondary xylem annual ring

Wood

primary xylem

Pith

Rays consist of parenchyma cells that permit lateral conduction of nutrients from the pith to the cortex as well as some storage of food. A phloem ray is actually a continuation of a xylem ray. Some phloem rays are much broader than other phloem rays.

Bark The bark of a tree contains both periderm (cork, cork cambium, and a single layer of cork cells filled with suberin) and phloem. Although secondary phloem is produced each year by vascular cambium, phloem does not build up from season to season.

5×

Figure 9.18  Three-year-old woody twig.  The buildup of

secondary xylem in a woody stem results in annual rings, which tell the age of the stem. The rings can be distinguished because each one begins with spring wood (large vessel elements) and ends with summer wood (smaller and fewer vessel elements).



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SCIENCE IN YOUR LIFE  ►

157

ECOLOGY

The Many Uses of Bamboo Due to its resilience, amazing rate of growth, and ability to grow in a variety of climates, bamboo is quickly becoming a valuable crop. Certain varieties of bamboo are capable of growing up to a foot per day and can reach their full height within one year in the right conditions. Bamboo is classified as a grass that contains varieties ranging in height from one foot to over 100 feet. Globally there are over 1,400 species of bamboo. Approximately 900 can be  found in tropical climates, whereas the remaining 500 are found in temperate environments. Several varieties of bamboo are even found in the eastern and southeastern United States. Bamboo is recognized for its versatility as a building material, commercial food product, and clothing material. It can be processed into roofing material, flooring, and support beams, as well as a variety of construction materials. The mature stalks can be used as support columns in “Green” construction. It can also be used to replace steel reinforcing rods that are typically used in concrete-style construction. The bamboo goods industry started increasing

in popularity in the United States during the mid-1990s and is expected to exceed $25 billion in the next several years. Bamboo products are three times harder than oak. While not used extensively in the commercial food market, bamboo does have a variety of uses (Fig. 9A). The shoots are often used in a variety of Asian dishes as a vegetable. They can be boiled and added to a variety of dishes or eaten raw. In China, bamboo is made into a variety of alcoholic drinks; other Asian countries make soups, pancakes, and broths. The hollow bamboo stalk can be used to cook rice and soups within it. This will give the foods a subtle but distinct taste. Additional uses include modifying bamboo into cooking utensils, most notably chopsticks. Clothing products are now being made out of bamboo. When made into clothing, bamboo is reported as being very light and extremely soft. It also has the ability to wick moisture away from the skin, making it ideal to wear during exercise. Several lines of baby clothing are being made from bamboo. Bamboo is being recognized as one of the most eco-friendly crops. It requires lesser

amounts of chemicals or pesticides to grow. It will remove nearly five times more greenhouse gases and produce nearly 35% more oxygen than an equivalent stand of trees. Harvesting can be done starting at the second to third year of growth all the way through to the fifth to seventh year of growth. Because bamboo is a perennial, it allows for a yearly regrowth of the stand after harvesting instead of requiring yearly replanting. Versatility, hardiness, ease of growing, and greater awareness of environmentally friendly products are quickly making bamboo an ideal product that may someday replace metal, wood, and plastics.

Questions to Consider 1. Would you purchase products made from bamboo, even if it cost more than products made from nonrenewable materials? 2. Should the government provide financial incentives for farmers to switch their crops over to bamboo? 3. What are the negative consequences of growing and using bamboo products?

Figure 9A  The many uses of bamboo.  Bamboo is quickly becoming a multifunctional product in today’s society. Uses range from building materials to food to clothing.



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growing larger and summer wood contains numerous thick-walled tracheids. At the end of the growing season, just before the cambium becomes dormant again, only heavy fibers with especially thick secondary walls may develop. When the trunk of a tree has spring wood followed by summer wood, the two together make up one year’s growth, or an annual ring. You can tell the age of a tree by counting the annual rings (Fig. 9.19a). The outer annual rings, where transport occurs, are called sapwood. In older trees, the inner annual rings, called the heartwood, no longer function in water transport. The cells become plugged with deposits such as resins, gums, and other substances that inhibit the growth of bacteria and fungi. Heartwood may help support a tree, although some trees stand erect and live for many years after the heartwood has rotted away. Figure 9.19b shows the layers of a woody stem in relation to one another.

annual rings

The annual rings are important not only in telling the age of a tree, but also in serving as a historical record of tree growth. For example, if rainfall and other conditions were extremely favorable during a season, the annual ring may be wider than usual.

Woody Plants Is it advantageous to be woody? With adequate rainfall, woody plants can grow taller than herbaceous plants and increase in girth because they have adequate vascular tissue to support and service their leaves. However, it takes energy to produce secondary growth and prepare the body for winter if the plant lives in the temperate zone. Also, woody plants need more defense mechanisms because a long-lasting plant that stays in one spot is likely to be attacked by herbivores and parasites. Then, too, trees don’t usually reproduce until they have grown for several seasons, by which time they may have succumbed to an accident or disease. In certain habitats, it is more advantageous for a plant to put most of its energy into simply reproducing rather than being woody.

Stem Diversity

a. Tree trunk, cross-sectional view

heartwood sapwood vascular cambium phloem cork

Stems exist in diverse forms, or modifications (Fig. 9.20). Above­ ground horizontal stems, called stolons or runners, produce new plants where the nodes touch the ground. The strawberry plant contains this type of stem, which functions in vegetative reproduction. Aboveground vertical stems can also be modified. For example, cacti have succulent stems specialized for water storage, and the tendrils of grape plants (which are stem branches) allow them to climb. Morning glory and its relatives have stems that twine around support structures. Such tendrils and twining shoots help plants expose their leaves to the sun. Underground horizontal stems, called rhizomes, may be long and thin, as in sod-forming grasses, or thick and fleshy, as in irises. Rhizomes survive the winter and contribute to asexual reproduction because each node bears a bud. Some rhizomes have enlarged portions called tubers, which function in food storage. Potatoes are an example of tubers. Corms are bulbous underground stems that lie dormant during the winter, just as rhizomes do. They have thin, papery leaves and a thick stem. They also produce new plants the next growing season. Gladiolus corms are referred to as bulbs by laypersons, but the  botanist reserves the term bulb for a structure composed of thick modified leaves attached to a short vertical stem. An onion is a bulb. Humans make use of stems in many ways. The stem of the sugarcane plant is a primary source of table sugar. The spice cinnamon and the drug quinine are derived from the bark of Cinnamomum verum and various Cinchona species, respectively.  

b. Tree trunk, longitudinal view

Check Your Progress  9.5

Figure 9.19  Tree trunk.  a. A cross section of a 39-year-old tree

1. Compare and contrast eudicot and monocot stems. 2. Explain why there are two portions to every ring in a tree’s

trunk. The xylem within the darker heartwood is inactive. The xylem within the lighter sapwood is active. b. The relationship of bark, vascular cambium, and wood is retained in a mature stem. The pith has been buried by the growth of layer after layer of new secondary xylem.

annual rings.

3. Describe various stem modifications.



Chapter 9  Plant Organization and Function

rhizome branch adventitious roots stolon

papery leaves

corm

axillary bud

node

axillary bud

159

rhizome

adventitious roots

a. Stolon

tuber adventitious roots

b. Rhizome

c. Tuber

d. Corm

Figure 9.20  Stem diversity.  a. A strawberry plant has aboveground horizontal stems called stolons. Every other node produces a new shoot system.

b. The underground horizontal stem of an iris is a fleshy rhizome. c. The underground stem of a potato plant has enlargements called tubers. We call the tubers potatoes. d. The corm of a gladiolus is a thick stem covered by papery leaves.

9.6  Organization of Leaves Learning Outcomes Upon completion of this section, you should be able to 1. Identify the structures of a leaf. 2. Describe some types of leaf modifications.

Figure 9.21 shows a cross section of a generalized foliage leaf of a temperate-zone eudicot plant. The functions of a foliage leaf are to carry on photosynthesis, regulate water loss, and be protective against parasites and predators. Epidermal tissue is located on the leaf’s upper and lower surfaces, where it is well situated to be protective. Epidermis contains trichomes, little hairs that are highly protective, especially when they secrete irritating substances against predators. Epidermal cells play a significant role in regulating water loss. Closely packed epidermal cells secrete an outer, waxy cuticle that helps prevent water loss but does not allow the uptake of CO2 needed for photosynthesis. However, the stomata, located particularly in the lower epidermis, do allow the uptake of CO2 and can also be closed by guard cells to prevent water loss. Trichomes can also help prevent

parasites from entering stomata, and their presence cuts down on water loss when stomata are open. Leaves are the chief photosynthesizing organs of a plant. Therefore, leaves must absorb solar energy, take up CO2, and receive water by way of leaf veins. For the reception of solar energy, foliage leaves are generally flat and thin—this shape allows solar energy to penetrate the entire width of the leaf. The body of a leaf is composed of mesophyll. The elongated cells of the palisade mesophyll carry on most of the photosynthesis and are situated to allow its chloroplasts to efficiently absorb solar energy. The irregular cells of spongy mesophyll are bounded by air spaces that receive CO2 when stomata are open. The loosely packed arrangement of the cells in spongy mesophyll increases the amount of surface area for absorption of CO2 and for water loss. Evaporation of water is actually the means by which water rises in leaf veins, as discussed in section 9.7. Leaf veins branch, and water needed for photosynthesis diffuses from narrow terminations that also take up the products of photosynthesis for distribution to other parts of the plant. Bundle sheaths are layers of cells surrounding vascular tissue. Most bundle sheaths are parenchyma cells, sclerenchyma cells, or a combination of the two. The parenchyma cells help regulate the entrance and exit of materials into and out of the leaf vein.



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UNIT 2  Plant Biology

trichomes

Water and minerals enter leaf through xylem.

cuticle upper epidermis palisade mesophyll

Sugar exits leaf through phloem.

air space

bundle sheath cell

spongy mesophyll lower epidermis cuticle leaf vein

stoma chloroplast

central vacuole

upper ep de s epidermis

epidermal cell

nucleus chloroplast

O2 and H2O exit leaf through stoma.

palisade mesophyll

leaf vein

nucleus guard cell CO2 enters leaf through stoma.

spongy mesophyll

mitochondrion

stoma Leaf cell

Stoma and guard cells

lower epidermis

stoma 803 Micrograph of leaf cross section

Figure 9.21  Leaf structure.  Photosynthesis takes place in the mesophyll tissue of leaves. The leaf is enclosed by epidermal cells covered with a waxy layer, the cuticle. The veins contain xylem and phloem for the transport of water and solutes. Stoma in the epidermis permit the exchange of gases.

Leaf Diversity The blade of a leaf can be simple or compound (Fig. 9.22a). A simple leaf has a single blade in contrast to a compound leaf, which is divided into leaflets. For example, a magnolia tree has simple leaves, and a buckeye tree has compound leaves. In pinnately compound leaves, the leaflets occur in pairs, as in a black walnut tree, while in palmately compound leaves, all of the leaflets are attached to a single point, as in a buckeye tree. Plants such as the mimosa have bipinnately compound leaves, with leaflets subdivided into even smaller leaflets. Leaves can be arranged on a stem in three ways: alternate, opposite, or whorled (Fig. 9.22b). The American beech has alternate leaves; the maple has opposite leaves, being attached to the same node; and bedstraw has a whorled leaf arrangement, with several leaves originating from the same node. Leaves are adapted to environmental conditions. Shade plants tend to have broad, wide leaves, and desert plants tend to have reduced leaves with sunken stomata. The leaves of a cactus are the spines attached to the succulent (fleshy) stem (Fig. 9.23a). Other succulents have leaves adapted to hold moisture.

An onion bulb is composed of leaves surrounding a short stem. In a head of cabbage, large leaves overlap one another. The petiole of a leaf can be thick and fleshy, as in celery and rhubarb. Leaves of climbing plants, such as those of peas and cucumbers, are modified into tendrils that can attach to nearby objects (Fig. 9.23b). Some plants have leaves that are specialized for catching insects. A sundew has sticky trichomes that trap insects and others that secrete digestive enzymes. The Venus flytrap has hinged leaves that snap shut and interlock when an insect triggers sensitive trichomes that project from inside the leaves (Fig. 9.23c). Insectivorous plants commonly grow in marshy regions, where the supply of soil nitrogen is severely limited. The digested insects provide the plants with a source of organic nitrogen.

Check Your Progress  9.6 1. Describe the basic structure of a leaf, including the tissues where photosynthesis occurs and those that regulate the movement of water and gases. 2. Identify how leaves are adapted to their environment.



Chapter 9  Plant Organization and Function

161

axillary bud Alternate leaves, American beech Simple leaf, magnolia

Palmately compound leaf, buckeye

axillary buds Whorled leaves, bedstraw

axillary bud Pinnately compound leaf, black walnut

Opposite leaves, maple

a. Simple versus compound leaves

b. Arrangement of leaves on stem

Figure 9.22  Classification of leaves.  a. Leaves are simple or compound, being either pinnately compound or palmately compound. Note the one axillary bud per compound leaf. b. The leaf arrangement on a stem can be alternate, opposite, or whorled. spine

stem

Figure 9.23  Leaf diversity.  a. The spines of a cactus are modified

leaves that protect the fleshy stem from animal predation. b. The tendrils of a cucumber are modified leaves that attach the plant to a physical support. c. The modified leaves of the Venus flytrap serve as a trap for insect prey. When triggered by an insect, the leaf snaps shut. Once shut, the leaf secretes digestive juices that break down the soft parts of the insect’s body.

tendril a. Cactus, Opuntia

b. Cucumber, Cucumis

9.7  Uptake and Transport of Nutrients Learning Outcomes Upon completion of this section, you should be able to 1. Explain the movement of water in a plant according to the cohesion-tension model of xylem transport. 2. Explain the movement of organic nutrients in a plant according to the pressure-flow model of phloem transport.

c. Venus flytrap, Dionaea

In order to produce a carbohydrate, a plant requires carbon dioxide from the air and water from the soil. The transport system of the plant consists of the vascular tissue: the xylem and phloem. Water and minerals are transported through a plant in xylem; the products of photosynthesis are transported in phloem.

Water Uptake and Transport Water and minerals enter the plant through the root, primarily through the root hairs. Osmosis of water and diffusion of minerals



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UNIT 2  Plant Biology

aid the entrance of water and minerals from the soil into the plant. Eventually, however, a plant uses active transport to bring minerals into root cells. From there, water, along with minerals, moves across the tissues of a root until it enters xylem. Once water enters xylem, it must be transported upward to the leaves of the plant. This can be a daunting task. Water entering root cells creates a positive pressure called root pressure that tends to push xylem sap upward. Atmospheric pressure can support a column of water to a maximum height of approximately 10.3 m. However, some trees can exceed 90 m in height, so other factors must be involved in causing water to move from the roots to the leaves. Figure 9.24 illustrates the accepted model for the transport of water and, therefore, minerals in a plant. It is called the cohesion-tension model.

Leaves • Transpiration creates tension. • Tension pulls the water column upward from the roots to the leaves.

Cohesion-Tension Model of Xylem Transport

stoma intercellular space H2O

cohesion due to hydrogen bonding between water molecules

The vascular tissue of xylem contains the hollow conducting cells called tracheids and vessel elements (see Fig. 9.6). The vessel elements are larger than the tracheids, and they are stacked one on top of the other to form a pipeline that stretches from the roots to the leaves. It is an open pipeline because the vessel elements have no end walls, just perforation plates, separating one from the other. The tracheids, which are elongated with tapered ends, form a less obvious means of transport. Water can move across the end and side walls of tracheids because of pits, or depressions, where the secondary wall does not form.

Explanation of the Model  Unlike animals, which rely on a pumping heart to move blood through their vessels, plants utilize a passive, not active, means of transport to move water in xylem. The cohesion-tension model of xylem transport relies on the properties of water (see section 2.3). The term cohesion refers to the tendency of water molecules to cling together. Because of hydrogen bonding, water molecules interact with one another, forming a continuous water column in xylem from the roots to the leaves that is not easily broken. Adhesion refers to the ability of water, a polar molecule, to interact with the molecules making up the walls of the vessels in xylem. Adhesion gives the water column extra strength and prevents it from slipping downward. What causes the continuous water column to move passively upward? Consider the structure of a leaf. When the sun rises, stomata open, and carbon dioxide enters a leaf. Within the leaf, the mesophyll cells—particularly the spongy layer—are exposed to the air, which can be quite dry. Water now evaporates from mesophyll cells. Evaporation of water from leaf cells is called transpiration. At least 90% of the water taken up by the roots is eventually lost by transpiration. This means that the total amount of water lost by a plant over a long period of time is surprisingly large. A single Zea mays (corn) plant loses somewhere between 135 and 200 liters (l) of water through transpiration during a growing season. The water molecules that evaporate are replaced by other water molecules from the leaf veins. In this way, transpiration exerts a driving force—that is, a tension—which draws the water column up in vessels from the roots to the leaves. As transpiration occurs, the water column is pulled upward, first within the leaf, then from the stem, and finally from the roots. Tension can reach from the leaves to the root only if the water column is continuous. What happens if the water column within

mesophyll cells

xylem

adhesion due to polarity of water molecules cell wall

water molecule Stem • Cohesion makes water column continuous. • Adhesion keeps water column in place.

xylem

root hair

Roots • Water enters xylem at root. • Water column extends from leaves to root.

xylem

3

Figure 9.24  Cohesion-tension model of xylem transport. 

Tension created by evaporation (transpiration) at the leaves pulls water along the length of the xylem from the root hairs to the leaves.

xylem is broken, as by cutting a stem? The water column “snaps back” in the xylem vessel, away from the site of breakage, making it more difficult for conduction to occur. This is why it is best to maximize water conduction by cutting flower stems under water. This effect has also allowed investigators to measure the tension in stems. A device called the pressure bomb measures how much



Chapter 9  Plant Organization and Function

SCIENCE IN YOUR LIFE  ►

163

BIOETHICAL

Using Plants to Clean Up Toxic Messes Phytoremediation uses plants like poplar, ­mustard, and mulberry to clean up lead, uranium, and other environmental pollutants. The genetic makeup of these plants allows them to absorb, store, degrade, or transform substances that normally kill or harm other plants and ­animals. “It’s an elegantly simple solution to pollution problems,” says Louis Licht, who runs Ecolotree, an Iowa City phytoremediation company. The idea behind phytoremediation is not new, but the idea of using these plants on contaminated sites has just gained support in  the last decade. Different plants work on ­different contaminants. The mulberry bush, for instance, is effective on industrial sludge; some grasses attack petroleum wastes; and sunflowers (together with soil additives) remove lead. The plants clean up sites in different ways, depending on the substance involved. The plants or microbes around their roots break down organic substances, allowing the remains to be absorbed by the plant or left in the soil or water. Inorganic contaminants are trapped within the body of the plant, which must then be harvested and disposed of, or processed to reclaim the trapped contaminant.

Figure 9B 

Phytoremediation of nitrates.  Poplars and various

other plants can be used to absorb excess nitrates that run off from farm fields.

nitrate levels of streams by more than 90% (Fig. 9B).

Mustard Plants Take Up Uranium Phytoremediation has also helped clean up badly polluted sites, in some cases at a fraction of the usual cost. Uranium was removed from a Superfund site on an Army firing range in Aberdeen, Maryland. Mustard plants removed the uranium at as little as 10% of the cost of traditional cleanup methods.

Poplars Take Up Excess Nitrates

Limitations of Phytoremediation

The poplars act like vacuum cleaners, sucking up nitrate-laden runoff from a fertilized cornfield before this runoff reaches nearby waterways. Nitrate runoff into the Mississippi River from Midwest farms, after all, is a major cause of the large “dead zone” of oxygen-depleted water that develops each summer in the Gulf of Mexico. Poplars planted along the edge of farm fields have been shown to reduce the

One of the main limitations to phytoremediation is the pace. Depending on the contaminant, it can take several growing seasons to clean a site—much longer than conventional methods. Phytoremediation is also only effective at depths that plant roots can reach, making it useless against deep-lying contamination unless the soil is excavated. Phytoremediation will not work on lead and other metals unless chemicals

pressure it takes to push the xylem sap back to the cut surface of the stem. There is an important consequence to the way water is transported in plants. When a plant is under water stress, the stomata close. Now the plant loses little water because the leaves are protected against water loss by the waxy cuticle of the upper and lower epidermis. When stomata are closed, however, carbon dioxide cannot enter the leaves, and plants are unable to photosynthesize. (CAM plants are a notable exception. See section 8.4.) Photosynthesis, therefore, requires an abundant supply of water so that the stomata remain open and allow carbon dioxide to enter.

are added to the soil. Plants will also vary in the amount of pollutant that they can absorb. Despite its shortcomings, experts see a bright future for this technology because, for one reason, the costs are relatively small compared to those of traditional remediation ­technologies. Traditional methods of cleanup require much energy input and therefore have higher cost. In general, phytoremediation is a low-cost alternative to traditional methods because less energy is required for operation and maintenance.

Questions to Consider 1. What happens to the pollutants when the plant dies? 2. Why would one plant be more adapted to  absorbing a particular nutrient than another? 3. How does this process relate to the ­cohesion-tension and pressure-flow models?

guard cells and turgor pressure decreases, the stoma closes. Notice in Figure 9.25 that the guard cells are attached to each other at their ends and that the inner walls are thicker than the outer walls. When water enters, a guard cell’s radial expansion is restricted because of cellulose microfibrils in the walls, but lengthwise expansion of the outer walls is possible. When the outer walls expand lengthwise, they buckle out from the region of their attachment, and the stoma opens. Since about 1968, it has been clear that potassium ions (K+) accumulate within guard cells when stomata open. In other words, active transport of K+ into guard cells causes water to follow by osmosis and stomata to open. Opening and Closing of Stomata If plants are kept in the dark, stomata open and close about Each stoma, a small pore in the leaf epidermis, is bordered by two every 24 hours, just as if they were responding to the presence of guard cells (Fig. 9.25). When water enters the guard cells and sunlight in the daytime and the absence of sunlight at night. This turgor pressure increases, the stoma opens. When water exits the means that some sort of internal biological clock must be keeping

164

UNIT 2  Plant Biology

Open Stoma H2O

H2O

vacuole

K+

guard cell H+

stoma K+ enters guard cells, and water follows. a.

3433

Closed Stoma H2O

H2O

K+

K+ exits guard cells, and water follows. b.

3703

Figure 9.25  Opening and closing of stomata.  a. A stoma opens when turgor pressure increases in guard cells due to the entrance of K+ followed by the entrance of water. b. A stoma closes when turgor pressure decreases due to the exit of K+ followed by the exit of water. time. Circadian rhythms (behaviors that occur nearly every 24 hours) and biological clocks are areas of intense investigation at this time. Other factors that influence the opening and closing of stomata include temperature, humidity, and stress.

Organic Nutrient Transport Plants transport water and minerals from the roots to the leaves. They also transport organic nutrients from the leaves to the parts of plants that need them. This includes young leaves that have not yet reached their full photosynthetic potential, flowers that are in the process of making seeds and fruits, and the roots, whose location in the soil prohibits them from carrying on photosynthesis.

Role of Phloem As long ago as 1679, Marcello Malpighi suggested that bark is involved in moving sugars from leaves to roots. He observed that if a strip of bark is removed below the level of the majority of the tree’s leaves, the bark swells just above the cut, and sugar accumulates in the swollen tissue. We know today that this is because the

phloem is being removed, but the xylem is left intact. Therefore, the results suggest that phloem is the tissue that transports sugars. Radioactive tracer studies with carbon 14 (14C) have confirmed that phloem transports organic nutrients. When 14C-labeled carbon dioxide (CO2) is supplied to mature leaves, radioactively labeled sugar is soon found moving down the stem into the roots. It’s difficult to get samples of sap from phloem without injuring the phloem, but this problem is solved by using aphids, small insects that are phloem feeders. The aphid drives its stylet, a sharp mouthpart that functions like a hypodermic needle, between the epidermal cells, and sap enters its body from a sieve-tube member (Fig. 9.26). If the aphid is anesthetized using ether, its body can be carefully cut away, leaving the stylet. Phloem can then be collected and analyzed. The use of radioactive tracers and aphids has revealed that the movement of nutrients through phloem can be as fast as 60–100 cm per hour and possibly up to 300 cm per hour.

Pressure-Flow Model of Phloem Transport The pressure-flow model is the current explanation for the ­movement of organic materials in phloem. Consider the following



Chapter 9  Plant Organization and Function

165

mesophyll cell of leaf Leaf

water sugar

phloem xylem

a. An aphid feeding on a plant stem Leaves • Leaves are the main source of sugar production. • Sugar (pink) is actively transported into sieve tubes. • Water (blue) follows by osmosis.

Figure 9.26  Acquiring phloem sap.  Aphids are

small insects that remove nutrients from phloem by means of a needlelike mouthpart called a stylet. a. Excess phloem sap appears as a droplet after passing through the aphid’s body. b. Micrograph of a stylet in plant tissue. When an aphid is cut away from its stylet, phloem sap becomes available for collection and analysis.

xylem

phloem Stems • Phloem contents flow from a source to a sink. • Xylem flows from the roots to the leaves.

b. Aphid stylet in place Roots • Sugar is stored in the sink. • Cells can use it for cellular respiration. • Water exits by osmosis and returns to the xylem.

experiment in which two bulbs are connected by a glass tube. The left-hand bulb contains solute at a higher concentration than the right-hand bulb. Each bulb is bounded by a differentially permeable membrane, and the entire apparatus is submerged in distilled water:

cortex cell of root

flow of solution

Figure 9.27  Pressureflow model of phloem transport.  Sugars are

concentrated sugar solution

1

H2O

H2O

2

H2O

differentially permeable membranes

dilute sugar solution

produced at the source (leaves) and dissolve in water to form phloem. In the sieve tubes, water is pulled in by osmosis. The phloem follows positive pressure and moves toward the sink (root system).

xylem

phloem

Root



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UNIT 2  Plant Biology

Distilled water flows into the left-hand bulb to a greater extent because it has the higher solute concentration. The entrance of water creates a positive pressure, and water flows toward the second bulb. This flow not only drives water toward the second bulb, but it also provides enough force for water to move out through the membrane of the second bulb—even though the second bulb contains a higher concentration of solute than the distilled water. In plants, sieve tubes are analogous to the glass tube that connects the two bulbs. Sieve tubes are composed of sieve-tube members, each of which has a companion cell. It is believed that the companion cells assist the sieve-tube members in some way. The sieve-tube members align end to end, and strands of plasmodesmata (cytoplasm) extend through sieve plates from one sieve-tube member to the other. Sieve tubes, therefore, form a continuous pathway for organic nutrient transport throughout a plant. During the growing season, photosynthesizing leaves are producing sugar (Fig. 9.27). Therefore, they are a source of sugar. This sugar is actively transported into phloem. Again, transport is dependent on an electrochemical gradient established by a proton pump, a form of active transport. Sugar is carried across the membrane in conjunction with hydrogen ions (H+), which are moving down their concentration gradient. After sugar enters sieve tubes, water follows passively by osmosis. The buildup of water within sieve tubes creates the positive pressure that starts a flow of phloem contents. The roots (and other growth areas) are a sink for sugar, meaning that they are removing sugar, storing it, or using it for cellular respiration. After sugar is actively transported out of sieve tubes, water exits phloem passively by osmosis (which

reduces pressure at the sink) and is taken up by xylem, which transports water to leaves, where it is used for photosynthesis. Now, phloem contents continue to flow from the leaves (source) to the roots (sink). The pressure-flow model of phloem transport can account for any direction of flow in sieve tubes if we consider that the direction of flow is always from source to sink.

Check Your Progress  9.7 1. Describe cohesion and adhesion and how they are involved in moving water up a plant.

2. Explain how transpiration is involved in moving water throughout a plant.

3. Explain how osmosis is involved in moving organic nutrients in the plant.

Conclusion The preservation of biodiversity is not only important to preserve the health of ecosystems, such as rain forests, but for the health of humans as well. Plants, such as bamboo and the neem tree, play an important role in the health and welfare of humans. In addition to providing wood and building materials, plants can be a source of chemicals that may be used to fight diseases, from hypertension and pain to heart disease. 

MEDIA STUDY TOOLS http://connect.mheducation.com Maximize your study time with McGraw-Hill SmartBook®, the first adaptive textbook.

 

Animations

9.4  Root Nodule Formation

 

3D Animation

  Tutorial

9.7  Plant Transport: Water Transport in Xylem • Plant Transport: Translocation in Phloem

SUMMARIZE 9.1  Cells and Tissues of Plants ■ There are over 346,000 known species of plants. Of these, the majority

are the flowering plants, or angiosperms.

■ A plant has the ability to grow its entire life because it possesses meri-

stematic tissue. Apical meristems are located at or near the tips of stems and roots, where they increase the length of these structures. This increase in length is called primary growth. The apical meristems

9.7  Cohesion-Tension Model • Pressure-Flow Model

continually produce three types of meristem: protoderm, ground meristem, and procambium. ■ The entire body of both nonwoody (herbaceous) and young woody plants is covered by a layer of epidermis, which in most plants contains a single layer of closely packed epidermal cells. The walls of epidermal cells that are exposed to air are covered with a waxy cuticle to minimize water loss. ■ Ground tissue forms the bulk of a plant and contains parenchyma, collenchyma, and sclerenchyma cells. Parenchyma cells are



Chapter 9  Plant Organization and Function

thin-walled and capable of photosynthesis when they contain chloroplasts. Collenchyma cells have thicker walls for flexible support. Sclerenchyma cells are hollow, nonliving support cells with secondary walls fortified by lignin. ■ Vascular tissue consists of xylem and phloem. Xylem contains two types of conducting cells: vessel elements and tracheids. Xylem transports water and minerals. In phloem, sieve tubes are composed of sieve-tube members, each of which has a companion cell. Phloem transports sugar and other organic compounds, including hormones.

9.2  Plant Organs and Systems ■ A flowering plant has two main organ systems, the shoot system and

the root system.

■ Vegetative organs are those that are involved in growth and nutrition.

Roots are the organ that anchor a plant, absorb water and minerals, and store the products of photosynthesis. Stems  are organs that support leaves, conduct materials to and from roots and leaves, and help store plant products. Leaves are organs that are specialized for gas exchange, and they carry on most of the photosynthesis in the plant.

167

cork cambium, which produces new cork cells when needed. Cork, a part of the bark, replaces epidermis in woody plants. ■ In a cross section of a woody stem, the bark is composed of all the tissues outside the vascular cambium: secondary phloem, cork cambium, and cork. Wood consists of secondary xylem, which builds up year after year and forms annual rings. ■ Stems are diverse. Modified stems include horizontal aboveground stolons and underground stems, rhizomes, corms, and some tendrils.

9.6  Organization of Leaves ■ The bulk of a leaf is mesophyll tissue bordered by an upper and lower

layer of epidermis. The epidermis is covered by a cuticle and may bear trichomes. Stomata tend to be in the lower layer. Vascular tissue is present within leaf veins. The leaf veins conduct water and the products of photosynthesis to various parts of the plant body. Bundle sheaths are the cells surrounding the vascular tissue. ■ Leaves are diverse. The spines of a cactus are leaves. Other succulents have fleshy leaves. An onion is a bulb with fleshy leaves, and the tendrils of peas are leaves. The Venus flytrap has leaves that trap and digest insects.

9.3  Monocot versus Eudicot Plants

9.7  Uptake and Transport of Nutrients

■ Flowering plants are divided into monocots and eudicots according to

■ Water transport in plants occurs within xylem. The cohesion-tension

the number of cotyledons in the seed; the arrangement of vascular tissue in roots, stems, and leaves; number of flower parts; and number of pores or slits in the pollen grains.

9.4  Organization of Roots ■ Primary growth occurs in both the shoot and root systems. In the root

■

■

■

■

system, a root tip has a zone of cell division (protected by a root cap), a zone of elongation, and a zone of maturation. A cross section of a herbaceous eudicot root reveals the epidermis, which protects; the cortex, which stores food; the endodermis, which regulates the movement of minerals; and the vascular cylinder, which is composed of vascular tissue. The Casparian strip regulates the movement of water and minerals. The first layer of cells in the vascular cylinder is called the peri­ cycle. In the vascular cylinder of a eudicot, the xylem appears starshaped, and the phloem is found in separate regions, between the arms of the xylem. In contrast, a monocot root has a ring of vascular tissue with alternating bundles of xylem and phloem surrounding the pith. Roots are diversified. Taproots are specialized to store the products of photosynthesis. A fibrous root system covers a wider area. Prop roots are adventitious roots specialized to provide increased anchorage. Roots may enter into symbiotic relationships with bacteria (root ­nodules) and fungi.

9.5  Organization of Stems ■ The activity of the shoot apical meristem within a terminal bud

accounts for the primary growth of a stem. A terminal bud contains internodes and leaf primordia at the nodes. When stems grow, the internodes lengthen. ■ In a cross section of a nonwoody eudicot stem, epidermis is the outermost layer of cells, followed by cortex tissue, vascular bundles in a ring, and an inner pith. Monocot stems have scattered vascular bundles, and the cortex and pith are not well defined. ■ Secondary growth of a woody stem is due to activities of the vascular cambium, which produces new xylem and phloem every year, and

model of xylem transport states that transpiration (evaporation of water at stomata) creates tension, which pulls water upward in xylem. This method works only because water molecules are cohesive and adhesive. Transpiration and carbon dioxide uptake occur when stomata are open. ■ Stomata open when guard cells take up potassium (K+) ions and water follows by osmosis. Stomata open because the entrance of water causes the guard cells to buckle out. ■ Transport of organic nutrients in plants occurs within phloem. The pressure-flow model of phloem transport states that sugar is actively transported into phloem at a source, and water follows by osmosis. The resulting increase in pressure creates a flow, which moves water and sugar to a sink.

ASSESS Testing Yourself Choose the best answer for each question.

9.1  Cells and Tissues of Plants 1. All of the tissues of a plant originate from: a. epidermal tissue b. meristem tissue c. ground tissue d. vascular tissue 2. _____ forms the outer protective covering of a plant a. meristem tissue b. ground tissue c. vascular tissue d. epidermal tissue 3. Tracheids and vessel elements would be found in: a. vascular tissue. b. ground tissue. c. epidermal tissue. d. meristem tissue.



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UNIT 2  Plant Biology

9.2  Plant Organs and Systems 4. Which of the following would not be found in the shoot system of a plant? a. stem c. roots b. leaves d. All of these are correct. 5. Nodes and internodes are found on the _____ of a plant. a. roots c. stems b. leaves d. All of these are correct. 6. All of the following are vegetative organs except a. leaves. d. roots. b. stems. e. All of these are correct. c. flowers.

9.3  Monocot versus Dicot Plants 7. Which of these is an incorrect contrast between monocots (stated first) and eudicots (stated second)? a. one cotyledon—two cotyledons b. leaf veins parallel—net veined c. pollen with three pores—pollen with one pore d. flower parts in threes—flower parts in fours or fives e. All of these are correct.

13. Gases, such as CO2, enter the leaf through: a. stomata. c. the mesophyll b. bundle sheath cells d. the phloem

9.7  Uptake and Transport of Nutrients 14. What role do cohesion and adhesion play in xylem transport? a. Like transpiration, they create a tension. b. Like root pressure, they create a positive pressure. c. Like sugars, they cause water to enter xylem. d. They create a continuous water column in xylem. 15. The sugar produced by mature leaves moves into sieve tubes by way of _____, while water follows by _____. a. osmosis, osmosis b. active transport, active transport c. osmosis, active transport d. active transport, osmosis

ENGAGE BioNOW Want to know  how this science is relevant to your life? Check out the BioNow videos below:

9.4  Organization of Roots

■ Saltwater Filter

8. Root hairs are found in the zone of a. cell division. b. elongation. c. maturation. d. apical meristem. e. All of these are correct. 9. The Casparian strip is found a. between all epidermal cells. b. between xylem and phloem cells. c. surrounding endodermal cells. d. within the secondary wall of parenchyma cells. e. in both endodermis and pericycle.

■ Properties of Water

9.5  Organization of Stems 10. Between the bark and the wood in a woody stem, there is a layer of meristem called a. cork cambium. b. vascular cambium. c. apical meristem. d. the zone of cell division. e. procambium preceding bark. 11. Which of these is a stem? a. carrot b. stolon of strawberry plants c. spine of cacti d. sweet potato e. pneumatophore

9.6  Organization of Leaves 12. Which part of a leaf carries on most of the photosynthesis of a plant? a. vascular bundle b. mesophyll c. epidermal layer d. guard cells e. trichomes

■ Cell Division

Thinking Critically 1. If you were given an unfamiliar vegetable, how could you tell if it was a root or a stem, based on its external features and microscopic examination of its cross section?  2. What are some of the potential evolutionary advantages of being woody? 3. Compare and contrast the transport of water and organic nutrients in plants with the transport of blood in humans. 4. Welwitschia is a genus of plant that lives in the Namib and Mossamedes Deserts in Africa. Annual rainfall averages only 2.5 cm (1 inch) per year. Welwitschia plants contain a large number of stomata (22,000 per cm2), which remain closed most of the time. Can you suggest how a large number of stomata would be beneficial to these desert plants? 

PHOTO CREDITS Opener (Neem tree): © Dinodia Photos/Alamy; opener (Neem products): © bdspn/iStock/360/ Getty RF; 9.2a: © Nigel Cattlin/Alamy; 9.4b: © Biophoto Associates/Science Source; 9.2c: © Kingsley Stern; 9.3 (all): © Biophoto Associates/Science Source; 9.4a: © N.C. Brown Center for Ultrastructure Studies, SUNY, College of Environmental Science & Forestry, Syracuse, NY; 9.4b: © Dr. Michael Clayton, University of Wisconsin, Madison, Dept. of Botany; 9.6a: © Dorling Kindersley/Getty Images; 9.6b–c: © Dwight Kuhn; 9.8a(root tip): © Ray F. Evert/University of Wisconsin Madison; 9.8b: © Carolina Biological Supply Company/Phototake; 9.9: © Lee Wilcox; 9.10a: © Al Telser/McGraw Hill Education; 9.10b: © George Ellmore/Tufts University; 9.11a: © Brett Stevens/Getty Images; 9.11b: © NokHoOkNoi/iStock/360/GettyRF; 9.11c: © FLPA/Mark Newman/age fotostock; 9.11d: © DEA/S Montanari/age fotostock; 9.12a: © Dwight Kuhn; 9.12b: © Dr. Keith Wheeler/ Science Source; 9.14a: © Steven P. Lynch; 9.15(top): © Ed Reschke; 9.15(bottom): © Ray F. Evert/University of Wisconsin Madison; 9.16(top): © Carolina Biological Supply Company/ Phototake; 9.16(bottom): © Kingsley Stern; 9.18(circular cross section): © Ed Reschke/Getty Images; 9.19a: © Ardea London/Ardea.com; 9A(flooring): © Roman Borodaev/Alamy RF; 9A(shoots): © Koki Iino/Getty RF; 9A(bamboo grove): © Michele Westmorland/Getty Images; 9.20a: © Evelyn Jo Johnson; 9.20b: © Science Pictures Limited/Science Source; 9.20c–d: © Carlyn Iverson/McGraw-Hill Education; 9.21: © Ray F. Evert, University of Wisconsin, Madison; 9.23a: © Nature Picture Library RF; 9.23b–c: © Steven P. Lynch; 9.25a–b: © Jeremy Burgess/SPL/Science Source; 9B: © Yann Arthus-Bertrand/Corbis; 9.26a: © Dr. M.H. Zimmerman/Harvard University; 9.26b: © Steven P. Lynch.

CASE STUDY Seedless Plants It is nice to be able to eat a cold watermelon on a hot summer day without being bothered by the seeds. Seedless watermelons, once a novelty, are now a common convenience. But how are seedless watermelons produced? Seeded watermelons have a diploid number (2n) of 22 chromosomes. Genetic manipulation of a 2n plant can produce a 4n plant with 44 chromosomes. When a 2n plant is crossed with a 4n plant , the resulting fruit has 3n seeds. The plant that germinates from this 3n seed is sterile (unable to produce seeds) because meiosis cannot proceed as usual. Hence the seedless watermelon! Although seedless watermelons do not produce viable seeds, they develop small white “seeds.” These are actually soft, tasteless seed coats that can be eaten with the watermelon. Since these seed coats can’t be used to start next year’s crop, the next batch of 3n seeds needed for planting must be produced each year by crossing 2n and 4n plants. Expense, time, and energy are required in order to save us the inconvenience of spitting out the seeds! Seedless watermelons are just one example of the capability we have to genetically modify plants and other organisms, often simply called GMOs. Unlike seedless watermelons, there is controversy surrounding the potential health effects of eating GM (genetically modified) foods. Nonetheless, humans modify plants all the time. The fundamentals of agriculture and supplying plant-based products to the human population include a better understanding of how flowering plant reproduction occurs, seed production and development, and how plants operate under natural conditions. As you read through the chapter, think about the following questions:

1. What hormones are associated with the normal growth and development of flowering plants?

10

Plant Reproduction and Responses

CHAPTER OUTLINE 10.1  Sexual Reproduction in Flowering Plants

10.2 Growth and Development 10.3 Asexual Reproduction and Genetic Engineering in Plants

10.4 Control of Growth and Responses

BEFORE YOU BEGIN Before beginning this chapter, take a few moments to review the following discussions: Section 5.4  What role does meiosis play in sexual reproduction? Section 5.5  What are the differences between mitosis and meiosis? Section 9.3  What is the difference between a monocot and eudicot plant?

2. What are some examples of how humans have manipulated plants to better serve our needs?

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10.1  Sexual Reproduction in Flowering Plants Learning Outcomes S SI TO MI

Upon completion of this section, you should be able to 1. Describe the alternation-of-generations life cycle including the role of the sporophyte and gametophyte generations.  2. Identify the reproductive parts of a flower and describe the function of each part. 3. Diagram and describe the development of male and female gametophytes and the development of the sporophyte of flowering plants.

anther sporophyte

seed

diploid (2n)

zygote

ovule ovary MEIOSIS

FERTILIZATION

Adaptation to a Land Environment The life cycle of flowering plants is adapted to a land existence because all stages of the life cycle are protected from drying out (see section 30.1). For example, the microscopic gametophytes develop within the sporophyte. Pollen grains are not released until they have a thick wall, and they are carried by wind or an animal

megaspore

egg sperm Male gametophyte (pollen grain)

IS

In flowering plants, the sporophyte is dominant. Dominant simply means that this is the stage where the plant spends the majority of its life cycle and conducts the majority of photosynthesis. In the case of the angiosperms, this is the stage of the life cycle that bears flowers (Fig. 10.1). A flower is a reproductive structure and it produces two types of spores: microspores and megaspores. A microspore undergoes mitosis and becomes a pollen grain, which is the male gametophyte. Meanwhile, the megaspore has undergone mitosis to become a microscopic embryo sac, which is the female gametophyte. The female gametophyte is retained within the flower. A pollen grain is either windblown or carried by an animal to the vicinity of the embryo sac. At maturity, a pollen grain contains two nonflagellated sperm that travel down a pollen tube to the embryo sac. Upon fertilization, the structure surrounding the embryo sac develops into a seed. The seeds are enclosed by a fruit, which aids in dispersing the seeds. When a seed germinates, a new sporophyte emerges and develops into a mature organism.

microspore

S TO

Alternation of Generations in Flowering Plants

haploid (n)

MI

Like the animals, sexual reproduction in plants is advantageous because it generates variation among the offspring through the process of meiosis and random fertilization. When plants reproduce sexually, they undergo an alternation of generations, in which they alternate between two multicellular stages, one diploid (2n) and one haploid (n). In this chapter, we will use the life cycle of a flowering plant, or angiosperm, as our example (Fig. 10.1). In the plant life cycle there are two stages. The diploid stage of the life cycle is called the sporophyte. The sporophyte produces haploid spores by the process of meiosis. These spores then divide by mitosis to become the gametophyte. The gametophyte is haploid and it divides by mitosis to produce haploid gametes (egg and sperm). Fertilization of an egg by a sperm produces a new diploid sporophyte.

Female gametophyte (embryo sac)

Figure 10.1  Alternation of generations in flowering plants. 

The sporophyte bears flowers. The flower produces microspores within anthers and megaspores within ovules by meiosis. A megaspore becomes a female gametophyte, which produces an egg within an embryo sac, and a microspore becomes a male gametophyte (pollen grain), which produces sperm. Fertilization results in a seed-enclosed zygote and stored food.

(usually an insect) to another flower, where they develop a pollen tube that carries the sperm to the egg. Following fertilization, the seed coat protects the embryo until conditions favor regrowth.

Flowers The flower is unique to the group of plants known as the angiosperms. The evolution of the flower was a major factor leading to the success of angiosperms, which now number over 346,000 described species. Aside from producing the spores and protecting the gametophytes, flowers often attract pollinators. Flowers also produce the fruits that enclose the seeds. A flower develops in response to various environmental signals (see  section 10.4). In many plants, a shoot apical meristem stops producing leaves and starts producing a bud that contains a  flower. In other plants, axillary buds develop directly into flowers. A typical flower has four whorls of modified leaves attached to a receptacle at the end of a flower stalk. The receptacle bearing



Chapter 10  Plant Reproduction and Responses

171

carpel (pistil) stigma

stamen anther filament

style ovary

petal

sepal

ovule

receptacle

peduncle

Figure 10.2  Anatomy of a flower.  A complete flower has all flower parts: sepals, petals, stamens, and at least one carpel.

a single flower is attached to a structure called a peduncle, while the structure that bears one of several flowers is called a pedicel. Figure 10.2 shows the following basic floral structures: 1. Sepals are the most leaflike of all the flower parts, are usually green, and protect the bud as the flower develops within. Sepals can also be the same color as the flower petals. 2. Petals are the structures used to attract a variety of pollinators. Wind-pollinated flowers may have no petals at all. 3. Stamens are the “male” portion of the flower. Each stamen has two parts: the anther, a saclike container, and the filament, a slender stalk. Pollen grains develop from the microspores produced in the anther. 4. At the very center of a flower is the carpel (also called the pistil), a vaselike structure that represents the “female” portion of the flower. A carpel usually has three parts: the stigma, an enlarged sticky knob; the style, a slender stalk; and the ovary, an enlarged base that encloses one or more ovules. In monocots, the petals and sepals occur in threes and multiples of three. In eudicots, the petals and sepals are in fours or fives and multiples of four or five (Fig. 10.3). A flower can have a single carpel or multiple carpels. Sometimes several carpels are fused into a single structure, in which case this compound ovary has several chambers containing ovules, each of which will house a megaspore. For example, an orange develops from a compound ovary, and every section of the orange is a chamber. Not all flowers have sepals, petals, stamens, and carpels. Those that do, such as the flower in Figure 10.2, are said to be

complete, and those that do not are said to be incomplete. Flowers that have both stamens and carpels are called perfect (bisexual) flowers. Those with only stamens or only carpels are imperfect (single-sex) flowers. If staminate and carpellate (pistillate) flowers are on one plant, the plant is monoecious (one house) (Fig. 10.4). If staminate and carpellate flowers are on separate plants, the plant is dioecious (two houses).

Life Cycle of Flowering Plants In plants, the sporophyte produces haploid spores by meiosis. The haploid spores grow and develop into haploid gametophytes, which produce gametes by mitotic division. Flowering plants are heterosporous, meaning that they produce microspores and megaspores. Microspores become mature male gametophytes (sperm-bearing pollen grains), and megaspores become mature female gametophytes (egg-bearing embryo sacs).

Development of Male Gametophyte Microspores are produced in the anthers of flowers (Fig. 10.5). An anther has four pollen sacs, each containing many microspore mother cells. A microspore mother cell undergoes meiosis to produce four haploid microspores. In each, the haploid nucleus divides mitotically, followed by unequal cytokinesis, and the result is two cells enclosed by a finely sculptured wall. This structure, called the pollen grain, is at first an immature male gametophyte that consists of a tube cell and a generative cell. The larger tube cell will eventually produce a pollen tube. The smaller generative cell



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UNIT 2  Plant Biology

stamen

p2 s1 s2

carpel petal

p1 p3

sepal s3 a. Daylily

a. Staminate flowers

Figure 10.4  Corn plants are monoecious.  A corn plant has (a) clusters of staminate flowers and (b) clusters of carpellate flowers. Staminate flowers produce the pollen that is carried by wind to the carpellate flowers, where an ear of corn develops.

p3 p2 carpel stamen p4

petal

b. Carpellate flowers

p1

p5

b. Festive azalea

Figure 10.3  Monocot versus eudicot flowers.  a. Monocots, such as daylilies, have flower parts usually in threes. In particular, note the three petals and three sepals. b. Azaleas are eudicots. They have flower parts in fours or fives; note the five petals of this flower. (p = petal; s = sepal)

divides mitotically either now or later to produce two sperm. Once these events take place, the pollen grain has become the mature male gametophyte.

Development of Female Gametophyte The ovary contains one or more ovules. An ovule has a central mass of parenchyma cells almost completely covered by layers of  tissue called integuments, except where there is an opening called a micropyle. One parenchyma cell enlarges to become a megaspore mother cell, which undergoes meiosis, producing four haploid megaspores (Fig. 10.5). Three of these megaspores are

nonfunctional; one is functional. Typically, the nucleus of the functional megaspore divides mitotically until there are eight nuclei in the female gametophyte. When cell walls form later, seven cells are formed, one of which contains two nuclei (binucleate). This collection of seven cells is collectively called the female gametophyte, or embryo sac.

Pollination and Fertilization The walls separating the pollen sacs in the anther break down when the pollen grains are ready to be released (Fig. 10.6). ­Pollination is simply the transfer of pollen from an anther to the stigma of a carpel. Self-pollination occurs if the pollen is from the same plant, and cross-pollination occurs if the pollen is from a ­different plant of the same species. Angiosperms often have adaptations to foster cross-pollination. For example, the carpels may mature only after the anthers have released their pollen. Cross-pollination may also be brought about with the assistance of a particular pollinator. The Ecology feature, “The Coevolution of Plants and Their Pollinators,” explores the importance of insects and other pollinators to the life cycle of a flowering plant. Plants have evolved mechanisms, such as color, odors, or production of nectar, to attract specific pollinators to their flowers. If a pollinator goes from flower to flower of only one type of plant, cross-pollination is more likely to occur in an efficient manner. Over time, certain pollinators have become adapted to reach the nectar of only one type of flower. The process by which the evolution of two species is connected is called coevolution, and it is considered to be a major factor in the success of the angiosperms. 

SCIENCE IN YOUR LIFE  ►

ECOLOGY

The Coevolution of Plants and Their Pollinators Plants and their pollinators have adapted to one another. They have a mutualistic relationship in which each benefits—the plant uses its pollinator to ensure that cross-pollination takes place, and the pollinator uses the plant as a source of food. This mutualistic relationship came about through the process of coevolution—that is, the interdependency of the plant and the pollinator is the result of suitable changes in the structure and function of each. 

Bee- and Wasp-Pollinated Flowers There are 20,000 known species of bees that pollinate flowers. The best-known pollinators are the honeybees (Fig. 10Aa). As noted in the text, bee eyes see ultraviolet (UV) wavelengths. Therefore, bee-pollinated flowers are usually brightly colored and are predominantly blue or yellow; they are not entirely red. They may also have ultraviolet shadings called nectar guides, which highlight the portion of the flower that contains the reproductive structures.  The mouthparts of bees are fused into a long tube that contains a tongue. This tube is an adaptation for sucking up nectar provided by the plant, usually at the base of the flower. Bees also collect pollen as a food.  Bee-pollinated flowers are delicately sweet and fragrant, advertising that nectar is present. The nectar guides often point to a narrow floral tube large enough for the bee’s feeding apparatus but too small for other insects to reach the nectar. Bee-pollinated flowers may be irregular in shape, and are sturdy because they often have a landing platform where the bee can alight. The flower structure requires the bee to brush up against the anther and stigma as it moves toward the floral tube to feed.  One type of orchid, Ophrys, has evolved a unique adaptation. The flower resembles a female wasp, and when the male of that species attempts to copulate with the flower, the flower spring loads pollen on the wasp’s head. When the frustrated wasp attempts to “copulate” with another flower, the pollen is perfectly positioned to come in contact with the stigma of the second flower. 

Moth- and Butterfly-Pollinated Flowers Both moths and butterflies have a long, thin, hollow proboscis, but they differ in other characteristics. Moths usually feed at night and have a well-developed sense of smell. The flowers they visit are visible at night, because they are lightly shaded (white, pale yellow, or pink), and they have strong, sweet perfume, which helps attract moths. Moths hover when they feed, and

a.

b.

c.

d.

Figure 10A  Types of pollinators.  a. A bee-pollinated flower is a color other than red. b. A butterfly-pollinated flower is often a composite, containing many individual flowers. c. Hummingbirdpollinated flowers are curved back, allowing the bird to insert its beak to reach the rich supply of nectar. d. Bat-pollinated flowers are large, sturdy flowers that can take rough treatment.  their flowers have deep tubes with open margins that allow the hovering moths to reach the nectar with their long proboscis.  Butterflies, in comparison, are active in the daytime and have good vision but a weak sense of smell. Their flowers have bright ­colors—even red, because butterflies can see the color red—but the flowers tend to be odorless. Unable to hover, butterflies need a place to land. Flowers that are visited by butterflies often have flat landing platforms (Fig. 10Ab). Composite flowers (composed of a compact head of numerous individual flowers) are especially favored by butterflies. Each flower has a long, slender floral tube, accessible to the long, thin butterfly proboscis. 

Bird- and Bat-Pollinated Flowers In North America, the most well-known bird pollinators are the hummingbirds. These small animals have good eyesight but do not have a well-developed sense of smell. Like moths, they hover when they feed. Typical flowers pollinated by hummingbirds are red, with a slender floral tube and margins that are curved back and out of the way. And although they produce

copious amounts of nectar, the flowers have little odor. As a hummingbird feeds on nectar with its long, thin beak, its head comes in contact with the stamens and pistil (Fig. 10Ac).  Bats are adapted to gathering food in various ways, including feeding on the nectar and pollen of plants. Bats are nocturnal and have an acute sense of smell. Those that are pollinators also have keen vision and a long, extensible, bristly tongue. Typically, bat-pollinated flowers open only at night and are light-colored or white. They have a strong, musky smell similar to the odor that bats produce to attract one another. The flowers are generally large and sturdy and are able to hold up when a bat inserts part of its head to reach the nectar. While the bat is at the flower, its head becomes dusted with pollen (Fig. 10Ad). 

Questions to Consider 1. Why do you think pollinators are often very specific to the plants they pollinate? 2. What are the potential consequences if honeybees were to go extinct? 3. What are some other examples of coevolution that you can think of?

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UNIT 2  Plant Biology

anther

Mature Seed seed coat The ovule develops into a seed containing the embryonic sporophyte and endosperm.

mitosis

ovule ovary

embryo endosperm (3n)

Sporophyte

Seed

diploid (2n)

DOUBLE FERTILIZATION

haploid (n)

tube cell

Pollination During double fertilization, one sperm from the male gametophyte will fertilize the egg; another sperm will join with polar nuclei to produce the 3n endosperm.

Development of the sporophyte: pollen tube

Pollination occurs; a pollen grain germinates and produces a pollen tube.

generative cell Pollen grain (male gametophyte)

ovule wall

sperm

antipodals

Mature male gametophyte polar nuclei egg

polar nuclei

tube cell nucleus

egg cell synergids Embryo sac (mature female gametophyte)

sperm

Figure 10.5  Life cycle of flowering plants.  Starting with the generation of the gametophytes (upper right) and moving clockwise, a pollen sac in the anther contains a microspore mother cell, which produces microspores by meiosis. A microspore develops into a pollen grain, which germinates and has two sperm. An ovule in an ovary contains a megaspore mother cell, which produces a megaspore by meiosis. A megaspore develops into an embryo sac containing seven cells, one of which is an egg. A pollen grain contains two sperm by the time it germinates and forms a pollen tube. During double fertilization, one sperm fertilizes the egg to form a diploid zygote, and the other fuses with the polar nuclei to form a triploid (3n) endosperm cell. A seed contains the developing sporophyte embryo plus stored food.

When a pollen grain lands on the stigma of the same species, it germinates, forming a pollen tube (see Fig. 10.5). The germinated pollen grain, containing a tube cell and two sperm, is the mature male gametophyte. As it grows, the pollen tube passes between the cells of the stigma and the style to reach the micropyle, a pore of the ovule. Now double fertilization occurs. One sperm nucleus unites with the egg nucleus, forming a 2n zygote, and the other sperm nucleus migrates and unites with the polar nuclei of the central cell, forming a 3n endosperm cell. The zygote divides mitotically to become the embryo, a young sporophyte, and the endosperm cell divides mitotically to become the

endosperm. Endosperm is the tissue that will nourish the embryo and seedling as they develop.

Check Your Progress  10.1 1. Identify which generation produces two types of spores and what will happen to the spores.

2. List the reproductive parts of a flower and give a function for each part.

3. Describe what is meant by the term heterosporous. 4. Describe the process of double fertilization.



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Chapter 10  Plant Reproduction and Responses

Development of the male gametophyte:

Development of the female gametophyte:

In pollen sacs of the anther, a microspore mother cell undergoes meiosis to produce 4 microspores each.

In an ovule within an ovary, a megaspore mother cell undergoes meiosis to produce 4 megaspores.

anther ovary

Pollen sac

microspore mother cell

Ovule b.

1,484×

c.

2,000×

megaspore mother cell

MEIOSIS

MEIOSIS

ovule wall Microspores

s

megaspore

Figure 10.6  Pollen grains.  a. Grass releasing pollen. b. Pollen

grains of Canadian goldenrod, Solidago canadensis. c. Pollen grains of asparagus, Asparagus officinalis. The shape and pattern of pollen grain walls are quite distinctive, and experts can use them to identify the genus, and sometimes even the species, that produced a particular pollen grain.

3 megaspores disintegrate

s

osi mit

One megaspore becomes the embryo sac (female gametophyte).

osi mit

Microspores develop into male gametophytes (pollen grains).

a.

Megaspores

integument micropyle

10.2  Growth and Development Learning Outcomes Upon completion of this section, you should be able to 1. Recognize the developmental steps of a eudicot embryo and compare the function of its cotyledons to that of a cotyledon in monocots. 2. Identify different types of fruits. 3. Label seed structure and describe germination and dispersal.

Development of the Eudicot Embryo The endosperm cell, shown in Figure 10.7a, divides to produce the endosperm tissue, shown in Figure 10.7b. The zygote also divides, but asymmetrically. One of the resulting cells is small, with dense cytoplasm. This cell is destined to become the embryo, and it divides repeatedly in different planes, forming a ball of cells. The other, larger cell also divides repeatedly, but it forms an elongated structure called a suspensor, which has a basal cell. The suspensor pushes the embryo deep into the endosperm tissue. The suspensor then disintegrates as the seed matures.

During the globular stage, the embryo is a ball of cells (Fig. 10.7c). The root-shoot axis of the embryo is already established at this stage because the embryonic cells near the suspensor will become a root, whereas those at the other end will ultimately become a shoot. The embryo has a heart shape when the cotyledons, or seed leaves, appear (Fig. 10.7d). As the embryo continues to enlarge and elongate, it takes on a torpedo shape (Fig. 10.7e). Now the root tip and shoot tip are distinguishable. The shoot apical meristem in the shoot tip is responsible for aboveground growth, and the root apical meristem in the root tip is responsible for belowground growth. In a mature embryo, the epicotyl is the portion between the cotyledon(s) that contributes to shoot development (Fig. 10.7f ). The hypocotyl is that portion below the cotyledon(s) that contributes to stem development. The radicle contributes to root development. Now, the embryo is ready to develop the two main parts of a plant: the shoot system and the root system. The cotyledons are quite noticeable in a eudicot embryo and may fold over. As the embryo develops, the wall of the ovule becomes the seed coat.

Monocot Versus Eudicot Cotyledons Eudicots have two cotyledons; monocots have only one. In monocots, the cotyledon rarely stores food. Instead, it absorbs food molecules from the endosperm and passes them to the embryo. In eudicots, the cotyledons usually store the nutrient molecules that the embryo uses. Therefore, the endosperm disappears because it has been taken up by the two cotyledons.



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UNIT 2  Plant Biology

endosperm nucleus zygote

endosperm

a.

b.

embryo

suspensor basal cell c.

cotyledons appearing

shoot tip epicotyl bending cotyledons hypocotyl bending cotyledons

radicle f.

root tip

d.

e.

Figure 10.7  Development of a eudicot embryo.  a. The singled-celled zygote lies beneath the endosperm nucleus. b, c. The endosperm is a mass of tissue surrounding the embryo. The embryo is located above the suspensor. d. The embryo becomes heart-shaped as the cotyledons begin to appear. e. There is progressively less endosperm as the embryo differentiates and enlarges. As the cotyledons bend, the embryo takes on a torpedo shape. f. The embryo consists of the epicotyl, the hypocotyl, and the radicle.

Fruit Types and Seed Dispersal A fruit is derived from an ovary and sometimes other flower parts. It serves to protect and help disperse the offspring. As a fruit develops, the ovary wall thickens to become the pericarp (Fig. 10.8a). The pericarp can contain up to three layers: exocarp, mesocarp, and endocarp. Simple fruits are derived from a simple ovary of a single carpel or from a compound ovary of several fused carpels. A pea pod is an example of a simple fruit. At maturity, the pea pod breaks open on both sides of the pod to release the seeds. Peas and beans are legumes. A legume is a fruit that splits along two sides when mature. Like legumes, cereal grains of wheat, rice, and corn are dry fruits. Sometimes, these grainlike fruits are mistaken for seeds because a dry pericarp adheres to the seed within. These dry fruits are indehiscent, meaning that they don’t split open. Humans gather grains before they are released from the plant and then process them to acquire their nutrients. In some simple fruits, the mesocarp becomes fleshy. When ripe, fleshy fruits often attract animals and provide them with food

(Fig. 10.8b). Peaches and cherries are examples of fleshy fruits that have a hard endocarp. This type of endocarp protects the seed so it can pass through the digestive system of an animal and remain unharmed. In a tomato, the entire pericarp is fleshy. If you cut open a tomato, you see several chambers because the flower’s carpel is composed of several fused carpels. Among fruits, apples are an example of an accessory fruit because the bulk of the fruit is not from the ovary, but from the receptacle. Only the core of an apple is derived from the ovary. If you cut an apple crosswise, it is obvious that an apple, like a tomato, came from a compound ovary with several chambers. Compound fruits develop from several individual ovaries. For example, each little part of a raspberry or blackberry is derived from a separate ovary. Because the flower had many separate carpels, the resulting fruit is called an aggregate fruit (Fig. 10.8c). The strawberry is also an aggregate fruit, but each ovary becomes a one-seeded fruit called an achene. The flesh of a strawberry is from the receptacle. In contrast, a pineapple comes from many different carpels that belong to separate flowers. As the ovaries mature, they fuse to form a large, multiple fruit (Fig. 10.8d).



Chapter 10  Plant Reproduction and Responses

177

Figure 10.8  Structure and function of fruits.  a. Maple trees produce a winged fruit. The wings rotate in the wind and keep the fruit aloft. b. Strawberry plants produce an accessory fruit. Each “seed” is actually a fruit on a fleshy, expanded receptacle. c. Like strawberries, blackberries are aggregate fruits. Each “berry” is derived from an ovary within the same flower. d. A pineapple is a multiple fruit derived from the ovaries of many flowers. seed covered by pericarp

wing

one fruit one fruit

fruits from ovaries of many m flowers

d. A multipl multiple fruit contains many fused fruits produced from simple ovaries of individual flowers.

a.

c.

one fruit

fruits from ovaries of one flower

has seed pods that swell as they mature. When the pods finally burst, the ripe seeds are hurled out.

Germination of Seeds b.

Dispersal of Seeds To increase a plant’s chance of success, it needs to be widely ­distributed. In order to achieve success, plant seeds need to be dispersed away from the parent plant. Plants have various means of ensuring that dispersal takes place. The hooks and spines of clover, bur, and cocklebur attach to the fur of animals and the clothing of humans. Birds and mammals sometimes eat fruits, including the seeds, which are then defecated (passed through the digestive tract with the feces) some distance from the parent plant. Squirrels and other animals gather seeds and fruits, which they bury some distance away. Some plants have fruits with trapped air or seeds with inflated sacs that help them float in water. The fruit of the coconut palm, for example, which can be dispersed by ocean currents, may land hundreds of kilometers away from the parent plant. Many plants have seeds with structures such as woolly hairs, plumes, and wings that adapt them for dispersal by wind. The seeds of an orchid, however, are so small and light that they need no special adaptation to carry them far away. The somewhat heavier dandelion fruit uses a tiny “parachute” for dispersal. The winged fruit of a maple tree, which contains two seeds, has been known to travel up to 10 kilometers (km) from its parent (see Fig. 10.8a). A touch-me-not plant

Provided the environmental conditions are right seeds g­ erminate— that is, they begin to grow so that a seedling appears (Fig. 10.9). Some seeds do not germinate until they have been dormant for a specific period of time. For seeds, dormancy is the time during which no growth occurs, even though conditions may be favorable for growth. In the temperate zone, seeds often have to be exposed to a period of cold weather before dormancy is broken. In deserts, germination does not occur until there is adequate moisture. This requirement helps ensure that seeds do not germinate until the most favorable growing conditions are present. Germination takes place if there is sufficient water, warmth, and oxygen to sustain growth. Germination requires regulation, and both inhibitors and stimulators are known to exist. Fleshy fruits (e.g., apples and tomatoes) contain inhibitors so that germination does not occur until the seeds are removed and washed. In contrast, stimulators are present in the seeds of some temperate-zone woody plants. Mechanical action may also be required. Water, bacterial action, and even fire can act on the seed coat, allowing it to become permeable to water. The uptake of water causes the seed coat to burst.

Eudicot Versus Monocot Seed Germination Eudicot embryos have two cotyledons that have absorbed the endosperm (Fig. 10.9). The cotyledons supply nutrients to the embryo and seedling and eventually shrivel and disappear. If the two cotyledons of a bean seed are parted, you can see a rudimentary plant. The epicotyl bears young leaves and is called a



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UNIT 2  Plant Biology

first leaf

plumule

pericarp

cotyledons (two)

Seed coat endosperm

hypocotyl

Cotyledon (one) coleoptile Embryo: plumule radicle coleorhiza

radicle seed coat cotyledon

primary root

a. Corn seed

a. Bean seed

true leaf

first true leaves (primary leaves) seed coat

coleoptile

cotyledons (two)

hypocotyl

primary root

epicotyl withered cotyledons hypocotyl

first leaf coleoptile

coleoptile

prop root

radicle secondary root primary root

adventitious root coleorhiza

primary root

b. Bean germination and growth

b. Corn germination and growth

Figure 10.9  Eudicot seed structure and germination.  The common garden bean is a eudicot. a. Eudicot seed structure. b. Germination and development of the seedling.

Figure 10.10  Monocot seed structure and germination. 

plumule. As the eudicot seedling emerges from the soil, the shoot is hook-shaped to protect the delicate plumule. The hypocotyl becomes part of the stem, and the radicle develops into the roots. In monocots, the endosperm is the food-storage tissue, and the single cotyledon does not have a storage role (Fig. 10.10). Corn is a monocot, and its kernels are actually fruits, therefore, the outer covering is the pericarp. The plumule and radicle are enclosed in protective sheaths called the coleoptile and the coleorhiza, respectively. The plumule and the radicle burst through these coverings during germination.

10.3  Asexual Reproduction and Genetic Engineering in Plants

Check Your Progress  10.2 1. Compare the function of endosperm in monocots versus eudicots.

2. Identify examples of plant adaptations for seed dispersal by wind.

3. Explain why seeds don’t germinate immediately after dispersal. 4. Describe the differences between eudicot and monocot seed germination.

A corn plant is a monocot. a. Seed structure. b. Germination and development of the seedling.

Learning Outcomes Upon completion of this section, you should be able to 1. Recognize how asexual reproduction in plants differs from sexual reproduction. 2. Describe how plants are propagated in tissue culture. 3. Explain how genetic engineering can be used to alter plant traits.

In asexual reproduction, there is only one parent involved. Because plants contain nondifferentiated meristem tissue, they routinely reproduce asexually by vegetative propagation. For example, complete strawberry plants will grow from the nodes of stolons (see Fig. 9.20a) and irises will grow from the nodes of rhizomes (see Fig. 9.20b). White potatoes are actually portions of belowground stems, and each eye is a bud that will produce a new potato plant if it is planted with a portion of the swollen tuber. Sweet potatoes are



Chapter 10  Plant Reproduction and Responses

modified roots. They can be propagated by planting sections of the root. You may have noticed that the roots of some fruit trees, such as cherry and apple trees, produce “suckers,” small plants that can grow into new trees. Many types of plants are now propagated from stem cuttings. The discovery that the plant hormone auxin can cause roots to develop from a stem, as discussed in section 10.4, has expanded the list of plants that can be propagated from stem cuttings.

Propagation of Plants in Tissue Culture Tissue culture is the growth of a tissue in an artificial liquid or solid culture medium. Tissue culture now allows botanists to breed a large number of plants from somatic tissue and to select any that have superior hereditary characteristics. These and any plants with desired genotypes can be propagated rapidly in tissue culture. Plant cells are totipotent, which means that an entire plant can be produced from most plant cells. The only exceptions are the plant cells that lose their nuclei (sieve-tube members) or that are dead at maturity (xylem, sclerenchyma, and cork cells). Commercial micropropagation methods now produce thousands, even millions, of identical seedlings in a small vessel. One favorite such method is meristem culture. If the correct proportions of hormones are added to a liquid medium, many new shoots will develop from a single shoot tip. When these are removed, more shoots form. Because the shoots are genetically identical, the adult plants that develop from them, called clonal plants, all have the same traits. Another advantage to meristem culture is that meristem, unlike other portions of a plant, is virus-free. Therefore, the plants produced are also virus-free. When mature plant cells are used to grow entire plants, enzymes are often used to digest the cell walls of mesophyll tissue from a leaf. The result is

179

cells without walls, called protoplasts (Fig. 10.11a). The protoplasts regenerate a new cell wall (Fig. 10.11b) and begin to divide, forming aggregates of cells and then a callus (Fig. 10.11c, d). These clumps of cells can be manipulated to produce somatic embryos (asexually produced embryos) (Fig. 10.11e). It’s possible to produce millions of somatic embryos at once in large tanks called bioreactors. This is done for certain vegetables, such as tomatoes, celery, and asparagus, and for ornamental plants, such as lilies, begonias, and African violets. Somatic embryos that are encapsulated in a protective hydrated gel (and sometimes called artificial seeds) can be shipped anywhere. A mature plant develops from each somatic embryo (Fig. 10.11f ). Plants generated from the somatic embryos vary somewhat because of mutations that arise during the production process. These are called somaclonal variations and represent another way to produce new plants with desirable traits. Recall that pollen grains are produced in the anthers of a flower. Anther culture is a direct way to produce a line of plants whose homologous chromosomes have the same genes. Anther culture involves mature anthers that are cultured in a medium containing vitamins and growth regulators. The haploid tube cells within the pollen grains divide, producing proembryos consisting of as many as 20 to 40 cells. Finally, the pollen grains rupture, releasing haploid embryos. The experimenter can now generate a haploid plant, or chemical agents can be added that encourage chromosomal doubling. The resulting plants are diploid, and the homologous chromosomes carry the same genes. The culturing of plant tissues has led to a technique called cell suspension culture. Rapidly growing calluses are cut into small pieces and shaken in a liquid nutrient medium so that single cells or small clumps of cells break off and form a suspension. These

Figure 10.11  Tissue culture of

plants.  a. When plant cell walls are

a. Protoplasts

b. Cell wall regeneration

c. Aggregates of cells

d. Callus (undifferentiated mass)

e. Somatic embryo

f. Plantlet

removed by digestive enzyme action, the resulting cells are called protoplasts. b. Cell walls regenerate, and cell division begins. c. Cell division produces an aggregate of cells. d. An undifferentiated mass, called a callus, forms. e. Somatic cell embryos appear. f. The embryos develop into plantlets that can be transferred to soil for growth into adult plants.



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UNIT 2  Plant Biology

a.

b.

Figure 10.12  Transgenic plants.  a. These soybean plants have been given a gene that causes them to be resistant to a particular herbicide. b. The corn on the left is not a GMO and does not develop as well as the GMO corn plant on the right. 

cells will produce the same chemicals as the entire plant. For example, cell suspension cultures of Cinchona ledgeriana produce quinine, and those of Digitalis lanata produce digitoxin.

Genetic Engineering of Plants Traditionally, hybridization, the crossing of different varieties of plants or even species, was used to produce plants with desirable traits. Hybridization, followed by vegetative propagation of the mature plants, generated a large number of identical plants with these traits. Today, it is possible to directly alter the genes of organisms and, in that way, produce new varieties with desirable traits.

Tissue Culture and Genetic Engineering Protoplasts can be genetically altered. A gene of interest isolated from any type of organism—plant, animal, or bacteria—can be inserted into protoplasts by a variety of mechanisms, including high-voltage electric pulses and chemical treatments. Each of these methods creates temporary pores in the plasma membrane that allow foreign genes to enter the cells. In one such procedure, a gene for the production of the firefly enzyme luciferase was inserted into tobacco protoplasts, and the adult plants glowed when sprayed with the substrate luciferin. Regeneration of cereal grains from protoplasts is typically difficult. As a result, other methods are often used to introduce DNA into plant cells with intact cell walls. In one method, a gene gun is used to bombard a callus (undifferentiated mass of cells) with DNA-coated microscopic metal particles that pierce the cell wall and plasma membrane, delivering DNA into the cell and then the nucleus. The genetically altered callus can then be induced to develop into a genetically altered adult plant. Many plants, including corn and wheat varieties, have been genetically engineered by these methods. Such plants are called

transgenic organisms (or genetically modified organisms) because they carry a foreign gene and have new and different traits. Figure 10.12 shows two types of transgenic plants. In a more recent method, foreign DNA is inserted into the plasmid of the bacterium Agrobacterium, which normally infects plant cells. The plasmid then contains recombinant DNA because it has genes from different sources—namely, those of the plasmid and also the isolated genes of interest. When a bacterium with a recombinant plasmid infects a plant, the recombinant plasmid enters the cells of the plant and can naturally incorporate into the plant’s DNA.

Agricultural Plants with Improved Traits Corn and cotton plants, in addition to soybean and potato plants, have been engineered to be resistant to either herbicides or insect pests. Some corn and cotton plants have been developed that are both insect- and herbicide-resistant. According to the USDA, over 89% of corn and 91% of cotton planted in 2014 was geneticallymodified. These biotech crops represent one of the most significant crop technologies in the history of modern agriculture. If crops are resistant to a broad-spectrum herbicide and weeds are not, then the herbicide can be used to kill the weeds. When herbicideresistant plants were planted, weeds were easily controlled, less tillage was needed, and soil erosion was minimized. However, opponents of this approach have expressed concern that these methods may endanger wildlife and native plant species. Another concern is the potential transfer of herbicide-resistant genes from the modified crop plant to a related wild, “weedy” species through natural hybridization. Crops with other improved agricultural and food-quality traits are desirable (Fig. 10.13). Irrigation, even with fresh water, inevitably leads to salinization of the soil, which reduces crop yields. Thus, the development of salt-tolerant crops would increase yields



Chapter 10  Plant Reproduction and Responses

181

Transgenic Crops of the Future Improved Agricultural Traits Herbicide resistant Salt tolerant Drought tolerant Cold tolerant Improved yield Modified wood pulp Disease protected

Wheat, rice, sugar beets, canola Cereals, rice, sugarcane, canola Cereals, rice, sugarcane Cereals, rice, sugarcane Cereals, rice, corn, cotton Trees Wheat, corn, potatoes

Improved Food-Quality Traits Fatty acid/oil content Protein/starch content Amino acid content

Corn, soybeans Cereals, potatoes, soybeans, rice, corn Corn, soybeans

a. Desirable traits

b.

WT

X10E

Wild-type (WT) and transgenic tomato (X1OE) grown in the presence of 200 mM NaCl

Figure 10.13  Transgenic crops of the future.  a. Transgenic crops of the future include those with improved agricultural or food quality traits.

b. Salt-tolerant tomato plants have been engineered. The plant to the left does poorly when watered with a salty solution, but the engineered plant to the right is tolerant of the solution. The development of additional salt-tolerant crops (such as rice) is helping  to increase food production.

on such land. Salt-tolerant tomatoes have been developed to address this situation. First, scientists identified a gene coding for a channel protein that transports Na+ into a vacuole, preventing it from interfering with plant metabolism. Then the scientists used the gene to engineer tomato plants that overproduce the channel protein. The modified plants thrived despite being watered with a salty solution. Salt- and also drought- and cold-tolerant cereals, rice, and sugarcane might help provide enough food for a growing world population. Potato blight is the most serious potato disease in the world. In the mid-1840s, it was responsible for the Irish potato famine that caused the death of millions of people. By placing a gene from a naturally blight-resistant wild potato into a farmed variety, researchers have now made potato plants that are no longer vulnerable to a range of blight strains. Some progress has also been made in increasing the food quality of crops. Soybeans have been developed that mainly produce monounsaturated fatty acid, a change that may improve human health. These altered plants also produce acids that can be used as hardeners in paints and plastics. The necessary genes were derived from Vernonia and castor bean seeds and were transferred into the soybean DNA. Other types of genetically engineered plants are also expected to increase productivity. Stomata might be altered to take in more carbon dioxide or lose less water. A team of Japanese scientists is working on introducing the C4 photosynthetic cycle into rice (see the Ecology feature, “The New Rice,” in section 8.4). Unlike C3 plants, C4 plants do well in hot, dry weather. These modifications

would require a more complete reengineering of plant cells than the single-gene transfers that have been done so far. Some people have expressed health and environmental concerns regarding the growing of transgenic crops, as discussed in the Health feature, “Are Genetically Engineered Foods Safe?” 

Commercial Products Single-gene transfers have allowed plants to produce various products, including human hormones, clotting factors, and antibodies. One type of antibody made by corn can deliver radioisotopes to tumor cells, and another made by soybeans may be developed to treat genital herpes. The tobacco mosaic virus has been used as a vector to introduce a human gene into adult tobacco plants in the field. (Note that this technology bypasses the need for tissue culture completely.) Tens of grams of α-galactosidase, an enzyme needed for the treatment of a human lysosomal storage disease, were harvested per acre of tobacco plants. And it took only 30 days to get tobacco plants to produce antibodies to treat non-Hodgkin lymphoma after being sprayed with a genetically engineered virus.

Check Your Progress  10.3 1. List two ways in which plants can reproduce asexually. 2. Identify how plants are grown in tissue culture. 3. Discuss two examples of how and why plants are genetically engineered.



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UNIT 2  Plant Biology

SCIENCE IN YOUR LIFE  ►

HEALTH

Are Genetically Engineered Foods Safe? A series of focus groups conducted by the Food and Drug Administration (FDA) in 2000 showed that although most participants believed that genetically engineered foods, now also called genetically modified organisms (GMOs), might offer benefits, they also feared possible unknown long-term health consequences. The discovery by activists that a type of genetically engineered corn called StarLink had inadvertently made it into the food supply triggered the recall of taco shells, tortillas, and many other corn-based foodstuffs from supermarkets. Further, the makers of StarLink were forced to buy back StarLink from farmers and to compensate food producers at an estimated cost of several hundred ­million dollars in late 2000. StarLink is a type of “Bt” corn. It contains a foreign gene taken from a common soil organism, Bacillus thuringiensis, which makes a protein that is toxic to many insect pests. About a dozen Bt varieties, including corn, potato, and even tomato, have now been approved for human consumption. These strains contain a gene for an insecticidal protein called CryIA. Instead, StarLink contained a gene for a related protein called Cry9C, which researchers thought might slow down the chances of pest resistance to Bt corn. In order to get FDA approval for use in foods, the makers of StarLink performed the required tests. Like the other now-approved strains, StarLink wasn’t poisonous to rodents, and its biochemical structure is not similar to those of most chemicals in food that commonly cause allergic reactions in humans (called allergens). But the Cry9C protein resisted digestion longer than the other Bt proteins when it was put in simulated stomach acid and subjected to heat. Because most food allergens resist digestion in a similar fashion, StarLink was not approved for human consumption. The scientific community is now trying to devise more tests for allergens, because it has not been possible to determine conclusively whether Cry9C is or is not an allergen. Also, it is unclear how resistant to digestion a protein must be in order to be an allergen, and it is also unclear what degree of amino acid sequence similarity a potential allergen must have to a known allergen to raise concern. It is well recognized among health professionals that there is a need not only to understand the physiological thresholds for sensitization to food allergens, but also to identify the thresholds in the

immune system that elicit a reaction to food allergens. Other scientists are concerned about the following potential drawbacks to the planting of Bt corn: (1) resistance among populations of the target pest, (2) exchange of genetic material between the transgenic crop and related plant species, and (3) Bt crops’ impact on nontarget species. They feel that many more studies are needed before stating for certain that Bt corn has no ecological drawbacks. Over the past decade, the planting of genetically engineered corn has increased significantly. Many crops, such as corn, soybeans, and cotton, are now predominantly GMOs (Fig. 10B). However, there has been a considerable push for regulation by organizations that are concerned with food safety. By 2014, eight states had passed legislation that requires GMO foods to be labeled as such. In almost every state some form of legislation has been introduced for consideration.  But the controversy continues. Unfortunately for both agricultural producers and consumer groups, there is still not an adequate definition as to what defines a GMO; and an

identification as to what extent genetic modification is acceptable. Research is also underway to determine what types of GMOs are necessary for human society to feed an increasingly large human population and to counter the consequences of climate change. At the same time, researchers are attempting to determine if some forms of GMOs are dangerous to  human physiology or the stability of an ecosystem.

Questions to Consider 1. Do you think genetically modified organisms (GMOs) should be labeled? Construct an argument both for and against labeling GMOs. 2. In some developing countries, vitamin A deficiency is a major cause of blindness in small, malnourished children. Scientists have developed a form of rice, called golden rice, that is genetically modified to assist with the metabolism of vitamin A and might prevent millions of cases of blindness. The scientists who created golden rice claim it is safe for consumption, although critics say the health effects are not yet fully understood. Given its nutritional potential, should golden rice be planted and distributed on a wide scale? What information would you need to make your decision?

a.

b.

c.

Figure 10B  Examples of genetically modified crops.  Over 89% of corn (a), 94% of soybeans (b), and 91% of cotton (c), planted in 2014 were genetically modified.



Chapter 10  Plant Reproduction and Responses

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10.4  Control of Growth and Responses Learning Outcomes Upon completion of this section, you should be able to 1. Explain the importance of plant hormones. 2. Identify the various types of plant hormones and their function. 3. Recognize how plants respond to stimuli.

Plants respond to environmental stimuli such as light, gravity, and seasonal changes, usually by a change in their pattern of growth. Hormones are involved in these responses.

axillary buds

Plant Hormones Plant hormones are small organic molecules produced by the plant that serve as chemical signals between cells and tissues. The five commonly recognized groups of plant hormones are auxins, gibberellins, cytokinins, abscisic acid, and ethylene. Other chemicals, some of which differ only slightly from the natural hormones, also affect the growth of plants. These and the naturally occurring hormones are sometimes grouped together and called plant growth regulators. Plant hormones bring about a physiological response in target cells after binding to a specific receptor protein in the plasma membrane. As an example, Figure 10.14 shows how the hormone auxin brings about elongation of a plant cell, a necessary step toward differentiation and maturation.

Golgi apparatus

H + pump

cell wall materials

auxin receptor second messenger and DNA-binding protein plasma membrane

4 1 3

DNA

2

ATP H+

H+

apical bud removed lateral branches

a. Plant with apical bud intact

b. Plant with apical bud removed

Figure 10.15  Apical dominance.  a. Auxin in the apical bud inhibits axillary bud development, and the plant exhibits apical dominance. b. When the apical bud is removed, lateral branches develop.

Auxins The most common naturally occurring auxin is indoleacetic acid (IAA). It is produced in shoot apical meristem and is found in young leaves, flowers, and fruits. Therefore, you would expect auxin to affect many aspects of plant growth and development.

Effects of Auxin  Auxin is present in the apical meristem of a plant shoot, where it causes the shoot to grow from the top, a phenomenon called apical dominance. Only when the terminal bud is removed, deliberately or accidentally, are the axillary buds able to grow, allowing the plant to branch (Fig. 10.15). Pruning the top (apical meristem) of a plant generally achieves a fuller look because of increased branching of the main body of the plant. The application of a weak solution of auxin to a woody cutting causes adventitious roots to develop more quickly than they would otherwise. Auxin production by seeds also promotes the growth of fruit. As long as auxin is concentrated in leaves or fruits rather than in the stem, leaves and fruits do not fall off. Therefore, trees can be sprayed with auxin to keep mature fruit from falling to the ground.

How Auxins Work  When a plant is exposed to unidirectional light, auxin moves to the shady side, where it binds to receptors mRNA cell wall H+ and activates an ATP-driven pump that transports hydrogen ions (H+) out of the cell (see Fig. 10.14). The acid environment weakens cellulose fibrils, and activated enzymes further degrade the cell wall. Water now enters the cell, and the resulting increase in turgor pressure causes the cell to elongate and the stem to bend toward the light. This growth response to unidirectional light is called phototropism. Besides phototropism, auxins are also Figure 10.14  Auxin mode of action.  (1) The binding of auxin to a involved in gravitropism, in which roots curve downward and receptor stimulates an H+ pump, moving hydrogen ions into the cell wall. stems curve upward in response to gravity, as discussed later in this (2) The resulting acidity causes the cell wall to weaken, and water enters the section. cell. (3) At the same time, a second messenger stimulates production of Synthetic auxins are used today in a number of applications. growth factors, and (4) new cell wall materials are synthesized. The result of These auxins are sprayed on plants, such as tomatoes, to induce the all these actions is that the cell elongates, causing the stem to elongate. growth factors

H+

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UNIT 2  Plant Biology

development of fruit without pollination, creating seedless varieties. Synthetic auxins have been used as herbicides to control broadleaf weeds, such as dandelions and other plants. These substances have little effect on grasses. Agent Orange is a powerful synthetic auxin that was used in extremely high concentrations to defoliate the forests of Vietnam during the Vietnam War. This powerful auxin proved to be carcinogenic and harmed many of the local people.

Gibberellins Gibberellins were discovered in 1926 while a Japanese scientist was investigating a fungal disease of rice plants called “foolish seedling disease.” The plants elongated too quickly, causing the stem to weaken and the plant to collapse. The fungus infecting the plants produced an excess of a chemical called gibberellin, named after the fungus Gibberella fujikuroi. It was not until 1956 that gibberellic acid was able to be isolated from a flowering plant. Sources of gibberellin in flowering plant parts are young leaves, roots, embryos, seeds, and fruits. Gibberellins are growth-promoting hormones that bring about elongation of the resulting cells. We know of about 136 gibberellins that differ slightly in their chemical makeup. The most common of these is gibberellic acid, GA3 (the subscript designation distinguishes it from other gibberellins).

Effects of Gibberellins  When gibberellins are applied externally to plants, the most obvious effect is stem elongation (Fig. 10.16). Gibberellins can cause dwarf plants to grow, cabbage plants to become 2 m tall, and bush beans to become pole beans. During dormancy a plant does not grow, even though conditions may be favorable for growth. The dormancy of seeds and buds can be broken by applying gibberellins, and research with barley seeds has shown how GA3 influences the germination of seeds. Barley seeds have a large, starchy endosperm that must be broken down into sugars to provide energy for the embryo to grow. It is hypothesized that after GA3 attaches to a receptor in the plasma membrane, calcium ions (Ca2+) combine with a protein. This complex is believed to activate the gene that codes for amylase. Amylase then acts on starch to release the sugars needed for

the seed to germinate. Gibberellins have been used to promote barley seed germination for the beer brewing industry.

Cytokinins Cytokinins were discovered as a result of attempts to grow plant tissue and organs in culture vessels in the 1940s. It was found that cell division occurred when coconut milk (a liquid endosperm) and yeast extract were added to the culture medium. Although the effective agent or agents could not be isolated, they were collectively called cytokinins because cytokinesis refers to division of the cytoplasm. A naturally occurring cytokinin was not isolated until 1967. Because it came from the kernels of maize (Zea), it was called zeatin.

Effects of Cytokinins  The cytokinins promote cell division. These substances, which are derivatives of adenine, one of the purine bases in DNA and RNA, have been isolated from actively dividing tissues of roots and also from seeds and fruits. A synthetic cytokinin, called kinetin, also promotes cell division. Researchers have found that senescence (aging) of leaves can be prevented by applying cytokinins. When a plant organ, such as a leaf, loses its natural color, it is most likely undergoing senescence. During senescence, large molecules within the leaf are broken down and transported to other parts of the plant. Senescence does not always affect the entire plant at once. For example, as some plants grow taller, they naturally lose their lower leaves. Not only can cytokinins prevent the death of leaves, but they can also initiate leaf growth. Axillary buds begin to grow despite apical dominance when cytokinin is applied to them. Researchers are well aware that the ratio of auxin to cytokinin and the acidity of the culture medium determine whether a plant tissue forms an undifferentiated mass, called a callus, or differentiates to form roots, vegetative shoots, leaves, or floral shoots. Researchers have reported that chemicals called oligosaccharins (chemical fragments released from the cell wall) are effective in directing differentiation. They hypothesize that reception of auxin and cytokinins, which leads to the activation of enzymes, releases these fragments from the cell wall.

Abscisic Acid Abscisic acid (ABA) is produced by any “green tissue” with chloroplasts, but also by the monocot endosperm, and in roots. Abscisic acid was once thought to function in abscission, the dropping of leaves, fruits, and flowers from a plant. But even though the external application of abscisic acid promotes abscission, researchers no longer believe this hormone functions naturally in this process. Instead, they think the hormone ethylene, discussed next, brings about abscission.

a.

b.

Figure 10.16  Effect of gibberellins.  a. The plant on the right was treated with gibberellins; the plant on the left was not treated. b. The grapes are larger on the right, because gibberellins caused an increase in the space between the grapes, allowing them to grow larger. 

Effects of Abscisic Acid  Abscisic acid is sometimes called the stress hormone because it initiates and maintains seed and bud dormancy and brings about the closure of stomata when a plant is under water stress (Fig. 10.17). Dormancy has begun when a plant stops growing and prepares for adverse conditions (even though conditions at the time are favorable for growth). For example, researchers believe that abscisic acid moves from leaves to vegetative buds in the fall, and thereafter, these



Chapter 10  Plant Reproduction and Responses

inside

outside H2O

K+

185

K+

K+

Ca2+

ABA

Open stoma

Guard cell plasma membrane

Closed stoma a. No abscission

b. Abscission

Figure 10.17  Abscisic acid control of stoma closing.  K+ is

concentrated inside the guard cells, and the stoma is open (left). When ABA binds to its receptor in the guard cell plasma membrane, calcium ions (Ca2+) enter (middle). Then, potassium (K+) channels open and K+ exits the cell. Water follows and the stoma closes (right).

buds are converted to winter buds. A winter bud is covered by thick, hardened scales. A reduction in the level of abscisic acid and an increase in the level of gibberellins are believed to break seed and bud dormancy. Then seeds germinate, and buds send forth leaves. Ultimately abscisic acid causes potassium ions (K+) to leave guard cells, causing the guard cells to lose water, and the stomata to close.

Ethylene Ethylene is a gas that works with other hormones to bring about certain effects.

Effects of Ethylene  Ethylene is involved in abscission. Low levels of auxin and perhaps gibberellin in the leaf, compared to the stem, probably initiate abscission. But once the process of abscission has begun, ethylene stimulates certain enzymes, such as cellulase, which cause leaf, fruit, or flower drop (Fig. 10.18a, b). Cellulase hydrolyzes cellulose in plant cell walls. In the early 1900s, it was common practice to prepare citrus fruits for market by placing them in a room with a kerosene stove. Only later did researchers realize that ethylene, an incomplete combustion product of kerosene, was ripening the fruit (Fig. 10.18c). Because it is a gas, ethylene can act from a ­distance. A barrel of ripening apples can induce the ripening of a bunch of bananas, even if they are in different containers. ­Ethylene is released at the site of a plant wound due to physical damage or infection (which is why one rotten apple spoils the whole bushel).

Plant Responses to Environmental Stimuli Environmental signals determine the seasonality of growth, reproduction, and dormancy in plants. Plant responses are strongly influenced by such environmental stimuli as light, day length, gravity, and touch. The ability of a plant to respond to environmental signals fosters the survival of the plant and the species in a particular environment.

c. Ripening

Figure 10.18  Functions of ethylene.  a. Normally, there is no abscission when a holly twig is placed under a glass jar for a week. b. When an ethylene-producing ripe apple is also under the jar, abscission of the holly leaves occurs. c. Similarly, ethylene given off by this one ripe tomato will cause the others to ripen. Plant responses to environmental signals can be rapid, as when stomata open in the presence of light, or they can take some time, as when a plant flowers in season. Despite their variety, most plant responses to environmental signals are due to growth and sometimes differentiation, brought about at least in part by particular hormones.

Plant Tropisms Plant growth toward or away from a directional stimulus is called a tropism. Tropisms are due to differential growth—one side of an organ elongates faster than the other, and the result is a curving toward or away from the stimulus. The following three well-known tropisms were each named for the stimulus that causes the response: phototropism gravitropism thigmotropism

growth in response to a light stimulus growth in response to gravity growth in response to touch

Growth toward a stimulus is called a positive tropism, and growth away from a stimulus is called a negative tropism. For example, in positive phototropism, stems curve toward the light, and in



186

UNIT 2  Plant Biology

Figure 10.19  Tropisms.  a. In positive phototropism, the stem of a plant curves toward the light. This response is due to the accumulation of auxin on the shady side of the stem. b. In negative gravitropism, the stem of a plant curves away from the direction of gravity 24 hours after the plant was placed on its side. This response is due to the accumulation of auxin on the lower side of the stem. c. In thigmotropism, contact directs the growth of the plant, as is shown in this morning glory plant.

a.

according to the photoperiod, which is the ratio of the length of day to the length of night over a 24-hour period. Plants can be divided into the following three groups:

b.

1. Short-day plants flower when the day length is shorter than a critical length. (Examples are cocklebur, poinsettia, and chrysanthemum.) 2. Long-day plants flower when the day length is longer than a critical length. (Examples are wheat, barley, clover, and spinach.) 3. Day-neutral plants do not depend on day length for flowering. (Examples are tomato and cucumber.)

c.

negative gravitropism, stems curve away from the direction of gravity (Fig. 10.19). Roots, of course, exhibit positive gravitropism. The role of auxin in the positive phototropism of stems has been studied for quite some time. Because blue light, in particular, causes phototropism to occur, researchers believe that a yellow pigment related to the vitamin riboflavin acts as a photoreceptor for light. Following reception, auxin migrates from the bright side to the shady side of a stem. The cells on that side elongate faster than those on the bright side, causing the stem to curve toward the light. Negative gravitropism of stems occurs when auxin moves to the lower part of a stem when a plant is placed on its side. Figure 10.14 explains how auxin brings about elongation.

Flowering The flowering of angiosperms is a striking response to environmental seasonal changes. In some plants, flowering occurs Cocklebur

Experiments have shown that the length of continuous darkness, not light, controls flowering. For example, the cocklebur does not flower if a suitable length of darkness is interrupted by a flash of light. In contrast, clover does flower when an unsuitable length of darkness is interrupted by a flash of light (Fig. 10.20). Interrupting the light period with darkness has no effect on flowering.

Phytochrome and Plant Flowering If flowering is dependent on day and night length, plants must have some way to detect these periods. This appears to be the role of phytochrome, a blue-green leaf pigment that alternately exists in two forms—Pr and Pfr (Fig. 10.21a). Direct sunlight contains more red light than far-red light; therefore, Pfr (far red) is apt to be present in plant leaves during the day. In the shade and at sunset, there is more far-red light than red light. Therefore, Pfr is converted to Pr (red) as night approaches. Clover

night flash of light critical length

24 hours

day

a. Short-day (long-night) plant

b. Long-day (short-night) plant

Figure 10.20  Photoperiodism and flowering.  a. Short-day plant.

When the day is shorter than a critical length, this type of plant flowers. The plant does not flower when the day is longer than the critical length. It also does not flower if the longer-than-critical-length night is interrupted by a flash of light. b. Long-day plant. The plant flowers when the day is longer than a critical length. When the day is shorter than a critical length, this type of plant does not flower. However, it does flower if the slightly longer-than-criticallength night is interrupted by a flash of light.



Chapter 10  Plant Reproduction and Responses

lightsensitive region

187

There is also a slow metabolic replacement of Pfr by Pr during the night. It is possible that phytochrome conversion is the first step in a signaling pathway that results in flowering. A flowering hormone has never been discovered.

red light

Other Functions of Phytochrome far-red light

kinase

inactive Pr

a.

active Pfr

The Pr → Pfr conversion cycle has other functions. The presence of Pfr indicates to seeds of some plants that sunlight is present and conditions are favorable for germination. Such seeds must be only partly covered with soil when planted. Following germination, the presence of Pr indicates that stem elongation may be needed to reach sunlight. Seedlings that are grown in the dark etiolate—that is, the stem increases in length, and the leaves remain small (Fig. 10.21b). Once the seedling is exposed to sunlight and Pr is converted to Pfr, the seedling begins to grow normally—the leaves expand, and the stem branches.

Check Your Progress  10.4 1. Identify the functions of auxins, gibberellins, cytokinins, abscisic acid, and ethylene.

2. Recognize the different types of plant growth tropism. 3. Compare the differences between short-day and long-day plants.

4. Identify the potential role of phytochrome in plant flowering.

Conclusion

b.

Normal growth

Etiolation

Figure 10.21  Phytochrome control of growth pattern.  a. The inactive form of phytochrome (Pr) is converted to the active form (Pfr) in the presence of red light. b. In sunlight, phytochrome is active and normal growth occurs. In the shade, etiolation occurs.

Genetic manipulation of watermelon is capable of producing a 3n seedless watermelon. It is also possible to genetically modify plants by using tissue culture techniques and various ways of giving them foreign genes. Some of these plants become GM (genetically modified) foods. Although it is economically beneficial to produce GM foods, some are concerned about their health effects. Scientists are interested in learning more about how plants grow, seeds develop, and plants respond to stimuli in the natural environment.

MEDIA STUDY TOOLS http://connect.mheducation.com Maximize your study time with McGraw-Hill SmartBook®, the first adaptive textbook.

 

Animations

10.4  Phytochrome Signaling

  Tutorials 10.1  Alternation of Generations • Angiosperm Life Cycle



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UNIT 2  Plant Biology

SUMMARIZE 10.1  Sexual Reproduction in Flowering Plants ■ Flowering plants have an alternation of generations life cycle. ■ Flowers borne by the sporophyte produce microspores and mega-

■

■

■

■

■

spores by meiosis. Microspores develop into a male gametophyte, and megaspores develop into the female gametophyte. Gametophytes produce gametes by mitosis. Flowers contain sepals that form an outer whorl; petals, which are the next whorl; and stamens that form a whorl around the base of at least one carpel. The carpel, in the flower’s center, consists of a stigma, style, and ovary, which contains ovules. Anthers contain microspore mother cells, each of which divides meiotically to produce four haploid microspores. Each microspore divides mitotically to produce a two-celled pollen grain—a tube cell and a generative cell. The generative cell later divides mitotically to produce two sperm cells. The pollen grain is the male gametophyte. After pollination, the pollen grain germinates, and as the pollen tube grows, sperm cells travel to the embryo sac. Plants have undergone coevolution with many of their pollinators to ensure fertilization occurs. Each ovule contains a megaspore mother cell, which divides meiotically to produce four haploid megaspores, only one of which survives. This megaspore divides mitotically to produce the female gametophyte (embryo sac), which usually has seven cells. Flowering plants undergo double fertilization. One sperm nucleus unites with the egg nucleus, forming a 2n zygote, and the other unites with the polar nuclei of the central cell, forming a 3n endosperm cell. The zygote becomes the sporophyte embryo, and the endosperm cell divides to become endosperm tissue. Collectively these structures are known as the seed, which is often found within a fruit.

10.2  Growth and Development ■ Prior to seed formation, the zygote undergoes growth and development

to become an embryo. ■ The seeds enclosed by a fruit contain the embryo (hypocotyl, epicotyl, plumule, radicle) and stored food (endosperm and/or cotyledons). ■ Following dispersal, a seed germinates. Dormancy is the period in which no growth occurs.

10.3  Asexual Reproduction and Genetic Engineering in Plants

■ ■

■

■

∙ Gibberellins promote stem elongation and break seed dormancy so that germination occurs. ∙ Cytokinins promote cell division, prevent senescence of leaves, and along with auxin, influence differentiation of plant tissues. ∙ Abscisic acid (ABA) initiates and maintains seed and bud dormancy and closes stomata under water stress. ∙ Ethylene causes abscission of leaves, fruits, and flowers. It also causes some fruits to ripen. Environmental signals play a significant role in plant growth and development. Tropisms are growth responses toward or away from unidirectional stimuli. When a plant is exposed to light, auxin moves laterally from the bright to the shady side of a stem. Then, cells on the shady side elongate, and the stem bends toward the light. Similarly, auxin is responsible for negative gravitropism and growth upward, opposite gravity. Flowering is a striking response to seasonal changes. Phytochrome, a plant pigment that responds to daylight, is believed to be part of a biological clock system that in some unknown way brings about flowering. Phytochrome has various other functions, such as seed germination, leaf expansion, and stem branching. The photoperiods of plants can be divided into three groups: ∙ Short-day plants flower when days are shorter (nights are longer) than a critical length, and  ∙ long-day plants flower when days are longer than a critical length.  ∙ Some plants are day-neutral.

ASSESS Testing Yourself Choose the best answer for each question.

10.1  Sexual Reproduction in Flowering Plants 1. Stigma is to carpel as anther is to a. sepal.  b. stamen.  c. ovary.  d. style. 2. Label the diagram below of alternation of generations in flowering plants.

■ Many flowering plants reproduce asexually (e.g., buds give rise to

S TO

MI sporophyte

j. a. (2n)

zygote

MEIOSIS

e. (n) microspore egg

f.

i.

h.

IS

g.

S TO

MI

responses to environmental stimuli. ■ There are five commonly recognized groups of plant hormones: ∙ Auxins cause apical dominance, the growth of adventitious roots, and two types of tropisms—phototropism and gravitropism.

c. d.

FERTILIZATION

10.4  Control of Growth and Responses ■ Plant hormones are chemical signals involved in plant growth and

b.

IS

entire plants, or roots produce new shoots). Traditionally, hybridization produced new varieties of plants. ■ Asexual reproduction allows for clonal propagation in tissue culture. Plant tissue culture allows production of new plants because most plant cells are totipotent. ■ Transgenic organisms  (including many genetically modified organisms) are produced by genetic engineering procedures that insert genes from other species into the host organism.



Chapter 10  Plant Reproduction and Responses

3. Label the following diagram of a flower.

h.

10.4  Control of Growth and Responses a. b. c. d.

j. i.

e.

g.

189

f.

10.2  Growth and Development 4. Fruits a. nourish embryo development. b. help with seed dispersal. c. signal gametophyte maturity. d. attract pollinators. e. stay where they are produced.  5. The seed leaves of a plant are called the a. meristem. c. cotyledons. b. endosperm. d. zygote. 6. A seed contains a. a seed coat.  d. stored food. b. cotyledon(s). e. All of these are correct. c. an embryo. 

10.3  Asexual Reproduction and Genetic Engineering in Plants 7. A plant that contains genes from another plant species is called: a. totipotent c. an angiosperm b. an eudicot d. transgenic 8. Which of the following techniques allows researchers to grow large numbers of identical plants because the majority of cells in a plant are totipotent? a. genetic engineering c. tissue culture b. hybridization d. None of these are correct.

For questions 9–14, match each statement with a hormone in the key. Answers can be used more than once.  KEY: a. auxin d. ethylene b. gibberellin e. abscisic acid c. cytokinin  9. It is present in a gaseous form.  10. Grapes can grow larger and exhibit stem elongation.  11. Stomata close when a plant is water-stressed.  12. Stems bend toward the sun.  13. It inhibits plant senescence.  14. It causes leaf abscission. 

ENGAGE BioNOW Want to know  how this science is relevant to your life? Check out the BioNow videos below: ■ Nut Fungus ■ Cell Division

Thinking Critically 1. Explain the similarities and differences on how mitosis and meiosis are used in the plant and human (animal) life cycle. 2. How might a plant benefit from having just one type of pollinator? 3. You hypothesize that abscisic acid (ABA) is responsible for the turgor pressure changes that permit a plant to track the sun (such as a patch of sunflowers on a sunny day). What observations could you make to support your hypothesis?  4. In most parts of the world, commercial potato crops are produced asexually by planting tubers. However, in some regions, such as Southeast Asia and the Andes, some potatoes are grown from true seeds. Discuss the advantages and disadvantages of growing potatoes from true seeds.

PHOTO CREDITS Opener: © Mitch Hrdlicka/Getty RF; 10.3a: © Gilles Delacroix/Garden World Images/Age fotostock; 10.3b: © Pat Pendarvis; 10.4a: © Adam Hart-Davis/SPL/Science Source; 10.4b: © Design Pics Inc./Alamy RF; 10.5(pollen grain): © Graham Kent; 10.5(embryo sac): © Ed Reschke; 10.6a: © Tim Gainey/Alamy; 10.6b: © Medical-on-line/Alamy; 10.6c: © Steve Gschmeissner/Science Photo Library/Getty RF; 10Aa: © Steven P. Lynch; 10Ab: © Creatas/PunchStock RF; 10Ac: © Rolf Nussbaumer Photographer/Alamy RF; 10Ad: © Dr. Merlin D. Tuttle/Bat Conservation International; 10.8a: © James Mauseth; 10.8b: © MIXA/Getty RF; 10.8c: © David Marsden/Getty Images; 10.8d: © Ingram Publishing/ Alamy RF; 10.9: © Ed Reschke; 10.10: © Ed Reschke/Getty Images; 10.11(all): © Prof. Dr. Hans-Ulrich Koop, from Plant Cell Reports, 17: 601–604; 10.12a: © Bloomberg/Getty Images; 10.12b: © Sven Kaestner/AP; 10.13b: © Eduardo Blumwald; 10Ba: © USDA/Doug Wilson, photographer; 10Bb: © Norm Thomas/Science Source; 10Bc: © Pixtal/Age fotostock RF; 10.15(both): © Prof. Malcolm B. Wilkins; 10.16a: © Science Source; 10.16b: © Amnon Lichter, The Volcani Center; 10.18(both): © Kingsley Stern; 10.18c: © Kent Knudson/ PhotoLink/Getty RF; 10.19a: © Dorling Kindersley/Getty Images; 10.19b: © Kingsley Stern; 10.19c: © Alison Thompson/Alamy; 10.21b(both): © Nigel Cattlin/Alamy.



UNIT 3  Maintenance of the Human Body

11

Human Organization CHAPTER OUTLINE 11.1  Types of Tissues 11.2 Body Cavities and Body Membranes 11.3 Organ Systems 11.4 Integumentary System 11.5 Homeostasis BEFORE YOU BEGIN Before beginning this chapter, take a few moments to review the following discussions: Section 1.1  Why is homeostasis a characteristic of life? Figure 1.2  What levels of biological organization are found in humans? Section 3.3  What major organelles make up animal cells?

CASE STUDY The Biology of Performing A performance of Taylor Swift is a complex production, and not only from the perspective of the people with her on the stage. Whether she is singing, or dancing complicated routines with her fellow performers, she is demonstrating how the human body is able to perform vastly complicated feats. Her nervous system must coordinate a variety of tasks, including: her breathing rate, remembering the words and tempo to each song, and the patterns of her motions onstage. Sensory input from her ears and eyes must monitor all the sounds and sights around her, while also maintaining her balance and contracting and relaxing her muscles in order to produce the desired choreography. Of course, in biological terms, very similar observations could be made about humans engaging in almost any other type of physical activity, like playing sports, building a house, performing surgery, creating a sculpture, or driving a car. Even while you are just sitting quietly, perhaps reading a textbook, your body systems are undergoing a flurry of activity. Your muscles and bones work together to keep your body upright. Your respiratory and cardiovascular systems are providing oxygen to your tissues, and wastes are being transported to your kidneys for elimination. Your digestive system is providing nutrients for various tissues to use, and your immune and lymphatic systems are keeping infections at bay. Your nervous and endocrine systems are monitoring and controlling all of these functions. And of course, all these activities must continue even while your body is performing other highly skilled activities. As you read through the chapter, think about the following questions:

1. When a musician like Taylor Swift is performing on stage, which of her body systems are most actively engaged? Which systems are less important?

2. What types of tissues make up each of the human body’s major organ systems?

3. How do the different body systems interact with, and depend on, each other?

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11.1  Types of Tissues Learning Outcomes Upon completion of this section, you should be able to 1. List the four major types of tissues found in the human body, and describe the structural features of each. 2. Identify the common locations of each tissue type in the human body. 3. Explain how the cells that make up various types of tissues are specialized to perform their functions.

Recall from the biological levels of organization (see section 1.1) that cells are composed of molecules; a tissue has similar types of cells; an organ contains several types of tissues; and several organs make up an organ system. In this chapter, we will take a closer look at the tissue, organ, and organ system levels of organization. Tissues are composed of similarly specialized cells that perform a common function in the body. The tissues of the human body can be categorized into the following four major types: Epithelial tissue  covers body surfaces and lines body cavities. Connective tissue  binds and supports body parts. Muscular tissue  moves the body and its parts. Nervous tissue  receives stimuli, processes information, and conducts nerve impulses. We will now examine the structure and function of each of these four types of tissues.

Epithelial Tissue Epithelial tissue, also called epithelium, consists of tightly packed cells that form a continuous layer. Epithelial tissue covers surfaces and lines body cavities. Epithelial tissue has numerous functions in  the body. Usually, it has a protective function, but it can also be  modified to carry out secretion, absorption, excretion, and filtration. On the external surface, epithelial tissue protects the body from injury, drying out, and possible invasion by microbes such as bacteria and viruses. On internal surfaces, modifications help epithelial tissue carry out both its protective and specific functions. Epithelial tissue secretes mucus along the digestive tract and sweeps up impurities from the lungs by means of cilia (sing., cilium). It efficiently absorbs molecules from kidney tubules and from the intestine because of minute cellular extensions called microvilli (sing., microvillus). A basement membrane usually joins an epithelium to underlying connective tissue. We now know that the basement membrane consists of glycoprotein secreted by epithelial cells and collagen fibers that belong to the connective tissue. Epithelial tissue is classified according to the shape of cell it is composed of (squamous, cuboidal, or columnar) and the number of layers in the tissue (Fig. 11.1). Squamous epithelium is characterized by flattened cells and is found lining the lungs and blood vessels. Cuboidal epithelium contains cube-shaped cells and lines the kidney tubules. Columnar epithelium has cells resembling rectangular pillars or columns, with nuclei usually located

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near the bottom of each cell. This epithelium lines the digestive tract. Ciliated columnar epithelium lines the uterine tubes, where it propels the egg toward the uterus, or womb. Classification is also based on the number of layers in the tissue. Simple epithelium has a single layer of cells, whereas stratified epithelium has layers of cells piled one on top of another. The walls of the smallest blood vessels, called capillaries, are composed of simple squamous epithelium. The permeability of this single layer of cells allows exchange of substances between the blood and tissue cells. Simple cuboidal epithelium lines kidney tubules and the cavities of many internal organs. Stratified squamous epithelium lines the nose, mouth, esophagus, anal canal, and vagina. The outer layer of skin is also stratified squamous epithelium, but the cells have been reinforced by keratin, a protein that provides strength. Pseudostratified epithelium appears to be layered, but true layers do not exist because each cell touches the basement membrane. The lining of the windpipe, or trachea, is pseudostratified ciliated columnar epithelium. A secreted covering of mucus traps foreign particles, and the upward motion of the cilia carries the mucus to the back of the throat, where it either may be swallowed or expelled. Smoking can cause a change in mucous secretion and inhibit ciliary action, and the result is a chronic inflammatory condition called bronchitis. When an epithelium secretes a product, it is said to be glandular. A gland can be a single epithelial cell, as are the mucus-­ secreting goblet cells within the columnar epithelium lining the digestive tract, or a gland can contain many cells. Glands that secrete their product into ducts are called exocrine glands, and those that secrete their product into the bloodstream are called endocrine glands. The pancreas is both an exocrine gland, because it secretes digestive juices into the small intestine via ducts, and an endocrine gland, because it secretes insulin into the bloodstream.

Junctions Between Epithelial Cells The cells of a tissue can function in a coordinated manner when the plasma membranes of adjoining cells interact. Three common types of junctions link epithelial cells (see section 4.3). These are the tight junctions, which form an impermeable barrier between the cells; gap junctions, which serve to strengthen connections while allowing small molecules to pass; and adhesion junctions, which act like rivets or “spot welds” to anchor tissues in place, increasing their overall strength.

Connective Tissue Connective tissue binds organs together, provides support and protection, fills spaces, produces blood cells, and stores fat. As a rule, connective tissue cells are widely separated by a matrix, consisting of a noncellular material that varies in consistency from solid to jellylike to fluid. A nonfluid matrix may have fibers of three possible types. White collagen fibers contain collagen, a protein that gives them flexibility and strength. Reticular fibers are very thin collagen fibers that are highly branched and form delicate supporting networks. Yellow elastic fibers contain elastin, a protein that is not as strong as collagen but more elastic.



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Figure 11.1  Epithelial tissue.  Certain

types of epithelial tissue—squamous, cuboidal, and columnar—are named for the shapes of their cells. They all have a protective function in addition to other specific functions.

Pseudostratified, ciliated columnar • lining of trachea • sweeps impurities toward throat

Simple squamous • lining of lungs, blood vessels • protects 2503

Stratified squamous • skin (epidermis) • lining of nose, mouth, esophagus, anal canal, vagina • protects

cilia goblet cell secretes mucus basement membrane 1003

2503

basement membrane basement membrane

Simple cuboidal • lining of kidney tubules, various glands • absorbs molecules

Simple columnar • lining of small intestine, uterine tubes • absorbs nutrients

2503

2503 goblet cell secretes mucus

basement membrane

basement membrane



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which connect muscles to bones, and in ligaments, which connect bones to other bones at joints. Both loose fibrous and dense fibrous connective tissues have cells called fibroblasts located some distance from one another and separated by a jellylike matrix containing white collagen fibers and yellow elastic fibers.

Loose Fibrous and Dense Fibrous Tissues Loose fibrous connective tissue supports epithelium and also many internal organs (Fig. 11.2a). Its presence in lungs, arteries, and the urinary bladder allows these organs to expand. It forms a protective covering enclosing many internal organs, such as muscles, blood vessels, and nerves. Dense fibrous connective tissue contains many collagen fibers that are packed together (Fig. 11.2b). This type of tissue has more specific functions than does loose connective tissue. For example, dense fibrous connective tissue is found in tendons,

Loose fibrous connective tissue • has space between components. • occurs beneath skin and most epithelial layers. • functions in support and binds organs. fibroblast

elastic fiber

collagen fiber

In adipose tissue (Fig. 11.2c), the fibroblasts enlarge and store fat. These cells are commonly called adipocytes. The body uses this

250×

collagen fibers

nucleus

nuclei of fibroblasts

400×

c.

b.

chondrocyte within lacunae

Dense fibrous connective tissue • has collagenous fibers closely packed. • in dermis of skin, tendons, ligaments. • functions in support.

Adipose tissue • cells are filled with fat. • occurs beneath skin, around heart and other organs. • functions in insulation, stores fat.

Hyaline cartilage • has cells in lacunae. • occurs in nose; in the walls of respiratory passages; at ends of bones, including ribs. • functions in support and protection.

d.

Adipose Tissue and Reticular Connective Tissue

250×

a.

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Compact bone • has cells in concentric rings. • occurs in bones of skeleton. • functions in support and protection. central canal

250×

osteon

osteocyte canaliculi within a lacuna

matrix e.

Figure 11.2  Connective tissue

320×

examples.  a. In loose fibrous connective tissue, cells called fibroblasts are separated by a jellylike matrix, which contains both collagen and elastic fibers. b. Dense fibrous connective tissue contains tightly packed collagen fibers for added strength. c. Adipose tissue cells (adipocytes) have nuclei pushed to one side because the cells are filled with fat. d. In hyaline cartilage, the flexible matrix has a white, translucent appearance. e. In compact bone, the hard matrix contains calcium salts. Concentric rings of cells in lacunae form an elongated cylinder called an osteon. An osteon has a central canal that contains blood vessels and nerve fibers.



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stored fat for energy, insulation, and organ protection. Adipose tissue is found beneath the skin, around the kidneys, and on the surface of the heart. Reticular connective tissue forms the supporting meshwork of lymphoid tissue in lymph nodes, the spleen, the thymus, and the bone marrow. All types of blood cells are produced in red bone marrow, but a certain type of lymphocyte (T lymphocyte) completes its development in the thymus. The lymph nodes are sites of lymphocyte responses (see Chapter 13).

Cartilage Cartilage is a specialized form of dense fibrous connective tissue, which most commonly forms the smooth surfaces that allow bones to slide against each other in joints. Cartilage is found in many other locations, however. The cells in cartilage lie in small chambers called lacunae (sing., lacuna), separated by a matrix that is solid yet flexible. Unfortunately, because this tissue lacks a direct blood supply, it heals very slowly. There are three types of cartilage, distinguished by the main type of fiber in the matrix. Hyaline cartilage (Fig. 11.2d), the most common type of cartilage, contains only very fine collagen fibers. The matrix has a white, translucent appearance. Hyaline cartilage is found in the nose and at the ends of the long bones and the ribs, and it forms rings in the walls of respiratory passages. The fetal skeleton is also made of this type of cartilage. Later, the cartilaginous fetal skeleton is replaced by bone. Elastic cartilage has more elastic fibers than hyaline cartilage. For this reason, it is more flexible and is found, for example, in the framework of the outer ear. Fibrocartilage has a matrix containing strong collagen fibers. Fibrocartilage is found in structures that withstand tension and pressure, such as the pads between the vertebrae in the backbone and the wedges in the knee joint.

Blood Blood is unlike other types of connective tissue in that the matrix (i.e., plasma) is not made by the cells. The internal environment of  the body consists of blood and the fluid between the body’s cells. The systems of the body help keep the composition and chemistry of blood within normal limits, and blood, in turn, creates interstitial fluid, which is the fluid between the cells. Blood transports nutrients and oxygen to the interstitial fluid and removes carbon dioxide and other wastes. It helps distribute heat and also plays a role in fluid, ion, and pH balance. Various components of blood help protect us from disease, and blood’s ability to clot prevents fluid loss. If blood is transferred from a person’s vein to a test tube and prevented from clotting, it separates into two layers (Fig. 11.3a). The upper, liquid layer, called plasma, represents about 55% of the volume of whole blood and contains a variety of inorganic and organic substances dissolved or suspended in water (Table 11.1). The lower layer consists of red blood cells (erythrocytes), white blood cells (leukocytes), and blood platelets (thrombocytes) (Fig.  11.3b). Collectively, these are called the formed elements, and they represent about 45% of the volume of whole blood. Formed elements are manufactured in the red bone marrow of the skull, ribs, vertebrae, and ends of the long bones. The red blood cells are small, biconcave, disk-shaped cells without nuclei. The presence of the red pigment hemoglobin makes the cells red and, in turn, makes the blood red. Hemoglobin is composed of four units. Each contains the protein globin and a complex,

Figure 11.3  Blood, a liquid connective tissue.  a. Blood

Bone Bone is the most rigid connective tissue. It consists of an extremely hard matrix of inorganic salts, notably calcium salts, deposited around protein fibers, especially collagen fibers. The inorganic salts give bone rigidity, and the protein fibers provide elasticity and strength, much as steel rods do in reinforced concrete. Compact bone (Fig. 11.2e) makes up the shaft of a long bone. It consists of cylindrical structural units called osteons. The central canal of each osteon is surrounded by rings of hard matrix. Bone cells are located in spaces called lacunae between the rings of matrix. In the central canal, nerve fibers carry nerve impulses and blood vessels carry nutrients that allow bone to renew itself. Nutrients can reach all of the bone cells because they are connected by thin processes within canaliculi (minute canals) that also reach to the central canal. The shaft of long bones such as the femur (thigh bone) has a hollow chamber filled with bone marrow, a site where blood cells develop. The ends of a long bone contain spongy bone, which contains numerous bony bars and plates, separated by irregular spaces. Although lighter than compact bone, spongy bone is still designed for strength. Just as braces are used for support in buildings, the solid portions of spongy bone follow lines of stress. Section 19.1 provides more information about bone and cartilage.

plasma Formed elements: white blood cells and platelets red blood cells

white blood cells

a. Blood sample

platelets red blood cell

plasma

b. Blood smear

is often classified as connective tissue because the cells are separated by a matrix—plasma. The formed elements consist of several types of cells. b. A drawing of the components of blood: plasma, red blood cells, white blood cells, and platelets (which are actually fragments of a larger cell).



TABLE 11.1  Components of Blood Plasma Water (90–92% of total) Solutes (8–10% of total) Inorganic ions (electrolytes)

Na+, Ca2+, K+, Mg2+, Cl–, HCO3–, HPO42–, SO42–

Gases

O2, CO2

Plasma proteins

Albumin, globulins, fibrinogen, transport proteins

Organic nutrients

Glucose, lipids, phospholipids, amino acids, etc.

Nitrogen-containing waste products Urea, ammonia, uric acid Regulatory substances

Hormones, enzymes

iron-containing structure called heme. The iron forms a loose association with oxygen, and in this way, red blood cells transport oxygen. White blood cells differ from red blood cells in that they are usually larger, have a nucleus, and without staining would appear translucent. When smeared onto microscope slides and stained, the nuclei of white blood cells typically look bluish (Fig. 11.3b). White blood cells fight infection in a number of different ways. Some white blood cells are phagocytic and engulf infectious pathogens, while others are responsible for the adaptive immunity that develops after an individual is exposed to various pathogens or toxins, either through natural infection or by vaccination. One important feature of acquired immunity is the production of antibodies, molecules that combine with foreign substances to inactivate them.

Skeletal muscle • has striated cells with multiple nuclei. • occurs in muscles attached to skeleton. • functions in voluntary movement of body.

striation

nucleus

Platelets are not complete cells. Rather, they are fragments of large cells present only in bone marrow. When a blood vessel is damaged, platelets help to form a plug that seals the vessel, and injured tissues release molecules that stimulate the clotting process. For more details on red blood cells, white blood cells, and platelets, see section 12.2 and Chapter 13.

Muscular Tissue Muscular tissue is composed of cells called muscle fibers. Muscle fibers contain actin filaments and myosin filaments, whose interaction accounts for movement. The three types of muscle tissue are skeletal, smooth, and cardiac. Skeletal muscle (Fig. 11.4a) is usually attached by tendons to the bones of the skeleton, and when it contracts, body parts move. Contraction of skeletal muscle is under voluntary control. Skeletal muscle fibers are cylindrical and quite long—sometimes they run the length of the muscle. They arise from fusion of several cells, resulting in one fiber with multiple nuclei. The fibers have alternating light and dark bands that give them a striated appearance. Smooth muscle is so named because the cells lack striations. The spindle-shaped fibers, each with a single nucleus, form layers in which the thick middle portion of one cell is opposite the thin ends of adjacent cells. Consequently, the nuclei form an irregular pattern in the tissue (Fig. 11.4b). Smooth muscle is not under voluntary control, and therefore is said to be involuntary. Smooth muscle is found in the walls of viscera (intestine, stomach, and other internal organs) and blood vessels.

Cardiac muscle • has branching, striated cells, each with a single nucleus. • occurs in the wall of the heart. • functions in the pumping of blood. • is involuntary.

Smooth muscle • has spindle-shaped cells, each with a single nucleus. • cells have no striations. • occurs in blood vessel walls and walls of the digestive tract. • functions in movement of substances in lumens of body. • is involuntary.

250× smooth muscle cell

a.

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nucleus

b.

400×

intercalated disk

nucleus

250×

c.

Figure 11.4  Muscular tissue.  a. Skeletal muscle is voluntary and striated. b. Smooth muscle is involuntary and nonstriated. c. Cardiac muscle is involuntary and striated. Cardiac muscle cells branch and fit together at intercalated disks.



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Cardiac muscle (Fig. 11.4c) is found only in the walls of the heart. Cardiac muscle combines features of both smooth muscle and skeletal muscle. Like skeletal muscle, it has striations, but the contraction of the heart is involuntary. Cardiac muscle cells also differ from skeletal muscle cells in that they usually have a single, centrally placed nucleus. The cells are branched and seemingly fused one with the other, and the heart appears to be composed of one large interconnecting mass of muscle cells. Actually, cardiac muscle cells are separate and individual, but they are bound end to end at intercalated disks, areas where folded plasma membranes between two cells contain adhesion junctions and gap junctions. These areas promote the flow of electrical current when the heart muscle contracts by allowing ions to flow freely between cells. See  section 19.4 to learn more about the mechanism of muscle contraction.

dendrite

Neuron

nucleus

cell body

Nervous Tissue Nervous tissue, which contains nerve cells called neurons (Fig. 11.5a), is present in the brain and spinal cord. A neuron is a specialized cell that has three parts: a cell body, dendrites, and an axon (Fig. 11.5a, b). The cell body contains the major concentration of the cytoplasm and the nucleus of the neuron. A dendrite is a process that conducts signals toward the cell body. An axon is a process that typically conducts nerve impulses away from the cell body. Long axons are covered by myelin, a white fatty substance, which increases the speed of nerve impulses. The term fiber1 is used here to refer to an axon along with its myelin sheath, if it has one. Outside the brain and spinal cord, fibers bound by connective tissue form nerves. The nervous system has just three functions: sensory input, integration of data, and motor output. Nerves conduct impulses from sensory receptors to the spinal cord and the brain where integration occurs. The phenomenon called sensation occurs only in the brain, however. Nerves also conduct nerve impulses away from the spinal cord and brain to the muscles and glands, causing them to contract and secrete, respectively. In this way, a coordinated response to the stimulus is achieved.

Neuroglia

Microglia Astrocyte

Oligodendrocyte

myelin sheath axon

Capillary a. Neuron and neuroglia

Figure 11.5  Neurons

In addition to neurons, nervous tissue contains neuroglia. Neuroglia are cells that outnumber neurons nine to one and take up more than half the volume of the brain (Fig. 11.5a). Although the primary function of neuroglia is to support and nourish neurons, research is currently being conducted to determine how much they directly contribute to brain function. The four types of neuroglia in the brain are microglia, astrocytes, oligodendrocytes, and ependymal cells. Microglia, in addition to supporting neurons, engulf bacterial and cellular debris. Astrocytes provide nutrients to neurons and produce a hormone known as glia-derived growth factor, which has potential as a treatment for Parkinson disease and other diseases caused by neuron degeneration. Oligodendrocytes form myelin sheaths. Outside the brain, Schwann cells are the type of neuroglia that encircle long nerve fibers and form a myelin sheath.

and neuroglia. 

b. Micrograph of a neuron In connective tissue, a fiber is a component of the matrix; in muscle tissue, a fiber is a muscle cell; in nervous tissue, a fiber is an axon and its myelin sheath. 1

200×

a. Neurons conduct nerve impulses. Neuroglia consist of cells that support and nourish neurons and have various functions. Astrocytes lie between neurons and a capillary. Therefore, nutrients entering neurons from the blood must first pass through astrocytes. Oligodendrocytes form the myelin sheaths around fibers in the brain and spinal cord. b. A neuron cell body as seen with a light microscope.



Chapter 11  Human Organization

Ependymal cells line the fluid-filled spaces of the brain and spinal cord. Neuroglia don’t have long processes, but even so, researchers are now beginning to gather evidence that they do communicate among themselves and with neurons. To learn more about the nervous system, see Chapter 17.

Cranial cavity Dorsal cavity

Check Your Progress  11.1

Vertebral canal

1. List the five types of epithelium, and identify a location where each could be found in the human body.

Thoracic cavity

2. Compare and contrast the three types of connective tissue. 3. Describe the structure and function of skeletal, smooth,

diaphragm

and cardiac muscle. 4. Distinguish between a neuron and the neuroglia.

11.2  Body Cavities and Body Membranes

Ventral cavity

spinal cord Abdominal cavity

Learning Outcomes Upon completion of this section, you should be able to 1. List the two main cavities of the human body, and the major organs found in each. 2. Compare and contrast the location(s) and function(s) of the different types of body membranes.

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vertebrae

Pelvic cavity

a.

When referring to anatomical parts of humans, certain standard pleura terms are used. Many of the same terms apply to other organisms, although there is some variation because the human terms always pericardium refer to a body that is in the upright, standing position, as in Figure 11.6. The term ventral, which means the same thing as anterior in humans, refers to the front, and dorsal or posterior both mean peritoneum toward the back. Superior means toward the head, and inferior means toward the feet. Other anatomical terms are relative to other b. body parts; something that is medial is closer to the midline of the body, whereas lateral means away from the midline. Similarly, Figure 11.6  Mammalian body cavities.  a. Side view. The dorsal when referring to an appendage like an arm or a leg, proximal (toward the back) cavity contains the cranial cavity and the vertebral canal. means closer to the trunk of the body, while distal means away The brain is in the cranial cavity, and the spinal cord is in the vertebral canal. from the trunk. In the ventral (toward the front) cavity, the diaphragm separates the thoracic cavity and the abdominal cavity. The heart and lungs are in the thoracic Keeping these definitions in mind, the human body can be cavity, and most other internal organs are in the abdominal cavity. b. Types divided into two main body cavities: the ventral cavity and the of serous membranes. dorsal cavity (Fig. 11.6a). The ventral cavity, which is called a coelom during development, becomes divided into the thoracic, abdominal, and pelvic cavities (the latter two are sometimes Body Membranes grouped together as the abdominopelvic cavity). The thoracic cavBody membranes line cavities and the internal spaces of organs ity contains the right and left lungs and the heart. It is separated and tubes that open to the outside. from the abdominal cavity by a horizontal muscle called the diaMucous membranes line the tubes of the digestive, respiraphragm. The stomach, liver, spleen, gallbladder, and most of the tory, urinary, and reproductive systems. They are composed of an small and large intestines are in the upper portion of the abdominal epithelium overlying a loose fibrous connective tissue layer. The cavity. The pelvic cavity contains the rectum, the urinary bladder, epithelium contains goblet cells that secrete mucus. This mucus the internal reproductive organs, and the rest of the large intestine. ordinarily protects the body from invasion by bacteria and viruses. Males have an external extension of the abdominal wall, called the More mucus is secreted and expelled when a person has a cold and scrotum, containing the testes. has to blow her or his nose. The dorsal cavity also has two parts: the cranial cavity within Serous membranes, which line the thoracic and abdominal the skull contains the brain; the vertebral canal, formed by the cavities and cover the organs they contain, are also composed of vertebrae, contains the spinal cord.

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epithelium and loose fibrous connective tissue. They secrete a watery fluid that keeps the membranes lubricated. Serous membranes support the internal organs and compartmentalize the large thoracic and abdominal cavities. This helps hinder the spread of any infection. Serous membranes have specific names according to their location (Fig. 11.6b). The pleurae (sing., pleura) line the thoracic cavity and cover the lungs; the pericardium covers the heart; the peritoneum lines the abdominal cavity and covers its organs. A double layer of peritoneum, called mesentery, supports the abdominal organs and attaches them to the abdominal wall. Peritonitis is a potentially life-threatening infection of the peritoneum that may occur if an inflamed appendix bursts before it is removed, or if the digestive tract is perforated for any other reason. Synovial membranes, composed only of loose connective tissue, line freely movable joint cavities. They secrete synovial fluid that lubricates the cartilage at the ends of the bones so that they can move smoothly in the joint cavity. The meninges are membranes within the dorsal cavity. They are composed only of connective tissue and serve as a protective covering for the brain and spinal cord.

Check Your Progress 11.2 1. Identify the two cavities that are separated by the

diaphragm. 2. Describe the function of the fluids produced by various body membranes.

11.3  Organ Systems

blood is moving throughout the body, it distributes heat produced by the muscles. Blood transports nutrients and oxygen to the cells and removes their waste molecules, including carbon dioxide. Despite the movement of molecules into and out of the blood, it has a fairly constant volume and pH. The blood is also a route by which cells of the immune system can be distributed throughout the body.

Lymphatic and Immune Systems The lymphatic system (see Chapter 13) consists of lymphatic vessels (which transport lymph), lymph nodes, and other lymphatic (lymphoid) organs. This system protects the body from disease by purifying lymph and storing lymphocytes, the white blood cells responsible for adaptive immunity. Lymphatic vessels absorb fat from the digestive system and collect excess interstitial fluid, which is returned to the cardiovascular system. The immune system (see Chapter 13) consists of all the cells in the body that protect us from disease, especially those caused by infectious agents. The lymphocytes, in particular, belong to this system.

Digestive System The digestive system (see Chapter 14) includes the mouth, esophagus, stomach, small intestine, and large intestine (colon), along with associated organs such as the teeth, tongue, salivary glands, liver, gallbladder, and pancreas. This system receives food and digests it into nutrient molecules, which can enter the  body’s cells. The nondigested remains are eventually eliminated.

Learning Outcomes

Respiratory System

Upon completion of this section, you should be able to 1. List the major organs that make up each organ system. 2. Describe the general function(s) of each organ system.

The respiratory system (see Chapter 15) consists of the lungs and the tubes that take air to and from them. The respiratory system brings oxygen into the body for cellular respiration and removes carbon dioxide from the body at the lungs, restoring pH.

Figure 11.7 illustrates the organ systems of the human body. Just as organs work together in an organ system, organ systems work together in the body. Therefore, while a particular organ may be assigned to one system, it may assist in the function of other organ systems.

Integumentary System The integumentary system (see section 11.4) contains skin, which is made up of two main types of tissue: the epidermis is composed of stratified squamous epithelium, and the dermis is composed of fibrous connective tissue. The integumentary system also includes nails, located at the ends of the fingers and toes (collectively called the digits); hairs and muscles that move hairs; the oil and sweat glands; blood vessels; and nerves leading to sensory receptors. Besides having a protective function, skin also synthesizes vitamin D, collects sensory data, and helps regulate body temperature.

Urinary System The urinary system (see  Chapter 16) contains the kidneys, the urinary bladder, and the tubes that carry urine. This system rids the body of metabolic wastes, and helps regulate the fluid balance and pH of the blood.

Nervous System The nervous system  (see  Chapters 17 and 18) consists of the brain, spinal cord, and associated nerves. The nerves conduct nerve impulses from sensory receptors to the brain and spinal cord, where integration occurs. Nerves also conduct nerve impulses from the brain and spinal cord to the muscles and glands, allowing us to respond to both external and internal stimuli.

Cardiovascular System

Musculoskeletal System

In the cardiovascular system, (see Chapter 12) the heart pumps blood and sends it under pressure into the blood vessels. While

Within the musculoskeletal system,  (see  Chapter 19) the bones provide a scaffolding that helps hold and protect body parts. For



Chapter 11  Human Organization

Integumentary system

Cardiovascular system

Lymphatic and immune systems

• protects body • provides temperature homeostasis • synthesizes vitamin D • receives sensory input Organ: Skin

• transport system for nutrients, waste • provides temperature, pH, and fluid homeostasis Organ: Heart

• defends against infectious diseases • provides fluid homeostasis • assists in absorption and transport of fats Organs: Lymphatic vessels, lymph nodes, spleen

Digestive system

Respiratory system

• ingests, digests, and • exchanges gases at both lungs and tissues processes food • absorbs nutrients and • assists in pH homeostasis eliminates waste Organs: Lungs • involved in fluid homeostasis Organs: Oral cavity, esophagus, stomach, small intestine, large intestine, salivary glands, liver, gallbladder, pancreas

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Urinary system • excretes metabolic wastes • provides pH and fluids homeostasis Organs: Kidneys, urinary bladder

Skeletal system

Muscular system

Nervous system

Endocrine system

Reproductive system

• provides support and protection • assists in movement • stores minerals • produces blood cells Organ: Bones

• assists in movement and posture • produces heat Organ: Muscles

• receives, processes, and stores sensory input • provides motor output • coordinates organ systems Organs: Brain, spinal cord

• produces hormones • coordinates organ systems • regulates metabolism and stress responses • involved in fluid and pH homeostasis Organs: Testes, ovaries, adrenal glands, pancreas, thymus, thyroid, pineal gland

• produces and transports gametes • nurtures and gives birth to offspring in females Organs: Testes, penis, ovaries, uterus, vagina

Figure 11.7  Organ systems of the body.

example, the skull forms a protective encasement for the brain. The skeleton also helps move the body because it serves as a place of attachment for the skeletal muscles. It stores minerals, and it produces blood cells within the bone marrow. Skeletal muscle contraction maintains posture and accounts for the movement of the body and its parts. Cardiac muscle contraction results in the

heartbeat. The walls of internal organs contract due to the presence of smooth muscle.

Endocrine System The endocrine system (see Chapter 20) consists of the hormonal glands, which secrete chemical messengers called hormones.



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Hormones have a wide range of effects, including regulating cellular metabolism, regulating fluid and pH balance, and helping us respond to stress. Both the nervous and endocrine systems coordinate and regulate the functioning of the body’s other systems. The endocrine system also helps maintain the functioning of the male and female reproductive organs.

Reproductive System The reproductive system (see Chapter 21) has different organs in the male and female. The male reproductive system consists of the testes, other glands, and various ducts that conduct semen to and through the penis. The testes produce sex cells called sperm. The female reproductive system consists of the ovaries, uterine tubes, uterus, vagina, and external genitals. The ovaries produce sex cells called eggs, or oocytes. When a sperm fertilizes an oocyte, an offspring begins to develop.

11.4  Integumentary System Learning Outcomes Upon completion of this section, you should be able to 1. Identify the two main regions of skin, and how these are distinguished from the subcutaneous layer. 2. Describe the makeup and function of the accessory structures of human skin. 3. List some common disorders of human skin, and how these may be treated.

Check Your Progress  11.3

The skin and its accessory organs (nails, hair, oil glands, and sweat glands) are collectively called the integumentary system. Skin protects underlying tissues from physical trauma, pathogen invasion, and water loss. It also participates in homeostasis by helping to regulate body temperature. The skin even synthesizes vitamin D with the aid of ultraviolet radiation. Skin also contains sensory receptors, which help us to be aware of our surroundings and to communicate through touch.

1. List a major organ that is found in each system. 2. Identify two organ systems that protect the body from

Regions of the Skin

disease.

The skin has two regions: the epidermis and the dermis (Fig. 11.8). A subcutaneous layer is present between the skin and any

Figure 11.8  Human skin anatomy.  Skin consists of two regions, the epidermis and the dermis. A subcutaneous layer lies below the dermis. hair shaft

sweat pore

Epidermis

basal cells melanocytes sensory receptor capillaries oil gland arrector pili muscle

Dermis

free nerve endings hair follicle hair root sweat gland artery vein nerve adipose tissue

Subcutaneous layer



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HEALTH

UV Rays: Too Much Exposure or Too Little? The sun is the major source of energy for life on Earth. Without the sun, most organisms would quickly die out. But the sun’s energy can also be damaging. In addition to visible light, the sun emits ultraviolet (UV) radiation, which has a shorter wavelength (and thus a higher energy) than visible light. Based on wavelength, UV radiation can be grouped into three types: UVA, UVB, and UVC, but only UVA and UVB reach the Earth’s surface. Scientists have developed a UV index to determine how powerful the solar rays are in different U.S. cities. In general, the more southern the city, the higher the UV index and the greater the risk of skin cancer. Maps indicating the daily risk levels for various U.S. regions can be viewed at http://www.nws.noaa.gov/om/heat/uv.shtml. Both UVA and UVB rays can damage skin cells. Tanning occurs when melanin granules increase in keratinized cells at the surface of the skin as a way to prevent further damage by UV rays. Too much exposure to UVB rays can directly damage skin cell DNA, leading to mutations that can cause skin cancer. UVB is also responsible for the pain and redness characteristic of a sunburn. In contrast, UVA rays do not cause sunburn, but penetrate more deeply into the skin, damaging collagen fibers and inducing premature aging of the skin, as well as certain types of skin cancer. UVA rays can also induce skin cancer by causing the production of highly reactive chemicals that can indirectly damage DNA. Skin cancer is the most commonly diagnosed type of cancer in the United States, outnumbering cancers of the lung, breast, prostate, and colon combined. The most common type of skin cancer is basal cell carcinoma (Fig. 11Aa), which is rarely fatal, but can be disfiguring if allowed to grow. Squamous cell carcinoma (Fig. 11Ab), the second most common type, is fatal in about 1% of cases. The most deadly form is melanoma (Fig. 11Ac), which occurs in adolescents and young adults as well as in older people. If detected early, over 95% of patients survive at least five years, but if the cancer cells have already spread throughout the body, only 10% to 20% can expect to live this long. Melanoma affects pigmented cells and often has the appearance of an unusual mole (Fig. 11Ac). Any moles that become malignant are removed surgically. If the cancer has spread, chemotherapy and a number of other

a. Basal cell carcinoma

b. Squamous cell carcinoma

c. Melanoma

Figure 11A  Skin cancer.  a. Basal cell carcinomas are the most common type of skin cancer.

b. Squamous cell carcinomas arise from the epidermis, and usually occur in areas exposed to the sun. c. Malignant melanomas result from a proliferation of pigmented cells. Warning signs include a change in the shape, size, or color of a normal mole, as well as itching, tenderness, or pain.

treatments are also available. In March 2007, the USDA approved a vaccine to treat melanoma in dogs, which was the first time a vaccine was approved as a treatment for any cancer in animals or humans. Clinical trials are under way to test a similar vaccine for use in humans. Advances have also been made in the field of immunotherapy, where the cells of a person’s immune system (see Chapter 13) are directed to attack melanoma cells. According to the Skin Cancer Foundation, about 90% of nonmelanoma skin cancers, and 65% of melanomas, are associated with exposure to UV radiation from the sun. So how can we protect ourselves? First, try to minimize sun exposure between 10 A.M. and 4 P.M. (when the UV rays are most intense) by wearing protective clothing, hats, and sunglasses. Second, use sunscreen. Sunscreens generally do a better job of blocking UVB than UVA rays. In fact, the SPF, or sun protection factor printed on sunscreen labels, refers only to the degree of protection against UVB. Many sunscreens don’t provide as much protection against UVA, and because UVA doesn’t cause sunburn, people may have a false sense of security. Some sunscreens do a better job of blocking UVA—look for those that contain zinc oxide, titanium dioxide, avobenzone, or Mexoryl SX. Unfortunately, tanning salons use lamps that emit UVA rays that are two to three times more powerful than the UVA rays emitted by the sun. Because of the potential damage to deeper layers of skin, most medical experts recommend avoiding indoor tanning salons altogether. Because UV light is potentially damaging, why haven’t all humans developed the

more protective dark skin that is common to humans living in tropical regions? It turns out that vitamin D is produced in the body only when UVB rays interact with a form of cholesterol found mainly in the skin. This “sunshine vitamin” serves several important functions in the body, including keeping bones strong, boosting the immune system, and reducing blood pressure. Certain foods also contain vitamin D, but it can be difficult to obtain sufficient amounts through diet alone. Therefore, in more temperate areas of the planet, lighterskinned individuals have the advantage of being able to synthesize sufficient vitamin D. Interestingly, dark-skinned people living in such regions may be at increased risk for vitamin D deficiency. So how much sun exposure is enough in temperate regions of the world? During the summer months, an average fair-skinned person will synthesize plenty of vitamin D after exposure to 10–15 minutes of midday sun. During winter months, however, anyone living north of Atlanta probably receives too few UV rays to stimulate vitamin D synthesis, and therefore they must fulfill their requirement through their diet.

Questions to Consider 1. How would skin cancer negatively affect homeostasis? 2. If caught early, melanoma is rarely fatal. What are some factors that could delay detection of a melanoma? 3. There is considerable controversy about the level of dietary vitamin D intake that should be recommended. Why is this the case?



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underlying structures, such as muscle or bone. The subcutaneous layer is not a part of the skin. The epidermis is made up of stratified squamous epithelium. New cells derived from basal cells become flattened and hardened as they are pushed to the surface by cells forming underneath them. Hardening takes place because the cells produce keratin, a waterproof protein. A thick layer of dead keratinized cells, arranged in spiral and concentric patterns, forms fingerprints and footprints. Specialized cells in the epidermis called melanocytes produce melanin, the main pigment responsible for skin color. Different amounts of melanin in the skin provide protection from damage by excessive ultraviolet (UV) radiation, while allowing sufficient UV activation of vitamin D. As the Health feature, “UV Rays: Too Much Exposure or Too Little?,” explains, too much exposure to UV light may be dangerous. The dermis is a region of fibrous connective tissue beneath the epidermis. The dermis contains collagen and elastic fibers. The collagen fibers are flexible but offer great resistance to overstretching. They prevent the skin from being torn. The elastic fibers maintain normal skin tension but also stretch to allow movement of underlying muscles and joints. The number of collagen and elastic fibers decreases with exposure to the sun, causing the skin to become less supple and more prone to wrinkling. The dermis also contains blood vessels that nourish the skin. When blood rushes into these vessels, a person blushes, and when blood flow is reduced, or if there is not adequate oxygen in the blood, the skin turns “blue.” Sensory receptors are specialized free nerve endings in the dermis that respond to external stimuli. There are sensory receptors for touch, pressure, pain, and temperature. The fingertips contain the most touch receptors, and these add to our ability to use our fingers for delicate tasks. The subcutaneous layer is composed of loose connective tissue and adipose tissue, which stores fat. Fat is a stored source of energy in the body. Adipose tissue also helps thermally insulate the body from either gaining heat from the outside or losing heat from the inside.

Accessory Organs of the Skin Nails, hair, and glands are structures of epidermal origin, even though some parts of hair and glands are largely found in the dermis. Nails are a protective covering of the distal part of the digits. Nails can help pry things open or pick up small objects. Nails grow from special epithelial cells at the base of the nail in the portion called the nail root. These cells become keratinized as they grow out over the nail bed. The visible portion of the nail is called the nail body. The cuticle is a fold of skin that hides the nail root. The whitish color of the half-moon-shaped base, or lunula, results from the thick layer of cells in this area (Fig. 11.9). Hair follicles are in the dermis and continue through the epidermis where the hair shaft extends beyond the skin. Epidermal cells form the root of hair, and their division causes a hair to grow. As the cells become keratinized and die, they are pushed farther from the root. Interestingly, chemical substances in the body such  as illicit drugs and by-products of alcohol metabolism are

nail root cuticle lunula nail bed nail body

Figure 11.9  Nail anatomy.  Cells produced by the nail root become

keratinized, forming the nail body.

incorporated into growing hair shafts, where they can be detected by laboratory tests. The Bioethical feature, “Just a Snip, Please: Testing Hair for Drugs,” explores how hair can be used to diagnose for drug use. When we are scared or cold, contraction of the arrector pili muscles attached to hair follicles may cause the hairs to “stand on end” and goose bumps to develop. This trait has no real function in humans, although from an evolutionary perspective it may have helped fur-covered mammals to look larger, as well as to keep warm by trapping air within the fur. The color of hair is due to the variable presence of different forms of melanin. In general, the more melanin, the darker the hair. A decreasing melanin content with age results in the various shades of gray hair. White or gray hair contains little or no melanin. Hair loss is most often agerelated, but also may occur following many types of illnesses. Each hair follicle has one or more oil glands (also called ­sebaceous glands), which secrete sebum, an oily substance that lubricates the hair within the follicle and the skin itself. With the exception of the palms of the hands and soles of the feet, all areas of human skin have oil glands. The skin of an average adult also has about 3,000  sweat glands (also called sudoriferous glands) per square inch. A sweat gland is a coiled tubule within the dermis that straightens out near its opening, or pore. Some sweat glands open into hair follicles, but most open onto the surface of the skin. Sweat glands play a role in modifying body temperature. When body temperature starts to rise, sweat glands become active. Sweat absorbs body heat as it evaporates. Once body temperature lowers, sweat glands are no longer active.

Disorders of the Skin The integumentary system is susceptible to a number of diseases. Perhaps more than animals with skin covered by tough scales or a thick fur, human skin can be easily traumatized. It is also prone to certain infections, although its dryness, mildly acidic pH, and the presence of dead cells on its outermost layers renders skin resistant



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203

BIOETHICAL

Just a Snip, Please: Testing Hair for Drugs Imagine you have just started your first real full-time job, with health insurance, a pension plan, and other benefits. The first day, after you settle into your cubicle, a manager comes by and asks for a sample of your hair. “We normally just snip a bit off in the back, where it will hardly show,” he says, smiling. When you ask why, he replies that it is company policy to  screen employees for drug use every six months. Tests that detect illicit drugs or their metabolic products in urine or saliva are of limited value, because most of these chemicals are secreted by the body for only a few days. In contrast, many illicit drugs and/or their metabolites are incorporated into growing hair shafts throughout the body, where they remain indefinitely. Especially in the first 1.5 inches of hair growth from the scalp, each 0.5 inch is considered to represent 30 days’ worth of growth (and thus, potential drug use). Hair from anywhere on the body can be used, although the growth of body hair is usually slower, so the time of any drug use cannot be determined. It generally takes four to five days from the time a drug is taken into the body until it begins to appear in hair. Many commercial laboratories now offer hair testing (Fig. 11B). In general, these labs contend that they can distinguish between environmental exposure to a particular drug—from being in the vicinity of someone smoking marijuana, for example— and actual drug use by an individual. In recent years, several court decisions have supported the idea that hair testing can accurately distinguish actual drug use from such “passive” exposure. This has led to the marketing of several shampoos for cleaning or

“detoxifying” the hair shafts, but these may not be effective. And even if they were, a lab could conceivably test hair for common contaminants expected to be found in everyone’s hair. If these contaminants were not found, this could be used as evidence that a person has attempted to hide prior drug usage. Even if one accepts that hair testing is an accurate way to prove that a person has used drugs, is it ethical for businesses to require their employees to undergo such tests? Does it matter what type of job a person has? For example, would it be more difficult to argue against mandatory drug testing for school bus drivers than for stockbrokers? And for those of you still living with your parents, how upset would you be to find out that your mom or dad had slipped into your bedroom at night, snipped off a small bit of hair from the back of your head, and mailed it to one of several companies that now offer hair testing to the general public?

Questions to Consider

Figure 11B  Hair testing kit.  Kits for testing hair for illicit or prescription drugs can be purchased at retail stores or online.

1. The Fourth Amendment to the U.S. Constitution guards against “unreasonable searches and seizures” by the government. Do you believe that the types of drug testing described here are unconstitutional?

to many pathogens. Several types of cancer can arise from the skin, often secondary to damage from UV rays (see the Health feature, “UV Rays: Too Much Exposure or Too Little?”). Allergic reactions and various irritating chemicals can cause dermatitis, or inflammation of the skin. Acne is a common skin disorder that usually first appears during adolescence. It occurs mostly on the face, shoulders, chest, and back—areas where the skin has the highest density of sebaceous glands. Around the time of puberty, increasing levels of certain hormones cause an increased production of sebum. This, in turn, can lead to a blockage of the exit of the sebum from the gland, and an increase in the growth of Propionibacterium acnes, a bacterium that is almost universally found on human skin. This causes an

2. In which of the following additional situations would you support mandatory drug testing: To test airline pilots for hallucinogens? To test high school athletes for steroids? To test NBA players for marijuana?

inflammatory response, commonly known as a pimple. In many cases white blood cells called neutrophils will migrate into the area, forming pus (see Chapter 13), and sometimes deeper tissues become involved, which increases the possibility of permanent scarring. Most medications for acne work by drying out and unclogging blocked pores, killing the bacteria, and/or reducing inflammation. Over-the-counter medications can be useful for mild acne, but stronger, prescription medications may be needed for more severe cases. In 2005, the FDA approved Zeno, a hand-held medical device that kills P. acnes by heating the skin to 120 degrees for 2.5 minutes. Other available treatments rely on lasers or light to kill the bacteria and/or reduce the production of sebum.



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Warts are small areas of skin proliferation caused by the human papillomavirus. Though they can occur at any age, nongenital warts most commonly occur between the ages of 12 and 16. Though sometimes embarrassing, these are generally harmless and disappear without treatment—more than half do so within two years. Warts that cause cosmetic disfigurement or are painful (such as plantar warts on the foot) can be surgically removed, frozen with liquid nitrogen, or treated with various pharmaceutical compounds. One example is cantharidin, an extract from the blister beetle. ­Genital warts are discussed in section 21.5.

Check Your Progress  11.4

Control center

data to control center

response to stimulus

Effect

Sensor

change of internal conditions

1. Compare the structure of the epidermal and dermal layers

negative feedback and return to normal

stimulus

of the skin.

2. Discuss why a dark-skinned individual living in northern

Canada might develop bone problems. 3. List three accessory organs of the skin, and describe the major function(s) of each.

11.5  Homeostasis Learning Outcomes Upon completion of this section, you should be able to 1. Define homeostasis, and describe why it is essential to living organisms. 2. Differentiate between positive and negative feedback mechanisms, and list specific examples of each. 3. Discuss the roles of specific organ systems in homeostasis, and the health consequences if homeostasis is disrupted.

too m

uch

Homeostasis

too litt le

Figure 11.10  Negative feedback mechanism.  The sensor and control center of a feedback mechanism work together to keep a variable close to a particular value. Let’s take a simple example. When the pancreas detects that the blood glucose level is too high, it secretes insulin, a hormone that causes cells to take up glucose. Now the blood sugar level returns to normal, and the pancreas is no longer stimulated to secrete insulin.

Mechanical Example

Negative Feedback Negative feedback is the primary homeostatic mechanism that keeps a variable close to a particular value, or set point. A homeostatic mechanism has at least two components: a sensor and a control center (Fig. 11.10). The sensor detects a change in internal conditions. The control center then directs a response that brings conditions back to normal again. Now, the sensor is no longer activated. In other words, a negative feedback mechanism is present when the output of the system dampens the original stimulus.

A home heating system is often used to illustrate how a more complicated negative feedback mechanism works (Fig. 11.11). You set the thermostat at, say, 68°F. This is the set point. The thermostat contains a thermometer, a sensor that detects when the room temperature is above or below the set point. The thermostat also contains a control center. It turns the furnace off when the room is too hot and turns it on when the room is too cold. When the furnace is off, the room cools, and when the furnace is on, the room warms. In other words, typical of negative feedback mechanisms, there is a fluctuation above and below normal. Room Temperature (°F)

Homeostasis is the maintenance of a relatively constant internal ­environment by an organism, or even by a single cell. Even though external conditions may change dramatically, internal conditions stay within a narrow range. For example, regardless of how cold or hot it gets, the temperature of the body stays around 98.6°F (37°C). Even if you consume an acidic food such as yogurt (with a pH around 4.5), the pH of your blood is usually about 7.4, and even if you eat a candy bar, the amount of sugar in your blood stays between 0.05% and 0.08%. It is important to realize that internal conditions are not absolutely constant. They tend to fluctuate above and below a particular value. Therefore, the internal state of the body is often described as one of dynamic equilibrium. If internal conditions change to any great degree, illness results. This makes the study of homeostatic mechanisms medically important.

75

furnace turns off

70

set point 65 60

furnace turns on Time

Human Example: Regulation of Body Temperature The sensor and control center for body temperature are located in a part of the brain called the hypothalamus. When the body temperature is above normal, the control center directs the blood



Chapter 11  Human Organization

sends data to thermostat

0

50

60 70

80

5

Control center

directs furnace to turn off

60 7 0 80

68°F set point

50

0

60 7 0 80

Notice that a negative feedback mechanism prevents change in the same direction; that is, body temperature does not get warmer and warmer because warmth brings about a change toward a lower body temperature. Also, body temperature does not get colder and colder because a body temperature below normal brings about a change toward a warmer body temperature.

Positive Feedback

60 7 0

80

5

Sensor

205

70°F too hot

Positive feedback is a mechanism that brings about an ever greater change in the same direction. One example is the process of blood

furnace off

Control center

negative feedback and return to normal temperature

stimulus

sends data to control center

too m

uch

Homeostasis

directs response to stimulus

98.6°F set point

Sensor

too litt le

negative feedback and return to normal temperature

Effect

Blood vessels dilate; sweat glands secrete.

stimulus change of internal conditions

60 7 0

5

50

0

negative feedback and return to normal temperature

stimulus

60 7 0 80

above

Control center

norm

60 7 0 80

al

Normal body temperature

60 7 0

80

directs furnace to turn on

50

0

80

66°F too cold

5

Sensor

furnace on

below

sends data to thermostat

norm

al

68°F set point

Figure 11.11  Complex negative feedback mechanism.  When a room becomes too warm, negative feedback allows the temperature to return to normal. A contrary cycle, in which the furnace turns on and gives off heat, returns the room temperature to normal when the room becomes too cool.

vessels of the skin to dilate (Fig. 11.12). This allows more blood to flow near the surface of the body, where heat can be lost to the environment. In addition, the nervous system activates the sweat glands, and the evaporation of sweat helps lower body temperature. Gradually, body temperature decreases to 98.6°F (37°C). When the body temperature falls below normal, the control center (via nerve impulses) directs the blood vessels of the skin to constrict. This conserves heat. If body temperature falls even lower, the control center sends nerve impulses to the skeletal muscles, and shivering occurs. Shivering generates heat, and gradually body temperature rises to 98.6°F. When the temperature rises to normal, the control center is inactivated.

negative feedback and return to normal

stimulus change of internal conditions

Effect

Sensor

Blood vessels constrict; sweat glands are inactive.

directs response to stimulus

Control center sends data to control center 98.6°F  set point

Figure 11.12  Regulation of body temperature.  Normal body

temperature is maintained by a negative feedback system.



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clotting, during which injured tissues release chemical factors that activate platelets. These activated platelets initiate the clotting process, and also release factors that stimulate further clotting (see section 12.2). Positive feedback loops tend to be involved in processes that have a definite cutoff point. Consider that when a woman is giving birth, the head of the baby begins to press against the cervix, stimulating sensory receptors there. When nerve impulses reach the brain, the brain causes the pituitary gland to secrete the hormone oxytocin. Oxytocin travels in the blood and causes the uterus to contract. As labor continues, the cervix is ever more stimulated, and uterine contractions become ever stronger until birth occurs.

Homeostasis and Body Systems All systems of the body contribute toward maintaining homeostasis.

The Transport Systems The cardiovascular system conducts blood to and away from capillaries, where exchange of gases, nutrients, and wastes occurs. Interstitial fluid, which bathes all the cells of the body, is refreshed when molecules such as oxygen and nutrients move into the interstitial fluid from the blood and when carbon dioxide and wastes move from the interstitial fluid into the blood (Fig. 11.13).

The lymphatic system is an accessory to the cardiovascular system. Lymphatic capillaries collect excess interstitial fluid, which is returned via lymphatic vessels to the cardiovascular system. Lymph nodes are sites where the immune system responds to invading microorganisms.

The Maintenance Systems The respiratory system adds oxygen to and removes carbon dioxide from the blood. It also plays a role in regulating blood pH because removal of CO2 causes the pH to rise, just as CO2 retention helps to lower the pH. The digestive system takes in and digests food, providing nutrient molecules that enter the blood to replace the nutrients that are constantly being used by the body cells. The liver, an organ that assists the digestive process by producing bile, also plays a significant role in regulating blood composition. Immediately after glucose enters the blood, any excess is removed by the liver and stored as glycogen. Later, the glycogen can be broken down to replace the glucose used by the body cells. In this way, the glucose composition of the blood remains relatively constant. The liver also removes toxic chemicals, such as ingested alcohol and other drugs, from the blood. The liver makes urea, a nitrogenous end product of protein metabolism. Urea and other metabolic waste molecules are excreted by the kidneys, which are a part of the urinary system. Urine formation by the kidneys is extremely critical to the body, not only because it rids the body of unwanted substances, but also because urine formation offers an opportunity to carefully regulate blood volume, salt balance, and pH.

The Support Systems blood flow

red blood cell

arteriole

capillary

tissue cell

The integumentary and musculoskeletal systems protect the in­ternal organs. In addition, the integumentary system produces vitamin D, while the skeleton stores minerals and produces the blood cells.

The Control Systems The nervous system and the endocrine system work together to control other body systems so that homeostasis is maintained. We have already seen that in negative feedback mechanisms, sensory receptors send nerve impulses to control centers in the brain, which then rapidly direct effectors to become active. Effectors can be muscles or glands. Muscles bring about an immediate change. Endocrine glands secrete hormones that bring about a slower, more lasting change that keeps the internal environment relatively stable.

oxygen and nutrients

carbon dioxide and wastes

Disease venule blood flow

Figure 11.13  Regulation of tissue fluid composition.  Cells are surrounded by interstitial fluid, which is continually refreshed because oxygen and nutrient molecules constantly exit, and carbon dioxide and waste molecules continually enter the bloodstream.

A disease is an abnormality in the body’s normal processes that significantly impairs homeostasis. As will be seen in physiology chapters of this text, diseases (disorders) can affect virtually every part of the human body. The major causes of human diseases include blood vessel problems, cancers, infections, and inflammatory conditions. A particular disease may be described as systemic, meaning that it affects the entire body or at least several organ systems.



Chapter 11  Human Organization

Other diseases, including many infections or inflammatory diseases such as dermatitis or arthritis, are more localized to a specific part of the body. Diseases may also be classified on the basis of their severity and duration. Acute diseases, such as poison ivy dermatitis or influenza, tend to occur suddenly and generally last a short time, although some can be life threatening. Chronic diseases, such as multiple sclerosis, AIDS, and most cancers, tend to develop slowly and last a long time, even for the rest of a person’s life, unless an effective cure is available. The term cancer refers to a group of disorders in which the usual controls on cell division fail, resulting in the production of abnormal cells that invade and destroy healthy body tissue. Cancers are classified according to the type of tissue from which they arise. Carcinomas, the most common type, are cancers of epithelial tissue. The Health feature, “UV Rays: Too Much Exposure or Too Little,” in section 11.4 describes one type of carcinoma, the melanoma. Sarcomas are cancers arising in muscle or connective tissue, especially bone or cartilage. Leukemias are cancers of the blood cells, and lymphomas are cancers that originate in lymph nodes. The chance of developing cancer in a particular tissue shows a positive correlation to the rate of cell division. Epithelial cells reproduce at a high rate, and 2.5 million new blood cells appear each second. Therefore, carcinomas and leukemias are common types of cancers.

207

Check Your Progress 11.5 1. Explain how positive feedback differs from negative feedback.

2. Describe how several body systems can interact to maintain homeostasis.

3. List several specific diseases that result when a particular body system fails to perform its function.

Conclusion The human body consists of an incredibly complex system of molecules, cells, tissues, organs, and organ systems functioning together. Ultimately, the interaction of these components provides humans with the ability to achieve the highest levels of physical and mental functions, as well as maintaining general homeostasis. We are aware of only a few of these processes, such as the activity of our muscles when we are trying to learn a new skill or perform a challenging physical task. When these homeostatic processes fail, various types of diseases can result.

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11.1  Overview of Tissues • Epithelial Tissue • Connective Tissue • Muscle Tissue • Nervous Tissue 11.2  Body Cavities 11.4  Human Skin 11.5  Homeostasis • Temperature Regulation

 

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11.5  Homeostasis

SUMMARIZE 11.1  Types of Tissues In the human body, most cells are organized into tissues. Human tissues are categorized into four groups: ■ Epithelial tissue covers the body and lines its cavities. Squamous epithelium, cuboidal epithelium, and columnar epithelium can be simple, stratified, or pseudostratified and may also have cilia or microvilli. Epithelial cells sometimes form glands that secrete either into ducts or into the blood.

  Tutorials 11.5  Negative Feedback

■ Connective tissue, in which cells are separated by a matrix, often

binds body parts together. In most cases, the matrix contains collagen fibers, reticular fibers, and elastic fibers. ∙ Loose fibrous connective tissue supports epithelium and encloses organs. Dense fibrous connective tissue, such as that of tendons and ligaments, contains a closely packed matrix. Cartilage, a type of dense fibrous connective tissue in which cells are found in lacunae, can be of three types: hyaline cartilage, elastic cartilage, or fibrocartilage.



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∙ Bone has a hard matrix due to the presence of calcium salts. Compact bone is found in the shafts of long bones; spongy bone is found on the ends. ∙ Adipose tissue stores fat, while reticular connective tissue forms a supportive mesh in some organs. ∙ Blood is a connective tissue in which the matrix is a liquid. Blood consists of the liquid part, called plasma, and the formed elements, which include the red blood cells, white blood cells, and platelets. Interstitial fluid  is created when the liquid portions of the blood surround the individual cells of the body. ■ Muscular tissue is of three types. Skeletal muscle is found in muscles attached to bones, and smooth muscle is found in internal organs. Both skeletal muscle and cardiac muscle are striated. Both cardiac and smooth muscle are involuntary. ■ Nervous tissue has one main type of conducting cell, the neuron, and several types of supporting cells, the neuroglia. Each neuron has a cell body, dendrites, and an axon. Axons are specialized to conduct nerve impulses. Bundles of axons bound by connective tissue are called nerves.

11.2  Body Cavities and Body Membranes The internal organs occur within two main body cavities. ■ The dorsal cavity includes the cranial cavity and vertebral canal. ■ The ventral cavity contains the thoracic, abdominal, and pelvic cavities, which house organs of the respiratory, digestive, urinary, and reproductive systems, among others. ■ Four types of membranes line body cavities and the internal spaces of organs. Mucous membranes line the tubes of the digestive system; serous membranes line the thoracic and abdominal cavities and cover the organs they contain; synovial membranes line movable joint cavities; and meninges cover the brain and spinal cord.

11.3  Organ Systems The organ systems of the human body can be grouped according to their major function(s): ■ The integumentary system protects deeper tissues and also makes vitamin D, collects sensory data, and helps regulate body temperature. ■ The cardiovascular, lymphatic, digestive, respiratory, and urinary systems perform processing and transport functions that maintain the normal conditions of the body. ■ The immune system defends against infectious disease. ■ The musculoskeletal system supports and protects the body and permits movement. ■ The nervous system receives sensory input and directs muscles and glands to respond. ■ The endocrine system produces hormones, which influence the functions of the other systems. ■ The reproductive system produces sex cells (gametes), and ultimately, offspring.

11.4  Integumentary System

toward the surface; and (2) the dermis contains glands, hair follicles, blood vessels, and sensory receptors. ■ The accessory organs of skin include the nails, hair follicles, oil glands (sebaceous glands), and sweat glands. ■ The skin is susceptible to several types of disorders, including infections, cancers, and various inflammatory conditions.

11.5  Homeostasis ■ The body’s internal environment consists of blood and tissue fluid. ■ Homeostasis is the maintenance of a relatively constant internal envi-

ronment, mainly by two mechanisms: (1) negative feedback mechanisms keep the environment relatively stable (when a sensor detects a change above or below a set point, a control center brings about an effect that reverses the change and brings conditions back to normal again); and (2) positive feedback mechanisms bring about rapid change in the same direction as the stimulus. Disease results when one or more body systems fail.

ASSESS Testing Yourself Choose the best answer for each question.

11.1  Types of Tissues 1. Tissues are formed from ______ and are arranged together to form ______ . a. organs; organ systems c. cells; organs b. cells; molecules d. molecules; cells 2. Which choice is true of both cardiac and skeletal muscle? a. striated c. multinucleated cells b. single nucleus per cell d. involuntary control 3. Which of the following is not a connective tissue? a. blood b. bone c. cartilage d. adipose e. All of these are connective tissues.

11.2  Body Cavities and Body Membranes 4. Label the diagram of the body cavities below. f. e. g. b. h. a.

c.

d.

■ The integumentary system is composed of skin and the accessory

organs.

■ The main functions of the integumentary system are to protect under-

lying tissues while providing a barrier to pathogen invasion and water loss. Skin also functions in the sense of touch. ■ Human skin has two regions: the epidermis contains basal cells that produce new epithelial cells, which become keratinized as they move

11.3  Organ Systems 5. Which system(s) help control pH balance? a. digestive c. urinary b. respiratory d. Both b and c are correct.



Chapter 11  Human Organization

6. Which system plays the biggest role in fluid balance? a. cardiovascular b. urinary c. digestive d. integumentary

209

10. Which of the following is an example of negative feedback? a. Uterine contractions increase as labor progresses. b. Insulin decreases blood sugar levels after a meal is eaten. c. Sweating increases as body temperature drops. d. Platelets continue to plug an opening in a blood vessel until blood flow stops.

11.4  Integumentary System 7. Label the indicated structures in the diagram of human skin below:

ENGAGE BioNOW Want to know  how this science is relevant to your life? Check out the BioNow videos below:

a.

l. k. j. i. h. g. f.

b.

■ Properties of Water ■ Characteristics of Life

Thinking Critically c.

d.

e.

8. Keratinization of epithelial cells occurs in which layer of the skin? a. subcutaneous layer b. dermis c. epidermis d. All of these are correct.

11.5  Homeostasis 9. Which of the following allows rapid change in one direction but does not achieve stability? a. homeostasis b. positive feedback c. negative feedback d. All of these are correct.

1. In what way(s) is blood like a tissue? 2. Which of these homeostatic mechanisms in the body are examples of positive feedback, and which are examples of negative feedback? Why? a. The adrenal glands produce epinephrine in response to a hormone produced by the pituitary gland in times of stress; the pituitary gland senses the epinephrine in the blood and stops producing the hormone. b. As the bladder fills with urine, pressure sensors send messages to the brain with increasing frequency, signaling that the bladder must be emptied. The more the bladder fills, the more messages are sent. c. When you drink an excess of water, specialized cells in your brain as well as stretch receptors in your heart detect the increase in blood volume. Both signals are transmitted to the kidneys, which increase the production of urine. 3. Explain how a failure of homeostasis leads to death.

PHOTO CREDITS Opener: © David M. Benett/Getty Images; 11.1(all)–11.2a–b, d–e: © Ed Reschke; 11.2c: © Dennis Strete/McGraw-Hill Education; 11.4a, c: © Ed Reschke; 11.4b: © Dennis Strete/McGraw-Hill Education; 11.5b: © Ed Reschke; 11Aa–b: © Dr. P. Marazzi/Science Source; 11Ac: © James Stevenson/SPL/Science Source; 11B: © MCT via Getty Images.



12

Cardiovascular System CHAPTER OUTLINE 12.1  The Blood Vessels 12.2 Blood 12.3 The Human Heart 12.4 Two Cardiovascular Pathways 12.5 Cardiovascular Disorders BEFORE YOU BEGIN Before beginning this chapter, take a few moments to review the following discussions: Section 4.2  What molecules must diffuse across the plasma membrane of cells lining capillaries? Section 11.1  How does the structure of cardiac muscle differ from skeletal and smooth muscle? Section 11.5  The cardiovascular system interacts with which other body systems?

CASE STUDY A Silent Killer On any visit to the doctor’s office you have probably had your blood pressure checked. While you may think that this is just related to the reason for your visit, one of the primary things that the doctor is looking for is an indication of the overall health of your cardiovascular system. And increasingly, doctors are detecting hypertension, or elevated blood pressure. Most young adults consider hypertension to be a disease of middle age, since they probably know someone who takes blood pressure medication. However recent studies suggest that almost one in five young adults may have high blood pressure, making it one of the major health risks facing young adults. Hypertension is a disease of the cardiovascular system. Its characteristics are a blood pressure above 140/90 mm Hg. But even if your blood pressure is below this, you may have what is called pre-hypertension, which may indicate future cardiovascular problems. Hypertension is caused by a variety of factors, including lifestyle, diet, and genetics. Long-term elevated blood pressure is correlated with an increase in heart disease, stroke, kidney disease, and vision problems.  Fortunately, there are steps that we can take to reduce the risk of hypertension. Eating a healthy diet, exercising regularly, and avoiding smoking are all effective approaches to improving the overall health of the cardiovascular system. While new technologies and medicine are constantly being developed, the single best preventative measure that can be taken is personal monitoring. To do that requires an understanding of the factors that influence blood pressure. In this chapter, we will explore the structure and function of the cardiovascular system in the human body and explore why this organ system is so essential to life. As you read through the chapter, think about the following questions:

1. What is the difference between the diastolic and systolic values? 2. How is blood pressure normally regulated in the body? 3. What factors may cause an individual to develop hypertension?

210



Chapter 12  Cardiovascular System

12.1  The Blood Vessels Learning Outcomes Upon completion of this section, you should be able to 1. Identify the structural features and function of each of the three main types of blood vessels. 2. Explain the process of capillary exchange.

The cardiovascular system has three types of blood vessels: the arteries (and arterioles), which carry blood away from the heart to the capillaries; the capillaries, which permit exchange of material with the tissues; and the veins (and venules), which return blood from the capillaries to the heart. The structure of each of these blood vessels is well adapted to its function in the body.

The Arteries An arterial wall has three layers (Fig. 12.1a). As is the case with all of the blood vessels, arteries have an inner endothelium, a simple squamous epithelium attached to a connective tissue basement membrane that contains elastic fibers (see Chapter 11). The middle layer is the thickest layer and consists of smooth muscle that can contract to regulate blood flow and blood pressure. The outer layer is fibrous connective tissue near the middle layer, but it becomes loose connective tissue at its periphery. The largest artery in the human body is the aorta. It is approximately 25 mm wide and carries O2-rich blood from the heart to other parts of the body. Smaller arteries branch off from the aorta, eventually forming a large number of arterioles. Arterioles are small arteries just visible to the naked eye, averaging about 0.5 mm in diameter. The inner layer

arteriole

venule

211

of arterioles is endothelium. The middle layer is composed of some elastic tissue, but mostly of smooth muscle with fibers that encircle the arteriole. When these muscle fibers are contracted, the vessel has a smaller diameter (is constricted). When these muscle fibers are relaxed, the vessel has a larger diameter (is dilated). Whether arterioles are constricted or dilated affects blood pressure. The greater the number of vessels dilated, the lower the blood pressure.

The Capillaries Capillaries join arterioles to venules (Fig. 12.1b). Capillaries are extremely narrow—only 8–10 µm wide—and have thin walls composed only of a single layer of endothelium with a basement membrane. Although each capillary is small, they form vast networks. Their total surface area in a human body is about 6,300 square meters (m2). Capillary beds (networks of many capillaries) are present in nearly all regions of the body. Consequently, a cut to almost any body tissue draws blood. One region of the body that is nearly capillary-free is the cornea of the eye, so that light can pass through. Therefore, the cells of the cornea must obtain nutrients by diffusion from the tears on the outside surface, and from the aqueous humor on the inside surface (see Chapter 18). Capillaries play a very important role in homeostasis because an exchange of substances takes place across their thin walls. Oxygen and nutrients, such as glucose, diffuse out of a capillary into the interstitial fluid that surrounds cells. Wastes, such as carbon dioxide, diffuse into the capillary. Some water also leaves a capillary. Any excess is picked up by lymphatic vessels, as shown in Figure 12.9 and discussed further in Chapter 13. The relative constancy of interstitial fluid is absolutely dependent upon capillary exchange.

Figure 12.1  Blood vessels.  The walls of arteries and veins have three layers. The inner layer is composed largely of endothelium, with a basement membrane that has elastic fibers; the middle layer is smooth muscle tissue; the outer layer is connective tissue (largely collagen fibers). a. Arteries have a thicker wall than veins because they have a larger middle layer than veins. b. Capillary walls are one-cell-thick endothelium. c. Veins are generally larger in diameter than arteries, so collectively, veins have a larger holding capacity than arteries. d. Micrograph of an artery and a vein. outer layer middle layer inner layer

outer layer middle layer inner layer

b. Capillary

artery

vein

valve inner layer middle layer outer layer a. Artery

c. Vein

d. Micrograph of artery and vein

1003



212

UNIT 3  Maintenance of the Human Body

Because capillaries serve the cells, the heart and the other vessels of the cardiovascular system can be thought of as the means by which blood is conducted to and from the capillaries. Only certain capillary beds are completely open at any given time. Rings of muscles, called precapillary sphincters, control the flow of blood into a capillary bed. When the precapillary sphincters are closed, blood flow into that capillary bed is reduced. For example, after eating, the capillary beds that serve the digestive system are mostly open, and those that serve the muscles are mostly closed. Each capillary bed has arteriovenous shunts that allow blood to go directly from arterioles to venules, bypassing the bed (Fig. 12.2).  Contrary to a popular misconception, it is not a decrease in blood flow to the brain after meals that causes “postprandial somnolence,” or the sleepiness that many people feel after eating. The blood supply to the brain is maintained under most physiological conditions, including ingestion of a heavy meal. Instead, it is recognized that hormones released by the digestive tract may instead be the culprit.

veins, causing them to enlarge and be visible as varicose veins. These most commonly occur in the lower legs of older individuals. Varicose veins of the anal canal are known as hemorrhoids. Because the walls of veins are thinner (see Fig. 12.1d), they can expand to a greater extent. At any one time, about 70% of the blood is in the veins. In this way, the veins act as a blood reservoir. If blood is lost due to hemorrhaging, nervous stimulation causes the veins to constrict, providing more blood to the rest of the body. The largest veins are the venae cavae, which include the superior vena cava (20 mm wide) and the inferior vena cava (35 mm wide). Both veins deliver O2-poor blood into the heart.

Check Your Progress  12.1 1. Describe how blood flow is controlled in each of the three major types of blood vessels.

2. List several specific substances that diffuse across capillary walls.

The Veins Veins and venules take blood from the capillary beds to the heart. First, the venules (small veins) drain blood from the capillaries and then join to form a vein. The walls of veins (and venules) have the same three layers as arteries, but there is less smooth muscle and connective tissue (see Fig. 12.1c). Therefore, the wall of a vein is thinner than that of an artery. Veins often have valves, which allow blood to flow only toward the heart when open and prevent blood from flowing backward when closed. Valves are found in the veins that carry blood against the force of gravity, especially the veins of the lower limbs. Unlike blood flow in the arteries and arterioles, which is kept moving by the pumping of the heart, blood flow in veins is primarily due to skeletal muscle contraction. If these valves become damaged by disease or through the normal wear-and-tear of aging, blood may begin pooling in the

12.2  Blood Learning Outcomes Upon completion of this section, you should be able to 1. List the major types of blood cells, and summarize their functions. 2. Identify the major molecular and cellular events that result in a blood clot. 3. Define capillary exchange, and describe the two major forces involved.

artery capillary bed

arteriole

O2-rich blood flow

precapillary sphincter arteriovenous shunt

venule O2-poor blood flow

vein

Figure 12.2  Anatomy of a capillary bed.  A capillary bed forms

a maze of capillary vessels that lies between an arteriole and a venule. When precapillary sphincter muscles are relaxed, the capillary bed is open, and blood flows through the capillaries. When sphincter muscles are contracted, blood flows through the arteriovenous shunt that carries blood directly from an arteriole to a venule. As blood passes through a capillary in the tissues, it gives up its oxygen (O2). Therefore, blood goes from being O2-rich in the arteriole (red color) to being O2-poor in the vein (blue color).

The body contains a number of different types of fluids, some of which are noted in Table 12.1. In this section, we will explore the function of blood. As noted in Chapter 11, blood is a liquid connective tissue. Not only does blood have transport functions, but it also acts in regulation (homeostasis) and protection. In addition to nutrients and wastes, blood also transports hormones. Blood helps regulate body temperature by dispersing body heat and helps regulate blood pressure because the plasma proteins contribute to the osmotic pressure of blood. Buffers in blood help maintain blood pH at about 7.4. Blood helps protect the body against invasion by disease-causing pathogens. Clotting mechanisms protect the body against potentially life-threatening loss of blood. If blood is collected from a person’s vein into a test tube and prevented from clotting, it separates into three layers (Fig. 12.3a). The upper layer is plasma, the liquid portion of

TABLE 12.1  Body Fluids Related to Blood Name

Composition

Blood

Formed elements and plasma

Plasma

Liquid portion of blood

Serum

Plasma minus fibrinogen

Interstitial fluid

Plasma minus most proteins

Lymph

Interstitial fluid within lymphatic vessels



Chapter 12  Cardiovascular System

Figure 12.3  Composition of blood. 

a. When blood is collected into a test tube containing an anticoagulant to prevent clotting and then centrifuged, it consists of three layers. The transparent straw-colored or yellow top layer is the plasma, the liquid portion of the blood. The thin, middle layer consists of leukocytes and platelets. The bottom layer contains the erythrocytes. b. Breakdown of the components Formed elements of plasma. c. Micrograph of the formed elements, which are also listed in (d).

a.

213

Formed Elements Type

Plasma (about 55% of whole blood)

Function and Description Transport O2 and help transport CO2

Red blood cells (erythrocytes)

Leukocytes and platelets ( 50

50–230

50–230

> 230

a. Young male with fragile X syndrome

Same individual when mature

b. Fragile X chromosome

Key = affected

> 230

= unaffected

< 50

50–230

c. Inheritance pattern for fragile X syndrome

Figure 24A  TRED in fragile X syndrome.  a. A young male with fragile X syndrome appears normal but with age develops an elongated face with a prominent jaw and ears that noticeably protrude. b. An arrow points out the fragile site of this fragile X chromosome. c. The number of repeats at the fragile site are given for each person, showing that the incidence and severity increase with each succeeding generation. A grandfather who has a pre-mutation with 50 to 230 repeats has no symptoms but transmits the condition to his grandsons through his daughter. Two grandsons have full-blown mutations with more than 230 repeats.



Chapter 24  Chromosomal Basis of Inheritance

Unaffected female

Victoria

1

Alice Louis IV

?

2

?

Hemophiliac male

Albert

3 Alexandra Nicholas II ?

Unaffected male

Victoria Edward

Carrier female

10

4

Leopold

Beatrice

Mary

7 ?

?

Helena

8

?

?

Olga Marie Alexi Tatiana Anastasia

9 ?

?

?

Juan Carlos 5

All were assassinated 1. Victoria 2. Edward VII 3. Irene 4. George V 5. George VI 6. Margaret 7. Victoria 8. Alfonso XIII 9. Juan

487

6 Philip Elizabeth II

12

11

13

16

14

15

Kate William Harry

10. Alexandra 11. Charles 12. Diana 13. Andrew 14. Edward 15. Anne 16. Sarah

George Alexander Louis Charlotte

Figure 24.6  A pedigree showing X-linked inheritance of hemophilia in European royal families.  Because Queen Victoria was a carrier, each of her sons had a 50% chance of having the disorder, and each of her daughters had a 50% chance of being a carrier. This pedigree shows only a portion of the European royal families. because the affected person’s blood either does not clot or clots very slowly. Although hemophiliacs bleed externally after an injury, they also bleed internally, particularly around joints. Hemorrhages can be stopped with transfusions of fresh blood (or plasma) or concentrates of the missing clotting protein. ­Factors VIII and IX are also now available as biotechnology products. At the turn of the century, hemophilia was prevalent among the royal families of Europe, and all of the affected males could trace their ancestry to Queen Victoria of England (Fig. 24.6). Of Queen Victoria’s 26 grandchildren, four grandsons had hemophilia, and four granddaughters were carriers. Because none of Queen Victoria’s ancestors were affected, it seems that  the faulty allele she carried arose by mutation, either in Victoria or in one of her parents. Her carrier daughters, Alice and Beatrice, introduced the allele into the ruling houses of Russia and Spain, respectively. Alexei, the last heir to the ­Russian throne before the Russian Revolution, was a hemophiliac. There are no hemophiliacs in the present British royal family because Victoria’s eldest son, King Edward VII, did not receive the allele.

Check Your Progress  24.2 1. Explain how sex-linked patterns of inheritance differ from autosomal inheritance.

2. Describe the phenotypic ratio that is expected for a cross in which both parents have one X-linked recessive allele.

3. Summarize the causes of the X-linked recessive disorders discussed in this chapter.

24.3  Changes in Chromosome Number Learning Outcomes Upon completion of this section, you should be able to 1. Describe how chromosome number disorders arise. 2. Give several examples of chromosome number disorders.

The Karyotype Physicians and prospective parents sometimes want to view an unborn child’s chromosomes to determine whether a chromosomal



488

UNIT 5  Continuance of the Species

abnormality exists. Any cell in the body except red blood cells, which lack a nucleus, can be a source of chromosomes for examination. In adults, it is easiest to obtain and use white blood cells separated from a blood sample for the purpose of looking at the chromosomes. For an examination of fetal chromosomes, a physician may recommend procedures such as chorionic villus sampling or amniocentesis (see section 22.3).  After a cell sample has been obtained, the cells are stimulated to divide in a culture medium. When a cell divides, chromatin condenses to form chromosomes. The nuclear envelope fragments, liberating the chromosomes. Next, a chemical is used to stop the division process when the chromosomes are most compacted and visible microscopically. Stains are applied to the slides, and the cells can be photographed with a camera attached to a microscope. Staining causes the chromosomes to have dark and light crossbands of varying widths, and these can be used—in addition to size and shape—to help pair up the chromosomes. A computer is used to arrange the chromosomes in pairs (Fig. 24.7). The display is called a karyotype.

Changing the Number of Chromosomes Figure 24.7  Karyotype of human chromosomes.  In body

cells, the chromosomes occur in pairs. In a karyotype, the pairs have been numbered and arranged by size from largest to smallest. These chromosomes are duplicated, and each one is composed of two sister chromatids. This karyotype illustrates the 46 chromosomes of a male.

Normally, a human receives 22 pairs of autosomes and two sex chromosomes. Sometimes individuals are born with either too many or too few autosomes or sex chromosomes, most likely due to nondisjunction during meiosis. This occurs as a result of ­nondisjunction, which is a failure of the chromosomes or sister chromatids to separate during meiosis. As shown in Figure 24.8, pair of homologous chromosomes

pair of homologous chromosomes Meiosis I

nondisjunction

normal

Meiosis II

normal

nondisjunction

Fertilization

Zygote

2n + 1 a.

2n + 1

2n – 1

2n – 1

2n

2n

2n + 1

2n – 1

b.

Figure 24.8  Nondisjunction of chromosomes during oogenesis.  a. Nondisjunction can occur during meiosis I and results in abnormal eggs that also have one more or one less than the normal number of chromosomes. Fertilization of these abnormal eggs with normal sperm results in a zygote with abnormal chromosome numbers. b. Nondisjunction can also occur during meiosis II if the sister chromatids separate but the resulting daughter chromosomes go into the same daughter cell. Then the egg will have one more or one less than the usual number of chromosomes. Fertilization of these abnormal eggs with normal sperm produces a zygote with abnormal chromosome numbers.



Chapter 24  Chromosomal Basis of Inheritance

nondisjunction may occur during meiosis I, when both members of a homologous pair go into the same daughter cell, or during meiosis II, when the sister chromatids fail to separate and both daughter chromosomes go into the same gamete.  Notice in Figure 24.8 that nondisjunction causes some of the egg cells to be missing a chromosome, while others have two copies of the same chromosome. When these eggs are fertilized by a sperm cell, the result is a zygote with an incorrect complement of chromosomes. If the zygote has an extra chromosome (2n+1), the result is called a trisomy because one type of chromosome is present in three copies. If the zygote is missing a chromosome (2n-1), the result is called a monosomy because one type of chromosome is present in a single copy.

Changes in Autosome Chromosome Number Normal development depends on the presence of exactly two of each kind of chromosome. Too many chromosomes are tolerated better than a deficiency of chromosomes, and several autosome trisomies are known to occur in humans (see Table 24.1). Among autosomal trisomies, only trisomy 21 (Down syndrome) has a reasonable chance of survival after birth. This is most likely due to the fact that chromosome 21 is one of the smallest chromosomes. Trisomies of chromosome 13 (Patau syndrome) and 18 (Edwards syndrome) also occur, but the individuals rarely survive past the first few months of life.

Down Syndrome The most common autosomal trisomy seen among humans is trisomy 21, also called Down syndrome (Fig. 24.9a). This syndrome

489

TABLE 24.1  Syndromes from Abnormal Chromosome Numbers Syndrome

Sex

Chromosomes

Chromosome Number

Patau

M or F

Trisomy 13

47

Edwards

M or F

Trisomy 18

47

Down

M or F

Trisomy 21

47

Poly-X

F

XXX (or XXXX)

47 or 48

Klinefelter

M

XXY (or XXXY)

47 or 48

Jacobs

M

XYY

47

Turner

F

X

45

is easily recognized by the following characteristics: short stature; an eyelid fold; a flat face; stubby fingers; a wide gap between the first and second toes; a large, fissured tongue; a round head; a palm crease (called the simian line); and mental impairment, which may range from mild to severe. Persons with Down syndrome usually have three copies of chromosome 21 because the egg had two copies instead of one. However, in around 23% of the cases the extra chromosome 21 came from the sperm. The chances of a woman having a Down syndrome child increase rapidly with age, starting at about age 40, and the reasons for this are still being determined. Although an older woman is more likely to have a Down syndrome child, most babies with Down syndrome are born to women younger than age 40 because this is the age group having the most babies. A karyotype can detect changes in chromosome

Gart gene

a. Down syndrome karyotype with an extra chromosome 21

b.

Figure 24.9  Trisomy 21.  a. Karyotype of an individual with Down syndrome shows an extra chromosome 21. b. More sophisticated technologies allow

investigators to pinpoint the location of specific genes associated with the syndrome. An extra copy of the Gart gene, which leads to a high level of purines in the blood, may account for the mental impairment seen in persons with Down syndrome.



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UNIT 5  Continuance of the Species

number such as occurs with Down syndrome. However, young women are not routinely encouraged to undergo the procedures necessary to get a sample of fetal cells (i.e., amniocentesis or chorionic villus sampling) because the risk of complications is greater than the risk of having a Down syndrome child. Fortunately, a test based on substances in maternal blood can help identify fetuses that may need to be karyotyped. The Health feature, “Preventing and Testing for Birth Defects,” (see section 22.3) describes the processes of amniocentesis and chorionic villus sampling. The genes that cause Down syndrome are located on the long arm of chromosome 21 (Fig. 24.9b), and extensive investigative work has been directed toward discovering the specific genes responsible for the characteristics of the syndrome. Thus far, investigators have discovered several genes that may account for various conditions seen in persons with Down syndrome. For example, they have located genes most likely responsible for the increased tendency toward leukemia, cataracts, and accelerated rate of aging. Researchers have also discovered that an extra copy of the Gart gene causes an increased level of purines in the blood, a finding associated with mental impairment. One day, it may be possible to control the expression of the Gart gene even before birth so that at least this symptom of Down syndrome does not appear.

Changes in Sex Chromosome Number An abnormal sex chromosome number is the result of inheriting too many or too few X or Y chromosomes. Figure 24.8 can be used to illustrate nondisjunction of the sex chromosomes ­during oogenesis, if you assume that the chromosomes shown represent X chromosomes. Nondisjunction during oogenesis or  spermatogenesis can result in gametes that have too few or too  many X or Y chromosomes. After fertilization, the syndromes associated with the sex chromosomes in Table 24.1 are possibilities. The chances of survival are greater when trisomy or monosomy involves the sex chromosomes. In normal XX females, one of the X chromosomes becomes a darkly stained mass of chromatin called a Barr body. A Barr body is an inactive X chromosome. Therefore, we now know that the cells of females function with a single X chromosome just as those of males do. This is most likely the reason that a zygote with one X chromosome (Turner syndrome) can survive. Then, too, all extra X chromosomes beyond a single one become Barr bodies, and this explains why poly-X females and XXY males are seen fairly frequently. An extra Y chromosome, called Jacobs syndrome, is tolerated in humans, most likely because the Y chromosome carries few genes.  A person with only one sex chromosome, an X (Turner syndrome), is a female, and a person with more than one X chromosome plus a Y (Klinefelter syndrome) is a male. This shows that in humans, the presence of a Y chromosome, not the number of X chromosomes, determines maleness. The SRY (sex-determining region of Y) gene, on the short arm of the Y chromosome, produces a hormone called testis-determining

factor, which plays a critical role in the development of male genitals.

Turner Syndrome Females with Turner syndrome have only one sex chromosome, an X. They are usually short and may have malformed features, such as a webbed neck, high palate, and small jaw. Many have congenital heart and kidney defects. Most have ovarian failure and do not undergo puberty or menstruate without sex hormone replacement therapy. However, pregnancy has been achieved through in vitro fertilization using donor eggs. Women with Turner syndrome have a normal range of intelligence but often have a nonverbal learning disability. They can lead very successful, fulfilling lives if they receive appropriate care.

Klinefelter Syndrome About 1 in 650 males is born with two X chromosomes and one Y chromosome. The symptoms of this condition (referred to as “47, XXY”) are often so subtle that only 25% are ever diagnosed, and those are usually not diagnosed until after age 15. Earlier diagnosis opens the possibility for educational accommodations and other interventions that can help mitigate common symptoms, which include speech and language delays. Those 47, XXY males who develop more severe symptoms as adults are referred to as having “Klinefelter syndrome.” All 47, XXY adults will require assisted reproduction in order to father children. Affected individuals commonly receive testosterone supplementation beginning at puberty.

Poly-X Females A poly-X female has more than two X chromosomes and thus extra Barr bodies in the nucleus. Females with three X chromosomes (sometimes called triplo-X) have no distinctive phenotype, aside from a tendency to be tall and thin. Although some have delayed motor and language development, most poly-X females are not mentally impaired. Some may have menstrual difficulties, but many menstruate regularly and are fertile. Their children usually have a normal karyotype. Females with more than three X chromosomes occur rarely. Unlike XXX females, XXXX females are tall and more likely to be mentally impaired. They exhibit various physical abnormalities, but may menstruate normally.

Jacobs Syndrome XYY males, who have Jacobs syndrome, can only result from nondisjunction during spermatogenesis, specifically due to a nondisjunction event during meiosis II of spermatogenesis. We know this because two Ys are present only during meiosis II in males. Affected males are usually taller than average, suffer from persistent acne, and tend to have speech and reading problems. Despite the extra Y chromosome, there is no difference in behavior between XYY males and XY males.



Chapter 24  Chromosomal Basis of Inheritance

Deletions and Duplications

Check Your Progress  24.3 1. Explain why the chances of survival are greater for a

trisomy or monosomy of the sex chromosomes than for autosomes. 2. Explain the nondisjunction event that would cause a Turner or Klinefelter syndrome individual.

24.4  Changes in Chromosome Structure Learning Outcomes Upon completion of this section, you should be able to 1. Describe a chromosomal deletion, duplication, translocation, and inversion. 2. Summarize the disorders caused by changes in chromosome structure.

Chromosomal mutations occur when breaks occur in the backbone of the DNA molecules within a chromosome (see section 25.1). Various environmental agents—radiation, certain organic chemicals, or even viruses—can cause chromosomes to break apart. Ordinarily, when breaks occur, the segments reunite to give the same sequence of genes. But their failure to reunite correctly can result in one of several types of mutations: deletion, duplication, translocation, or inversion. Chromosomal mutations can occur during meiosis, and if the offspring inherits the abnormal chromosome, a syndrome may develop.

a

a

b

b +

c

deletion

491

c

d

d

e

e

f

f

g

g

A deletion occurs when a single break causes a chromosome to lose an end piece or when two simultaneous breaks lead to the loss of an internal chromosomal segment. An individual who inherits a normal chromosome from one parent and a chromosome with a deletion from the other parent no longer has a pair of alleles for each trait, and a syndrome can result. Williams syndrome occurs when chromosome 7 loses a tiny end piece (Fig. 24.10). Children who have this syndrome look like pixies because they have turned-up noses, wide mouths, small chins and large ears. Although their academic skills are poor, they exhibit excellent verbal and musical abilities. The gene that governs the production of the protein elastin is missing, which affects the health of the cardiovascular system and causes their skin to age prematurely. Such individuals are very friendly but need an ordered life, perhaps because of the loss of a gene for a protein that is normally active in the brain. Cri du chat (cat’s cry) syndrome is seen when chromosome 5 is missing an end piece. The affected individual has a small head, is mentally impaired, and has facial abnormalities. Abnormal development of the glottis and larynx results in the most characteristic symptom—the infant’s cry resembles that of a cat. In a duplication, a chromosomal segment is repeated in the same chromosome or in a nonhomologous chromosome. In any case, the individual has more than two alleles for certain traits. An  inverted duplication is known to occur in chromosome 15. Inversion means that a segment joins in the direction opposite from normal. Children with this syndrome, called inv  dup 15 syndrome, have poor muscle tone, mental impairment, seizures, a curved spine, and autistic characteristics, including poor speech, hand flapping, and lack of eye contact (Fig. 24.11).

h lost

h a.

b.

Figure 24.10  Deletion.  a. When chromosome 7 loses an end piece, the result is Williams syndrome. b. These children, although unrelated, have similar appearance, health, and behavioral problems.



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UNIT 5  Continuance of the Species

a

a

b

b

c

c

both of the involved chromosomes has the normal amount of genetic material and is healthy, unless the chromosome exchange breaks an allele into two pieces. The person who inherits only one of the translocated chromosomes will have only one copy of certain alleles and three copies of certain other alleles. In 5% of cases, a translocation that occurred in a previous generation between chromosomes 21 and 14 is the cause of one type of Down syndrome. The affected person inherits two normal chromosomes 21 and an abnormal chromosome 14 that contains a segment of chromosome 21. In these cases, Down syndrome is not related to the age of the mother, but instead tends to run in the family of either the father or the mother. Figure 24.12 shows the phenotype of an individual with a translocation between chromosomes 2 and 20. Although they have the normal amount of genetic material, the rearrangement of the material produces the phenotypic changes associated with Alagille syndrome. People with this syndrome ordinarily have a deletion on chromosome 20. Therefore, it can be deduced that the translocation disrupted an allele on chromosome 20. The symptoms of ­Alagille syndrome range from mild to severe, so some people may not be aware they have the syndrome.

duplication d

d e

inversion

e e

f

d

g f g

a.

b.

Figure 24.11  Duplication.  a. When a piece of chromosome 15 is

duplicated and inverted, (b) a syndrome results (inv dup 15) that is characterized by poor muscle tone and autistic characteristics.

The Health feature, “Trinucleotide Repeat Expansion Disorders (TREDs),” (section 24.2) describes a type of duplication called a trinucleotide repeat expansion. In the case of Huntington disease, the trinucleotide repeat consists of the sequence CAG repeated over and over again. In the case of fragile X syndrome, the trinucleotide repeat is CGG. Persons with duplications over a particular number, which is specific for each disease, exhibit symptoms of the disease. In the case of Huntington disease, sufferers have over 40 copies of the CAG sequence within the huntingtin protein gene.

Inversion An inversion occurs when a segment of a chromosome is turned 180 degrees. You might think this is not a problem because the same genes are present, but the reverse sequence of alleles can lead to altered gene activity. Crossing-over between an inverted chromosome and the noninverted homologue can lead to recombinant chromosomes  that have both duplicated and deleted segments. This happens because alignment between the two homologues is  only possible when the inverted chromosome forms a loop (Fig. 24.13).

Translocation A translocation is the exchange of chromosomal segments between two nonhomologous chromosomes. A person who has

a c d

b

l m n

d

l m n

c translocation

a.

e

o

e

o

f

p

f

p

g

q

q

g

h

r

r

h b.

Figure 24.12  Translocation.  a. When chromosomes 2 and 20 exchange segments, (b) Alagille syndrome may result in both cyanosis (blue coloration) and clubbing (widening of the fingers) because the translocation disrupts an allele on chromosome 20.



Chapter 24  Chromosomal Basis of Inheritance

A

a

B

b

C

e

D

d

inverted segment

a

C

B

c b

c E f

G

g

A C

D

g f c

d e

E F

f g

F

Check Your Progress  24.4

region of crossing-over

B A

493

G

homologous chromosomes

d

1. Explain why a duplication on one chromosome is usually

associated with a deletion on the corresponding homologous chromosome. 2. Explain how it is possible for a person with a translocation or an inversion to be phenotypically normal.

D

e b a

E F G

duplication and deletion in both

Figure 24.13  Inversion.  Left: A segment of one homologue is inverted. Notice that in the inverted segment edc occurs instead of cde. Middle: The two homologues can pair only when the inverted sequence forms an internal loop. After crossing-over, a duplication and a deletion can occur. Right: The homologue on the left has AB and ba sequences and neither gf nor FG genes. The homologue on the right has gf and FG sequences and neither AB nor ba genes.

Conclusion Research on the causes of Down syndrome has indicated that 95% of Down syndrome cases are a result of nondisjunction of chromosome 21, resulting in three copies of this chromosome. However, we now also know that some cases are caused by translocations. Because of this research, scientists have been able to identify specific genes that are associated with the symptoms of the syndrome. For Down syndrome individuals, such as Brandon, these medical advances have ensured that he can lead a long, productive life.

MEDIA STUDY TOOLS http://connect.mheducation.com Maximize your study time with McGraw-Hill SmartBook®, the first adaptive textbook.

 

Animations

24.3 X-Inactivation

SUMMARIZE 24.1  Gene Linkage ■ All the genes on one chromosome form a linkage group, which is

broken only when crossing-over occurs. Genes that are linked tend to go together into the same gamete. ■ If crossing-over occurs, a dihybrid cross yields all possible phenotypes among the offspring, but the expected ratio is greatly changed because recombinant phenotypes are reduced in number.

24.2  Sex-Linked Inheritance In humans, there are 22 pairs of autosomes (1 to 22), and one pair of sex chromosomes (X and Y). Some traits are sex-linked, meaning that although they do not determine gender, they are carried on the sex chromosomes. Most of the alleles for these traits are carried on the X

  Tutorials 24.1 Linkage

chromosome (X-linked), whereas the Y does not bear alleles for those same traits. ■ Color blindness, Duchenne muscular dystrophy, fragile X syndrome,

and hemophilia are all X-linked recessive disorders.

24.3  Changes in Chromosome Number ■ Nondisjunction during meiosis can result in an abnormal number of

autosomes or sex chromosomes in the gametes. In a trisomy, individuals have an extra chromosome, whereas in a monosomy they are missing a chromosome. If nondisjunction occurs with the X chromosome, extra X chromosomes may be inactivated, forming Barr bodies. Monosomies and trisomies are often detected by karyotype analysis. ■ Down syndrome results when an individual inherits three copies of chromosome 21.



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UNIT 5  Continuance of the Species

■ Females who possess a single X have Turner syndrome, and those who

are XXX are poly-X females. Males with Klinefelter syndrome are XXY. Males who are XYY have Jacobs syndrome.

24.4  Changes in Chromosome Structure ■ Changes in chromosome structure also affect the phenotype. Chromo-

somal mutations include deletions, duplications, translocations, and inversions. ■ Translocations do not necessarily cause any difficulties if the person has inherited both translocated chromosomes. However, the translocation can disrupt a particular allele, and then a syndrome will follow. ■ An inversion can lead to chromosomes that have a deletion and a duplication when the inverted piece loops back to align with the noninverted homologue, and crossing-over follows between the nonsister chromatids.

ASSESS Testing Yourself Choose the best answer for each question.

24.1  Gene Linkage 1. All the genes on one chromosome are said to form a a. chromosomal group. b. recombination group. c. linkage group. d. crossing-over group. 2. If alleles R and s are linked on one chromosome and r and S are linked on the homologous chromosome, what gametes will be produced? (Assume that no crossing-over occurs.) a. RS, Rs, rS, rs b. RS, rs c. Rs, rS d. R, S, r, s

24.2  Sex-Linked Inheritance 3. The following pedigree pertains to color blindness. Using the letter B for the normal allele, what is the genotype of the individual with the asterisk?

*

Key = affected = unaffected

a. Xb Xb b. XB Xb c. XB XB

4. Assume that two parents with normal vision have a son who has red-green color blindness. Which parent is responsible for the son’s color blindness? a. the mother b. the father c. either parent d. None of these are correct.

24.3  Changes in Chromosome Number For questions 5–9, match the chromosome disorder to its description in the key. Key: a. female with undeveloped ovaries and uterus, unable to undergo puberty, normal intelligence, can live normally with hormone replacement b. XXY male, can inherit more than two X chromosomes c. male or female, mentally impaired, short stature, flat face, stubby fingers, large tongue, simian palm crease d. XXX or XXXX female e. caused by nondisjunction during spermatogenesis 5. Klinefelter syndrome 6. Poly-X female 7. Down syndrome 8. Turner syndrome 9. Jacobs syndrome

24.4  Changes in Chromosome Structure For questions 10–13, match the chromosomal mutation to its description in the key. Key: a. turned-up nose, wide mouth, small chin, large ears, poor academic skills, excellent verbal and musical abilities, prematurely aging cardiovascular system b. deletion in chromosome 5 c. poor muscle tone, mental impairment, seizures, curved spine, autistic characteristics, poor speech, hand flapping, lack of eye contact d. translocation between chromosomes 2 and 20 10. Alagille syndrome 11. Inv dup 15 syndrome 12. Williams syndrome 13. Cri du chat syndrome



ENGAGE BioNOW Want to know  how this science is relevant to your life? Check out the BioNow video below: ■ Glowing Fish Genetics

Thinking Critically 1. Although most men with Klinefelter syndrome are infertile, some are able to father children. It was found that most fertile individuals with Klinefelter syndrome exhibit mosaicism, in which some cells are normal (46, XY) but others contain the extra chromosome (47, XXY). How might this mosaicism come about? What effects might result? Why might expectant parents want to undergo fetal genetic testing if they know that, regardless of the findings, they plan to have the baby?

Chapter 24  Chromosomal Basis of Inheritance

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2. Why do you think there are no viable trisomies of chromosome 1? 3. Why can a person carrying a translocation be normal except for the inability to have children? 4. Both the husband and wife have normal vision. The wife gives birth to a color-blind daughter. Is it more likely the father had normal vision or is color blind? What does this lead you to deduce about the girl’s parentage?

PHOTO CREDITS Opener: © moodboard/Alamy; 24.5(left, right): © Dr. Rabi Tawil, Director, Neuromuscular Pathology Laboratory, University of Rochester Medical Center; 24.5(center): © Muscular Dystrophy Association; 24Aa(both): © R. Simensen and R. Curtis Rogers, “Fragile X Syndrome,” American Family Physician 39(5): 186, May 1989; 24Ab: © Monica Schroeder/ Science Source; 24.7, 24.9a: © NRI/SPL/Science Source; 24.10b: © The Williams Syndrome Association; 24.11b: © Kathy Wise; 24.12b: © Wellcome Image Library/ Custom Medical Stock Photo.



25

DNA Structure and Gene Expression CHAPTER OUTLINE 25.1  DNA Structure 25.2 DNA Replication 25.3 Gene Expression 25.4 Control of Gene Expression 25.5 Gene Mutations and Cancer BEFORE YOU BEGIN Before beginning this chapter, take a few moments to review the following discussions: Section 2.8  What are the three components of a nucleotide? Section 2.8  How does the structure of DNA differ from that of RNA? Section 23.1  What is the relationship between the genotype and phenotype?

CASE STUDY Xeroderma Pigmentosum Sometimes they are called “children of the night” because they cannot play outside in the sunlight. They even have a summer camp named Camp ­Sundown in which they do all the things that other children do at summer camp such as hiking, horseback riding, and swimming, except they do it after sundown. Whenever these children are outside during daylight, they wear protective clothing and eyewear, use sunblock, and travel in cars with tinted windows. These are children that suffer from a genetic disease called xeroderma pigmentosum (XP). This is a genetic disease in which the enzymes that are needed to repair DNA damage due to ultraviolet (UV) light are defective. Therefore, those who suffer with XP cannot be exposed to UV light.  XP is very rare, only about 1 in a million individuals in the United States have XP, although those with Japanese descent have a higher incidence. There is no cure. DNA damage cannot be repaired and mutations accumulate throughout the lifetime of the patient. These individuals have a 1,000-fold higher risk of skin cancer than those with normal DNA repair enzymes. Exposure to UV light results in blistering and freckling, and individuals suffer from premature aging of the skin along with eye tumors. More severe cases of XP may result in progressive neurological complications such as intellectual disability and hearing loss. Fewer than 40% of individuals with XP survive beyond the age of 20, although those with milder cases may survive into middle age. The most common cause of death is skin cancer, either squamous cell carcinoma or metastatic melanoma.  This chapter describes the structure of DNA and how mutations could affect the function of RNA and proteins. It also describes the progression of cancer from a single mutation to a metastatic tumor. As you read through the chapter, think about the following questions:

1. How is the information in the DNA interpreted into a functional protein, such as an enzyme?

2. How might a mutation in the DNA result in the formation of cancer?

496



Chapter 25  DNA Structure and Gene Expression

25.1  DNA Structure Learning Outcomes Upon completion of this section, you should be able to 1. Summarize the experiments that enabled scientists to determine that DNA made up the genetic material. 2. Describe the structure of a DNA molecule.

The middle of the twentieth century was an exciting period of scientific discovery. On one hand, geneticists were busy determining that DNA (deoxyribonucleic acid) is the genetic material of life. On the other hand, biochemists were in a frantic race to describe the structure of DNA. The classic experiments performed during this era set the stage for an explosion in our knowledge of modern molecular biology. When researchers began their work, they knew that the genetic material must be (1) able to store information that pertains to the development, structure, and metabolic activities of the cell or organism; and (2) stable so that it can be replicated with high accuracy during cell division and be transmitted from generation to generation. In this section, we will explore how the structure of a DNA molecule satisfies both of these requirements.

The Nature of the Genetic Material During the late 1920s, the bacteriologist Frederick Griffith was attempting to develop a vaccine against a form of bacteria (Streptococcus pneumoniae) that causes pneumonia in mammals. In 1931, he performed a classic experiment with the bacterium. He noticed that when these bacteria are grown on culture plates, some, called S strain bacteria, produce shiny, smooth colonies,

497

and others, called R strain bacteria, produce colonies that have a rough appearance. Under the microscope, S strain bacteria have a capsule (mucous coat) that makes them smooth, but R strain bacteria do not. When Griffith injected mice with the S strain of bacteria, the mice died (Fig. 25.1a), and when he injected mice with the R strain, the mice did not die (Fig. 25.1b). In an effort to determine whether the capsule alone was responsible for the virulence (ability to kill) of the S strain bacteria, he injected mice with heat-killed S strain bacteria. The mice did not die (Fig. 25.1c). Finally, Griffith injected the mice with a mixture of heatkilled S strain and live R strain bacteria. Most unexpectedly, the mice died—and living S strain bacteria were recovered from the bodies! Griffith concluded that some substance necessary for the bacteria to produce a capsule and be virulent must have passed from the dead S strain bacteria to the living R strain bacteria, thus transforming the R strain bacteria (Fig. 25.1d). This change in the phenotype of the R strain bacteria must be due to a corresponding change in their genotype. Thus, the transforming substance was potentially the genetic material. Reasoning such as this prompted investigators at the time to begin looking for the transforming substance to determine the chemical nature of the genetic material. By the 1940s, scientists recognized that genes are on chromosomes and that chromosomes contain both proteins and nucleic acids. However, there was some debate about whether protein or DNA was the genetic material. Many thought that the protein component of chromosomes must be the genetic material because proteins contain up to 20 different amino acids that can be sequenced in any particular way. On the other hand, nucleic acids—DNA and RNA—contain only four types of nucleotides as basic building blocks. Some argued that DNA did not have enough variability to be able to store information and be the genetic material.

capsule

+ Injected live R strain has no capsule and mice do not die.

Injected live S strain has capsule and causes mice to die.

a.

b.

Injected heatkilled S strain does not cause mice to die.

c.

Injected heat-killed S strain plus live R strain causes mice to die.

Live S strain is withdrawn from dead mice.

d.

Figure 25.1  Griffith’s experiment.  a. Encapsulated S strain is virulent and kills mice. b. Nonencapsulated R strain is not virulent and does not kill

mice. c. Heat-killed S strain bacteria do not kill mice. d. If heat-killed S strain and R strain are both injected into mice, they die because the R strain bacteria have been transformed into the virulent S strain.



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UNIT 5  Continuance of the Species

In 1944, after 16 years of research, Oswald Avery and his coinvestigators, Colin MacLeod and Maclyn McCarty, published a paper demonstrating that the transforming substance that allows S. pneumoniae to produce a capsule and be virulent is DNA. This meant that DNA is the genetic material. Here is what they found out: 1. DNA from S strain bacteria causes R strain bacteria to be transformed so that they can produce a capsule and be virulent. 2. The addition of DNase, an enzyme that digests DNA, prevents transformation from occurring. This supports the hypothesis that DNA is the genetic material. 3. The molecular weight of the transforming substance is large. This suggests the possibility of genetic variability. 4. The addition of enzymes that degrade proteins has no effect on the transforming substance nor does RNase, an enzyme that digests RNA. This shows that neither protein nor RNA is the genetic material.

DNA labeled with 32P

These experiments showed that DNA is the transforming substance and, therefore, the genetic material. Although some scientists remained skeptical, many felt that the evidence for DNA being the genetic material was overwhelming. An experiment by Alfred Hershey and Martha Chase in the early 1950s helped to firmly establish DNA as the genetic material. Hershey and Chase used a virus called a T phage, composed of radioactively labeled DNA and capsid coat proteins, to infect Escherichia coli bacteria (Fig 25.2). They discovered that the radioactive tracers for DNA, but not protein, ended up inside the bacterial cells, causing them to become transformed. As only the genetic material could have caused this transformation, Hershey and Chase determined that DNA must be the genetic material.

Structure of DNA The structure of DNA was determined by James Watson and Francis Crick in the early 1950s. The data they used and how they

capsid Viruses in liquid are not radioactive.

virus

Bacteria in sediment are radioactive.

bacterium centrifuge 1. When bacteria and viruses are cultured together, radioactive viral DNA enters bacteria.

2. Agitation in blender dislodges viruses. Radioactivity stays inside bacteria.

3. Centrifugation separates viruses from bacteria and allows investigator to detect location of radioactivity.

a. Viral DNA is labeled (yellow). capsid labeled with 35S Viruses in liquid are radioactive. Bacteria in sediment are not radioactive. centrifuge 1. When bacteria and viruses are cultured together, radioactive viral capsids stay outside bacteria.

2. Agitation in blender dislodges viruses. Radioactivity stays outside bacteria.

3. Centrifugation separates viruses from bacteria and allows investigator to detect location of radioactivity.

b. Viral capsid is labeled (yellow).

Figure 25.2  Hershey-Chase experiments.  These experiments concluded that viral DNA, not protein, was responsible for directing the production

of new viruses.



499

Chapter 25  DNA Structure and Gene Expression

interpreted the data to deduce DNA’s structure are reviewed in the Scientific Inquiry feature, “Finding the Structure of DNA.” DNA is a chain of nucleotides. Each nucleotide is a complex of three subunits—phosphoric acid (phosphate), a pentose sugar (deoxyribose), and a nitrogen-containing base. There are four possible bases: two are purines with a double ring, and two are pyrimidines with a single ring. Adenine (A) and guanine (G) are purines; thymine (T) and cytosine (C) are pyrimidines. A DNA polynucleotide strand has a backbone made up of alternating phosphate and sugar molecules. The bases are attached to the sugar but project to one side. DNA has two such strands, and the two strands twist about one another in the form of a double helix (Fig. 25.3a). The strands are held together by hydrogen bonding between the bases: A always pairs with T by forming two hydrogen bonds, and G always pairs with C by forming three hydrogen bonds. Notice that a purine is always bonded to a pyrimidine. This is called complementary base pairing. When the DNA helix unwinds, it resembles a ladder (Fig. 25.3b). The sides of the

Check Your Progress  25.1 1. Summarize the significance of the Griffith and Avery experiments.

2. Explain how, at the completion of the Hershey-Chase experiment, the results suggested that DNA was the genetic material. 3. Describe the structure of the DNA molecule.

3′ end T

C

G

S

S

A S

4′ P

C

3′ end

P

A

1′

S 3′

OH

T

5′

S

1′

2′

G P

T

P a. Double helix

b. Ladder structure

5′ O

C 4′

S

3′ end

4′

S

P

S

S

3′

5′

C A

2′

OH

S P

A

T

5′ end P

T

P

purine base

A

P

pyrimidine base

P

S

S

P

phosphate

P

5′ end P

ladder are the sugar-phosphate backbones, and the rungs of the ladder are the complementary paired bases. The two DNA strands are antiparallel, meaning that they are oriented in opposite directions, which you can verify by noticing that the sugar molecules are oriented differently. The carbon atoms in a sugar molecule are numbered, and the fifth carbon atom (5′) is uppermost in the strand on the left, while the third carbon atom (3′) is uppermost in the strand on the right (Fig. 25.3c).

C

S C 3′

C 2′

1′

deoxyribose

5′ end c. One pair of bases

Figure 25.3  Overview of DNA structure.  a. DNA double helix. b. Unwinding the helix reveals a ladder configuration in which the

uprights are composed of sugar and phosphate molecules and the rungs are complementary bases. The bases in DNA pair in such a way that the sugarphosphate backbones are oriented in different directions. c. Notice that 3′ and 5′ are part of the system for numbering the carbon atoms that make up the sugar.



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UNIT 5  Continuance of the Species

SCIENCE IN YOUR LIFE  ►

SCIENTIFIC INQUIRY

Finding the Structure of DNA In 1953, James Watson, an American biologist, and Francis Crick, a British physicist, set out not only to determine the structure of DNA, but also to build a model that would explain how DNA, the genetic material, can vary from species to species and even from individual to individual. In the process, they also proposed how DNA replicates (makes a copy of itself) so that daughter cells receive an identical copy. The bits and pieces of data available to Watson and Crick were like puzzle pieces they had to fit together. This is what they knew from the research of others: 1. DNA is a polymer of nucleotides, each one having a phosphate group, the sugar deoxyribose, and a nitrogen-containing base. There are four types of nucleotides because there are four different bases: adenine (A) and guanine (G) are purines, while cytosine (C) and thymine (T) are pyrimidines. 2. A chemist, Erwin Chargaff, had determined in the late 1940s that regardless of the species under consideration, the number of purines in DNA always equals the number of pyrimidines. Further, the amount of adenine equals the amount of thymine (A = T), and the amount of guanine equals the amount of cytosine

(G  =  C). These findings came to be known as Chargaff’s rules. 3. Rosalind Franklin (Fig. 25Aa), working with Maurice Wilkins at King’s College, London, had just prepared an X-ray diffraction photograph of DNA (Fig. 25Ab, c). It showed that DNA is a double helix of constant diameter and that the bases are regularly stacked on top of one another. Using these data, Watson and Crick deduced that DNA has a twisted, ladderlike structure. The sugar-phosphate molecules make up the sides of the ladder, and the bases make up the rungs. The double helices of the DNA molecule are antiparallel, meaning that they are oriented in opposite directions. Further, they determined that if A is normally hydrogen-bonded with T, and G is normally hydrogen-bonded with C (Chargaff’s rules), then the rungs always have a constant width, consistent with the X-ray photograph. Watson and Crick built an actual model of DNA out of wire and tin. This double-helix model does indeed allow for differences in DNA structure between species because the base pairs can be in any order. Also, the model suggests that complementary base pairing plays a role in the replication of DNA. As Watson and Crick pointed out in their original

Figure 25A  X-ray

diffraction of DNA.  a. Rosalind Franklin, 1920–1958. b. The X-ray diffraction system used by Franklin. c. The diffraction pattern of DNA produced by Rosalind Franklin. The crossed (X) pattern in the center told investigators that DNA is a helix, and the dark portions at the top and the bottom told them that some feature is repeated over and over. Watson and Crick determined that this feature was the result of the hydrogen-bonded bases.

Rosalind Franklin diffraction pattern

a.

diffracted X-rays X-ray beam

crystalline DNA b.

c.

paper, “It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.” When the Nobel Prize for the discovery of the double helix was awarded in 1962, the honor went to James Watson, Francis Crick, and Maurice Wilkins. Rosalind Franklin was not listed as one of the recipients. Tragically, Franklin developed ovarian cancer in 1956, and she died in 1958 at the age of 37. Franklin had not been nominated for the award, and according to the rules at that time, was ineligible for the Nobel Prize.

Questions to Consider 1. Based on Chargaff’s rules, if a segment of DNA is composed of 20% adenine (A) bases, what is the percentage of guanine (G)? 2. Watson and Crick’s discovery of DNA is clearly one of the most important biological discoveries in the last century. What advances in medicine and science can you think of that are built on knowing the structure of DNA? 3. Describe why the structure of DNA led Watson and Crick to point out “a possible copying mechanism for the genetic material.”



Chapter 25  DNA Structure and Gene Expression

2. New complementary DNA nucleotides fit into place by the process of complementary base pairing. These are positioned and joined by the enzyme DNA polymerase. The DNA polymerase uses each original strand as a template for the formation of a complementary new strand. 3. Because the strands of DNA are oriented in an antiparallel configuration, and the DNA polymerase may add new nucleotides only to one end of the chain, DNA synthesis occurs in

25.2  DNA Replication Learning Outcomes Upon completion of this section, you should be able to 1. Explain why DNA replication is semiconservative. 2. Summarize the events that occur during the process of DNA replication.

When the body grows or heals itself, cells divide. Each new cell requires an exact copy of the DNA contained in the chromosomes. The process of copying one DNA double helix into two identical double helices is called DNA replication. During the S phase of interphase during mitosis when DNA is replicated (see section 5.1), the double-stranded structure of DNA allows each original strand to serve as a template for the formation of a complementary new strand (Fig. 25.4a). As a result, DNA replication is termed semiconservative because a new double helix has one conserved old strand and one new strand (Fig. 25.4b). Replication results in two DNA helices that are identical to each other and to the original molecule. At the molecular level, several enzymes and proteins participate in the synthesis of the new DNA strands. This process is summarized in Figure 25.5: G 1. The enzyme DNA helicase unwinds and “unzips” the double-stranded DNA by breaking the weak hydrogen bonds between the paired bases.

5′ T C G A T

5′

Replication fork

A T

A

T

C

G

A

T C

G C

3′

C

T A

3′ C G A T C G T A G C G C T A A T C G A T G C G C T A

A T C G C G

C

5′

T A 5′ T A C G

Incoming nucleotides

T A

T A

G C

C

G

C G 3′ T A Original Newly (template) synthesized strand daughter strand a. The mechanism of DNA replication

C

G A

5′ Original (template) strand

3′

5′

3′ C G A T C G T A G C G C T A A T C G A T G C G C T A

C G A T C G T A G C G C T A A T C G A T G C G C T A

A T

A T

replication.  a. After the DNA double helix

unwinds, each parental strand serves as a template for the formation of the new daughter strands. Complementary free nucleotides hydrogen bond to a matching base (e.g., A with T; G with C) in each parental strand and are joined to form a complete daughter strand. b. Two helices, each with a daughter and parental strand (semi-conservative), are produced during replication.

3′

C G

T A C G

Figure 25.4  Overview of DNA

501

3′ 5′ 3′ b. The products of replication

5′

Figure 25.5  Molecular mechanisms of DNA replication. 

A general diagram of the process of DNA replication, showing the major enzymes and proteins that are involved in the process. template strand

leading new strand

DNA polymerase helicase at replication fork

lagging strand

template strand

Okazaki fragment

parental DNA helix DNA ligase

DNA polymerase



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UNIT 5  Continuance of the Species

opposite directions. The leading strand follows the helicase enzyme, while synthesis on the lagging strand results in the formation of short segments of DNA called Okazaki fragments. 4. To complete replication, the enzyme DNA ligase connects the Okazaki fragments and seals any breaks in the sugarphosphate backbone. 5. The two double helix molecules are identical to each other and to the original DNA molecule.

coding strand 5′

DNA

3′ A

G C

G

A

C

C

C

T

C

C

T

G G G

G

G

3′

5′ template strand

transcription in nucleus

Chemotherapeutic drugs for cancer treatment stop replication and, therefore, cell division. Some chemotherapeutic drugs are analogs that have a similar, but not identical, structure to one of the four nucleotides in DNA. When these are mistakenly used by the cancer cells to synthesize DNA, replication stops, and the cancer cells die off.

5′ mRNA

translation at ribosome

3′ A

G C

codon 1

Check Your Progress  25.2 1. Explain why DNA replication is said to be semiconservative. 2. Summarize the sequence of events that occur during DNA

polypeptide

replication.

N

C R1

G

A

C

codon 2

O

O

C N C

C

R2

Serine Aspartic acid

25.3  Gene Expression Learning Outcomes Upon completion of this section, you should be able to 1. Describe the roles of RNA molecules in gene expression. 2. Summarize the sequence of events that occurs during gene expression. 3. Determine the sequence of amino acids in a peptide, given the messenger RNA sequence. 4. Explain the purpose of mRNA processing.

The process of using the information within a gene to synthesize a protein is called gene expression. Gene expression relies on the participation of several different forms of RNA (ribonucleic acid) molecules (see section 2.8), most important of which are messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA). We will take a closer look at each one of these molecules in the sections below. Overall, gene expression requires two processes called transcription and translation (Figure 25.6). In eukaryotes, transcription takes place in the nucleus and translation takes place in the cytoplasm. During transcription, a portion of DNA serves as a template for mRNA formation. During translation, the sequence of mRNA bases (which are complementary to those in the template DNA) determines the sequence of amino acids in a polypeptide. So, in effect, genetic information lies in the sequence of the bases in DNA, which through mRNA determines the sequence of amino acids in a protein. Transfer RNA assists mRNA during protein synthesis by bringing amino acids to the ribosomes. Proteins differ from one another by the sequence of their amino acids, and proteins determine the structure and function of cells and the phenotype of the organism.

C

C

C

C

codon 3

O

N

C

C

R3 Proline

Figure 25.6  Overview of gene expression.  One strand of DNA acts as a template for mRNA synthesis, and the sequence of bases in mRNA determines the sequence of amino acids in a polypeptide.

Transcription During transcription, a segment of the DNA called a gene serves as a template for the production of a RNA molecule. Historically, molecular genetics considered a gene to be a nucleic acid sequence that codes for the sequence of amino acids in a protein. We now know that not all genes contain instructions for protein formation. Some genes include instructions for the formation of DNA molecules, such as mRNA, tRNA, and rRNA. We also know that protein-coding regions can be interrupted by regions that do not code for a protein. Thus, the definition of a gene is changing. Mark Gerstein (a bioinformatic professor at Yale University) has suggested a gene be defined as follows: “A gene is a genomic sequence (either DNA or RNA) directly encoding functional products, either RNA or protein.”1 Although all three classes of RNA are formed by transcription, we will focus on transcription to form messenger RNA (mRNA), the first step in protein synthesis.

Messenger RNA The purpose of messenger RNA (mRNA) is to carry genetic information from the DNA to the ribosomes for protein synthesis. Messenger RNA is formed by the process of transcription which, in eukaryotes, occurs in the nucleus. Transcription begins when the enzyme RNA polymerase binds tightly to a promoter, a region of DNA that contains a special sequence of nucleotides. This enzyme opens up the DNA helix just in front of it so that Gerstein, M. B., Bruce, C., and Rozowsky, J. S. et al. “What is a gene. post-ENCODE? History and updated definition,” Genomic Research 17:669–681 (2007). 1

39

59

C C

terminator

T

C

C

A

A

A

C G

exon

G

5′

C

direction of polymerase movement

T

C

G

G

C

RNA polymerase

A

U C

C

T

intron

promoter

intron

cap

A

59

T

intron

3′ poly-A tail

spliceosome exon

exon 3′

cap

poly-A tail pre-mRNA splicing

59

39

exon

5′ G

T

A

exon

exon

C

A

T

3′

intron

exon

5′

A C

exon

T

mRNA transcript

C

exon

A

DNA template strand

G

T

exon

pre-mRNA

U

T

39

G

template strand

G

A

exon intron intron transcription

A

C

G

coding strand T

exon DNA

G

T

A

U

503

Chapter 25  DNA Structure and Gene Expression

to RNA processing

Figure 25.7  Transcription of DNA to form mRNA.  During transcription, complementary RNA is made from a DNA template. At the point of attachment of RNA polymerase, the DNA helix unwinds and unzips, and complementary RNA nucleotides are joined together. After RNA polymerase has passed by, the DNA strands rejoin and the mRNA transcript is released.

intron RNA mRNA 5′

3′ cap

complementary base pairing can occur in the same way as in DNA replication. Then, RNA polymerase inserts the RNA nucleotides, and an mRNA molecule results. When mRNA forms, it has a sequence of bases complementary to that of the DNA; wherever A, T, G, or C are present in the DNA template, U, A, C, or G, respectively, are incorporated into the mRNA molecule (Fig. 25.7). Now, mRNA is a faithful copy of the sequence of bases in DNA.

poly-A tail nuclear pore in nuclear envelope

nucleus

cytoplasm

Processing of mRNA  In eukaryotic cells, the mRNA molecule that was produced by transcription, called pre-mRNA, is typically processed before exiting the nucleus and becoming a mature mRNA molecule. Not all of the DNA in the pre-mRNA molecule contains inforFigure 25.8  mRNA processing.  During processing, a cap and tail mation that is needed for the final product. The information that is are added to the pre-mRNA molecule, and the introns are removed so that needed to produce the functional product, such as a protein, is cononly exons remain. tained within DNA sequences called exons. The regions between the exons are called introns, or intergenic sequences (Fig. 25.8). While the DNA within the intron does not contain information that

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UNIT 5  Continuance of the Species

contributes to the final structure of the protein, we now know that these regions often have important regulatory functions. During the processing of the pre-mRNA molecule: 1. The 5′ end of the mRNA is modified by the addition of a cap, composed of an altered guanine nucleotide. 2. The 3′ end is modified by the addition of a poly-A tail, a series of adenosine nucleotides.  3. The introns are removed, and the exons are joined to form a mature mRNA molecule consisting of continuous exons. 

Second Base

First Base

U

Third Base

U

C

A

G

UUU phenylalanine UUC phenylalanine UUA leucine

UCU serine

UGU cysteine

U

UGC cysteine

C

UUG leucine CUU leucine

UCG serine CCU proline

UAU tyrosine UAC tyrosine UAA stop UAG stop

CUC leucine CUA leucine

CCC proline CCA proline

CUG leucine

UCC serine UCA serine

CAU histidine

UGA stop UGG tryptophan CGU arginine

CAC histidine

CGC arginine

C

CAA glutamine

CGA arginine

A

CCG proline

CAG glutamine

CGG arginine

G

AUU isoleucine AUC isoleucine

ACU threonine ACC threonine

AAU asparagine AAC asparagine

AGU serine AGC serine

U

AUA isoleucine

ACA threonine

AAA lysine

AGA arginine

A

AUG (start) methionine

ACG threonine

AAG lysine

AGG arginine

G

The Genetic Code

GUU valine

GCU alanine

GAU aspartic acid

GGU glycine

U

The sequence of bases in DNA is transcribed into mRNA, which ultimately codes for a particular sequence of amino acids to form a polypeptide. Can four mRNA bases (A, C, G, U) provide enough combinations to code for 20 amino acids? If only one base stood for an amino acid (i.e., a “singlet code”), then only four amino acids would be possible. If two bases stood for one amino acid, there would only be 16 possible combinations (4 × 4). If the code is a triplet, then there are 64 possible triplets of the four bases (4 × 4 × 4). Each triplet of nucleotides is called a codon. The code is also degenerate, meaning that most amino acids are coded for by more than one codon (Fig. 25.9). For example, leucine has six codons and serine has four codons. This degeneracy offers some protection against possibly harmful mutations that change the sequence of bases. Of the 64 codons, 61 code for amino acids, whereas the remaining three are stop codons (UAA, UGA, UAG), codons that do not code for amino acids but instead signal polypeptide termination. The genetic code is just about universal in all living organisms. This means that a codon in a fruit fly codes for the same amino acid as in a bird, a fern, or a human. There are some minor variations in some of the oldest forms of prokaryotes (see section 28.3). The universal nature of the genetic code suggests that it dates back to the very first organisms on Earth and that all living organisms have a common evolutionary history.

GUC valine GUA valine GUG valine

GCC alanine GCA alanine GCG alanine

GAC aspartic acid GAA glutamic acid GAG glutamic acid

GGC glycine GGA glycine GGG glycine

C

Ordinarily, processing brings together all the exons of a gene. In some instances, cells use only certain exons to form a mature RNA transcript. The result can be a different protein product in each cell. Alternate mRNA splicing accounts for the ability of white blood cells to produce a specific antibody for each type of bacteria and virus we encounter on a daily basis (see section 13.3)

C

Translation Translation is the second process by which gene expression leads to protein synthesis. Translation occurs at the ribosome and requires several enzymes, and several different types of RNA molecules, including mRNA, tRNA, and rRNA.

A

G

G U

C

A G

Figure 25.9  The genetic code.  Notice that in this chart, each of the codons (in boxes) is composed of three letters representing the first base, second base, and third base of a codon on the mRNA molecule. For example, find the box where C for the first base and A for the second base intersect. You will see that U, C, A, or G can be the third base. The bases CAU and CAC are codons for histidine.

arginine amino acid

amino acid

anticodon

Transfer RNA Transfer RNA (tRNA) molecules bring amino acids to the ribosomes, the site of protein synthesis. Each tRNA molecule is a ­single-stranded polynucleotide that doubles back on itself such that complementary base pairing creates a bootlike shape. On one end is an amino acid, and on the other end is an anticodon, a triplet of three bases complementary to a codon of mRNA (Fig. 25.10).

A

a.

G C U

anticodon b.

Figure 25.10  Transfer RNA: amino acid carrier.  a. A tRNA is a polynucleotide that folds into a bootlike shape because of complementary base pairing. At one end of the molecule is its specific anticodon—in this case, GCU (which hybridizes to the codon CGA). At the other end, an amino acid attaches that corresponds to this anticodon—in this case, arginine. b. tRNA is represented like this in the illustrations in this text. 



Chapter 25  DNA Structure and Gene Expression

Although there are 64 possible codons, there are only 40 different tRNA molecules. This is because of the wobble effect, which states that for some tRNAs, the third nucleotide in the mRNA codon may vary. This is believed to provide additional degeneracy to the genetic code, and help protect against mutations that may alter the amino acid sequence of a protein. When a tRNA–amino acid complex comes to the ribosome, its anticodon pairs with an mRNA codon. For example, if the codon is CGG, what is the anticodon, and what amino acid will be attached to the tRNA molecule? Based on Figure 25.9, the answer to this question is as follows: Codon (mRNA)

Anticodon (tRNA)

Amino Acid (protein)

CGG

GCC

Arginine

endoplasmic reticulum. Ribosomes are composed of many proteins and several ribosomal RNAs (rRNAs). In eukaryotic cells, rRNA is produced in a nucleolus within the nucleus. Then the rRNA joins with proteins manufactured in and imported from the cytoplasm to form two ribosomal subunits, one large and one small (Fig. 25.11a). The subunits leave the nucleus and join together in the cytoplasm to form a ribosome just as protein synthesis begins. A ribosome has a binding site for mRNA as well as binding sites for three tRNA molecules (Fig. 25.11b). These binding sites facilitate complementary base pairing between tRNA anticodons and mRNA codons. As the ribosome moves down the mRNA molecule, new tRNAs arrive, and a polypeptide forms and grows longer (Fig. 25.11c). Translation terminates once the polypeptide is fully formed and an mRNA stop codon is reached. The ribosome then dissociates into its two subunits and falls off the mRNA molecule. As soon as the initial portion of mRNA has been translated by one ribosome and the ribosome has begun to move down the mRNA, another ribosome attaches to the same mRNA. Therefore, several ribosomes are often attached to and translating a single mRNA, thus forming several copies of a polypeptide simultaneously. The entire complex is called a polyribosome (Fig. 25.11d).

The order of the codons of the mRNA determines the order that tRNA–amino acids come to a ribosome and, therefore, the final sequence of amino acids in a protein. During translation, tRNAs bring amino acids to the ribosomes in the order dictated by the base sequence of mRNA. 

Ribosomes and Ribosomal RNA Ribosomes are the site of translation. They are small structural bodies that may be found both in the cytoplasm and on the rough

a. Structure of a ribosome

505

b. Binding sites of ribosome

large subunit 39

59

mRNA

E

P

A tRNA binding sites

small subunit outgoing tRNA

polypeptide

incoming tRNA

mRNA

c. Function of ribosomes

d. Polyribosome

Figure 25.11  Polyribosome structure and function.  a. Structure of a ribosome. b. Internal view of a ribosome showing the tRNA binding sites. c. Several ribosomes, collectively called a polyribosome, move along an mRNA at one time. Therefore, several polypeptides can be made at the same time. d. Electron micrograph of a polyribosome.



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UNIT 5  Continuance of the Species

joins to the small subunit (Fig. 25.12). Although similar in many ways, initiation in eukaryotes is much more complicated and will not be discussed here. A ribosome has three binding sites for tRNAs. One of these is called the P (for peptide) site, and the other is the A (for amino acid) site. The tRNA exits at the E site. The initiator tRNA happens to be capable of binding to the P site, even though it carries only the amino acid methionine (see Fig. 25.9). The A site is for tRNA carrying the next amino acid.

amino acid methionine Met

initiator tRNA U A A U C G

5′

E site P site A site

mRNA

3′

Met

Elongation

small ribosomal subunit

large ribosomal subunit

U A C A U G

5′

A small ribosomal subunit binds to mRNA; an initiator tRNA pairs with the mRNA start codon AUG.

start codon

The large ribosomal subunit completes the ribosome. Initiator tRNA occupies the P site. The A site is ready for the next tRNA.

Initiation

Figure 25.12  Initiation.  During initiation, participants in the translation process assemble as shown. The start codon, AUG, also codes for the first amino acid, methionine.

Translation Requires Three Steps During translation, the codons of an mRNA base pair with the anticodons of tRNA molecules carrying specific amino acids. The order of the codons determines the order of the tRNA molecules at a ribosome and the sequence of amino acids in a polypeptide. The process of translation must be extremely orderly so that the amino acids of a polypeptide are sequenced correctly. Protein synthesis involves three steps: initiation, elongation, and termination. Enzymes are required for each of the three steps to function properly. The first two steps, initiation and elongation, require energy. In our discussion of the process we will use an example from a prokaryotic cell, such as a bacteria.

Initiation Initiation is the step that brings all the components of the translation machinery together. Proteins called initiation factors are required to assemble the small ribosomal subunit, mRNA, initiator tRNA, and the large ribosomal subunit for the start of protein synthesis. Initiation is shown in Figure 25.12. To begin, a small ribosomal subunit attaches to the mRNA in the vicinity of the start codon (AUG). The first or initiator tRNA pairs with this codon because its anticodon is UAC. Then, a large ribosomal subunit

3′

Elongation is the protein synthesis step in which a polypeptide increases in length one amino acid at a time (Fig. 25.13). In addition to the participation of tRNAs, elongation requires elongation factors, which facilitate the binding of tRNA anticodons to mRNA codons at a ribosome. Elongation consists of a series of four steps (Fig. 25.13): 1. A tRNA with an attached peptide is already at the P site, and a tRNA carrying the next amino acid in the chain is just arriving at the A site. 2. Once the next tRNA is in place at the A site, the peptide chain will be transferred to this tRNA. 3. Energy and part of the ribosomal subunit are needed to bring about this transfer. The energy contributes to peptide bond formation, which makes the peptide one amino acid longer by adding the peptide from the A site. 4. Next, translocation occurs—the mRNA moves forward one codon length, and the peptide-bearing tRNA is now at the ribosome P site. The “spent” tRNA now exits. The new codon is at the A site and is ready to receive the next complementary tRNA. The complete cycle in steps 1–4 is repeated at a rapid rate (e.g., about 15 times each second in the bacteria E. coli).

Termination Termination is the final step in protein synthesis. During termination, as shown in Figure 25.14, the polypeptide and the assembled components that carried out protein synthesis are separated from one another. Termination of polypeptide synthesis occurs at a stop codon. Termination requires a protein called a release factor, which cleaves the polypeptide from the last tRNA. After this occurs, the polypeptide is set free and begins to take on its three-dimensional shape. The ribosome dissociates into its two subunits. Properly functioning proteins are of paramount importance to the cell and to the organism. For example, if an organism inherits a faulty gene, the result can be a genetic disorder (such as Huntington disease) caused by a malfunctioning protein or a propensity toward cancer. Proteins are the link between genotype and phenotype. The DNA sequence underlying these proteins distinguishes different types of organisms. In addition to accounting for the difference between cell types, proteins account for the differences between organisms.



Chapter 25  DNA Structure and Gene Expression

met

peptide bond

ser

tRNA

met ser

val val

C A U G U A G A C

3′

asp

C A U C U G G U A G A C

C A U CU G G U A G A C

3′

asp

C

A

3′

5′ 3. Peptide bond formation attaches the peptide chain to the newly arrived amino acid.

2. Two tRNAs can be at a ribosome at one time; the anticodons are paired to the codons.

U G G

val

asp

5′

1. A tRNA–amino acid approaches the ribosome and binds at the A site.

tryp

tryp

tryp

val

5′

ala

ala

ala

anticodon

tryp

thr

ser

ser

C U G

ala

met

met

asp

507

U C U G G UA G A C A C C

5′

3′

4. The ribosome moves forward; the “empty” tRNA exits from the E site; the next amino acid–tRNA complex is approaching the ribosome.

Elongation

Figure 25.13  Elongation.  Note that a polypeptide is already at the P site. During elongation, polypeptide synthesis occurs as amino acids are added one at a time to the growing chain.

Review of Gene Expression

asp ala tryp

asp

release factor

val

ala

glu tryp val C U U G A A

5′

glu

U G A

stop codon

3′

The ribosome comes to a stop codon on the mRNA. A release factor binds to the site.

C

U

U

A G A

A U G

3′

5′

The release factor hydrolyzes the bond between the last tRNA at the P site and the polypeptide, releasing them. The ribosomal subunits dissociate.

A gene is expressed when product, in our case a protein, has been synthesized. Protein synthesis requires the process of transcription and translation (Fig. 25.15). During transcription, a segment of a DNA strand serves as a template for the formation of messenger RNA (mRNA). The bases in mRNA are complementary to those in DNA. Every three mRNA bases is a codon (a triplet code) for a certain amino acid. Messenger RNA is processed before it leaves the nucleus, during which time the introns are removed and the ends are modified. Messenger RNA carries a sequence of codons to the ribosomes.  During translation, tRNAs bring attached amino acids to the ribosomes. Because tRNA anticodons pair with codons, the amino acids become sequenced in the order originally specified by DNA. The genes we receive from our parents determine the proteins in our cells and these proteins are responsible for our inherited traits!

Check Your Progress  25.3 1. Explain the role of mRNA, tRNA, and rRNA in gene expression.

Termination

2. Describe the movement of information from the nucleus to the formation of a functional protein.

Figure 25.14  Termination.  During termination, the finished

3. Discuss why the genetic code is said to be degenerate.

polypeptide is released, as are the mRNA and the last tRNA.



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TRANSCRIPTION

TRANSLATION

1. DNA in nucleus serves as a template for mRNA.

DNA 2. mRNA is processed before leaving the nucleus. primary mRNA

mRNA

introns

amino acids

peptide 3′

mature mRNA

nuclear pore

3. mRNA moves into cytoplasm and 5′ becomes associated with ribosomes.

large and small ribosomal subunits

CC C

6. Polypeptide synthesis takes place one amino acid at a time.

tRNA C C C UG G U U U GG G A C C A A A G UA

C A U

anticodon

5′ ribosome

4. tRNAs with anticodons carry amino acids to mRNA. 3′

codon 5. Anticodon–codon complementary base pairing occurs.

Figure 25.15  Review of gene expression.  Messenger RNA is produced and processed in the nucleus during transcription, and protein synthesis occurs at the ribosomes (in cytoplasm and rough ER) during translation.

25.4  Control of Gene Expression Learning Outcomes Upon completion of this section, you should be able to 1. Describe the relationship between gene regulation and gene activity in a cell. 2. Compare regulation in prokaryotes to regulation in eukaryotes. 3. Discuss the many ways genes are regulated in eukaryotes.

The human body contains many types of cells that differ in structure and function. Each cell type must contain its own mix of proteins that make it different from all other cell types. Therefore, only certain genes are active in cells that perform specialized functions, such as nerve, muscle, gland, and blood cells. Some of these active genes are called housekeeping genes because they govern functions that are common to many types of cells, such as glucose metabolism. But otherwise, the activity of selected genes accounts for the specialization of cells: Cell type

Gene type Housekeeping Hemoglobin Insulin Myosin

Red blood

Muscle

Pancreatic

In other words, gene expression is controlled in a cell, and this control accounts for its specialization. Let’s begin by examining a simpler system—the control of transcription in prokaryotes.

Control of Gene Expression in Prokaryotes The bacterium Escherichia coli that lives in your intestine can use various sugars as a source of energy and carbon. This organism can quickly adjust its gene expression to match your diet. The enzymes that are needed to break down lactose, the sugar present in milk, are found encoded together in an operon in the bacterial DNA. An operon is a cluster of genes usually coding for proteins related to a particular metabolic pathway, along with the short DNA sequences that coordinately control their transcription. The control sequences consist of a promoter, a sequence of DNA where RNA polymerase first attaches to begin transcription, and an operator, a sequence of DNA in the lac operon where a repressor protein binds (Fig. 25.16). In the lac operon, the structural genes for three enzymes that are needed for lactose metabolism are under the control of one promoter/operator complex. If lactose is absent, a protein called a repressor binds to the operator. When the repressor is bound to the operator, RNA polymerase cannot transcribe the three structural genes of the operon. The lac repressor is encoded by a regulatory gene located outside of the operon. When lactose is present, the lactose binds with the lac repressor so that the repressor is unable to bind to the operator. Then RNA polymerase is able to transcribe the structural genes into a single mRNA, which is then translated into the three different enzymes. In this way, the enzymes needed to break down lactose are only synthesized when lactose is present. The lac operon is considered an inducible operon. It is only activated when lactose induces its expression. Other bacterial



Chapter 25  DNA Structure and Gene Expression

509

RNA polymerase cannot bind to promoter.

regulator gene

promoter operator

structural genes

3′

5′

DNA

active repressor active repressor

a. Lactose absent. Enzymes needed to metabolize lactose are not produced. RNA polymerase can bind to promoter.

3′

5′

DNA inactive repressor 5′

mRNA

3′

mRNA

active repressor lactose

enzymes for lactose metabolism b. Lactose present. Enzymes needed to metabolize lactose are produced only when lactose is present.

Figure 25.16  The lac operon.  a. When lactose is absent, the regulator gene codes for a repressor that is normally active. When it binds to the operator, RNA polymerase cannot attach to the promoter, and structural genes are not expressed. b. When lactose is present, it binds to the repressor, changing its shape so that it is inactive and cannot bind to the operator. Now, RNA polymerase binds to the promoter, and the structural genes are expressed.

operons are repressible operons. They are usually active until a repressor turns them off.

Control of Gene Expression in Eukaryotes In prokaryotes, a single promoter serves several genes that make up a transcription unit or operon. In eukaryotes, each gene has its own promoter where RNA polymerase binds. In contrast to the prokaryotes, eukaryotes employ a variety of mechanisms to regulate gene expression. These mechanisms affect whether the gene is expressed, the speed with which it is expressed, and how long it is expressed.

Levels of Gene Control Eukaryotic genes exhibit control of gene expression at five different levels: (1) pretranscriptional control, (2) transcriptional control, (3) posttranscriptional control, (4) translational control, and (5) posttranslational control.

Genes within darkly staining, highly condensed portions of chromatin, called heterochromatin, are inactive. A dramatic example of this occurs with the X chromosome in mammalian females. Females have two X chromosomes, while males have only one X chromosome. This inequality is balanced by the inactivation of one X chromosome in every cell of the female body. Each inactivated X chromosome, called a Barr body in honor of its discoverer, can be seen as a small, darkly staining mass of condensed chromatin along the inner edge of the nuclear envelope. During early prenatal development, one or the other X chromosome in a cell is randomly shut off by chromatin condensation into heterochromatin. All of the cells that form from division of that cell will have the same X chromosome inactivated. Therefore, females have patches of tissue that differ in which X chromosome is being expressed. If that female is heterozygous for a particular X-linked gene, she will be a mosaic, containing patches of cells expressing different alleles. This can be clearly seen in calico cats (Fig. 25.17). Active genes in eukaryotic cells are associated with more loosely packed chromatin, called euchromatin. However, even euchromatin must be “unpacked” before it can be transcribed. The

Pretranscriptional Control  Eukaryotes use DNA methylation and chromatin packing as a way to keep genes turned off.

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UNIT 5  Continuance of the Species

Coats of calico cats have patches of orange and black.

active X chromosome allele for orange color

inactive X Barr bodies

cell division inactive X allele for black color active X chromosome Females have two X chromosomes.

One X chromosome is inactivated in each cell. Which one of the pair is inactivated is random.

Figure 25.17  X-inactivation.  In cats, the alleles for black or orange coat color are carried on the X chromosome. Random X-inactivation occurs in

females. Therefore, in heterozygous females, some of the cells express the allele for black coat color, while other cells express the allele for orange coat color. The white color on calico cats is provided by another gene.

presence of nucleosomes limits access to the DNA by the transcription machinery. A chromatin remodeling complex pushes the nucleosomes aside to open up sections of DNA for expression: histone DNA

nucleosome

Chromatin remodeling complex

Transcriptional Control  As in prokaryotes, eukaryotic transcriptional control is dependent on the interaction of proteins with particular DNA sequences. The proteins are called transcription factors and activators, while the DNA sequences they are associated with are called enhancers and promoters. In eukaryotes, each gene has its own promoter. Transcription factors are proteins that help RNA polymerase bind to a promoter. Several transcription factors per gene form a transcription initiation complex that also helps pull double-stranded DNA apart and even acts to release RNA polymerase so that transcription can begin. The same transcription factors are used over again at other promoters, so it is easy to imagine that if one malfunctions, the result could be disastrous to the cell. In eukaryotes, transcription activators are proteins that speed transcription dramatically. In general, transcription activators are themselves activated in response to a signal received and transmitted by a signal transduction pathway. They bind to enhancer regions in the DNA and stimulate transcription.

Posttranscriptional Control  Following transcription, messenger RNA (mRNA) is processed before it leaves the nucleus and passes into the cytoplasm. The primary mRNA is converted to the mature mRNA by the addition of a poly-A tail and a guanine cap, and by the removal of the introns and splicing back together of the exons. The same mRNA can be spliced in different ways to make slightly different products in different tissues. For example, both the hypothalamus and the thyroid gland produce the hormone calcitonin, but the calcitonin mRNA that exits the nucleus contains different combinations of exons in the two tissues. The speed of transport of mRNA from the nucleus into the cytoplasm can ultimately affect the amount of gene product following transcription. There is a difference in the length of time it takes various mRNA molecules to pass through a nuclear pore. Translational Control  The longer an mRNA remains in the cytoplasm before it is broken down, the more gene product can be translated. Differences in the poly-A tail or the guanine cap can determine how long a particular transcript remains active before it is destroyed by a ribonuclease associated with ribosomes. Hormones can cause the stabilization of certain mRNA transcripts. For example, the mRNA for vitelline, an egg membrane protein, can persist for three weeks if it is exposed to estrogen, as opposed to 15 hours without estrogen. Posttranslational Control  Some proteins are not active immediately after synthesis. After translation, insulin is folded into a three-dimensional structure that is inactive. Then a sequence of about 30 amino acids is enzymatically removed from the middle of the molecule, leaving two polypeptide chains that are bonded together by disulfide (S—S) bonds. This activates the protein. Other modifications, such as phosphorylation, also affect the activity of a protein. Many proteins only function a short time before they are degraded or destroyed by the cell. The various levels of gene control are summarized in Figure 25.18.



Chapter 25  DNA Structure and Gene Expression

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Check Your Progress  25.4

histones

1. Explain why gene expression would not be the same in all cells of an organism.

2. Describe how the lac operon represents an on/off switch for gene expression.

3. Summarize the different levels of gene regulation in eukaryotes.

Pretranscriptional control

25.5  Gene Mutations and Cancer Learning Outcomes Transcriptional control

3′

primary mRNA

intron

5′

exon

Posttranscriptional control mature mRNA

5′ 3′

nuclear pore

nuclear envelope Translational control

polypeptide chain

Posttranslational control

plasma membrane functional protein

Figure 25.18  Levels at which control of gene expression

occurs in eukaryotic cells.  The five levels of control are (1) pretranscriptional control, (2) transcriptional control, and (3) posttranscriptional control, which occur in the nucleus; and (4) translational and (5) posttranslational control, which occur in the cytoplasm.

Upon completion of this section, you should be able to 1. Summarize the causes of gene mutations. 2. Describe why cancer is a failure of genetic control. 3. Describe the characteristics of cancer cells.

A gene mutation is a permanent change in the sequence of bases in DNA. Gene mutations may have a variety of effects on a cell. For example, it may change how the gene is expressed in the cell. A mutation may also change the structure of a protein so that its function is altered, or completely inactivated.  Mutations that occur in the sex cells, also called germ cells, may be passed on from generation to generation. This is the basis of heredity, and may be responsible for susceptibility to some forms of cancer. Mutation in the remainder of the cells of the body, called somatic cells, are passed on to daughter cells by cell division. These are not passed on to future generations, but may sometimes lead to the development of cancer.

Causes of Mutations Some mutations are spontaneous—they happen for no apparent reason—whereas others are induced by environmental influences. In most cases, spontaneous mutations arise as a result of abnormalities in normal biological processes. Induced mutations may result from exposure to toxic chemicals or radiation, which induce (cause) changes in the base sequence of DNA. 

Errors in DNA Replication  DNA replication errors are a rare source of mutations. DNA polymerase, the enzyme that carries out replication, proofreads the new strand against the old strand. Usually, mismatched pairs are then replaced with the correct nucleotides. In the end, there is typically only one mistake for every 1 billion nucleotide pairs replicated. Mutagens  Environmental influences called mutagens cause mutations in humans. Mutagens include radiation (e.g., radioactive elements, X rays, ultraviolet [UV] radiation) and certain organic chemicals (e.g., chemicals in cigarette smoke and certain pesticides). The rate of mutations resulting from mutagens is generally low because DNA repair enzymes constantly monitor and repair any irregularities.

Transposons  Transposons are specific DNA sequences that have the remarkable ability to move within and between

512

UNIT 5  Continuance of the Species Normal gene codes for purple pigment a. Mutated gene cannot code for purple pigment transposon b.

c.

Figure 25.19  Transposon.  a. A purple coding gene ordinarily codes for a purple pigment. b. A transposon “jumps” into the purple-coding gene. This mutated gene is unable to code for purple pigment and a white kernel results. c. Indian corn displays a variety of colors and patterns due to transposon activity.

chromosomes. Their movement to a new location sometimes alters neighboring genes, particularly by increasing or decreasing their expression. Although “movable elements” in corn were described over 50 years ago by Barbara McClintock, their significance was only realized recently. Also called jumping genes, transposons have now been discovered in almost every species, including bacteria, fruit flies, and humans. McClintock described how the presence of white kernels in corn is due to a transposon located within a gene coding for a pigment-producing enzyme (Fig. 25.19a, b). “Indian corn” displays a variety of colors and patterns because of transposons (Fig. 25.19c). In a rare human neurological disorder called Charcot-Marie-Tooth disease, a transposon called Mariner causes the muscles and nerves of the legs and feet to gradually wither away.

Normal

DNA

T A C A A A T G C T C T T A G

mRNA

A U G U U U A C G A G A A U C

Amino acids

Base substitution

Met

Phe

Arg

Iso

DNA

T A C G A A T G C T C T T A G

mRNA

A U G C U U A C G A G A A U C

Amino acids

Met

Leu

Thr

Arg

Iso

Added

Effect of Mutations on Protein Activity Point mutations involve a change in a single DNA nucleotide. That change alters transcription and possibly changes the specific amino acid. One type of point mutation is a base substitution resulting in one DNA nucleotide being replaced with another incorrect nucleotide. Notice the base difference in the second row of Figure 25.20  and how it changes the resultant amino acid sequence.  Sometimes a base substitution has little or no effect on the final protein produced, but in some cases early stop codons can be introduced, or coding for the wrong amino acid can severely alter the protein shape. Such is the case with the genetic disorder sickle cell disease (Fig. 25.21). In this gene, there is a base substitution that alters the mRNA codon for glutamic acid. Instead, the codon for valine is present, altering the final shape of hemoglobin—the protein that carries oxygen in the blood. The abnormal hemoglobin molecules form semirigid rods, and the red blood cells become sickle-shaped, resulting in decreased blood flow through tiny blood vessels. Frameshift mutations (see Fig. 25.20) occur most often because one or more nucleotides are either inserted or deleted from DNA. The result of a frameshift mutation can be a completely new sequence of codons and nonfunctional protein. Here is how this occurs: The sequence of codons is read from a specific starting

Thr

Addition

DNA

T A C A A A T T G C T C T T A G

mRNA

A U G U U U A A C G A G A A U C

Amino acids

Met

Phe

Asp

Glu

Asp

Deleted

Deletion

DNA

T A C A A T G C T C T T A G

mRNA

A U G U U A C G A G A A U C

Amino acids

Met

Leu

Arg

Glu

Figure 25.20  Point mutations.  The effect of a point mutation can vary. Starting at the top: Normal sequence of bases results in a normal sequence of amino acids; next, a base substitution can result in the wrong amino acid; in the final two rows, an addition or deletion can result in a frameshift mutation, altering all the codons downstream of the point mutation. point, as in this sentence, THE CAT ATE THE RAT. If the letter C is deleted from this sentence and the reading frame is shifted, we read THE ATA TET HER AT—something that doesn’t make sense.



Chapter 25  DNA Structure and Gene Expression

Normal hemoglobin

Mutant hemoglobin

DNA

G G A C T T C T T

DNA

G G A C A T C T T

mRNA

C C U G A A G A A

mRNA

C C U G U A G A A

Amino acids

Pro

Glu

Amino acids

Glu

Pro

7,400×

Val

Glu

513

The development of cancer involves a series of accumulating mutations that can be different for each type of cancer. As discussed in section 5.2, tumor suppressor genes ordinarily act as brakes on cell division, especially when it begins to occur abnormally. Proto-oncogenes stimulate cell division but are usually turned off in fully differentiated nondividing cells. When protooncogenes mutate, they become oncogenes that are active all the time. Carcinogenesis begins with the loss of tumor suppressor gene activity and/or the gain of oncogene activity.  It often happens that tumor suppressor genes and protooncogenes code for transcription factors or proteins that control transcription factors. As we have seen, transcription factors are a part of the rich and diverse types of mechanisms that control gene expression in cells. They are of fundamental importance to DNA replication and repair, cell growth and division, control of

7,400×

Figure 25.21  Mutation and sickle cell disease.  Due to a base substitution in the hemoglobin gene, the DNA now codes for valine instead of glutamic acid, and the result is that normal red blood cells become sickle-shaped. 

New mutations arise, and one cell (brown) has the ability to start a tumor. primary tumor

Nonfunctional Proteins A single nonfunctioning protein can have a dramatic effect on the phenotype, because enzymes are often a part of metabolic pathways. One particular metabolic pathway in cells is as follows: A (phenylalanine)

EA

B (tyrosine)

EB

C (melanin)

If a faulty code for enzyme EA is inherited, a person is unable to convert the molecule A to B. Phenylalanine builds up in the system, and the excess causes mental impairment and the other symptoms of the genetic disorder phenylketonuria (PKU). In the same pathway, if a person inherits a faulty code for enzyme EB, then B cannot be converted to C, and the individual is an albino. A rare condition called androgen insensitivity is due to a faulty receptor for androgens, which are male sex hormones such as testosterone. Although there is plenty of testosterone in the blood, the cells are unable to respond to it. Female instead of male external genitals form, and female instead of male secondary sex characteristics occur. The individual, who appears to be a normal female, may be prompted to seek medical advice when menstruation never occurs. The karyotype is that of a male rather than a female, and the individual does not have the internal sexual organs of a female.

lymphatic vessel

blood vessel

Cancer in situ. The tumor is at its place of origin. One cell (purple) mutates further.

lymphatic vessel

blood vessel

Cancer cells now have the ability to invade lymphatic and blood vessels and travel throughout the body.

Mutations Can Cause Cancer It is estimated that one-third of us will develop cancer at some time in our lives. While the five-year survival rate for individuals with cancer has increased from 50% in 1977 to 68% today, around 1,600 people each day will die of the disease. In the United States, the three deadliest forms of cancer are lung cancer, colorectal cancer, and breast cancer.

New metastatic tumors are found some distance from the primary tumor.

Figure 25.22  Progression of cancer.  A single abnormal cell begins the process, and the most aggressive cell, thereafter, becomes the one that divides the most and forms the tumor. Eventually, cancer cells gain the ability to invade underlying tissue and travel to other parts of the body, where they develop new tumors.



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UNIT 5  Continuance of the Species

apoptosis (programmed cell death), and cellular differentiation. Therefore, it is not surprising that inherited or acquired defects in transcription factor structure and function contribute to the development of cancer. To take an example, a major tumor suppressor gene called p53 is more frequently mutated in human cancers than any other known gene. It has been found that the p53 protein acts as a transcription factor, and as such is involved in turning on the expression of genes whose products are cell cycle inhibitors. p53 also promotes apoptosis when it is needed. The retinoblastoma (RB) protein controls the activity of a transcription factor for cyclin D and other genes whose products promote entry into the S stage of the cell cycle. When the tumor suppressor gene p16 mutates, it is unable to control the cell cycle, the RB protein is continuously expressed, and the result is too much active cyclin D in the cell. Mutations in many other genes also contribute to the development of cancer. Several proto-oncogenes code for ras proteins, which are needed for cells to grow, to make new DNA, and to not grow out of control. A point mutation is sufficient to turn a normally functioning ras proto-oncogene into an oncogene. Abnormal growth results. Although cancers vary greatly, they usually follow a common multistep progression (Fig. 25.22). Most cancers begin as an

SCIENCE IN YOUR LIFE  ►

abnormal cell growth that is benign, or not cancerous, and usually does not grow larger. However, additional mutations may occur, causing the abnormal cells to fail to respond to inhibiting signals that control the cell cycle. When this occurs, the growth becomes malignant, meaning that it is cancerous and possesses the ability to spread. The Health feature, “Benign Versus Malignant Tumors,” explores these differences in more detail.

Characteristics of Cancer Cells The primary characteristics of cancer cells are as follows: Cancer cells are genetically unstable. Generation of cancer cells appears to be linked to mutagenesis. A cell acquires a mutation that allows it to continue to divide. Eventually one of the progeny cells will acquire another mutation and gain the ability to form a tumor. Further mutations occur, and the most aggressive cell becomes the dominant cell of the tumor. Tumor cells undergo multiple mutations and also tend to have chromosomal aberrations and rearrangements. Cancer cells do not correctly regulate the cell cycle. Cancer cells continue to cycle through the cell cycle. The normal controls of the cell cycle do not operate to stop the cycle and allow the

HEALTH

Benign Versus Malignant Tumors Cancer is a disease that is characterized by unrestricted cell growth. As a cancer cell continues its unregulated division, it may form a population of cells called a tumor. There are two different types of tumors: benign tumors and malignant tumors. A benign tumor (Fig. 25A) is usually surrounded by a fibrous capsule (usually made of connective tissue). Because of this capsule, a benign tumor does not invade the adjacent tissue. The cells of a benign tumor

resemble normal cells fairly closely. Many people have benign tumors. For example, a mole (or nevus) is a benign tumor of skin cells called melanocytes. However, not all benign tumors are harmless. Sometimes these tumors restrict a normal tissue’s blood supply, or cause normal tissue to not function correctly. If this is in a critical area of the body, such as areas of the brain that control the heartbeat or breathing, the results may be fatal. The cells of malignant tumors do not resemble normal cells, and are able to invade surrounding tissues (Fig 25B). As the cells of the tumor invade nearby tissues, they may come in contact with either blood or lymph vessels, allowing the tumor to spread throughout the body. The tumors associated with lung cancer are often malignant, and may easily spread to other organs, such as the brain or liver. The more deformed a cancer cell, the greater the chance that it will be malignant, and thus dangerous to Figure 25A  Benign Tumor.  A mole is an example of a the individual. benign tumor.

Figure 25B  A malignant tumor. 

Melanoma is a form of skin cancer that is an example of a malignant tumor.

Questions to Consider 1. Why might oncologists (cancer physicians) look for abnormal cells in the blood of a suspected cancer patient? 2. What differences in the cell cycle might occur between the cells of a benign and malignant tumor?



Chapter 25  DNA Structure and Gene Expression

SCIENCE IN YOUR LIFE  ►

515

HEALTH

Prevention of Cancer prevents the conversion of nitrates and nitrites into carcinogenic nitrosamines in the digestive tract.

Be tested for cancer. Do a shower test for breast cancer or testicular cancer. Women should get a Pap smear for cervical cancer annually if they are over 21 or are sexually active. Have a friend or physician check your skin annually for any unusual moles. Have other exams done regularly by a physician.

Include vegetables from the cabbage family in your diet. The cabbage family includes cabbage, broccoli, Brussels sprouts, kohlrabi, and cauliflower. These vegetables may reduce the risk of gastrointestinal and respiratory tract cancers.

Be aware of occupational hazards. Exposure to several different industrial agents (nickel, chromate, asbestos, vinyl chloride, etc.) and/or radiation increases the risk of various cancers. Your employer should notify you of potential hazards in your workplace. The risk of several of these cancers is increased when combined with cigarette smoking.

Limit consumption of salt-cured, smoked, or nitrite-cured foods.

Use sunscreen. Almost all cases of basal cell and squamous cell skin cancers are sun-related. Use a sunscreen of at least SPF 30 (Fig. 25C) and wear protective clothing if you are going to be out during the brightest part of the day. Don’t sunbathe on the beach or in a tanning salon.

Check your home for radon. Excessive radon exposure in homes increases the risk of lung cancer, especially in cigarette smokers. It is best to test your home and take the proper remedial actions.

Avoid unnecessary X rays. Even though most medical and dental X rays are adjusted to deliver the lowest dose possible, unnecessary X rays should be avoided. Sensitive areas of the body that are not being X-rayed should be protected with lead screens.

Practice safe sex. Human papillomaviruses (HPVs) cause genital warts and are associated with cervical cancer, as well as tumors of the vulva, vagina, anus, penis, and mouth. HPVs may be involved in 90–95% of all cases of cervical cancer, and 20 million people in the United States have an infection that can be transmitted to others.

Carefully consider hormone therapy. Estrogen therapy to control menopausal symptoms increases the risk of endometrial cancer.

Figure 25C  UV protection.  Use of sunscreen with SPF of 30 or higher can help prevent skin cancer, as can limiting midday exposure. However, including progesterone in estrogen replacement therapy helps minimize this risk.

Maintain a healthy weight. The risk of cancer (especially colon, breast, and uterine cancers) is 55% greater among obese women and 33% greater among obese men, compared to people of normal weight. Eating a low-fat, healthy diet helps maintain your weight as well as reducing your risk of cancer.

Exercise regularly. Regular activity can reduce the incidence of certain cancers, especially colon and breast cancer.

Increase consumption of foods that are rich in vitamins A and C. Beta-carotene, a precursor of vitamin A, is found in dark-green, leafy vegetables, carrots, and various fruits. Vitamin C is present in citrus fruits. These vitamins are called antioxidants because they prevent the formation of chemicals called free radicals that can damage the DNA in the cell. Vitamin C also

Salt-cured or pickled foods may increase the risk of stomach and esophageal cancer. Smoked foods, such as ham and sausage, contain chemical carcinogens similar to those in tobacco smoke. Nitrites are sometimes added to processed meats (e.g., hot dogs and cold cuts) and other foods to protect them from spoilage. These are converted to carcinogenic nitrosamines in the digestive tract.

Be moderate in the consumption of alcohol. Cancers of the mouth, throat, esophagus, larynx, and liver occur more frequently among heavy drinkers, especially when accompanied by smoking cigarettes or chewing tobacco.

Don’t smoke. Cigarette smoking accounts for about 30% of all cancer deaths. Smoking is responsible for 90% of lung cancer cases among men and 79% among women—about 87% altogether. People who smoke two or more packs of cigarettes a day have lung cancer mortality rates 13 to 23 times greater than those of nonsmokers. Cigars and smokeless tobacco (chewing tobacco or snuff) increase the risk of cancers of the mouth, larynx, throat, and esophagus.

Questions to Consider 1. Which of these cancer-preventive measures are you currently practicing? 2. Why do you think tobacco use increases the risk of other types of cancer besides lung cancer? 3. Why does the use of tanning beds also increase the incidence of skin cancer?



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UNIT 5  Continuance of the Species

cells to differentiate. Because of that, cancer cells tend to be nonspecialized. Both the rate of cell division and the number of cells increase. Cancer cells escape the signals for cell death. A cell that has genetic damage or problems with the cell cycle will initiate apoptosis, or programmed cell death. However, cancer cells do not respond to internal signals to die, and they continue to divide even with genetic damage. Cells from the immune system, when they detect an abnormal cell, will send signals to that cell, inducing apoptosis. Cancer cells also ignore these signals. Most normal cells have a built-in limit to the number of times they can divide before they die. One of the reasons normal cells stop entering the cell cycle is that the telomeres become shortened. Telomeres are sequences at the ends of the chromosomes that keep them from fusing with each other. With each cell division, the telomeres shorten, eventually becoming short enough to signal apoptosis. Cancer cells turn on the gene that encodes the enzyme telomerase, which is capable of rebuilding and lengthening the telomeres. Cancer cells thus show characteristics of “immortality” in that they can enter the cell cycle repeatedly. Cancer cells can survive and proliferate elsewhere in the body. Many of the changes that must occur for cancer cells to form tumors elsewhere in the body are not understood. The cells apparently disrupt the normal adhesive mechanism and move to another place within the body. They travel through the blood and lymphatic vessels and then invade new tissues, where they form

tumors. This process is known as metastasis. As a tumor grows, it must increase its blood supply by forming new blood vessels, a process called angiogenesis. Tumor cells switch on genes that code for the production of growth factors, encouraging blood vessel formation. These new blood vessels supply the tumor cells with the nutrients and oxygen they require for rapid growth, but they also rob normal tissues of nutrients and oxygen.

Check Your Progress  25.5 1. Explain how gene mutations occur. 2. Distinguish between a point mutation and a frameshift mutation.

3. Explain the characteristics of cancer.

Conclusion Mutations are the result of changes in the DNA of an individual. In some cases, these changes affect specific proteins. For example, children who suffer from xeroderma pigmentosum are missing the enzymes that repair DNA after exposure to UV light. These enzymes constantly search the DNA for abnormalities and then either directly repair the damage, or remove a section of the DNA with the abnormality and resynthesize a new strand of DNA.

MEDIA STUDY TOOLS http://connect.mheducation.com Maximize your study time with McGraw-Hill SmartBook®, the first adaptive textbook.

 

MP3 Files

25.3  Protein Synthesis • Transcription • Translation

 

Animations

25.1  Hershey-Chase Experiment • DNA Structure 25.2  DNA Replication • Meselson and Stahl Experiment 25.3  Stages of Transcription • How Spliceosomes Process RNA • How Translation Works 25.4  Combination of Switches: The lac Operon • Control of Gene Expression in Eukaryotes • X-inactivation • Transcription Factors 25.5  Transposons • Mutation by Base Substitution • Addition and Deletion Mutations • How Tumor Suppressor Genes Block Cell Division • Telomerase Function

 

3D Animations

25.1  DNA Replication: DNA Structure 25.2  DNA Replication 25.3  Molecular Biology of the Gene: Transcription • Molecular Biology of the Gene: mRNA Modifications • Molecular Biology of the Gene: Translation

  Tutorials 25.2  DNA Replication 25.3  Overview of Gene Expression 25.4  lac Operon



Chapter 25  DNA Structure and Gene Expression

SUMMARIZE 25.1  DNA Structure

■

■ In a series of experiments, researchers were able to determine that

DNA (deoxyribonucleic acid) was the genetic material.

■ DNA is structured as a double helix, with two sugar-phosphate back-

bones and paired nitrogen-containing bases (which may be either purines or pyrimidines). Complementary base pairing of A (adenine) with T (thymine), and G (guanine) with C (cytosine) occurs between the strands. The DNA strands of the double helix are oriented in an antiparallel configuration.

25.2  DNA Replication ■ DNA replication is semiconservative, meaning that following replica-

tion each double helix contains one old strand and one new strand. Enzymes, including DNA polymerase, helicase, and DNA ligase are involved in the replication process.

25.3  Gene Expression Genes contain the information for the synthesis of proteins or RNA (ribonucleic acid) molecules. RNA plays an active role in all stages of gene expression. Gene expression involves two processes: transcription and translation. ■ Transcription produces an RNA sequence complementary to one of the DNA strands. Transcription uses an RNA polymerase enzyme, which binds to a region of the DNA called a promoter. Messenger RNA (mRNA) produced in eukaryotes must be processed by the addition of a cap and tail, and removal of introns, before being transported to the cytoplasm where the information in the exons can be translated. ■ DNA specifies the synthesis of proteins using a codon consisting of three bases of RNA. Each codon codes for one amino acid. ■ During translation, transfer RNA (tRNA) molecules, attached to their own particular amino acid, travel to a ribosome, and through complementary base pairing between anticodons of tRNA and codons of mRNA, the tRNAs, and the amino acids they carry, are sequenced in a predetermined way to form a polypeptide chain. ■ Translation consists of three stages: initiation, elongation, and termination. A ribosome, which contains ribosomal RNA (rRNA), has a binding site for two tRNAs at a time. The tRNA at the P site passes a peptide to a newly arrived tRNA–amino acid at the A site. Then translocation occurs: the ribosome moves, and the tRNA with the peptide is at the P site. In this way, an amino acid grows one amino acid at a time until a polypeptide is formed.

25.4  Control of Gene Expression ■ Each cell type in the body contains its own mix of proteins that make

it different from all other cell types. Only certain genes are active in cells that perform specialized functions. In prokaryotes, genes are clustered into operons. An operon contains a group of genes, usually coding for proteins related to a particular metabolic pathway, along with the promoter, a DNA sequence where RNA polymerase binds to begin transcription, and an operator, where a repressor protein binds. In the case of the lac operon, the structural genes code for enzymes that metabolize lactose. When lactose is absent, the repressor binds to the operator and shuts off the operon. When lactose is present, it binds to the repressor so that it can no longer bind to the operator, and this allows RNA polymerase to transcribe the structural genes. ■ Control of gene expression is more complicated in eukaryotes. Eukaryotes have five levels of control: pretranscriptional control, transcriptional

■

■ ■ ■

517

control, posttranscriptional control, translational control, and posttranslational control. Genes that are found within highly condensed heterochromatin are inactive. A dramatic example of this is inactivated X chromosomes, called Barr bodies. Even genes that are found in looser-packed DNA, called euchromatin, must have the nucleosomes shifted in order to be expressed. The transcriptional level of control includes transcription factors that assist the RNA polymerase and transcriptional activators that greatly increase the rate of transcription. In posttranscriptional control, differences in mRNA processing affect gene expression. Translational controls occur in the cytoplasm and involve the length of time the mRNA is functional. Posttranslational controls also occur in the cytoplasm and involve the length of time the protein is functional. Proteins can be activated by a variety of methods.

25.5  Gene Mutations and Cancer ■ A gene mutation is a permanent change in the sequence of bases in

■ ■

■

■

DNA. The effect of a DNA base sequence change on protein activity can range from no effect to complete inactivity. Gene mutations result in a change in the sequence of nucleotides in the DNA and can be caused by errors in replication, mutagens, and transposons. Point mutations involve a change in a single DNA nucleotide and, therefore, a possible change in a specific amino acid. Frameshift mutations occur most often because one or more nucleotides are either inserted or deleted from DNA. The development of cancer involves a series of accumulating mutations that can be different for each type of cancer. Carcinogenesis begins with the loss of tumor suppressor gene activity and/or the gain of oncogene activity. Mutations in many other genes also contribute to the development of cancer. Although cancers vary greatly, they usually follow a common multistep progression. Cancer cells defy the normal regulation of the cell cycle and can invade and colonize other areas of the body. Cancer cells do not exhibit contact inhibition and thus form benign tumors. When tumor cells gain the ability to invade surrounding tissues, they are said to be malignant. In terms of their primary characteristics, cancer cells are genetically unstable, do not correctly regulate the cell cycle, cause angiogenesis, escape the signals for cell death, and can survive and proliferate elsewhere in the body, a process called metastasis. Multiple mutations and chromosomal aberrations are present in cancer cells. They continue to cycle through the cell cycle and tend to be nonspecialized. They do not respond to either internal or external signals for apoptosis, and they induce the expression of telomerase to repair their telomeres.

ASSESS Testing Yourself Choose the best answer for each question.

25.1  DNA Structure 1. The double helix model of DNA resembles a twisted ladder in which the rungs of the ladder are a. a purine paired with a pyrimidine. b. A paired with G and C paired with T. c. sugar-phosphate paired with sugar-phosphate. d. a 5′ end paired with a 3′ end. e. Both a and b are correct.



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UNIT 5  Continuance of the Species

2. If 30% of an organism’s DNA is thymine, then a. 70% is purine. b. 20% is guanine. c. 30% is adenine. d. 70% is pyrimidine. e. Both b and c are correct. 3. If the sequence of bases in one strand of DNA is 5′ TAGCCT 3′, then the sequence of bases in the other strand is a. 3′ TCCGAT 5′. b. 3′ TAGCCT 5′. c. 3′ ATCGGA 5′. d. 3′ AACGGUA 5′. e. None of these are correct.

25.2  DNA Replication 4. DNA replication is said to be semiconservative because a. one of the new molecules conserves both of the original DNA strands. b. the new DNA molecule contains two new DNA strands. c. both of the new molecules contain one new strand and one old strand. d. DNA polymerase conserves both of the old strands. 5. The enzyme responsible for separating double-stranded DNA into single-stranded DNA is a. DNA helicase. b. DNA primase. c. DNA polymerase. d. DNA ligase. 6. The enzyme responsible for adding new nucleotides to a growing DNA chain during DNA replication is a. helicase. b. RNA polymerase. c. DNA polymerase. d. DNA ligase.

25.3  Gene Expression 7. Which of the following processes occurs in the nucleus and forms a complementary copy of one strand of the DNA molecule for gene expression? a. DNA replication b. translation c. transcription d. RNA processing e. None of these are correct. 8. RNA processing a. removes the exons, leaving only the introns. b. is the same as transcription. c. is an event that occurs after RNA is transcribed. d. is the rejection of old, worn-out RNA. e. All of these are correct. 9. If the sequence of bases in the coding strand of a DNA molecule is TAGC, then the sequence of bases in the mRNA will be a. AUCG. b. TAGC. c. UAGC. d. ATCG.

25.4  Control of Gene Expression 10. An operon is a short sequence of DNA a. that prevents RNA polymerase from binding to the promoter. b. that prevents transcription from occurring. c. and the sequences that control its transcription. d. that codes for the repressor protein. e. that functions to prevent the repressor from binding to the operator. 11. How is transcription directly controlled in eukaryotic cells? a. through the use of phosphorylation b. by means of apoptosis c. using transcription factors and activators d. when chromatin is packed to keep genes turned on e. None of these are correct.

25.5  Gene Mutations and Cancer 12. Mutation rates are typically low due to a. proofreading by DNA polymerase. b. DNA repair enzymes. c. silent mutations. d. None of these are correct. e. All of these are correct. 13. Which of these is a characteristic of cancer cells? a. They do not exhibit contact inhibition. b. They lack specialization. c. They have abnormal chromosomes. d. They fail to undergo apoptosis. e. All of these are correct.

ENGAGE BioNOW Want to know  how this science is relevant to your life? Check out the BioNow videos below: ■ Quail Hormones ■ Metamorphosis ■ Glowing Fish Genetics

Thinking Critically 1. What kind of genes do you think would be included in the category of “housekeeping” genes? 2. When the enzyme telomerase was first discovered, some people thought it might be “the fountain of youth” because it could immortalize cells. Why was this not found to be true?

3. Why is it only the risk for cancer that is inherited?

PHOTO CREDITS Opener: © AP Images/Jim McKnight; 25Aa, 25Ac: © Science Source; 25.11d: © Science Source; 25.17: © Photodisc/Getty RF; 25.19c: © Mondae Leigh Baker; 25.21(blood cells): © Eye of Science/Science Source; 25A: © Dr. P. Marazzi/Science Source; 25B: © Mediscan/ Alamy; 25C: © Jeff Maloney/Getty RF.

CASE STUDY Biotechnology and Diabetes Type 1 diabetes results from the body’s failure to either produce insulin or produce enough insulin for the body’s needs. Insulin is a hormone that helps move glucose from the bloodstream into the cells of the body, where it can be used to make the fuel, ATP, for our cells. This type of diabetes is a chronic disease that has to be controlled throughout the individual’s life. Left untreated, type 1 diabetes can lead to blindness, kidney failure, nerve damage, cardiovascular disease, and death. Over 25 million people in the United States are afflicted with diabetes and approximately 5% of them have type 1. In fact, diabetes is the third leading major cause of death in the United States behind heart disease and cancer. To treat this condition, individuals need to monitor their blood glucose level several times per day and use insulin injections multiple times per day when needed.  Where does this insulin come from? At one time it was derived from the pancreas of cows and pigs, but now with the advances in biotechnology, human insulin is used to treat diabetes patients. The gene for human insulin can be inserted into a bacteria cell and the bacteria can produce the insulin needed by diabetics. In this chapter, we will explore the fundamentals of one of the most rapidly changing areas of science—that of biotechnology. In addition, we will see how the processes of gene therapy may make it possible to one day replace the defective genes that cause many forms of human disease. As you read through the chapter, think about the following questions:

1. What procedures are used to introduce a bacterial gene into a plant? 2. What is the difference between a genetically modified and transgenic organism?

26

Biotechnology and Genomics CHAPTER OUTLINE 26.1  DNA Technology 26.2 Biotechnology Products 26.3 Gene Therapy 26.4 Genomics, Proteomics, and Bioinformatics

BEFORE YOU BEGIN Before beginning this chapter, take a few moments to review the following discussions: Section 25.1  What are the structural characteristics of a DNA molecule? Section 25.2  What is the role of the DNA polymerase in DNA replication? Section 25.3  What are the stages in gene expression?

3. How are animals being genetically modified?

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UNIT 5  Continuance of the Species

26.1  DNA Technology

human DNA

Learning Outcomes Upon completion of this section, you should be able to 1. Describe the steps in forming recombinant DNA. 2. Discuss how the polymerase chain reaction works. 3. Explain what is meant by a DNA “fingerprint.”

plasmid (vector)

insulin gene

Knowledge of DNA biology has led to an ability to manipulate the genes of organisms. We can clone genes and then use them to alter the genome (the complete genetic makeup of an organism) of viruses and cells, whether bacterial, plant, or animal cells. This practice, called genetic engineering, has innumerable uses, from producing a product to treating cancer and genetic disorders.

Forms of Cloning We often think of cloning as the production of identical copies of an organism through some asexual means. The members of a bacterial colony on a petri dish are clones because they all came from the division of the same original cell. Human identical twins are also considered clones, because the first two cells of the embryo separated and each became a complete individual. The Bioethical feature, “Forms of Cloning,” explores some of the different ways that this term may be used in biology.

bacterium

human cell Restriction enzyme cleaves DNA.

DNA ligase seals the insulin gene into the plasmid.

recombinant DNA

Host cell takes up recombined plasmid.

Gene cloning occurs.

Bacteria produce a product.

Cloning a Gene Another major biological application of cloning is gene cloning, which is the production of many identical copies of a single gene. Biologists clone genes for a number of reasons. They might want to produce large quantities of the gene’s protein product, such as human insulin, learn how a cloned gene codes for a particular protein, or use the genes to alter the phenotypes of other organisms in a beneficial way. When cloned genes are used to modify a human, the process is called gene therapy (see section 26.3). Otherwise, organisms with foreign DNA or genes inserted into them are called transgenic organisms, which are frequently used to produce a product desired by humans. While a variety of techniques now exist to produce cloned DNA, most processes rely on recombinant DNA technology and the polymerase chain reaction (PCR).

Recombinant DNA Technology Recombinant DNA (rDNA) contains DNA from two or more ­different sources, such as the human cell and the bacterial cell in Figure 26.1. To make rDNA, a researcher needs a vector, a piece of DNA that can be manipulated such that foreign DNA can be added to it. One common vector is a plasmid. Plasmids are small accessory rings of DNA from bacteria that are not part of the bacterial chromosome and are capable of self-replicating. ­ ­Plasmids were discovered by investigators studying the bacterium ­Escherichia coli.

insulin

Figure 26.1  Cloning a human gene.  Human DNA and plasmid DNA are cleaved by a specific type of restriction enzyme. For example, human DNA containing the insulin gene is spliced into a plasmid by the enzyme DNA ligase. Gene cloning is achieved after a bacterium takes up the plasmid. If the gene functions normally as expected, the product (e.g., insulin) may also be retrieved.

Two enzymes are needed to introduce foreign DNA into v­ ector DNA: (1) a restriction enzyme to cleave the vector DNA and (2) DNA ligase to seal DNA into an opening created by the restriction enzyme. Hundreds of restriction enzymes occur naturally in bacteria, where they act as a primitive immune system by cutting up any viral DNA that enters the cell. They are called restriction enzymes because they restrict the growth of viruses, but they can also be used as molecular scissors to cut doublestranded DNA at a specific site. For example, the restriction enzyme called EcoRI always recognizes and cuts double-stranded



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Forms of Cloning The term cloning may be used in several different ways in biology. In addition to gene cloning (see Fig. 26.1), there is reproductive cloning— the ability to clone an adult animal from a normal body cell—and therapeutic cloning, which allows the rapid production of mature cells of a specific type. Both types of cloning are a direct result of recent discoveries about how the cell cycle is controlled. Reproductive cloning, or the cloning of adult animals, was once thought to be impossible, because investigators found it difficult to have the nucleus of an adult cell “start over” with the cell cycle, even when it was placed in an egg cell that had had its own nucleus removed. In 1997, Dolly the sheep demonstrated that reproductive cloning is indeed possible. The donor cells were starved before the cell’s nucleus was placed in an enucleated egg. This caused them to stop dividing and go into a G0 (resting) stage, and this made the nuclei amenable to cytoplasmic signals for initiation of development (Fig. 26Aa). This advance has made it possible to clone all sorts of farm animals that have desirable traits and even to clone rare animals that might otherwise become

fuse egg with G0 nucleus

G0 cells from animal to be cloned a. Reproductive cloning

example, the bone marrow has stem cells that produce new blood cells. However, adult stem cells are limited in the number of cell types they may become. Nevertheless, scientists are beginning to overcome this obstacle. In 2006, by adding just four genes to adult skin stem cells, Japanese scientists were able to coax the cells, called fibroblasts, into becoming induced pluripotent stem cells (iPS), a type of stem cell that is similar to an ESC. The researchers were then able to create heart and brain cells from the adult stem cells. Other researchers have used this technique to reverse Parkinson-like symptoms in rats. Although questions exist on the benefits of iPS cells, these advances demonstrate that scientists are actively investigating methods of overcoming the current limitations and ethical concerns of using embryonic stem cells.

Questions to Consider 1. How might the study of therapeutic cloning benefit scientific studies of reproductive cloning? 2. What types of diseases might not be treatable using therapeutic cloning?

remove and discard egg nucleus

egg

remove G0 nucleus

extinct. Despite the encouraging results, however, there are still obstacles to be overcome, and a ban on the use of federal funds in experiments to clone humans remains firmly in place. In therapeutic cloning, however, the objective is to produce mature cells of various cell types rather than an individual organism. The purposes of therapeutic cloning are (1) to learn more about how specialization of cells occurs and (2) to provide cells and tissues that could be used to treat human illnesses, such as diabetes, or major injuries, such as strokes or spinal cord injuries. There are two possible ways to carry out therapeutic cloning. The first way is to use the same procedure as reproductive cloning, except that embryonic stem cells (ESCs) are separated and each is subjected to a treatment that causes it to develop into a particular type of cell, such as red blood cells, muscle cells, or nerve cells (Fig. 26Ab). Some have ethical concerns about this type of therapeutic cloning, because if the embryo were allowed to continue development it would become an individual. The second way to carry out therapeutic cloning is to use adult stem cells. Stem cells are found in many organs of an adult’s body; for

culture

remove and discard egg nucleus

egg

embryonic stem cells

Implant embryo into surrogate mother Clone is born

nervous remove G0 nucleus G0 somatic cells

fuse egg with G0 nucleus

blood culture embryonic stem cells

muscle

b. Therapeutic cloning

Figure 26A  Reproductive and therapeutic cloning.  a. The purpose of reproductive cloning is to produce an individual that is genetically

identical to the one that donated a nucleus. The nucleus is placed in an enucleated egg, and after several mitotic divisions, the embryo is implanted into a surrogate mother for further development. b. The purpose of therapeutic cloning is to produce specialized tissue cells. A nucleus is placed in an enucleated egg, and after several mitotic divisions, the embryonic cells (called embryonic stem cells) are separated and treated to become specialized cells.



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DNA in the following manner when DNA has the sequence of bases GAATTC:

associated with a disease and thus facilitate the development of medicines or treatments. This information also serves as the foundation for the study of forensic biology and even contributes to our understanding of our evolutionary history (see Chapter 32). When DNA technology was in its inception in the early 1970s, this technique was performed manually using dye-terminator substances or radioactive tracer elements attached to each of the four nucleotides during DNA replication, with results being deciphered from their pattern on a gel plate. Modern-day sequencing involves dyes attached to the nucleotides and use of a laser to detect the different dyes by an automated sequencing machine, which shows the order of nucleotides on a grid computer screen. To begin sequencing a segment of DNA, many copies of the ­segment are made, or replicated, using a procedure called the polymerase chain reaction.

DNA A G A A T T C G C duplex T C T T A A G C G restriction enzyme A A T T C G C A G

G C G

"sticky ends"

T C T T A A

Notice that a gap now exists into which a piece of foreign DNA can be placed if it ends in bases complementary to those exposed by the restriction enzyme. To ensure this, it is necessary only to cleave the foreign DNA with the same type of restriction enzyme. The single-stranded, but complementary, ends of the two DNA molecules are called “sticky ends” because they can bind a piece of foreign DNA by complementary base pairing. Sticky ends facilitate the insertion of foreign DNA into vector DNA. DNA ligase, an enzyme that functions in DNA replication, is then used to seal the foreign piece of DNA into the vector. Bacterial cells take up recombinant plasmids, especially if the cells are treated to make them more permeable. Thereafter, as the plasmid replicates, so does the foreign DNA and thus the gene is cloned.

The Polymerase Chain Reaction The polymerase chain reaction (PCR) can create billions of copies of a segment of DNA in a test tube in a matter of hours. PCR is very specific—it amplifies (makes copies of) a targeted DNA sequence, usually a few hundred bases in length. PCR requires the use of DNA polymerase, the enzyme that carries out DNA replication, and a supply of nucleotides for the new DNA strands. PCR involves three basic steps that occur repeatedly (Fig. 26.2), usually for about 35 to 40 cycles: (1) a denaturation step at 95°C, where DNA is heated to become single stranded; (2) an annealing step at a temperature usually between 50° and 60°C, where an oligonucleotide primer hybridizes to each of the single DNA strands; and (3)  an extension step at 72°C, where an engineered DNA polymerase adds complementary bases to each of the single DNA strands, creating double-stranded DNA. PCR is a chain reaction because the targeted DNA is repeatedly replicated, much in the same way natural DNA replication occurs, as long as the process continues. Notice in Figure 26.2 that

DNA Sequencing DNA sequencing is a procedure that determines the order of nucleotides in a segment of DNA, often within a specific gene. DNA sequencing allows researchers to identify specific alleles that are

1. Sample is first heated to denature DNA.

2. DNA is cooled to a lower temperature to allow annealing of primers.

DNA strand DNA segment to be amplified

DNA is denatured into single strands

5′

3′ 5′

3′

5′ 3′

5′

3′

3. DNA is heated to 72°C, the optimal 5′ temperature for Taq DNA 3′ polymerase to 5′ extend primers. 3′ 5′

3′

3′

3′ 5′ Primers anneal to DNA Taq DNA polymerase 5′ 3′ 5′ 3′

5′

3′

Cycle 2: 4 copies

5′

5′

3′

3′ 3′ 5′

5′ 3′

5′ 3′ 3′ 5′

5′ 3′

5′ 3′ 5′

5′ 3′

3′ 5′

Cycle 3: 8 copies

5′ 3′

5′

3′ 3′ 5′

5′ 3′

3′ 5′

5′ 3′ 3′ 5′

5′ 3′

3′

5′ 3′ 5′

5′ 3′

3′

5′

Figure 26.2  Polymerase chain reaction (PCR).  PCR allows the production of many identical copies of DNA in a laboratory setting. Assuming you start with only one copy, you can create millions of copies with only 20 to 25 cycles. For this reason, only a tiny DNA sample is necessary for forensic genetics.



523

Collect DNA

marker

DNA Analysis

24

Crime scene

Suspect A

16 repeats

suspect B

12 repeats

12 repeats

suspect A

12 repeats

Perform PCR on repeats

16 repeats

crime scene evidence

12 repeats

the amount of DNA doubles with each replication cycle. Thus, assuming you start with only one copy of DNA, after one cycle, you will have two copies, after two cycles four copies, and so on. PCR has been in use since its development in 1985 by Kary Banks Mullis, and now almost every laboratory has automated PCR machines to carry out the procedure. Automation became possible after a temperature-insensitive (thermostable) DNA polymerase was extracted from the bacterium Thermus aquaticus, which lives in hot springs. The enzyme can withstand the high temperature used to denature double-stranded DNA. Therefore, replication does not have to be interrupted by the need to add more enzyme. DNA amplified by PCR is often analyzed for various purposes. For example, mitochondrial DNA base sequences have been used to decipher the evolutionary history of human populations. Because so little DNA is required for PCR to be effective, it is commonly used as a forensic method for analyzing DNA found at crime scenes—only a drop of semen, a flake of skin, or the root of a single hair is necessary!

Chapter 26  Biotechnology and Genomics

Suspect B

Number of repeats

Analysis of DNA following PCR has undergone improvements 22 over the years. At first, the entire genome was treated with restriction enzymes, and because each person has their own restriction 18 enzyme sites, they would have a unique collection of DNA frag16 ment sizes. During a process called gel electrophoresis, whereby 14 an electrical current is used to force DNA through a porous gel 12 material, these fragments are separated according to their size. Smaller fragments move farther through the gel than larger frag10 ments, and result in a pattern of distinctive bands, called a DNA 8 profile or DNA fingerprint. Now, short tandem repeat (STR) profiling is the method of 2 choice. STRs are the same short sequence of DNA bases that recur several times, as in GATAGATAGATA. STR profiling is advantaUse gel electrophoresis to identify criminals geous because it doesn’t require the use of restriction enzymes. Figure 26.3  DNA fingerprinting.  To establish a DNA fingerprint, Instead, PCR is used to amplify target sequences of DNA, which short segments of DNA from samples are amplified by a PCR reaction. are fluorescently labeled. The PCR products are placed in an autoThese fragments are then separated using gel electrophoresis (or a detector) mated DNA sequencer. As the sequences move through the to look for small variations in the length of the fragments. sequencer, the fluorescent labels are picked up by a laser. A detector then records the length of each DNA fragment. The fragments DNA fingerprints from blood or tissues at a crime scene have been are different lengths because each person has their own number of successfully used in convicting criminals. DNA fingerprinting repeats at the particular location of the STR on the chromosome through STR profiling was extensively used to identify the victims (i.e., each STR locus). That is, the greater the number of STRs at a of the tsunamis in the past few years in Indonesia and Japan. Relalocus, the longer the DNA fragment amplified by PCR. If inditives can be found, paternity suits can be settled, and genetic disorviduals are homozygotes, they will have a single fragment, and ders can be detected. PCR has also shed new light on evolutionary heterozygotes will have two fragments of different lengths studies by comparing DNA extracted from human mummies thou(Fig.  26.3). The more STR loci employed, the more confident sands of years old or animal fossils millions of years old. The ­scientists can be of distinctive results for each person.  National Football League uses synthetic DNA to mark each of the The FBI has collected a large database of human STR profiles Super Bowl footballs to be able to authenticate them. for current and future crime scene analyses, and genetic profiles may someday be on peoples’ driver’s licenses in the United States. Check Your Progress  26.1 The Bioethical feature, “DNA Fingerprinting and the Criminal Justice System,” explores some of the pros and cons associated 1. Summarize the two required steps for producing with DNA forensics that may result from these trends. recombinant DNA. Applications of DNA technology are limited only by our imag 2. Explain how the PCR reaction amplifies a segment of DNA. inations. When the DNA matches that of a virus or mutated gene, it 3. Explain why STRs may be used for identification. is known that a viral infection, genetic disorder, or cancer is present.

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DNA Fingerprinting and the Criminal Justice System Traditional fingerprinting has been used for years to identify criminals and to exonerate those wrongly accused of crimes. The opportunity now arises to use DNA fingerprinting in the same way. DNA fingerprinting requires only a small DNA sample, which can come from blood left at the scene of the crime, semen from a rape case, or even a single hair root! Advocates of DNA fingerprinting claim that identification is “beyond a reasonable doubt.” But how can investigators be certain? Much of the forensic DNA fingerprinting done today uses short tandem repeats (STRs)— stretches of noncoding DNA in our genome that contain repeated DNA sequences. Most commonly, these repeats are four bases in length— for example, CATG. You may have 11 copies of this repeat on a particular chromosome inherited from your father and only 3 copies on the homologous chromosome from your mother. When analyzed by electrophoresis, greater numbers of repeats correspond to increasing lengths on the DNA. People have unique repeat patterns, so these STRs can be used to discriminate between individuals. A particular STR pattern on a single chromosome may be shared by

a number of people. However, by studying multiple STR sites, a statistically unique pattern can be developed for everyone—unless you share your DNA with an identical twin! In the United States, the FBI’s Combined DNA Index System (CODIS) uses 13 STR sites (plus a marker for sex) to identify individuals. Opponents of this technology, however, point out that it is not without its problems. Police or laboratory negligence can invalidate the evidence. For example, during the O. J. Simpson trial, the defense claimed that the DNA evidence was inadmissible because it could not be proven that the police had not “planted” O. J.’s blood at the crime scene. There have also been reported problems with sloppy laboratory procedures and the credibility of forensic experts. In one case, Curtis McCarty had been placed on death row three times by the same prosecutor and police lab analyst. After 21 years in prison, he was exonerated. The prosecutor has been accused of misconduct, and the police lab analyst was fired for falsifying laboratory data to obtain convictions. In addition to identifying criminals, DNA fingerprinting can be used to establish

paternity and maternity; determine nationality for immigration purposes; and identify victims of a national disaster, such as the terrorist attacks of September 11, 2001, the tsunamis in Indonesia (2007) and Japan (2011), and the earthquakes in China (2009) and Haiti (2010). There have been recent suggestions that personal identification (such as passports) and even digital passwords (such as you use for your computer) should be based on a DNA fingerprint profile.

Questions to Consider 1. Would you be willing to provide your DNA for a national DNA databank? Why or why not? 2. If not everyone, do you think that convicted felons, at least, should be required to provide DNA for a databank? 3. Should all defendants have access to DNA fingerprinting (at government expense) to prove they didn’t commit a crime? Should this include those already convicted of crimes who want to reopen their cases using new DNA evidence?

26.2  Biotechnology Products Learning Outcomes Upon completion of this section, you should be able to 1. Identify ways in which human society benefits from genetically modified bacteria, plants, and animals. 2. Describe the steps involved in the production of a transgenic animal.

Today, transgenic bacteria, plants, and animals are often called genetically modified organisms (GMOs), and the products they produce are called biotechnology products (Fig. 26.4).

Transgenic Bacteria Recombinant DNA technology is used to produce transgenic bacteria, which are grown in huge vats called bioreactors. The bacteria express the cloned gene, and the gene product is usually collected from the medium in which the bacteria are grown. Biotechnology products produced by bacteria include insulin, human growth hormone, tPA (tissue plasminogen activator), and hepatitis B vaccine. Transgenic bacteria have many other uses as well. Some have been produced to promote the health of plants. For example, bacteria

Figure 26.4  Biotechnology products.  Products, such as the biodegradable plastic poncho shown here, or fuels like bioethanol, are becoming increasingly common.

that normally live on plants and encourage the formation of ice crystals have been changed from frost-plus to frost-minus bacteria. As a result, new crops such as frost-resistant strawberries are being developed. Also, a bacterium that normally colonizes the roots of corn



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Are Genetically Engineered Foods Safe? A series of focus groups conducted by the Food and Drug Administration (FDA) in 2000 showed that although most participants believed that genetically engineered foods, now also called GMOs, might offer benefits, they also feared possible unknown long-term health consequences. The discovery by activists that a type of genetically engineered corn called StarLink™ had inadvertently made it into the food supply triggered the recall of taco shells, tortillas, and many other corn-based foodstuffs from supermarkets. Further, the makers of StarLink™ were forced to buy back StarLink™ from farmers and to compensate food producers at an estimated cost of several hundred million dollars in late 2000. StarLink™ is a type of “Bt” corn. It contains a foreign gene taken from a common soil organism, Bacillus thuringiensis, which makes a protein that is toxic to many insect pests. About a dozen Bt varieties, including corn, potato, and even a tomato, have now been approved for human consumption. These strains contain a gene for an insecticidal protein called CryIA. Instead, StarLink™ contained a gene for a related protein called Cry9C, which researchers thought might slow down the chances of pest resistance to Bt corn. In order to get FDA approval for use in foods, the makers of StarLink™ performed the required tests. Like the other now-approved strains, StarLink™ wasn’t poisonous to rodents, and its biochemical structure is not similar to those of most chemicals in food that commonly cause allergic reactions in humans (called allergens). But the Cry9C protein resisted digestion longer than the other Bt proteins when it was put in simulated stomach acid and subjected to heat. Because most food allergens resist digestion in a

similar fashion, StarLink™ was not approved for human consumption. The scientific community is now trying to devise more tests for allergens because it has not been possible to determine conclusively whether Cry9C is or is not an allergen. Also, at this point, it is unclear how resistant to digestion a protein must be in order to be an allergen, and it is also unclear what degree of amino acid sequence similarity a potential allergen must have to a known allergen to raise concern. Dean D. Metcalfe, chief of the Laboratory of Allergic Diseases at the National Institute of Allergy and Infectious Diseases, said “We need to understand thresholds for sensitization to food allergens and thresholds for elicitation of a reaction with food allergens.” Other scientists are concerned about the following potential drawbacks to the planting of Bt corn: (1) resistance among populations of the target pest, (2) exchange of genetic material between the transgenic crop and related plant species, and (3) Bt crops’ impact on nontarget species. They feel that many more studies are needed before stating for certain that Bt corn has no ecological drawbacks. Despite controversies, the planting of genetically engineered corn has increased annually. The USDA reports that U.S. farmers planted genetically engineered corn on between 80% to 89% of all corn acres in 2014, up from 26% in 2001 (Fig. 26B). Today, almost 94% of soybean acreage, and 84% to 91% of cotton acreage is genetically modified. Some groups advocate that GMOs should be labeled as such, but this may not be easy to accomplish because, for example, most cornmeal is derived from

both conventional and genetically engineered corn. So far, there has been no attempt to sort out one type of food product from the other. However, at many health food stores, foods that do not contain GMOs are now labeled.

Questions to Consider 1. Do you think genetically modified organisms (GMOs) should be labeled? Construct an argument both for and against labeling GMOs. 2. Some people are strongly advocating the complete removal of GMOs from the market. A few people, called ecoterrorists, are taking such drastic actions as burning crops or even setting biotechnology labs on fire. Do you think GMOs should be removed from the market? Why or why not? What further information would you need to make your decision? 3. Rice is a staple in the diet of millions of people worldwide, many of them living in less-developed countries. In some of those same countries, vitamin A deficiency is a major cause of blindness in small, malnourished children. Scientists have developed a new form of rice, called “golden rice,” which is genetically modified to assist with the metabolism of vitamin A and might prevent millions of cases of blindness. The scientists who created golden rice claim it is safe for consumption, although critics say the health effects are not yet fully understood. Given its nutritional potential, should golden rice be planted and distributed on a wide scale? What information would you need to make your decision?

Figure 26B  Genetically engineered crops.  Genetically engineered (a) corn, (b) soybeans, and (c) cotton crops are increasingly being planted by today’s farmers.

a.

b.

c.

b.

c.



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One altered plant known as the pomato is the result of these technologies. This plant produces potatoes belowground and tomatoes aboveground. Foreign genes transferred to cotton, corn, and potato strains have made these plants resistant to pests because their cells now produce an insect toxin. Similarly, soybeans have been made resistant to a common herbicide that is sprayed to kill weeds that compete with soybean growth. Some corn and cotton plants are both pest- and herbicide-resistant. These and other genetically engineered crops that are expected to have increased yields are now commonly sold commercially. However, as discussed in the Health feature, “Are Genetically Engineered Foods Safe?,” the public is concerned about the possible effect of genetically modified organisms, also called GMOs, on human health, as well as the environment. Like bacteria, plants are also being engineered to produce human proteins, such as hormones, clotting factors, and antibodies, in their seeds. One type of antibody made by corn can deliver radioisotopes to tumor cells, and another made by soybeans can be used to treat genital herpes. Recently, antibodies against the Ebola virus are being produced by genetically-engineered tobacco plants.

Figure 26.5  Bioremediation.  Bacteria capable of decomposing oil have been engineered and patented by researchers such as Dr. Chakrabarty.

Transgenic Animals

plants has now been endowed with genes (from another bacterium) that code for an insect toxin. The toxin protects the roots from insects. Bacteria can be selected for their ability to degrade a particular substance, and this ability can then be enhanced by bioengineering. For instance, naturally occurring bacteria that eat oil can be genetically engineered to do an even better job of cleaning up beaches after oil spills (Fig. 26.5), such as the 2010 Deep Water Horizon spill in the Gulf of Mexico. Bacteria can also remove sulfur from coal before it is burned, resulting in cleaner emissions. One bacterial strain was given genes that allowed it to clean up levels of toxins that would have killed other bacterial strains. Further, these bacteria were given “suicide” genes that caused them to self-destruct when their job was done. Organic chemicals are often synthesized by having catalysts act on precursor molecules or by using bacteria to carry out the synthesis. Today, it is possible to go one step further and manipulate the genes that code for these enzymes. For instance, biochemists discovered a strain of bacteria that is especially good at producing phenylalanine, an organic chemical needed to make aspartame, better known as NutraSweet®. They isolated, altered, and cloned the appropriate genes so that various bacteria could be genetically engineered to produce phenylalanine.

Techniques have been developed to insert genes into the eggs of animals. It is possible to microinject foreign genes into eggs by hand, but another method uses vortex mixing. The eggs are placed in an agitator with DNA and silicon-carbide needles. The needles make tiny holes in the eggs through which the DNA can enter. When these eggs are fertilized, the resulting offspring are transgenic animals. Using this technique, many types of animal eggs have acquired the gene for bovine growth hormone (BGH). The procedure has been used to produce larger fishes, cows, pigs, rabbits, and sheep. Gene pharming, the use of transgenic farm animals to produce pharmaceuticals, is being pursued by a number of firms. Genes that code for therapeutic and diagnostic proteins are incorporated into an animal’s DNA, and the proteins appear in the animal’s milk. Plans are under way to produce drugs for the treatment of cystic fibrosis, cancer, blood diseases, and other disorders by this method. Figure 26.6 outlines the procedure for producing transgenic animals: DNA containing the gene of interest is injected into donor eggs. Following in vitro fertilization, the zygotes are placed in host females, where they develop. After female offspring mature, the product is secreted in their milk. Eliminating a gene is another way to study a gene’s function. A knockout mouse has had both alleles of a gene removed or made nonfunctional. For example, scientists have constructed a knockout mouse lacking the CFTR gene, the same gene mutated in cystic fibrosis patients. The mutant mouse has a phenotype similar to a human with cystic fibrosis and can be used to test new drugs for the treatment of the disease.

Transgenic Plants Techniques have been developed to introduce foreign genes into immature plant embryos or into plant cells called protoplasts that have had their cell wall removed. It is possible to treat protoplasts with an electric current while they are suspended in a liquid containing foreign DNA. The electric current makes tiny, self-sealing holes in the plasma membrane through which the desired genetic material can enter. Protoplasts go on to develop into mature plants containing and expressing the foreign DNA.

Check Your Progress  26.2 1. List some of the beneficial applications of transgenic bacteria, plants, and animals.

2. Distinguish between a transgenic organism and a cloned organism.



Chapter 26  Biotechnology and Genomics

human gene for growth hormone

Brain (gene transfer by injection)* • Huntington disease • Alzheimer disease • Parkinson disease • brain tumors

microinjection of human gene

Skin (gene transfer by modified blood cells)** • skin cancer

donor of egg

development within a host goat

human growth hormone

Lungs (gene transfer by aerosol spray)* • cystic fibrosis • hereditary emphysema Liver (gene transfer by modified implants)** • familial hypercholesterolemia

Transgenic goat produces human growth hormone.

milk

Blood (gene transfer by bone marrow transplant)* • sickle-cell disease

a.

Endothelium (blood vessel lining) (gene transfer by implantation of modified implants)** • hemophilia • diabetes mellitus

transgenic goat cells with gene for human growth hormone microinjection of these 2n nuclei into enucleated donor eggs enucleated eggs

Muscle (gene transfer by injection)* • Duchenne muscular dystrophy

donor of eggs

Bone marrow (gene transfer by implantation of modified stem cells)** • SCID • sickle-cell disease

development within host goats

milk

527

Cloned transgenic goats produce human growth hormone.

* in vivo ** ex vivo

Figure 26.7  Gene therapy.  Sites of ex vivo and in vivo gene therapy to cure the conditions noted.

b.

Figure 26.6  Production of transgenic animals.  a. A genetically engineered egg develops in a host to create a transgenic goat that produces a biotechnology product in its milk. b. Nuclei from the transgenic goat are transferred into donor eggs, which develop into cloned transgenic goats.

26.3  Gene Therapy Learning Outcome Upon completion of this section, you should be able to 1. Compare and contrast in vivo and ex vivo gene therapy.

Gene Therapy Once a genetic disorder is detected, gene therapy is a potential course of treatment. Gene therapy is the insertion of genetic

­ aterial into human cells for the treatment of genetic disorders and m various other human illnesses, such as cardiovascular disease and cancer.  Figure 26.7 shows regions of the body that have received copies of normal genes by various methods of gene transfer. Viruses genetically modified to be safe can be used to ferry a normal gene into the body, and so can liposomes, which are microscopic globules of lipids specially prepared to enclose the normal gene (i.e., ex vivo gene therapy). On the other hand, sometimes the gene is injected directly into a particular region of the body (i.e., in vivo gene therapy). Note that despite its promise for treating disorders, gene therapy may have detrimental side effects for some patients, such as causing leukemia or invoking an immune response. Nonetheless, some strides are being made for a number of human diseases, some of which are described in the following sections.



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2. Use retroviruses to bring the normal gene into the bone marrow stem cells.

1. Remove bone marrow stem cells.

retrovirus

defective gene

4. Return genetically engineered cells to patient.

viral recombinant DNA reverse transcription viral recombinant RNA

viral recombinant RNA normal gene

3. Viral recombinant DNA carries normal gene into genome.

normal gene

Figure 26.8  Ex vivo gene therapy in humans.  Bone marrow stem cells are withdrawn from the body, an RNA retrovirus is used to insert a normal gene into them, and they are then returned to the body.

Ex Vivo Gene Therapy

In Vivo Gene Therapy

Figure 26.8 describes an ex vivo methodology for treating children who have SCID (severe combined immunodeficiency). These children lack the enzyme ADA (adenosine deaminase), which is involved in the maturation of T and B cells. Therefore, these c­ hildren are prone to constant infections and may die without ­treatment. To carry out gene therapy, bone marrow stem cells are removed from the bone marrow of the patient and infected with a virus that carries a normal gene for the enzyme into their DNA. Then the cells are returned to the patient, where it is hoped they will divide to produce more blood cells with the same genes. Patients who have undergone this procedure show significantly improved immune function associated with a sustained rise in the level of ADA enzyme activity in the blood. Another example of ex vivo gene therapy has been used to treat familial hypercholesterolemia, a condition that develops when liver cells lack a receptor protein for removing cholesterol from the blood. The high levels of blood cholesterol make the patient subject to fatal heart attacks at a young age. A small portion of the liver is surgically excised and then infected with a virus containing a normal gene for the receptor before being returned to the patient. Patients are expected to experience lowered serum cholesterol levels following this procedure.

Cystic fibrosis patients lack a gene that codes for the transmembrane carrier of the chloride ion (for genetics of this disease see section 23.2). They often die due to numerous infections of the respiratory tract because a thick mucus forms in the lungs and attracts bacteria and other antigens. In gene therapy trials, the gene needed to cure cystic fibrosis is sprayed into the nose or delivered to the lower respiratory tract by an adenovirus vector or by using liposomes. So far, these treatments have met with limited success, but investigators are trying to improve uptake by using a combination of different vectors. Gene therapy is increasingly relied upon as a part of cancer treatment. Genes are being used to make healthy cells more tolerant of chemotherapy, while making tumor cells more sensitive. Knowing that the tumor suppressor gene p53 brings about apoptosis (cell death), researchers are interested in finding a way to selectively introduce p53 into cancer cells, and in that way, kill them.

Check Your Progress  26.3 1. Summarize the methods that are being used to introduce genes into humans for gene therapy.

2. Provide examples of ex vivo and of in vivo gene therapy.



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Testing for Genetic Disorders Prospective parents know if either of them has an autosomal dominant disorder because the person will show it. However, genetic testing is required to detect if either is a carrier for an autosomal recessive disorder. If a woman is already pregnant, the parents may want to know if the unborn child has the disorder. If the woman is not pregnant, the parents may opt for testing of an embryo or egg before she does become pregnant. One way to detect genetic disorders is to test the DNA for mutated genes.

Testing the DNA DNA testing typically uses procedures that test for a specific genetic marker, or probe the genome for sequences of interest using DNA microarrays. Testing for a genetic marker is similar to the traditional procedure for DNA fingerprinting (see section 26.2). As an example, consider that individuals with Huntington disease have an abnormality in the sequence of their bases at a particular location on a chromosome. This abnormality in sequence is a genetic marker. Huntington disease, specifically, results from a STR that is so long that it actually causes a frameshift mutation within a gene even though the STR itself occurs outside, but nearby the gene. In this and similar cases, the length of the STR can be detected with PCR and analysis on an automated DNA sequencer.

DNA Microarrays With advances in robotic technology, it is now possible to place the entire human genome onto a single microarray (Fig. 26C). The mRNA from the organism or the cell to be tested is labeled with a fluorescent dye and added to the chip. When the mRNAs bind to the microarray, a fluorescent pattern results that is recorded by a computer. Now the investigator knows what DNA is active in that cell or organism. A researcher can use this method to determine the difference in gene expression between two different cell types, such as between liver cells and muscle cells. A mutation microarray, the most common type, can be used to generate a person’s genetic profile. The microarray contains hundreds to thousands of known disease-associated mutant

DNA probe array

tagged DNA did bind to probe DNA probe

tagged DNA

tagged DNA did not bind to probe

testing subject's DNA

Figure 26C  Use of a DNA microarray to test for a genetic disorder.  This DNA

chip contains rows of DNA sequences for mutations that indicate the presence of particular genetic disorders. If DNA fragments derived from an individual’s DNA bind to a sequence representing a mutation on the DNA chip, that sequence fluoresces, and the individual has the mutation.

gene alleles. Genomic DNA from the individual to be tested is labeled with a fluorescent dye, and then added to the microarray. The spots on the microarray fluoresce if the individual’s DNA binds to the mutant genes on the chip, indicating that the individual may have a particular disorder or is at risk for developing it later in life. This technique can generate a genetic profile much more quickly and inexpensively than older methods involving DNA sequencing. DNA microarrays also promise to hasten the identification of genes associated with diseased tissues. In the first instance, mRNA derived from diseased tissue and normal tissue is labeled with different fluorescent dyes. The normal tissue serves as a control. The investigator applies the mRNA from both normal and abnormal tissue to the microarray. The relative intensities of fluorescence from a spot on the microarray indicate the amount of mRNA originating from that gene in the diseased tissue relative to the normal tissue. If a gene is activated in the disease, more copies of mRNA will bind to the microarray than from the control tissue, and the spot will appear more red than green.

Genomic microarrays are also used to identify links between disease and chromosomal variations. In this instance, the chip contains genomic DNA that is cut into fragments. Each spot on the microarray corresponds to a known chromosomal location. Labeled genomic DNA from diseased tissues and control tissues bind to the DNA on the chip, and the relative fluorescence from both dyes is determined. If the number of copies of any particular target DNA has increased, more sample DNA will bind to that spot on the microarray relative to the control DNA, and a difference in fluorescence of the two dyes will be detected.

Questions to Consider 1. What benefits are there when using a DNA microarray over a genetic marker such as a STR? 2. Why might a researcher want to know what genes are being expressed in different cell types? 3. How might the information from a DNA microarray be used to develop new drugs to treat disease?



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UNIT 5  Continuance of the Species

26.4  Genomics, Proteomics, and Bioinformatics Learning Outcomes Upon completion of this section, you should be able to 1. Discuss the implications of knowing the human genome sequence. 2. Describe a major insight from comparative genomics. 3. Compare and contrast functional genomics and proteomics.

In the preceding century, researchers discovered the structure of DNA, how DNA replicates, and how DNA and RNA are involved in the process of protein synthesis. Genetics in the twenty-first century largely concerns genomics, the study of the complete genetic sequences of humans and other organisms. Knowing the sequence of bases in genomes is the first step, and mapping their location on the chromosomes is the next step. The enormity of the task can be appreciated by knowing not only that we have approximately 23,000 genes that code for proteins (the actual number has yet to be determined), but also that nearly 98% of the approximately 3.2 billion bases of our genome is noncoding and contains many repetitive sequences of unidentified function. Many other organisms have an even larger number of protein-coding genes, but fewer noncoding regions when compared to the human genome.

Sequencing the Genome We now know the sequence of the roughly 3.2 billion pairs of DNA bases in our genome. Stretched out, the DNA in each of our cells is about 5 feet long, and the nucleotides would make a book over a half million pages if printed as text. This feat, which has been compared to completion of the periodic table of the elements in chemistry, was accomplished by the Human Genome Project (HGP), a 13-year effort that involved both university and private laboratories around the world. How did they do it? First, investigators developed a laboratory procedure that would allow them to decipher a short sequence of base pairs, and then instruments became available that could carry out sequencing automatically. Over the 13-year span, DNA sequencers were constantly improved, and today’s instruments can automatically analyze up to 120 million base pairs of DNA in a 24-hour period. So, new genomes are being sequenced all the time, and at a much faster rate than the human genome. For example, the genome of the African clawed frog, Xenopus laevis, which is roughly the same size as the human genome, was sequenced in under a year. Completion of the human genome sequence has opened up great possibilities for biomedical research and treatment. These methods are widely being used to screen individuals for risk associated with specific diseases (cancer, diabetes, etc). The HGP also led to the discovery of many small regions of DNA that vary among individuals (polymorphisms). Most of these are single nucleotide polymorphisms (SNPs), meaning that they have a difference of only one nucleotide. Many SNPs have no effect. Others may contribute to protein-coding differences affecting the phenotype. It’s possible that certain SNP patterns change an individual’s susceptibility to disease and alter their response to medical treatments. These discoveries have now led pharmaceutical companies to consider producing “designer drugs,” which are tailored for an individual’s genotype.

Determining that humans have approximately 23,000 genes required a number of techniques, many of which relied on identifying RNAs in cells and then working backward to find the DNA that can pair with that RNA. Structural genomics—knowing the sequence of the bases and how many genes we have—is now being followed by functional genomics. Most of the known human genes are expected to code for proteins. However, most of the human genome is noncoding because it does not specify the order of amino acids in a polypeptide. This noncoding DNA, once dismissed as “junk DNA,” is now known to serve many important functions, including the regulation of genes that code for proteins.

Genome Architecture Initially, researchers were somewhat surprised to discover that nearly 98% of the human genome is DNA that does not directly code for amino acid sequences. Some of the DNA that does not specify polypeptides is transcribed into ribosomal RNA and transfer RNA, both structural molecules involved in protein assembly. The rest of the genome consists of transposable elements (or transposons), repetitive DNA elements, and sequences with unknown function. Transposable elements make up approximately 44% of the human genome. Transposable elements, originally discovered by Barbara McClintock in 1950 (who later won a Nobel Prize for this work), are short sequences of DNA that are able to jump from one location on a chromosome to another. Their movement to a new location sometimes alters neighboring genes, particularly decreasing their expression. In other words, a transposon sometimes acts like a regulator gene. The movement of transposons throughout the genome is thought to be a driving force in the evolution of life. Around 44% of the human genome is made up of repetitive elements, which occur when the same sequence of two or more nucleotides (e.g., CACACA) are repeated many times along the length of one or more chromosomes. Although many scientists still dismiss them as having no function, others point out that the centromeres and telomeres of chromosomes are composed of repetitive elements and, therefore, repetitive DNA elements may not be as useless as once thought. Telomeres are repetitive DNA sequences found near the ends of chromosomes and are thought to help maintain their structural stability. In addition, perhaps repetitive sequences in centromeres may help with segregating sister chromatids during cell division.

Redefining the Gene Knowledge of the human genome sequences has changed the way researchers think about the concept of a “gene.” Historically, a gene was thought of as a particular location (locus) of a chromosome. While prokaryotes typically possess a single circular chromosome with genes that are tightly packed together, eukaryotic chromosomes are much more complex. The genes are seemingly randomly distributed along the length of a chromosome and are fragmented into exons, with intervening sequences called introns scattered throughout the length of the gene. In fact, 95% or more of most human genes is composed of introns. Recall that after transcription, introns need to be spliced out and exons joined together to form a functional mRNA transcript that will next be translated into a protein. Once regarded as merely intervening sequences, introns are now attracting attention as regulators of gene expression. The presence of introns allows exons to be put together in



Chapter 26  Biotechnology and Genomics

various sequences so that different mRNAs and proteins can result from a single gene. It could also be that introns function to regulate gene expression and help determine which genes are to be expressed and how they are to be spliced. In fact, entire genes have been found embedded within the introns of other genes. Thus, perhaps the modern definition of a gene should take the emphasis away from the chromosome and place it on the results of transcription. Previously, molecular genetics considered a gene to be a nucleic acid sequence that codes for the sequence of amino acids in a protein. In contrast to this definition, we have known for some time that all three types of RNA (rRNA, mRNA, and tRNA) are transcribed from DNA and that these RNAs are useful products. We also know that protein-coding regions can be interrupted by regions that do not code for a protein but do produce RNAs with various functions. In light of these new findings, Mark Gerstein and associates suggested a new definition in 2007: “A gene is a genomic sequence (either DNA or RNA) directly encoding functional products, either RNA or protein.” This definition takes into account three new things we have learned by investigating genomic sequences: (1) a gene product may not necessarily be a protein; (2) a gene may not be found at a particular locus on a chromosome; and (3) the genetic material need not be only DNA—some prokaryotes have RNA genes.

Functional and Comparative Genomics In addition to the human genome, the genomes of many other organisms, including a common bacterium, a yeast, and a mouse, are also complete (Table 26.1). Because we now know the nucleotide sequence of many genomes, we can now focus on comparative and functional genomics. Using comparative genomics, researchers have identified many similarities between the sequence of human bases and those of other organisms. Model organisms (e.g., those found in Table 26.1) can be used in these types of genetic analyses because they share many mechanisms and cellular pathways with other organisms, including humans. For example, scientists inserted a human gene associated with Parkinson disease into the fruit fly, Drosophila melanogaster, and the flies showed symptoms similar to those seen in humans with the disorder. These studies confirmed that the suspected gene is involved in Parkinson disease and suggested we might be able to use fruit flies to test potential therapies. Comparative genomics also offers a way to study changes in the genome through time because some diseases, as well as model

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organisms, have a shorter generation time than humans. In this way, we have been able to track the evolution of the human immunodeficiency virus (HIV), the virus that causes AIDS, in individual patients. Tracking the genome sequence of the virus through time has allowed scientists and doctors to understand how the virus responds to different drug therapy regimens and, in some cases, modify treatment to improve a patient’s life span. Comparing genomes will also help us understand the evolutionary relationships among organisms. One surprising discovery is that the genomes of all vertebrates are similar. Researchers were not surprised to find that the genomes of humans and chimpanzees were approximately 98% alike, but they did not expect to find that the human and mouse sequence were 85% similar. Genomic comparisons will likely yield improved ability to reconstruct evolutionary relationships among organisms, as discussed in Chapter 27. Comparative genomics also allows researchers to infer function of unknown genes in one species through amino acid similarity of those genes in another species. The aim of functional genomics is to understand the function of the various genes discovered within each genomic sequence and how these genes interact. In fact, functional genomics has utilized comparative genomics to assess similarities between human genes and genes of other organisms to help deduce the probable function of many of our estimated 23,000 genes. Functional genomics also uses DNA microarrays to monitor the expression of thousands of genes simultaneously. The use of a microarray can tell what genes are turned on in a specific cell or tissue type in a particular organism at a particular point in time and under certain environmental circumstances. For example, we could compare gene expression of a patient in different stages of cancer growth to assist with treatment. As discussed in section 26.3, DNA microarrays can also be used to identify various mutations in a human’s genome. This is called the person’s genetic profile, which can be used to determine if various genetic diseases are likely, as well as to suggest which drug therapy may be most appropriate based on the individual’s genotype.

Proteomics Now that entire genomes are being published for different species, there is a race to sequence their proteomes, or a species’ entire collection of proteins. Proteomics is the study of the structure, function, and interaction of cellular proteins, which differ depending on each

TABLE 26.1  Comparison of Sequenced Genomes

Drosophila melanogaster (fruit fly)

Arabidopsis thaliana (flowering plant)

Caenorhabditis elegans (roundworm)

Saccharomyces cerevisiae (yeast)

Organism

Homo sapiens (human)

Mus musculus (mouse)

Estimated Size

3.2 billion bases

2.5 billion bases

180 million bases

120 million bases

100 million bases

12.1 million bases

Approximate Number of Genes

~23,000

~23,000

~14,000

~26,000

~19,000

~6,300

Chromosome Number

46

40

8

10

12

32



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UNIT 5  Continuance of the Species

cell type. Each cell produces hundreds of different proteins that can vary between cells and within the same cell, depending on conditions. Therefore, the goal of proteomics is an overwhelming endeavor. Computer modeling of the three-dimensional shape of these proteins is an important part of proteomics. The study of protein shape and function is essential to the discovery of better drugs so that their chemical structure can match that of different protein shapes. One day, it may be possible to correlate drug treatment to the particular genome of the individual to increase efficiency and decrease side effects.

Bioinformatics Bioinformatics is the application of computer technologies, specially developed software, and statistical techniques to the study of biological information, particularly databases that contain much genomic and proteomic information (Fig. 26.9). The new data produced by structural genomics and proteomics have produced literally terabytes of raw data stored in databases that are readily available to research scientists. It is called raw data because billions of base pairs of DNA nucleotide sequence have little meaning by themselves. Functional genomics and proteomics are dependent on computer analysis to find significant patterns in the raw data. For example, BLAST, which stands for basic local alignment search tool, is a computer program that can identify homologous genes among the genomic sequences of model organisms. Homologous genes are genes that code for the same proteins, although the base sequence may be slightly different. Finding these differences can help identify the putative function of genes as new organisms’ genomes are sequenced, and also help to trace the history of evolution among a group of organisms. For example, researchers found the function of the protein that causes cystic fibrosis by using the computer to search for genes in model organisms that have the same sequence. Because they knew the function of this same gene in model organisms, they could deduce the function in humans. This was a necessary step toward possibly developing specific treatments for cystic fibrosis. Bioinformatics has various applications in human genetics. The human genome has 3.2 billion known base pairs, and without the computer it would be almost impossible to make sense of these data. For example, it is now known that an individual’s genome often contains multiple copies of a gene. But individuals may differ as to the number of copies—called copy number variations.

Figure 26.9  Bioinformatics.  New computer programs are being developed to make sense out of the raw data generated by genomics and proteomics. Bioinformatics allows researchers to study both functional and comparative genomics in a meaningful way. Now it seems that the number of copies of a gene in a genome can be associated with specific diseases. The computer can help make correlations between genomic differences among large numbers of people and certain diseases. It is safe to say that without bioinformatics, our progress in assembling DNA sequences into genomes; determining the function of DNA sequences; mapping genes on chromosomes; comparing our genome to model organisms; knowing how genes and proteins interact in cells; and so forth, would be extremely slow. Instead, with the help of bioinformatics, progress should proceed rapidly in these and other areas.

Check Your Progress  26.4 1. Explain the difference between genomics and proteomics. 2. Explain how comparative genomics can provide insights into gene function.

3. Discuss the importance of bioinformatics to the study of genomics and proteomics.

Conclusion Insulin for diabetes patients has been made since the late 1970s in large vats called bioreactors. A non-disease-producing strain of Escherichia coli has been made that contains the human gene for the production of insulin through recombinant DNA technology. In the bioreactor with the recombinant cells is also a medium that contains a food source for the bacteria. The bacteria stay alive and make billions of copies of themselves, while at the same time producing human insulin. The insulin is retrieved from the medium and used for injections. Insulin lispro (Humalog®) and insulin human recombinant (Humulin® N) are two very common types of medical insulin made in this manner.

Through reading this chapter, it should be clear that advances in genomics and biotechnology will allow us to better understand the causes of human diseases, as well as to develop potential treatments and cures. One of the great challenges of the twenty-first century will be to learn how to assemble and interpret the massive quantities of data generated through genome sequencing projects, comparative genomics, and proteomics. An even greater challenge to society is to use these data and the technologies that result in an ethically responsible manner. There are many ethical dilemmas facing the use of biotechnology, and thus the need for everyone to understand the principles of the material presented in this chapter.



Chapter 26  Biotechnology and Genomics

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MEDIA STUDY TOOLS http://connect.mheducation.com Maximize your study time with McGraw-Hill SmartBook®, the first ­adaptive textbook.

 

Animations

26.1  Restriction Endonucleases • Polymerase Chain Reaction 26.2  Early Genetic Engineering Experiment 26.3  DNA Microarray 26.4  Transposons

SUMMARIZE 26.1  DNA Technology ■ Knowledge of the genome has enabled scientists to conduct genetic

engineering and several types of cloning. Gene cloning can be used to isolate a gene and produce many copies of it. The gene can be studied in the laboratory or inserted into a bacterium, plant, or animal, producing a transgenic organism. Common methods of studying DNA molecules include recombinant DNA (rDNA) technology and the polymerase chain reaction (PCR). Recombinant DNA contains DNA from two different sources. A restriction enzyme cleaves both vector (often a plasmid) DNA and foreign DNA. The resulting “sticky ends” facilitate insertion of foreign DNA into vector DNA. The foreign gene is sealed into the vector DNA by DNA ligase. ■ PCR uses a heat-resistant DNA polymerase to quickly make multiple copies of a specific piece (target) of DNA. PCR is a chain reaction because the targeted DNA is replicated over and over again. Analysis of DNA segments following PCR may generate a DNA fingerprint (or profile). These have multiple uses from assisting genomic research to DNA forensics studies. Increasingly, short tandem repeat (STR) profiling is being used to identify individuals or for forensic studies.

  Tutorials 26.1  Polymerase Chain Reaction

26.3  Gene Therapy ■ Gene therapy, by either ex vivo or in vivo methods, is used to correct

the genotype of humans and to cure various human ills by giving the patient a foreign gene. ■ During ex vivo therapy, cells are removed from the patient, treated, and returned to the patient. Ex vivo gene therapy has apparently helped children with SCID lead normal lives. ■ In vivo therapy consists of directly giving the patient a foreign gene that will improve his or her health. Although it has limited success for treating cystic fibrosis, a number of in vivo therapies are being employed in the war against cancer and other human illnesses, such as cardiovascular disease.

26.4  Genomics, Proteomics, and Bioinformatics ■ Genomics is the study of the complete genetic sequences of a spe-

■

26.2  Biotechnology Products ■ Transgenic organisms, also called genetically modified organisms ■

■

■ ■

(GMOs), have had a foreign gene inserted into them. Genetically modified bacteria, agricultural plants, and farm animals now produce biotechnology products of interest to humans, such as hormones and vaccines. Bacteria usually secrete the product, but the seeds of plants and the milk of animals contain the product. Transgenic bacteria have also been engineered to promote the health of plants, extract minerals, and produce medically important chemicals. Transgenic crops, engineered to resist herbicides and pests, are commercially available. Transgenic animals have been given various genes, in particular the one for bovine growth hormone (BGH). Cloning of whole animals is now possible.

■

■

■ ■

cies. Because of the Human Genome Project (HGP), researchers now know the sequence of all the base pairs of the human genome. So far, between approximately 23,000 genes that code for proteins have been found; the rest of our DNA consists of noncoding regions. Noncoding regions include introns, transposable elements, and repetitive DNA. Noncoding DNA, once dismissed as “junk,” is now thought to have important functions that may include maintaining structural integrity of chromosomes and gene regulation. Currently, researchers are placing an emphasis on functional and comparative genomics. Functional genomics aims to understand the function of protein-coding regions and noncoding regions of our genome. To that end, researchers are utilizing new tools such as DNA microarrays to generate genetic profiles. Comparative genomics has revealed little difference between the DNA sequence of our bases and those of many other organisms. Genome comparisons have revolutionized our understanding of evolutionary relations by revealing previously unknown relationships between organisms. Proteomics is the study of which genes are active in producing proteins in which cells and under which circumstances. Bioinformatics is the use of computers to assist with analysis of data from proteomics and functional and comparative genomics.



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UNIT 5  Continuance of the Species

ASSESS Testing Yourself Choose the best answer for each question.

26.1  DNA Technology 1. Which of the following enzymes are needed to introduce foreign DNA into a vector? a. DNA gyrase and DNA ligase b. DNA ligase and DNA polymerase c. DNA gyrase and DNA polymerase d. restriction enzyme and DNA gyrase e. restriction enzyme and DNA ligase 2. In this process, a gene of interest is inserted into the chromosome of a bacterium, allowing the gene to be expressed within the cell. a. polymerase chain reaction b. DNA sequencing c. DNA cloning d. microarray technology e. DNA replication 3. The polymerase chain reaction a. uses RNA polymerase. b. takes place in huge bioreactors. c. uses a temperature-insensitive enzyme. d. makes lots of nonidentical copies of DNA. e. All of these are correct.

26.2  Biotechnology Products 4. Which of the following statements is incorrect? a. Bacteria usually secrete the biotechnology product into the medium. b. Plants are being engineered to have human proteins in their seeds. c. Animals are engineered to have a human protein in their milk. d. Animals can be cloned, but plants and bacteria cannot. 5. Bacteria are able to successfully transcribe and translate human genes because a. both bacteria and humans contain plasmid vectors. b. bacteria can replicate their DNA, but humans cannot. c. human and bacterial ribosomes are vastly different. d. the genetic code is nearly universal.

26.3  Gene Therapy 6. In this process, cells are removed from the body and a vector is used to insert a gene of interest. a. DNA fingerprinting c. ex vivo gene therapy b. in vivo gene therapy d. DNA sequencing 7. When a cloned gene is used to modify a human disease, the process is called a. bioremediation. d. gene pharming. b. gene therapy. e. DNA fingerprinting. c. genetic profiling.

26.4  Genomics, Proteomics, and Bioinformatics 8. This field of study examines the function and interaction of proteins within a cell or an organism. a. genomics b. bioengineering c. proteomics d. gene therapy e. None of these are correct. 9. Comparative genomics a. is the application of computer technologies to the study of the genome. b. is the study of the structure, function, and interaction of cellular proteins. c. can be used to understand human gene function by investigating genes in other species. d. involves studying all the genes that occur in a cell. e. is the study of a person’s complete genotype, or genetic profile. 10. Bioinformatics can a. assist genomics and proteomics. b. compare our genome to that of a monkey. c. depend on computer technology. d. match up genes with proteins. e. All of these are correct.

ENGAGE BioNOW Want to know  how this science is relevant to your life? Check out the BioNow video below: ■ Glowing Fish Genetics

Thinking Critically 1. We can use transgenic viruses to infect humans and help treat genetic disorders. That is, the viruses are genetically modified to contain “normal” human genes to try to replace nonfunctional human genes. Using this type of gene therapy, a person is infected with a particular virus, which then delivers the “normal” human gene to cells by infecting them. What are some pros and cons of viral gene therapy? 2. In a genomic comparison between humans and yeast, what genes would you expect to be similar?

PHOTO CREDITS Opener: © Clynt Garnham Medical/Alamy; 26.4: © Europics/Newscom; 26.5(bacteria): © Medical-on-Line/Alamy; 26.5(researcher): © CEK/AP Images; 26Ba: © USDA/Doug Wilson, photographer; 26Bb: © Norm Thomas/Science Source; 26Bc: © Pixtal/Age fotostock RF; 26C(array, right): © Deco/Alamy RF; Table 26.1 (human): © Image Source/ Jupiterimages RF; (mouse): © ImageBroker/Superstock RF; (fly): © Hermann Eisenbeiss/ Science Source; (plant): © USDA/Peggy Greb, photographer; (roundworm): © Sinclair Stammers/Science Source; (yeast): © G. Wanner/Getty Images; 26.9: © Geno Zhou/ FeatureChina/Newscom.

UNIT 6  Evolution and Diversity

CASE STUDY Evolution of Antibiotic Resistance You have probably heard of antibiotic-resistant bacteria such as MRSA ­(methicillin-resistant Staphylococcus aureus), a concern of hospitals and medical facilities. Unfortunately, examples of antibiotic resistant bacteria are becoming more common. One of these is Shigella sonnei (shown above), a bacteria that is commonly found in human feces, but when it is ingested, can cause diarrhea and intestinal problems. Globally, around 100 million people per year are infected with Shigella, and over 600,000 die from it. Normally, regular hand washing with soap and warm water prevents most Shigella infections. Historically, antibiotics have been used to treat Shigella, but an antibiotic-resistance strain of Shigella has developed that is becoming a concern of the health industry.  Use and overuse of antibiotics has resulted in evolution of resistant bacterial strains. Although we tend to think of evolution as happening over long timescales, human activities can accelerate the process of evolution quite rapidly. In fact, evolution of resistance to the antibiotic methicillin occurred in just one year! Antibiotic-resistant strains of bacteria are generally hard to treat, and treatment of infected patients can cost thousands of dollars. Some scientists believe that “superbugs,” or bacteria that have evolved antibiotic resistance, will be a far bigger threat to human health than emerging diseases such as H1N1 flu and AIDS. The good news is that our understanding of evolutionary biology has helped change human behavior to deal with superbugs. For example, doctors no longer prescribe antibiotics unless they are relatively certain a patient has a bacterial infection. Antibiotic resistance is an example of why evolution is important in people’s everyday lives. In this chapter, you will learn about evidence that indicates evolution has occurred and about how the evolutionary process works. As you read through the chapter, think about the following questions:

1. In the case of evolution of antibiotic resistance, what type of selection is

Evolution of Life

27

CHAPTER OUTLINE 27.1  Theory of Evolution 27.2 Evidence of Evolution 27.3 Microevolution 27.4 Processes of Evolution 27.5 Macroevolution and Speciation 27.6 Systematics BEFORE YOU BEGIN

Before beginning this chapter, take a few moments to review the following discussions: Sections 3.2 and 3.3  What are the differences between eukaryotic and prokaryotic cells? Section 23.1  What is the genetic basis of inheritance? Section 25.1  How does the structure of DNA relate to its role as the genetic material?

operating?

2. Why would resistance to antibiotics evolve more quickly in some species than others?

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UNIT 6  Evolution and Diversity

27.1  Theory of Evolution Learning Outcomes Upon completion of this section, you should be able to 1. List two observations Darwin made on his voyage that influenced the development of his theory. 2. Identify how Darwin’s and Lamarck’s theories of evolution are different. 3. Summarize the four main observations that make up Darwin’s theory of natural selection.

In December 1831, a new chapter in the history of biology had its humble origins. A 22-year-old naturalist, Charles Darwin, set sail on a journey of a lifetime aboard the British naval vessel HMS Beagle (Fig. 27.1). Darwin’s primary mission on this voyage was to serve as the ship’s naturalist—to collect and record the geological and biological diversity he saw during the voyage. Initially, Darwin was a supporter of the long-held idea that species had remained unchanged since the time of creation. During the five-year voyage of the Beagle, Darwin’s observations challenged his own belief that species do not change over time. His observations of geological formations and species variation

Figure 27.1  Voyage of the HMS Beagle.  The map shows the journey of the HMS Beagle around the world. As Darwin traveled along the east coast of South America, he noted that a bird called a rhea looked like the African ostrich. On the Galápagos Islands, marine iguanas, found no other place on Earth, use their large claws to cling to rocks and their blunt snouts for eating marine algae.

led him to propose a new process by which species arise and change. This process, called evolution, proposes that species arise, change, and become extinct due to natural, not supernatural, forces. This new view was not readily accepted by Darwin’s peers, but it gained gradual credibility as a result of a scientific and intellectual revolution that began in Europe in the late 1800s. Today, over 150 years since Darwin first published his idea of natural selection, the principle he proposed has been subjected to rigorous scientific tests, so much so that it is now considered one of the unifying theories of biology. Darwin’s theory of evolution by natural selection explains both the unity and diversity of life on Earth, how all living organisms share a common ancestor, and how species adapt to various habitats and ways of life. Although many have believed that Darwin forged this change in worldview by himself, several biologists during the preceding century and some of Darwin’s contemporaries had a large influence on Darwin as he developed his theory. The European scientists of the eighteenth and nineteenth centuries were keenly interested in understanding the nature of biological diversity. This

Charles Darwin

HMS Beagle

marine iguana

Galápagos Islands

rhea



Chapter 27  Evolution of Life

537

was a time of exploration and discovery as the natural history of new lands was mapped and documented. Shipments of new plants and animals from newly explored regions were arriving in England to be identified and described by biologists—it was a time of rapid expansion of our understanding of the Earth’s biological diversity. In this atmosphere of discovery, ­Darwin’s theory first took root and grew. One scientist that influenced Darwin as he developed his theory was Jean-Baptiste de Lamarck. Lamarck, a predecessor of Darwin, proposed one of the first hypotheses to explain how species evolve and Early giraffes probably had adapt to the environment. Lamarck developed the theEarly giraffes probably had necks of various lengths. short necks that they stretched ory of inheritance of acquired characteristics—that to reach food. the environment can bring about inherited change. One example he gave, and the one for which he is most famous, is that the long neck of a giraffe developed over time because animals stretched their necks to reach food high in trees and then passed gradually longer necks to their offspring (Fig. 27.2a). This hypothesis for the inheritance of acquired characteristics has never been verified. The reason is due to the molecular mechanism of inheritance. Changes to an organism’s visible characteristics, or phenotype (see section 23.1), acquired during an organism’s lifetime do not result in genetic changes that are heritable. As an example, ­consider tail cropping in Doberman pinschers. All Doberman puppies are born with tails, even though Natural selection due to Their offspring had longer competition led to survival of their parents’ tails are most often cropped. That is, tail necks that they stretched to the longer-necked giraffes and cropping is a phenotypic change that is not inherited in reach food. their offspring. the DNA.  While Lamark’s inheritance of acquired characteristics does not adequately explain the overall process of evolution, his work was important in advancing the discussion about evolutionary change over time. Recent findings in genetics also suggest that ­epigenetic inheritance, where chemical modifications to the DNA that occur in one ­generation may be passed on to later generations, actually represents a form of Larmarkian inheritance. In contrast to Lamarck, Darwin proposed that traits that provided an advantage must be passed on to the next generation, that is, must be heritable for adaptation Eventually, only long-necked Eventually, the continued to occur (Fig. 27.2b). In the example of the giraffe, giraffes survived the stretching of the neck led Darwin’s theory would predict that giraffe populations, competition. to today's giraffe. which have variation in the length of necks, would contain some individuals with longer necks than others. a. Lamark b. Darwin Those with the longer necks would be able to reach Figure 27.2  A comparison of Lamarck’s and Darwin’s theories more leaves, acquire more energy, and potentially proof evolution.  a. Jean-Baptiste de Lamarck’s proposal of the inheritance of duce more offspring. These offspring would inherit the acquired characteristics. b. Charles Darwin’s theory of natural selection. long neck trait from their parents. Over time, the longer neck trait would become more common in a population, resulting in an adaptation to the environment. selection. Darwin observed that some animals, such as the Darwin called this process of adaptation evolution by natural African ostrich and the South American rhea, looked very simiselection. Darwin’s theory developed over a long period of time. lar, but could not be related because they lived on different conDuring his voyage on the HMS Beagle, he made many observatinents. He reasoned that the similarity between these two large, tions that shaped the development of his theory of natural



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UNIT 6  Evolution and Diversity

a. Large, ground-dwelling finch

b. Warbler-finch

c. Cactus-finch

Figure 27.3  Three Galápagos finches.  Each of the 13 species of finches has a beak adapted to a particular way of life. For example: (a) the heavy beak of the large ground-dwelling finch (Geospiza magnirostris) is suited to a diet of large seeds; (b) the beak of the warbler-finch (Certhidea olivacea) is suited to feeding on insects found among ground vegetation or caught in the air; and (c) the longer beak, somewhat decurved, and the split tongue of the cactus-finch (Cactornis scandens) are suited to extracting the flesh of cactus fruit. flightless birds was due to adaptation to similar environments. While visiting the Galápagos Islands Darwin observed that the finches on each island were similar in overall appearance, but had differences in beak size and shape (Fig. 27.3). To explain these observations, Darwin speculated whether these different species of finches could have descended from a mainland finch species. In other words, he wondered if a finch from South America was the common ancestor to all the types on the Galápagos Islands. Perhaps new species had arisen because the geographic distance between the islands isolated populations of birds long enough for them to evolve independently. And perhaps the present-day species had resulted from accumulated changes that occurred within each of these isolated populations. With this and other types of evidence, Darwin returned home from his voyage and spent the next 20 years developing and finetuning his theory of natural selection. His theory can be summarized as follows (see Fig. 27.2b): ■■

■■

Individual organisms within a species exhibit variation that can be passed from one generation to the next—that is, they have heritable variation. Darwin emphasized that the members of a population vary in their functional, physical, and behavioral traits. He emphasized that variation, abundant in natural populations, must be heritable for the process of natural selection to operate. We now know that genes, along with the environment, determine the phenotype of individuals and trait variation within populations. Organisms compete for available resources. Darwin read an essay by Thomas Malthus in which he proposed that human population growth is kept in check because an environment can support only a limited number of people. Darwin realized that if all offspring born to a population were to survive, there would not be sufficient resources to support it indefinitely. He calculated the reproductive potential of elephants, assuming an average life span of 100 years and that each female bears no fewer than six young over her lifetime. If all these young were to survive and reproduce, then after

■■

■■

only 750 years, each breeding pair would have 19 million descendants! Obviously, no environment has the resources to support an elephant population of this magnitude, and no such elephant population has ever existed. Individual organisms within a population differ in terms of their reproductive success. Darwin observed that there are some individuals in a population that have favorable traits that enable them to better compete for limited resources, and thus devote more energy to reproduction. Darwin called this ability to have more offspring differential reproductive success. Natural selection occurs because certain members of a population happen to have an advantageous trait that allows them to survive and reproduce to a greater extent than do other members. For example, a mutation in a wild dog that increases its sense of smell may help it find prey. Organisms become adapted to conditions as the environment changes. Darwin’s theory proposes that natural selection through differential reproductive success shapes traits, inherited from predecessors, in response to the environment. The result of this descent with modification is adaptation. An adaptation is any evolved trait that helps an organism be more suited to its environment. Adaptations are especially recognizable when unrelated organisms living in a particular environment display similar characteristics. For example, manatees, penguins, and sea turtles all have flippers, which help them move through the water. Such adaptations to specific environments result from natural selection. Over time, selection can cause adaptive traits to be increasingly represented in the population. Evolution includes other processes in addition to natural selection (see sections 27.3 and 27.4), but natural selection is the only process that results in adaptation to the environment.



Chapter 27  Evolution of Life

Check Your Progress  27.1 1. Describe the four critical elements of Darwin’s theory. 2. Explain why Lamarck’s “inheritance of acquired

characteristics” does not explain evolutionary change.

3. Summarize the process of natural selection.

27.2  Evidence of Evolution Learning Outcomes Upon completion of this section, you should be able to 1. Define biological evolution. 2. Describe, with examples, four lines of evidence for evolution.

Biological evolution, or simply evolution, is all the changes that have occurred, due to differential reproductive success, in living organisms over geological time. As we discussed in section 27.1, differential reproductive success indicates that some individuals reproduce more than others because they are better suited to their environment. The Earth is about 4.6 billion years old and prokaryotes, probably the first living organisms, evolved about 3.5 billion years ago. The eukaryotic cell arose about 2.1 billion years ago, but multicellularity did not begin until perhaps 700 million years ago. This means that only single-celled

wing

wing

head

tail with vertebrae

tail a.

feet b.

Figure 27.4  Transitional fossils.  a. A fossil of Archaeopteryx,

now considered the first bird, has features of both birds and dinosaurs. Fossils indicate it had feathers and wing claws. Most likely, it was a poor flier. Perhaps it ran over the ground on strong legs and climbed into trees with the assistance of these claws. b. Archaeopteryx also had a feathercovered, reptilelike tail that shows up well in this artist’s representation.

539

organisms were present for 80% of the time that life has existed on Earth. Consequently, most evolutionary events we will be discussing in the next few chapters occurred in less than 20% of the history of life! Because of descent with modification, all living organisms share the same fundamental characteristics: They are made of cells, take chemicals and energy from the environment, respond to external stimuli, and reproduce (see section 1.1). Life is diverse because living organisms are adapted to different environments and the features that enable them to survive in those environments vary tremendously. Many fields of biology provide evidence that evolution through descent with modification occurred in the past and is still occurring. Let us look at the various types of evidence for evolution.

Fossil Evidence Fossils are the remains and traces of past life or any other direct evidence of past life. Most fossils consist only of hard parts of organisms, such as shells, bones, or teeth, because these are usually preserved after death. The soft parts of a dead organism are often consumed by scavengers or decomposed by bacteria. Occasionally, however, an organism is buried quickly and in such a way that decomposition is never completed or is completed so slowly that the soft parts leave an imprint of their structure. Traces include trails, footprints, burrows, worm casts, or even preserved droppings. The great majority of fossils are found embedded in sedimentary rock. Sedimentation, a process that has been going on since Earth formed, can take place on land or in bodies of water. The weathering and erosion of rocks produces particles that vary in size and are called sediment. As such particles accumulate, the sediments form strata (sing., stratum), or recognizable layers of rock. Any given stratum is older than the one above it and younger than the one immediately below it, so that the relative age of fossils can be determined based on their depth. feathers Paleontologists are biologists who study the fossil record and from it draw conclusions about the history of life. When fossils are arranged from oldest to youngest, they can provide evidence of evolutionary change through time. A particularly good example of this teeth is the horse, which evolved from a dog-sized, forest-dwelling tree browser with forward-looking eyes claws and teeth geared toward chewing leaves, to the animal we recognize as a horse today. Modern horses are adapted for an open field-type habitat, with eye sockets more to the sides of their head to allow for better peripheral vision and thereby detection of possible predators, as well as teeth geared toward grinding grasses. Particularly interesting are the fossils that serve as t­ ransitional links between groups. A famous example are the fossils of Archaeopteryx that lived about 165 million years ago (Fig. 27.4). The



540

UNIT 6  Evolution and Diversity

fossil clearly seems to be an intermediate form between dinosaurs and birds. Archaeopteryx had dinosaur-like features including jaws with teeth and a long, jointed tail, but it also had feathers and wings similar to those of modern birds. The fossils of Archaeopteryx are similar to other transitional fossils in that they have some traits like their ancestors and others like their descendants, rather than expressing intermediate traits.  Fossils have been discovered that support the hypothesis that whales had terrestrial ancestors. Ambulocetus natans (meaning the walking whale that swims) was the size of a large sea lion, with broad, webbed feet on its forelimbs and hindlimbs that enabled it to both walk and swim. It also had tiny hooves on its toes and the primitive skull and teeth of early whales (Fig. 27.5). Modern whales still have remnants of a hindlimb consisting of only a few bones that are very reduced in size. As the ancestors of whales adopted an increasingly aquatic lifestyle, the location of the nasal opening underwent a transition, from the tip of the snout as in Ambulocetus, to midway between the tip of the snout and the skull in Basilosaurus, to the very top of the head in modern whales (Fig.  27.5). Other transitional links among fossil vertebrates s­uggest that fishes evolved before amphibians, which evolved before reptiles, which evolved before mammals in the history of life.

a. Ambulocetus

50 MYA

b. Basilosaurus

40 MYA

Geological Timescale As a result of studying strata, scientists have divided Earth’s history into eras, and then periods and epochs (Table 27.1). The fossil record has helped determine the dates given in the table. There are two ways to age fossils. The relative dating method determines the relative order of fossils and strata depending on the layer of rock in which they were found, but it does not determine the actual date they were formed. The absolute dating method relies on radioactive dating techniques to assign an actual date to a fossil. All radioactive isotopes have a particular half-life, the length of time it takes for half of the radioactive isotope to change into another stable element. Carbon 14 (14C) is the only radioactive isotope in organic matter. Assuming a fossil contains organic matter, half of the 14C will have changed to nitrogen 14 (14N) in 5,730 years. For example, if a fossil has one-fourth the amount of radioactive 14C as a modern sample, then the fossil is approximately 11,460 years old (two half-lives).

Biogeographical Evidence The field of biogeography is the study of the range and distribution of plants and animals in different places throughout the world. Such distributions are consistent with the hypothesis that, when forms are related, they evolved in one locale and then spread to accessible regions. Therefore, a different mix of plants and animals would be expected whenever geography separates continents, islands, seas, and so on. Darwin’s observations of biogeography while on the HMS Beagle helped to form his ­theory of evolution. One observation in particular was that of the variation among the finches on the Galápagos Islands (see section 27.1).

modern

c. Right whale

Figure 27.5  Anatomical transitions during the evolution

of whales.  Fossil evidence suggests that modern whales evolved from

terrestrial ancestors that walked on four limbs. Transitional fossils show a gradual reduction in the hindlimb and a movement of the nasal openings from the tip of the nose to the top of the head—both are adaptations to living in water. a. Ambulocetus, an ancestor to whales (50 mya), had a distinct hindlimb and a nose at the tip of its snout. b. Basilosaurus, a more recent ancestor of whales (40 mya), had a greatly reduced hindlimb and a nasal opening mid-snout. c. The modern Right whale has a vestigial hindlimb that is inside the body. Its nasal opening is far back on the top of the head, from which it breathes as it surfaces from under the water.



Chapter 27  Evolution of Life

541

TABLE 27.1  The Geological Timescale: Major Divisions of Geological Time and Some of the Major Evolutionary Events of Each Time Period

Era

Period

Epoch

Millions of Years Ago (mya)

Cenozoic

Quaternary

Holocene

current

Plant Life

Animal Life

Humans influence plant life.

Age of Homo sapiens

Significant Extinction Event Underway Quaternary

Pleistocene

0.01

Herbaceous plants spread and diversify.

Presence of ice age mammals. Modern humans appear.

Neogene

Pliocene

2.6

Herbaceous angiosperms flourish.

First hominids appear.

Neogene

Miocene

5.3

Grasslands spread as forests contract.

Apelike mammals and grazing mammals flourish; insects flourish.

Neogene

Oligocene

23.0

Many modern families of flowering plants evolve; appearance of grasses.

Browsing mammals and monkeylike primates appear.

Paleogene

Eocene

33.9

Subtropical forests with heavy rainfall thrive.

All modern orders of mammals are represented.

Paleogene

Paleocene

55.8

Flowering plants continue to diversify.

Ancestral primates, herbivores, carnivores, and insectivores appear.

Mass Extinction: 50% of all species, dinosaurs and most reptiles Mesozoic

Cretaceous

65.5

Flowering plants spread; conifers persist.

Placental mammals appear; modern insect groups appear.

Jurassic

145.5

Flowering plants appear.

Dinosaurs flourish; birds appear.

Mass Extinction: 48% of all species, including corals and ferns Triassic

199.6

Forests of conifers and cycads dominate.

First mammals appear; first dinosaurs appear; corals and molluscs dominate seas.

Mass Extinction (“The Great Dying”): 83% of all species on land and sea Paleozoic

Permian

251.0

Gymnosperms diversify.

Reptiles diversify; amphibians decline.

Carboniferous

299.0

Age of great coal-forming forests: ferns, club mosses, and horsetails flourish.

Amphibians diversify; first reptiles appear; first great radiation of insects.

Mass Extinction: Over 50% of coastal marine species, corals Devonian

359.2

First seed plants appear. Seedless vascular plants diversify.

First insects and first amphibians appear on land.

Silurian

416.0

Seedless vascular plants appear.

Jawed fishes diversify and dominate the seas.

Mass Extinction: Over 57% of marine species Ordovician

443.7

Nonvascular land plants appear.

Invertebrates spread and diversify; first jawless and then jawed fishes appear.

Cambrian

488.3

Marine algae flourish.

All invertebrate phyla present; first chordates appear.

630

First soft-bodied invertebrates evolve.

1,000

Protists diversify.

2,100

First eukaryotic cells evolve.

2,700

O2 accumulates in atmosphere.

3,500

First prokaryotic cells evolve.

4,570

Earth forms.



542

UNIT 6  Evolution and Diversity

Many of the barriers between landmasses arose through a process called continental drift. That is, the position of the continents has never been fixed. Rather, their positions and the positions of the oceans have changed over time (Fig. 27.6). During the Permian period, all the present landmasses belonged to one continent and then later drifted apart. As evidence of this, fossils of one species of seed fern (Glossopteris) have been found on all the southern continents separated by oceans. This species’ presence on Antarctica is evidence that this continent was not always frozen. In contrast, many Australian species are restricted to that continent, including the majority of marsupials (pouched mammals such as the kangaroo). What is the explanation for these distributions? Some organisms must have evolved and spread out before the continents broke up. Then they became extinct. The world’s six major biogeographical regions each have their own distinctive mix of living organisms. Darwin noted that South America lacked rabbits, even though the environment was quite suitable to them. He concluded there are no rabbits in South America because rabbits evolved somewhere else and had no means of reaching South America. To take another example, both cacti and spurges (Euphorbia) are plants adapted to a hot, dry environment— both are succulent, spiny, flowering plants. Why do cacti grow in the American deserts and most Euphorbia grow in African deserts when each would do well on the other continent? It seems obvious that they just happened to evolve on their respective continents. What is the best explanation for this phenomenon? Different mammals and flowering plants evolved separately in each biogeographical region, and barriers such as mountain ranges and oceans prevented them from migrating to other regions.

Mass Extinctions Extinction is the death of every member of a species. During mass extinctions, a large percentage of species become extinct within a relatively short period of time. So far, there have been five major mass extinctions. These occurred at the ends of the Ordovician, Devonian, Permian, Triassic, and Cretaceous periods (see Table 27.1), and a sixth is likely occurring now, probably as a result of human activities (discussed in Chapter 37). Following mass extinctions, the remaining groups of organisms are likely to spread out and fill the habitats vacated by those that have become extinct. It was proposed in 1977 that the Cretaceous extinction (or “Cretaceous crisis”) was due to an asteroid that exploded, producing meteorites that fell to Earth. A large meteorite striking Earth could have produced a cloud of dust that mushroomed into the atmosphere, blocking out the sun and causing plants to freeze and die. In support of this hypothesis, a huge crater that could have been caused by a meteorite involved in the Cretaceous extinction was found in the Caribbean–Gulf of Mexico region on the Yucatán Peninsula. During the Cretaceous period, great herds of dinosaurs roamed the plains, but all dinosaur species went extinct near the end of the Cretaceous period. Certainly, continental drift contributed to the Ordovician extinction. This extinction occurred after Gondwana arrived at the South Pole. Immense glaciers, which drew water from the oceans, chilled even once-tropical land. Marine invertebrates and coral reefs, which were especially hard hit, didn’t recover until Gondwana drifted away from the pole and warmth returned. The mass extinction at the end of the Devonian period saw an end to 70% of marine invertebrates. Other scientists believe that this mass

Laurasia

Laurasia

a ae ng

Pa

Go

Go nd wa na

nd

wa n

a

Permian period ~250 million years ago

Figure 27.6  Continental drift.  During the Permian period, all the continents were joined into a supercontinent called Pangaea. During the Triassic period, the joined continents of Pangaea began moving apart, forming two large continents called Laurasia and Gondwana. Then all the continents began to separate. This process is continuing today. North America and Europe are presently drifting apart at a rate of about 2 cm per year.

Triassic period ~220 million years ago

North America

Jurassic period 144 million years ago

Eurasia

North America

Eurasia

Africa South America

Africa

India

South America

Australia

Australia Antarctica

Cretaceous period 65 million years ago

India

Antarctica

Present day



Chapter 27  Evolution of Life

extinction could have been due to movement of Gondwana back to the South Pole.

Anatomical Evidence The concept of common descent offers a plausible explanation for anatomical similarities among organisms. Vertebrate forelimbs are used for flight (birds and bats), orientation during swimming (whales and seals), running (horses), climbing (arboreal lizards), or swinging from tree branches (monkeys). Yet all vertebrate forelimbs contain the same sets of bones organized in similar ways, despite their dissimilar functions. The most plausible explanation for this unity is that the basic forelimb plan belonged to a common ancestor, and then the plan was modified in the succeeding groups as each continued along its own evolutionary pathway. Structures that are anatomically similar because they are inherited from a common ancestor are called homologous structures (Fig. 27.7). In contrast, analogous structures serve the same function, but are not constructed similarly, nor do they share a common ancestry. The wings of birds and insects and the eyes of octopi and humans are analogous structures and are similar due to a common environment, not common ancestry. The presence of homology, not analogy, is evidence that organisms are related. Vestigial structures are anatomical features that are fully developed in one group of organisms but that are reduced and may have no function in similar groups. Most birds, for example, have well-developed wings for flight. However, some bird species (e.g., ostrich) have greatly reduced wings and do not fly. Similarly, snakes have no use for hindlimbs, and yet some have remnants of hindlimbs in a pelvic girdle and legs. The presence of vestigial bird

humerus ulna radius metacarpals phalanges

bat

543

structures can be explained by common descent. Vestigial structures occur because organisms inherit their anatomy from their ancestors. They are traces of an organism’s evolutionary history. The homology shared by vertebrates extends to their embryological development (Fig. 27.8). At some time during development, all vertebrates have a postanal tail and exhibit paired pharyngeal pouches. In fishes and amphibian larvae, these pouches develop into functioning gills. In humans, the first pair of pouches becomes the cavity of the middle ear and the auditory tube. The second pair becomes the tonsils, while the third and fourth pairs become the thymus and parathyroid glands. Why should terrestrial vertebrates develop and then modify structures like pharyngeal pouches that have lost their original function? The most likely explanation is that fishes are ancestral to other vertebrate groups.

fish

salamander

tortoise

chicken pharyngeal pouches

whale

cat

horse

human

human postanal tail

Figure 27.7  Significance of homologous structures. 

Although the specific design details of vertebrate forelimbs are different, the same bones are present (note color-coding). Homologous structures provide evidence of a common ancestor.

Figure 27.8  Significance of developmental similarities.  At these comparable developmental stages, vertebrate embryos have many features in common, which suggests they evolved from a common ancestor. (These embryos are not drawn to scale.)



544

UNIT 6  Evolution and Diversity

In 1859, Charles Darwin speculated that whales evolved from a land mammal. His hypothesis has now been substantiated. In recent years the fossil record has yielded an incredible parade of fossils that link modern whales and dolphins to land ancestors. The presence of a vestigial pelvic girdle and legs in modern whales is also significant evidence that ancestors of these creatures once walked on land. However, that these structures are severely reduced in modern whales is consistent with the fact that they are exclusively aquatic today (see Fig. 27.5).

acid sequence of cytochrome c show that the sequence in a human differs from that in a monkey by only one amino acid, from that in a duck by 11 amino acids, and from that in a yeast by 51 amino acids. These data are consistent with other data regarding the anatomical similarities of these organisms and, therefore, how closely they are related. In addition, we now know that chromosomal changes, such as translocations and inversions (see Chapter 24), play an important role in evolutionary change. Evolution is no longer considered a hypothesis. It is one of the great unifying theories of biology. In science, the word theory is reserved for those conceptual schemes that are supported by a large number of observations and scientific experiments (see section 1.3). The theory of evolution has the same status in biology that the theory of gravity has in physics.

Biochemical Evidence Almost all living organisms use the same basic biochemical molecules, including DNA, ATP (adenosine triphosphate), and many identical or nearly identical enzymes. Further, organisms use the same DNA triplet code for the same 20 amino acids in their proteins. Because the sequences of DNA bases in the genomes of many organisms are now known, it has become clear that humans share a large number of genes with much simpler organisms. It appears that life’s vast diversity has come about by only a slight difference in many of the same genes and regulatory genes often found in introns and other regions of the genome. The result has been widely divergent types of bodies. When the degree of similarity in DNA nucleotide sequences or the degree of similarity in amino acid sequences of proteins is examined, the more similar the DNA sequences are, generally the more closely related the organisms are. For example, humans and chimpanzees are about 97% similar! Cytochrome c is a molecule that is used in the electron transport chain of all the organisms appearing in Figure 27.9. Data regarding differences in the amino

yeast

Number of Amino Acid Differences Compared to Human Cytochrome c

0

moth

fish

We Can Observe Selection at Work Table 27.1 outlines major events in the evolution of life over a timescale of millions and billions of years. But evolution does not necessarily happen over long periods of time. Researchers have recorded the evolution of traits in natural populations over decades. Over the course of history, humans have used the power of evolution to shape traits of livestock, agricultural plants, and even our animal companions, over relatively short periods of time.

Humans as Agents of Evolution Darwin noted that humans could artificially modify desired traits in plants and animals by selecting to breed individuals with preferred traits. For example, the diversity of domestic dogs has

turtle

duck

pig

monkey

human

10

20

30

40

Cytochrome c is a small protein that plays an important role in the electron transport chain within mitochondria of all cells.

50

Figure 27.9  Significance of biochemical differences.  The branch points in this diagram indicate the number of amino acids that differ between human cytochrome c and the organisms depicted. These biochemical data are consistent with those provided by a study of the fossil record and comparative anatomy.



Chapter 27  Evolution of Life

Evolution in Natural Populations The Galápagos finches have beaks adapted to the food they eat, with different species of finches on each island (see Fig. 27.3). Today, many investigators, including Peter and Rosemary Grant of Princeton University, are documenting natural selection as it occurs on the Galápagos Islands. In 1973, the Grants began a study of the various finches on Daphne Major, an island near the center of the Galápagos Islands. The rainfall on this island varies widely from wet years to dry years. The Grants measured the beaks of each finch on the island and found that the beak depth of the medium ground finch, Geospiza fortis, changed between wet and dry years (Fig. 27.11). Medium ground finches like to eat small, tender seeds, but when the weather turns dry, they must eat larger, drier seeds that are harder to crush. During dry years, the majority of finches died from starvation because their beaks were not well equipped to feed on the harder seeds. But the few finches with deeper beaks were better able to crush larger, harder seeds, and these finches survived to reproduce.

wet year

Beak Depth

resulted from prehistoric humans selectively breeding wolves with particular traits, such as hair length, height, and guarding behavior. This type of human-controlled breeding to increase the frequency of desired traits is called artificial selection. Artificial selection, like natural selection, is possible only because the original population exhibits a variety of characteristics, allowing humans to select traits they prefer. For dogs, the result of artificial selection is the existence of many breeds of dogs all descended from the wolf (Fig. 27.10). Darwin surmised that if humans could create such a wide variety of organisms by artificial selection, then natural selection could also produce diversity. But in this case, the environment, and not humans, is the force selecting for particular traits.

545

dry year

dry year

dry year

medium ground finch 1977

1980

1982

1984

Figure 27.11  Evolution in action.  The average beak depth of ground finches varies from generation to generation, according to the amount of rainfall. The amount of rainfall affects the hardness and size of seeds on the islands, and different beak depths were better suited to eating different types of seeds. Average beak features were observed to change many times over a period of a decade. This is one way in which evolution by natural selection has been observable over short periods of time.

Therefore, among the next generation of G. fortis birds, the mean, or average, beak was deeper than the previous generation. The Grants’ research demonstrates that evolutionary change can sometimes be observed within the time frame of a human life span.

Check Your Progress  27.2 1. Summarize how the fossil record, geological timescale, and biogeography each provide evidence in support of the theory of evolution. 2. Explain how a breeder might produce a new breed of dog with long ears and a snub nose and how this new breed is evidence of evolution. 3. Determine which would have fewer protein and DNA base sequence differences with humans—chimpanzees or cows.

27.3  Microevolution Learning Outcomes Boston terrier Irish wolfhound

Wolf

Figure 27.10  Artificial selection.  All dogs, Canis lupus familiaris, are descended from the gray wolf, Canis lupus, which humans started to domesticate about 14,000 years ago. The process of selective breeding by humans has led to extreme differences among breeds.

Upon completion of this section, you should be able to 1. List the five conditions necessary for the allele frequencies of a population to be in Hardy-Weinberg equilibrium. 2. Calculate genotype frequencies of a population in HardyWeinberg equilibrium. 3. Identify an evolving population from a change in its allele frequencies over generations.

Many traits can change temporarily in response to a varying environment. For example, the color change in the fur of an Arctic fox from brown to white in winter, the increased thickness of your dog’s fur in cold weather, or the bronzing of your skin when exposed to the sun last only for a season.



546

UNIT 6  Evolution and Diversity

These are not evolutionary changes. Changes to traits over an individual’s lifetime are not evidence that an individual has evolved, because these traits are not heritable (see Fig. 27.2). In order for traits to evolve, they must have the ability to be passed on to subsequent generations. Evolution is about change in a heritable trait within a population, not within an individual, over many generations. Darwin observed that populations, not individuals, evolve, but he could not explain how traits change over time. Now we know that genes interact with the environment to determine traits. Because genes and traits are linked, evolution is really about genetic change—or more specifically, evolution is the change in allele frequencies in a population over time. This type of evolution is called microevolution. In this chapter, we use the example of the peppered moth to examine how populations evolve.

Microevolution in the Peppered Moth Population genetics, as its name implies, is the field of biology that studies the diversity of populations at the level of the gene. A population is all the members of a single species that occupy a particular area at the same time and that interbreed and exchange genes. Population geneticists are interested in how genetic diversity in populations changes over generations, and in the forces that cause populations to evolve. Population geneticists study microevolution by measuring the diversity of a population in terms of allele and genotype frequencies over generations. You may recall from Chapter 23 that diploid organisms, such as moths, carry two copies of each chromosome, with one copy of each gene on each chromosome. A single gene can come in many forms, or alleles, that encode variations of a single trait. In the peppered moth, a single gene for body color has two alleles, D (dark color) and d (light color), with D dominant to d (Fig. 27.12). We know that with only two alleles, there are three possible genotypes (the combinations of alleles in an individual) for the color gene in the peppered moth: DD (homozygous dominant),

Phenotype:

Genotype:

DD homozygous

Dd heterozygous

dd homozygous

Alleles:

D

D

d

D

d

d

chromosome

Figure 27.12  The genetic basis of body color in the

peppered moth.  Light or dark body color in the peppered moth is

determined by a gene with two alleles, D and d. Genotypes DD and Dd produce dark body color, and dd produces light body color. The D and d alleles are variants of a gene at a particular locus on a chromosome.

Dd (heterozygous), or dd (homozygous recessive). DD or Dd genotypes produce dark moths, and the dd genotype produces light moths (Fig. 27.12).

Allele Frequencies Suppose that a population geneticist collected a population of 25 peppered moths, some dark and some light, from a forest outside London (Fig. 27.13). In this population, you would expect to find a mixture of D and d alleles in the gene pool—the alleles of all genes in all individuals in a population. The population geneticist ran tests to determine the alleles present in each moth. Of the 50 alleles in the peppered moth gene pool (2 alleles × 25 moths), 10 were D and 40 were d. Thus the frequency of the D and the d alleles would be 10/50, or 0.20, and 40/50, or 0.80. The allele frequency, as illustrated in this example, is the proportion of each allele in a population’s gene pool (Fig. 27.13). Notice that the frequencies of D and d add up to 1. This relationship is true of the sum of allele frequencies in a population for any gene of any diploid organism. This relationship is described by the expression p + q = 1, where p is the frequency of one allele, in this case D, and q is the frequency of the other allele, d (Fig. 27.13). For the next three seasons, samples of 25 moths were collected from the same forest, and the allele frequencies were always the same: 0.20 D, and 0.80 d. Because allele frequencies in this population did not change over generations, we could conclude that this population has not evolved.

Hardy-Weinberg Equilibrium A population in which allele frequencies do not change over time, such as in the moth population just described, is said to be in genetic equilibrium, or Hardy-Weinberg equilibrium—a stable, nonevolving state. Hardy-Weinberg equilibrium is derived from the work of British mathematician Godfrey H. Hardy and German physician Wilhelm Weinberg, who in 1908 developed a mathematical model to estimate genotype frequencies of a population that is in genetic equilibrium. Their mathematical model proposes that the genotype frequencies of a nonevolving population can be described by the equation p2 + 2pq + q2, again with p and q representing the frequency of alleles D and d. Recall that in our moth population, D = p and d = q, so that D2 is the frequency of the DD genotype, 2Dd is the frequency of the Dd genotype, and d2 is the frequency of the dd genotype (Fig. 27.13).

Conditions of Hardy-Weinberg Equilibrium The principles of the Hardy-Weinberg equation indicate that allele frequencies in a gene pool will remain at equilibrium, and thus constant, after one generation of random mating in a large, sexually reproducing population as long as five conditions are met: 1. No mutations. Genetic mutations are an alteration in an allele, due to a change in DNA composition. Under Hardy-Weinberg assumptions, allele changes do not occur, or changes in one direction are balanced by changes in the opposite direction.



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Chapter 27  Evolution of Life

Population = 25 moths, 50 alleles

Gene pool: D = 10

Equilibrium genotype frequencies:

Allele frequencies:

d = 40 DD

DD

Dd

Dd

Dd

DDDDDddd D

Dd

Dd

Dd

Dd

DDDDdddd

Dd

d

dd

p 2 + 2pq + q 2 = 1

p+ q = 1 Dd

dd

Dd

dd

dd

dd

dddddddd

p = frequency of D

p = frequency of D

q = frequency of d

q = frequency of d

Frequency of D

Frequency of DD

p = 10/50 alleles

dd

dd

dd

dd

dddddddd

Frequency of d q = 40/50 alleles

dd

dd

dd

dd

= 0.20

p2

dd

dd

dd

= 0.80

2pq = 2(freq D x freq d) = 2(0.20 x 0.80) = 0.32 Frequency of dd

dddddddd

dddddddd

= 0.04

Frequency of Dd

q2 dd

= freq D 2 = (0.2)(0.2)

= freq d 2 = (0.08)(0.80)

= 0.64

1.00

1.00

Figure 27.13  How Hardy-Weinberg equilibrium is estimated.  A population of 25 moths contains a gene pool of D and d alleles. The frequencies of the D and d alleles can be estimated from the gene pool. Under Hardy-Weinberg equilibrium, the frequencies of D and d alleles should produce in the next generation a predictable frequency of genotypes that can be calculated with the Hardy-Weinberg equation.

2. No genetic drift. Genetic drift is random changes in allele frequencies by chance. If a population is very large, changes in allele frequencies due to chance alone are insignificant. 3. No gene flow. Gene flow is the sharing of alleles between two populations through interbreeding. If there is no gene flow, migration of individuals, and therefore their genes, into or out of the population does not occur. 4. Random mating. Random mating occurs when individuals pair by chance, not according to their genotypes or phenotypes. 5. No selection. Often, the environment selects certain phenotypes to reproduce and have more offspring than other phenotypes. If selection does not occur, no phenotype is favored over another to reproduce. Although possible in theory, Hardy-Weinberg equilibrium is never achieved in wild populations because all of the five required conditions are never met in the real world. The peppered moth

population we are using as an example obeys all five of the conditions for genetic equilibrium, but in reality, populations are constantly evolving from generation to generation. Hardy-Weinberg equilibrium does not typically occur in natural populations, but it is an important tool for population geneticists because the violation of one or more of the five conditions causes the allele and/or genotype frequencies of a population to change in predictable ways (Table 27.2). For example, change in genotype frequencies without a change in allele frequencies over generations suggests that a population is not mating randomly.

Check Your Progress  27.3 1. List the five conditions necessary to maintain a HardyWeinberg equilibrium in a population.

2. Explain what deviations from Hardy-Weinberg equilibrium tell us about a population.



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UNIT 6  Evolution and Diversity

TABLE 27.2  Hardy-Weinberg Proportions Can Be Used to Determine if Evolution Has Occurred. Hardy-Weinberg Condition

Deviation from Condition

Effect of Deviation on Population

Expected Deviation from HWE

Evolution Occurred?

Random mating

Nonrandom mating

Allelles do not assort randomly

Change in genotype frequencies

No

X DD/Dd

DD/Dd

No selection

Selection

Certain allelles are selected for or against

Change in allelle frequencies

Yes

No mutation

Mutation

Addition of new allelles

Change in allelle frequencies

Yes

No migration

Immigration or emigration

Individuals carry allelles into, or out of, the population

Change in allelle frequencies

Yes

Large population (no genetic drift)

Small population (genetic drift) 1.  bottleneck effect 2.  founder effect

Loss of allelle diversity; some allelles may disappear

Change in allelle frequencies

Yes

27.4  Processes of Evolution Learning Outcomes Upon completion of this section, you should be able to 1. Define the five agents of evolutionary change. 2. List the four requirements of evolution by natural selection. 3. Identify examples of the three types of natural selection.

Five Agents of Evolutionary Change In section 27.3, we outlined the five conditions for a population to be in genetic equilibrium. The opposite of these conditions can cause evolutionary change—namely mutations, genetic drift, gene flow, nonrandom mating, and natural selection. In this section, we define each of these five agents of evolutionary change.

Mutations The principles of Hardy-Weinberg equilibrium recognize that new mutations can cause the allele frequencies in a population to change. Mutations, which are genetic changes, are the only source

of new variation in a population. Without mutations, there could be no new heritable genetic diversity to be shaped by the forces of natural selection. It is important to realize that mutations are random events, they do not arise because the organism “needs” one. For example, a mutation that gives bacteria resistance to antibiotics appears by chance. It is because the mutation provides a survival advantage that it becomes increasingly common in the bacteria population over generations, not because the bacteria needed to evolve resistance to antibiotics. Not all mutations provide an advantage. Some mutations are harmful. But in reality, most mutations have little to no effect on an organism’s fitness. For example, one type of mutation, called a point mutation, is a change in a single nucleotide in a gene. A point mutation is called “silent” if it does not result in a change to the function of the protein encoded by the gene. Mutations that cause a malfunctioning protein can be harmful to an organism.

Genetic Drift Genetic drift refers to changes in the allele frequencies of a gene pool due to the random meeting of gametes during fertilization. Each time an organism reproduces, one in millions of sperm will fertilize one of many potential eggs. The random selection and



Chapter 27  Evolution of Life

assortment of gametes in a population causes allele frequencies to shift each generation. Thus, populations are always undergoing genetic drift. As you can imagine, genetic drift has greater effects in smaller populations because there are fewer gametes to assort. Removal of gametes from a population due to random events can have an effect on allele frequencies in the next generation. For example, the chance death of one individual in a population of a million will not have an appreciable effect on allele frequencies, but the chance death of one individual in a population of ten could change the frequency of an allele by 10% or even cause its loss altogether (if that individual was the only one with that allele) (Fig. 27.14).  In nature, the founder effect and bottleneck effect occur when populations are drastically reduced in size. For example, a bottleneck effect can occur following a natural disaster that kills a large proportion of a population (Fig. 27.15). Both of these effects result in a severe reduction in the gene pool that can drastically affect allele frequencies. The founder effect occurs when a few individuals form a new colony, and only a fraction of the total genetic diversity of the original gene pool is represented in these individuals (Fig. 27.15). The particular alleles carried by the founders are dictated by chance alone.

Gene Flow Gene flow is the movement of alleles among populations when individuals or their gametes migrate from one population to another and breed in that new population. For example, adult plants are not able to migrate, but their pollen is often blown by the wind or carried by insects into different populations. Gene flow mixes genetic diversity and keeps the gene pools of populations similar.

10% of population

natural disaster kills five green frogs 20% of population

Figure 27.14  Genetic drift.  Genetic drift occurs when, by chance, only certain members of a population (in this case, green frogs) reproduce and pass on their alleles to the next generation. A natural disaster can cause the allele frequencies of the next generation’s gene pool to be markedly different from those of the previous generation.

a.

b.

c.

Original population gene pool = 3,800 alleles*

549

d.

Remnant population gene pool = 90 alleles* 11%

13% 8%

26%

44% 45%

53% *1 marble = 10 alleles

Figure 27.15  Bottleneck and founder effects change allele frequencies.  a. The gene pool of a large population contains four

different alleles, represented by colored marbles in a bottle, each with a different frequency. b. A population bottleneck occurs. The marbles, or alleles, that exit the bottle must pass through the narrow neck into the cup. The new gene pool will have a fraction of the alleles from the original population. c. The gene pool of the new population has changed from the original. Some alleles are in high frequency, while some are not present. d. A founder event is the same as a bottleneck, except that in the founder event, the original population still exists.

Nonrandom Mating Nonrandom mating occurs when individuals are selective about choosing a mate with a preferred trait. Because most sexually reproducing organisms select their mates based on some trait, random mating is never observed in natural populations. Inbreeding, or mating between close relatives, is an example of nonrandom mating. Inbreeding changes genotype frequencies by decreasing the proportion of heterozygotes and increasing the proportion of homozygotes for all genes. This increase in the frequency of homozygotes also increases the frequency of recessive homozygous disorders in a population. For example, the Amish population of Lancaster, Pennsylvania, is an isolated religious sect descended from a few German founders. Intermarriage has increased the frequency of Ellis–van Creveld syndrome, a rare form of dwarfism, that occurs in individuals who are homozygous recessive for a mutation to the EVC gene (for more information on the effects of inbreeding in human populations, see the Scientific Inquiry feature, “Inbreeding in the Pingelapese”).

Natural Selection In section 27.1 we introduced natural selection as the process that allows some individuals, with an advantage over others, to produce more offspring. Here, we restate these steps in the context of



550

UNIT 6  Evolution and Diversity

SCIENCE IN YOUR LIFE  ►

SCIENTIFIC INQUIRY

Inbreeding in the Pingelapese One of the requirements of a population in Hardy-Weinberg equilibrium is that mates are chosen at random—that is, without preference for a particular trait. Most populations, however, do not meet this requirement because some traits are more attractive in a mate than others. Humans, for example, select mates based on a set of traits that we find appealing in a mate. This type of assortative mating based on trait preference is common in many species. Inbreeding is a unique form of nonrandom, or assortative, mating where individuals mate with close relatives, such as cousins. One consequence of inbreeding is an increase in the frequency of homozygous genotypes in a population. For the most part, an increase in homozygotes does not have a large detrimental effect, especially if the population is very large. But when populations are small, inbreeding can have a large impact on the health of a population. Many human diseases are caused by the inheritance of two recessive alleles, such that the disease appears only in persons who are homozygous recessive for the

disease-causing allele. In very small populations, the probability that individuals carrying the recessive allele will mate increases because the mating pool is very small. In this case, inbreeding significantly increases the frequency of homozygous recessive genotypes, and thus those afflicted with a disease. Human cultures tend to have social rules that discourage inbreeding, but in very small populations, such as those following a bottleneck or founder event, inbreeding is sometimes unavoidable. One example of the effects of inbreeding on human populations is the occurrence of a rare form of non-sex-linked color blindness, achromatopsia, on the island of Pingelap, a small coral atoll in the Pacific Ocean. People who are achromatic are completely color blind (Fig. 27A). Normal human color vision is possible because of cells in the back of the eye, called cones. Those with achromatopsia do not have any cone cells because they are homozygous for a rare recessive allele that prevents cone cells from developing. Complete color blindness is so common

on Pingelap that it is considered part of everyday life for most families. Experts propose that the high frequency of achromatopsia appeared following a severe population bottleneck in 1775 when a typhoon killed 90% of the inhabitants. Approximately 20 people survived the typhoon. Four generations after the typhoon struck, achromatopsia began to appear frequently in the population. How could this happen? Figure 27B shows how inbreeding in a population can produce homozygous recessive genotypes. Geneticists explain that the high frequency of this rare genetic disorder on Pingelap is consistent with a large degree of intermarriage among relatives, or inbreeding, following the typhoon. Mwanenised, a male survivor of the typhoon, was a carrier, or a heterozygote, for the achromatopsia allele. He had ten children, which was a large proportion of the first generation of Pingelapese after the typhoon. Geneticists would predict that on average 50% of his children would have been homozygous

Figure 27A  Achromatopsia.  Complete achromatopsia is a recessive genetic disorder that causes complete color blindness. People with complete achromatopsia see no color, only shades of gray. This image shows how a person with this form of color blindness would see the world.

modern evolutionary theory. Evolution by natural selection requires the following: 1. Individual variation. The members of a population differ from one another. 2. Inheritance. Many of these differences are heritable genetic differences. 3. Overproduction. Individuals in a population are engaged in a struggle for existence because breeding individuals in a population tend to produce more offspring than the environment can support.

4. Differential reproductive success. Individuals that are better adapted to their environment produce more offspring than those that are not as well adapted, and consequently, their fertile offspring will make up a greater proportion of the next generation. In biology, the fitness of an individual is measured by the number of fertile offspring produced throughout its lifetime. Gene mutations are the ultimate source of variation because they provide new alleles. Sometimes, these mutations result in positive fitness consequences for an organism, perhaps providing variation for adaptation to a new environmental change. However, in sexually reproducing



Chapter 27  Evolution of Life

for the normal allele, and 50% would have been heterozygous carriers of the color-blind allele. Thus, none of Mwanenised’s children would have been color blind (Fig. 27B). However, intermarriage of the next generations would unavoidably have brought together men and women who were carriers for the achromatopsia allele. Thus, in subsequent generations the homozygous recessive genotype, and complete color blindness, did appear due to inbreeding within a small population (Fig. 27B). On

Pingelap, an increase in color blindness was observed by the fourth generation.1 The effect of inbreeding can be long-term, especially if a population remains relatively small and isolated. Today 1 in 12 Pingelapese suffers from achromatopsia, more than 3,000 times more frequently than in the United States, where it occurs in approximately 1 in 40,000 births. See Sacks, Oliver (1998). The Island of the Colorblind. (Vintage/Anchor Books, New York). An interesting, nontechnical account of the Pingelapese. 1

X

Cc

CC noncarrier

Questions to Consider 1. What might have happened to the colorblind allele in the Pingelapese population following the typhoon if Mwanenised had not had any children? Only five children? 2. Would you predict that the Pingelapese population is in Hardy-Weinberg equilibrium? How would you measure this? 3. Has the Pingelapese population evolved? Explain. (Hint: Does inbreeding cause a change in allele frequencies?)

Founder generation

CC

Mwanenised

Cc

carrier

cc

color-blind Cc

close relative C CC

X

CC

Cc

50% CC normal vision

C CC

Cc

50% Cc carrier

X

CC

Cc

C

Cc

c CC

Cc

CC

Produces 1st generation genotypes (all can see color)

X

Cc

C 50% CC 50% Cc

CC

C

C CC

CC

C CC

CC

Cc

Subsequent generations of carriers and noncarriers inbreed

25% CC 50% Cc 25% cc

First appearance of color-blind genotype and phenotype

Cc

CC

C

C CC

c

C CC

CC

Cc

551

C 100% normal

Cc

c

C CC

Cc

Cc

cc

c

Figure 27B  Inbreeding in small populations increases the frequency of recessive genetic disorders.  On the island of Pingelap, a high level of incidental inbreeding in a small population has resulted in a higher than expected occurrence of complete achromatopsia.

organisms, genetic variation can also result from crossing-over and independent assortment of chromosomes during meiosis, and also fertilization, when gametes are combined. A different combination of alleles can lead to a new and different phenotype. In this context, consider that most of the traits on which natural selection acts are polygenic, and thus, controlled by more than one gene. Such traits have a range of phenotypes that often follow a bell-shaped curve. The three main types of natural selection are stabilizing selection, directional selection, and disruptive selection.

Stabilizing Selection  With stabilizing selection, extreme phenotypes are selected against, and individuals near the average are favored. Stabilizing selection can improve adaptation of the population to those aspects of the environment that remain constant. Human birth weight is an example of stabilizing selection. Over many years, hospital data have shown that human infants born with an intermediate birth weight (3–4 kg) have a better chance of survival than those at either extreme (either much less or much greater than average). When a baby is small, its systems may not be fully functional, and when a baby is large, it may have experienced a



552

UNIT 6  Evolution and Diversity

difficult delivery. Stabilizing selection reduces the variability in birth weight in human populations (Fig. 27.16). Percent of Births in Population

50

15

30 20 10

10

7 5

5

3 2 .9

1.4

1.8 2.3 2.7 3.2 3.6 4.1 4.5 Birth Weight (in kilograms)

Figure 27.16  Human birth weight.  The birth weight (blue) is

influenced by the mortality rate (red). Babies that are born weighing 3–4 kg are more likely to survive.

Disruptive Selection  In disruptive selection, two or more extreme phenotypes are favored over any intermediate phenotype (Fig. 27.18). For example, British land snails (Cepaea nemoralis) have a wide habitat range that includes grass fields, hedgerows, and forested areas. In areas with low-lying vegetation, thrushes feed mainly on snails with dark shells that lack light bands, and in forested areas, they feed mainly on snails with light-banded shells. Therefore, the two different habitats have resulted in two different phenotypes in the population.

After More Time

Body Size

Number of Individuals

After Time Number of Individuals

Number of Individuals

Initial Distribution

70 Percent Infant Mortality

Directional Selection  Directional selection occurs when an extreme phenotype is favored and the distribution curve shifts in that direction (Fig. 27.17). This changes the average phenotype in a population. Such a shift can occur when a population is adapting to a changing environment. For example, the gradual increase in the size of the modern horse, Equus, can be correlated with a change in the environment from forest conditions to grassland conditions. Hyracotherium, the ancestor of the modern horse, was about the size of a dog and was adapted to the forestlike environment of the Eocene epoch of the Paleocene period. This animal could have hidden among the trees for protection, and its lowcrowned teeth would have been appropriate for browsing on leaves. Later, in the Miocene and Pliocene epochs, grasslands began to replace the forests. Then, the ancestors of Equus were subject to selective pressure for the development of strength, intelligence, speed, and durable grinding teeth. A larger size provided the strength needed for combat, elongated legs ending in hooves gave speed for escaping from enemies, and the durable grinding teeth enabled the animals to feed efficiently on grasses. Nevertheless, the evolution of the horse should not be viewed as a straight line of descent; there were many side branches that became extinct. Another example of directional selection is the development of antibiotic resistance in a species of bacteria. This is because the population is moving from a phenotype of being susceptible to the antibiotic, toward a phenotype of being antibiotic resistant. As discussed in the chapter opener, and the Health Feature, “Evolution of Antibiotic Resistance,” new species of bacteria are becoming resistant to antibiotics every year as a result of directional selection.

100

20

Body Size

Body Size

a.

Hyracotherium Merychippus

Figure 27.17  Directional selection.  a. Directional selection occurs when natural selection

favors one extreme phenotype, resulting in a shift in the distribution curve. b. For example, Equus, the modern-day horse, which is adapted to a grassland habitat, is much larger than its ancestor, Hyracotherium, which was adapted to a forest habitat.

b.

Equus



Chapter 27  Evolution of Life

SCIENCE IN YOUR LIFE  ►

553

HEALTH

Evolution of Antibiotic Resistance The antibiotic penicillin was discovered by Alexander Fleming in 1928. On November 25, 1930, penicillin successfully cured a bacterial infection for the first time. Since the discovery of penicillin, modern science has produced an array of antibiotics effective against most bacterial infections. Prior to the discovery of penicillin, bacterial infections were a major cause of death worldwide, so much so that today it is difficult to imagine dying from a bacterial infection. Yet we are now coming full circle; many of the antibiotics that have saved millions of lives have evolved resistance to antibiotics (Fig. 27C). In 1945, the first resistance to penicillin was discovered in a strain of Staphylococcus aureus. Today, over 90% of S. aureus strains are resistant to penicillin. Through Darwin’s theory of natural selection, we have come to understand how bacteria become resistant to antibiotics. The process by which bacteria evolve antibiotic resistance is the same as natural selection—antibiotics kill bacteria that are susceptible, but bacterial populations are usually so large that some resistant individuals are likely to survive and reproduce. Through time, the frequency of resistant individuals increases in the population to the point that certain antibiotics are no longer effective. Overall, antibiotic resistance is evolving at an alarming rate, to the point where bacterial infections that just decades ago were treatable with antibiotics can now be fatal. In fact, resistance often evolves soon after the introduction of new antibiotics. This is the type of “accelerated evolution” that creates “superbugs,” such as methicillin-resistant Staphylococcus aureus, or MRSA, which you learned about in the beginning of this chapter (Fig. 27Ca). Resistant bacteria, such as MRSA, can cause abscessed lesions on the skin that are difficult

a. Staphylococcus aureus

SEM 9,560×

b. Pseudomonas aeruginosa

SEM 4,590×

resistance creates the need for even newer antibiotics. The development of a single new antibiotic is estimated to cost between $400 and $500 million. Antibiotic resistance adds over $30 billion to annual medical costs in the United States alone! Our understanding of evolutionary biology has helped doctors treat patients appropriately. Doctors generally no longer prescribe antibiotics for colds or flu because they are viral infections. Antibiotics kill only bacteria, not viruses. In the case of tuberculosis (Fig. 27Cc), the strain infecting a patient is tested for antibiotic resistance so that an appropriate antibiotic treatment can be administered. The antibiotic isoniazid is used to treat patients with nonresistant strains, whereas a four-drug regimen is recommended for treatment of resistant strains.

Questions to Consider

c. Mycobacterium tuberculosis SEM 6,200×

Figure 27C  Bacteria with antibioticresistant strains.  a. Methicillin-resistant

Staphylococcus aureus (MRSA). b. Pseudomonas aeruginosa. c. Mycobacterium tuberculosis.

to treat. In some cases, resistant bacteria cause a systemic infection of the blood called bacterial septicemia. Pseudomonas aeruginosa (Fig. 27Cb) produces a toxin that causes abscesses to erupt on the skin in patients who have P. aeruginosa septicemia. Bacterial

Maintenance of Variation A population nearly always shows some genotypic variation. The maintenance of variation is beneficial because populations with limited variation may not be able to adapt to new conditions should the environment change, and may become extinct. How can variation be maintained in spite of selection constantly working to reduce it? First, we must remember that the forces that promote variation are still at work: mutation still generates new alleles, recombination and independent assortment still shuffle the alleles during gametogenesis, and fertilization still creates new combinations of alleles from those present in the gene pool. Second, gene

1. In New York City, state health officials have the power to quarantine tuberculosis patients who do not take their medicine. That is, they can essentially lock them up for as long as needed (often up to 18 months) to treat their illness. Note also that some of the medications can have serious side effects. What do you think about this policy? 2. Many people in less-developed countries die from tuberculosis, not because their disease is incurable, but simply because they do not have health insurance and cannot afford the medications. Why might it be advantageous for the United States to pay more for our medications so that pharmaceutical companies can provide them to lower-income people at a reduced cost or for free?

flow might still occur. If the receiving population is small and mostly homozygous, gene flow can be a significant source of new alleles. Finally, natural selection favors certain phenotypes, but the other types may still remain in reduced frequency. Disruptive selection even promotes polymorphism in a population. Genetic variation can be maintained in diploid organisms in heterozygote individuals that can maintain recessive alleles in the gene pool.

The Heterozygote Advantage Only alleles that are expressed (cause a phenotypic difference) are subject to natural selection. In diploid organisms, this fact makes



UNIT 6  Evolution and Diversity

Initial Distribution

Figure 27.18  Disruptive selection.  a. Disruptive selection favors two or more extreme phenotypes. b. Today, British land snails primarily possess one of two different phenotypes, each adapted to a different habitat. Snails with dark shells are more prevalent in forested areas, and light-banded snails are more prevalent in areas with low-lying vegetation.

Number of Individuals

554

After Time

Number of Individuals

Banding Pattern

After More Time

Number of Individuals

Banding Pattern

Banding Pattern a.

b.

the heterozygote a potential protector of recessive alleles that might otherwise be weeded out of the gene pool by natural selection. Because the heterozygote remains in a population, so does the possibility of the recessive phenotype, which might have greater fitness in a changing environment. When natural selection favors the ratio of two or more phenotypes in generation after generation, it is called balanced polymorphism. Sickle cell disease offers an example of a balanced polymorphism (Fig. 27.19). Individuals with sickle cell disease have the genotype HbSHbS (Hb = hemoglobin, the oxygen-carrying protein in red blood cells; S = sickle cell) and tend to die at an early age due to hemorrhaging and organ destruction. Those who are heterozygous and have sickle cell trait (HbAHbS; A = normal) are better off because their red blood cells usually become sickle-shaped only when the oxygen content of the environment is low. Ordinarily, those with a normal genotype (HbAHbA) are the most fit. Geneticists studying the distribution of sickle cell disease in Africa have found that the recessive allele (HbS) has a higher frequency (0.2 to as high as 0.4 in a few areas) in regions where malaria is also prevalent. What is the connection between higher frequency of the recessive allele and malaria? Malaria is caused by a parasite that lives in and destroys the red blood cells of the normal homozygote (HbAHbA). However, the parasite is unable to live in the red blood cells of the heterozygote (HbAHbS) because the infection causes the red blood cells to become sickle-shaped. Sickle-shaped red blood cells lose potassium, and this causes the parasite to die. Thus, in an environment where malaria is prevalent, the heterozygote is favored. Each of the homozygotes is selected against but is maintained because the heterozygote is favored in parts of Africa subject to malaria. Figure 27.19 summarizes the effects of the three possible genotypes.

malaria sickle cell overlap of both

a.

Genotype

b.

Phenotype

Result

Hb Hb

Normal

Dies due to malarial infection

HbAHbS

Sickle cell trait

Lives due to protection from both

HbSHbS

Sickle cell disease

Dies due to sickle cell disease

A

A

Figure 27.19  Sickle cell disease.  a. Sickle cell disease is more

prevalent in areas of Africa where malaria is more common. b. Whether or not a person dies from the sickle cell disease, malaria, or survives depends upon the genotype. Heterozygotes for the sickle cell trait are protected against both diseases.



Chapter 27  Evolution of Life

they occur. Macroevolution is the result of the accumulation of microevolutionary change that results in the formation of new species.  There are several different ways of defining a species. In this section, we will focus on the biological species concept, which states that a  species is a group of organisms that are capable of interbreeding and are isolated reproductively from other organisms. Under the biological species concept, if organisms cannot mate and produce offspring in nature, or if their offspring are sterile, then they are defined as different species.  Reproductive isolation of similar species is accomplished by the isolating mechanisms illustrated in Figure 27.20. Prezygotic isolating mechanisms are in place before fertilization, and thus reproduction is never attempted. Postzygotic isolating ­mechanisms are in place after fertilization, so reproduction may take place, but it does not produce fertile offspring.

Check Your Progress  27.4 1. Summarize the five agents of evolutionary change. 2. Explain how the agents of evolutionary change relate to the process of natural selection.

3. Describe how malaria makes having a sickle cell allele advantageous, and explain if having a sickle cell allele would be advantageous in all parts of the world.

27.5  Macroevolution and Speciation Learning Outcomes Upon completion of this section, you should be able to 1. Distinguish between prezygotic and postzygotic isolation mechanisms. 2. Explain how adaptive radiation can lead to speciation. 3. Compare and contrast gradualism with punctuated equilibrium.

The Process of Speciation

In this section, we turn our attention to evolution on a large scale, that is, macroevolution. The history of life on Earth is a part of macroevolution. Macroevolution involves speciation, or the splitting of one species into two or more species. The same microevolutionary agents that are at play within populations—genetic drift, natural selection, mutation, and migration—also shape the evolution of new species. Thus, microevolution and macroevolution are the result of the same agents, differing only in the scale at which

Speciation has occurred when one species gives rise to two species, each of which continues on its own evolutionary pathway. How can we recognize speciation? Whenever reproductive isolation develops between two formerly interbreeding groups or populations, speciation has occurred. One type of speciation, called allopatric speciation, usually occurs when populations become separated by a geographic barrier and gene flow is no longer possible. Figure 27.21 illustrates an example of allopatric speciation that has been extensively studied in California. Apparently, members of an ancestral population of Ensatina salamanders existing in

Postzygotic Isolating Mechanisms

Prezygotic Isolating Mechanisms Premating Habitat isolation Species at same locale occupy different habitats. species 1 Temporal isolation Species reproduce at different seasons or different times of day. species 2 Behavioral isolation In animal species, courtship behavior differs, or individuals respond to different songs, calls, pheromones, or other signals.

555

Mating

Mechanical isolation Genitalia between species are unsuitable for one another.

Gamete isolation Sperm cannot reach or fertilize egg.

Fertilization Zygote mortality Fertilization occurs, but zygote does not survive.

Hybrid sterility Hybrid survives but is sterile and cannot reproduce.

hybrid offspring

F2 fitness Hybrid is fertile, but F2 hybrid has reduced fitness.

Figure 27.20  Reproductive barriers.  Prezygotic isolating mechanisms prevent mating attempts, or a successful outcome should mating take place. No zygote ever forms. Postzygotic isolating mechanisms prevent the zygote from developing—or should an offspring result, it is not fertile.



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UNIT 6  Evolution and Diversity Ensatina ring species E. eschscholtzii oregonensis

E. eschscholtzii picta central valley mountains

E. eschscholtzii platensis

central valley barrier

E. eschscholtzii xanthoptica

coastal mountains

Sierra Nevada mountains

South America. As Darwin proposed, these 13 species of finches are descended from a species of mainland finch that migrated to one of the islands. It is likely that after the original species colonized a single island, some individuals dispersed to other islands where they adapted, via natural selection, to feed in various habitats on ecologically different islands. Remarkably, the shape of the beak of each finch species is well matched to the abundant food type on each island. Today, there are seed-eating ground finches, cactus-eating ground finches, insect-eating tree finches, all with different-sized beaks; and a warbler-type tree finch, with a beak adapted to eating insects and gathering nectar. Among the tree finches is a woodpecker type, which lacks the long tongue of a true woodpecker, but makes up for this by using a cactus spine or a twig to ferret out insects. Darwin’s finches are an example of adaptive radiation, or the proliferation of a species by adaptation to different ways of life.

E. eschscholtzii croceater

The Pace of Speciation

Currently, there are two hypotheses about the pace of speciation and, therefore, evolution. One hypothesis is called the phyletic gradualism model, and the other is called the punctuated equilibrium model. Each model gives a different answer to the question of why so few transitional links are found in the fossil record. Traditionally, evolutionary biologists, including Darwin, have supported a process called the E. eschscholtzii E. eschscholtzii ­gradualistic model, which states that change is very eschscholtzii klauberi slow but steady within a lineage before and after a divergence (splitting of the line of descent) (Fig. 27.22a). ThereFigure 27.21  Allopatric speciation.  In this example of allopatric fore, it is not surprising that few transitional links such as Archaespeciation, the Central Valley of California is reproductively separating a opteryx (see Fig. 27.4) have been found. Indeed, the fossil record, range of populations of Ensatina eschscholtzii that are all descended from the even if it were complete, might be unable to show when speciation same northern ancestral species.  has occurred. Why? Because a new species comes about after reproductive isolation, and reproductive isolation cannot be detected in the fossil record! Only when a new species evolves and displaces the existing species is the new species likely to show up the Pacific Northwest migrated southward, establishing a series of in the fossil record. populations. Each population was exposed to its own selective A model of evolution called punctuated equilibrium has also pressures along the coastal and Sierra Nevada Mountains. Due to been proposed (Fig. 27.22b). It says that long periods of stasis, or the presence of the Central Valley of California, which is largely no visible change, are followed by rapid periods of speciation. dry and thus an unsuitable habitat for amphibians, gene flow rarely With reference to the length of the fossil record (about 3.5 billion occurs between eastern and western populations of Ensatina. years), speciation occurs relatively rapidly, and this can explain Genetic differences also increased from north to south, resulting in why few transitional links are found. Mass extinction events are two distinct forms of Ensatina salamanders in Southern California often followed by rapid (relative to the age of the Earth) periods of that differ dramatically in color. speciation. It is also possible that a single population could suddenly divide into two reproductively isolated groups without being geographically isolated. The best evidence for this type of speciation, called sympatric speciation, is found among plants, where multiCheck Your Progress  27.5 plication of the chromosome number in one plant prevents it from 1. Distinguish between a prezygotic isolation mechanism and successfully reproducing with others of its kind. Self-reproduction a postzygotic isolation mechanism. can maintain such a new plant species. 2. Explain the following: You find evidence of an adaptive

Adaptive Radiation One of the best examples of speciation is provided by the finches on the Galápagos Islands, located 600 miles west of Ecuador,

radiation in the fossil record, followed by a long period of little change. Which model of evolution would this support?

ancestral species

new species 2 Time



Chapter 27  Evolution of Life

a.

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new species 1

new species 1

ancestral species

ancestral species

transitional link

ancestral species

new species 2

new species 2 Time

Time b.

a.

Figure 27.22  Gradualism compared to punctuated equilibrium.  a. Supporters of the phyletic gradualism model hypothesize that speciation takes place gradually and many transitional links occur. b. Supporters of the more recent, punctuated equilibrium model hypothesize that speciation occurs rapidly, with no transitional links.

27.6  Systematics

new species 1

Learning Outcomes Upon completion of this section, you should be able to 1. Recognize how phylogenetics, the theory of evolution, and classification are interrelated. ancestral ancestral 2. Define the goal of systematic biology and its twospecies species branches of study: taxonomy and phylogenetics. 3. Interpret the information provided in both a cladogram and a phylogenetic tree.

In section 27.5 we examined macroevolution, or evolutionary change that results in the formation of new species. Macroevolunew tion is the source of past and present biodiversity.species Systematic 2 ­biology (or systematics) is the study of the evolutionary history of Time biodiversity. Systematic biology is a quantitative science that uses b. characteristics of living and fossil organisms, or traits, to infer the evolutionary relationships among organisms, and to then organize biodiversity based upon these relationships.

lupus) and its close relative, the domestic dog (Canis familiaris). The binomial system of nomenclature is used to assign a two-part name (genus and species) to each taxon. For example, the scientific name for modern humans is Homo sapiens. Notice that the scientific name is in italics and only the genus is capitalized. The name of an organism usually tells you something about the organism. In this instance, the species name, sapiens, refers to a large brain. Today, taxonomists, scientists that study taxonomy, use several categories of classification created by Swedish biologist Carl Linnaeus in the 1700s to show varying levels of similarity: species, genus, family, order, class, phylum, and kingdom. Recently, the domain, a higher taxonomic category, has been added to this list. There can be several species within a genus, several genera within a family, and so forth. In this hierarchy, the higher the category, the more inclusive it is. Therefore species in the same domain have general traits in common, whereas those in the same genus have quite specific traits in common.

Phylogeny

Phylogenetics is the branch of systematic biology that studies the evolutionary relatedness among groups of organisms. PhylogenetiTaxonomy is the branch of systematic biology concerned with cists use characters from the fossil record, comparative anatomy identifying, naming, and classifying organisms. A taxon (pl., and development, and the sequence, structure, and function of taxa) is the general name for organisms that exhibit a set of shared RNA and DNA molecules to construct a phylogeny, a hypothesis traits. Classification is the process of naming and assigning organof evolutionary relatedness among taxa represented by a “family isms or groups of organisms to a taxon. For example, the taxon tree.” In essence, systematic biology is the study of the evolutionVertebrata contains organisms with a bony spinal column. As ary history of biodiversity, and a phylogeny is the estimation, and another example, the taxon Canidae contains the wolf (Canis visual representation, of this history. Taxonomists can then use a

Classification

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UNIT 6  Evolution and Diversity

phylogeny to classify organisms into taxa based upon shared evolutionary history.  Cladistics is one method phylogeneticists use to construct a phylogeny called a cladogram. Branches in a cladogram are called clades, and each clade contains a most recent common ancestor and all its descendants. Cladistics uses only those traits that are shared among all individuals to define a clade. These shared derived traits are not found in organisms outside of the clade, called “outgroups.” Figure 27.23 depicts a cladogram for seven groups of vertebrates. Only the lamprey, the “outgroup,” lacks jaws, but the other six groups of vertebrates are in the same clade because they all have jaws, a shared derived trait. In the same way, the vertebrates in the clade without the lamprey and the shark all have the shared derived trait of lungs, and so forth. Figure 27.23 is somewhat misleading because, although single traits are noted on the tree, cladistics uses much more data to arrange groups of organisms into clades. Cladistics is based on the premise of parsimony, which considers the simplest explanation—that is, the cladogram, which requires the fewest number of evolutionary changes—to be the best hypothesis of evolutionary history. It is important to note that a phylogeny (or in the case of cladistics, a cladogram), represents evolutionary relatedness only if homologous traits, or those that share a common ancestry (see Fig. 27.7), are used. Deciphering homology is sometimes d­ ifficult because of convergent evolution. Convergent evolution is the independent evolution of analogous traits, or the same or similar traits, in distantly related lines of descent (see section  27.2). Analogous structures have the same function in different groups, but organisms with these structures do not share a recent common ancestor. Instead, analogous structures arise because of adaptations to similar environments. DNA nucleotide sequences are traits that are commonly used to construct a phylogeny. Modern computer systems and laboratory techniques have made acquiring DNA sequences easy and economical. Homology of nucleotides in a DNA sequence is determined by alignment, or matching sections of sequence from the same section of the genome among all the taxa under study. Figure 27.24 is a phylogeny of primates based on DNA data, if we assume that DNA

differences, or mutations, occur at a relatively constant rate. This is called a  molecular clock, since the number of differences can be used to estimate the absolute age of each branch in the phylogeny.

Linnaean Classification Versus Phylogenetics The Linnaean classification system was developed long before Darwin proposed his theory of evolution, and even longer before the field of phylogenetics was born. The Linnaean classification system still carries historical taxa that were assembled without an understanding of evolutionary relatedness. This can cause problems for the taxonomist, because in some cases analogous, and not homologous, traits were used to classify organisms. Figure 27.25 illustrates the types of problems that arise when trying to reconcile Linnaean classification with the principles of phylogenetics. The phylogeny in this figure proposes that birds are in a clade with crocodiles because they share a recent common ancestor. This ancestor had a gizzard, among other shared derived traits of the skull. Yet Linnaean classification places birds in their own group, separate from crocodiles and separate from reptiles in general. In many other instances, also, Linnaean classification is not consistent with new understandings about phylogenetic relationships. Therefore, some phylogeneticists have proposed a different system of classification called the International Code of Phylogenetic Nomenclature, or PhyloCode, which sets forth rules to follow in naming of clades. Other biologists are hoping to modify Linnaean classification to be consistent with the principles of modern systematic biology.

Three-Domain Classification System Classification systems change over time. Historically, most biologists have utilized the five-kingdom system of classification: plants, animals, fungi, protists, and monerans. Organisms were classified into these kingdoms based on type of cell (prokaryotic or eukaryotic), level of organization (unicellular or multicellular), and type of nutrition. In the five-kingdom system, the monerans were distinguished by their structure—they were prokaryotic (lack

Figure 27.23  Cladogram. 

A cladogram gives comparative information about relationships. Organisms in the same clade share the same derived traits. Humans and all the other vertebrates shown, except lampreys, are in the same clade as sharks because they all have jaws. However, humans are alone on a branch because only they are bipedal. This cladogram is simplified, because phylogeneticists actually use a great deal more data to construct cladograms.

lamprey

shark

salamander

lizard

tiger

gorilla

human bipedal

loss of tail hair amniotic membrane lungs jaws



Chapter 27  Evolution of Life

Millions of Years Ago (MYA)

0

Galago

Capuchin

Green monkey

Rhesus monkey

Gibbon

Chimpanzee

559

Human

10

20

30

40

Figure 27.24  Molecular data.  The relationships of certain primate species are based on a study of their genomes. The length of the branches indicates the relative number of DNA base-pair differences between the groups. These data make it possible to suggest a date for the origin of other branches in the tree by using a molecular clock.

Figure 27.25  Cladistic classification. 

Taxonomic designations are based upon evolutionary history. Each taxon includes a common ancestor and all of its descendants.

mammals

turtles

snakes and d lizard li ds

crocodiles

birds

gizzard

epidermal scales hair and mammary glands amniotic egg, internal fertilization

a membrane-bound nucleus)—whereas the organisms in the other kingdoms were eukaryotic (have a membrane-bound nucleus). The kingdom Monera contained all prokaryotes, which according to the fossil record evolved first (see section 28.1 for discussion about the origin of the first cell).

Ribosomal RNA (rRNA) genes and new data about cells challenged the five-kingdom system of classification. These data support that there are two, not one, groups of prokaryotes. Based on this information, a three-domain system of classification was developed (Fig. 27.26). A domain is a classification category higher than the kingdom. The two prokaryote groups were classified into the domain Bacteria and the domain Archaea because they are so fundamentally different from each other that they warrant being assigned to separate domains. A third domain, the eukaryotes, were classified into the domain Eukarya, which contains kingdoms for all eukaryotes, including protists, animals, fungi, and plants. Interestingly, the Archaea and Eukarya are more closely related to each other than either is to domain Bacteria.



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Check Your Progress  27.6 1. Explain the differences between the study of taxonomy and phylogenetics. How do they relate to the classification of organisms? 2. Describe the differences between a cladogram and a phylogenetic tree. 3. List the three domains of life.

fungi plants EUKARYA

animals

protists protists cyanobacteria heterotrophic bacteria

BACTERIA

ARCHAEA

common ancestor

Figure 27.26  The three-domain system of classification. 

Representatives of each domain are depicted. The phylogenetic tree shows that domain Archaea is more closely related to domain Eukarya than either is to domain Bacteria.

Conclusion In this chapter, you have learned about both macroevolution and microevolution. Although people tend to think of evolution occurring over long timescales, it can occur quickly, particularly under human influence. The evolution of antibiotic resistance and pesticide resistance are examples of how such rapid evolution can be important in your everyday life. The evolution of antibiotic-resistant strains of bacteria, such as Shigella sonnei, is an increasing health problem, costing billions in increased medical costs and even resulting in deaths of some affected patients. By the same token, pesticideresistant insects are costing billions in crop damage and decreasing yields. An evolutionary approach to treating bacterial infections involves careful use of antibiotics, thereby slowing the rate at which newly resistant strains evolve.

MEDIA STUDY TOOLS http://connect.mheducation.com Maximize your study time with McGraw-Hill SmartBook®, the first adaptive textbook.

 

Animations

27.2  Evolution of Homologous Genes 27.4  Mutation by Base Substitution • Simulation of Genetic Drift 27.6  Phylogenetic Trees • Three Domains

SUMMARIZE 27.1  Theory of Evolution Charles Darwin served as the naturalist aboard the HMS Beagle during its five-year voyage. Darwin’s observations of geology and species variation led him to his theory of evolution: how species change over time. ■ Jean-Baptiste de Lamarck proposed, before Darwin, a theory of evolution called inheritance of acquired characteristics, which proposed that changes to the phenotype acquired during a lifetime could be passed on to subsequent generations.

  Tutorials 27.3  Hardy-Weinberg Equilibrium

■ Darwin proposed the theory of natural selection, which proposes

that adaptation to the environment is due to heritable changes that, if advantageous, are passed to future generations differentially because of high reproductive success.

27.2  Evidence of Evolution Darwin’s theory of evolution by natural selection is supported by the following: ■ The fossil record and biogeography, as well as studies of comparative anatomy and development, and biochemistry, all provide evidence for



Chapter 27  Evolution of Life

evolution. Transitional links are fossils that have traits of both ancestor and descendant, and represent evolutionary shift from one trait to another. ■ Biogeography shows that the distribution of organisms on Earth can be influenced by a combination of evolutionary and geological processes. Continental drift is one geological process that affected the biogeographical distribution of organisms. ■ Comparing the anatomy and the development of organisms distinguishes homologous structures from analogous structures. Only homologous structures provide information about common ancestry. Vestigial structures are remnants of past adaptations, but which are no longer used in the current environment. Artificial selection by humans for preferred traits has led to evolution of many plants and animals. In natural populations, evolution can be observed to occur over periods as short as decades, such as the beak size of the Galápagos medium ground finch. ■ All organisms have certain biochemical molecules in common, and these chemical similarities indicate the degree of relatedness.

27.3  Microevolution ■ Microevolution is a process that involves a change in allele frequencies

within the gene pool of a sexually reproducing population. Population genetics is the study of the change in allele frequencies in the gene pool of a population. The work of Hardy-Weinberg indicates that gene pool frequencies arrive at a genetic equilibrium, or Hardy-Weinberg equilibrium, that is maintained generation after generation unless disrupted by mutations, genetic drift, gene flow, nonrandom mating, or natural selection. Any change from the initial allele frequencies in the gene pool of a population signifies that evolution has occurred.

27.4  Processes of Evolution ■ The principles of the Hardy-Weinberg equation state that gene pool

frequencies arrive at an equilibrium that is maintained generation after generation unless disrupted by mutations, genetic drift, gene flow, nonrandom mating, or natural selection. ■ Mutations may influence the fitness, or reproductive success, of the individual. The founder effect and bottleneck effect are special types of genetic drift due to severe reduction in population size. ■ Stabilizing selection, directional selection, and disruptive selection are different ways in which the distribution of a trait changes in response to natural selection.

27.5  Macroevolution and Speciation ■ Macroevolution is the origin of new species, or speciation. A new

■

■ ■

■

species forms when the two populations can no longer interbreed because of either (or both) prezygotic or postzygotic isolating mechanisms. Allopatric speciation, the most common form of speciation, occurs when populations become reproductively isolated because of a geographic barrier. Sympatric speciation occurs when organisms overlap in their distributions. The evolution of several species of finches on the Galápagos Islands is an example of speciation caused by adaptive radiation, because each one has a different way of life. Currently, there are two models about the pace of speciation. The gradualistic model represents a slow, steady change over time, while during punctuated equilibrium, long periods of no change are interrupted by periods of rapid change.

561

27.6  Systematics ■ Systematic biology, or systematics, involves phylogenetics, the study

of evolutionary relatedness (or phylogeny) among taxa (taxon), and taxonomy, the classification of taxa into a hierarchy of categories: domain, kingdom, phylum, class, order, family, genus, and species. Taxonomists are scientists who classify biodiversity. ■ Cladistics is one method of phylogenetics that applies the principle of parsimony to shared derived traits to develop a cladogram, or a treelike hypothesis of evolutionary history. Cladistics strives to classify organisms based upon evolutionary relatedness. This may be complicated by convergent evolution and analogous traits. ■ Each clade contains the most recent common ancestor and all its descendants. Modern phylogenetic methods most commonly analyze similarities and differences among nucleotide sequences, aided by computers, to reconstruct phylogenies. ■ The three-domain system (domain Bacteria, domain Archaea, and domain Eukarya), based on molecular data, is currently preferred to the formerly used five-kingdom system (Monera, Protista, Fungi, Plantae, Animalia). Both bacteria and archaea are prokaryotes. Members of the kingdoms Protista, Fungi, Plantae, and Animalia are eukaryotes.

ASSESS Testing Yourself Choose the best answer for each question.

27.1  Theory of Evolution 1. Inheritance of acquired characteristics is associated with a. Darwin. b. Lamarck. c. Malthus. d. None of these are correct. 2. A(n) _____ makes a species better suited to its environment. a. homologous trait b. adaptation c. analogous trait d. mutation

27.2  Evidence of Evolution 3. Fossils that serve as transitional links allow scientists to a. determine how prehistoric animals interacted with each other. b. deduce the order in which various groups of animals arose. c. relate climate change to evolutionary trends. d. determine why evolutionary changes occur. 4. The flipper of a dolphin and the fin of a tuna are a. homologous structures. b. homogeneous structures. c. analogous structures. d. reciprocal structures.

27.3  Microevolution 5. Assuming a Hardy-Weinberg equilibrium, 21% of a population is homozygous dominant, 50% is heterozygous, and 29% is homozygous recessive. What percentage of the next generation is predicted to be homozygous recessive? a. 21% d. 42% b. 50% e. 58% c. 29%



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UNIT 6  Evolution and Diversity

6. Which of the following would not be true of a population that is exhibiting Hardy-Weinberg equilibrium for a trait? a. there is no genetic drift b. there is random mating c. there is no evidence of selection d. there is no gene flow with another population e. it is evolving with regards to that trait

27.4  Processes of Evolution For questions 7–11, match the description with the appropriate term in the key. Key: a. mutation b. natural selection c. founder effect d. bottleneck e. gene flow f. nonrandom mating 7. The northern elephant seal went through a severe population decline as a result of hunting in the late 1800s. As a result of a hunting ban, the population has rebounded but is now homozygous for nearly every gene studied. 8. A small, reproductively isolated religious sect called the Dunkers was established by 27 families that came to the United States from Germany over 200 years ago. The frequencies for blood group alleles in this population differ significantly from those in the general U.S. population. 9. Turtles on a small island tend to mate with relatives more often than turtles on the mainland. 10. Within a population, plants that produce an insect toxin are more likely to survive and reproduce than plants that do not produce the toxin. 11. The gene pool of a population of bighorn sheep in the southwest United States is altered when several animals cross over a mountain pass and join the population. 12. People who are heterozygous for the cystic fibrosis gene are more likely than others to survive a cholera epidemic. This heterozygote advantage seems to explain why homozygotes are maintained in the human population, and is an example of a. disruptive selection. b. balanced polymorphism. c. high mutation rate. d. nonrandom mating.

27.5  Macroevolution and Speciation 13. The formation of new species due to geographic barriers is called a. isolation speciation. b. allopatric speciation. c. allelomorphic speciation. d. sympatric speciation. e. symbiotic speciation. 14. Which of the following models is supported by the observation that few transitional links are found in the fossil record? a. gradualism b. punctuated equilibrium c. Hardy-Weinberg d. None of these are correct.

27.6  Systematics 15. The three-domain classification system has recently been developed based on a. mitochondrial biochemistry and plasma membrane structure. b. cellular and rRNA sequence data. c. plasma membrane and cell wall structure. d. rRNA sequence data and plasma membrane structure. e. nuclear and mitochondrial biochemistry. 16. The branch of systematic biology that assigns organisms to specific classifications (domain, phyla, class, etc.) is called a. phylogenetics. b. taxonomy. c. systematics. d. cladistics.

ENGAGE BioNOW Want to know  how this science is relevant to your life? Check out the BioNow videos below: ■ Characteristics of Life ■ Quail Evolution

Thinking Critically 1. The frequency of a rare disorder expressed as an autosomal recessive trait is 0.0064. Using the Hardy-Weinberg equation, determine the frequency of carriers for the disease in this population. 2. Viruses such as HIV are rapidly replicated and have very high mutation rates. Thus, evolution of the virus can be observed in a single infected person. Using what you have learned in this chapter, explain why HIV is so hard to treat, even though multiple drugs to treat HIV have been developed. 3. You observe a wasting disease in cattle that you know is genetically caused and thus heritable. The disease is fatal in young cattle. The allele frequency for the gene that causes the disease is 0.05 in the United States, but 0.35 in South America. Explain why such a difference in allele frequencies might exist. 4. If p2 = 0.36, what percentage of the population has the recessive phenotype, assuming a Hardy-Weinberg equilibrium? 5. In a population of snails, 10 had no antennae (aa); 180 were heterozygous with antennae (Aa); and 810 were homozygous with antennae (AA). What is the frequency of the a allele in the population?

PHOTO CREDITS Opener: © Mediscan/Alamy; 27.1(Darwin): © National Library of Medicine; 27.1(rhea): © Nicole Duplaix/National Geographic/Getty Images; 27.1(ship): © Mary Evans Picture Library/The Image Works; 27.1(iguana): © FAN Travelstock/Alamy RF; 27.3a: © Miguel Castro/Science Source; 27.3b: © Celia Mannings/Alamy; 27.3c: © Michael Stubblefield/ Alamy RF; 27.4a: © Jason Edwards/Getty RF; 27.4b: © Joe Tucciarone/Interstellar Illustrations; 27.10(Boston terrier): © Robert Dowling/Corbis; 27.10(wolf): © DLILLC/ Corbis RF; 27.10(Irish wolfhound): © Ralph Reinhold/Index Stock Imagery/Photolibrary RF; 27A (both): © Ingram Publishing/SuperStock RF; 27Ca(MRSA): © Science Source; 27Cb(P. aeruginosa): © Steve Gschmeissner/Science Source; 27Cc(M. tuberculosis): © SPL/Science Source; 27.18b(dark snail): © NHPA/Superstock; 27.18b(light snail): © W. Layer/ Blickwinkel/Age fotostock.

CASE STUDY The West Africa Ebola Outbreak  In 2013, an outbreak of Ebola, one of the most feared viruses on the planet, began in the West African nation of Guinea. It is believed that a one-year-old boy contracted the disease while playing near a tree that housed a species of bat that is known to carry the virus. By early 2014, the disease had become widespread in the neighboring countries of Sierra Leone and Liberia, with cases in Nigeria, Mali, and Senegal.  According to the CDC, there have been over 25,000 confirmed cases of Ebola in West Africa, and over 10,000 confirmed deaths. But most agencies believe that this is an underestimate and that the complete toll of this outbreak may never be known. What makes Ebola so feared is that it belongs to a family of viruses that cause hemorrhagic fever, a disease that targets several different cell types of the body, including macrophages of the immune system, and the endothelial cells in the circulatory system and liver. While Ebola is frequently described as a disease the causes widespread bleeding, most deaths are actually due to fluid loss, organ failure (such as the liver) or an overall failure of the immune system. It is transmitted through direct contact with the body fluids of an infected person. Like many viruses, there are many misconceptions regarding the Ebola virus. These include that the disease is airborne, you can get the virus from contact with cats and dogs, and that antibiotics are an effective treatment. In fact, in many ways Ebola is similar to any virus, it must invade specific cells of the body in order to hijack the cell's metabolic machinery to make more copies of itself.   In this chapter, we will not only examine the interaction of viruses with living organisms, but also other members of the microbial world, such as bacteria and prions.  As you read through the chapter, think about the following questions:

28

Microbiology

CHAPTER OUTLINE 28.1  The Microbial World 28.2 Origin of Microbial Life 28.3 Archaea 28.4 Bacteria 28.5 Viruses, Viroids, and Prions BEFORE YOU BEGIN

Before beginning this chapter, take a few moments to review the following discussions: Section 1.2  To which two domains do the prokaryotes belong? Section 3.2  What are some basic structural features of prokaryotic cells? Table 27.1  When did prokaryotic cells first appear on Earth?

1. How do viruses, such as Ebola, infect the cells of the body? 2. Are viruses classified as living organisms? 3. What are some other human diseases that are caused by viruses?

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28.1  The Microbial World Learning Outcomes Upon completion of this section, you should be able to 1. Describe the contributions of Leeuwenhoek and Pasteur to the science of microbiology. 2. Explain what is meant by the term microbiota. 3. Provide several specific examples of the beneficial effects of microbes.

Antonie van Leeuwenhoek was an early eighteenth-century Dutch tradesman and scientist. He was apparently very skilled at working with glass lenses, which enabled him to greatly improve the microscopes that already existed in the late 1600s. Using these instruments, he was among the first to view microscopic life-forms in a drop of water, which he called “animalcules” and described in this way: [They] were incredibly small, nay so small, in my sight, that I judged that even if 100 of these very wee animals lay stretched out one against another, they could not reach to the length of a grain of coarse sand.

Leeuwenhoek and others after him believed that the “wee animals” he had observed could arise spontaneously from inanimate matter. For about 200 years, scientists carried out various experiments to determine the origin of microscopic organisms in laboratory cultures. Finally, in about 1859, Louis Pasteur devised the experiment shown in Figure 28.1. In the first experiment, when flasks containing sterilized broth were exposed to either outdoor or indoor air, they often became contaminated with microbial growth. In the second experiment, FIRST EXPERIMENT

however, if the neck of the flask was curved so that microbes could not enter the broth from the air, no growth occurred. Thus, the bacteria were not arising spontaneously in the broth, but rather were coming from the air. In 1884, Pasteur also suggested that something even smaller than a bacterium was the cause of rabies, and it was he who chose the word virus from a Latin word meaning poison. Pasteur went on to develop a vaccine for rabies, which he used to save the life of a young French boy who had been bitten by a rabid dog. Microbiology is the study of microbes, a term that includes the bacteria, archaea, protists, fungi, viruses, viroids, and prions. Most, but not all, of these organisms are so small that they require a microscope to be seen. Today we know that bacteria and other microbes are incredibly numerous in air, water, soil, and on objects. A single spoonful of soil can contain 1010 bacteria, and the total number of bacteria on Earth has been estimated at around 1030, which clearly exceeds the numbers of any other type of living organism on Earth. It is a good thing bacteria are microscopic—if they were the size of beetles, the Earth would be covered in a layer of bacteria several miles deep! We need only visit almost any news site to hear about diseases caused by microbes. Viral diseases such as avian flu and Ebola, and new strains of antibiotic-resistant bacteria, such  as Staphylococcus aureus and Mycobacterium tuberculosis will make you think that all microbes cause disease. However, many microbes have beneficial, and sometimes essential, roles to play in human health as well as in the proper functioning of the biosphere. For example, for every cell in your body, there are 10 bacterial cells living on or within you. While you are generally unaware of them,  most of these microbes, also known as the microbiota, have beneficial effects. Bacteria that live on your skin help crowd out harmful microbes that might grow in those areas, and bacteria in the intestines aid in digestion and SECOND EXPERIMENT

flasks outside building opened briefly boiling to sterilize broth

89% show growth

flask is open to air

boiling to sterilize broth

air here is pure

air enters here

flasks inside building opened briefly boiling to sterilize broth

32% show growth

bacteria collect here 100% have no growth

Figure 28.1  Pasteur’s experiments.  Pasteur disproved the theory of spontaneous generation of microbes by performing these types of experiments.



Chapter 28  Microbiology

Biological Evolution

extant organisms Common ancestor of all life on Earth or LUCA (last universal common ancestor)

LUCA extinct lineages

x

x

Origin of Life: first self-replicating cell

x

Stage 4

cell

Biological Evolution

synthesize vitamin K and vitamin B12. Our bodies are actually a finely balanced ecosystem for microbes. Bacteria and fungi are the decomposers that play essential roles in various nutrient cycles on Earth by breaking down organic and inorganic materials that can be reused by plants and animals. Photosynthetic algae and other protists are primary producers that capture energy from the sun or inorganic material, providing nutrients for more complex organisms. Some bacteria also perform photosynthesis, often in ways very different from algae or plants. We have also learned to use bacteria to clean up our environment. Wastewater that leaves your bathroom and kitchen is probably cleaned at a treatment plant by processes that rely on the activity of bacteria. It seems that bacteria can consume virtually any substance found on Earth. Soil contaminated by oil spills or just about any other toxic compounds can be cleaned by encouraging the growth of bacteria that eat these compounds. Not only do many microbes play essential roles in the environment, but they are also valuable for industrial processes, particularly food processing. Also, most antibiotics known today were first discovered in soil bacteria or fungi. Genetic techniques can be used to alter the products generated by bacterial cultures. A variety of valuable products can be made in this way, including insulin and vaccines. Although it might not seem so to an individual suffering from tuberculosis, AIDS, or another serious infectious disease, most microbes are far more beneficial than harmful—in fact, we could not live without them! In this chapter, we will explore the prokaryotic bacteria, viruses, viroids and prions. We will take a closer look at the fungi and protists in Chapter 29. 

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cell origin of genetic code protocell

DNA RNA

Stage 4

Stage 3

plasma membrane

Check Your Progress  28.1

polymers

Stage 2

1. Describe the experiments Pasteur used to disprove the idea of spontaneous generation.

polymerization

microbiology. 3. Explain what is meant by the term microbiota.

28.2  Origin of Microbial Life Learning Outcomes Upon completion of this section, you should be able to 1. Distinguish between chemical and biological evolution. 2. List and briefly describe the four stages that are thought to have led to the formation of the first cells. 3. Explain how the Miller-Urey experiment (among others) provided support for the “primordial soup” hypothesis. 4. Compare the implications of the protein-first, RNA-first, and membrane-first hypotheses for the evolution of the first living cell.

As noted in section 27.1, Darwin’s theory of evolution by natural selection is guided by the principle of common ancestry, that all life on Earth can be traced back to a single ancestor. This ancestor, also called the last universal common ancestor (LUCA), is common to all organisms that live, and have lived, on Earth since life began (Fig. 28.2). What were the properties of this common ancestor?

Chemical Evolution

2. List the major groups of microbes that are studied in

small organic molecules energy capture

Stage 1

abiotic synthesis

inorganic chemicals

early Earth

Figure 28.2  Stages of the origin of life.  The first organic molecules (bottom) originated from chemically altered inorganic molecules present on early Earth (Stage 1). More complex organic macromolecules were synthesized to create polymers (Stage 2) that were then enclosed in a plasma membrane to form the protobiont, or protocell (Stage 3). The protobiont underwent biological evolution to produce the first true, selfreplicating, living cell (Stage 4). This first living cell underwent continued biological evolution (top), with a single surviving lineage, the LUCA, which became the common ancestor of all life on Earth.



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UNIT 6  Evolution and Diversity

How did life first begin on Earth? Darwin mused that perhaps the first living organism arose in a “warm little pond with all sorts of ammonia and phosphoric salts, light, heat, electricity, etc.” With typical insight, Darwin was describing some of the actual conditions of ancient Earth that gave rise to the first forms of life. In Chapter 1, we considered the characteristics shared by all organisms. Organisms acquire energy through metabolism, or the chemical reactions that occur within cells. They also respond and interact with their environment, self-replicate, and are subject to the forces of natural selection that drive adaptation to the environment. The molecules of organisms, called biomolecules, are organic molecules. The first organisms on Earth would have had all of these characteristics. Yet early Earth was very different from the Earth we know today, and consisted mainly of inorganic substances. How, then, did life get started in this inorganic “warm little pond”? Advances in chemistry, evolutionary biology, paleontology, microbiology, and other branches of science have helped to develop new, and test old, hypotheses about the origin of life. These studies contribute to an ever-growing body of scientific evidence that life originated 3.5–4 billion years ago (bya) from nonliving matter in a series of four stages: Stage 1. Organic monomers. Simple organic molecules, called monomers, evolved from inorganic compounds prior to the existence of cells. Amino acids, the basis of proteins, and nucleotides, the building blocks of DNA and RNA, are examples of organic monomers. Stage 2. Organic polymers. Organic monomers were joined, or polymerized, to form organic polymers, such as DNA, RNA, and proteins. Stage 3. Protobionts. Organic polymers became enclosed in a membrane to form the first cell precursors, called protobionts (protocells). Stage 4. Living cells. Protobionts acquired the ability to selfreplicate, as well as other cellular properties. Scientists have performed experiments to test hypotheses at each stage of the origin of life. Stages 1–3 involve the processes of a “chemical evolution,” before the origin of life (see Fig. 28.2). Stage 4 is when life first evolved through the processes of “biological evolution.”

Evolution of Monomers In the 1920s, Russian biochemist Alexander Oparin and British geneticist J. B. S. Haldane independently hypothesized that the first stage in the origin of life was the evolution of simple organic molecules from the inorganic compounds that were present in the Earth’s early atmosphere. The Oparin-Haldane hypothesis, sometimes called the “primordial soup” hypothesis, proposes that early Earth had very little oxygen (O2), but instead was made up of water vapor (H2O), hydrogen gas (H2), methane (CH4), and ammonia (NH3). Methane and ammonia are reducing agents because they readily donate their electrons. In the absence of oxygen, their reducing capability is powerful. Thus, the early Earth had a reducing atmosphere in which oxidation-reduction (redox) reactions could have driven the chemical evolution, or abiotic synthesis, of organic monomers from inorganic molecules in the presence of strong

energy sources. Note that “oxidation” used in this context refers to a chemical redox reaction (see section 6.4), and not to oxygen gas. In 1953, the American chemist Stanley Miller, working under Harold Urey, performed a famous experiment to test the primordial soup hypothesis of early chemical evolution (Fig. 28.3). He reasoned that the energy sources on early Earth included heat from volcanoes and meteorites, radioactivity from isotopes, powerful electric discharges in lightning, and solar radiation. For his experiment, Miller placed a mixture of methane (CH4), ammonia (NH3), hydrogen (H2), and water (H2O), in a closed system, heated the mixture, and circulated it past an electric spark (simulating lightning). After a week’s run, Miller discovered that a variety of amino acids and other organic acids had been produced. The Miller-Urey experiment has been repeatedly tested over the decades since it was first performed. In 2008, a group of scientists examined 11 vials of compounds produced from variations of the Miller-Urey experiment, and found a greater variety of organic molecules than Miller reported, including all 22 amino acids. If early atmospheric gases did react with one another to produce small organic compounds, neither oxidation (no free oxygen was present) nor decay (no decomposers existed) would have destroyed these molecules, and rainfall would have washed them into the ocean, where they would have accumulated for hundreds of millions of years. Therefore, the oceans would have been a thick, warm organic soup—much like Darwin’s “warm little pond.” The Oparin-Haldane hypothesis was an important contribution to our understanding of the early stages of life’s origins, but other hypotheses have also been proposed and tested. In the late 1980s, German chemist Günter Wächtershäuser proposed that thermal vents at the bottom of the Earth’s oceans (Fig. 28.4) provided all the elements and conditions necessary to synthesize organic monomers. According to his “iron-sulfur world” electrode

stopcock for adding gases

stopcock for withdrawing liquid

electric spark CH4 NH3 H2 H2O condenser

gases hot water out cool water in liquid droplets

boiler

heat

small organic molecules

Figure 28.3  Stanley Miller’s experiment.  Gases that were

thought to be present in the early Earth’s atmosphere were admitted to the apparatus, circulated past an energy source (electric spark), and cooled to produce a liquid that could be withdrawn. Upon chemical analysis, the liquid was found to contain various small organic molecules, which could serve as monomers for large cellular polymers.



Chapter 28  Microbiology

plume of hot water rich in iron-nickel sulfides

hydrothermal vent

Figure 28.4  Chemical evolution at hydrothermal vents. 

Minerals that form at deep-sea hydrothermal vents like this one can catalyze the formation of ammonia and even monomers of larger organic molecules that occur in cells.

hypothesis, dissolved gases emitted from thermal vents, such as carbon monoxide (CO), ammonia, and hydrogen sulfide, pass over iron and nickel sulfide minerals, also present at thermal vents. The iron and nickel sulfide molecules act as catalysts that drive the chemical evolution from inorganic to organic molecules. A very different line of thinking involves the comets and meteorites that have pelted the Earth throughout history. In recent years, scientists have confirmed the presence of organic molecules in some meteorites. Many scientists feel that these organic molecules could have seeded the chemical origin of life on early Earth. Others even hypothesize that bacterium-like cells evolved first on another planet and then were carried to Earth, an idea known as panspermia. A meteorite from Mars, labeled ALH84001, landed on Earth some 13,000 years ago. When examined, experts found tiny rods similar in shape to fossilized bacteria. The fact that Mars possessed liquid water in its past, and that life may have evolved there and then was carried to Earth, is one hypothesis that is being examined.

Evolution of Polymers Within a cell’s cytoplasm, organic monomers join to form polymers in the presence of enzymes—such as the synthesis of protein polymers from amino acids by ribosomes. Enzymes themselves are proteins, but presumably there were no proteins present on early Earth. How did the first organic polymers form if no enzymes were present? Building on the Miller-Urey experiment, American biochemist Sidney Fox suggested that once amino acids were present in the oceans, they could have collected in shallow puddles along the rocky shore. Then, the heat of the sun could have caused them to

567

form proteinoids, small polypeptides that have some catalytic properties. The formation of proteinoids has been simulated in the laboratory. When placed in water, proteinoids form microspheres, structures composed only of protein that have many properties of a cell. According to the protein-first hypothesis, some of these newly formed polypeptides had enzymatic properties. Moreover, if a certain level of enzyme activity provided an advantage over others, this would have set the stage for natural selection to shape the evolution of these first organic polymers. Those that evolved to be part of the first cell would have had a selective advantage over those that did not become part of a cell. Fox’s protein-first hypothesis assumes that protein enzymes arose prior to the first DNA molecule. Thus, the genes that encode proteins followed the evolution of the first polypeptides. In contrast, the RNA-first hypothesis suggests that only the macromolecule RNA was needed to progress toward formation of the first cell or cells. Thomas Cech and Sidney Altman shared a Nobel Prize in 1989 for their discovery that RNA can be both a substrate and an enzyme. Some viruses today have RNA genes; therefore, the first genes could have been RNA. It would seem, then, that RNA could have carried out the processes of life commonly associated today with DNA and proteins. Those who support this hypothesis say that it was an “RNA world” some 4 bya.

Evolution of Protobionts Before the first true cell arose, a protobiont (also called a protocell) would have emerged. Protobionts are characterized as having an outer membrane. This membrane would have provided a boundary between the inside of the cell and its outside world, which was critical to the proper regulation and maintenance of cellular activities. Therefore, the evolution of a membrane was a critical step in the origin of life. The plasma membrane of modern cells is made up of phospholipids assembled in a bilayer. The first plasma membranes were likely made up of fatty acids, which are smaller than phospholipids but like phospholipids have a hydrophobic “tail” and hydrophilic “head” (Fig. 28.5). Fatty acids are one of the organic polymers that could have formed from chemical reactions at deep-water thermal vents early in the history of life. In water, fatty acids assemble into small spheres called micelles, consisting of a single layer of fatty acids organized with their heads pointing out and tails pointing toward the center of the sphere. Under appropriate conditions micelles can merge to form vesicles. Vesicles are larger than micelles and are surrounded by a bilayer (two layers) of fatty acids (Fig. 28.6), similar to the phospholipid bilayer of modern cell membranes. An important feature of a vesicle lipid bilayer is that the individual fatty acids can flip between the two layers, which helps to move select molecules, such as amino acids, from outside to the inside of the vesicle. The first protobiont would likely have been a type of vesicle with this type of fatty acid bilayer membrane. Interestingly, if lipids are made available to protein microspheres, lipids tend to become associated with microspheres, producing a lipid-protein membrane. Lipid-protein microspheres share some interesting properties with modern cells: they resemble bacteria, they have an



568

UNIT 6  Evolution and Diversity a. Protocell Membrane a.

Cell Membrane b.

b. hydrophilic “head”

vesicle 2 fatty acids

hydrophobic “tail”

individual fatty acid c.

outside protocell

phospholipid d.

outside cell

inside protocell

inside cell

fatty acid bilayer

phospholipid bilayer

Figure 28.5  A comparison of protobiont and modern cell

plasma membranes.  The first lipid membrane was likely made of a

single layer of fatty acids. These first protobiont (protocell) membranes and the modern cell membrane have features in common. a. Individual fatty acids have a single fatty acid chain with a hydrophilic head and hydrophobic tail. b. Phospholipids of modern cell membranes are made of two hydrophobic fatty acid chains (the “tails”) attached to a hydrophilic head. c. Protobiont membranes were likely made up of a bilayer of fatty acids, with hydrophilic heads pointing outward, and hydrophobic tails pointing inward. d. Modern cells are organized in a similar fashion. Both bilayers create a semipermeable barrier between the inside and outside of a cell.

electrical potential difference, and they divide and perhaps are subject to selection. In the early 1960s, British biophysicist Alec Bangham discovered that when he extracted lipids from egg yolks and placed them in water, the lipids would naturally organize themselves into double-layered bubbles roughly the size of a cell. Bangham’s bubbles soon became known as liposomes. Bangham and others soon realized that liposomes might have provided life’s first membranous boundary. Perhaps liposomes with a phospholipid membrane engulfed early molecules that had enzymatic, even replicative, abilities. The liposomes would have protected the molecules from their surroundings and concentrated them so they could react (and evolve) quickly and efficiently. These investigators called this the membrane-first hypothesis, meaning that the first cell had to have a plasma membrane before any of its other parts. A protobiont would have had to acquire nutrition, that is, other molecules, so that it could grow. One hypothesis suggests that protobionts were heterotrophs, organisms that consume preformed organic molecules. However, if the protobiont evolved at hydrothermal vents, it may have carried out chemosynthesis—the synthesis of organic molecules from inorganic molecules and nutrients. Indeed, many modern bacteria are chemoautotrophs that obtain energy by oxidizing inorganic compounds, such as hydrogen sulfide (H2S), a molecule that is abundant at thermal vents. When hydrothermal

micelle

Figure 28.6  Structure and growth of the first plasma

membrane.  The first plasma membrane was likely made of a fatty acid

bilayer, similar to that seen in vesicles. Protobionts (protocells), the ancestor of modern cells, are thought to have had this type of membrane. a. A cross section of a vesicle reveals the fatty acid bilayer of a vesicle membrane. Micelles are spherical droplets formed by a single layer of fatty acids, and are much smaller than vesicles. b. Under proper conditions, micelles can merge to form vesicles. As micelles are added to the growing vesicle, individual fatty acids flip their hydrophilic heads toward the inside and outside of the vesicle, and their hydrophobic tails toward each other. This process forms a bilayer of fatty acids.

vents in the deep and extremely dark ocean were first discovered in the 1970s, investigators were surprised to discover complex vent ecosystems supported by organic molecules formed by chemosynthesis, a process that does not require the energy of the sun. ATP (adenosine triphosphate) is the most important energycarrying molecule in living organisms. Because all life on Earth uses ATP to fuel cellular metabolism, the evolution of a means to synthesize ATP must have occurred very early in the history of life. The first protobionts evolved in the oxygen-poor environment of early Earth. Thus it is probable that ATP was likely synthesized first by fermentation (see section 7.3). The subsequent evolution of oxidative phosphorylation would have provided an advantage because it greatly increased the amount of ATP synthesized per unit of energy. Mitochondria share a common ancestor with a group of bacteria that synthesize ATP via an electron transport chain. Oxidative phosphorylation is possible in eukaryotes because mitochondria provide an electron transport chain ATP factory. At first, the protobiont must have had limited ability to break down organic molecules, and scientists speculate that it took millions of years for complex biochemical pathways to evolve completely.

Evolution of Living Cells Today’s cell is able to carry on protein synthesis in order to produce the enzymes that allow DNA to replicate. The central dogma of genetics states that DNA directs protein synthesis and that information flows from DNA to RNA to protein. It is possible that this sequence developed in stages. According to the RNA-first hypothesis, RNA would have been the first to evolve, and the first true cell would have had RNA genes. These genes would have directed and enzymatically carried



Chapter 28  Microbiology

out protein synthesis. As noted, today we know a number of viruses have RNA genes. These viruses have a protein enzyme called reverse transcriptase that uses RNA as a template to form DNA. Perhaps with time, reverse transcription occurred within the protobiont, and this is how DNA-encoded genes arose. If so, RNA was responsible for both DNA and protein formation. According to the protein-first hypothesis, proteins, or at least polypeptides, were the first of the three (DNA, RNA, and protein) to arise. Only after the protobiont developed a plasma membrane and sophisticated enzymes did it have the ability to synthesize DNA and RNA from small molecules provided by the ocean. Because a nucleic acid is a complicated molecule, it is unlikely that RNA arose spontaneously from simple chemicals. It seems more likely that enzymes were needed to guide the synthesis of nucleotides and then nucleic acids. Once there were DNA genes, protein synthesis would have been carried out in the manner dictated by the central dogma of genetics. The Scottish chemist Alexander Cairns-Smith proposed that polypeptides and RNA evolved simultaneously. Therefore, the first true cell would have contained RNA genes that could have replicated because of the presence of proteins. This eliminates the baffling chicken-or-egg paradox: assuming a plasma membrane, which came first, proteins or RNA? It means, however, that two unlikely events would have had to happen at the same time. After DNA formed, the genetic code had to evolve before DNA could store genetic information. The present genetic code is subject to fewer errors than a million other possible codes. Also, the present code is among the best at minimizing the effect of mutations. A single-base change in a present codon is likely to result in the substitution of a chemically similar amino acid and, therefore, minimal changes in the final protein. This evidence suggests that the genetic code did undergo a natural selection process before finalizing into today’s code.

Check Your Progress  28.2 1. Explain the role of biomolecules in chemical and biological evolution.

2. List the four stages of the evolution of life, and explain why

they likely occurred in a particular order. 3. Compare and contrast the “primordial soup” and the “iron-sulfur world” hypotheses. 4. Explain why the protein-first versus RNA-first debate can be compared to a chicken-or-egg paradox.

28.3  Archaea Learning Outcomes Upon completion of this section, you should be able to 1. List some of the major criteria that are used to distinguish the domains Archaea, Bacteria, and Eukarya. 2. Review the structural features of the plasma membranes and cell walls of archaea. 3. Distinguish between halophiles, thermoacidophiles, and methanogens.

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TABLE 28.1  Comparison of Domains Archaea and Eukarya

Domain Feature

Archaea

Eukarya

Nucleus

No

Yes

Organelles

No

Yes

Introns

Sometimes

Yes

Histones

Yes

Yes

RNA polymerase

Several types

Several types

Methionine is at start of protein synthesis

Yes

Yes

As noted in section 1.2, the bacteria (domain Bacteria) and archaea (domain Archaea) are prokaryotes, but each is placed in its own domain because of molecular and cellular differences. P ­ rokaryotes are single-celled organisms that lack a nucleus and membranebound cytoplasmic organelles that are found in eukaryotic cells. Archaea and bacteria are not close relatives, even though both are prokaryotes. Based on a number of criteria, including nucleic acid similarities, the eukarya are more closely related to archaea than to bacteria (Table 28.1).

Archaeal Size and Structure Archaea usually range from 0.1–15 μm in size. Their genome is a single, closed, circular DNA molecule, often smaller than a bacterial genome. Like bacteria, archaea reproduce asexually by binary fission. The plasma membrane of archaea differs markedly from those of bacteria and eukaryotes. Rather than a lipid bilayer, archaea often have a monolayer of lipids with branched side chains. These chemical characteristics help many archaea tolerate acid and heat. The cell walls of archaea may also facilitate their survival in extreme environments. The cell walls of archaea lack peptidoglycan, which is a distinguishing feature of bacterial cell walls (see section 28.4). In some archaea, the cell wall is largely composed of polysaccharide, and in others, it is pure protein. A few have no cell wall.

Types of Archaea Three main types of archaea are distinguished based on their unique habitats and metabolic activities: halophiles, thermoacidophiles, and methanogens (Fig. 28.7). Other archaea are found in moderate environments, such as lake sediments and soil, where they are likely involved in nutrient cycling. Several archaea have been found living in symbiotic relationships with animals, including sponges, sea cucumbers, and in the digestive tracts of humans and other animals (see next section on “Methanogens”). However, no parasitic archaea have yet been found—that is, they are not known to cause any infectious disease.

Halophiles The halophiles (salt lovers) usually thrive at salt concentrations of around 12–15%; in contrast, ocean water is about 3.5% salt.



570

UNIT 6  Evolution and Diversity

Halophiles have been isolated from environments such as the Great Salt Lake in Utah, the Dead Sea, and hypersaline soils, where most other organisms cannot survive. These archaea have evolved a number of mechanisms enabling them to survive in very salty environments. Their plasma membranes often have chloride pumps that contain a lightpowered protein called halorhodopsin (related to the rhodopsin pigment found in the human retina). This unique protein pumps chloride and water into the cell, thereby preventing excessive dehydration. Some halophiles also perform a type of photosynthesis that uses a pigment called bacteriorhodopsin instead of chlorophyll. When the pigment absorbs solar energy, it moves a hydrogen ion to outside the plasma membrane. This establishes an H+ gradient that promotes ATP synthesis. a.

Thermoacidophiles

33,200× b.

Another major type of archaea, the thermoacidophiles, are found in extremely hot, acidic, aquatic environments such as hot springs, geysers, and underwater volcanoes. Unlike the great majority of life-forms on Earth, the lipid membranes and proteins of these archaea have evolved to withstand and function optimally at temperatures as high as 80°C; some can even grow at 105°C (remember that water boils at 100°C)! One thermoacidophilic species, Picrophilus torridus, has been particularly well studied for its ability to survive at a pH of less than 1, similar to that found in a 1.2-molar sulfuric acid solution (car battery acid is around 4.5 molar). Sequencing of the genome of this organism has shown that a relatively high number (12%) of its genes are involved in transport functions of its plasma membrane. Many of these transporter proteins are presumably involved in pumping excess H+ ions out of the cell, as well as in taking advantage of the high density of H+ outside the cell to transport other desired molecules into the cell. As one of the most acid-tolerant organisms known, P. torridus will likely provide enzymes for various biotechnology processes that require acidic conditions.

Methanogens

25,000× c.

Figure 28.7  Extreme habitats.  Many archaea thrive in unusual

environmental conditions. a. Halophilic archaea can live in salt lakes. b. Thermophilic archaea can live in the hot springs of Yellowstone National Park. c. Methanogens live in anaerobic swamps and in the guts of animals.

The methanogens (methane-makers) mostly use carbon dioxide and hydrogen as energy sources, producing methane as a by-product. Methanogens are found in anaerobic environments such as swamps, lake sediments, rice paddies, and the intestines of animals. Cows, which have large populations of methanogens in their digestive tracts, release a significant amount of methane into the environment. Because methane is a greenhouse gas that may contribute to global warming and climate change, some scientists are suggesting that the amount of methane produced by cattle should be reduced by changing the animals’ diets, adding substances to their feed to alter the microbial populations, or developing a vaccine to specifically inhibit the growth of methanogens. However, the relative contribution of cattle, compared to other potential sources of methane, remains controversial.



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Chapter 28  Microbiology capsule

Check Your Progress  28.3

ribosomes

cell wall

plasma membrane

1. List some key characteristics of archaea, bacteria, and eukarya.

2. Explain how the plasma membranes of archaea differ from those of bacteria and eukaryotes.

3. Describe adaptations that allow archaea to survive in extremely hot or salty environments.

4. Discuss why a dedicated environmentalist might give up eating beef.

28.4  Bacteria Learning Outcomes Upon completion of this section, you should be able to 1. Identify the major structural features of bacteria. 2. Describe bacterial reproduction, including three mechanisms by which bacteria increase variation. 3. Explain why bacteria are considered to be metabolically diverse. 4. List several major bacterial diseases of humans and describe how they are treated.

Bacteria (domain Bacteria) are the most common type of prokaryote on Earth. To date over 11,000 different species have been named, but the number of unnamed species is probably in the millions. There are estimated to be over 10,000 different species of bacteria in a single gram of soil. The Harvard paleontologist Steven Jay Gould once labeled the Earth as “Planet of the Bacteria.” Indeed, bacteria are found in virtually every environment on Earth.

Bacterial Size and Structure Most bacteria are between 0.2–10 μm in size. A few, however, are quite large, including one species that is about the same size as the period at the end of this sentence. Bacteria have three basic shapes: rod (bacillus, pl., bacilli); spherical (coccus, pl., cocci); and spiralshaped or helical (spirillum, pl., spirilla) (Fig. 28.8). A bacillus or coccus can occur singly or may occur in particular arrangements.

a. Bacilli (rod): Escherichia coli

SEM 13,300×

storage granule

chromosome (DNA)

nucleoid

fimbriae

flagellum

Figure 28.9  Typical bacterial cell.  Bacteria are prokaryotes and thus have no membrane-bound nuclei or organelles. For example, when cocci form a cluster, they are called staphylococci, whereas cocci that form chains are called streptococci. The typical structure of a bacterium is shown in Figure 28.9. All bacterial cells have a plasma membrane, which is a lipid bilayer, similar to the plasma membrane in plant and animal cells. Most bacterial cells are further protected by a cell wall that contains the unique molecule peptidoglycan.  Bacteria can be classified by differences in their cell walls, which are detected using a staining procedure devised more than 100 years ago by Hans Christian Gram. When you go to the doctor for a bacterial infection, one of the most common tests performed is the Gram stain, because different antibiotics tend to be more effective against Gram-positive or Gram-negative bacteria. Cell walls that have a thick layer of peptidoglycan outside the plasma membrane stain purple with the Gram stain procedure, and are called Gram-positive bacteria. If the peptidoglycan layer is either thin or lacking altogether, the cells stain pink and are considered Gram-negative. In addition to their plasma membrane, Gramnegative bacteria have an outer membrane that contains lipopolysaccharide molecules. When these Gram-negative cells are killed by

b. Cocci: Streptococcus thermophilus

SEM 6,250×

c. Spirillum: Spirillum volutans

LM 400×

Figure 28.8  Typical shapes of bacteria.  a. Bacilli, rod-shaped bacteria. b. Cocci (spherical). c. Spirillum (curved).

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UNIT 6  Evolution and Diversity

your immune system, these molecules are released, stimulating inflammation and fever. Beyond the cell wall, some bacteria have a slimy polysaccharide capsule that can protect the cell from dehydration and the immune system. Motile bacteria have flagella for locomotion but never cilia. The flagellum is a stiff, curved filament that rotates like a propeller. Bacterial flagella are structurally distinct from eukaryotic flagella. Some bacteria have fimbriae that bind to various surfaces. For example, bacteria that cause urinary tract infections can bind to urinary tract cells. Drinking cranberry juice inhibits this binding. Most bacteria have a single circular chromosome, which is located in a nucleoid region of the cytoplasm, rather than in a membrane-bound nucleus. Many bacteria also harbor accessory rings of DNA called plasmids that can carry genes for antibiotic resistance, among other things, and are commonly used to carry foreign DNA into other bacteria during genetic engineering (see section 26.1). Bacterial cells also contain an abundance of ribosomes, as well as various types of granules that store nutrients such as glycogen and lipids. Notice that bacteria do not have an endoplasmic reticulum, a Golgi apparatus, mitochondria, or chloroplasts. This diagram summarizes the major structural features of bacteria:

Outside the cell

Flagella Fimbriae Capsule Cell wall Plasma membrane

Inside the cell

Cytoplasm Ribosomes Nucleoid

Bacterial cell

Bacterial Reproduction and Gene Transfer Bacteria reproduce asexually. After a period of sufficient growth, the bacterial cell simply divides into two new cells, with each cell getting a copy of the genome and approximately half of the cytoplasm. This process is known as binary fission (Fig. 28.10). Each daughter cell is a clone, or exact copy, of the parent cell. When bacteria are spread out and grown on agar plates, each individual cell can give rise to a colony of millions of cells. In the absence of genetic mutations, each of these cells is a clone of the initial cell. Some bacteria need only 20 minutes to reproduce, whereas others grow more slowly, with generation times of a day or more. Under unfavorable conditions, some bacteria, such as Clostridium tetani, the cause of tetanus, can produce resistant structures called endospores, thick-walled, dehydrated structures capable of surviving the harshest conditions, perhaps even for thousands of years. Endospores are not for reproduction, but are simply a way to survive unfavorable conditions. Sexual reproduction does not occur among prokaryotes, but at least three means of horizontal gene transfer have been observed. Conjugation takes place when a donor cell passes DNA to a recipient cell by way of a sex pilus. Transformation

cytoplasm

cell wall nucleoid region

Figure 28.10  Binary fission.  When conditions are favorable for

growth, prokaryotes divide to reproduce. Binary fission is a form of asexual reproduction because the daughter cells have exactly the same genetic material as the parent.

occurs when a bacterium takes up DNA released into the medium by dead bacteria. During transduction, viruses carry portions of bacterial DNA from one bacterium to another. As discussed in section 26.2 and in the Bioethical feature, “DIY Bio,” scientists are using these processes to build geneticallymodified organisms (GMOs).

Bacterial Metabolism Although most bacteria are structurally similar to each other, they demonstrate a remarkable range of metabolic abilities. Most bacteria are heterotrophs that require an outside source of organic compounds in the same way that animals do. Some heterotrophic bacteria are anaerobic and cannot use oxygen to capture electrons at the end of their electron transport chain (which is physically located in their plasma membrane, as all prokaryotes lack mitochondria). Instead, they use a variety of substances as final electron acceptors. Sulfate-reducing bacteria transfer the electrons to sulfate, producing hydrogen sulfide, which smells like rotten eggs. Denitrifying bacteria use nitrate, whereas others use minerals, such as iron or manganese, as electron receivers. Other bacteria are chemoautotrophs. They reduce carbon dioxide to an organic compound by using energetic electrons derived from chemicals, such as ammonia, hydrogen gas, and hydrogen sulfide. Electrons can also be extracted from certain minerals, such as iron. Some bacteria are photosynthesizers that use solar energy to produce their own food. Cyanobacteria are believed to have arisen some 3.8 billion years ago and to have produced much of the oxygen in the atmosphere at that time. Sometimes erroneously called blue-green algae, these organisms contain green chlorophyll and have other pigments that give them a bluish-green color (Fig. 28.11). Other bacterial photosynthesizers split hydrogen sulfide, instead of water, in anaerobic environments. Therefore, they produce sulfur rather than oxygen as a by-product of photosynthesis.



Chapter 28  Microbiology

SCIENCE IN YOUR LIFE  ►

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BIOETHICAL

DIY Bio Picture yourself becoming a scientist. Do you see years of education, a white lab coat, and high-tech equipment? That image of a scientist is changing. For about $250, you can purchase materials to perform a sophisticated experiment to create a glowing plant right in your own home with no experience.  A project like this is part of a movement called DIY biology, where amateurs are performing modern biology experiments in their kitchens, garages, and community lab spaces, rather than academic or corporate research laboratories. A main focus of DIY biologists is synthetic biology, in which new biological processes or organisms are genetically designed or constructed to serve useful purposes, such as creating plants that could replace street lights or your desk lamp to save energy. 

One necessary material for the project is a gene that codes for green fluorescent protein (GFP), which glows when exposed to ultraviolet light. You’ll also need a tool to get the genes into the plant, but it isn’t a mechanical device. Instead, you can buy a culture of Agrobacterium tumefaciens, a Gram-negative bacillus soil bacterium commonly used for a genetic engineering technique called Agrobacteriummediated plant transformation.  Natural Agrobacterium is pathogenic to many agricultural crops, including grapes, nuts, and beets, because it inserts a tumor-causing plasmid, or extra piece of DNA, into plant cells. Agrobacterium used in the synthetic biology process doesn’t have the tumor-causing genes in its plasmid; instead, the goal is to add the GFP genes to the plasmid in a process called transformation as the first step of the experiment. 

a.

Once the Agrobacterium incorporates the foreign genes into its own, it can be used to deliver the genes to plant cells. Flowering tips of the plant are dipped into the Agrobacterium, which naturally infects the plant’s reproductive cells with newly transformed GFP plasmid. If the procedure is completed properly, some of the seeds produced by the original plant will grow into glowing plants. 

Questions to Consider 1. Is it safe to manipulate the genes of an organism?  2. Should scientific experiments be left to trained scientists in regulated research laboratories?  3. What are other applications of Agrobacteriummediated plant transformation? 

Aside from producing oxygen, cyanobacteria are often the first colonizers of rocks. Many are capable of both carbon fixation and nitrogen fixation, needing only minerals, air, sunlight, and water for growth. Cyanobacteria are also notorious for forming toxic blooms in waters enriched by nutrients, which may require that some lakes be closed to human use. Cyanobacteria (and in some cases eukaryotic algae) form a symbiotic relationship with fungi in a lichen. The fungi provide a place for the cyanobacteria to grow and obtain water and mineral nutrients. Some cyanobacteria even appear to live in a symbiotic relationship in the fur of sloths, providing the animals with a form of camouflage in their dense green forest home!

Bacterial Diseases of Humans

b.

Figure 28.11  Cyanobacteria.  Cyanobacteria are photosynthetic

bacteria that contain chlorophyll. a. In Chroococcus, single cells are grouped in a common gelatinous sheath. b. Filaments of cells occur in Oscillatoria, a common inhabitant in freshwater environments.

Most types of bacteria do not cause disease, but a significant number do. Why does one microbe cause disease, whereas another, closely related species is completely harmless? Pathogenic microbes often carry genes that code for specific virulence factors that determine the type and extent of illness they are capable of causing. For example, right now you have billions of Escherichia coli living in your large intestine, without causing any problems. However, some strains of E. coli have acquired genes that make them dangerous invasive pathogens. The strain of E. coli called O157:H7 has the ability to generate a toxin that damages the lining of the intestine, resulting in a bloody diarrhea. It has also obtained other virulence factors that help it stick to the lining of the gut more efficiently. In 2011, around 60 people in ten states became ill from E. coli O157:H7 contamination in Romaine lettuce. That same year an outbreak of pathogenic E. coli in Germany sickened 852 people and caused 32 deaths.



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By acquiring genes coding for virulence factors (i.e., via conjugation, transformation, and/or transduction) harmless bacteria may be converted into pathogens. Antibiotic resistance genes may pass between organisms in the same ways, creating antibioticresistant pathogens. Some bacterial diseases that are transmitted sexually were discussed in section 21.5. Next we discuss a few more common types of bacterial diseases in humans.

Streptococcal Infections  More different types of human disease are caused by bacteria from the genus Streptococcus than by any other type of bacterium. Streptococcus pneumoniae can cause pneumonia, meningitis, and middle ear infections. Up to 70% of apparently healthy adults are carriers of this potentially harmful bacterium, which causes disease mainly in children and the elderly. S. mutans is found on the teeth and contributes to tooth decay and the formation of dental caries. The most common illness caused by streptococci is pharyngitis, commonly called strep throat, which is usually due to infection by S. pyogenes (Fig. 28.12a). S. pyogenes also causes relatively mild skin diseases, such as impetigo in infants (Fig. 28.12b). A number of more serious and invasive diseases can occur after or during infections with S. pyogenes. The nature and severity of these diseases depend on which virulence factors the pathogen carries. Scarlet fever is a strep infection caused by a Streptococcus strain that produces a toxin causing a red rash. An intense immune response to the infection can lead to rheumatic fever, which is characterized by a high temperature, swollen joints that form nodules, and heart damage. Although rarely seen in the United States today, rheumatic fever killed more school-age children than all other diseases combined in the early twentieth century. By releasing enzymes that destroy connective and muscle tissues and kill cells, S. pyogenes infection can lead to large-scale cell lysis and tissue destruction. “Flesh-eating” bacteria cause necrotizing fasciitis (Fig. 28.12c), in which rapid tissue damage can require amputation of infected limbs or, in 40% of cases, rapid death. Staphylococcus aureus and MRSA  Staphylococcus aureus is perhaps the bacterial species that has received the most media attention recently. About 20% of people are carriers of this bacterium (mostly on their skin or in their nostrils) without any

a. Streptococcus pyogenes

20,0003

symptoms. When S. aureus does cause disease, it is usually limited to skin infections. However, in people who are very young, very old, or immunocompromised for any other reason, it can invade the body and cause life-threatening disease. Moreover, a strain of S. aureus that is resistant to methicillin, called MRSA, is killing an increasing number of young, otherwise healthy individuals. Besides being resistant to many antibiotics, MRSA strains often possess genes coding for toxins not found in other S. aureus strains. Some of these toxins can be very damaging to tissues. Since the peak in MRSA infections between 2006 and 2008, the number of annual cases have been declining, mostly because health-care facilities have initiated infection-control procedures and increased awareness among health professionals. 

Tuberculosis Tuberculosis (TB) is one of the leading worldwide causes of death due to infectious disease. When first described in 1882 by the father of medical microbiology, Robert Koch, TB caused one of every seven deaths in Europe, and one-third of all deaths of young adults. Today, it is estimated that one-third of the world’s population is infected with the TB bacterium, causing approximately 2 million deaths each year. Tuberculosis is a chronic disease caused by Mycobacterium tuberculosis, a pathogen closely related to M. leprae, the causative agent of leprosy. Generally, M. tuberculosis infection occurs in the lungs, but it can occur elsewhere as well. M. tuberculosis is very slow growing, and the extent of the disease is determined by host susceptibility. An immune response results in inflammation of the lungs. Active lesions can produce dense structures called tubercles (Fig. 28.13). These can persist for years, causing symptoms such as coughing and spreading the bacteria to other areas of the lungs and body as well as to other individuals. Eventually, the damaged lung tissue hardens and calcifies, leaving characteristic spots that can be observed with chest X rays. The number of TB cases in the United States surged in the late 1980s, mainly due to increased rates of infection with HIV, which damages helper T cells that are needed to fight the TB bacterium. Since 1992, however, the prevalence of TB in the United States has steadily declined, to a rate of 3.4 cases per 100,000 people in 2011. This reduction is mainly attributed to better testing and treatment of infected persons. This trend is also occurring worldwide,

b. Impetigo

c. Flesh-eating disease

Figure 28.12  Streptococcus pyogenes.  a. Streptococcus pyogenes, shown here in an electron micrograph, often forms chains of cells, as seen here. b. Impetigo is a common mild skin infection. c. Flesh-eating disease is rare but life-threatening.



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in the intestines for several days. A recent large outbreak of salmonellosis in the United States occurred between late summer of 2008 and January 2009, prompting the FDA to recall every product made from peanuts processed by a Georgia factory. Over 700 people became ill, and at least nine died after ingesting contaminated products.

tubercle

Drug Control of Bacterial Diseases

a.

b.

43

Figure 28.13  Tuberculosis.  Tuberculosis, an infection caused by

the bacterium Mycobacterium tuberculosis, usually settles in the lungs. a. Lung tissue with a large cavity surrounded by tubercles, which are hard, calcified nodules where the bacteria are trapped. b. Photomicrograph shows a cross section of one large tubercle, with a diseased center.

although numbers of TB cases continue to increase in Africa and Southeast Asia. The 2007 story of a 31-year-old Atlanta lawyer with TB shows how easily an infectious agent can spread around the world. Despite knowing that he was infected with M. tuberculosis, he flew from Atlanta to Paris to get married, traveling through five countries in the process. Making matters worse, he was infected with a strain of M. tuberculosis called XDR, which is resistant to nearly all drugs available to treat TB. Upon returning to the United States, he was taken into forced quarantine for four weeks, the first time an American had been forcibly isolated by the Centers for Disease Control and Prevention (CDC) in several decades. He was released after it was determined that he was no longer infectious. It should be noted, however, that there is no way to know how many people infected with M. tuberculosis are traveling on airplanes every day.

Food Poisoning  Whether the source was mishandled food at a salad bar, or potato salad that sat in the sun too long at a picnic, all of us have probably experienced the intestinal discomfort known as food poisoning. Two basic types of bacteria cause food poisoning: those that produce toxins while they are growing in food, and those that cause infections once they are in the intestine. Several species of bacteria can produce toxins in foods, especially those containing dairy, eggs, or meat products. The symptoms, which consist mainly of vomiting and diarrhea, tend to appear suddenly within a few hours of ingestion, and are usually self-limiting. In contrast, Clostridium botulinum, the causative agent of botulism, produces one of the most toxic substances on Earth. When people are canning and don’t heat foods to a high enough temperature, Clostridium can produce endospores that survive the canning process. These endospores then germinate in the airless environment of the can or bottle and become toxin-producing cells. If untreated, about 25% of people who ingest botulism toxin die from respiratory paralysis. Salmonella is a classic example of a bacterial food poisoning agent that does not produce gastroenteritis until it has reproduced

Most antibiotics kill or inhibit bacteria by interfering with their unique metabolic pathways. Therefore, they are not expected to harm human cells. For example, erythromycin and tetracyclines inhibit bacterial protein synthesis by binding to bacterial ribosomes, whereas penicillins and cephalosporins inhibit bacterial cell wall synthesis. There are problems associated with antibiotic therapy. Some individuals are allergic to certain antibiotics, and the reaction may even be fatal. Antibiotics not only kill off disease-causing bacteria, but they may also reduce the number of beneficial bacteria in the intestinal tract and vagina. This may allow the overgrowth of certain harmful bacteria or yeast, especially in the intestine, vagina, or mouth. See the Health feature, “Antibiotics and Probiotics,” for a discussion of how probiotics can be used to replenish these beneficial organisms. Most important perhaps is the growing resistance of bacteria to antibiotics, as discussed in the story that opened Chapter 27. Antibiotics were introduced in the 1940s, and for several decades they worked so well it appeared that infectious diseases had been brought under control. However, we now know that bacterial strains can mutate and become resistant to a particular antibiotic. Worse yet, when bacteria exchange genetic material, resistance can pass between different bacteria. Penicillin and tetracycline now have a failure rate of more than 22% against Neisseria gonorrhoeae, which causes gonorrhea. Bacteria with multiple drug resistances, such as MRSA and multidrug-resistant M. tuberculosis, are an especially serious threat in hospitals, prisons, and longterm care facilities. With the rise of antibiotic resistance, many companies are now working to develop innovative kinds of antibacterial therapies. Recent advances in the study of genomics (see section 26.4) has enabled scientists to rapidly screen the genomes of bacteria for the genes that produce novel compounds that may be used to produce new classes of antibiotics. There has been some recent success in these efforts, but it still takes years of research and development to develop these new drugs for widespread use.

Check Your Progress  28.4 1. Describe the three basic shapes of bacteria. 2. Explain how bacterial conjugation differs from transformation and transduction.

3. Compare the nutritional strategy of a heterotrophic bacterium with that of a chemoautotroph.

4. Consider how antibiotics work, and then describe two specific mechanisms bacteria could use to become resistant.



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SCIENCE IN YOUR LIFE  ►

HEALTH

Antibiotics and Probiotics In 1928 at a London lab, Scottish scientist Alexander Fleming was researching substances that might inhibit bacterial growth when he took a two-week vacation. Upon returning, he noticed some petri dishes that had bacteria growing on them were contaminated with Penicillium fungus. In one of the most famous moments in medical history, Fleming realized that the fungus was producing a chemical that inhibited the growth of the bacteria (Fig. 28Aa). Of course that substance turned out to be penicillin, and the practice of medicine was changed forever. Many bacterial infections that had previously meant serious illness or even death were now treatable. As often happens, however, the widespread use of Fleming’s new drug, and many others like it, had unintended consequences. Especially in patients who take antibiotics frequently or for a prolonged period, normal microbial populations in the body can be disrupted, resulting in diarrhea, yeast infections, or worse. A bacterium called Clostridium difficile, which is normally present in the intestines at relatively low levels, tends to overgrow in these patients, and may cause a severe, or even fatal, inflammation of the colon. Treatment for this disease is usually more antibiotics, but some authorities are suggesting that probiotics may be helpful in preventing this and many similar diseases. Grocery store shelves and Internet health sites are full of products claiming to contain “probiotics,” usually containing bacteria of the genera Lactobacillus or Bifidobacterium.

According to the U.N. Food and Agricultural Organization, probiotics are “live microorganisms, which, when administered in adequate amounts, confer a health benefit on the host.” By this definition, foods like yogurt, cheeses, and other fermented dairy products, are probiotics. But because of the increased interest in the health benefits of “friendly” bacteria, live bacteria are being added to an increasing variety of products (Fig. 28Ab). Americans spent over $1 billion on probiotic products in 2011, and are expected to spend twice as much on probiotics by 2016. How do probiotics work? As already mentioned in this chapter, the normal microflora of the body can produce vitamins and aid digestion. They also inhibit harmful microbes by simply taking up space and nutrients, and in some cases, by secreting chemicals that directly kill pathogens. But the beneficial effects of normal microflora go beyond competing with other microbes. Body surfaces such as the intestinal wall are fortified with large populations of immune cells that guard against invasion of the tissues by pathogens. An ongoing interaction between these cells and the normal microflora is necessary to maintain a healthy immune system. Experimentally, animals that are raised under gnotobiotic (germ-free) conditions have poorly developed immune systems, and replenishing their bodies with “good” bacteria restores healthy immune function, sort of like priming a pump. That is one reason why probiotics are also being tested for treatment of inflammatory conditions such as irritable

Figure 28A  Changing attitudes about microbes. 

a. When Fleming discovered the inhibition of bacterial growth by Penicillium (a fungal contaminant) in 1928, few would have predicted the (b) benefits of bacteria less than 100 years later.

a.

b.

bowel syndrome, ulcerative colitis, and Crohn’s disease. Future applications may also include treating urinary tract and vaginal infections, inhibiting food allergies, preventing tooth decay, and improving the effectiveness of vaccines! Despite their potential, however, one aspect of probiotics to be aware of is quality control. Currently there is very little regulation of which bacterial strains, or even the number of beneficial organisms, a product must contain to be labeled as probiotic. And of course, it is extremely unlikely that all the potential benefits of probiotics will pan out. In a period of less than 100 years, however, we have gone from discovering a new way to inhibit or kill harmful bacteria that invade our bodies, to realizing how closely our health is linked to our relationship with harmless microflora.

Questions to Consider 1. If no antibiotics existed, how might your life be different? 2. Besides taking antibiotics, what other factors might influence the numbers and types of microflora in your body? 3. High numbers of “good” bacteria are found in the intestine and on the skin. The immune system needs to protect these areas from invading microbes, but cannot respond as strongly to the normal microflora without causing problems. What are some possible ways that immune cells could distinguish “good” from “bad” bacteria?



Chapter 28  Microbiology

28.5  Viruses, Viroids, and Prions Learning Outcomes Upon completion of this section, you should be able to 1. Identify the major structural features of viruses. 2. Describe the steps in a typical viral reproductive cycle, and explain how some viruses become latent. 3. Describe several viral diseases of humans, and explain why it is difficult to produce vaccines against some viruses. 4. Distinguish between viruses, viroids, and prions.

As we learned in Chapter 1, all living organisms are composed of cells. Viruses are acellular, meaning that they are not composed of cells. Also, viruses are obligate parasites, meaning that they can reproduce only inside a living cell (called the host cell) by utilizing at least some of the machinery (ribosomes, certain enzymes, etc.) of that cell. Therefore, the question arises, “Are viruses alive?” Scientists and philosophers have long argued this question. Some say that viruses are not alive. After all, not only are they acellular, but also some have been synthesized in the lab from chemicals! Moreover, when viruses are outside a host cell, they are totally quiescent, exhibiting no metabolic activity. Others argue

TEM 60,000× Adenovirus: DNA virus with a polyhedral capsid and a fiber at each corner. fiber protein fiber

577

that viruses are alive because they have a genome that directs their reproduction when they are inside the host cell. If viruses are not considered alive, then certainly the simpler viroids and prions are not alive either. Viroids are strands of RNA that can reproduce inside a cell, and prions are protein molecules that cause other proteins to become prions.

Viral Size and Structure Most viruses are much smaller than bacteria. Viruses typically measure between 0.03–0.2 μm, whereas most bacteria measure at least 0.5 μm. Interestingly, the largest virus was recently discovered in Siberia. Called Pithovirus, it is about 1.5 μm in size, which is larger than the smallest known bacterium. Viruses come in a variety of shapes, including helices, spheres, polyhedrons, and more complex forms. A virus always has at least two parts—an outer capsid composed of protein subunits, which protects an inner core of nucleic acid (Fig. 28.14). The viral genome can be single- or double-stranded DNA, or single- or double-stranded RNA. This diversity in genetic material is different from all cellular organisms, which always have a doublestranded DNA genome. Special viral enzymes that help a virus reproduce can also be inside the capsid.

Influenza virus: RNA virus with a spherical capsid surrounded by an envelope with spikes. spikes

capsid

protein unit capsid

DNA

RNA envelope

a.

b.

Figure 28.14  Viruses.  a. Despite their diversity, all viruses have an outer capsid composed of protein subunits and a nucleic acid core that is composed of either DNA or RNA. b. Some types of viruses also have a membranous envelope.



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viral enzyme

6.

RNA genome

2.

capsid spike

1. 5.

5. Assembly: New viruses are now present.

host DNA

Rubella virus

1. Attachment: Spike combines with receptor.

6. Budding: Virus acquires an envelope and spikes.

Host cell

envelope

viral RNA

2. Entry: Virus enters cell and uncoating occurs.

proteins

4. 3.

4. Biosynthesis: Viral components are synthesized.

3. Replication: Many copies of viral RNA genome are made.

Figure 28.15  Reproductive cycle of an animal virus.  The rubella virus, like many others that infect animal cells, has an RNA genome. Notice also that the mode of entry requires uncoating and that the virus acquires an envelope and spikes when it buds from the host cell.

In some viruses, especially those that infect animals, the capsid is surrounded by a membrane called an envelope. This envelope is made of lipid, and is usually derived from the host cell’s plasma membrane when the virus buds from the host cell. Viral glycoproteins called spikes often extend from the envelope. These spikes are critical to viral infection because they help the virus bind to the surface of the host cell before entering it.

Viral Reproduction Viruses infect almost every type of organism on Earth. Viruses called bacteriophages infect bacterial cells, usually just a particular group or species of bacteria. Some viruses infect only plants, whereas others infect only animals. Some viruses can infect and cause disease only in humans. The specificity of a virus for a host occurs because the spike of a virus and a receptor molecule on a cell’s plasma membrane fit together like a hand fits a glove, which triggers events allowing viral entry into the cell. Figure 28.15 illustrates the reproductive cycle of a typical enveloped animal RNA virus. This reproductive cycle has six steps: 1. During attachment, the spikes of the virus bind to a specific receptor molecule on the surface of a host cell. The host cell normally uses this receptor for another purpose, but it is effectively “hijacked” by the virus for its own purposes. 2. During entry, also called penetration, the viral envelope fuses with the host’s plasma membrane, and the rest of the virus

(capsid and viral genome) enters the cell. The genome is freed when cellular enzymes remove the capsid, a process called uncoating. 3. Replication occurs when a viral enzyme makes complementary copies of the genome, which is RNA in this case. 4. During biosynthesis, some of these RNA molecules serve as mRNA for the production of more capsid and spike proteins, using host ribosomes. 5. At assembly, a mature capsid forms around a copy of the viral genome. 6. During budding, new viruses are released from the cell surface. During this process, they acquire a portion of the host cell’s plasma membrane and spikes, which were specified by viral genes during biosynthesis. The enveloped viruses are now free to spread the infection to other cells.

Latency  Some animal viruses can become latent (hidden) inside the host cell. Herpesviruses and retroviruses are well known for using this strategy, which helps them avoid detection by the host immune system. During latency, new viruses are not produced, but the viral genome is reproduced along with the host cell. Environmental stresses, such as ultraviolet radiation, can induce the latent virus to enter the biosynthesis stage, leading to the production of new virus particles. Borrowing terms that were first defined in bacteriophages, latent viruses are sometimes said to be “lysogenic,” whereas viruses that are actively reproducing are known as “lytic.”



Chapter 28  Microbiology

Retroviruses add an interesting twist to the story of viral reproduction. The genome of a retrovirus is RNA, but these viruses are able to convert their genome into DNA because they contain an enzyme called reverse transcriptase. This enzyme, which is not found in host cells, is so named because it catalyzes a process that is the reverse of normal transcription, which goes from DNA to RNA. The DNA copy (cDNA) of the retroviral genome can be integrated into the host DNA, where it is called a provirus (see Fig. 21.14). Not only is the integrated provirus resistant to antiviral medications taken by the host, but it is also able to escape detection by the host immune system.

Viral Diseases of Humans Viruses cause many important human diseases, but we have room to discuss only a few of them: colds, influenza, measles, and four herpes viral infections. Some viral diseases that are transmitted sexually, including HIV/AIDS, were discussed in section 21.5. The best protection against most viral diseases is immunization utilizing a vaccine. Of the viral diseases we will be discussing, vaccines are currently available for influenza, measles, chickenpox, and shingles, but not for the common cold, herpes simplex, or Epstein-Barr viruses.

The Common Cold and Influenza Colds are most commonly caused by rhinoviruses, and the symptoms usually include a runny nose, mild fever, and fatigue. The flu, caused by the influenza virus, is characterized by more severe

capsid

symptoms, such as a high fever, body aches, and severe fatigue. Cold symptoms tend to subside within a week, but the flu may last for two or three weeks, and can result in death, especially in elderly patients or those with weakened immune systems. While most common cold viruses are endemic (always present) in the human population, flu outbreaks tend to be epidemic, affecting large numbers of people in more limited geographic areas at any one time. Why can you get several colds or the flu year after year? There are over 100 different strains of rhinoviruses, plus several other viruses that cause very similar symptoms. In the case of colds, you become immune only to those strains you have contracted before. However, the reason you may get the flu each year is that the influenza virus can change rapidly. Small changes called antigenic drift, especially those affecting the surface spikes, may be enough to make the virus capable of temporarily evading the immune response of individuals who were immune to the original virus (Fig. 28.16a). Therefore, manufacturers of influenza vaccines try to incorporate the strains that are predicted to cause the highest number of cases each year. Still, antigenic drift can lead to local epidemics. The genome of the influenza virus is composed of eight segments of RNA. When two different influenza viruses infect the same cell, these RNA segments can get mixed up as the viruses reproduce. When new virus particles are assembled that contain RNA from both original viruses, this reassortment event, called an antigenic shift, may lead to new combinations of surface spikes (Fig. 28.16b). Because the human population has not previously been exposed to this combination of antigens, the result can be a

mutation

RNA genome

579

mutations

envelope

spikes

modified spike

Antigenic drift

Human influenza virus

a.

Animal influenza virus

Human influenza virus

reassortment combination of spikes

Antigenic shift

in host cell

b.

Figure 28.16  Antigenic drift and shift.  a. In antigenic drift, small mutations gradually change surface antigens so that antibodies to the original virus become less effective. As time goes by, people are more likely to become ill upon exposure to the virus. b. In antigenic shift, even more serious major changes take place on surface antigens as genome segments are reassorted between two influenza viruses that infect the same cell. Now, antibodies are more likely to be ineffective, and most people will become ill when they are exposed to the virus.



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pandemic, or worldwide epidemic. The “avian flu” virus, which goes by the names H5N1 and H7N9, which originated in Southeast Asia, contains genes from a bird virus, as well as a human virus, and thus may have resulted from a reassortment event. Fortunately, neither of these viruses currently seem to be efficiently transmitted to humans. However, the most recent “swine flu” outbreak of 2009 serves to remind us it is impossible to predict what future reassortment events may occur, so vaccine manufacturers must play catchup whenever a new virus evolves.

Measles Measles is one of the most contagious human diseases. An unvaccinated person can become infected simply by breathing viral particles in the air of a room where an infected person has been, even two hours later. After exposure, there is a seven- to twelve-day incubation period before the onset of flulike symptoms, including fever. A red rash develops on the face and moves to the trunk and limbs. The rash lasts for several days. In the more-developed nations, the fatality rate is about 1 in 3,000. However, in the less-developed countries, the fatality rate is 10–15%. The number of deaths is decreasing in many areas, however, due to efforts to vaccinate children, who still make up the majority of measles fatalities. In the United States, the number of measles cases dropped from 5 million each year to a few thousand after the introduction of the measles vaccine in 1963. Between 2000 and 2013, measles deaths worldwide fell by 75%, from an estimated 750,000 to 146,000. The measles vaccine is usually given to infants in combination with vaccines against mumps and rubella, collectively called the MMR vaccine. However, recently there has been an increase in the rates of measles in the United States, which is largely attributed to the decision by some parents not to vaccinate their children with the MMR vaccine due to religious or personal choice.

many states are now requiring that children receive this vaccine. In May 2006, the FDA approved the first vaccine for prevention of shingles. It is for use in people over age 60. Epstein-Barr virus (EBV) is the herpesvirus associated with infectious mononucleosis, or “mono.” Like HSV-1, EBV is very common—as many as 95% of adults aged 35 to 40 have been infected. When infected with EBV as children, most people have no symptoms. In contrast, those infected as teenagers and young adults often develop infectious mononucleosis. The disease is named after the tendency of the cells infected by the virus, which are also known as mononuclear cells, to become more numerous in the blood. The symptoms of mono usually include fever, fatigue, and swollen lymph nodes. Like all herpesviruses, EBV establishes a lifelong latent infection. EBV has also been associated with other conditions, such as chronic fatigue syndrome and certain rare cancers.

Antiviral Drugs Because viruses use the machinery of host cells for viral replication, it is difficult to develop drugs that affect viral replication without harming host cells. As noted earlier, however, many viruses use their own enzymes to copy their genetic material, and many antiviral drugs inhibit these enzymes. As discussed in section 21.5, a variety of antiretroviral agents have been developed to inhibit the reverse transcriptase and protease enzymes used by HIV, which can delay the onset of AIDS. Other drugs, such as acyclovir and valacyclovir (or Valtrex), are commonly used to control recurrences of HSV-1 and HSV-2 infections. These compounds mimic nucleotides, and thus can inhibit viral genome synthesis. Other antiviral drugs may affect virus attachment, entry, or assembly. If no effective antiviral drugs are available for a particular viral infection, patients are often told to simply let the virus run its course, because antibiotics are not effective against viral infections.

Herpesviruses

Viroids and Prions

Herpesviruses cause chronic infections that remain latent for much of the time. Eight herpesviruses capable of infecting humans have been described. Some of these infect epithelial cells and neurons, whereas others infect blood cells. Some herpesviruses do not seem to cause any pathology, but we will discuss four herpesviruses that cause disease in humans. Herpes simplex virus type 1 (HSV-1) is usually associated with cold sores and fever blisters around the mouth. It is estimated that 70–90% of the adult population is infected with HSV-1. Herpes infections of the genitals are generally caused by HSV-2 and are transmitted through sexual contact. Painful blisters fill with clear fluid that is very infectious (see Fig. 21.15). Genital herpes infections are widespread in the United States, affecting 20–25% of the adult population. Most individuals infected with genital herpes will experience recurrences of symptoms induced by various stresses. Another herpesvirus that infects humans is the varicella-­ zoster virus, which causes chickenpox. Later in life, usually after age 60, the latent virus can reemerge as a related disease called shingles. Painful blisters form in an area innervated by a single sensory neuron, often on the upper chest or face. The symptoms may last for several weeks or, in some cases, months. The FDA approved a vaccine for the prevention of chickenpox in 1995, and

Viroids and prions are also acellular pathogens. A viroid consists of a circular piece of naked RNA. The viroid RNA is ten times shorter than that of most viral genomes, and apparently it does not code for any proteins. Viroid replication causes diseases in plants, the only known hosts. The mechanism of viroid diseases, such as potato spindle tuber and apple scar skin, are not known. Prions are proteinaceous infectious particles that cause degenerative diseases of the nervous system in humans and other animals. They are derived from normal proteins of unknown function in the brains of healthy individuals. Disease occurs when the normal proteins change into the abnormal, prion shape. This forms more prion proteins, which go on to convert other normal proteins into the wrong shape. These abnormal proteins seem to build up in the brain, causing a loss of neurons and what appear to be holes in the tissue (see Fig. 17.22). The first prion disease to be described was scrapie, which occurs in sheep. It is called scrapie because, as the disease affects the animal’s brain, an affected sheep begins to scrape off much of its wool. Scrapie is not capable of causing disease in humans. However, another prion disease in cattle, called bovine spongiform encephalopathy (BSE), or mad cow disease, does appear capable of infecting humans and has caused over 150 human deaths in



Chapter 28  Microbiology

Great Britain and a few other countries. This disease is now called variant Creutzfeldt-Jakob disease (vCJD), to distinguish it from the “classic” form of CJD, which occurs spontaneously in a very low percentage of people. In April 2012, a California dairy cow was diagnosed with BSE, which was the fourth case of BSE detected in U.S. cattle since 2003. Three human cases of vCJD have been documented in the United States, but all three victims were thought to have acquired the disease in other countries prior to moving to the United States. Chronic wasting disease is another prion disease that occurs in deer, elk, and moose, but there have been no documented cases of transmission to humans. One prion disease that passes directly from human to human is kuru, which was transmitted among a cannibalistic tribe in New Guinea due to their traditional practice

581

of consuming their dead relatives. Even though prion diseases are as frightening as they are fascinating, their incidence in humans remains very low.

Check Your Progress  28.5 1. Describe the basic structure of an enveloped virus,

including capsid, nucleic acid, envelope, and spikes.

2. Explain the six stages of a typical animal virus reproductive cycle.

3. List three viral diseases for which a vaccine is available, and three for which there is no vaccine.

4. Compare how viruses differ from viroids and prions.

Conclusion Historically, Ebola epidemics and outbreaks have only been very short-lived and affected a relatively small number of individuals. The duration and extent of the 2014–2015 Ebola outbreak in West Africa caused immediate concern throughout the globe. The fact that, for the first time, cases were reported in both Europe and the United States, increased public awareness and concern about Ebola. The attention to this disease has focused both governments and private agencies on the need to develop methods of managing outbreaks of highly contagious diseases, as well

as investing in research on how to treat and prevent these diseases. Advances in medical research, specifically in the field of genomics, has already had an impact on how outbreaks of infectious diseases will be addressed in the future. Even before the disease had peaked in West Africa, researchers were testing new vaccines and using the data from these trials to possibly prevent future widespread outbreaks of Ebola and other hemorrhagic fevers.

MEDIA STUDY TOOLS http://connect.mheducation.com Maximize your study time with McGraw-Hill SmartBook®, the first ­adaptive textbook.

 

Animations

28.2  Miller-Urey Experimentation 28.3  Binary Fission 28.4  Gram Stain • Bacterial Locomotion • Binary Fission • Bacterial Conjugation • Bacterial Transformation • Antibiotic Inhibition of Protein Synthesis 28.5  Lambda Phage Replication Cycle • Entry of a Virus into a Host Cell • Replication Cycle of a Retrovirus • Antiviral Agents • How Prions Arise

  Tutorials 28.5  Viral Life Cycle



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Microbiology is primarily the study of bacteria, archaea, protists, fungi, and viruses. Most microbes are microscopic, and some are extremely numerous. ■ Two of the most significant contributors to the early discipline of microbiology were Antonie van Leeuwenhoek, who was the first to visualize microbes using his microscopes, and Louis Pasteur, whose many accomplishments include disproving the idea that microbes could arise spontaneously. ■ Microbes are perhaps best known for causing infectious diseases, which can range from the merely annoying to the swiftly fatal. However, most microorganisms are far more beneficial than harmful, not only to humans, but to Earth’s ecosystem. These include the microbiota that reside on and in the human body, as well as decomposers that break down organic and inorganic matter.

using the Gram stain procedure. The outer membrane of Gramnegative bacteria contains a toxic substance called lipopolysaccharide. Many bacteria have flagella for motility. ■ All bacteria have a DNA genome, usually found on a single, circular chromosome in the nucleoid region. Bacteria reproduce by binary fission, producing identical daughter cells. Horizontal gene transfer also occurs, by conjugation, transformation, or transduction. Some bacteria can survive for long periods by producing endospores. ■ Most bacteria are heterotrophs, requiring carbon in the form of organic molecules. Some are chemoautotrophs, which acquire carbon from carbon dioxide and energy from chemicals. Cyanobacteria are an important group of photosynthetic bacteria. ■ Bacterial species that cause important diseases of humans include members of the genera Staphylococcus, Streptococcus, Mycobacterium, and Salmonella. Antibiotics are chemicals that either kill or inhibit the growth of bacteria.

28.2  Origin of Microbial Life

28.5  Viruses, Viroids, and Prions

SUMMARIZE 28.1  The Microbial World

A central principle of evolutionary theory is that all life on Earth has arisen from a last universal common ancestor (LUCA). Chemical reactions are hypothesized to have led to the formation of biomolecules as well as the first true cells in the following stages: ■ In stage 1, simple organic molecules, or monomers, arise from inorganic chemicals. The “primordial soup” hypothesis suggested by Oparin and Haldane is an influential idea about this stage, which is supported by the work of Miller, Urey, and others. ■ In stage 2, monomers join to form polymers such as proteins, RNA, or membranes. The protein-first hypothesis suggests that some early proteins were enzymes that could synthesize other molecules. The RNA-first hypothesis holds that RNA was the first macromolecule. ■ In stage 3, the evolution of a protobiont (protocell) very likely involved the formation of micelles, which would have fused to form larger vesicles. The membrane-first hypothesis focuses on the need for an outer membrane to contain other biochemical reactions. The first membranes may have been similar to liposomes, which form spontaneously from lipids in water. The protobiont may have been a heterotroph that consumed preformed organic molecules, or a chemoautotroph that required only inorganic nutrients. ■ Stage 4 produced living cells that were able to reproduce. According to the central dogma of genetics, information flows from DNA to RNA to protein. However, this may not have been the case in the earliest cells.

28.3  Archaea Archaea and bacteria are prokaryotes, single-celled microbes that lack a nucleus or membrane-bound organelles. However, the two groups are classified into different domains of life due to nucleic acid sequence and biochemical differences. ■ Structurally, archaeal plasma membranes have unique lipids that help some to survive in extreme environments. ■ Archaea are often associated with extreme habitats, but actually are widespread in the environment. Three major types of archaea are the halophiles, thermoacidophiles, and methanogens.

28.4  Bacteria Bacteria are the most widespread, and in many ways successful, type of organism on Earth. ■ Most bacteria have a cell wall that contains peptidoglycan, and most bacteria can be classified as either Gram-positive or Gram-negative

Viruses are minute, acellular pathogens that reproduce as obligate intracellular parasites. ■ Structural features shared by all viruses include an outer capsid composed of protein and an inner core of nucleic acid. The viral genome can be single- or double-stranded DNA or RNA. Many animal viruses also have an envelope with spikes. ■ The reproductive cycle of an animal virus typically has six steps: attachment, entry, genome replication, biosynthesis, assembly, and budding. During budding, enveloped viruses usually acquire a portion of the host cell’s plasma membrane. Retroviruses (e.g., HIV) have RNA genomes that are converted to DNA by reverse transcriptase, before integrating into the host cell’s chromosome as a provirus. ■ Viruses infect almost all types of organisms and cause many important diseases. Viral diseases of humans include the common cold, influenza, measles, and chickenpox. Antiviral drugs usually inhibit viral enzymes. Like viruses, viroids and prions are acellular pathogens. ■ Viroids are obligate, intracellular plant pathogens that are autonomously replicating short RNA molecules. They contain no protein. ■ Prions contain no nucleic acid. They are proteinaceous, infectious particles that convert a normal cellular protein into the abnormal form, which accumulates and damages brain tissue, inducing dementia. Diseases caused by prions include scrapie, kuru, and mad cow disease.

ASSESS Testing Yourself Choose the best answer for each question.

28.1  The Microbial World 1. Decomposers a. break down dead organic matter in the environment by secreting digestive enzymes. b. break down living organic matter by secreting digestive enzymes. c. destroy living cells and then break them down with digestive enzymes. d. live in close association with another species.



2. Which of the following best describes the term microbiota? a. The microbes that are smaller than a eukaryotic cell. b. The microbes that cause disease. c. The microbes that are decomposers. d. The microbes that are naturally found on or within our bodies.

28.2  Origin of Microbial Life 3. The RNA-first hypothesis for the origin of cells is supported by the discovery of a. ribozymes. b. proteinoids. c. polypeptides. d. nucleic acid polymerization. 4. Which of these steps probably occurred first in the chemical and biological evolution of life? a. The cell membrane of a protobiont formed. b. Formation of the first organic molecules c. The genetic material was RNA. d. Simple organic monomers formed from inorganic materials.

28.3  Archaea 5. Archaea differ from bacteria in that they a. have a nucleus. b. have membrane-bound organelles. c. have peptidoglycan in their cell walls. d. are often photosynthetic. e. None of these are correct. 6. While studying an ancient lake you discover that the water in the lake had an exceptionally high concentration of salt. What type of bacteria would you have expected to live in this lake? a. cyanobacteria b. thermoacidophiles c. methanogens d. halophiles

28.4  Bacteria 7. Which of the following describes a bacterium that is spherical in shape? a. bacillus b. coccus c. spirillum 8. In this process, bacteria pick up DNA from the environment. a. binary fission b. transduction c. conjugation d. transformation 9. The DNA of a bacteria is contained where in the cell? a. in the nucleus b. a region of the cytoplasm called the nucleoid c. at the ribosomes d. within the lipopolysaccharide layer

Chapter 28  Microbiology

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28.5  Viruses, Viroids, and Prions 10. Prions contain a. DNA only. c. RNA only. b. protein only. d. DNA, RNA, and protein. 11. Small changes in influenza surface antigens lead to a. antigenic drift. c. antigenic recombination. b. antigenic shift. d. antigenic schism. 12. The envelope of an animal virus is usually derived from the _____ of its host cell. a. cell wall c. capsule b. plasma membrane d. receptors 13. Capsid proteins are synthesized during which phase of viral replication? a. replication d. proteination b. biosynthesis e. All of these are correct. c. assembly

ENGAGE Thinking Critically 1. Model organisms are those widely used by researchers who wish to understand basic processes that are common to many species. Bacteria such as Escherichia coli are model organisms for modern geneticists. Give three reasons why bacteria would be useful in genetic experiments. 2. Many viruses contain their own enzymes for replicating their genetic material, whereas others use the host cell’s enzymes. Would DNA viruses or RNA viruses be more likely to produce their own enzyme(s) for this purpose, and why? 3. Explain a method by which an antiviral drug could interfere with the replication cycle of a virus. 4. Antibiotic medications work by targeting specific structures and functions in bacterial cells. Side effects on the patient are usually minimal, because their eukaryotic cells do not possess the same structures and characteristics as the prokaryotic pathogens. What structures or functions of the prokaryotic cell would serve as good targets for new antibiotics? 

PHOTO CREDITS Opener: © epa european pressphoto agency b.v./Alamy; 28.4: © Ralph White/Corbis; 28.7a(main): © Marco Regalia Sell/Alamy RF; 28.7a and b (insets): © Eye of Science/ Science Source; 28.7b(main): © Alfredo Mancia/Getty RF; 28.7c(main): © Susan Rosenthal/ Corbis; 28.7c(inset): © Dr. M. Rohde, GBF/Science Source; 28.8a: © Eye of Science/Science Source; 28.8b: © SciMAT/Science Source; 28.8c: © Ed Reschke/Getty Images; 28.10: © CNRI/SPL/Science Source; 28.11a: © Biology Pix/Science Source; 28.11b: © M.I. (Spike) Walker/Alamy; 28.12a: © Dr. Kari Lounatmaa/Science Source; 28.12b: © SPL/Science Source; 28.12c: © ZUMA Press, Inc./Alamy; 28.13a: © McGraw-Hill Education; 28.13b: © Carolina Biological Supply Company/Phototake; 28Aa: © Bettmann/Corbis; 28Ab: © McGraw-Hill Education; 28.14a(top): © Biophoto Associates/Science Source; 28.14b(top): © CDC/Cynthia Goldsmith.



29

Protists and Fungi CHAPTER OUTLINE 29.1  Protists 29.2 Fungi

BEFORE YOU BEGIN Before beginning this chapter, take a few moments to review the following discussions: Section 1.2  Protists and fungi belong to which domain of life? Section 3.5  How does the endosymbiotic theory explain the origin of energy-producing organelles in the eukaryotic cell? Section 5.4  What is the difference between a haploid and a diploid eukaryotic cell?

CASE STUDY Dangerous Protists Protected from the heat wave in an air-conditioned lab, a pathologist pulls a petri plate covered in E. coli bacteria from an incubator. A boy has suddenly passed away, and a sample of his brain tissue is in the center of the plate. Radiating from the tissue are tiny pathways cut through the film of bacteria, pathways characteristic of the “brain-eating amoeba” Naegleria fowleri. The tiny protists must have been the cause of death, and they were feeding their way through the E. coli, just as they had fed through the boy’s brain. Five days ago, the boy suddenly experienced a high fever, stiff neck, and vomiting. Doctors suspected bacterial meningitis, but antibiotics didn’t help, and he died before more tests could be performed.  But how was the boy infected? At the start of the oppressive heat wave, he and his family had gone to the lake to cool off. Stirred up from the bottom by the boy’s boisterous play, the amoeba likely entered his nose during a handstand attempt and burrowed toward his brain, virtually guaranteeing his death. Fortunately, N. fowleri is just one rare species among thousands of other beneficial protists that live around us, on us, and in us. In the same lake where the boy was infected, there are critical species producing oxygen, recycling nutrients, and forming the base of the aquatic food chain.  In this chapter, will not only explore the diversity of life found in Kingdom Protista, but also that of Kingdom Fungi. While we often hear about the negative aspects of organisms in these kingdoms, we will learn that many species of protistans and fungi benefit humans. As you read through this chapter, think about the following questions:

1. Are all protistans parasites? 2. How are protists related to other eukaryotes, such as humans? 3. How do protists and fungi impact human heath and welfare?

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Chapter 29  Protists and Fungi

29.1  Protists Learning Outcomes Upon completion of this section, you should be able to 1. Summarize what is known regarding the evolution of the first single-celled protist. 2. Describe the general characteristics of a protist. 3. Summarize the unique and specific structural features of each group of protists, including how they reproduce and obtain food. 4. Identify several human diseases caused by protists, and explain why many are difficult to treat.

The domain Eukarya is divided into four kingdoms: Protista, Fungi, Plantae, and Animalia. For the most part, protists are ­single-celled eukaryotic microbes, although some exist as colonies of cells or are multicellular. They are such a diverse group, however, that perhaps the best definition of a protist is any eukaryotic organism that is not a plant, animal, or fungus. Protists offer us a glimpse into the past because they are most likely related to the first eukaryotic cell to have evolved. The first eukaryotic cell is thought to have arisen from a prokaryotic cell around 1.7 billion years ago.  According to the endosymbiotic theory, mitochondria may have resulted when a nucleated cell engulfed aerobic bacteria, and chloroplasts may have originated when a nucleated cell with mitochondria engulfed cyanobacteria (see Fig. 3.16). Protists also bridge the gap between the first eukaryotic cells and multicellular organisms—the other three types of eukaryotes (fungi, plants, and animals) all trace their ancestry to a protist.

Characteristics of Protists Protists vary in size from microscopic algae and protozoans to kelp that can exceed 200 m in length. Kelp, a brown alga, is multicellular; Volvox, a green alga, is colonial; Spirogyra, also a green alga, is filamentous. Most protists are single-celled, but despite their small size they have attained a high level of complexity. The amoeboids and ciliates possess unique organelles—their contractile vacuole is an organelle that assists in water regulation.  Protists are sometimes grouped according to how they acquire organic nutrients. The algae are a diverse group of photoautotrophic protists that synthesize organic compounds via photosynthesis. Protozoans are a group of heterotrophic protists that obtain organic compounds from the environment. Some protozoans, such as Euglena, are able to combine autotrophic and heterotrophic nutritional modes.  Protists reproduce sexually and asexually. Asexual reproduction by mitosis is the norm in protists. Sexual reproduction generally occurs only when environmental conditions are unfavorable. Protists can form spores or cysts, which are dormant phases of the protist life cycle, that can survive until favorable conditions return. Parasitic protists form cysts for the transfer to a new host. Many protists cause diseases in humans, but many others have significant ecological importance. Aquatic photoautotrophic protists produce oxygen and are the foundation of the food chain in

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both freshwater and saltwater ecosystems. They are a part of plankton, organisms suspended in the water that serve as food for heterotrophic protists and animals. Interestingly, whales, the largest animals in the sea, feed on plankton, one of the smallest.  

Diversity of Protists Protists were once classified together as a single kingdom. However, DNA evidence has suggested that protists do not all belong to the same evolutionary lineage, meaning that they did not evolve from a single common ancestor. In fact, protists and other eukaryotes, including the plants, fungi, and animals, are currently classified into six supergroups (Table 29.1). A supergroup is a high-level taxonomic group below domain and above kingdom. Each supergroup represents a separate evolutionary lineage.  The DNA evidence supports these multiple protist lineages, but the relationships among the lineages are difficult to decipher. Protist lineages are very long and old, dating back to the origin of the first eukaryotes. As lineages stretch back in time, we can be less and less certain about how they are related to each other, just as the history of humans is less complete the further back in time we look. New research in the evolution of protists has helped clarify some of the evolutionary relationships among eukaryote lineages (Fig. 29.1), but much research still needs to be done.  In the following sections, we examine the various supergroups into which protists and other eukaryotes have been placed based on our current understanding.

The Archaeplastids The archaeplastids include land plants and other photosynthetic organisms, such as green and red algae, that have chloroplasts (also called plastids) derived from endosymbiotic cyanobacteria (see Fig. 3.16).  The green algae are protists that contain both chlorophylls a and b. They inhabit a variety of environments, including oceans, fresh water, snowbanks, the bark of trees, and the backs of turtles. Some of the 8,000 species of green algae also form symbiotic relationships with plants, animals, and fungi in lichens (see section 29.2).  Green algae occur in many different forms. The majority are single-celled; however, filamentous and colonial forms exist. Seaweeds are multicellular green algae that resemble lettuce leaves. Despite the name, green algae are not always green; some have additional pigments that give them an orange, red, or rust color. 

TABLE 29.1  Eukaryotic Supergroups Supergroup

Types of Organisms

Archaeplastids

Land plants as well as green and red algae

Chromalveolates

Brown algae, diatoms, ciliates, sporozoans, and water molds

Excavates

Euglenids and certain other flagellates

Amoebozoans

Amoeboids, as well as plasmodial and cellular slime molds

Rhizarians

Foraminiferans and radiolarians

Opisthokonts

Animals, fungi, and certain flagellates



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UNIT 6  Evolution and Diversity

Chlorophytes Land plants

Archaeplastids

Red algae

Charophytes Apicomplexans

Ciliates Brown algae

Chromalveolates

Dinoflagellates

Diatoms

Parabasalids

prokaryotic ancestors

Euglenids

Excavates

Diplomonads

Domain Eukarya

Water molds

Amoeboids Animals Choanoflagellates Fungi Foraminiferans Radiolarians

Opisthokonts

Plasmodial slime molds

Rhizaria

Cellular slime molds

Amoebozoans

Kinetoplastids

Figure 29.1  Evolutionary relationship of the eukaryotic supergroups.  Molecular data are used to determine the relatedness of the supergroups and their constituents. This is a simplified tree that does not include all members of each supergroup.

Biologists propose that land plants are closely related to the green algae, because both land plants and green algae have chlorophylls a and b, a cell wall that contains cellulose, and food reserves made of starch. Molecular data suggest that the green algae are subdivided into two groups, the chlorophytes and the charophytes

(see Figure 29.1). Charophytes are thought to be the green algae group most closely related to land plants.   Chlamydomonas is an example of a charophyte that inhabits still, freshwater pools. Its fossil ancestors date back over a billion years. The anatomy of Chlamydomonas is best seen using an



Chapter 29  Protists and Fungi

eyespot nucleus with nucleolus

chloroplast

flagellum

pyrenoid starch granule

Figure 29.2  Chlamydomonas.  Chlamydomonas is a motile,

single-celled green alga.

electron microscope, because it is less than 25 μm long (Fig. 29.2). It has a defined cell wall and a single, large, cup-shaped chloroplast that contains a pyrenoid, a dense body where starch is synthesized. In many species, a bright red, light-sensitive eyespot helps guide individuals toward light for photosynthesis. 

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A number of colonial forms occur among the chlorophytes. Volvox is a well-known colonial green alga (Fig. 29.3). A colony is a loose association of independent cells. A Volvox colony is a hollow sphere with thousands of cells arranged in a single layer surrounding a watery interior. Volvox cells move the colony by coordinating the movement of their flagella. Some Volvox cells are specialized for reproduction, and each of these can divide asexually to form a new daughter colony. This daughter colony resides for a time within the parent colony, but then it escapes by releasing an enzyme that dissolves away a portion of the parent colony. Spirogyra is an example of a charophyte (see Fig. 29.4) found in green masses on the surfaces of ponds and streams. It has ribbonlike, spiraled chloroplasts. Spirogyra undergoes sexual reproduction via conjugation, a temporary union during which the cells exchange genetic material. Two haploid filaments line up parallel to each other, and the cell contents of one filament move into the cells of the other filament, forming diploid zygospores. Diploid zygospores survive the winter, and in the spring they undergo meiosis to produce new haploid filaments. Most red algae are multicellular charophytes, ranging from simple filaments to leafy structures. As seaweeds, red algae often resemble the brown algae seaweeds, although red algae can be more delicate (Fig. 29.5). In addition to chlorophyll, the red algae contain red and blue pigments that give them characteristic colors. Coralline algae have calcium carbonate in their cell walls and contribute to the formation of coral reefs. Red algae produce a number of useful gelling agents. Agar is commonly used in microbiology laboratories to solidify culture media and commercially to encapsulate vitamins or drugs. Agar is also a gelatin substitute for vegetarians, an antidrying agent in baked goods, and an additive in cosmetics, jellies, and desserts.

cell wall chloroplast vacuole nucleus zygote cytoplasm pyrenoid

17× a. Cell anatomy daughter colony

vegetative cells

b. Conjugation

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Figure 29.4  Spirogyra.  a. Spirogyra is a filamentous green alga in which each cell has a ribbonlike chloroplast. b. During conjugation, the cell Volvox colony often contains daughter colonies, which are asexually contents of one filament enter the cells of another filament. Zygote formation produced by special cells. follows.

Figure 29.3  Volvox.  Volvox is a colonial chlorophyte. The adult

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Figure 29.5  Red algae.  Red algae contain accessory pigments in addition to chlorophyll. Represented here by Chondrus crispus, they are smaller and more delicate than brown algae. Carrageenan is a related product used in cosmetics and chocolate manufacture. The species Porphyra, a red seaweed, is popular as a sushi wrap in Japan.

The Chromalveolates The chromalveolates represent a very large, diverse group of protistans. Members of this group are typically photosynthetic, but have a different evolutionary lineage from the green and red algae. Brown algae are the conspicuous multicellular seaweeds that dominate rocky shores along cold and temperate coasts. The color of brown algae is due to accessory pigments that actually range from pale beige to yellow-brown to almost black. These pigments allow the brown algae to extend their range down into deeper waters because the pigments are more efficient than green chlorophyll in absorbing the sunlight away from the ocean surface. The alga produces a slimy matrix that retains water when the tide is out and the seaweed is exposed. This gelatinous material, algin, is used in ice cream, cream cheese, and some cosmetics. The Sargasso Sea in the North Atlantic Ocean commonly has large floating mats of brown algae called sargassum. The most familiar brown algae are kelp (genus Laminaria) in coastal regions and giant marine kelp (genus Macrocystis) in deeper waters (Fig. 29.6). Tissue differentiation in kelp results in blades, stalks, and holdfasts, which are analogous to the leaves, stems, and roots of plants. Diatoms are tiny, single-celled chromalveolates with an ornate silica shell. The shell is made up of upper and lower shelves, called valves, that fit together. Diatoms have a photosynthetic accessory pigment that gives them an orange-yellow color. Diatoms make up a significant part of plankton, which serves as a source of oxygen and food for heterotrophs in both freshwater and marine ecosystems.  Diatoms reproduce asexually and sexually. Asexual reproduction occurs by diploid parents undergoing mitosis to produce two diploid daughter cells. Each time a diatom reproduces asexually, the size of the daughter cells decreases until diatoms are about 30% of their original size. At this point, they begin to reproduce sexually. The diploid cell produces gametes by meiosis. Gametes fuse

Figure 29.6  Brown algae.  Brown algae have accessory pigments that allow them to obtain their light energy while living in deeper waters compared to other algae. This is a type of brown algae known as bull kelp, Nereocystis luetkeana. The photo on the upper right shows flotation bladders that brown algae form to keep their blades close to the surface. to produce a diploid zygote, which grows and then divides via mitosis to produce new diploid diatoms of normal size.  Diatoms are easy to recognize under the microscope because they have a wide variety of elaborate shells made of silica (Fig.  29.7). The two overlapping shells have intricately shaped depressions, pores, and passageways that bring the diatom’s plasma membrane in contact with the environment. The fossilized remains of diatoms, also called diatomaceous earth, accumulate on the ocean floor and can be mined for use as filtering agents and abrasives. Most water molds are saprotrophs, meaning that they feed on dead organic matter. They usually live in water, where they decompose remains and form furry growths when they parasitize fish (Fig. 29.8). In spite of their common name, some water molds live on land and parasitize insects and plants. The water mold Phytophthora was responsible for the 1840s potato famine in Ireland. Water molds, also called oomycetes (“egg fungi”), used to be grouped with fungi, because they are similar to fungi in many

Figure 29.7  Assorted fossilized diatoms.  Diatoms have overlapping shells made of silica. Scientists use their delicate markings to identify the particular species.



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filaments of water mold

dead goldfish cellulose plate flagella

Figure 29.8  Water mold.  Saprolegnia, a water mold, feeding on a dead goldfish. Water molds are not fungi. ways. The funguslike water molds have a filamentous body similar to the hyphae of fungi, but their cell walls are composed of cellulose instead of chitin. The life cycle of water molds also differs from that of the fungi. Water molds have a filamentous body, as do fungi, but the cell walls of water molds are largely composed of cellulose, whereas fungi have cell walls made of chitin. During asexual reproduction, water molds produce flagellated spores. During sexual reproduction, they produce eggs and sperm.  Dinoflagellates are best known for the red tide they cause when they greatly increase in number, an event called an algal bloom. Gonyaulax, one species implicated in red tides, produces a very potent toxin (Fig. 29.9). This can be harmful by itself, but it also accumulates in shellfish. The shellfish are not damaged, but people can become quite ill when they eat them. Despite some harmful effects, dinoflagellates are important members of the phytoplankton in marine and freshwater ecosystems. Generally, dinoflagellates are photosynthetic, although colorless heterotrophic forms are known to live as symbionts inside other organisms. Corals, which build coral reefs, contain large numbers of symbiotic dinoflagellates. Many dinoflagellates have protective cellulose plates that become encrusted with silica, turning them into hard shells. Dinoflagellates have two flagella. One is located in a groove that encircles the protist, and the other is in a longitudinal groove and has a free end. The arrangement of the flagella makes a dinoflagellate whirl as it moves. In fact, the name dinoflagellate is derived from the Greek dino-, for whirling. It is interesting to note that luminous dinoflagellates produce a twinkling light that can give seas a phosphorescent glow at night, particularly in the tropics. Ciliates are the largest group of animal-like protists, also called protozoans. All of them have cilia, hairlike structures that rhythmically beat, moving the cell forward or in reverse. Typically, the cell rotates as it moves. Cilia also help capture prey and particles and then move them toward the mouthparts. After phagocytic vacuoles engulf food, they combine with lysosomes, which supply the enzymes needed for digestion. A contractile vacuole helps to maintain water balance with the surrounding environment. Some ciliates are up to 3 mm long, which is large enough to be seen with the naked eye. Most are freely motile, but some can be anchored to a surface using a stalk and collecting food with cilia. Paramecium is the most widely known ciliate, and it is commonly

Figure 29.9  Dinoflagellates.  Dinoflagellates have cellulose plates. These belong to Gonyaulax, a dinoflagellate that contains a reddish-brown pigment and is responsible for occasional “red tides.”

used for research and teaching. It is shaped like a slipper and has visible contractile vacuoles (Fig. 29.10a). Members of the genus Paramecium have a large macronucleus and a small micronucleus. The macronucleus produces mRNA and directs metabolic functions. The micronucleus is important during sexual reproduction. Paramecium has been important for studying ciliate sexual reproduction, which involves conjugation, with interactions between micronuclei and macronuclei (Fig. 29.10b). The apicomplexa are commonly called sporozoans because they produce spores. All phases of the life cycle are generally nonmotile, with the exception of male gametes and zygotes. All sporozoans are either intercellular or extracellular parasites. An apical complex composed of fibrils, microtubules, and organelles is found at one end of the single cell. Enzymes for attacking host cells are secreted through a pore at the end of the apical complex. Specific diseases of humans caused by sporozoans are discussed at the end of this section.

The Excavates The excavates have atypical or absent mitochondria and distinctive flagella and/or deep (excavated) oral grooves. They are sometimes referred to as flagellates due to the fact that they propel themselves using one or more flagella. Euglenids are freshwater single-celled organisms that typify the problem of classifying protists. Many euglenids have chloroplasts, but some do not. Those that lack chloroplasts ingest or absorb their food. Those that have chloroplasts are believed to have originally acquired them by ingestion and subsequent endosymbiosis of a green algal cell. Three, rather than two, membranes surround these chloroplasts. The outermost membrane is believed to



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pellicle

anal pore

micronucleus macronucleus

trichocyst long flagellum short flagellum

eyespot photoreceptor carbohydrate granule

contractile vacuole

contractile vacuole (full)

gullet

oral groove

food vacuole

cilia

contractile vacuole (partially full)

nucleolus nucleus pellicle pyrenoid chloroplast

a. Paramecium

nuclei

b. During conjugation two paramecia first unite at oral areas

100×

Figure 29.10  Ciliates.  Ciliates are the most complex of the protists.

a. Structure of Paramecium, adjacent to an electron micrograph. Note the oral groove, the gullet, and the anal pore. b. A form of sexual reproduction called conjugation occurs periodically. 

represent the plasma membrane of an original host cell that engulfed a green alga. Euglenids have two flagella, one of which is typically much longer than the other and projects out of the anterior, vase-shaped invagination (Fig. 29.11). Near the base of this flagellum is an eyespot apparatus, which is a photoreceptive organelle for detecting light. Because euglenids are bounded by a flexible pellicle composed of protein strips lying side by side, they can assume different shapes as the underlying cytoplasm undulates and contracts. Parabasalids and diplomonads are single-celled, flagellated excavates that are endosymbionts of animals. They are able to survive in anaerobic, or low-oxygen, environments. These protozoans lack mitochondria; instead, they rely on fermentation for the production of ATP. A variety of forms are found in the guts of termites, where they assist with the breakdown of cellulose.  Parabasalids have a unique, fibrous connection between the Golgi apparatus and flagella. The most common sexually transmitted ­disease, trichomoniasis, is caused by the parabasalid Trichomonas

960×

Figure 29.11  Euglena.  Euglenoids have a flexible pellicle and a long

flagellum that propels the body.

vaginalis. Infection causes vaginitis in women. The parasite may also infect the male genital tract; however, the male may have no symptoms.  A diplomonad cell has two nuclei and two sets of flagella. The diplomonad Giardia lamblia forms cysts, which are transmitted by contaminated water. Giardia attaches to the human intestinal wall, causing severe diarrhea (Fig. 29.12). This protozoan lives in the digestive tracts of a variety of other mammals as well. Beavers are known to be a reservoir of Giardia infection in the mountains of the western United States, and many cases of infection have been acquired by hikers who filled their canteens at a beaver pond. 

The Amoebozoa Amoeboids move by pseudopods, processes that form when cytoplasm streams forward in a particular direction (Fig. 29.13). They usually live in aquatic environments, such as oceans and freshwater lakes and ponds, where they are a part of the zooplankton. When amoeboids feed, their pseudopods surround and engulf their prey, which may be algae, bacteria, or other protists. Digestion then occurs within a food vacuole.



Chapter 29  Protists and Fungi circular marking

Giardia

591

surface

Plasmodium, Physarum

Sporangia, Hemitrichia

Figure 29.12  Giardia lamblia.  This protist adheres to any surface, including epithelial cells, by means of a sucking disk. Characteristic markings can be seen on the host cell after the disk detaches.

mature plasmodium

plasma membrane contractile vacuole

young plasmodium

sporangia formation begins

food vacuole nucleus

nucleolus cytoplasm

mitochondrion

young sporangium

zygote

FERTILIZATION

diploid (2n)

MEIOSIS

haploid (n)

pseudopod

mature sporangium

Figure 29.13  Amoeba.  This amoeboid is common in freshwater

ponds. Bacteria and other microorganisms are digested in food vacuoles, and contractile vacuoles rid the body of excess water. 

In forests and woodlands, slime molds feed on dead plant material. They also feed on bacteria, keeping their population under control. The vegetative cells of slime molds are mobile and amoeboid. They ingest their food by phagocytosis. Plasmodial slime molds exist as a plasmodium, a diploid, multinucleated, cytoplasmic mass enveloped by a slime sheath, that creeps along, phagocytizing decaying plant material in a forest or agricultural field (Fig. 29.14). At times unfavorable to growth, such as during a drought, the plasmodium develops many sporangia, reproductive structures that produce spores. The spores produced by a sporangium can survive until moisture is sufficient for them to germinate. In plasmodial slime molds, spores release a haploid flagellated cell or an amoeboid cell. Eventually, two of these fuse to form a zygote that feeds and grows, producing a multinucleated plasmodium once again.

spores fusion amoeboid cells

germinating spore

flagellated cells

Figure 29.14  Plasmodial slime molds.  The diploid adult forms sporangia during sexual reproduction, when conditions are unfavorable to growth. Haploid spores germinate, releasing haploid amoeboid or flagellated cells that fuse.



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UNIT 6  Evolution and Diversity

Cellular slime molds can exist as individual amoeboid cells. They are common in soil, where they feed on bacteria and yeasts. Dictyostelium discoideum is the commonly researched example. As food supplies dwindle, the cells release a chemical that causes them to aggregate into a sluglike pseudoplasmodium, which eventually gives rise to a fruiting body that produces spores. When favorable conditions return, the spores germinate, releasing haploid amoeboid cells, and this cycle, which is asexual, begins again. A sexual cycle is known to occur under very moist conditions. Genetic analyses of several slime mold species has revealed that these may have been some of the earliest eukaryotes to adapt to life on land, perhaps a billion years ago. Recent research has also shown that slime mold behavior can be surprisingly complex. Japanese researchers showed that when placed into a “maze” constructed on an agar plate, slime molds could find the shortest path to food. When presented with different types of food, the slime molds chose the most nutritious type.

The Opisthokonta Animals and fungi are opisthokonts, along with several closely related protists. This supergroup includes both single-celled and multicellular protozoans. Among the opisthokonts are the choanoflagellates, animal-like protozoans that are closely related to sponges. The choanoflagellates, including single-celled as well as colonial forms, are filter-feeders with cells that bear a striking resemblance to the choanocytes that line the inside of sponges (see section 31.2). Each choanoflagellate has a single posterior flagellum surrounded by a collar of slender microvilli (Fig 29.15). Beating of the flagellum creates a water current that flows through the collar, where food particles are taken in by phagocytosis.

The Rhizaria The rhizarians  consist of the foraminiferans and the radiolarians, organisms with fine, threadlike pseudopods. Although rhizarians were once classified with amoebozoans, they are now assigned to a different supergroup. Foraminiferans and radiolarians both have a skeleton, called a test, made of calcium carbonate. Because each geological time period has a distinctive form of foraminiferan, they can be used to date sedimentary rock. Depositions over millions of years followed by geological upheaval formed the White Cliffs of Dover along the southern coast of England (Fig. 29.16a). Also, the great Egyptian pyramids are built of foraminiferous limestone. In radiolarians, the test is internal (Fig. 29.16b). The tests of dead foraminiferans and radiolarians form a deep layer of sediment on the ocean floor. Their presence is used as an indicator of oil deposits on land or sea.

Figure 29.15  Choanoflagellates.  The choanoflagellates are most closely related to the animals.

160×

a. Foraminiferan, Globigerina, and the White Cliffs of Dover, England

b. Radiolarian tests

SEM 150×

Figure 29.16  Foraminiferans and radiolarians.  a. Pseudopods

of a live foraminiferan project through holes in the calcium carbonate shell. Fossilized shells were so numerous they became a large part of the White Cliffs of Dover when a geologic upheaval occurred. b. Skeletal tests of radiolarians.

Protistans and Human Disease  Compared to the number of diseases caused by bacteria and viruses, a relatively few protists cause human diseases, and all of these are caused by heterotrophic protozoans. However, some cause significant sickness and mortality.

Malaria The most widespread and dangerous protozoan disease is malaria. According to the World Health Organization (WHO), about half of the world’s population lives in areas where malaria is endemic. In 2013, an estimated 198 million people were infected with malaria worldwide, and 584,000 of these people died. As significant as those numbers are, they are actually decreasing, at least in part due to the concerted efforts of many governments and private organizations. Malaria can be caused by several sporozoan parasites in the genus Plasmodium. All of these have a complex life cycle that involves transmission by a mosquito vector (Fig. 29.17). The sexual reproduction phase in the mosquito ends with the migration of a developmental stage called sporozoites into the salivary glands. These sporozoites are then transmitted by the mosquito’s bite to a human, where they reproduce asexually in the liver and red blood



Chapter 29  Protists and Fungi

cells, forming merozoites. The infected red blood cells frequently rupture, releasing merozoites and toxins, and causing the person to experience the chills and fever that are characteristic of malaria. Some of these merozoites become gametocytes, which, if ingested by another mosquito, can start the sexual reproduction phase again. People who survive malaria have only a limited immunity to reinfection. Interestingly, the sickle cell trait found in up to one-third of sub-Saharan Africans provides some protection against malaria. Efforts to control mosquito populations have been successful in the United States, where malaria is rarely seen. The control of mosquitoes worldwide has now been hampered by the rise of resistance to pesticides like DDT. Scientists are working to find new ways to inhibit mosquitoes, such as genetically engineering them to be resistant to Plasmodium infection. A Seattle company is even designing a laser system that could protect villages by zapping mosquitoes in flight! An intense research effort is under way to develop a malaria vaccine, funded largely by the Bill and Melinda Gates Foundation,

female gamete

Sexual phase in mosquito

593

which has contributed about $1.2 billion to malaria research since 2000. A 2011 study published in the New England Journal of Medicine reported the results of a malaria vaccine trial conducted in seven countries and involving 15,460 children. Overall, the vaccine reduced the incidence of malaria in children by approximately 50% during the 12 months after vaccination.

Toxoplasmosis The sporozoan called Toxoplasma gondii is commonly transmitted by cat feces. The disease toxoplasmosis generally causes no appreciable symptoms, but the parasite can be harmful to a developing fetus. Thus, pregnant women are advised not to empty cat litter boxes and to avoid working in gardens or other locations where cats may defecate. A related protozoal disease is caused by Cryptosporidium parvum. The organism and its cysts are common in surface waters and in the feces of animals and birds, and can pass through sand filters in water treatment plants, while being unaffected by chlorine treatment. Cryptosporidiosis usually causes a self-limiting gastroenteritis, but in some cases can lead to a fatal watery diarrhea.

African Sleeping Sickness male gamete food canal

Trypanosoma brucei, the cause of African sleeping sickness, is transmitted by the tsetse fly (see the Health feature, “African Sleeping Sickness”). It attacks the patient’s blood, causing inflammation that decreases oxygen flow to the brain. Chagas disease, caused by T. cruzi, is transmitted by the kissing bug, so called because it tends to bite lips. 

Figure 29.17  Life cycle of Plasmodium vivax, a species that

zygote

causes malaria.  Asexual reproduction of this sporozoan occurs in humans, while sexual reproduction takes place within the Anopheles mosquito.

sporozoite 1. In the gut of a female Anopheles mosquito, gametes fuse, and the zygote undergoes many divisions to produce sporozoites, which migrate to her salivary gland.

salivary gland

2. When the mosquito bites a human, the sporozoites pass from the mosquito salivary glands into the bloodstream and then the liver of the host.

3. Asexual spores (merozoites) produced in liver cells enter the bloodstream and then the red blood cells.

6. Some merozoites become gametocytes, which enter the bloodstream. If taken up by a mosquito, they become gametes.

liver cell

gametocytes

Asexual phase in humans 4. When the red blood cells rupture, merozoites invade and reproduce asexually inside new red blood cells.

5. Merozoites and toxins pour into the bloodstream when the red blood cells rupture, causing chills and fever.



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UNIT 6  Evolution and Diversity

SCIENCE IN YOUR LIFE  ►

HEALTH

African Sleeping Sickness The World Health Organization (WHO) recognizes several neglected tropical diseases (NTDs) that affect more than 1 billion people exclusively in the most impoverished communities. Several of these NTDs, including African sleeping sickness, are caused by parasitic protists that are transmitted to humans from the bite of an infected insect. African sleeping sickness, also called African human trypanosomiasis, is the deadliest of all of the NTDs. It is caused by a type of parasitic, single-celled protist, Trypanosoma brucei, which is transmitted to humans in the bite of the blood sucking tsetse fly (Glossina). The tsetse fly is the transmission vector of the disease because it carries the trypanosomes and transmits the long, bladeshaped protists (Fig. 29A) into the human bloodstream while feeding. The flies first become infected with the parasite when they bite either an infected human or other mammal, such as domestic cattle and pigs, which carry T. brucei. Figure 29B illustrates the relationship between humans, the tsetse fly, and domestic livestock. Unlike humans, livestock can carry T. brucei without getting sick. Therefore, they act as a reservoir for the disease. Humans also can serve as a reservoir for new tsetse fly infections because the parasite can be picked up from humans by a fly bite and transmitted to others. It is estimated that as many as 50,000 people are plagued by this disease. In the later stages of infection, T. brucei attacks the brain, causing behavioral changes and a shift in sleeping patterns. If caught early enough, it can be cured with medication, but because it is most common in very poor nations, medication and treatment are difficult to come by. Without treatment, the disease is fatal. The tsetse fly prefers shaded, cool, moist habitats, such as the sandy edges of pools and rivers in central Africa. Regions that are plagued by sleeping sickness have no running water, so daily trips to local rivers and watering holes are necessary to collect water. The people who live and work in tsetse habitat are the most vulnerable to the disease. As a result, many communities have abandoned the areas infested with the tsetse fly. The abandonment of these fertile valleys has had a huge impact on the ecology of surrounding areas and the economy of communities and nations on a large scale.

Figure 29A 

Trypanosoma brucei. 

A micrograph showing T. brucei, a causal agent of African sleeping sickness, among red blood cells.

red blood cell

trypanosome

Recently, an effort to eliminate the tsetse fly from these lands has been successful. The WHO reports that the number of new cases of sleeping sickness has dropped to the lowest levels in 50 years. The treatment and control of sleeping sickness has allowed the resettlement of more than 25 million hectares of prime agricultural land.

Questions to Consider 1. How does poverty reinforce a high occurrence of African sleeping sickness? 2. How are the economy, ecology, and disease biology of African sleeping sickness interdependent?

African sleeping sickness

Unaffected

No transmission

Reservoir for fly infection

Vector for mammal infection

Reservoir for fly infection

Tsetse fly

Figure 29B  The transmission pattern of Trypanosoma brucei.  Tsetse flies can become infected with T. brucei by feeding on infected humans or livestock. The flies can then transmit the parasitic protist to other humans, causing African sleeping sickness.



Chapter 29  Protists and Fungi

Amoeboid-Related Diseases Parasitic amoeboids in the genus Entamoeba cause amoebic ­dysentery. Complications arise when the parasite invades the intestinal lining and reproduces there. If the parasite enters the body proper, liver and brain involvement can be fatal. Another example is an amoeboid in the genus Acanthamoeba which can cause corneal inflammation as well as serious infections. Even more foreboding, the amoeboid protist Naegleria fowleri can invade and attack the human nervous system, nearly always resulting in the death of the victim. It is usually acquired by swimming in warm bodies of fresh water, such as ponds, lakes, rivers, and unchlorinated swimming pools. Fortunately, this ­scenario is extremely rare.

common ancestor

595

Basidiomycota (club fungi) Ascomycota (sac fungi)

common ancestor

Glomeromycota (AM fungi) Zygomycota (zygospore fungi)** Chytridiomycota (zoospore fungi) Microsporidia (obligate parasitic fungi)*

Check Your Progress  29.1 1. Describe the endosymbiotic theory. 2. Describe the circumstances by which some protists

produce spores or cysts, and explain the benefit this provides to the organism. 3. Compare the type of nutrition that is typically required by an alga, a protozoan, a water mold, and a cellular slime mold. 4. Identify the type of protist that causes each of the following diseases: malaria, African sleeping sickness, and amoebic dysentery.

29.2  Fungi Learning Outcomes Upon completion of this section, you should be able to 1. Explain how most fungi obtain their nutrients. 2. Describe the structural components of a fungal body. 3. Compare the reproductive strategies of the major groups of fungi. 4. Identify several fungal diseases of humans, and explain why many can be difficult to treat.

Fungi (domain Eukarya, kingdom Fungi) are a structurally diverse group of eukaryotes that are strict heterotrophs. Unlike animals, fungi (sing., fungus) release digestive enzymes into their external environment and digest their food outside the body, while animals ingest their food and digest it internally. A few fungi are parasitic, but most are saprotrophs that decompose dead plants, animals, and microbes. Along with bacteria, fungi play an important role in ecosystems by breaking down complex organic molecules and returning inorganic nutrients to producers of food—that is, photosynthesizers. Fungi can degrade even cellulose and lignin in the woody parts of trees. It is common to see fungi (brown rot or white rot) on the trunks of fallen trees. The body of a fungus can become large enough to cover acres of land.

Evolution of Fungi Figure 29.18 illustrates the evolutionary relationships among the fungi. The evolutionary tree is a hypothesis about how these groups

* Recently placed in the kingdom Fungi ** Molecular data suggests the Zygomycota may have multiple

evolutionary origins.

Figure 29.18  Evolutionary relationships of the fungi.  While

DNA evidence is still unraveling the relationships among the fungi, this diagram presents the most accepted hypothesis on the evolutionary history of the fungi.

are related. The Microsporidia and chytrids are different from all other fungi, because they are single-celled. The chytrids are aquatic and have flagellated spores and gametes. Our description of fungal structure applies best to the zygospore fungi, sac fungi, and club fungi. The AM fungi are interesting because they exist only as mycorrhizae in symbiotic association with plant roots.  Protists evolved some 1.5 bya (billion years ago). Plants, animals, and fungi can all trace their ancestry to protists, but molecular data tell us that animals and fungi shared a more recent common ancestor than animals and plants. Therefore, animals and fungi, both in the supergroup Opisthokonta, are more closely related to each other than either is to plants (see Figure 29.1). The common ancestor of animals and fungi was most likely an aquatic, flagellated, single-celled protist. Multicellular forms evolved sometime after animals and fungi split into two different lineages. Fungi do not fossilize well, so it is difficult to estimate from the fossil record when they first evolved. The earliest known fossil fungi are dated at 460 mya (million years ago), but fungi probably evolved a lot earlier. Mycorrhizae are evident in plant fossils, also some 460 mya. Perhaps fungi were instrumental in the colonization of land by plants. Much of the fungal diversity most likely had its origin in an adaptive radiation when organisms began to colonize land. 

Biology of Fungi The body of a fungus is composed of a mass of individual filaments called hyphae (sing., hypha). Collectively, the mass of filaments is called a mycelium (pl., mycelia) (Fig. 29.19). Some fungi have cross walls that divide a hypha into a chain of cells. These hyphae are termed septate. Septa have pores that allow cytoplasm and even organelles to pass from one cell to the other along the length of the hypha. Nonseptate fungi have no cross walls, and their hyphae are multinucleated.



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UNIT 6  Evolution and Diversity

septate

aseptate

a. Fungal mycelia

b. Individual strands of hyphae

170×

c. Septate and aseptate hyphae

Figure 29.19  Fungal mycelia and hyphae.  a. Fungal mycelium made from hyphae, growing as a white mass on strawberries. b. Hyphal filaments growing on the surface of a plant. c. Hyphae are either septate (do have cross-walls) or aseptate (do not have cross-walls.) 

Hyphae give the mycelium quite a large surface area per volume of cytoplasm, which facilitates absorption of nutrients into the body of a fungus. Hyphae grow at their tips, and the mycelium absorbs and then passes nutrients on to the growing tips. Individual hyphae can grow quite rapidly, as much as 18 ft per day. Altogether, a single mycelium may add as much as a kilometer of new hyphae in a single day! Fungal cells are quite different from plant cells, not only because they lack chloroplasts, but also because their cell walls contain chitin, not cellulose. Chitin, like cellulose, is a polymer of glucose, but in chitin, each glucose molecule has a nitrogen-­ containing amino group attached to it. Chitin is also the major structural component of the exoskeleton of arthropods, such as insects, lobsters, and crabs. Unlike plants, the energy reserve of fungi is not starch, but glycogen, as in animals. Fungi are nonmotile and most do not have flagella at any stage in their life cycle. They move toward a food source by growing toward it. Although a few fungal species are aquatic, most have adapted to life on land by producing windblown spores during both asexual and sexual reproduction (Fig. 29.20). A spore is a haploid reproductive cell that develops into a new organism without the need to fuse with another reproductive cell. In fungi, spores germinate into new mycelia. Sexual reproduction in fungi involves conjugation of hyphae from two different mating types (usually designated + and –). Often, the haploid nuclei from the two hyphae do not immediately fuse to form a zygote. The hyphae contain + and – nuclei for long periods of time. Eventually, the nuclei fuse to form a zygote that undergoes meiosis, followed by spore formation.

Diversity of Fungi Fungi are traditionally classified based on their mode of sexual reproduction. Figure 29.18 is an evolutionary tree showing how the five major groups of fungi that will be discussed in this section are believed to be related. Major fungal groups (phyla) include the microsporidians, chytrids, zygospore fungi, sac fungi, club fungi, and AM fungi.

Microsporidians The single-celled Microsporidia are parasites of animal cells, most often seen in insects but also found in vertebrates, such as

b.

3,000×

a.

Figure 29.20  Fungi reproduce by spore formation.  a. Here a pear-shaped puffball, Lycoperdon pyriforme, is releasing perhaps billions of spores. b. The spores of a puffball fungus as viewed under an electron microscope.

fish, rabbits, and humans. Biologists once believed that Microsporida were an ancient line of protist due in part to their lack of mitochondria. However, genome sequencing of the microsporidian Encephalitozoon cuniculi revealed genes that are mitochondrial, leading to the hypothesis that this organism once had a mitochondrion, which later became greatly reduced. In addition,



Chapter 29  Protists and Fungi

microsporidians have the smallest known eukaryotic genome. New sequence information places E. cuniculi and other microsporida in fungi, rather than with the protists. 

597

algal cell wall

Chytrid Fungi The chytrid fungi (phylum Chytridomycota) may have been the first type of fungi to evolve. They are unique among the fungi because they are aquatic, and they produce flagellated reproductive cells. Most chytrids reproduce asexually through the production of zoospores, which grow into new chytrids. Many chytrids play a role in the decay and digestion of dead aquatic organisms, but some are parasitic on plants, animals, and protists (Fig. 29.21).

Zygospore Fungi The zygospore fungi (phylum Zygomycota) are mainly saprotrophs, but some are parasites of small soil protists or worms and even insects, such as the housefly. Rhizopus stolonifer (Fig. 29.22) is well known to many of us as the mold that appears on old bread, even when it has been refrigerated. In Rhizopus, the hyphae are specialized: some are horizontal and exist on the surface of the bread; others grow into the bread to anchor the mycelium and carry out digestion; and still others are stalks that bear sporangia. A sporangium is a capsule that produces spores. When Rhizopus reproduces sexually, the ends of + and – hyphae join, haploid nuclei fuse, and a thick-walled zygospore results. The zygospore undergoes a period of dormancy before meiosis and germination take place. Following germination, aerial hyphae, with sporangia at their tips, produce many spores. The spores are dispersed by air currents and give rise to new mycelia.

hyphae

chytrid

Figure 29.21  Chytrids parasitizing a protist.  These aquatic chytrids (Chytriomyces hyalinus) have penetrated the cell walls of this algal protist and are absorbing nutrients meant for their host. They will produce flagellated zoospores that will go on to parasitize other protists.

Sac Fungi Approximately 75% of all known fungi are sac fungi (phylum Ascomycota), named for their characteristic cuplike sexual reproductive structure called an ascocarp. Many sac fungi reproduce by producing chains of asexual spores called conidia (sing., conidium). Cup fungi, morels, and truffles have conspicuous ascocarps (Fig. 29.23). Truffles, underground symbionts of hazelnut and oak trees, are highly prized as gourmet delights. Pigs and dogs are trained to sniff out truffles in the woods, but they are also cultivated on the roots of seedlings.

zygospore

Figure 29.22  Black bread mold, Rhizopus stolonifer.  The mycelium of this mold utilizes sporangia to produce windblown spores. A zygospore forms during the sexual life cycle.

fertilization

Sexual

meiosis

sporangium

Asexual – strain

+ strain mycelium



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UNIT 6  Evolution and Diversity ascospores ascocarp

mature ascus a. Ascocarp

bud scar budding yeast cell a.

b.

425×

Figure 29.24  Asexual reproduction in sac fungi.  a. Yeasts, unique among fungi, reproduce by budding. b. The sac fungi usually reproduce asexually by producing spores called conidia.

ascocarp

b. Cup fungi

3,000×

c. Morels

Figure 29.23  Sexual reproduction in sac fungi.  Some sac

fungi are known to us by their ascocarp, the structure that produces spores. a. Diagram of an ascocarp containing asci, where spores are produced. b. The ascocarps of cup fungi. c. The ascocarp of a morel, which is a prized delicacy.

Morels (Fig. 29.23c) are often collected by chefs and other devotees because of their unique and delicate flavor. Collectors must learn, however, to avoid “false morels,” which can be poisonous. Interestingly, even true morels should not be eaten raw. As saprophytes, these organisms secrete digestive enzymes capable of digesting wood or leaves. If consumed prior to being inactivated by cooking, these enzymes can cause digestive problems. Most fungal plant pathogens are sac fungi. Examples of these pathogenic fungi include powdery mildews that grow on plant leaves, as well as chestnut blight and Dutch elm disease that destroy trees. Ergot, a parasitic sac fungus that infects rye, produces hallucinogenic compounds similar to LSD. Psychoses resulting from ingesting grain contaminated with rye ergot may have led to the Salem, Massachusetts, witch trials in 1692. Some of the most familiar fungi were formerly considered “imperfect fungi,” because their means of sexual reproduction is unknown. However, based on DNA sequencing and other recent information, these fungi are now classified as sac fungi. Examples include Penicillium, the original source of penicillin, a breakthrough antibiotic that led to the important class of “cillin” antibiotics that have saved millions of lives. Other species of Penicillium—P. roquefortii and P. camemberti—are necessary to the production of blue cheeses. Aspergillus is used widely for its ability to produce citric acid and various enzymes. Certain species also cause serious infections in people with compromised immune systems.

Yeasts  The term yeasts is generally applied to single-celled fungi, and many of these organisms are sac fungi. Saccharomyces cerevisiae, brewer’s yeast, is representative of budding yeasts. When unequal binary fission occurs, a small cell gets pinched off and then grows to full size. Asexual reproduction occurs when the food supply runs out, producing spores (Fig. 29.24).

When some yeasts ferment, they produce ethanol and carbon dioxide. In the wild, yeasts grow on fruits, and historically, the yeasts already present on grapes were used to produce wine. Today, selected yeasts are added to relatively sterile grape juice in order to make wine. Also, yeasts are added to grains to make beer and liquor. Both the ethanol and carbon dioxide are retained in beers and sparkling wines, while carbon dioxide is released from wines. In breadmaking, the carbon dioxide produced by yeasts causes the dough to rise, and the ethanol quickly evaporates. The gas pockets are preserved as the bread bakes.

Club Fungi Club fungi (phylum Basidiomycota) are named for their characteristic sexual reproductive structure called a basidium. Basidia are enclosed within a basidiocarp, which develops after + and – hyphae join. The union of + and – nuclei occurs in the basidium, which produces spores by meiosis. We recognize most club fungi by their basidiocarp (Fig. 29.25).  When you eat a mushroom, you are consuming a basidiocarp. Certain mushrooms are poisonous, especially those of the genus Amanitas. A. phalloides, also known as the death cap, causes 90% of fatalities related to mushroom poisoning, due to its production of a toxin that interferes with gene transcription by inhibiting RNA polymerase. Other club fungi, mainly of the genus Psilocybe, produce a hallucinogenic chemical called psilocybin that is a structural analog of LSD. These mushrooms have been used in religious ceremonies since ancient times, though their recreational use is currently illegal in the United States. See the Health feature, “Deadly Fungi,” for a further discussion of fungi that produce toxins that are dangerous to humans. Shelf or bracket fungi found on dead trees are also basidiocarps. Less well known are puffballs and stinkhorns. In puffballs, spores are produced inside parchmentlike membranes, and the spores are released through a pore or when the membrane breaks down. Stinkhorns resemble a mushroom, but they emit a very disagreeable odor. Flies are attracted by the odor. When they linger to feed on the sweet jelly, they pick up spores and later distribute them. Smuts and rusts are club fungi that parasitize cereal crops, such as corn, wheat, oats, and rye. They are of great economic



Chapter 29  Protists and Fungi

nuclei in basidium

spores fusion

meiosis

599

Figure 29.25  Sexual reproduction in club fungi.  Most club fungi are known to us by their basidiocarp, the structure that produces spores as a part of sexual reproduction. a. Sexual life cycle of a basidiocarp and a basidium (pl., basidia) where spores are produced. Also shown are basidiocarps of (b) scarlet hood mushroom, (c) chicken-of-the-woods shelf fungi, and (d) a giant puffball. Puffballs contain many spores, and their basidiocarps tend to be rounded. When they are mature, any pressure from outside, such as a raindrop or the kick of a shoe, ejects the spores through a hole as a cloud of dust.

gill of mushroom basidiocarp

−

+

a. Sexual reproduction

b. Mushroom

importance because of the crops they destroy each year. The corn smut mycelium grows between corn kernels and secretes substances that cause tumorlike swellings to develop (Fig. 29.26a). The life cycle of rusts often requires two different plant host species to complete the cycle, and one way to keep them in check is to eradicate the alternate host. Wheat rust (Fig. 29.26b) can also be controlled by producing new and resistant strains of wheat.

Symbiotic Relationships of Fungi Several instances in which fungi are parasites of plants have already been mentioned. Fungi also commonly form symbiotic

c. Shelf fungi

d. Giant puffball

relationships with other organisms, in which both partners benefit.

Lichens Lichens are associations between fungi and cyanobacteria or green algae. The different lichen species are identified according to the fungal partner. Lichens are efficient at acquiring nutrients and moisture, and therefore, they can survive in poor soils, as well as on rocks with no soil. These primary colonizers produce organic matter and create new soil, allowing plants to invade the area. Lichens exhibit three structures: compact crustose lichens, often

leaf fungus

a. Corn smut, Ustilago

b. Wheat rust, Puccinia

40×

Figure 29.26  Smuts and rusts.  a. Corn smut. b. Wheat rust. Both are important fungal diseases affecting agricultural crops in the United States and

other countries.



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UNIT 6  Evolution and Diversity

SCIENCE IN YOUR LIFE  ►

HEALTH

Deadly Fungi It is unwise for amateurs to collect mushrooms in the wild because certain mushroom species are poisonous. The red and yellow Amanita mushrooms are especially dangerous. These species are also known as fly agaric because they were once thought to kill flies (the mushrooms were gathered and then sprinkled with sugar to attract flies). Its toxins include muscarine and muscaridine, which produce symptoms similar to those of acute alcoholic intoxication. In one to six hours, the victim staggers, loses consciousness, and becomes delirious, sometimes suffering from hallucinations, manic conditions, and stupor. Luckily, it also causes vomiting, which rids the system of the poison, so death occurs in less than 1% of cases. The death angel mushroom (Amanita phalloides, Fig. 29C) causes 90% of the fatalities attributed to mushroom poisoning. When this mushroom is eaten, symptoms don’t begin until 10 to 12 hours later. Abdominal pain, vomiting, delirium, and hallucinations are not the real problem; rather, a poison interferes with RNA (ribonucleic acid) transcription by inhibiting RNA polymerase, and the victim dies from liver and kidney damage. Some hallucinogenic mushrooms are used in religious ceremonies, particularly among

Mexican Indians. Psilocybe mexicana contains a chemical called psilocybin that is a structural analogue of LSD and mescaline. It produces a dreamlike state in which ergot visions of colorful patterns and objects seem to fill up space and dance past in endless succession. Other ­ senses are also sharpened to produce a feeling of intense reality. The only reliable way to tell a nonpoisonous mushroom from a poisonous one is to be able to correctly identify the species. Poisonous mushrooms cannot be identified with simple tests, such as whether they peel easFigure 29D  Ergot infection of rye, caused by ily, have a bad odor, or blacken Claviceps purpurea. a silver coin during cooking. Only consume mushrooms identified by an expert! Like club fungi, some sac fungi also accused of practicing witchcraft in Salem, contain chemicals that can be dangerous to Massachusetts, during the seventeenth century people. Claviceps purpurea, the ergot fungus, were actually suffering from ergotism. It is also infects rye and replaces the speculated that ergotism is to blame for supgrain with ergot—hard, purple- posed demonic possessions throughout the black ­bodies consisting of tightly centuries. As recently as 1951, an epidemic cemented hyphae (Fig. 29D). of ergotism occurred in Pont-Saint-Esprit, When ground with the rye and France. Over 150 persons became hysterical, made into bread, the fungus and four died. Because the alkaloids that cause ergotism releases toxic alkaloids that cause the disease ergotism. In stimulate smooth muscle and selectively block humans, vomiting, feelings of the sympathetic nervous system, they can be intense heat or cold, muscle used in medicine to cause uterine contractions pain, a yellow face, and lesions and to treat certain circulatory disorders, on the hands and feet are accom- including migraine headaches. Although the panied by hysteria and halluci- ergot fungus can be cultured in petri dishes, no nations. Ergotism was common one has succeeded in inducing it to form ergot in Europe during the Middle in the laboratory. So far, the only way to obtain Ages. During this period, it was ergot, even for medical purposes, is to collect it known as St. Anthony’s Fire and in an infected field of rye. was responsible for 40,000 deaths in an epidemic in A.D. Questions to Consider 994. We now know that ergot 1. Why do you think that some species of fungi contain chemicals that are harmful to contains lysergic acid, from people? which LSD is easily synthesized. Based on recorded symp- 2. How might an understanding of fungal poisons aid in the development of new toms, historians believe that Figure 29C  Poisonous mushroom, Amanita drugs for humans? those individuals who were phalloides.



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algal cell reproductive unit

fungal hyphae

Figure 29.27  Lichen morphology.  a. A section of a compact

crustose lichen shows the placement of the algal cells and the fungal hyphae, which encircle and penetrate the algal cells. b. Fruticose lichens are shrublike. c. Foliose lichens are leaflike.

fungal hyphae a. Crustose lichen, Xanthoria

b. Fruticose lichen, Lobaria

c. Foliose lichen, Parmelia

sac fungi reproductive cups

seen on bare rocks or tree bark; shrublike fruticose lichens; and leaflike foliose lichens (Fig. 29.27). The body of a lichen has three layers. The fungus forms a thin, tough upper layer and a loosely packed lower layer. These shield the photosynthetic cells in the middle layer. Specialized fungal hyphae, which penetrate or envelop the photosynthetic cells, transfer organic nutrients directly to the fungus. The fungus protects the algae from predation and desiccation and provides them with minerals and water. Lichens can reproduce asexually by releasing fragments that contain hyphae and an algal cell. As with many symbioses, the relationship between fungi and algae was likely a pathogen-and-host interaction originally, but became mutually beneficial over evolutionary time.

Mycorrhizae Mycorrhizae are mutualistic relationships between soil fungi and the roots of most plants. Plants whose roots are invaded by mycorrhizae grow more successfully in dry or poor soils, particularly those deficient in phosphates. The relationship is very ancient, as it is seen in early plant fossils. Perhaps it helped plants adapt to life on dry land. Mycorrhizal fungi generally go unnoticed, except for the truffle, which is collected for food. Mycorrhizal fungi may live on the outside of roots, enter the cortex of roots, or penetrate root cells. The fungus and plant cells can easily exchange nutrients, with the plant providing organic nutrients to the fungus and the fungus bringing water and minerals

to the plant. The fungal hyphae greatly increase the surface area from which the plant can absorb water and nutrients. AM fungi (phylum Glomeromycota) are a group whose name stands for arbuscular mycorrhizal fungi. Arbuscules are branching invaginations the fungus makes when it invades plant roots. AM fungi are one type of mycorrhiza, fungi that form mutually beneficial relationships with the roots of plants.

Fungal Diseases of Humans For reasons that aren’t completely understood, fungi tend to cause disease mainly in people whose immune system isn’t working properly. Mycoses, fungal diseases of animals, vary in levels of seriousness.

Superficial Mycoses Most superficial mycoses are confined to the outer layers of the skin, hair, or nails. Fungi called dermatophytes cause infections of the skin called tineas. Athlete’s foot is a tinea characterized by itching and peeling of the skin between the toes (Fig. 29.28a). In ringworm, which is not caused by a worm but instead by several different fungi, the fungus releases enzymes that degrade keratin and collagen in skin. The area of infection becomes red and inflamed, and the fungal colony grows outward, forming a ring of inflammation. As the center of the lesion begins to heal, ringworm acquires its characteristic appearance, a red ring surrounding an



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UNIT 6  Evolution and Diversity

a.

b.

c.

Figure 29.28  Human fungal diseases.  a. Athlete’s foot and (b) ringworm are caused by fungi called dermatophytes. c. Thrush, or oral candidiasis, is characterized by the formation of white patches on the tongue.

area of healed skin (Fig. 29.28b). Some people can harbor and spread the fungus, but show no signs of disease. At any one time, an estimated 3–8% of the U.S. population is infected with scalp ringworm. Candida albicans causes a wide variety of fungal infections. Superficial C. albicans infections (“yeast infections”) tend to occur when the balance of the normal microbiota in an area of the body is disturbed, or the immune system is suppressed. For example, resident bacteria of the genus Lactobacillus normally produce organic acids that lower the pH of the vagina, and this inhibits C. albicans, a normal vaginal inhabitant, from proliferating. When lactobacilli are killed off by antibiotics, the yeast proliferates, resulting in inflammation, itching, and discharge. Thrush, a ­Candida infection of the mouth, is common in newborns and AIDS patients (Fig. 29.28c). In immunosuppressed individuals, Candida may cause an invasive infection that can damage the heart, the brain, and other organs.

Systemic Mycoses Systemic mycoses refer to fungal infections affecting the internal organs, mainly the lungs. Especially in immunocompromised individuals, the fungi can then spread through the bloodstream to multiple organs, eventually resulting in the death of the patient. This is well known in HIV infection leading to AIDS, and several fungal diseases are considered to be AIDS-defining illnesses (see the Health feature, “Opportunistic Infections and HIV,” in Chapter 13). Occasionally, systemic fungal infections occur in otherwise healthy patients. Due to an effective immune response, most of these infections are either asymptomatic, or cause only mild disease. In rare situations, however, an invasive and progressive infection occurs. Most serious systemic fungal infections start with respiratory symptoms such as coughing, chest pain, hoarseness, and perhaps blood in the sputum. Depending on which fungal species is involved, different organ systems like the skin, central nervous system, bones and joints, and even the heart can become involved. The majority of people living in the Midwest and eastern United States have been infected with Histoplasma capsulatum, a common soil fungus often associated with bird droppings. Only about 5% of infected individuals notice any symptoms, but about

3,000 a year develop a lung disease called histoplasmosis, which resembles tuberculosis. Similarly, Cryptococcus neoformans occurs throughout the world, mainly in soils contaminated with the droppings of pigeons and chickens. Coccidioidomycosis, also called valley fever, can range from benign to severe infections. The causative agent, Coccidioides immitis, is found mainly in desert areas, such as in the southwestern United States. Research has shown that 80% of people become infected with C. immitis within five years of moving to an area where the organism is prevalent. Also known as black mold, Stachybotrys chartarum grows well on building materials—especially those containing cellulose— that become moist. It grows best in dark, unventilated locations. S. chartarum is thought to play a role in “sick building syndrome,” in which individuals exposed to toxins produced by the fungus may experience allergies, flulike symptoms, headaches, fatigue, and dermatitis.

Antifungal Drugs As eukaryotes, fungal cells contain a nucleus, organelles, and ribosomes that are like those of human cells. These similarities make it a challenge to design antimicrobials against fungi that do not also harm humans. It is generally safer to treat fungal skin infections with a topical medication, which is not absorbed. For systemic fungal infections, medication must be taken into the body, and thus side effects are more of a problem. To minimize this, researchers exploit any biochemical differences they can discover. The biosynthesis of membrane sterols differs somewhat between fungi and humans. Therefore, a variety of fungicides are directed against sterol biosynthesis.

Check Your Progress  29.2 1. Explain how the type of nutrition required by fungal cells differs from that required by algae or amoeboids.

2. Define hypha, mycelium, and chitin. 3. Compare the modes of reproduction of the zygospore fungi, sac fungi, yeasts, and club fungi.

4. List two types of superficial mycoses, and two systemic mycoses.



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Conclusion The N. fowleri amoeba that infected the boy has certain ecological requirements, most notably a lake or river with high water temperature and low water level. Climate change has made those conditions more common. For example, in 2010 and 2012 the first-ever cases of N. fowleri infection were documented in Minnesota, over 500 miles north of any other recorded infections. Higher than normal summer temperatures allowed the pathogen to survive in a swimming lake and take the lives of two children.

Changing climate influences not only the pathogen but also the human behavior that makes infection more likely. During the summers of 2010 and 2012, in the same heat that allowed N. fowleri to survive, large numbers of people visited area lakes and rivers to cool down. More people in the water can create a higher chance of infection, especially for a protist, such as N. fowleri, that is stirred up from the bottom by splashing and jumping. 

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Animations

29.1 Endosymbiosis

SUMMARIZE 29.1  Protists

  Tutorials 29.1  Endosymbiotic Theory 29.2  Zygospore Life Cycle

The Chromalveolates ■ Brown algae, also called seaweeds, generally live at deeper depths that

other algae.

The domain Eukarya contains the protists,  a diverse group of mostly single-celled eukaryotes that are widespread in aquatic and moist environments. It is likely that the first eukaryotic cells to evolve were protists, and as stated by the endosymbiotic theory, that mitochondria and chloroplasts first arose when a nucleated cell engulfed an aerobic or photosynthetic bacterium, respectively. The biology of protists can be quite complex. Protists may carry out both sexual and asexual reproduction. Spores or cysts may be formed when reproduction is not possible.  Plankton are protists found suspended in water, which make up an important part of many food chains. The protists are currently classified based on molecular data into supergroups that include all eukaryotes. The following summarizes these supergroups:

■ Diatoms, which have an outer layer of silica, are extremely numerous

The Archaeplastids

The Excavates

■ Green algae are photosynthetic protists that derive energy from the

■ Euglenids are flagellated protists, or flagellates, with a pellicle

sun. There are two forms of green algae—the charophytes and the chlorophytes. All green algae possess chlorophyll, store reserve food as starch, and have cell walls of cellulose, as do plants. Green algae can be single-celled, form  colonies, or live as multicellular organisms. Some green algae reproduce by conjugation. ■ Red algae are charophytes that contain red and blue photosynthetic pigments. Red algae are used in a variety of consumer goods.

in both marine and freshwater ecosystems. 

■ Water molds, which ordinarily are saprotrophs, are sometimes

parasitic.

■ Dinoflagellates usually have cellulose plates and two flagella. They

are extremely numerous in the ocean and can produce a toxin when they form red tides.  ■ Ciliates are animal-like protistans that are commonly referred to as protozoans. They move by means of their many cilia. They are remarkably diverse in form—and as exemplified by Paramecium, they show how complex a protist can be, despite being a single cell. Many contain a contractile vacuole to regulate water in their cells. ■ Apicomplexans are spore-producing organisms (sporozoans) that are commonly parasites of other organisms.

instead of a cell wall. Their chloroplasts are most likely derived from a green alga, through endosymbiosis. They have an eyespot apparatus that acts as a photoreceptor. Euglenids are both autotrophic and heterotrophic. ■ Parabasalids and diplomonads are single-celled, flagellated excavates that are endosymbionts of animals. Trichomonas vaginalis and Giardia lamblia are examples.



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UNIT 6  Evolution and Diversity

The Amoebozoa ■ Amoeboids move and feed by forming pseudopods. Parasitic amoe-

boids, in the genus Entamoeba, cause amoebic dysentery. ■ Plasmodial slime molds are amoeboid and ingest food by phagocytosis. Cellular slime molds exist as individual amoeboid cells until nutrients become limited. The cells then fuse into a pseudoplasmodium that produces spores.

The Opisthokonta ■ This supergroup includes the multicellular fungi and animals. ■ Choanoflagellates are animal-like protistans closely related to the

sponges.

The Rhizaria ■ Foraminiferans and the radiolarians are organisms with fine, thread-

like pseudopods.

Protistans and Human Disease A few protists cause some important human diseases, as follows: ■ Sporozoans cause malaria, one of the most significant diseases in the world. Toxoplasmosis and cryptosporidiosis are other diseases caused by sporozoans. ■ The most important diseases caused by flagellates are African sleeping sickness and Chagas disease. Infections with Giardia lamblia and Trichomonas vaginalis are also very common. ■ The most common human disease caused by an amoeboid is amoebic dysentery. Rarely, amoeboids can cause a severe keratitis, and even a fatal brain infection.

29.2  Fungi Fungi are single-celled or multicellular eukaryotes that are strict heterotrophs. Many are saprotrophs, meaning they act as decomposers. ■ After external digestion, fungi absorb the resulting nutrient molecules. Most fungi are saprotrophs that aid the cycling of chemicals in ecosystems by decomposing dead remains. ■ The body of a fungus is composed of thin filaments, each of which is called a hypha. A mass of hyphae is termed a mycelium. Nonseptate hyphae have no cross walls, forming multinucleate cells. Septate hyphae have cross walls, but pores allow for exchange of cytoplasm. The cell wall contains chitin. ■ Most fungi produce nonmotile, and often windblown, spores during both asexual and sexual reproduction. During sexual reproduction, hyphae tips fuse so that hyphae containing both + and – nuclei usually result. Following nuclear fusion, meiosis occurs during the production of the sexual spores. Fungi can be classified into the following major groups: ■ The single-celled Microsporidia are parasites of animal cells. ■ Chytrid fungi are aquatic and have flagella at some stages. ■ Zygospore fungi, of which the common bread mold Rhizopus stolonifer is a member, reproduce by forming a capsule called a sporangium that produces thick-walled zygospores. ■ Sac fungi are named for their production of a cuplike ascocarp, which produces spores called conidia. Most known fungi are sac fungi, including morels, truffles, Penicillium, Aspergillus, and plant pathogens like Dutch Elm disease and ergot. Some sac fungi, like the brewer’s yeast Saccharomyces cerevisiae, produce single-celled forms called yeasts. ■ Club fungi are named for their production of a club-shaped reproductive structure called a basidium. Some club fungi produce toxins; those in the genus Psilocybe produce an LSD-like substance.

Fungi often form the following symbiotic relationships with other organisms: ■ Lichens are symbiotic associations between fungi and cyanobacteria or green algae. The algae provide organic nutrients to the fungi, while the fungi provide protection and enhanced absorption of water and minerals. ■ Mycorrhizae enjoy a mutualistic relationship with plant roots, such that the fungus helps the plant absorb minerals and water, while the plant supplies the fungus with organic nutrients. AM fungi are a type of mycorrhiza that invades plant roots. A few fungi are parasites of animals, causing fungal infections or mycoses. Most of these fungi are opportunists that mainly infect those with a compromised immune system. ■ Superficial mycoses affect the outermost layers of skin, or sometimes the nails or hair. Ringworm is a type of tinea, or fungal infection of the skin. Candida albicans, a normal inhabitant of several body areas, can overgrow and cause “yeast infections” of the vagina, or thrush in the oral cavity. ■ Systemic mycoses affect other body systems, especially the lungs. 

ASSESS Testing Yourself Choose the best answer for each question.

29.1  Protists 1. Which of these is a green alga? a. Volvox b. Gelidium c. Euglena d. Paramecium e. Plasmodium 2. Dinoflagellates a. usually reproduce sexually. b. have protective cellulose plates. c. are insignificant producers of food and oxygen. d. have cilia instead of flagella. e. tend to be larger than brown algae. 3. Ciliates a. move by pseudopods. b. are not as varied as other protists. c. feed and move using cilia. d. do not divide by binary fission. e. are closely related to the radiolarians. 4. Which of the following is most closely related to the land plants? a. dinoflagellates b. charophytes c. chlorophytes d. euglenids 5. Protists that may lack mitochondria and possess deep oral grooves are known as a. excavates. d. amoeboids. b. apicomplexans. e. ciliates. c. alveolates. 6. Plasmodial and cellular _____ are important decomposers that have an amoeboid vegetative state. a. brown algae d. slime molds b. water molds e. red algae c. filamentous algae



7. Through what process did eukaryotic cells gain chloroplasts and mitochondria? a. engulfment b. endosymbiosis c. endocytosis d. phagocytosis e. conjugation 8. All protists are a. photosynthetic. b. parasites. c. single-celled. d. eukaryotes. e. All of these are correct.

29.2  Fungi 9. Which of the following represents an organism that uses decomposition as its mode of nutrition? a. parasite b. saprotroph c. autotroph d. All of these are correct. 10. Which of the following is not a characteristic of Microsporidia? a. parasitic b. lives in host cells c. multicellular d. can infect invertebrates like insects e. can infect vertebrates like humans 11. Which of the following is not a characteristic of chytrids? a. They have flagellated spores. b. They have flagellated gametes. c. They can live on very dry land. d. They can be single cells. e. They can be animal parasites. 12. The collective mass of hyphae is called a a. sporophyte. b. mycelium. c. ascocarp. d. lichen. 13. Mycorrhizae a. are a type of lichen. b. are mutualistic relationships. c. help plants gather solar energy. d. help plants gather inorganic nutrients. e. Both b and d are correct. 14. Fungi are classified according to a. sexual reproductive structures. b. shape of their hyphae. c. mode of nutrition. d. type of cell wall. e. All of these are correct. 15. Lichens a. cannot reproduce. b. need a nitrogen source to live. c. are parasitic on trees. d. are able to live in extreme environments.

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ENGAGE BioNOW Want to know  how this science is relevant to your life? Check out the BioNow video below: ■ Nut Fungus

Thinking Critically 1. Sickle cell disease occurs mostly in people of African ancestry. When a person inherits two copies of the sickle cell gene, his or her hemoglobin is abnormal, causing red blood cells to be more fragile and sticky than usual. This leads to anemia and circulatory problems. About 70,000 Americans have sickle cell disease, and about 2 million carry one copy of the gene. Even though it can cause a serious disease, the sickle cell gene is thought to have been favored by natural selection, because it gives some protection against malaria. Considering the type of cells infected by Plasmodium species, how might this protection work? What do you think may happen to the prevalence of the sickle cell gene if an effective vaccine against malaria is ever developed and widely used? 2. You are trying to develop a new anti-termite chemical that will not harm environmentally beneficial insects. Because termites are adapted to eat only wood, they will starve if they cannot digest this food source. Termites have two symbiotic partners: the protozoan Trichonympha collaris and the bacteria it harbors that actually produce the enzyme that digests the wood. Knowing this, how might you prevent termite infestations without targeting the termites directly?  3. Many fungal infections of humans are considered to be opportunistic, meaning that fungi that are normally free-living (usually in soil) can sometimes survive, and even thrive, on or inside the human body. From the fungal “point of view,” what unique challenges would be encountered when trying to survive on human skin? What about inside human lungs?

PHOTO CREDITS Opener: © FLPA/SuperStock; 29.3(both): © Manfred Kage/Science Source; 29.4b: © Spike Walker/Science Source; 29.5-29.6(both): © Dan Ippolito; 29.7: © Darlyne A. Murawski/Getty Images; 29.8: © Noble Proctor/Science Source; 29.9(top): © Kevin Schafer/Alamy; 29.10a: © Carolina Biological Supply/Phototake; 29.10b: © Ed Reschke/Photolibrary/Getty Images; 29.11: © Kage Mikrofotograffie/Phototake; 29.12: © Cultura RM/Alamy; 29.14(left): © NHPA/SuperStock; 29.14(right): © NaturePL/SuperStock; 29.16a(cliffs): © Stockbyte/ Getty RF; 29.16a(foraminiferan): © NHPA/Superstock; 29.16b(tests), 29A: © Eye of Science/ Science Source; 29B(boy): © Author’s Image/PunchStock RF; 29B(tsetse fly): © Frank Greenaway/Getty Images; 29B(pig): © G.K. & Vikki Hart/Getty RF; 29B(goat): © Eureka/ Alamy RF; 29.19a: © Matt Meadows/Photolibrary/Getty Images; 29.19b: © Andrew Syred/ Science Source; 29.20a: © Richard Packwood/Getty Images; 29.20b: © Steve Gschmeissner/ Getty RF; 29.21: Reproduced with permission of the Freshwater Biological Association on behalf of The Estate of Dr. Hilda Canter-Lund. (c) The Freshwater Biological Association; 29.22: © Garry DeLong/Getty Images; 29.23b: © Carol Wolfe, photographer; 29.23c: © Bear Dancer Studios/Mark Dierker; 29.24a: © Science Photo Library RF/Getty RF; 29.24b: © Dennis Kunkel Microscopy, Inc./Phototake; 29.25b: © Biophoto Associates/Science Source; 29.25c: © Inga Spence/Science Source; 29.25d: © L. West/Science Source; 29C: © De Agostini/Getty Images; 29D: © Wildlife GmbH/Alamy; 29.26a: © Marvin Dembinsky Photo Associates/Alamy; 29.26b: © Patrick Lynch/Science Source; 29.27a: © Dr. Jeremy Burgess/SPL/Science Source; 29.27b: © Steven P. Lynch; 29.27c: © yogesh more/Alamy RF; 29.28a: © P. Marazzi/SPL/Science Source; 29.28b: © John Hadfield/SPL/Science Source; 29.28c: © Dr. M. A. Ansary/Science Source.



30 Plants

CHAPTER OUTLINE 30.1  Evolutionary History of Plants 30.2 Nonvascular Plants 30.3 Seedless Vascular Plants 30.4 Seed Plants BEFORE YOU BEGIN Before beginning this chapter, take a few moments to review the following discussions: Section 3.3  What cellular structures are unique to plants? Section 5.4  What is the role of meiosis in a cell? Section 9.1  How does vascular tissue contribute to the success of the various groups of plants?

CASE STUDY The Source of Coal Our industrial society runs on fossil fuels, such as coal. The term fossil fuel refers to the remains of organic material from ancient times. During the Carboniferous period more than 300 million years ago, a great swamp forest encompassed what is now northern Europe, the Ukraine, and the Appalachian Mountains in the United States. The weather was warm and humid, and the plants grew very tall. These were not plants that would be familiar to us; instead, they were related to various groups of plants that are less well known—the lycophytes, horsetails, and ferns.  The amount of biomass in a Carboniferous swamp forest was enormous, and occasionally the swampy water rose, covering the plants that had died. Dead plant material under water does not decompose well. The partially decayed remains became covered by sediment, which changed them into sedimentary rock. Exposed to significant amounts of pressure and over long periods of time, the organic material became coal. This process continued for millions of years, resulting in immense deposits of coal. Geologic upheavals raised the deposits to the levels where they can be mined today.  With a change of climate, many of the plants of the Carboniferous period became extinct. However, as we will see in this chapter, some of their smaller herbaceous relatives have evolved and survived to our time. We owe the industrialization of today’s society to these ancient forests. As you read through the chapter, think about the following questions:

1. Where do lycophytes, horsetails, and ferns fit in the evolutionary history of the plants?

2. What adaptations in plants are associated with the angiosperms?

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Chapter 30  Plants

30.1  Evolutionary History of Plants Learning Outcomes Upon completion of this section, you should be able to 1. Identify the features present in the ancestor of land plants that helped give rise to the land plants. 2. List the major events in the evolutionary history of plants. 3. Describe alternation of generations in plants.

Plants (domain Eukarya, kingdom Plantae) are vital to human survival. As explained by the Ecology feature, “Plants and Humans,” our dependence on them is nothing less than absolute.  Although a land environment does offer advantages such as plentiful light, it also has certain challenges such as the constant threat of desiccation (drying out). Most important, all stages of reproduction—gametes, zygote, and embryo—must be protected from the drying effects of air. To keep the internal environment of cells moist, a land plant must acquire water, and transport it to all parts of the body. The evolution of an internal vascular system has made certain groups of plants incredibly successful. The evolutionary history of plants begins in the water. Most likely land plants evolved from a type of freshwater green algae some 450 mya (million years ago): both green algae and plants (1) contain chlorophylls a and b and various accessory pigments, (2) store excess carbohydrates as starch, and (3) have cellulose in their cell walls. A comparison of DNA and RNA base sequences suggests that land plants are most closely related to a group of freshwater green algae known as charophytes. Although the common ancestor of modern charophytes and land plants no longer exists, if it did, it would have features resembling those of the Chara and Coleochaete (Fig. 30.1). Chara are commonly known as stoneworts because they are encrusted with calcium carbonate deposits. The body consists of a single file of very long cells anchored in mud by thin filaments. Whorls of branches occur at regions called nodes, located between the cells of the main axis. Male and female reproductive structures

Chara

Coleochaete

Figure 30.1  The charophytes.  The charophytes (represented here by Chara and Coleochaete) are the green algae most closely related to the land plants.

607

grow at the nodes. A Coleochaete  grows on flat, aquatic surfaces, such as rocks in streams. They are being used as a model organism in the study of biotechnology. A key feature in the evolution of plants was the protection of the zygote. Land plants not only protect the zygote, they also protect and nourish the resulting embryo. This may be the first derived feature that separates land plants from green algae. Figure 30.2 traces the evolutionary history of land plants and will serve as a backdrop as we discuss major groups of plants in the pages that follow. To better understand the evolution of land plants, it is possible to associate each group with one of the major evolutionary events representing adaptations to a land existence. ■■ ■■

■■ ■■

■■

Mosses were the first group to have some form of protection for the embryo to prevent it from drying out. Lycophytes were the first group to evolve vascular tissue, which transports water and nutrients throughout the plant body. Ferns evolved megaphylls, large leaves that increase the ­surface area for photosynthesis. Gymnosperms were the first plants with seeds. Seeds contain the embryo and supportive nutrients within a ­ ­protective coat. Angiosperms are the only group to have flowers. Flowers attract a variety of pollinators and give rise to fruits.

Alternation of Generations All plants have a life cycle that includes an alternation of ­generations. In this life cycle, two multicellular individuals alternate, each producing the other (Fig. 30.3). The two individuals are (1) a sporophyte, which represents the diploid (2n) ­generation, and (2) a gametophyte, which represents the haploid (n) generation. The sporophyte (2n) is the structure that produces spores by meiosis. A spore is a haploid reproductive cell that develops into a new organism without needing to fuse with another reproductive cell. In the plant life cycle, a spore undergoes mitosis and becomes a gametophyte. The gametophyte (n) is named because of its role in the production of gametes. In plants, eggs and sperm are produced by mitotic cell division. A sperm and egg fuse, forming a diploid zygote that undergoes mitosis and becomes the sporophyte. Two observations are in order. First, meiosis produces haploid spores. This is consistent with the realization that the sporophyte is the diploid generation and spores are haploid reproductive cells. Second, mitosis occurs as a spore becomes a gametophyte, and mitosis also occurs as a zygote becomes a sporophyte. Indeed, it is the occurrence of mitosis at these times that results in two generations. Plants differ as to which generation is the dominant one. In plants, the dominant generation carries out the majority of photosynthesis. In nonvascular plants, the gametophyte is dominant, but in the other three groups of plants, the sporophyte is dominant. In the history of plants, only the sporophyte evolves vascular tissue. Therefore, the shift to sporophyte dominance is an adaptation to life on land. Notice that as the sporophyte gains in dominance, the



flowers, double fertilization, endosperm, fruit Angiosperms Seed

seeds

megaphylls

Vascular

Gymnosperms vascular tissue Seedless

Ferns and allies microphylls apical growth

Lycophytes

embryo protection

Bryophytes

Hornworts Mosses

common green algal ancestor

Liverworts

Nonvascular

common ancestor

Charophytes 550

500

450

400 350 Million Years Ago (MYA)

300

250

PRESENT

Figure 30.2  Evolutionary history of plants.  The evolution of plants involves several significant innovations. SCIENCE IN YOUR LIFE  ►

ECOLOGY

Plants and Humans Humans derive most of their nourishment from three flowering plants: wheat, corn, and rice. All three of these plants are members of the grass family and are collectively called grains (Fig. 30A). Wheat is commonly used to produce flour and bread. It was first cultivated in the Middle East about 8000 b.c., and is thought to be one of the earliest cultivated plants. Early settlers brought wheat to North America, where many varieties of corn, more properly called maize, were already in existence. Rice originated in southeastern Asia several thousand years ago, where it grew in swamps. Many foods of both plant and animal origin are considered bland or tasteless without seasonings or spices. The Americas were discovered when Columbus was seeking a new route to the Far East to acquire spices, such as nutmeg, oregano, rosemary, and sage. In addition, our primary sweetener, sugar, comes almost exclusively from two plants—­ sugarcane (grown in South America, Africa, Asia, and the Caribbean) and sugar beets (grown mostly in Europe and North America). Our most popular drinks—coffee, tea, and cola—also come from flowering plants. Coffee originated in Ethiopia, whereas tea is thought to have been first used somewhere in central Asia. Cotton and rubber are two plants that still have many uses today (Fig. 30B). Until a few decades ago, cotton

608

grain head

Wheat plants, Triticum

grain head

ear Rice plants, Oryza

Corn plants, Zea

Figure 30A  Plants used for food. and other natural fibers were our only source of clothing. Interestingly, when Levi Strauss wanted to make a tough pair of jeans, he needed a stronger fiber than cotton, so he used hemp. Hemp comes from a plant that is closely related

to marijuana, but unlike marijuana, has extremely small amounts of the chemical tetrahydrocannabinol (THC), which causes the hallucinogenic effect when marijuana is smoked. Today, hemp is increasingly used to make clothing due to its



Chapter 30  Plants

sporophyte (2n)

is tos

Mi

sporangium (2n)

zygote (2n)

diploid (2n)

FERTILIZATION

MEIOSIS

haploid (n)

spore (n)

(n) sis to

Mi tos is

Mi

(n) gametes

609

gametophyte becomes microscopic (Fig. 30.4) and dependent upon the sporophyte. All plants have an alternation-of-generations life cycle, but the appearance of the generations varies widely. In ferns, the gametophyte is a small, independent, heart-shaped structure. Archegonia are female reproductive organs that produce eggs. Antheridia are male reproductive organs that produce motile sperm. The eggs are fertilized by flagellated sperm, which swim to the archegonia in a film of water. In contrast, the female gametophyte of an angiosperm, called the embryo sac, is retained within the body of the sporophyte plant and consists of seven cells within a structure called an ovule. Following fertilization, the ovule becomes a seed. In seed plants, pollen grains are mature, sperm-bearing male gametophytes. Pollen grains can be transported by wind, insects, bats, or birds, and therefore, they do not need free water to reach the egg. In seed plants, reproductive cells are protected from drying out in the land environment.

Check Your Progress  30.1 1. Summarize the major adaptations in the evolution of the

gametophyte (n)

land plants.

2. Identify the role of each generation in the alternation-of-

Figure 30.3  Alternation of generations.  Plants alternate

generations life cycle.

between a diploid sporophyte stage and a haploid gametophyte stage.

Rubber tree, Hevea

Cotton, Gossypium

Figure 30B  Plants used commercially. toughness and wearability. Rubber had its origin in Brazil from the thick, white sap of the rubber tree. To produce a stronger rubber, such as that in tires, sulfur is added, and the sap is heated in a process called vulcanization. This produces a flexible material less sensitive to temperature changes. However, at present, most rubber is synthetically produced. Plants have also been used for centuries for a number of important household items, including the house itself. We are most familiar with lumber as the major structural portion in buildings. This wood comes mainly from a variety of conifers: pine, fir, and spruce, among others. In

the tropics, trees and even herbs provide important components for houses. In rural parts of Central and South America, palm leaves are preferable to tin for roofs, because they last as long as ten years and are quieter during rainstorms. In the Middle East, numerous houses along rivers are made entirely of reeds. Today, plants are increasingly researched for their use in medicinal products. Currently, about 50% of all pharmaceutical drugs originate from plants or are derived from plant products. Some cancers are even treatable with medicinal plants, such as the rosy periwinkle, extracts from which have shown some

success in treating childhood leukemia. Indeed, the National Cancer Institute and most pharmaceutical companies have spent millions of dollars to send botanists out to collect and test plant samples from around the world. Tribal medicine men, or shamans, of South America and Africa have already been of great importance in developing numerous drugs. However, plant extracts also continue to be misused for their hallucinogenic or other effects on the human body; examples are coca, the source of cocaine and crack, and the opium poppy, the source of heroin. In addition to all these uses of plants, we should not forget their aesthetic value. Flowers brighten any yard, ornamental plants accent landscaping, and trees provide cooling shade during the summer as well as shelter from harsh winds during the winter.

Questions to Consider 1. Why do you think plants are such a good source of drugs for human use? What advantage does this give the plants? 2. With over 270,000 species of angiosperms alone, why do you think we get most of our nourishment from only three plants (wheat, corn, and rice)? 3. What are some uses of plants, other than those just discussed?

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spores

G a m e t o p h y t e (n)

seed

seed

spores

roots

roots rhizoids

roots

(2n)

rhizoids Moss

Fern

S p o r o p h y t e

Gymnosperm

Angiosperm

Figure 30.4  Reduction in the size of the gametophyte.  As plants became adapted to life on land, the size of the gametophyte decreased, and the size of the sporophyte increased, as evidenced by these representatives of today’s plants. In the moss and fern, spores disperse the gametophyte. In gymnosperms and angiosperms, seeds disperse the sporophyte. 

30.2  Nonvascular Plants Learning Outcomes Upon completion of this section, you should be able to 1. Identify the traits of nonvascular plants that enable them to survive on land. 2. Compare and contrast the features of liverworts and mosses.

The bryophytes—the liverworts, hornworts, and mosses—were the first plants to colonize land. They only superficially appear to have roots, stems, and leaves because, by definition, true roots, stems, and leaves must contain vascular tissue. Vascular tissue is specialized for the transport of water and organic nutrients throughout the body of a plant. Bryophytes, which lack vascular tissue, are often called the nonvascular plants. Vascular tissue also provides support to the plant body, so bryophytes typically are low-lying; some mosses reach a maximum height of only about 20 cm. Bryophytes do share other traits with the vascular plants. For example, they have an alternation-of-generations life cycle similar to all other plants. Their bodies are covered by a cuticle, which is interrupted in hornworts and mosses by stomata, and they have apical tissue that produces complex tissues. However, bryophytes are the only land plants in which the gametophyte is dominant (see Fig. 30.4). Antheridia produce flagellated sperm, which means they need a film of moisture in order to swim to eggs located inside archegonia. The bryophytes’ lack of vascular tissue and the presence of flagellated sperm mean that you are apt to find bryophytes in moist locations. Some bryophytes compete well in harsh environments because they can reproduce asexually. 

Liverworts Liverworts (phylum Hepaticophyta) come in two types: those that have a flat, lobed body called a thallus (pl., thalli) and those that are leafy. Marchantia is a familiar liverwort (Fig. 30.5) that has a smooth upper surface. The lower surface bears numerous rhizoids (rootlike hairs) that project into the soil. Marchantia reproduces both asexually and sexually. Gemmae cups on the upper surface of the thallus contain gemmae, groups of cells that detach from the thallus and can asexually start a new plant. ­Sexual reproduction depends on disk-headed stalks that bear antheridia (sing., antheridium), where flagellated sperm are produced, and on umbrella-headed stalks that bear archegonia (sing., archegonium), where eggs are produced. Following fertilization, tiny sporophytes composed of a foot, a short stalk, and a capsule begin growing within archegonia. Windblown spores are produced within the capsule.

Mosses Mosses (phylum Bryophyta) can be found from the Arctic through the tropics to parts of the Antarctic. Although most live in damp, shaded locations in the temperate zone, some survive in deserts, whereas others inhabit bogs and streams. In forests, mosses frequently form a mat that covers the ground or the surfaces of rotting logs. Mosses can store large quantities of water in their cells, but if a dry spell continues for long, they become dormant until it rains. Most mosses can reproduce asexually by fragmentation. Just about any part of the plant is able to grow and eventually produce leaflike thalli. Figure 30.6 describes the life cycle of a typical ­temperate-zone moss. The gametophyte of mosses has two stages. First, there is the algalike protonema, a branching filament of cells. Then, after about three days of favorable growing conditions,



Chapter 30  Plants male gametophyte

611

female gametophyte

gemma cup

thallus rhizoids a. Gemma cup

gemma

Thallus with gemmae cups

b. Male gametophytes bear antheridia

c. Female gametophytes bear archegonia

Figure 30.5  Liverwort, Marchantia.  a. A gemma is a group of cells that can detach and start a new plant. b. Antheridia are present in disk-shaped stalks. c. Archegonia are present in umbrella-shaped stalks.

upright leafy thalli appear at intervals along the protonema. ­Rhizoids anchor the thalli, which bear antheridia and archegonia. An antheridium consists of a short stalk, an outer layer of sterile cells, and an inner mass of cells that become the flagellated sperm. An archegonium, which looks like a vase with a long neck, has a single egg located inside its base. The dependent sporophyte consists of a foot, which grows down into the gametophyte tissue; a stalk; and an upper capsule, or sporangium, where windblown spores are produced. At first, the sporophyte is green and photosynthetic. At maturity, it is brown and nonphotosynthetic. Because the gametophyte is the dominant generation, it seems consistent for spores to be dispersal agents— that is, when the haploid spores germinate, the gametophyte is in a new location.

Adaptations and Uses of Nonvascular Plants Although mosses are nonvascular, they are better than flowering plants at living on stone walls, fences and even in the shady cracks of hot, exposed rocks. For these particular microhabitats, being small and simple seems to be a selective advantage. When bryophytes colonize bare rock, they help convert the rocks to soil that can be used for the growth of other organisms. In areas such as bogs, where the ground is wet and acidic, dead mosses, especially Sphagnum, do not decay. The accumulated moss, called peat or bog moss, can be used as fuel. Peat moss is also commercially important because it has special nonliving cells that can absorb moisture. Thus, peat moss is often used in gardens to improve the water-holding capacity of the soil.

Check Your Progress  30.2 1. List two features limiting the adaptation of nonvascular plants.

2. Describe two features of nonvascular plants that indicate they are adapted to the land environment.

3. Describe the differences between a liverwort and a moss.

30.3  Seedless Vascular Plants Learning Outcomes Upon completion of this section, you should be able to 1. Explain the importance of vascular tissue to plants. 2. Distinguish between a lycophyte and pteridophyte.

All the other plants we will study are vascular plants. Vascular tissue in these plants consists of xylem, which conducts water and minerals up from the soil, and phloem, which transports organic nutrients from one part of the plant to another. The vascular plants usually have true roots, stems, and leaves. The roots absorb water from the soil, and the stem conducts water to the leaves. Xylem, with its strong-walled cells, supports the body of the plant against the pull of gravity. The leaves are fully covered by a waxy cuticle except where it is interrupted by stomata, little pores whose size can be regulated to control water loss. The sporophyte is the dominant generation in vascular plants. Seedless vascular plants include two groups: lycophytes and ferns and their allies. Both of these groups disperse their offspring by producing wind-blown spores. This is advantageous because the sporophyte is the generation with vascular tissue. Another advantage of having a dominant sporophyte relates to its being diploid. If a faulty gene is present, it can be masked by a functional gene. Then, too, the greater the amount of genetic material, the greater the possibility of mutations that will lead to increased variety and complexity. Indeed, vascular plants are complex, extremely varied, and widely distributed. Some vascular plants do not produce seeds. Seedless vascular plants include whisk ferns, club mosses, horsetails, and ferns, which disperse their offspring by producing windblown spores. When the spores germinate, they produce a small gametophyte that is independent of the sporophyte for its nutrition. In these plants, antheridia release flagellated sperm, which swim in a film of external water to the archegonia, where fertilization occurs. Because spores are



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Figure 30.6  Moss life cycle.  This diagram illustrates the life cycle of Polytrichum, a species of moss. 4. The sporophyte:     The mature sporophyte has a foot buried in female gametophyte tissue, a stalk, and an upper capsule (the sporangium), where meiosis occurs and spores are produced.

developing sporophyte capsule

3. The zygote:     The zygote and developing sporophyte are retained within the archegonium.

capsule Sporangium

Mitosis Sporophyte

stalk teeth operculum

zygote

5. The spores:     When the calyptra and lid (operculum) of      a capsule fall off, the spores are mature. One or two rings of teeth project inward from the margin of the capsule. The teeth close the opening, except when the weather is dry.

diploid (2n) FERTILIZATION

2. Fertilization:     Flagellated sperm produced in antheridia swim in external water to archegonia, each bearing a single egg.

MEIOSIS

haploid (n)

foot (n)

egg

Spores Mitosis

sperm Archegonia

buds

6. Spore dispersal:      Spores are released when they are most      likely to be dispersed      by air currents.

Protonema

Antheridia 7. The immature      gametophyte:     A spore germinates into a male or female protonema, the first stage of the male and the female gametophytes.

1. The mature      gametophytes: In mosses, the leafy gametophyte shoots bear either antheridia or archegonia, where      gametes are produced by mitosis.

Gametophytes rhizoids

dispersal agents and the nonvascular gametophyte is independent of the sporophyte, these plants cannot wholly benefit from the adaptations of the sporophyte to a terrestrial environment. The seedless vascular plants formed the great swamp forests of the Carboniferous period (Fig. 30.7). A large number of

these plants died but did not decompose completely. Instead, they were compressed to form the coal (see the chapter opener) that we still mine and burn today. Oil has a similar origin, but it typically forms in marine sedimentary rocks and includes animal remains.



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strobili

leaves branches

aerial stem rhizome root

Figure 30.7  The Carboniferous period.  Vast swamp forests of

treelike club mosses and horsetails dominated the land during the Carboniferous period. The air contained insects with wide wingspans, such as the predecessors of dragonflies. Amphibians also diversified greatly in this environment.

Figure 30.8  Ground pine, Lycopodium.  Lycophytes such as Lycopodium have vascular tissue and thus true roots, stems, and leaves. The Lycopodium sporophyte develops an underground rhizome system. A rhizome is an underground stem; this rhizome produces roots along its length.

Ferns

Lycophytes Lycophytes (phylum Lycophyta) are also called club mosses and were among the first land plants to have vascular tissue. Unlike true mosses (bryophytes), the lycophytes have well-developed vascular tissue in roots, stems, and leaves. Typically, a fleshy underground and horizontal stem, called a rhizome, sends up upright aerial stems. Tightly packed, scalelike leaves cover the stems and branches, giving the plant a mossy look. The small leaves, called microphylls, have a single vein composed of xylem and phloem.  As we will see, most other vascular plants have larger leaves, called megaphylls, that contain branched vascular tissue.

Ferns (phylum Polypodiophyta) are the largest group of plants other than the flowering plants, and they display great diversity in form and habitat. Ferns are most abundant in warm, moist, tropical regions, but can also be found in northern regions and in relatively dry, rocky, places. They range in size from low-growing plants resembling mosses to tall trees. The fronds (leaves) that grow from a rhizome can vary. The royal fern has fronds that stand 6 ft tall; those of the maidenhair fern are branched, with broad leaflets; and those of the hart’s tongue fern are straplike and leathery. Figure 30.9 provides some examples of ferns. frond (undivided)

single strand of vascular tissue Microphyll

branched vascular tissue Megaphyll

The sporangia of the lycophytes are borne on terminal clusters of leaves, called strobili, which are club-shaped. The spores are sometimes harvested and sold as lycopodium powder, or vegetable sulfur, for use in pharmaceuticals and in fireworks because it is highly flammable. Members of the genus Lycopodium featured in Figure 30.8 are common in moist woodlands in temperate climates, where they are called ground pines; they are also abundant in the tropics and subtropics.

Ferns and Their Allies

spores on fertile frond

Cinnamon fern, Osmunda cinnamomea

Hart’s tongue fern, Campyloneurum scolopendrium

In this section, we will discuss the characteristics of the seedless Figure 30.9  Fern diversity.  The many different types of ferns grow vascular plants called the pteridophytes, which includes the ferns, in places that offer moisture. These photos show the dominant sporophyte. whisk ferns, and horsetails. Unlike the lycophytes, the pteridoThe separate, delicate, and almost microscopic gametophyte is water dependent phytes possess megaphylls as leaves. and produces sperm that require moisture to swim to the egg.

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UNIT 6  Evolution and Diversity

7. The fronds: The sporophyte develops a rootbearing rhizome from which the aerial fronds project.

1. The sporophyte:      The sporophyte is dominant in ferns.

Sporophyte

sori

Polystichum 6. The zygote:     The resulting sporophyte zygote begins its development inside an archegonium. As the distinctive first leaf appears above the prothallus, and as the roots develop below it, the sporophyte becomes visible.

leaflet sporangium Sorus young sporophyte on gametophyte

fiddlehead

rhizome

roots

2. The sporangia: In this fern, the sporangia are located within sori (sing., sorus), on the underside of the leaflets.

Mitosis annulus zygote Sporangium

diploid (2n) FERTILIZATION

5. Fertilization:     Fertilization takes place when moisture is present, because the flagellated sperm must swim in a film of water from the antheridia to the egg within the archegonium.

MEIOSIS

haploid (n)

egg sperm

Spores

Archegonium

Prothallus (underside) Mitosis germinating spore

Antheridium

Gametophyte

Figure 30.10  Fern life cycle.  Stages of the life cycle of a typical fern.  Life Cycle, Adaptations, and Uses of Ferns  Ferns have true roots, stems, and leaves that contain vascular tissue. The well-­ developed leaves fan out, capture solar energy, and photosynthesize. The life cycle of a typical temperate fern is shown in Figure 30.10. The dominant sporophyte produces windblown spores. When the spores germinate, a tiny green and independent gametophyte develops. The gametophyte is water d­ ependent because it lacks vascular tissue. Also, flagellated sperm produced within antheridia require an outside source of moisture to swim to the eggs in the archegonia. Upon fertilization the zygote develops into a sporophyte. In nearly

rhizoids

3. The spores: Within a sporangium, meiosis occurs and spores are produced. When a sporangium opens, the spores are released.

4. The gametophyte: A spore germinates into a prothallus (the gametophyte), which typically bears archegonia at the notch and antheridia at the tip between the rhizoids.

all ferns, the leaves of the sporophyte first appear in a curled-up form called a fiddlehead, which unrolls as it grows. Once established, some ferns, such as the bracken fern Pteridium aquilinum, can spread into drier areas by means of vegetative (asexual) reproduction. For example, ferns can spread when their rhizomes grow horizontally in the soil, producing fiddleheads that become new fronds. In terms of economic value, people use ferns in decorative bouquets and as ornamental plants in the home and garden. Wood from tropical tree ferns often serves as a building material because it resists decay, as well as damage by termites. Ferns, especially the



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strobilus

sporangium branches

scale aerial stem

node rhizome leaves

Figure 30.11  Whisk fern, Psilotum.  Whisk ferns have no roots or leaves—the branches carry on photosynthesis. The sporangia are yellow.

ostrich fern, are eaten as food, and in the Northeast, many restaurants feature fiddleheads as a special treat, although studies have shown that some are carcinogenic.

Whisk Ferns and Horsetails



Whisk ferns (phylum Psilotophyta), named for their resemblance to whisk brooms, are found in Arizona, Texas, Louisiana, and Florida, as well as Hawaii and Puerto Rico. Psilotum (Fig. 30.11) looks like a rhyniophyte, an ancient vascular plant that is known only from the fossil record. Its aerial stem forks repeatedly and is attached to a rhizome, a fleshy horizontal stem that lies underground. Whisk ferns have no leaves, so the branches carry on photosynthesis. Sporangia, located at the ends of short branches, produce spores that are dispersal agents. The Psilotum gametophyte is independent, small (less than a centimeter), and found underground associated with mutualistic mycorrhizal fungi. The resemblance of its life cycle to that of a fern suggests that Psilotum is actually a fern devoid of leaves and roots. If so, it could be said to be a vestigial fern. Horsetails (phylum Equisetophyta), which thrive in moist habitats around the globe, are represented by Equisetum, the only genus in existence today (Fig. 30.12). A perennial rhizome produces roots and photosynthetic aerial stems that can stand about 1.3 m high. In some species, the whorls of slender green side branches at the joints (nodes) of the stem bear a fanciful resemblance to a horse’s tail. The small, scalelike leaves are whorled at the nodes and are nonphotosynthetic. Many horsetails have strobili at the tips of branch-bearing stems. Others send up special buffcolored, naked stems that bear the strobili. The stems of horsetails are tough and rigid because of silica deposited in their cell walls. Early Americans, in particular, used horsetails to scour pots and called them “scouring rushes.” Today, they are still used as ingredients in a few abrasive powders. Equisetum is sometimes considered a weed in the garden, and it is difficult to control because any portion of the rhizome left in the soil will regenerate.

root

rhizome

Figure 30.12  Horsetail, Equisetum.  Whorls of branches and tiny leaves are at the nodes of the stem. Spore-producing sporangia are borne in strobili (cones).

Check Your Progress  30.3 1. Explain what makes lycophytes significant in plant evolution.

2. Describe the difference between a microphyll and a megaphyll.

3. Summarize the life cycle of a fern.

30.4  Seed Plants Learning Outcomes Upon completion of this section, you should be able to 1. List the features of seed plants that make them fully adapted to life on land. 2. Compare and contrast the life cycles of gymnosperms with angiosperms. 3. List the key features of monocots and eudicots.

Gymnosperms and angiosperms are categorized as seed plants and are the most plentiful plants on the Earth today. Seeds contain a sporophyte embryo and stored food within a protective seed coat. The seed coat and stored food allow an embryo to survive harsh conditions during long periods of dormancy (arrested state) until environmental conditions become favorable for growth. Seeds can even remain dormant for hundreds of years. When a seed germinates, the stored food is a source of nutrients for the growing s­ eedling. The survival value of seeds largely accounts for the ­dominance of seed plants today.

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UNIT 6  Evolution and Diversity

7. The sporophyte: After fertilization, the ovule matures and becomes the seed composed of the embryo, reserve food, and a seed coat. In the fall of the second season, the seed cone opens to release winged seeds. When a seed germinates, the sporophyte embryo develops into a new pine tree.

1. The pollen cones: Typically, the pollen cones are quite small and develop near the tips of lower branches.

The seed cones: The seed cones are larger than the pollen cones and are located near the tips of higher branches.

Sporophyte

wing

seed

seed cone

pollen cones

Ovule

2. The pollen sacs (microsporangia): A pollen cone has two pollen sacs that lie on the underside of each scale. The ovules (megasporangia): The seed cone has two ovules that lie on the upper surface of each scale. An ovule contains a megasporangium.

Pollen sac embryo seed cone scale

seed coat pollen cone scale

stored food seed (Female gametophyte) Mitosis 6. The zygote: Once a pollen grain reaches a seed cone, it becomes a mature male gametophyte. A pollen tube digests its way slowly toward a female gametophyte and discharges two nonflagellated sperm. One of these fertilizes an egg in an archegonium, and a zygote results. 

Ovule

zygote

microsporocyte diploid (2n)

FERTILIZATION

Ovule

MEIOSIS

haploid (n)

Mitosis

Mature female gametophyte archegonium integument

megasporocyte

MEIOSIS

Microspores

pollen grain

3. Microspores: Within pollen sacs, each microsporocyte (microspore mother cell) undergoes meiosis and produces four microspores. Megaspore: Within an ovule, a megasporocyte (megaspore mother cell) undergoes meiosis, producing four megaspores, of which three degenerate.

egg Megaspore Pollination Ovule

Mitosis Mature male gametophyte

4. The pollen grain: The pollen grain has two wings and is carried by the wind to the seed cone during pollination.

pollen tube sperm

5. The female gametophyte: Only one of the megaspores undergoes mitosis and develops into a mature female gametophyte, having two to six archegonia. Each archegonium contains a single large egg lying near the ovule opening.

pollen grain

Figure 30.13  Pine life cycle.  The sporophyte is dominant, and its sporangia are borne in cones. There are two types of cones: pollen cones (microstrobili) and seed cones (megastrobili).



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Seed plants, such as the gymnosperms, are heterosporous, meaning that they have two types of spores and produce a male and female gametophyte during the course of their life cycle (Fig. 30.13). Pollen grains, which resist drying out, become the multicellular male gametophyte. Pollination occurs when a pollen grain is brought to the vicinity of a female gametophyte by wind or, in the case of flowering plants, by either wind or a pollinator. A pollinator is an animal, often an insect, that carries pollen from flower to flower. After a pollen grain germinates, sperm move toward the female gametophyte through a growing pollen tube. External water is not needed to accomplish fertilization. The female gametophyte develops within an ovule, eventually becoming a seed that disperses the offspring.

Gymnosperms Most of the approximately 900 species of gymnosperms are conebearing plants. On the surfaces of their cone scales are ovules, which later become seeds. These ovules are said to be naked because they are not completely enclosed by diploid tissue (as the ovules of angiosperms are). This characteristic gives the gymnosperms their name; gymnos is a Greek word meaning naked, and sperma means seed. Botanists now divide the gymnosperms into four groups: conifers, cycads, ginkgos, and gnetophytes.

a. A northern coniferous forest

pollen cone seed cones

Conifers The better-known gymnosperms are evergreen, cone-bearing trees called conifers (phylum Pinophyta). The 630 species of conifers include the familiar pine, spruce, fir, cedar, hemlock, redwood, and cypress. Perhaps the oldest and largest trees in the world are conifers. Bristlecone pines in the Nevada mountains are known to be more than 4,900 years old, and a number of redwood trees in California are 2,000 years old and more than 100 meters tall.

Adaptations and Uses of Conifers  Conifers are adapted to cold, dry weather. Thus, vast areas of northern temperate regions are covered in coniferous forests (Fig. 30.14a). The tough, needlelike leaves of pines conserve water because they have a thick cuticle and recessed stomata. Note in the life cycle of the pine (see Fig. 30.13) that the dominant sporophyte produces two kinds of cones. Pollen cones (microstrobili) produce windblown pollen, and seed cones (megastrobili) produce windblown seeds (Fig. 30.14b, c). These cones are adaptations to a land environment. Conifers supply much of the wood used to construct buildings and to manufacture paper. They also produce many valuable chemicals, such as those extracted from resin (pitch), a substance that protects conifers from attack by fungi and insects, and is used for waterproofing and in varnish and paints.

Other Gymnosperms

seed cone b. Cones of lodgepole pine, Pinus contorta

c. Fleshy seed cones of juniper, Juniperus

Figure 30.14  Conifers.  a. Conifers are adapted to living in cold climates. b. In pine trees, the seed cones become woody, but the pollen cones do not. c. The seed cones of a juniper have fleshy scales that fuse into a berrylike structure.

weighing 40 kg (Fig. 30.15a). Cycads were very plentiful in the Mesozoic era at the time of the dinosaurs, and it’s likely that dinosaurs fed on cycad seeds. Now, cycads are in danger of extinction because they grow very slowly, a distinct disadvantage. Ginkgos (phylum Ginkgophyta), although plentiful in the fossil record, are represented today by only one surviving species, Ginkgo biloba, the maidenhair tree, so named because its leaves resemble those of the southern maidenhair fern, Adiantum capillusveneris. Female trees produce fleshy seeds, which ripen in the fall, and give off such a foul odor that male trees are usually preferred for planting (Fig. 30.15b). Ginkgo trees are resistant to pollution and do well along city streets and in city parks. The three living genera of gnetophytes don’t resemble one another. Gnetum, which occurs in the tropics, consists of trees or climbing vines with broad, leathery leaves arranged in pairs. Ephedra, occurring only in southwestern North America and Southeast Asia, is a shrub with small, scalelike leaves. Welwitschia, living in the deserts of southwestern Africa, has only two enormous, straplike leaves, which split lengthwise as the plant ages (Fig. 30.15c).

Cycads (phylum Cycadophyta, approximately 100 species) have large, finely divided leaves growing in clusters at the top of the stem. Depending on their height, they resemble palms or ferns. Pollen cones or seed cones, which grow at the top of the stem surrounded by the leaves, can be huge—more than a meter long and

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UNIT 6  Evolution and Diversity

pollen cone a.

seed cone Barbary fig cactus, Opuntia ficus-indica

Water lily, Nymphaea odorata

b. Blue flag iris, Iris versicolor

Amborella, Amborella trichopoda

c.

Figure 30.15  Gymnosperm diversity.  a. Cycad cones. b. The

female ginkgo trees are known for their malodorous seeds. c. Welwitschia is known for its enormous straplike leaves. Snow trillium, Trillium nivale

Apple blossom, Malus domestica

Angiosperms Angiosperms, the flowering plants (phylum Magnoliophyta), are an exceptionally large and successful group of plants. There are over 270,000 known species of angiosperms. The flowers of the angiosperms vary widely in appearance (Fig. 30.16). Angiosperms live in all sorts of habitats, from fresh water to desert, and from the frigid north to the torrid tropics. They range in size from the tiny, almost microscopic duck weed to Eucalyptus trees over 100 m tall. It would be impossible to exaggerate the importance of angiosperms in our everyday lives. As discussed in the Ecology feature, “Plants and Humans,” in section 30.1, they provide us with clothing, food, medicines, and other commercially valuable products. The name angiosperm is derived from the Greek words angio, meaning vessel, and sperma, meaning seed. The seed develops from an ovule within an ovary, which becomes a fruit. Therefore, angiosperms produce covered seeds (in contrast to the exposed seeds of gymnosperms). Although the oldest fossils of angiosperms date back no more than about 135 million years, they are most likely much older. Molecular evidence suggests the possible ancestors of angiosperms may have originated as long as 200 million years ago. Scientists hypothesize that angiosperms coevolved with the insects that act as

Butterfly weed, Asclepias tuberosa

Figure 30.16  Flower diversity.  Angiosperm flowers vary widely in their appearance, but their function is similar.

pollinators. As angiosperms became diverse, flying insects also enjoyed an adaptive radiation, increasing in morphological and ecological diversity.

Monocots and Eudicots Most flowering plants belong to one of two classes. There are about 60,000 known species of Monocotyledones (Liliopsida), often referred to as the monocots. The remainder of the flowering plants belong to the Eudicotyledones, commonly called



Chapter 30  Plants

TABLE 30.1  Monocots and Eudicots Monocots

Eudicots

One cotyledon

Two cotyledons

Flower parts in threes or multiples of three

Flower parts in fours or fives or multiples of four or five

Usually herbaceous

Woody or herbaceous

Usually parallel venation

Usually net venation

Scattered bundles in stem

Vascular bundles in a ring

Fibrous root system

Taproot system

Pollen grain with one pore

Pollen grain with three pores

the eudicots. Table 30.1 lists the differences between monocots and eudicots. Monocots are named because they have only one cotyledon in their seeds, whereas eudicots have two cotyledons. Cotyledons are the seed leaves—they contain nutrients that nourish the developing embryo.

The Flower The flowers of angiosperms have certain structures in common (Fig. 30.17). The flower stalk expands slightly at the tip into a

anther

stigma

filament

style

pollen tube

ovary

stamens

ovule

carpel (pistil)

receptacle

petals (corolla)

sepals (calyx)

Figure 30.17  Generalized flower.  A flower has four main parts:

sepals, petals, stamens, and carpels. Each stamen has an anther and a filament. A carpel has a stigma, a style, and an ovary. The ovary contains ovules.

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receptacle, which bears the other flower parts. These parts, called sepals, petals, stamens, and carpels, are attached to the receptacle in whorls (circles). The sepals, collectively called the calyx, protect the flower bud before it opens. The sepals may drop off or may be colored like the petals. Usually, however, sepals are green and remain attached to the receptacle. The petals, collectively called the corolla, are quite diverse in size, shape, and color. The corolla often attracts a particular pollinator. Each stamen consists of a saclike anther, where pollen is produced, and a stalk called a filament. In most flowers, the anther is positioned where the pollen can be carried away by wind or a pollinator. One or more carpels is at the center of a flower. A carpel has three major regions: ovary, style, and stigma. The combination of the ovary, style, stigma, and carpels is sometimes called a pistil. The swollen base is the ovary, which contains from one to hundreds of ovules. The style elevates the stigma, which is adapted to receive pollen grains. Glands in the region of the ovary produce nectar, a nutrient that pollinators gather as they go from flower to flower. The flower contributes to the success of the angiosperms.

Life Cycle, Adaptations, and Uses of Flowering Plants  Figure 30.18 depicts the life cycle of a typical flowering plant. Sexual reproduction in flowering plants is dependent on the flower, which produces both pollen and seeds. A gymnosperm, such as a pine tree, depends wholly on the wind to disperse both pollen and seeds. In contrast, among angiosperms, some species have windblown pollen, whereas others rely on a pollinator to carry pollen to another member of the same species. Bees, wasps, flies, butterflies, moths, beetles, and even bats can act as pollinators. The flower provides the pollinator with nectar, a sugary substance that, in the case of bees, will become honey in the hive. Several species of bees (such as honeybees and bumblebees) also have two “pollen baskets,” one on each of their hind legs. When bees make a brief stop on a flower, they suck up nectar with specialized mouthparts and gather pollen grains, which are stored in their pollen baskets. Honey and pollen are the main diets of immature bees (called larvae). When a bee visits another flower, some of the pollen grains are rubbed off, and in this way the bee delivers pollen from one flower to another. Other types of pollinators also carry pollen between flowers of the same species, because ­pollinators and flowers are adapted to one another. Flowering plants that have windblown pollen are usually not very showy, whereas insect-pollinated flowers and bird-pollinated flowers are often colorful. Many flowers are adapted to attract specific pollinators. For example, bee-pollinated flowers are usually blue or yellow and have ultraviolet shadings that lead the pollinator to the location of nectar at the base of the flower. Night-blooming flowers are usually aromatic and white or creamcolored, so that their smell alone, rather than color, can attract nocturnal pollinators such as bats. Fruits, the final product of a flower, aid in the dispersal of seeds (Fig. 30.19). The production of fruit is another advantage angiosperms have over gymnosperms. Dry fruits, such as pods, assist the dispersal of windblown seeds. Mature pods sometimes



620

UNIT 6  Evolution and Diversity Stamen

Carpel stigma

anther

style

filament

ovary ovule 7. The sporophyte: The embryo within a seed is the immature sporophyte. When a seed germinates, growth and differentiation produce the mature sporophyte of a flowering plant.

Mitosis

fruit (mature ovary)

Sporophyte 1. The stamen: An anther at the top of a stamen has four pollen sacs. Pollen grains are produced in pollen sacs.

seed (mature ovule) 6. The seed: The ovule now develops into the seed, which contains an embryo and food enclosed by a protective seed coat. The wall of the ovary and sometimes adjacent parts develop into a fruit that surrounds the seed(s).

The carpel: The ovary at the base of a carpel contains one or more ovules. The contents of an ovule change during the flowering plant life cycle.

seed coat embryo endosperm (3n) Seed diploid (2n)

FERTILIZATION

haploid (n)

pollen grain

(mature male gametophyte) 5. Double fertilization: On reaching the ovule, the pollen tube discharges the sperm. One of the two sperm migrates to and fertilizes the egg, forming a zygote; the other unites with the two polar nuclei, producing a 3n (triploid) endosperm nucleus. The endosperm nucleus divides to form endosperm, food for the developing plant.

tube cell nucleus

Pollination integument polar nuclei sperm

generative cell

pollen tube sperm

antipodals

egg

polar nuclei

pollen tube

egg synergids Embryo sac (mature female gametophyte)

4. The mature male gametophyte: A pollen grain that lands on the carpel of the same type of plant germinates and produces a pollen tube, which grows within the style until it reaches an ovule in the ovary. Inside the pollen tube, the generative cell nucleus divides and produces two nonflagellated sperm. A fully germinated pollen grain is the mature male gametophyte.

The mature female gametophyte: The ovule now contains the mature female gametophyte (embryo sac), which typically consists of eight haploid nuclei embedded in a mass of cytoplasm. The cytoplasm differentiates into cells, one of which is an egg and another of which contains two polar nuclei.

Figure 30.18  Flowering plant life cycle.  The parts of the flower involved in reproduction are the stamens and the carpel. Reproduction has been divided into significant stages: female gametophyte development, male gametophyte development, and sporophyte development.

explode and shoot out the seeds. Other pods, such as those of peas and beans, simply break open and scatter the seeds. The infamous dandelion produces a one-seeded fruit with an umbrella-like crown of intricately branched hairs. The slightest gust of wind catches the hairs, raising and propelling the fruit into the air like a parachute.

Fruits also help disperse angiosperm seeds in two other ways. One means is exemplified by the coconut, a fruit that can drift for thousands of kilometers across seas and oceans. A second method of seed dispersal occurs because the seeds of some fruits have a fleshy covering that animals eat as a source of food.



Chapter 30  Plants

2. The microsporangia: Pollen sacs of the anther are microsporangia, where each microsporocyte undergoes meiosis to produce four microspores.

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The megasporangium: First, an ovule within an ovary contains a megasporangium, where a megasporocyte undergoes meiosis to produce four megaspores. a.

b.

c.

d.

e.

f.

stigma

style

Anther: Ovule: (megasporangium)

ovary integument

pollen sac (microsporangium)

microsporocyte megasporocyte

MEIOSIS

s

osi Mit

Microspores

MEIOSIS

Figure 30.19  Fruits.  Angiosperms have diverse fruits. a. A tomato is technically a berry. b. A strawberry is an aggregate fruit formed from many carpels within a single flower. c. Apples have seeds inside a fleshy fruit. d. A corn cob carries many fruits (kernels). e. Peas have seeds in pods. f. Dandelion fruits are carried by wind. Check Your Progress  30.4

Megaspores

1. Recognize the two types of spores that occur in the seed

s

osi Mit

degenerating megaspores

plant life cycle, and describe where they develop and what role they play in the life cycle. 2. List the features of the flowering plant life cycle that are not found in any other group of plants. 3. Distinguish between a monocot and a eudicot plant.

Ovule

3. Microspores: Each microspore in a pollen sac undergoes mitosis to become an immature pollen grain with two cells: the tube cell and the generative cell. The pollen sacs open, and the pollen grains are windblown or carried by an animal carrier, usually to other flowers. This is pollination. Megaspores: Inside the ovule of an ovary, three megaspores disintegrate, and only the remaining one undergoes mitosis to become a female gametophyte.

Conclusion While we may consider coal to be an important resource for our modern societies, in fact it is part of an evolutionary history of one of the most important groups of life on the planet—the plants. Over millions of years, plants—like other organisms— have evolved unique features that provided them an advantage over the previous groups.  Coal is a reminder that plants dominated the Earth’s surface long before the appearance of humans, or even most animals we would recognize today. When coal deposits were forming over 300 million years ago, the reptiles were just beginning their age of dominance, although it would still be tens of millions of years before the dinosaurs walked the Earth. Coal is just one of the many benefits that plants provide for us. Plants, and particularly angiosperms, provide us important products, such as food, clothing, and medicines. Land plants also provide shelter for many animals, oxygen to the air, and help buffer noise in cities. Without plants, the majority of life on the planet would quickly become extinct.

Only the fleshy part is digested, so the stones or seeds pass through the animal’s digestive system and are deposited perhaps far away from the parent plant. Still other fruits, like those of the cocklebur, have seeds with hooks that catch on the fur of animals and are carried away.

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UNIT 6  Evolution and Diversity

MEDIA STUDY TOOLS http://connect.mheducation.com Maximize your study time with McGraw-Hill SmartBook®, the first ­adaptive textbook.

  Tutorials 30.1  Alternation of Generations 30.4  Angiosperm Life Cycle

SUMMARIZE 30.1  Evolutionary History of Plants ■ Plants are multicellular, photosynthetic organisms adapted to a land

existence that probably evolved from a multicellular, freshwater green alga about 500 mya. ■ Five significant events associated with plant adaptation to a land existence are (1) embryo protection, (2) vascular tissue, (3) megaphylls, (4) seeds, and (5) the flower. ■ The life cycle of plants is characterized by alternation of generations— some have a dominant gametophyte (haploid) generation and others have a dominant sporophyte (diploid) generation. ■ Land plants are divided into nonvascular plants, seedless vascular plants, and seed plants based on presence/absence of vascular tissue and their dispersal mechanism.

30.2  Nonvascular Plants ■ The bryophytes are the group of nonvascular plants that include the

liverworts and mosses. They lack vascular tissue—and therefore, they do not have true roots, stems, or leaves. ■ The gametophyte is dominant. Antheridia produce swimming sperm that use external water to reach eggs in the archegonia. Following fertilization, the dependent moss sporophyte consists of a foot, a stalk, and a capsule within which windblown spores are produced by meiosis. ■ Use of windblown spores to disperse the gametophyte is nonvascular plants’ chief adaptation for reproducing on land. Each spore germinates to produce a gametophyte.

gametophyte produces flagellated sperm within antheridia and eggs within archegonia. ■ Vegetative (asexual) reproduction is sometimes used to disperse ferns in dry habitats.

30.4  Seed Plants ■ Seed plants have a life cycle that is fully adapted to existence on land.

The spores develop inside the body of the sporophyte and are of two types. The male gametophyte is the pollen grain, which produces nonflagellated sperm. Pollination occurs when the pollen grain is transferred to the egg by the wind or carried by an animal (pollinator). The sperm use a pollen tube in order to reach the egg. The egg is located within an ovule and produced by the female gametophyte. ■ Gymnosperms, which include the conifers, cycads, ginkgos, and gnetophytes, produce seeds that are uncovered (naked). Male gametophytes develop in pollen cones. The female gametophyte develops within an ovule located on the scales of seed cones. Following pollination and fertilization, the ovule becomes a winged seed dispersed by wind. Seeds contain the next sporophyte generation. ■ Angiosperms, also called the flowering plants, produce seeds covered by fruits. The petals of flowers attract pollinators, and the ovary develops into a fruit, which aids seed dispersal. Monocotyledones (monocots) and Eudicotyledones (eudicots) are the two main classes of flowering plants. A flower consists of a corolla (petals), calyx (sepals), stamens (anther, filament), and carpel (stigma, style, ovary, and ovule). Angiosperms provide most of the food that sustains terrestrial animals, and they are the source of many products used by humans.

30.3  Seedless Vascular Plants ■ Vascular plants have vascular tissue—xylem and phloem. Xylem

transports water and minerals and also supports plants against the pull of gravity. Phloem transports organic nutrients from one part of the plant to another. ■ Lycophytes are often called club mosses. They contain true roots, stems, and leaves because they possess vascular tissue. Lycophytes have narrow leaves called microphylls. ■ The pteridophytes include ferns, whisk ferns, and horsetails. They have larger leaves, called megaphylls, which have branched vascular tissue.  ■ The sporophyte is dominant; in the life cycle of seedless vascular plants, windborne spores disperse the gametophyte, and the separate

ASSESS Testing Yourself Choose the best answer for each question.

30.1  Evolutionary History of Plants 1. The spores produced by a plant are a. haploid and genetically different from each other. b. haploid and genetically identical to each other. c. diploid and genetically different from each other. d. diploid and genetically identical to each other.



Chapter 30  Plants

2. The gametophyte is the dominant generation in a. ferns. c. mosses. b. gymnosperms. d. angiosperms. 3. Label the stages in the following diagram of the plant life cycle.

a.

d.

diploid (2n) haploid (n)

b.

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10. Which of these pairs is mismatched? a. anther—produces microspores b. carpel—produces pollen c. ovule—becomes seed d. ovary—becomes fruit e. flower—is a reproductive structure 11. Unlike eudicots, monocots have a. woody tissue. b. two seed leaves. c. scattered vascular bundles in their stems. d. flower parts in multiples of four or five. 12. Label the parts of the flower in the following diagram.

c. a.

30.2  Nonvascular Plants 4. In mosses, meiosis occurs in a. antheridia. b. archegonia. c. capsules. d. protonema. 5. In bryophytes, sperm usually move from the antheridium to the archegonium by a. swimming. b. flying. c. insect pollination. d. wind pollination. e. bird pollination.

30.3  Seedless Vascular Plants 6. Microphylls a. have a single strand of vascular tissue. b. evolved before megaphylls. c. evolved as extensions of the stem. d. are found in lycophytes. e. All of these are correct. 7. How are ferns different from mosses? a. Only ferns produce spores as dispersal agents. b. Ferns have vascular tissue. c. In the fern life cycle, the gametophyte and sporophyte are both independent. d. Ferns do not have flagellated sperm. e. Both b and c are correct. 8. Ferns have a. a dominant gametophyte generation. b. vascular tissue. c. seeds. d. Both a and b are correct. e. Choices a, b, and c are correct.

30.4  Seed Plants

b.

d.

c.

e.

ENGAGE Thinking Critically 1. What characteristics of angiosperms have allowed this group of plants to dominate Earth? 2. Compare and contrast the plant alternation-of-generations life cycle with the human life cycle.

PHOTO CREDITS Opener: © Monty Rakusen/Getty RF; 30A(wheat): © Pixtal/age fotostock; 30A(cornfield, rice plants): © Corbis RF; 30A(corn ear): © Nigel Cattlin/Science Source; 30A(rice grain heads): © Dex Image/Getty RF; 30B(rubber): © moodboard/Corbis RF; 30B(cotton): © Pixtal/Age fotostock RF; 30.1(Chara): © Heather Angel/Natural Visions; 30.1(Coleochaete): (c) Chloe Shaut/Ricochet Creative Productions LLC; 30.5a: © Ed Reschke; 30.5b: © J.M. Conrader/Nat’l Audubon Society/Science Source; 30.5c–30.6 (sporophyte): © Ed Reschke/Getty Images; 30.6(gametophyte): © Steven P. Lynch; 30.7: © Richard Bizley/Science Source; 30.8: © Steve Solum/Photoshot; 30.9(cinnamon): © Laszlo Podor Photography/Getty RF; 30.9(hart’s tongue): © Organics image library/Alamy RF; 30.10: © Steven P. Lynch; 30.11: © Carolina Biological Supply Company/Phototake; 30.12: © Robert P. Carr/Photoshot; 30.13(pollen): © Ed Reschke/Photolibrary/Getty Images; 30.14a: © Pete Ryan/National Geographic/Getty RF; 30.14b: © Kathy Merrifield/Science Source; 30.14c: © Evelyn Jo Johnson; 30.15a(both): © Steven P. Lynch; 30.15b: © Kathy Merrifield/ Science Source; 30.15c:Courtesy Fiona Norris; 30.16(cactus): © Author’s Image/PunchStock RF; 30.16(waterlily): © Dave Moyer; 30.16(iris): © Steffen Hauser/botanikfoto/Alamy; 30.16(Amborella): © Stephen McCabe/Arboretum at University of California Santa Cruz; 30.16(trillium): © Adam Jones/Science Source; 30.16(apple blossoms): © Inga Spence/ Science Source; 30.16(butterfly weed): © Evelyn Jo Johnson; 30.18(pollen grain): © Graham Kent; 30.18(embryo sac): © Ed Reschke; 30.19a: © Stockbyte/PunchStock; 30.19b–c: © Mondae Leigh Baker; 30.19d: © Photolink/Getty RF; 30.19e: © Corbis RF; 30.19f: © Brand X Pictures RF.

9. Which statement about the conifer life cycle is false? a. Meiosis produces spores. b. The gametophyte is the dominant generation. c. The seed is a dispersal agent. d. Male gametophytes are carried by the wind.



31

Animals: The Invertebrates CHAPTER OUTLINE 31.1  Evolutionary Trends Among Animals 31.2 The Simplest Invertebrates 31.3 The Lophotrochozoa 31.4 The Ecdysozoa 31.5 Invertebrate Deuterostomes BEFORE YOU BEGIN Before beginning this chapter, take a few moments to review the following discussions: Section 1.2  What is the basis for classifying an organism as a member of kingdom Animalia? Section 3.3  Which cellular features are unique to animals? Table 27.1  When did animals first appear in the tree of life?

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CASE STUDY Neglected Tropical Diseases A fad diet called the “tapeworm diet” proposes that people intentionally ingest human tapeworms, a parasitic invertebrate, to decrease their body’s absorption of nutrients, thus promoting weight loss. The side effects of tapeworm infection, however, are potentially severe. In underdeveloped nations the World Health Organization (WHO) lists cysticercosis, an advanced tapeworm infection, as a serious neglected tropical disease (NTD) that threatens more than 50 million people worldwide. Cysticercosis causes severe malnutrition in growing children, as well as anemia and organ damage in children and adults. This fad diet has been banned by the FDA in the United States.  Public education about the risks of parasite infection is our greatest defense against such a potentially life-threatening pursuit of an ideal body type. The invertebrates—animals that lack a backbone—are far more diverse and numerous than the vertebrates. Many invertebrates enhance the quality of our lives. For example, various types of insects act as essential pollinators of many food crops. However, not all invertebrates are beneficial. Mosquitoes can be vectors of malaria whereas some parasitic worms are responsible for other NTDs such as elephantiasis and schistosomiasis. In this chapter, you will read about the evolution and diversity of the major groups of invertebrates as well as the traits that have enabled them to successfully adapt to a wide range of habitats. As you read through the chapter, think about the following questions:

1. Where do invertebrates fit into the evolutionary history of animals? 2. What evolutionary advantages are there for invertebrates to have a variety of developmental stages, body plans, and lifestyles?



Chapter 31  Animals: The Invertebrates

31.1  Evolutionary Trends Among Animals

■■ ■■

Learning Outcomes Upon completion of this section, you should be able to 1. List the general characteristics of animals. 2. Compare and contrast the types of symmetry found in animals. 3. Describe the differences between protostomes and deuterostomes.

The traditional five-kingdom classification placed animals in the kingdom Animalia. The modern three-domain system places animals in the domain Eukarya. Within the Eukarya, they are placed in the supergroup Opisthokonta along with fungi and certain protistans, notably the choanoflagellates (see Chapter 29). While animals are extremely diverse, they share some important differences from the other multicellular eukaryotes, the plants and fungi. Unlike plants, which make their food through photosynthesis, animals are heterotrophs, and must acquire nutrients from an external source. Unlike fungi, which digest their food externally and absorb the breakdown products, animals ingest (eat) whole food and digest it internally. In general, animals share the following characteristics (Fig. 31.1): ■■ ■■

Typically have the power of movement or locomotion by means of muscle fibers Multicellular; most have specialized cells that form tissues and organs

■■

625

Have a life cycle in which the adult is typically diploid Usually undergo sexual reproduction and produce an embryo that goes through developmental stages Heterotrophic; usually acquire food by ingestion followed by digestion

The animal kingdom is currently divided into between 35 and 40 groups, or phyla. The more common of these are presented in Table 31.1. Animals are recognized to have evolved from a protistan ancestor approximately 600 million years ago.  In this chapter, we will explore the phyla of animals called invertebrates. Invertebrates lack an internal skeleton, or endoskeleton, of bone or cartilage. The invertebrates evolved first, and far outnumber the vertebrates, which are the animals with an endoskeleton. Since the fossil record is not always complete (see Chapter 27), most evolutionary trees (Figure 31.2) are based on molecular (DNA) comparisons. For molecular comparisons, it is assumed that the more closely related two organisms are, the more nucleotide sequences in their DNA they will have in common (see section 26.4).

Anatomical Data There are a number of anatomical criteria that may be used to classify animals (Fig. 31.2). In this section, we will focus on the general characteristics of symmetry and embryonic development.

Type of Symmetry Animals can be asymmetrical, radially symmetrical, or bilaterally symmetrical. Asymmetrical animals have no particular

a.

b.

c.

d.

Figure 31.1  General features of animals.  a. Animals are multicellular, with specialized cells that form tissues and organs. b. Animals are heterotrophs, meaning they obtain nutrition from external sources. c. Animals are typically motile, due to their well-developed nervous and muscular systems. d. Most animals reproduce sexually, beginning life as a 2n zygote, which undergoes development to produce a multicellular organism that has specialized tissues. 



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UNIT 6  Evolution and Diversity

TABLE 31.1  Animal Characteristics DOMAIN: Eukarya KINGDOM: Animalia CHARACTERISTICS Multicellular, usually with specialized tissues; ingest or absorb food; diploid life cycle INVERTEBRATES Sponges (bony, glass, spongin): Asymmetrical, saclike body perforated by pores; internal cavity lined by choanocytes; spicules serve as internal skeleton. 5,150 Radiata Comb jellies: Possess eight longitudinal rows of cilia (combs) that aid in locomotion; lack nematocysts. 150 Cnidarians (hydra, jellyfish, corals, sea anemones): Radially symmetrical with two tissue layers; incomplete digestive tract; tentacles with nematocysts. 10,000 Protostomia (Lophotrochozoa) Lophophorates (bryozoans): Filter feeders with a mouth surrounded by a ciliated tentacle-like structure. 5,000 Flatworms (planarians, tapeworms, flukes): Bilateral symmetry with cephalization; three tissue layers and organ systems; acoelomate with incomplete digestive tract that can be lost in parasites; hermaphroditic. 20,000 Rotifers (wheel animals): Microscopic animals with a corona (crown of cilia) that looks like a spinning wheel when in motion. 2,200 Molluscs (chitons, clams, snails, squids): Coelom, all have a foot, mantle, and visceral mass; foot is variously modified; in many, the mantle secretes a calcium carbonate shell as an exoskeleton; true coelom and all organ systems. 110,000 Annelids (polychaetes, earthworms, leeches): Segmented with body rings and setae; cephalization in some polychaetes; hydroskeleton; closed circulatory system. 16,000 Protostomia (Ecdysozoa) Roundworms (roundworms, pinworms, hookworms, filarial worms): Pseudocoelom and hydroskeleton; complete digestive tract; free-living forms in soil and water; parasites common. 25,000 Arthropods (crustaceans, spiders, scorpions, centipedes, millipedes, insects): Chitinous exoskeleton with jointed appendages undergoes molting; insects—most have wings—are most numerous of all animals. 1,000,000 Deuterostomes Echinoderms (sea stars, sea urchins, sand dollars, sea cucumbers): Radial symmetry as adults; unique water-vascular system and tube feet; endoskeleton of calcium plates. 7,000 Chordates (tunicates, lancelets, vertebrates): All have notochord, dorsal tubular nerve cord, pharyngeal pouches, and postanal tail at some time; contains mostly vertebrates in which notochord is replaced by vertebral column. 56,000 VERTEBRATES Fishes (jawless, cartilaginous, bony): Endoskeleton, jaws, and paired appendages in most; internal gills; single-loop circulation; usually scales. 28,000 Amphibians (frogs, toads, salamanders): Most have jointed limbs; lungs; three-chambered heart with double-loop circulation; moist, thin skin. 6,900 Reptiles (snakes, turtles, crocodiles): Amniotic egg; rib cage in addition to lungs; three- or four-chambered heart typical; scaly, dry skin; copulatory organ in males and internal fertilization. 8,000 Birds (songbirds, waterfowl, parrots, ostriches): Endothermy, feathers, and skeletal modifications for flying; lungs with air sacs; four-chambered heart. 10,000 Mammals (monotremes, marsupials, placental): Hair and mammary glands. 4,800 The number of species for each group is based on current estimates

symmetry, such as some species of sponges. Radial symmetry means that the animal is organized circularly, similar to a wheel, so that no matter how the animal is sliced longitudinally, mirror images are obtained. Bilateral symmetry means that the animal has definite right and left halves; only a longitudinal cut down the center of the animal will produce a mirror image. dorsal

ventral radial symmetry

posterior anterior

bilateral symmetry

Radially symmetrical animals have the advantage of being able to reach out in all directions from one center. Radially symmetrical animals may be sessile, which means that it is permanently attached to a substrate, or free floating, such as jellyfish. Bilaterally symmetrical animals tend to be more active, with movement directed toward an anterior end (head). During the evolution of animals, bilateral symmetry is accompanied by ­ ­cephalization, localization of a brain and specialized sensory organs at the anterior end.

Embryonic Development Sponges are multicellular animals, but unlike other animals, they do not have true tissues. Therefore, sponges have the cellular level of



627

Chapter 31  Animals: The Invertebrates

segmentation deuterostome development

bilateral symmetry 3 tissue layers body cavity

Deuterostomia

Chordates

Echinoderms

segmentation molting of cuticle

Arthropods

tissue layers

Roundworms

Ecdysozoa

common ancestor

Bilateria

segmentation

Flatworms

protostome development

Eumetazoa

choanoflagellate ancestor

Lophotrochozoa

Molluscs

trochophore

multicellularity

Protostomia

Annelids

Rotifers

“Lophophorans” lophophore

ctenophore

Radiata

Ctenophores

Cnidarians

Sponges

Parazoa

radial symmetry 2 tissue layers

Figure 31.2  Phylogenetic tree of the major animal phyla.  All animal phyla living today are most likely descended from an ancestral protist living about 600 million years ago. This evolutionary tree is based on molecular data (nucleotide sequences) and anatomical data used to indicate which phyla are most closely related to one another.



UNIT 6  Evolution and Diversity

1. Cleavage, the first event of development, is cell division without cell growth. In protostomes, spiral cleavage occurs, and daughter cells sit in grooves formed by the previous cleavages. The fate of these cells is fixed and determinate in protostomes; each can contribute to development in only one particular way. In deuterostomes, radial cleavage occurs, and the daughter cells sit right on top of the previous cells. The fate of these cells is indeterminate—that is, if they are separated from one another, each cell can go on to become a complete organism. 2. As development proceeds, a hollow sphere of cells, or blastula, forms and the indentation that follows develops into an opening called the blastopore. In protostomes, the mouth appears at or near the blastopore, hence the origin of their name. In deuterostomes, the anus appears at or near the blastopore, and only later does a second opening form the mouth, hence the origin of their name. 3. Certain members of the protostomes and all deuterostomes have a body cavity completely lined by mesoderm, called a true coelom. However, a true coelom develops differently in the two groups. In protostomes, the mesoderm arises from cells located near the embryonic blastopore, and a splitting occurs that produces the coelom. In deuterostomes, the coelom arises as a pair of mesodermal pouches from the wall of the primitive gut. The pouches enlarge until they meet and fuse. The deuterostomes include the echinoderms and the chordates, two groups of animals that will be examined in detail in Chapter 32. The protostomes are divided into two groups: the ecdysozoa and the lophotrochozoa. The ecdysozoans include the roundworms and arthropods. Both of these types of animals molt; they shed their outer covering as they grow. Ecdysozoa means molting animals. The lophotrochozoa contain the lophophorans, which have a mouth surrounded by ciliated tentacle-like structures, and the trochozoans, which either have presently, or their ancestors had, a trochophore larva.

Check Your Progress  31.1 1. Distinguish an animal from a plant or fungi. 2. List the general characteristics that animals have in common. 3. Using Figure 31.2, identify the general characteristics of a mollusc.

Deuterostomes

Cleavage

Protostomes

top view

side view

top view

side view

Cleavage is spiral and determinate.

Cleavage is radial and indeterminate.

Protostomes

Deuterostomes

blastopore

mouth

blastopore

anus

primitive gut

anus

primitive gut

mouth

Fate of blastopore

organization. Because of this, sponges are often classified as a separate group, called the parazoans (Fig. 31.2), from the other animals.  True tissues appear in the other animals, the eumetazoans, as they undergo embryological development. The first three tissue layers are often called germ layers because they give rise to the organs and organ systems of complex animals. Animals such as the cnidarians, are diploblastic, meaning that they have only two tissue layers (ectoderm and endoderm) as embryos. These animals display the tissue level of organization (see Fig. 1.2). Those animals that develop further and have all three tissue layers (ectoderm, mesoderm, and endoderm) as embryos are triploblastic and have the organ level of organization. Notice in the phylogenetic tree that the animals with three tissue layers are either protostomes (proto is Greek for first; stoma is Greek for mouth) or deuterostomes (deuter is Greek for second). Figure 31.3 shows that protostome and deuterostome development are differentiated by three major events:

Blastopore becomes mouth.

Blastopore becomes the anus.

Protostomes

Deuterostomes

ectoderm

Coelom formation

628

endoderm

gut

ectoderm

mesoderm

ectoderm

Coelom forms by a splitting of the mesoderm.

endoderm

gut

mesoderm

ectoderm

Coelom forms by an outpocketing of primitive gut.

Figure 31.3  Protostomes compared to deuterostomes. 

Left: In the embryo of protostomes, cleavage is spiral—new cells are at an angle to old cells—and each cell has limited potential and cannot develop into a complete embryo; the blastopore is associated with the mouth; and the coelom, if present, develops by a splitting of the mesoderm. Right: In deuterostomes, cleavage is radial—new cells sit on top of old cells—and each one can develop into a complete embryo; the blastopore is associated with the anus; and the coelom, if present, develops by an outpocketing of the primitive gut.



Chapter 31  Animals: The Invertebrates

31.2  The Simplest Invertebrates Learning Outcomes Upon completion of this section, you should be able to 1. Describe the anatomy and life cycle of sponges and cnidarians. 2. Contrast the way of life of a sponge with that of a cnidarian. 3. Identify the basic morphology of a hydra.

Porifera The organisms in the phylum Porifera have saclike bodies are perforated by many pores (Fig. 31.4). The sponges are an excellent example of a poriferan. Sponges are aquatic, largely marine animals that vary greatly in size, shape, and color. Sponges are multicellular, although they have few cell types and no nerve or muscle cells or organized tissues. However, despite their apparent simplicity, the molecular data supports the fact that they are at the base of the evolutionary tree of animals.  In sponges, the outer layer of the body wall contains flattened epidermal cells, some of which have contractile fibers; the middle layer is a semifluid matrix with wandering amoeboid cells; and the inner layer is composed of flagellated cells called collar cells, or choanocytes (Fig. 31.4). The beating of the flagella produces water currents that flow through the pores into the central cavity and out through the osculum, the upper opening of the body. Even a simple sponge only 10 cm tall is estimated to filter as much as 100 liters

629

of water each day. It takes this much water to supply the needs of the sponge. A sponge is a sessile filter feeder, an organism that filters its food from the water by means of a straining device—in this case, the pores of the walls and the microvilli making up the collar of collar cells. Microscopic food particles that pass between the microvilli are engulfed by the collar cells and digested by them in food vacuoles or are passed to the amoeboid cells for digestion. The amoeboid cells also act as a circulatory device to transport nutrients from cell to cell, and they produce the sex cells (the egg and the sperm) and spicules. Sponges can reproduce both asexually and sexually. They reproduce asexually by fragmentation followed by regeneration, gemmule formation, or budding. A gemmule is like a spore and is resistant to drying out, freezing, or the lack of oxygen. Gemmule formation occurs in freshwater sponges, and when conditions are favorable, a gemmule gives rise to an adult sponge. During sexual reproduction, sperm are released through the osculum and drawn into other sponges through the pores, where they fertilize eggs within the body. Most sponges are hermaphroditic, meaning they possess both male and female sex organs. However, they usually do not self-fertilize. After fertilization, the zygote develops into a flagellated larva that may swim to a new location. Sponges are capable of regeneration—if the cells of a sponge are separated, they are capable of reassembling and regenerating into a complete and functioning organism! Sponges are classified on the basis of their skeleton. Some sponges have an internal skeleton composed of spicules (Fig. 31.4), small needle-shaped structures with one to six rays. Chalk sponges have spicules made of calcium carbonate; glass sponges have

osculum sponge wall

H2O out

spicule

pore

H2O in through pores

amoebocyte epidermal cell

central cavity

c.

collar

nucleus amoebocyte

flagellum

a. Yellow tube sponge, Aplysina fistularis

b. Sponge organization

d.

collar cell (choanocyte)

Figure 31.4  Simple sponge anatomy.  a. A simple sponge. b. The wall of a sponge contains two layers of cells, the outer epidermal cells and the inner collar cells as shown in (c). The collar cells (d, enlarged) have flagella that beat, moving the water through pores as indicated by the blue arrows in (b). Food particles in the water are trapped by the collar cells and digested within their food vacuoles. Amoebocytes transport nutrients from cell to cell. Spicules form an internal skeleton in some sponges.



630

UNIT 6  Evolution and Diversity

SCIENCE IN YOUR LIFE  ►

ECOLOGY

Destruction of the Coral Reefs Coral reefs are among the most biologically diverse and productive communities on Earth. Coral reefs tend to be found in warm, clear, and shallow tropical waters worldwide and are typically formed by reef-building corals, which are cnidarians. Aside from being beautiful and giving shelter to many colorful species of fishes, coral reefs help generate economic income from tourism, protect ocean shores from erosion, and may serve as the source of medicines derived from antimicrobial compounds that reef-dwelling organisms produce.  However, coral reefs around the globe are Figure 31A  Coral bleaching. being destroyed for a variety of reasons, most of them linked to human development. Deforesta- suggests that climate change is one of the faction, for example, causes tons of soil to settle on tors that is leading to coral bleaching and death the top of coral reefs. This sediment prevents because corals can tolerate only a narrow range photosynthesis of symbiotic algae that provide of temperatures. As global temperatures rise, so food for the corals. When the algae die, so do do water temperatures, and corals can die as a the corals, which then turn white. This is called result. Global warming also contributes to coral “bleaching,” and it has been seen in the favorable conditions for various pathogens that Pacific Ocean and the Caribbean Sea. Evidence can kill corals, such as those similar to

spicules that contain silica. Most sponges have a skeleton composed of fibers of spongin, a soft protein. A bath sponge is the dried spongin skeleton. Today, however, commercial “sponges” are usually synthetic. Note that the popular “loofah” is not really a sponge; it is actually a spongy plant related to cucumbers.

Figure 31.5  Comb jellies. 

rows of cilia

a. Pleurobrachia pileus; b. Mnemiopsis leidyi

tentacle a.

tentacles b.

pathogens that cause cholera in humans. Increases in aquatic nutrients from fertilizers that wash into the ocean also make corals more susceptible to diseases, which can also kill them.  Scientists estimate that 90% of coral reefs in the Philippines are dead or deteriorating due to human activities such as pollution and, especially, overfishing. Fishing methods that employ dynamite or cyanide to kill or stun the fish for food or the pet trade can easily kill corals. Paleobiologist Jeremy Jackson of the Smithsonian Tropical Research Institute in Panama estimates that we may lose 60% of the world’s coral reefs by the year 2050.

Questions to Consider 1. What features of coral reefs help explain why they are so biologically diverse? 2. Considering what is causing the loss of coral reefs, is it possible to save them? How?

Comb Jellies and Cnidarians These two groups of animals have true tissues and, as embryos, they have ectoderm and endoderm as their germ layers. They are radially symmetrical as adults, which offers the advantage of being able to reach out in all directions for food.

Comb Jellies Comb jellies (phylum Ctenophora) are solitary, free-swimming marine invertebrates that are found primarily in warm waters. Ctenophores propel themselves along by beating their cilia (Fig. 31.5) and can range in size from a few centimeters to 1.5 meters (m) in length. Their body is made up of a transparent jellylike substance called mesoglea. Most ctenophores capture their prey by using sticky adhesive cells called colloblasts. Some ctenophores are bioluminescent, enabling them to produce their own light.

Cnidarians Cnidarians (phylum Cnidaria) are tubular or bell-shaped animals that reside mainly in shallow coastal waters. However, there are some freshwater, brackish, and oceanic forms. The term cnidaria is derived from the presence of specialized stinging cells called ­cnidocytes (see Fig. 31.7). Each cnidocyte has a toxin-filled ­capsule called a nematocyst that contains a long, spirally coiled hollow thread. When the trigger of the cnidocyte is touched, the nematocyst is discharged. Some threads merely trap prey, and ­others have spines that penetrate and inject paralyzing venom.



The body of a cnidarian is a two-layered sac. The outer tissue layer is a protective epidermis derived from ectoderm. The inner tissue layer, which is derived from endoderm, secretes digestive juices into the internal cavity, called the gastrovascular cavity, which is involved in the digestion of food and circulation of nutrients. The fluid-filled gastrovascular cavity also serves as a supportive hydrostatic skeleton. This type of skeleton offers some resistance to the contraction of muscle but permits flexibility. The two tissue layers are separated by mesoglea. Two basic body forms are seen among cnidarians (Fig. 31.6a). The mouth of a polyp is directed upward, while the mouth of a jellyfish, or medusa, is directed downward. The bell-shaped medusa has more mesoglea than a polyp, and the tentacles are concentrated on the margin of the bell. At one time, both body forms may have been a part of the life cycle of all cnidarians. When both are present, the animal is dimorphic: the sessile polyp stage produces a medusa by asexual budding, and the motile medusa stage produces egg and sperm. In some cnidarians, one stage is dominant and the other is reduced; in other species, one form is absent altogether.

Cnidarian Diversity Cnidarians are quite diverse (Fig. 31.6b–e). Sea anemones (Fig.  31.6b) are sessile polyps that live attached to a submerged substrate. Most sea anemones range in size from 0.5–20 cm in length and 0.5–10 cm in diameter and are often colorful. Their upward-turned oral disk that contains the mouth is surrounded by a large number of hollow tentacles containing nematocysts. Corals (Fig. 31.6c) resemble sea anemones encased in a calcium carbonate (limestone) house. The coral polyp can extend into the water to feed on microorganisms and retreat into the house for safety. Some corals can be solitary, but the vast majority live in colonies that vary in shape from rounded to branching. Many corals exhibit elaborate geometric designs and stunning colors, and are responsible for the building of coral reefs. The slow accumulation of limestone can result in massive structures, such as the Belize Barrier Reef along the eastern coast of Belize. Coral reef ecosystems are very productive, but also very fragile (see the Ecology feature, “Destruction of the Coral Reefs”). Their protection is important due to the fact that an extremely diverse group of marine life exists there. The hydrozoans have a dominant polyp. Both hydra (Fig. 31.7) and the Portuguese man-of war (Fig. 31.6d) are hydrozoans. You might think the Portuguese man-of-war is an odd-shaped medusa, but actually it is a colony of polyps. The original polyp becomes a gas-filled float that provides buoyancy, keeping the colony afloat. Other polyps, which bud from this one, are specialized for feeding or for reproduction. A long, single tentacle armed with numerous nematocysts arises from the base of each feeding polyp. Swimmers who accidentally come upon a Portuguese man-of-war can receive painful, even serious, injuries from these stinging tentacles. In true jellyfishes (Fig. 31.6e), the medusa is the primary stage, and the polyp remains small. Jellyfishes depend on tides and currents for their primary means of movement. They feed on a variety of invertebrates and fishes and are themselves food for marine animals.

Chapter 31  Animals: The Invertebrates

mouth

631

tentacle gastrovascular cavity

planula

mesoglea zygote

Polyp asexual budding

fertilization mesoglea gastrovascular cavity

sperm egg

mouth

Medusa

tentacle

a.

b. Sea anemone, Actinia

c. Cup coral, Tubastrea

d. Portuguese man-of-war, Physalia

e. Jellyfish, Aurelia

Figure 31.6  Cnidarian diversity.  a. The life cycle of a cnidarian. Some

cnidarians have both a polyp stage and a medusa stage. In others, one stage may be dominant or absent altogether. b. The anemone, which is sometimes called the flower of the sea, is a solitary polyp. c. Corals are colonial polyps residing in a calcium carbonate or proteinaceous skeleton. d. The Portuguese man-of-war is a colony of modified polyps and medusae. e. True jellyfishes undergo the complete life cycle. This is the medusa stage. The polyp is small.



632

UNIT 6  Evolution and Diversity

Hydra The body of a hydra (Fig. 31.7 top) is a small tubular polyp about one-quarter inch in length. Hydras are often studied in biology

mouth

gastrodermis

tentacle

gastrovascular cavity

bud epidermis

gastrodermis epidermis

nerve net

gastrovascular cavity

circular muscle fibers

nutritivemuscular cell

classes and labs as an example of a cnidarian. Hydras are likely to be found attached to underwater plants or rocks in most lakes and ponds. The only opening (the mouth) is in a raised area surrounded by four to six tentacles that contain a large number of nematocysts. The outer tissue layer is a protective epidermis derived from ectoderm. The inner tissue layer, derived from endoderm, is called a gastrodermis. The two tissue layers are separated by mesoglea. There are both circular and longitudinal muscle fibers. Nerve cells located below the epidermis, near the mesoglea, interconnect and form a nerve net that communicates with sensory cells throughout the body. The nerve net allows transmission of impulses in several directions at once. Because they have both muscle fibers and nerve fibers, cnidarians are capable of directional movement. The body of a hydra can contract or extend, and the tentacles that ring the mouth can reach out and grasp prey and discharge nematocysts (Fig. 31.7). Digestion begins within the central cavity but is completed within the food vacuoles of gastrodermal cells. Nutrient molecules are passed to the other cells of the body by diffusion. The large gastrovascular cavity allows digestion and gastrodermal cells to exchange gases directly with a watery medium. Although hydras exist only as polyps (there are no medusae), they can reproduce either sexually or asexually. When sexual reproduction is going to occur, an ovary or a testis develops in the body wall. Like the sponges, cnidarians have great regenerative powers, and hydras can grow an entire organism from a small piece. When conditions are favorable, hydras reproduce asexually by making small outgrowths, or buds, that pinch off and begin to live independently.

Check Your Progress  31.2

interstitial cell

1. Describe three main characteristics of sponges. 2. Explain how cnidarians are more anatomically complex

epitheliomuscular cell

3. Explain the basic anatomical characteristics of a hydra.

than sponges.

gland cell cnidocyte

sensory cell

31.3  The Lophotrochozoa

mesoglea

Learning Outcomes

coiled thread trigger cnidocyte before discharge of nematocyst

lid

nematocyst cnidocyte after discharge of nematocyst

filament spines barb

Figure 31.7  Anatomy of Hydra.  Top: The body of Hydra is a

small, tubular polyp whose wall contains two tissue layers. Hydra reproduces asexually by forming outgrowths called buds (see photo) that develop into a complete animal. Middle: Various types of cells in the body wall. Bottom: Cnidocytes are cells that contain nematocysts.

Upon completion of this section, you should be able to 1. Describe the basic features of the lophotrochozoa. 2. Identify the unique features of the molluscs. 3. Contrast the differences between the flatworms and annelids.

The lophotrochozoa are bilaterally symmetrical, at least in some stage of their development. As embryos, they have three germ layers, and as adults, they have the organ level of organization. Lophotrochozoans are protostomes and include the lophophorans (bryozoans, phoronids, and brachiopods) and the trochozoans (flatworms, rotifers, molluscs, and annelids). Lophophorans are aquatic and have a feeding apparatus called the lophophore, which is a mouth surrounded by a ciliated tentacle-like structure (Fig. 31.8). The trochophores either have a trochophore larva today (molluscs and annelids), or an ancestor had one in the past (flatworms and rotifers). 



Chapter 31  Animals: The Invertebrates

be incomplete, and when two openings are present, the digestive tract is said to be complete. Also, flatworms have no body cavity, and instead the third germ layer, mesoderm, fills the space between their organs. Among flatworms, planarians are free-living, whereas flukes and tapeworms are parasitic. Free-living flatworms have muscles and excretory, reproductive, and digestive systems. The worms lack respiratory and circulatory systems, however. Because the body is flat and thin, diffusion alone can pass needed oxygen and other substances from cell to cell.

lophophore

cilia

a.

b.

633

trochophore larva

Figure 31.8  Lophotrochozoan characteristics.  a. The

lophophore feeding apparatus. b. A trochophore larva.

Lophophorans The evolutionary story of the lophophorans is still being researched, with molecular evidence suggesting that the ancestry of this group is more complex than originally thought. For our purposes, we will focus on the traditional classification which includes the bryozoans, brachiopods, and the phoronids.  Bryozoans (phylum Bryozoa) are aquatic, colonial lophophorans. Colonies are made up of individuals called zooids. Zooids are not independent animals but, rather, single members of a colony that cooperate as a single organism. Some zooids specialize in feeding and they filter particles from the water with the lophophore; some specialize in reproduction; and some can perform both functions. Zooids coordinate functions within a colony by communicating through chemical signals. Zooids have protective exoskeletons, which they use to attach to substrates, including the bottoms of ships, where they cause a nuisance by increasing drag and impeding maneuverability.  Brachiopods (phylum Brachiopoda) are a small group of lophophorans that have two hinged shells, much as molluscs do— but instead of having a left and right shell, they have a top and a bottom shell. Brachiopods affix themselves to hard surfaces with a muscular pedicle. Like other lophophorans, brachiopods use their lophophore to feed by filtering particles from the water.  Phoronids (phylum Phoronida) live inside a long tube formed from their own chitinous secretions. The tube is buried in the ground and their lophophore extends from it, but it can retract very quickly when needed. Only about 15 species of phoronids exist worldwide. 

Free-Living Flatworms  Freshwater planarians, shown in ­Figure 31.9, are small (several millimeters to several centimeters) worms that live in lakes, ponds, streams, and springs. Some tend to be colorless; others have brown or black pigmentation. They feed on small living or dead organisms, such as worms and crustaceans. Planarians have an excretory system that consists of a network of interconnecting canals extending through much of the body. The beating of cilia in the flame cells (named because the beating of the cilia reminded early investigators of the flickering of a flame) keeps the water moving toward the excretory pores. Planarians have a ladderlike nervous system. They possess a small anterior brain and two lateral nerve cords that are joined by cross-branches. Planarians exhibit cephalization; in addition to a brain, they have light-sensitive organs (eyespots) and chemosensitive organs located on the auricles. Their three muscle layers—outer circular, inner longitudinal, and diagonal—allow for varied movement. A ciliated epidermis allows planarians to glide along a film of mucus. Planarians capture food by wrapping around the prey, entangling it in slime, and pinning it down. Then they extend a muscular pharynx, and by a sucking motion, tear up and swallow the food. The pharynx leads into a three-branched gastrovascular cavity within which digestion is completed. The digestive tract is considered incomplete because it has only one opening. Planarians can reproduce both sexually and asexually. Although hermaphroditic (possessing both male and female reproductive organs), they typically practice cross-fertilization, in which the penis of one is inserted into the genital pore of the other (and vice versa), and reciprocal transfer of sperm takes place. Fertilized eggs hatch in two to three weeks as tiny worms. Asexual reproduction occurs by regeneration—if you slice a planarian in half, two new planarians will grow. You can even make a two-headed planarian by slicing the head in half! Parasitic Flatworms  There are several classes of parasitic flatworms, two of which are discussed here: the tapeworms (class Cestoda) and the flukes (class Trematoda).

Tapeworms  As adults, tapeworms are endoparasites (internal parasites) of various vertebrates, including humans. They vary in The majority of the lophotrochozoans are trochozoans. length from a few millimeters to nearly 20 m. Tapeworms have a tough integument, a specialized body covFlatworms ering resistant to the host’s digestive juices. Their excretory, muscuFlatworms (phylum Platyhelminthes) are aptly named because lar, and nervous systems are similar to those of other flatworms. they have an extremely flat body. Like the cnidarians, flatworms Tapeworms have a well-developed anterior region, called the have an incomplete digestive tract and only one opening, the ­scolex, which bears hooks for attachment to the intestinal wall of mouth. When one opening is present, the digestive tract is said to the host and suckers for feeding. Behind the scolex are proglottids, a

Trochozoans

634

UNIT 6  Evolution and Diversity gastrovascular cavity eyespots

pharynx extended through mouth flame cell

auricle

fluid a. Digestive system: Three-branched system flame cell excretory pore

cilia

excretory canal

b. Excretory system: Flame-cell system ovary yolk gland

sperm duct

testis

genital pore

c. Reproductive system: Hermaphroditic system brain

lateral nerve cord

transverse nerve

d. Nervous system: Ladder-style system auricle

eyespots

e. Photomicrograph of a planarian

Figure 31.9  Planarian anatomy.  a. A planarian extends its

pharynx to suck food into a gastrovascular cavity that branches throughout the body. b. The excretory system includes flame cells. c. The reproductive system (shown in pink and blue) has both male and female organs. d. The nervous system has a ladderlike appearance. e. A flatworm, such as Dugesia, is bilaterally symmetrical and has a head region with eyespots.

series of reproductive units with a full set of male and female sex organs. Each proglottid fertilizes its own eggs, which number in the thousands. Immature proglottids are located closer to the scolex, while mature (or gravid) proglottids are farther away. The gravids, which contain fertilized eggs, break away and are eliminated in the host’s feces. A secondary host must ingest the eggs for the life cycle to continue. Figure 31.10 illustrates the life cycle of the pork tapeworm, Taenia solium, where the human is the primary host and the pig is the secondary host. The larvae burrow through the intestinal wall and travel in the bloodstream to finally lodge and encyst in muscle. The cyst is a small, hard-walled structure that contains a larva called a bladder worm. When humans eat infected meat that has not been thoroughly cooked, the bladder worm breaks out of the cyst, attaches itself to the intestinal wall, and grows to adulthood. Then the life cycle begins again.

Flukes  Flukes are endoparasites of various vertebrates. Their flattened and oval-to-elongated body is covered by a nonciliated integument. At the anterior end of these animals, an oral sucker is surrounded by sensory papillae, and there is at least one other sucker used for attachment to the host. Although the digestive system is reduced compared to that of free-living flatworms, the alimentary canal is well developed. The excretory and muscular systems are similar to those of free-living flatworms, but the nervous system is reduced, with poorly developed sense organs. Most flukes are hermaphroditic. Flukes are usually named for the type of vertebrate organ they inhabit; for example, there are blood, liver, and lung flukes. The blood fluke (Schistosoma spp.) occurs predominantly in the M ­ iddle East, Asia, and Africa. Nearly 800,000 infected persons die each year from schistosomiasis. Adult flukes are small (approximately 2.5 cm long) and may live for years in their human hosts. The ­Chinese liver fluke, Clonorchis sinensis, is a major parasite of humans, cats, dogs, and pigs. This 20 mm-long fluke is commonly found in many regions of the Orient. It requires two intermediate hosts, a snail and a fish. Eggs are shed into the water in feces of the human or other mammal host and enter the body of a snail, where they undergo development. Larvae escape into the water and bore into the muscles of a fish. When humans eat infected fish, the juveniles migrate into the bile duct, where they mature. A heavy infection can cause destruction of the liver and death.

Rotifers Rotifers (phylum Rotifera) are trochozoans related to the flatworms. With sizes ranging between 0.5 mm and 3 mm, rotifers are microscopic animals. Anton van Leeuwenhoek, an early eighteenth-­ century scientist, was one of the first to view rotifers (which he called “wheeled animalcules”) through a microscope. Rotifers have a crown of cilia, known as the corona, on their heads (Fig. 31.11). The corona looks like a spinning wheel when in motion. This disc of beating cilia serves as an organ of locomotion and also directs food into the mouth. The approximately 2,200 species primarily live in fresh water; however, some marine and terrestrial forms exist. The majority of rotifers are transparent, but some are very colorful. Many species of rotifers can desiccate during harsh conditions and remain dormant for lengthy periods of time. This characteristic has earned them the title “resurrection animalcules.”



Chapter 31  Animals: The Invertebrates

Figure 31.10  Life cycle of a

hooks

tapeworm, Taenia.  The life

cycle includes a human (primary host) and a pig (secondary host). The adult worm is modified for its parasitic way of life. It consists of a scolex and many proglottids, which become bags of eggs.

1. Primary host ingests meat containing bladder worms.

corona

635

proglottid

2. Bladder worm attaches to human intestine where it matures into a tapeworm.

6. Rare or uncooked meat from secondary host contains many bladder worms.

mouth

flame bulb

brain eyespot

salivary glands

gastric gland

stomach germovitellarium

scolex

sucker

20× 3. As the tapeworm grows, proglottids mature, and eventually fill with eggs.

5. Livestock may ingest the eggs, becoming a secondary host as each larva becomes a bladder worm encysted in muscle.

4. Eggs leave the primary host in feces, which may contaminate water or vegetation.

the tremendous diversity found in the phylum Mollusca (Fig. 31.12). Molluscs include chitons, limpets, slugs, snails, abalones, conchs, nudibranchs, clams, scallops, squid, and octopuses. Molluscs have a true coelom, and all coelomates have bilateral symmetry, three germ layers, the organ level of organization, and a complete digestive tract. The advantages of having a coelom include freer body movements, space for development of complex organs, greater surface area for absorption of nutrients, and protection of internal organs from damage.

The Unique Characteristics of Molluscs  Despite being a very large and diversified group, all molluscs have a body composed of at least three distinct parts:

intestine cloaca anus

foot toe

Figure 31.11  Rotifer.  Rotifers are microscopic animals. The beating of cilia on two lobes at the anterior end of the animal gives the impression of a pair of spinning wheels.

Molluscs The molluscs (phylum Mollusca) are the second most numerous group of animals, numbering over 110,000 species. They inhabit a variety of environments, including marine, freshwater, and terrestrial habitats. Although almost everyone enjoys looking at the intricate patterns and beauty of seashells, few people are aware of

1. The visceral mass is the soft-bodied portion that contains internal organs, including a highly specialized digestive tract, paired kidneys, and reproductive organs. The nervous system of a mollusc consists of several ganglia connected by nerve cords. The amount of cephalization and sensory organs varies from nonexistent in clams to complex in squid and octopuses. 2. The foot is a strong, muscular portion used for locomotion. Molluscan groups can be distinguished by modifications of the foot. Molluscs exhibit varying amounts of mobility. Oysters are sessile; snails are extremely slow moving; and squid are fast-moving, active predators. 3. The mantle, a membranous or sometimes muscular covering, envelops but does not completely enclose the visceral mass. The mantle cavity is the space between the two folds of the mantle. The mantle may secrete a shell, which is an exoskeleton. Some molluscs have a univalve (single shell) while others are bivalves (possessing two shells).



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UNIT 6  Evolution and Diversity

shell

eyes

tentacles on mantle

valve b. Scallop, Pecten sp.

a. a Chiton, Chiton Tonicella tentacle

eye

gills

arm

mantle

foot

c. Nudibranch, Glossodoris macfarlandi

suckers

d. Common octopus, Octopus vulgaris

Figure 31.12  Diversity of molluscs.  a. A chiton is believed to be most like the ancestral mollusc that gave rise to all the other types. Recent evidence

suggests that ancestral molluscs such as the chiton were segmented, but that this segmentation was lost throughout much of the phylum. b. Like clams, a scallop is a bivalve. c. Like snails, a nudibranch is a gastropod. d. Like squids, an octopus is a cephalopod.

Another feature often present is a rasping, tonguelike radula, an organ that bears many rows of teeth and is used to obtain food.

Gastropods  The gastropods (class Gastropoda) include nudibranchs, conchs, and snails. In gastropods, whose name means “stomach-footed,” the foot is ventrally flattened, and the animal moves by muscle contractions that pass along the foot. Many are herbivores that use their radulas to scrape algae from surfaces. Others are carnivores, using their radulas to bore through surfaces, such as bivalve shells, to obtain food. While nudibranchs, also called sea slugs, lack a shell, conchs and snails have a univalve coiled shell in which the visceral mass spirals. Land snails, such as Helix aspera, have a head with two pairs of tentacles; one pair bears eyes at the tips (Fig. 31.13a). The shell not only offers protection, but also prevents desiccation (drying out). While aquatic gastropods have gills, land snails have a mantle that is richly supplied with blood vessels and functions as a lung. Reproduction is also adapted to a land existence. Land snails are hermaphroditic. When two snails meet, each inserts its penis into the vagina of the other, and following fertilization and the deposit of eggs externally, development proceeds directly without the formation of swimming larvae. Cephalopods  In cephalopods (class Cephalopoda, meaning head-footed), including octopuses, squid, and nautiluses, the foot has evolved into a funnel or siphon about the head (Fig. 31.13b).

Aside from the tentacles, which seize prey, cephalopods have a powerful beak and a radula (toothy tongue) to tear prey apart. Cephalization is apparent. The eyes have a lens and a retina with photoreceptors similar to those of vertebrates. However, the eye is constructed so differently from the vertebrate eye that the so-called camera-type eye must have evolved independently in both the molluscs and in the vertebrates. In cephalopods, the brain is formed from a fusion of ganglia, with nerves leaving the brain that supply various parts of the body. An especially large pair of nerves controls the rapid contraction of the mantle, allowing these animals to move quickly by the jet propulsion of water. Rapid movement and the secretion of a dark ink help cephalopods escape their enemies. Octopuses have no shell, and squid have only a remnant of a shell concealed beneath the skin. Octopuses, like some other species of cephalopods, are thought to be among the most intelligent invertebrates and are even capable of learning!

Bivalves  Clams, mussels, oysters, and scallops are called bivalves (class Bivalvia) because their shells have two parts. In a clam, the shell, which is secreted by the mantle, is composed of protein and calcium carbonate, with an inner layer of mother-of-pearl. If a foreign body is placed between the mantle and the shell, pearls form as concentric layers of shell are deposited about the particle. Figure 31.14 shows the internal anatomy of the freshwater clam, Anodonta. The adductor muscles hold the valves of the shell

eye hermaphroditic gland

fin

cerebral ganglion

shell

pen

mantle

suckers gill tentacle

ink sac arm

radula anus funnel

pedal ganglion penis

eye

tentacles with suckers

eyes

foot

spiral shell

fins eye

foot

a. Snail, Helix is a gastropod.

b. Squid

Figure 31.13  Gastropod and cephalopod anatomy.  a. Snails have a long muscular foot that allows them to creep along slowly by way of muscular contraction. Note the absence of lungs in this land snail. b. Squids are torpedo-shaped and adapted for fast swimming by jet propulsion. pericardial cavity umbo

anterior aorta

heart kidney posterior ganglion posterior retractor muscle

digestive gland

posterior adductor muscle

stomach anterior adductor muscle

posterior aorta

esophagus

shell

anterior ganglion

anus

mouth

excurrent siphon labial palps

incurrent siphon

foot ganglion

foot

gill gonad

intestine

mantle

Figure 31.14  Clam, Anodonta.  In this drawing of a clam, one of the bivalve shells and the mantle have been removed from one side. Follow the path of food from the incurrent siphon to the gills, the mouth, the stomach, the intestine, the anus, and the excurrent siphon. Locate the three ganglia: anterior, foot, and posterior. The heart lies in the reduced coelom.

637

638

UNIT 6  Evolution and Diversity

together. Within the mantle cavity, the gills, which are the organs for gas exchange in aquatic forms, hang down on either side of the visceral mass, which lies above the foot. The clam is a filter feeder. Food particles and water enter the mantle cavity by way of the incurrent siphon, a posterior opening between the two valves. Mucous secretions cause smaller particles to adhere to the gills, and ciliary action sweeps them toward the mouth. The heart of a clam lies just below the hump of the shell within the pericardial cavity, the only remains of the reduced coelom. The heart pumps blood into a dorsal aorta that leads to the various organs of the body. Within the organs, however, blood flows through spaces, or sinuses, rather than through vessels. This is called an open circulatory system because the blood is not entirely contained within blood vessels. This type of circulatory system functions well in a slow-moving or sessile animal. The nervous system of a clam is composed of three pairs of ganglia (anterior, foot, and posterior), which are all connected by nerves. Clams lack cephalization. The “hatchet” foot projects anteriorly from the shell, and by expanding the tip of the foot with blood and pulling the body after it, the clam moves forward. The digestive system of the clam includes a mouth with labial palps, an esophagus, a stomach, and an intestine, which coils about in the visceral mass and then is surrounded by the heart as it extends to the anus. Two excretory kidneys remove waste from the pericardial cavity for excretion into the mantle cavity. The sexes are usually separate. The gonad (i.e., ovary or testis) is located around the coils of the intestine. All clams have some type of larval stage, and marine clams have a trochophore larva.

Annelids  Annelids (phylum Annelida, about 12,000 species) are segmented, as evidenced by the rings encircling the outside of their bodies. Segmentation is also seen in arthropods and chordates, although annelids are the only trochozoan with segmentation and a well-developed coelom. Internally, the segments of annelids are partitioned by septa. Worms, in general, do not have an internal or external skeleton, but most often have a hydrostatic skeleton, a fluid-filled interior that supports muscle contraction and enhances flexibility. Along with the partitioning of the fluidfilled coelom, this hydroskeleton permits each body segment to move independently. Locomotion occurs by contraction and expansion of each body segment, propelling the animal forward. Thus, a terrestrial annelid is capable of crawling on the surface in addition to burrowing in the mud. Although the most familiar members of this phylum are leeches and earthworms, the majority of annelids are marine. Annelids vary in size from microscopic to tropical earthworms as long as 4 m. The body plan in annelids has led to specialization of the digestive tract. The digestive system may include a pharynx, esophagus, crop, gizzard, intestine, and accessory glands. Annelids have an extensive closed circulatory system with blood vessels that run the length of the body and branch to every segment. The nervous system consists of a brain connected to a ventral solid nerve cord, with ganglia in each segment. The excretory system consists of nephridia in most segments. A nephridium (pl., nephridia) is a tubule that collects waste material and excretes it through an opening in the body wall.

Polychaetes  Marine annelids are the Polychaeta, which refers to the presence of many setae. Setae are bristles that anchor the worm or help it move. The setae are in bundles on parapodia, which are paddlelike appendages found on most segments. Parapodia are used in swimming, but can also be used as respiratory organs. Clam worms, such as Nereis (Fig. 31.15a), prey on crustaceans and other small animals, which they capture using a pair of strong, chitinous jaws that extend with the pharynx. Associated with its way of life, Nereis has a well-defined head region with eyes and other sense organs (Fig. 31.15b). Other polychaetes are sessile tube worms, with tentacles that form a funnel-shaped fan (Fig. 31.15c). Water currents, created by the action of cilia, trap food particles that are directed toward the mouth of these filter feeders. Polychaetes have breeding seasons, and only during these times do they possess functional sex organs. In Nereis, many worms concurrently shed a portion of their bodies containing either eggs or sperm, and these float to the surface, where fertilization takes place. The zygote rapidly develops into a trochophore

sensory projections

sensory projections

pharynx (extended) jaw

eyes

parapodia

parapodia a. Clam worm, Nereis

b. Head region of Nereis

spiraled tentacles

Figure 31.15  Annelid

diversity.  a. Clam worms are predatory polychaetes that have a well-defined head region. b. Note also the parapodia, which are used for swimming and as respiratory organs. c. Christmas tree worms (a type of tube worm) are sessile filter feeders whose ciliated tentacles spiral.

c. Christmas tree worms, Spirobranchus



Chapter 31  Animals: The Invertebrates

larva, just as in marine clams. The existence of this larval form in both the annelids and molluscs is evidence that these two groups of animals are evolutionarily related.

Oligochaetes  The oligochaetes (class Oligochaeta), which include earthworms, have few setae per segment. Earthworms (e.g., Lumbricus) (Fig. 31.16) do not have a well-developed head or parapodia. Their setae protrude in pairs directly from the surface of the body. Locomotion, which is accomplished section by section, utilizes muscle contraction and the setae. When longitudinal muscles contract, segments bulge, and their setae protrude into the soil. Then, when circular muscles contract, the setae are withdrawn, and these segments move forward in sequence, pushing the whole animal forward. Earthworms reside in soil where there is adequate moisture to keep the body wall moist for gas exchange. They are scavengers that do not have an obvious head, and feed on leaves or any other organic matter. Food drawn into the mouth by the action of the muscular pharynx is stored in a crop and ground up in a thick, muscular gizzard. Digestion and absorption occur in a long intestine, whose dorsal surface has an expanded region, called a typhlosole, that increases the surface for absorption. Segmentation  Earthworm segmentation is visible internally by the presence of septa. The long, ventral solid nerve cord leading from the brain has ganglionic swellings and lateral nerves in each segment. The paired nephridia in most segments have two openings: one is a ciliated funnel that collects coelomic fluid, and the other is an exit in the body wall. Between the two openings is a convoluted region where waste material is removed from the blood vessels about the tubule. Red blood moves anteriorly in the dorsal blood vessel, which connects to the ventral blood vessel by five pairs of connectives called “hearts.” Pulsations of the dorsal blood vessel and the five pairs of hearts are responsible for blood flow. As the ventral vessel takes the blood toward the posterior regions of the worm’s body, it gives off branches in every segment. Altogether, segmentation is evidenced by (1) body rings, (2) coelom divided by septa, (3) setae on most segments, (4) ganglia and lateral nerves in each segment, (5) nephridia in most segments, and (6) branch blood vessels in each segment. Reproduction  Earthworms are hermaphroditic: the male organs are the testes, the seminal vesicles, and the sperm ducts, and the female organs are the ovaries, the oviducts, and the seminal receptacles. When mating, two worms lie parallel to each other facing in opposite directions. The fused midbody segment, called a clitellum, secretes mucus that protects the sperm from drying out as they pass between the worms. After the worms separate, the clitellum of each produces a slime tube, which is moved along over the anterior end by muscular contractions. As it passes, eggs and the sperm received earlier are deposited, and fertilization occurs. The slime tube then forms a cocoon to protect the worms as they develop. There is no larval stage. Comparison with Clam Worms  Comparing the anatomy of marine clam worms (class Polychaeta) to that of terrestrial earthworms (class Oligochaeta) highlights the manner in which earthworms are adapted to life on land. A lack of cephalization is seen

mouth

pharynx brain

hearts (5 pairs) seminal vesicle

639

esophagus coelom crop dorsal blood vessel nephridium ventral blood vessel ventral nerve cord

anus clitellum

a.

dorsal blood vessel coelomic longitudinal lining muscles muscular wall of intestine

circular muscles

nephridium typhlosole setae

coelom ventral blood vessel

cuticle

ventral nerve cord

excretory pore

subneural blood vessel b.

clitellum anterior end

clitellum

anterior end

c.

Figure 31.16  Earthworm, Lumbricus.  a. Internal anatomy of the

anterior part of an earthworm. Each body segment has an internal septa that divides the coelom into compartments. b. Cross section of an earthworm. c. When earthworms mate, they are held in place by a mucus secreted by the clitellum. The worms are hermaphroditic, and when mating, sperm pass from the seminal vesicles of each to the seminal receptacles of the other.



640

UNIT 6  Evolution and Diversity

anterior sucker

posterior sucker Medicinal leech, Hirudo medicinalis

Figure 31.17  Medicinal leech, Hirudo medicinalis.  Leeches

have been used in medicinal practices for thousands of years.

in the nonpredatory earthworms that extract organic remains from the soil they eat. Their lack of parapodia helps reduce the possibility of water loss and facilitates burrowing in soil. The clam worm makes use of external water, while the earthworm provides a mucous secretion to aid fertilization. It is the aquatic form that has the swimming, or trochophore larva, not the land form.

Leeches  Leeches (class Hirudinea) are usually found in fresh water, but some are marine or even terrestrial. They have the same body plan as other annelids, but they have no setae, and each body ring has several transverse grooves. Most leeches are only 2–6 cm in length, but some, including the medicinal leech, are as long as 20 cm. Among their modifications are two suckers, a small one around the mouth and a large, posterior one. While some leeches are free-living, feeding on plant material or invertebrates, most are fluid feeders that attach themselves to open wounds. Some bloodsuckers, such as the medicinal leech (Fig. 31.17), can cut through tissue. Leeches are able to keep blood flowing and prevent clotting by means of a substance in their saliva known as hirudin, a powerful anticoagulant. Check Your Progress  31.3 1. List the characteristics that unite the flatworms, molluscs, and annelids.

2. Compare features of the flatworm, mollusc, and annelid body cavity, digestive tract, and circulatory system.

3. Identify three characteristics found in all molluscs.

31.4  The Ecdysozoa Learning Outcomes Upon completion of this section, you should be able to 1. Identify the characteristics unique to ecdysozoans. 2. Describe the characteristics contributing to the success and diversity of arthropods. 3. List the major features that distinguish the crustaceans, insects, and arachnids.

The ecdysozoans (Gk. ecdysis, “stripping off”) include the roundworms and arthropods. They are one of the major groups of animals, which also contains the largest number of species.  Ecdysozoans construct an outer covering called a cuticle, which protects and supports the animal and is periodically shed to allow growth. Unlike many other invertebrate species that reproduce by releasing large amounts of eggs and sperm into their aqueous environment, many ecdysozoans have evolved separate sexes, which come together and copulate to deliver sperm into the female body, or to directly fertilize eggs as they are released.

Roundworms Roundworms (phylum Nematoda, about 25,000 species) are nonsegmented worms that are prevalent in almost any environment. Generally, nematodes are colorless and range in size from microscopic to exceeding 1 m in length. The internal organs, including the tubular reproductive organs, lie within the pseudocoelom. A pseudocoelom is a body cavity that is incompletely lined by mesoderm. In other words, mesoderm occurs inside the body wall but not around the digestive cavity (gut). The fluid-filled pseudocoelom provides space for the development of organs, substitutes for a circulatory system by allowing easy passage of molecules, and provides a type of skeleton. Nematodes have developed a variety of lifestyles from freeliving to parasitic. One species, Caenorhabditis elegans, a freeliving nematode, is a model animal used in genetics and developmental biology as it was one of the first species to have its genome sequenced.

Biology of a Roundworm In the roundworm, Ascaris lumbricoides (Fig. 31.18a), females tend to be larger (20–35 cm in length) than males (Fig. 31.18b). Both sexes move by a characteristic whiplike motion because they have only longitudinal muscles and no circular muscles next to the body wall. Ascaris species are most commonly parasites of humans and pigs. A female Ascaris is very reproductively prolific, producing over 200,000 eggs daily. The eggs are passed with host feces and, under the right conditions, can develop into a new worm within two weeks. The eggs need to enter a new host’s body via uncooked vegetables, soiled fingers, or ingested fecal material and hatch in the intestines. The juveniles make their way into the veins and lymphatic vessels and are carried to the heart and lungs. From the lungs, the larvae travel up the trachea, where they are swallowed and eventually reach the intestines. There, the larvae mature and begin feeding on intestinal contents. The symptoms of an Ascaris infection depend upon the site and the stage of infection.

Other Roundworms Trichinosis is a fairly serious infection caused by Trichinella spiralis, a roundworm that rarely infects humans in the United States. Humans contract the disease when they eat undercooked pork that contains encysted larvae. After maturation, the female adult burrows into the wall of the host’s small intestine, where she deposits live larvae that are then carried by the bloodstream and encyst in



Chapter 31  Animals: The Invertebrates

641

Figure 31.18  Roundworm anatomy.  a. The roundworm Ascaris.

b. Roundworms such as Ascaris have a pseudocoelom and a complete digestive tract with a mouth and an anus. c. A filarial worm infection causes elephantiasis, which is characterized by a swollen body part when the worms block lymphatic vessels. a. Ascaris testis sperm duct lateral nerve cord

dorsal nerve cord brain pharynx

pseudocoelom cuticle excretory pore

mouth

c.

sperm

gut

muscle layer

seminal vesicle

ventral nerve cord

spicules that aid in sperm transfer

anus

b. Male Ascaris anatomy cloaca

the skeletal muscles. When the adults are in the small intestine, digestive disorders, fatigue, and fever occur. When the larvae encyst, the symptoms include aching joints, muscle pain, and itchy skin. Elephantiasis is caused by a roundworm called the filarial worm (Fig. 31.18c), which utilizes the mosquito as an intermediate host. When a mosquito bites an infected person, it transports larvae to a new host. Because the adult worms reside in lymphatic vessels, fluid return is impeded, and the limbs of an infected human can swell to an enormous size, even resembling those of an elephant. Other roundworm infections are more common in the United States. Children frequently acquire a pinworm infection, and hookworm is seen in the southern states, as well as worldwide. Good hygiene, proper disposal of sewage, and cooking meat thoroughly usually protect people from parasitic roundworms.

(Fig. 31.20a). Segmentation allows the attachment of specialized, jointed appendages.  2. Jointed appendages. Jointed appendages allow for specialization of the body segments. The jointed appendages of arthropods are basically hollow tubes moved by muscles (Fig 31.20b). Typically, the appendages are highly adapted for a particular function, such as food gathering, reproduction, and locomotion. In addition, many appendages are associated with ­sensory structures and used for tactile purposes. 3. An exoskeleton (Fig. 31.20c). The exoskeleton is composed primarily of chitin, a strong, flexible, nitrogenous polysaccharide. The exoskeleton serves many functions, including protection, attachment for muscles, locomotion, and prevention of desiccation. However, because it is hard and nonexpandable, arthropods must molt, or shed, the exoskeleton in order to grow larger.

Arthropods

In addition, arthropods have a well-developed nervous system that consists of a brain and a ventral nerve cord. The head bears various types of sense organs, including eyes of two types—simple and compound. The compound eye is composed of many complete visual units, each of which operates independently (Fig. 31.20d). The lens of each visual unit focuses an image on a small number of photoreceptors within that unit. The simple eye, like that of vertebrates, has a single lens that brings the image to focus onto many receptors, each of which receives only a portion of the image. In addition to sight, many arthropods have well-developed touch, smell, taste, balance, and hearing. Respiration in arthropods depends on the environment in which the organism lives. Marine forms use gills, which are vascularized, highly convoluted, thin-walled tissue specialized for gas

Arthropods (phylum Arthropoda) are extremely diverse (Fig. 31.19), ranging in size from less than 0.1 mm (mites) to 4 m (Japanese crab) in length. Over 1 million species have been discovered and described, mostly insects, but some experts suggest that many more species exist. Arthropods occupy almost every type of habitat and are considered the most successful group of all the animals.  The remarkable success of arthropods is dependent on these characteristics: 1. Segmentation. The body of an arthropod is divided into segments. In some arthropods, the individual segments are fused into functional regions called the head, thorax, and abdomen



642

UNIT 6  Evolution and Diversity

a. Flat-backed millipede, Sigmoria

b. Tarantula, Aphonopelma

c. Dungeness crab, Cancer

d. Paper wasp, Polistes

e. Indian giant tiger centipede, Scolopendra hardwickei

Figure 31.19  Arthropod diversity.  a. A millipede has only one pair of antennae, and the head is followed by a series of segments, each with two pairs of appendages. b. The hairy tarantulas of the genus Aphonopelma are dark in color and move carefully and steadily. Their bite is harmless to people. c. A crab is a crustacean with a calcified exoskeleton, one pair of claws, and four other pairs of walking legs. d. A wasp is an insect with two pairs of wings, both used for flying, and three pairs of walking legs. e. A centipede has only one pair of antennae, and the head is followed by a series of segments, each with a single pair of appendages. Figure 31.20  Characteristics of arthropods.  abdomen

a.

thorax

Dragonfly

head compound eye

opening to tegumental gland

a. The body of an arthropod is segmented. These segments are often fused into a head, thorax, and abdomen. b. Jointed appendages help specialize the body segments. In the hollow appendages, muscles allow movement. c. The exoskeleton is secreted by the epidermis and consists of the endocuticle; the exocuticle, hardened by the deposition of calcium carbonate; and the epicuticle, a waxy layer. Chitin makes up the bulk of the exoskeleton. d. Arthropods have a more complex nervous system, which consists of a compound eye that contains many individual units, each with its own lens and photoreceptors.

cornea seta epicuticle exocuticle

joint flexor muscle extensor muscle

rhabdom

epidermis

pigment cell optic nerve

basement membrane b. Jointed appendages

endocuticle

c. Exoskeleton

tegumental gland

photoreceptors ommatidium

d. Compound eye



Chapter 31  Animals: The Invertebrates

exchange. Terrestrial forms have book lungs (e.g., spiders) or air tubes called tracheae. Tracheae serve as a rapid way to transport oxygen directly to the cells. The majority of arthropods undergo metamorphosis, which is  the drastic change in form and physiology that occurs as an immature stage, called a larva, becomes an adult. Among arthropods, the larva eats different food and lives in a different environment than the adult. For example, larval crabs live among and feed on plankton, while adult crabs are bottom dwellers that catch live prey or scavenge dead organic matter. Among insects, such as butterflies, the caterpillar feeds on leafy vegetation, while the adult feeds on nectar.

Crustaceans Crustaceans (subphylum Crustacea) are a group of largely marine arthropods that include barnacles, shrimps, lobsters, and crabs. There are also some freshwater crustaceans, including the crayfish, and some terrestrial ones, including the sowbug, or pillbug. Crustaceans are named for their hard shells. The exoskeleton is calcified to a greater degree in some forms than in others. Although crustacean anatomy is extremely diverse, the head usually bears a pair of compound eyes and five pairs of appendages. The first two pairs, called antennae, lie in front of the mouth and have sensory functions. The other three pairs are mouthparts used in feeding. In crayfish, such as Cambarus, the thorax bears five pairs of walking legs. The first walking leg is a pinching claw (Fig. 31.21). The gills are situated above the walking legs. The head and thorax are fused into a cephalothorax, which is covered on the top and sides by a nonsegmented carapace. The abdominal segments, which contain much musculature, are equipped with small paddlelike structures called swimmerets. The last two segments bear the uropods and the telson, which make up a fan-shaped tail to propel the crayfish backward.

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The digestive system includes a stomach, which is divided into two main regions. The anterior portion consists of a gastric mill, a special grinding apparatus equipped with chitinous teeth. The posterior region acts as a filter to prevent coarse particles from entering the digestive glands where absorption takes place. Green glands lying in the head region, anterior to the esophagus, excrete metabolic wastes through a duct that opens externally at the base of the antennae. The coelom is reduced to a space around the reproductive system. A heart within a pericardial cavity pumps blood containing the respiratory pigment hemocyanin into a hemocoel consisting of sinuses (open spaces). Hemocyanin contains copper (compared to hemoglobin—the red, iron-containing pigment of humans), causing the blood to have a blueish color. These sinuses are common in an open circulatory system, where the hemolymph flows freely about the organs and is not typically contained within blood vessels. The nervous system of the crayfish is very similar to that of the earthworm. The crayfish has a brain and a ventral nerve cord that passes posteriorly. Along the length of the nerve cord, segmental ganglia give off 8 to 19 paired lateral nerves. The sexes are separate in the crayfish, with the gonads located just ventral to the pericardial cavity. In the male, a coiled sperm duct opens to the outside at the base of the fifth walking leg. Sperm transfer is accomplished by the first two pairs of swimmerets, which are enlarged and quite strong. In the female, the ovaries open at the bases of the third walking legs. A stiff fold between the bases of the fourth and fifth pairs of walking legs serves as a seminal receptacle. Following fertilization, the eggs are attached to the swimmerets of the female.

Insects Insects (subphylum Uniramia) are so numerous (well over 1 million identified species) and so diverse (Fig. 31.22) that the study of this one group is a major specialty in biology called entomology. Some

second walking leg first walking leg (modified as a pincerlike claw)

third walking leg

brain stomach

heart

dorsal abdominal artery

green gland

fourth walking leg

anus

fifth walking leg uropods swimmerets carapace mouth b.

antennae

compound eye mouth

claspers gills opening of sperm duct

digestive gland

testis

sperm ventral duct nerve cord

anus telson

Figure 31.21  Male crayfish, Cambarus.  a. Externally, it is

possible to observe the crayfish’s jointed appendages, including the swimmerets, and the walking legs, which include claws. These appendages, plus a portion of the carapace, have been removed from the right side so that the gills are visible. b. An internal view shows the parts of the digestive and Cephalothorax Abdomen circulatory systems. Note the ventral nerve cord. a.

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UNIT 6  Evolution and Diversity c. Bee, Apis

a. Housefly, Musca

d. Dragonfly, Aeshna

b. Walking stick, Diapheromera

e. Birdwing butterfly, Troides

Figure 31.22  Insect diversity.  a. Flies have a single pair of wings

c. Bee, Apis

insects show remarkable behavioral adaptations, as exemplified by the social systems of bees, ants, termites, and other colonial insects. Insects have certain features in common. The body is divided into a head, a thorax, and an abdomen. The head usually bears a pair of sensory antennae, a pair of compound eyes, and several simple eyes. The mouthparts are adapted to the specific insect’s way of life. For example, a grasshopper has mouthparts that chew, whereas a butterfly has a long tube for siphoning the nectar of flowers. The abdomen contains most of the internal organs. The thorax bears three pairs of legs and zero to three pairs of wings. Wings, if present, enhance an insect’s ability to survive by providing a way of escaping enemies, finding food, facilitating mating, and dispersing the offspring. The exoskeleton of an insect is lighter and contains less chitin than that of many other arthropods. d. Dragonfly, Aeshna

and lapping mouthparts. b. Walking sticks are herbivorous, with biting and chewing mouthparts. c. Bees have four translucent wings and a thorax separated from the abdomen by a narrow waist. d. Dragonflies have two pairs of similar wings. They catch and eat other insects while flying. e. Butterflies have forewings larger than their hindwings. Their mouthparts form a long tube for siphoning up nectar from flowers.

Here, we will discuss the features of a grasshopper as a representative example of insect form and function. In the grasshopper (Fig. 31.23), the third pair of legs is suited to jumping. There are two pairs of wings. The forewings are tough and leathery, and when folded back at rest, they protect the broad, thin hindwings. On the lateral surface, the first abdominal segment bears a large tympanum on each side for the reception of sound waves.

Internal Organs  The digestive system of a grasshopper is suitable for a herbivorous diet. The mouthparts chew the food, which is temporarily stored in the crop before passing into a gastric mill, where it is finely ground before digestion is completed in the stomach. Nutrients are absorbed into the hemocoel from outpockets called gastric ceca. The stomach leads into an intestine and a



Chapter 31  Animals: The Invertebrates

rectum, which empties by way of an anus. The excretory system consists of Malpighian tubules, which extend into the hemocoel and collect nitrogenous wastes that are excreted into the digestive tract. The formation of a solid nitrogenous waste, namely uric acid, conserves water. The respiratory system begins with openings in the exoskeleton called spiracles. From here, the air enters small tubules called tracheae (Fig. 31.23a). The tracheae branch and rebranch until they end intracellularly, where the actual exchange of gases takes place. The movement of air through this complex of tubules is not passive. Rather, air is actively pumped by alternate contraction and relaxation of the body wall. Breathing by tracheae is suitable to the small size of insects (most are less than 60 mm in length), as the tracheae would be crushed by any significant amount of weight. The circulatory system contains a slender, tubular heart that lies against the dorsal wall of the abdominal exoskeleton and pumps hemolymph into an aorta that leads to a hemocoel, where it circulates before returning to the heart again. The hemolymph is colorless and lacks a respiratory pigment because the tracheal system is responsible for gas exchange. Head

Thorax

antenna

Abdomen

forewing

tympanum

hindwing

compound eye

ovipositor air sac simple eye

spiracles labial palps

a. spiracle crop brain

aorta

Malpighian tubules

tracheae

ovary heart

rectum

intestine oviduct

vagina salivary gland stomach mouth gastric ceca

ventral nerve cord

seminal receptacle nerve ganglion

b.

Figure 31.23  Female grasshopper, Romalea.  a. Externally,

the body of a grasshopper is divided into three sections and has three pairs of legs. The tympanum receives sound waves, and the jumping legs and the wings are for locomotion. b. Internally, the digestive system is specialized. The Malpighian tubules excrete a solid nitrogenous waste (uric acid). A seminal receptacle receives sperm from the male, which has a penis.

645

Reproduction and Development  Grasshopper reproduction is adapted to life on land. The male grasshopper has a penis, and sperm passed to the female are stored in a seminal receptacle. Internal fertilization protects both gametes and zygotes from drying out. The female deposits the fertilized eggs in the ground with her ovipositor. Metamorphosis is a change in form and physiology that occurs as an immature stage, called a larva, becomes an adult. Grasshoppers undergo gradual metamorphosis as they mature. The immature grasshopper, called a nymph, looks like an adult grasshopper, even though it differs somewhat in shape and form. Other insects, such as butterflies, undergo complete metamorphosis, involving drastic changes in form. At first, the animal is a wormlike larva (caterpillar) with chewing mouthparts. It then forms a case, or cocoon, about itself and becomes a pupa. During this stage, the body parts are completely reorganized. The adult then emerges from the cocoon. This life cycle allows the larvae and adults to use different food sources. Most eating by insects occurs during the larval stage. The field of forensic science has found a way to put our knowledge of insect life cycle stages to good use, as presented in the Scientific Inquiry feature, “Maggots: A Surprising Tool for Crime Scene Investigation.” Comparison with Crayfish  The grasshopper and the crayfish share a common ancestor, but they have diverged in their morphology to adapt to aquatic versus terrestrial environments. In crayfish, gills take up oxygen from water, while in the grasshopper, tracheae allow oxygen-laden air to enter the body. Appropriately, the crayfish has an oxygen-carrying pigment, but a grasshopper has no such pigment in its blood. The crayfish excretes a liquid nitrogenous waste (ammonia), while the grasshopper excretes a solid nitrogenous waste (uric acid). Only grasshoppers have (1) a tympanum for the reception of sound waves, and (2) a penis in males for passing sperm to females without possible desiccation, and an ovipositor in females for laying eggs in soil. Crayfish utilize their uropods when they swim; a grasshopper has legs for hopping and wings for flying.

Arachnids The arachnids (subphylum Chelicerata) include scorpions, spiders, ticks, and mites (Fig. 31.24). In this group, the cephalothorax bears six pairs of appendages: the chelicerae and the pedipalps and four pairs of walking legs. The cephalothorax is followed by an abdomen that contains internal organs. Scorpions are the oldest terrestrial arthropods. Today, they are widely distributed in the tropics, subtropics, desert, and temperate regions, with a notable absence from New Zealand. They are nocturnal and spend most of the day hidden under a log or a rock. In scorpions, the pedipalps are large pincers, and the long abdomen ends with a stinger that contains venom. Ticks and mites are parasites. Ticks suck the blood of vertebrates and sometimes transmit diseases, such as Rocky Mountain spotted fever or Lyme disease. Chiggers, the larvae of certain mites, feed on the skin of vertebrates. Spiders, the most familiar arachnids, have a narrow waist that separates the cephalothorax from the abdomen. Each chelicera consists of a basal segment and a fang that delivers venom to



646

UNIT 6  Evolution and Diversity

SCIENCE IN YOUR LIFE  ►

SCIENTIFIC INQUIRY

Maggots: A Surprising Tool for Crime Scene Investigation A fresh human corpse is lying on the floor. Within 10 to 15 minutes, blowflies with blue and green metallic bodies arrive. An animal corpse is a perfect site for egg production. The flies lay eggs in the mouth, nostrils, ears, eyes, and any open wounds. Depending on the temperature, the eggs will hatch in the next 12 to 24 hours. The small hatchling larval flies, more commonly known as maggots, feed on fat and collagen in the corpse in order to grow. Their development occurs in three stages called instars, during which the larvae shed skin and grow. When the third instar grows, it leaves the corpse and forms a pupa that takes about 14 days (depending on the temperature) to emerge as an adult blowfly. The adult has to wait a day or two until it can fly because the wings need to expand and the body has to harden. Once capable of flight, the adult leaves

to find a mate as well as another corpse in which to lay eggs, thereby completing the life cycle. What do blowflies have to do with crime scene investigation? If the corpse involved is a human, the timing of the various stages of the life cycle of the flies (e.g., newly hatched eggs, maggot size, maggot weight, or pupa remains) can help forensic scientists determine the postmortem interval (or PMI) since the time of death. Forensic entomology uses insects such as blowflies to determine the time of death. A forensic entomologist can even determine if a human corpse has been moved based on the fact that different species of blowflies prefer to lay eggs in different environments. For example, some species lay their eggs in shade, whereas others lay eggs in sunny habitats; similarly, some species

inhabit urban areas, whereas others inhabit rural areas. Thus, the combination of life stage(s) and species of blowfly found on a corpse can help a forensic entomologist determine the time and possible location of a person’s death.

Questions to Consider 1. Given that the hatching of eggs and the growth rate of blowfly larvae depend on temperature, how sure do you think forensic entomologists can be about time of death? 2. As a juror, how would you feel about forensic entomology evidence entered into court? 3. When might forensic entomology evidence be useful in a murder case?

stinger pedipalp

pedipalp

chelicera with fang

chelicera

a. Kenyan giant scorpion, Pandinus

b. Wasp spider, Argiope bruennichi

body wall

ventral view of mouthparts

lamellae of lung blood flow between lamellae

air flowing in through spiracle c. Wood ticks, Ixodes

d. Book lung anatomy

Figure 31.24  Arachnid diversity.  a. A scorpion has pincerlike chelicerae and pedipalps. Its long abdomen ends with a stinger that contains venom.

b. The wasp spider is named because of its markings, it is not poisonous to humans. c. In the western United States, the wood tick carries a debilitating disease called Rocky Mountain spotted fever. d. Arachnids breathe by means of book lungs, in which the “pages” are double sheets of thin tissue (lamellae).

paralyze or kill prey. Spiders use silk threads for all sorts of purposes, from lining their nests to catching prey. Biologists have used web-building behavior to discover how spider families are related. The internal organs of spiders also show how they are adapted to a terrestrial way of life. Malpighian tubules work in conjunction with rectal glands to reabsorb ions and water before a relatively dry nitrogenous waste (uric acid) is excreted. Invaginations of the inner body wall form the lamellae (“pages”) of spiders’ so-called book lungs. Air flowing into the folded lamellae on one side exchanges gases with blood flowing in the opposite direction on the other side.

a. Brittle star, Ophiothrix

Check Your Progress  31.4 1. Name two ways in which the roundworms are anatomically similar to the arthropods.

2. Explain the features that account for the great success of

arthropods. 3. Identify the major anatomical characteristics of a crustacean.

31.5  Invertebrate Deuterostomes Learning Outcomes

b. Sea cucumber, Cucumaria

Upon completion of this section, you should be able to 1. Recognize the basic morphological characteristics of echinoderms. 2. Describe how sea stars move, feed, and reproduce.

Among animals, chordates are most closely related to the echinoderms (phylum Echinodermata) as witnessed by their similar embryological development, even though echinoderms are invertebrates. Both echinoderms and chordates are deuterostomes, in which the second embryonic opening becomes the mouth, and the coelom forms by outpocketing the primitive gut (see Fig. 31.3). Protostomes, in contrast, are characterized by a mouth that forms from the first embryonic opening (the blastopore).

c. Sea urchin, Strongylocentrotus

Characteristics of Echinoderms Echinoderms are a diverse group of marine animals. There are no terrestrial echinoderms. They have an endoskeleton (internal skeleton) consisting of spine-bearing, calcium-rich plates. The spines, which stick out through their delicate skin, account for their name. It may seem surprising that echinoderms, although related to chordates, lack those features we associate with vertebrates such as humans. For example, echinoderms are often radially, not bilaterally, symmetrical. Their larva is a free-swimming filter feeder with bilateral symmetry, but it typically metamorphoses into a radially symmetrical adult. Recall that with radial symmetry, it is possible to obtain two mirror images, no matter how the animal is sliced longitudinally, whereas in bilaterally symmetrical animals, only a longitudinal cut gives two mirror images.

Echinoderm Diversity Echinoderms are quite diverse (Fig. 31.25). They include sea lilies, motile feather stars, brittle stars, and sea cucumbers. Sea

d. Feather stars, Antedon

Figure 31.25  Echinoderm diversity.  a. Brittle stars have long, slender arms, which make them the most mobile echinoderms. b. Sea cucumbers look like a cucumber. They lack arms but have tentacle-like tube feet with suckers around the mouth. c. Sea urchins have large, colored, external spines for protection. d. A feather star extends its arms to filter suspended food particles from the sea.

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648

UNIT 6  Evolution and Diversity

cucumbers actually look like a cucumber, except they have feeding tentacles surrounding their mouth. You may be more familiar with sea urchins and sand dollars in the class Echinoidea, which have spines for locomotion, defense, and burrowing. Sea urchins are food for sea otters off the coast of California.

arm

aboral side

Sea Stars Sea stars (starfish) are commonly found along rocky coasts, where they feed on clams, oysters, and other bivalve molluscs. The fiverayed body has an oral, or mouth, side (the underside) and an aboral, or anus, side (the upper side) (Fig. 31.26). Various structures project through the body wall: (1) spines from the endoskeletal plates offer some protection; (2) pincerlike structures called pedicellariae keep the surface free of small particles; and (3) skin gills, tiny fingerlike extensions of the skin, are used for gas exchange. On the oral surface, each arm has a groove lined by small tube feet. To feed, a sea star positions itself over a bivalve such as a clam and attaches some of its tube feet to each side of the shell (Fig.  31.27). By working its tube feet in alternation, it pulls the shell open. A very small crack is enough for the sea star to evert its cardiac stomach and push it through the crack to contact the soft parts of the bivalve. The stomach secretes enzymes, and digestion begins even while the bivalve is attempting to close its shell. Later, partly digested food is taken into the sea star’s body, where digestion continues in the pyloric stomach, using enzymes from digestive glands found in each arm. A short intestine opens at the anus on the aboral side. In each arm, the well-developed coelomic cavity contains not only a pair of digestive glands, but also gonads (either male or female), which open on the aboral surface by very small pores. The nervous system consists of a central nerve ring from which radial nerves extend into each arm. A light-sensitive eyespot is at the tip of each arm. Sea stars are capable of coordinated but slow responses and body movements. Locomotion depends on their water vascular system. Water enters this system through a structure on the aboral side called the madreporite, or sieve plate. From there it passes down a stone canal into a ring canal, which surrounds the mouth. A radial canal in each arm branches off from the central ring canal. For locomotion, water enters the ampullae from the radial canals. Contraction of ampullae forces water into the tube foot, expanding it. When the foot touches a surface, the center is withdrawn, giving it suction so that it can adhere to the surface. By alternating the expansion and contraction of the tube feet, a sea star moves slowly along. Echinoderms don’t have a respiratory, excretory, or circulatory system. Fluids within the coelomic cavity and the water vascular system carry out many of these functions. For example, gas exchange occurs across the skin gills and the tube feet. Nitrogenous wastes diffuse through the coelomic fluid and the body wall. Cilia on the peritoneum lining the coelom keep the coelomic fluid moving. Sea stars reproduce both asexually and sexually. If the body is fragmented, each fragment can regenerate a whole animal as long as the fragment contains part of the central disk. Fishermen who try to get rid of sea stars by cutting them up and tossing them

bivalve mollusc a. central disc arm pedicellaria skin gill digestive gland

endoskeletal plates eyespot

ampula coelomic cavity

radial canal tube feet

b. Aboral side showing ray cross section

madreporite

rectum rectal cecum

stone canal

anus

pyloric stomach cardiac stomach esophagus mouth

gonads ring canal gonopore lateral canal central nerve ring

c. Aboral side showing internal cross section

Figure 31.26  Sea star anatomy and behavior.  a. A sea star

uses the suction of its tube feet to open a bivalve mollusc, its primary source of food. b. Each arm of a sea star contains digestive glands, gonads, and portions of the water vascular system. This system (colored orange) terminates in tube feet. c. A different cross section shows the orientation of mouth, digestive system, and anus.

overboard are merely propagating more sea stars! Sea stars also spawn, releasing either eggs or sperm. The larva is bilaterally symmetrical and metamorphoses to become the radially symmetrical adult.



Chapter 31  Animals: The Invertebrates

Figure 31.27 

Sea star attacking a mollusc.  Tube feet pry open the mollusc shell enough to allow the sea star to insert its stomach, allowing the digestive juices to kill the mollusc.

Check Your Progress  31.5 1. Explain why echinoderms are more closely related to chordates than invertebrates.

2. Identify the basic structures of an echinoderm.

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Conclusion A fad diet called the “tapeworm diet” has been banned by the FDA due to the harmful side effects that parasitic tapeworms can cause to their hosts. More than 50 million people in less developed nations around the world are threatened with cysticercosis, a parasitic tapeworm infection that can lead to weight loss, anemia, and severe malnutrition. The invertebrates—animals that lack a backbone—are far more diverse than the vertebrates. While various invertebrates are associated with disease and health problems, many of them are very beneficial to the survival of the human species. Various insects act as pollinators for a large number of commercially grown crops. Invertebrates possess a wide variety of traits that have allowed them to successfully adapt to a wide range of lifestyles and habitats.

MEDIA STUDY TOOLS http://connect.mheducation.com Maximize your study time with McGraw-Hill SmartBook®, the first adaptive textbook.

 

Animations

31.1  Three Domains

  Tutorials 31.1  Protostomes and Deuterostomes

SUMMARIZE

■ Porifera (such as sponges) have the cellular level of organization, lack

31.1  Evolutionary Trends Among Animals

■ They use collar cells to create a water current, enabling them to act as

Animals are motile, multicellular heterotrophs that ingest their food. They exhibit a diploid life cycle. ■ Animals are divided into two categories—invertebrates, those that lack an endoskeleton—and vertebrates, those that possess an endoskeleton. ■ Symmetry varies among animals. Some are asymmetrical, whereas others have radial symmetry or bilateral symmetry. Some animals live portions of their life as sessile individuals. Cephalization is often associated with bilateral symmetry. ■ Animals are further divided into the protostomes or deuterostomes based upon their developmental features and processes. Lophotrochozoa are examples of protostomes. This group is divided into the lophophores, which contain a lophophoran, or mouth surrounded by ciliated tentacles, as well as the trochozoan, or individuals that either have presently, or their ancestors had, a trochophore larva.

31.2  The Simplest Invertebrates Table 31.2 contrasts selected anatomical features used to help classify animals.

true tissues, and are typically asymmetrical. filter feeders.

■ Most sponges are hermaphroditic, meaning they have both male and

female sex organs.

■ The internal skeleton is composed of spicules, which is often used to

classify them. Comb jellies and cnidarians are radially symmetrical and have two tissue layers derived from the germ layers ectoderm and endoderm. The body of comb jellies is made up of a transparent jellylike substance called mesoglea. ■ The gastrovascular cavity serves as a location for digestion and circulation of nutrients. ■ The defining feature of the cnidarians is the presence of stinging cells called cnidocytes that contain a toxin-filled capsule called a nematocyst. ■ Cnidarians exist as either a polyp or a medusa, or they can alternate between the two.

31.3  The Lophotrochozoa Lophotrochozoans are protostomes and include the lophophorans and the trochozoans. ■ The lophophorans include the bryozoans, brachiopods, and phoronids. All have a ciliated feeding apparatus.



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UNIT 6  Evolution and Diversity

TABLE 31.2  Comparison of Invertebrates Sponges

Cnidarians

Flatworms

Roundworms

Other Phyla*

Level of organization

Cell

Tissue

Organs

Organs

Organs

Type of body plan

—

Incomplete digestive system

Incomplete digestive system

Complete digestive system

Complete digestive system

Type of symmetry

Usually asymmetrical

Radial

Bilateral

Bilateral

Bilateral except echinoderms

Type of body cavity

—

—

None

Pseudocoelom

Coelom

Segmentation

—

—

—

—

Only annelids, arthropods, and chordates

Jointed appendages

—

—

—

—

Only arthropods and chordates

*Molluscs, annelids, arthropods, echinoderms, chordates

■ The trochozoans have a stage of development that uses cilia for loco-

motion. The trochozoans include: ∙ Flatworms have three tissue layers and no coelom. Planaria have cephalization, muscles, and a ladder-type nervous system and also use flame cells to help propel water toward excretory pores. Tapeworms have a specialized structure called the scolex that enables them to attach onto their host, followed by multiple reproductive units called proglottids. ∙ Rotifers are microscopic and have a corona that resembles a spinning wheel when in motion. ∙ The body of a mollusc typically contains a visceral mass, a mantle, and a foot. Many molluscs also have a head and a radula. ∙ Molluscs often have a reduced coelom and an open circulatory system. ∙ Gastropods include the nudibranchs, conchs, and snails. ∙ Cephalopods, such as squid, display marked cephalization, move rapidly by jet propulsion, and have a closed circulatory system. ∙ Bivalves (e.g., clams), are generally filter feeders that possess an open circulatory system, a radula, and a hatchet foot. ∙ Annelids are segmented both externally and internally. Their hydrostatic skeleton is a fluid-filled interior that supports muscle contraction and enhances flexibility. The nervous system consists of a ventral solid nerve cord and brain. ∙ The nephridium functions as a waste collection and excretory organ. ∙ Polychaetes are marine worms that have parapodia that contain tiny bristlelike hairs called setae. ∙ Earthworms are oligochaetes that are segmented and hermaphroditic scavengers that feed on organic matter in the soil. ∙ Leeches lack the setae found in other annelids. They contain suckers on each end of the body that enable attachment and feeding.

31.4  The Ecdysozoa The ecdysozoans all possess a cuticle and undergo molting during their larval stages. They represent the most abundant form of animal on the planet. ■ Roundworms are usually small, very diverse, and are present almost everywhere in great numbers. Roundworms have a pseudocoelom. Trichinosis and elephantiasis are two diseases associated with roundworm infections. ■ Arthropods are the most diverse group of animals. Their success is largely attributable to a flexible exoskeleton, segmentation, a welldeveloped nervous system, respiratory organs, and metamorphosis. Like many other arthropods, crustaceans have a head that bears compound eyes, antennae, and mouthparts. Blood is pumped into the hemocoel, where hemolymph will flow about the organs.

■ Like many other insects, grasshoppers have wings and three pairs of

legs attached to the thorax. Grasshoppers also have several adaptations to a terrestrial life, including respiration by tracheae and an excretory system consisting of Malpighian tubules, which collect and excrete nitrogenous wastes. ■ Spiders are arachnids with chelicerae, pedipalps, and four pairs of walking legs attached to a cephalothorax. Spiders, too, are adapted to life on land, and they spin silk that is used in various ways. ■ The crayfish, a crustacean, also has an open circulatory system, respiration by gills, and a ventral solid nerve cord.

31.5  Invertebrate Deuterostomes Echinoderms and chordates are deuterostomes, whereby the second embryonic opening becomes the mouth and the coelom forms by outpocketing of the primitive gut. Echinoderms (e.g., sea stars, sea urchins, sea cucumbers, sea lilies) have radial symmetry as adults (not as larvae) and spines from endoskeletal plates. ■ Typical of echinoderms, sea stars have tiny skin gills, a central nerve ring with branches, and a water vascular system for locomotion. Each arm of a sea star contains components of the nervous, digestive, and reproductive systems.

ASSESS Testing Yourself Choose the best answer for each question.

31.1  Evolutionary Trends Among Animals 1. Which of these is not a characteristic of animals? a. heterotrophic d. have chlorophyll b. ingest their food e. multicellular c. cells lack cell walls 2. The phylogenetic tree of animals shows that a. rotifers are closely related to flatworms. b. both molluscs and annelids are protostomes. c. some animals have radial symmetry. d. sponges were the first to evolve from an ancestral protist. e. All of these are correct. 3. Which of these does not pertain to a protostome? a. spiral cleavage b. blastopore is associated with the anus c. coelom, splitting of mesoderm d. annelids, arthropods, and molluscs e. mouth is associated with first opening



Chapter 31  Animals: The Invertebrates

31.2  The Simplest Invertebrates

31.5  Invertebrate Deuterostomes

4. Comb jellies are most closely related to a. cnidarians. d. roundworms. b. sponges. e. rotifers.  c. flatworms. 5. Corals belong to which of the following? a. comb jellies d. rotifers b. sponges e. None of these are correct. c. cnidarians 6. Which of the following is responsible for the process of filter feeding in a sponge? a. flame cells c. cnidocytes b. collar cells d. mesoglea

14. The two phyla in the Deuterostomia are a. Arthropoda and Nematoda. b. Brachiopoda and Platyhelminthes. c. Cestoda and Rotifera. d. Cnidaria and Ctenophora. e. Echinodermata and Chordata.  15. Which is not a characteristic feature of sea stars? a. coelom b. eyespot c. skin gills d. trachea e. tube feet  16. The water vascular system of sea stars functions in a. circulation of gasses and nutrients.  b. digestion. c. locomotion. d. reproduction. e. Both a and c are correct.

31.3  The Lophotrochozoans 7. A ciliated feeding structure in the larval stage is a characteristic of a: a. trochozoan c. lophophoran b. parazoan d. deuterostome 8. Which of the following has a complete digestive system? a. tapeworms c. cnidarians b. sponges d. earthworms 9. Bivalves, cephalopods, and gastropods are all: a. arthropods d. annelids b. lophophorans e. molluscs c. cnidarians 10. Which of the following is a characteristic of an annelid? a. nephridia b. closed circulatory system c. a nerve cord d. All of the above are correct.

31.4  The Ecdysozoa 11. Which characteristic(s) account(s) for the success of arthropods? a. jointed appendages c. segmentation b. an exoskeleton d. All of the above are correct. 12. Which of the following is not an arthropod? a. arachnids b. crustaceans c. rotifers d. insects e. All of the above are arthropods. 13. Label the indicated parts of the diagram below: a.

b.

e.

g.

f. h. i. j.

d.

c. k.

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ENGAGE Thinking Critically 1. Cnidarians are radially symmetrical, whereas flatworms, roundworms, molluscs, annelids, and arthropods are bilaterally symmetrical. How does the type of symmetry relate to the lifestyle of each type of animal? 2. Multicellular animals are thought to have evolved from single-celled protists resembling the choanoflagellates that exist today. Describe some specific advantages to multicellularity that gave animals an advantage over their single-celled relatives.  3. Many species of invertebrates (e.g., flatworms, roundworms, arthropods) are parasites of other animals. List several advantages, as well as challenges, inherent in a parasitic lifestyle. 

PHOTO CREDITS Opener: © James H. Robinson/Science Source; 31.1a: © Salvanegra/iStock/Getty RF; 31.1b: © Mark Dierker/McGraw-Hill Education; 31.1c: © Steve Bloom/Taxi/Getty; 31.1d: © Carolina Biological Supply Company/Phototake; 31.4a: © Andrew J. Martinez/Science Source; 31A: © Secret Sea Visions/Getty Images; 31.5a: © Jeff Rotman/Alamy; 31.5b: © Andrey Nekrasov/Alamy/RF; 31.6b: © AlbyDeTweede/iStock/Getty RF; 31.6c: © Corbis RF; 31.6d: © Islands in the Sea 2002, NOAA/OER; 31.6e: © Amos Nachoum/Corbis; 31.7(hydra): © NHPA/M. I. Walker/Photoshot RF; 31.8a: © The Natural History Museum/ Alamy; 31.9e: © NHPA/M. I. Walker/Photoshot RF; 31.10(proglottid): © CDC; 31.10(scolex): © James Webb/Phototake; 31.12a: © Jeff Rotman/Photolibrary/Getty Images; 31.12b: © Larry S. Roberts; 31.12c: © Kenneth W. Fink/Bruce Coleman/Photoshot; 31.12d: © Juniors Bildarchiv GmbH/Alamy RF; 31.13a: © Rosemary Calvert/Photographer’s Choice/ Getty Images; 31.13b: © Georgette Douwma/Science Source; 31.15a: © Heather Angel/ Natural Visions; 31.15c: © Diane R. Nelson; 31.16c: © MikeLane45/iStock360/Getty RF; 31.17: © St. Bartholomew’s Hospital/SPL/Science Source; 31.18a: © Kim Scott/Ricochet Creative Productions LLC; 31.18c: © Vanessa Vick/The New York Times/Redux; 31.19a: © John MacGregor/Getty Images; 31.19b: © G.C. Kelley/Science Source; 31.19c: © Bob Evans/Getty Images; 31.19d: © James Robinson/Science Source; 31.19e: © Matthijs Kuijpers/ Alamy RF; 31.22a: © McPhoto/SHU/INSADCO Photography/Alamy RF; 31.22b: © Creatas Images/PictureQuest RF; 31.22c: © MedioImages/Photodisc/Getty RF; 31.22d: © Photos. comSelect/Index Stock Imagery RF; 31.22e: © Darlyne A. Murawski/Getty Images; 31.24a: © Tom McHugh/Science Source; 31.24b: © imagebroker.net/Superstock RF; 31.24c: © Scott Camazine/Science Source; 31.25a : © Diane R. Nelson; 31.25b–c (both): © David Wrobel/ Getty Images; 31.25d: © Philippe Bourseiller/Getty Images; 31.26a: © Randy Morse, GoldenStateImages.com; 31.27: © Andy Long/Painet.



32

Animals: Chordates and Vertebrates CHAPTER OUTLINE 32.1  Chordates 32.2 Vertebrates: Fish and Amphibians 32.3 Vertebrates: Reptiles and Mammals 32.4 Evolution of the Hominins 32.5 Evolution of Modern Humans BEFORE YOU BEGIN Before beginning this chapter, take a few moments to review the following discussions: Section 27.2  How does the fossil record support evolutionary theory? Section 27.6  How does a phylogenetic tree indicate the evolutionary relatedness of species? Section 31.1  What phyla of animals are most closely related to the chordates?

CASE STUDY Evolution of Man’s Best Friend Dogs are probably our most familiar and beloved domesticated animals, “man’s best friend.” They are our companions as well as our working partners in war, law enforcement, herding, service, rescue, and therapy. But when and where did domestic dogs first evolve? Scientists have long suspected that Canis familiaris evolved from Canis lupus, the gray wolf, but genetic analyses now indicate that modern dogs evolved from a species of wolf that is now extinct. And while previous studies supported the idea that dogs were first domesticated in China or the Middle East, comparisons of mitochondrial DNA isolated from modern dogs, wolves, and ancient dog fossils now indicate that the first domesticated dogs most likely originated in Europe.  However, the time frame of this event still remains in question. While initially the domestication of the dog is believed to have occurred around 16,000 years ago, newer molecular analytical techniques suggest that this may have happened as far back as 130,000 years ago. If this is the case, then humans and canines have been around each other for much longer than anyone thought. The New Guinea singing dog is named for its habit of howling at different pitches. It is also called Stone Age dog, after the stone tool–using people who took it to New Guinea 6,000 years ago. Living on an island, it has remained geographically and reproductively isolated ever since. Therefore, it might be a “living fossil,” an organism with the same genes and characteristics as its original ancestor. If so, the New Guinea singing dog offers an opportunity to study what early domesticated dogs were like. However, like many other vertebrates, which are the topic of this chapter, the New Guinea singing dog is threatened with extinction. Expeditions into the highlands of New Guinea have yielded only a few droppings, tracks, and haunting howls in the distance.  As you read through the chapter, think about the following questions:

1. When in the history of life did vertebrates evolve?  2. What characteristics set vertebrates apart from other animals?  3. What other vertebrates may be considered “living fossils” ?

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Chapter 32  Animals: Chordates and Vertebrates

32.1  Chordates postanal tail

Learning Outcomes Upon completion of this section, you should be able to 1. Identify the four morphological characteristics unique to the chordates. 2. Describe the features of the two groups of nonvertebrate chordates.

To be classified as a chordate (phylum Chordata, around 60,000 species), an animal must have, at some time during its life history, the following characteristics (Fig. 32.1): 1. A dorsal supporting rod called a notochord. The notochord is located just below the nerve cord, toward the back (i.e., dorsal). Vertebrates have an embryonic notochord that is replaced by the vertebral column during development. 2. A dorsal tubular nerve cord. Tubular means that the cord contains a canal filled with fluid. In vertebrates, the nerve cord is protected by the vertebrae. Therefore, it is called the spinal cord because the vertebrae form the spine. 3. Pharyngeal pouches. Most vertebrates have pharyngeal pouches only during embryonic development. In the nonvertebrate chordates, the fishes, and some amphibian larvae, the pharyngeal pouches develop into functioning gills. Water passing into the mouth and the pharynx goes through the gill slits, which are supported by gill arches. In terrestrial vertebrates that breathe by lungs, the pouches are modified for various

notochord

dorsal tubular nerve cord

pharyngeal pouches

Figure 32.1  Chordate characteristics.  All chordates possess these traits at some point in their life cycle.

purposes. In humans, the first pair of pouches becomes the auditory tubes. The second pair becomes the tonsils, while the third and fourth pairs become the thymus and the parathyroids. 4. Postanal tail. The tail extends beyond the anus, hence the term postanal.

Evolutionary Trends Among the Chordates Figure 32.2 depicts the phylogenetic tree of the chordates, and lists at least one main evolutionary trend that distinguishes each group of animals from the preceding group.

lungs Amphibians bony skeleton Lobe-finned Fishes jaws

Chordates

Reptiles*

Vertebrates

4 limbs

Tetrapods

Mammals

Gnathostomes

amniotic egg

Amniotes

mammary gland

common ancestor

Ray-finned Fishes vertebrae Cartilaginous Fishes

notochord ancestral chordate

Jawless Fishes

Tunicates

Lancelets *includes birds

Figure 32.2  Phylogenetic tree of chordates.  Each of the innovations listed is an evolutionary trend shared by the classes beyond the branch point.

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UNIT 6  Evolution and Diversity

The tunicates and lancelets are nonvertebrate chordates, meaning that they do not have vertebrae. The vertebrates include the fishes, amphibians, reptiles, birds, and mammals. The cartilaginous fishes were the first to have jaws, and some early bony fishes had lungs. However, not all bony fishes have lungs. Amphibians were the first group to clearly have jointed appendages and to invade land. However, the fleshy appendages of lobe-finned fishes from the Devonian era contained bones homologous to those of terrestrial vertebrates. These fishes are believed to be ancestral to the amphibians. Reptiles and mammals have a means of reproduction suitable for land. During development, an amnion and other extraembryonic membranes are present. The amnion and extraembryonic membranes support the embryo and prevent it from drying out as it develops into a particular species’ offspring.

Nonvertebrate Chordates Nonvertebrate chordates are characterized by the fact that the notochord never becomes a vertebral column. Lancelets (subphylum Cephalochordata, about 23 species), all belong to one genus,  Branchiostoma (formally Amphioxus). They are marine chordates only a few centimeters long. They are rostrum pharynx notochord oral hood with tentacles

named for their resemblance to a lancet—a small, two-edged surgical knife (Fig. 32.3). Lancelets are found in shallow water along most coasts, where they usually lie partly buried in sandy or muddy substrates with only their anterior mouth and gill apparatus exposed. They feed on microscopic particles filtered out of a constant stream of water that enters the mouth and exits through the gill slits. Lancelets retain the four chordate characteristics as adults. In addition, segmentation is present, as evidenced by the segmental arrangement of the muscles and the periodic branches of the dorsal tubular nerve cord. Tunicates (subphylum Urochordata, about 1,250 species) live on the ocean floor and take their name from a tunic that makes the adults look like thick-walled, squat sacs (Fig. 32.4). They are also called sea squirts because they squirt water from one of their siphons when disturbed. The tunicate larva is bilaterally symmetrical and has the four chordate characteristics. Metamorphosis produces the sessile adult, which has incurrent and excurrent siphons. The pharynx is lined by numerous cilia. Beating of these cilia creates a current of water that moves into the pharynx and out the numerous gill slits, the only chordate characteristic that remains in the adult. Microscopic particles adhere to a mucous secretion and are digested. Some biologists have suggested that tunicates are directly related to vertebrates. They hypothesize that a larva with the four chordate characteristics may have become sexually mature, and evolution thereafter produced a fishlike vertebrate.

dorsal tubular nerve cord excurrent siphon

incurrent siphon

dorsal fin

gill bars and slits

caudal fin

atrium atriopore

gill slit

ventral fin anus

tunic

Figure 32.4  Sea squirt, Halocynthia.  The only chordate characteristic remaining in the adult is gill slits.

Check Your Progress  32.1 1. Identify the four features that distinguish chordates from other animal phyla.

2. Identify the morphological differences between a tunicate and a lancelet.

Figure 32.3  Lancelet, Branchiostoma.  Lancelets are filter

feeders. Water enters the mouth and exits at the atriopore after passing through the gill slits.

3. Describe three innovations that distinguish mammals from cartilaginous fishes (see Fig. 32.2).



Chapter 32  Animals: Chordates and Vertebrates

655

32.2  Vertebrates: Fish and Amphibians Learning Outcomes

dorsal fin

Upon completion of this section, you should be able to 1. Compare and contrast the characteristics of a jawless fish to those of a jawed fish. 2. Describe the major evolutionary innovations that distinguish the fishes from the amphibians. 3. Identify the features of the three groups of living amphibians.

gill slits

pelvic fin

jaw with teeth

At some time in their life history, all vertebrates (subphylum Vertebrata, about 58,000 species) have the characteristics of a chordate. The embryonic notochord, however, is generally replaced by a vertebral column composed of individual vertebrae. The vertebral column, which is part of the flexible but strong jointed endoskeleton, provides evidence that vertebrates are segmented. The skeleton protects the internal organs and serves as a place of attachment for muscles. Together, the skeleton and muscles form a system that permits rapid, and a more efficient, movement. Two pairs of appendages are typical. The pectoral and pelvic fins of fishes evolved into the jointed appendages that allowed vertebrates to move onto land. The main axis of the endoskeleton consists of not only the vertebral column, but also a skull that encloses and protects the brain. The high degree of cephalization is accompanied by complex sense organs. The eyes develop as outgrowths of the brain. The ears are primarily equilibrium devices in aquatic vertebrates, but they also function as sound-wave receivers in land vertebrates. The evolution of jaws in vertebrates has allowed them to diversify into a variety of biological roles. Vertebrates have a complete digestive tract and a large coelom. Their circulatory system is closed, meaning that the blood is contained entirely within blood vessels. Vertebrates have an efficient means of obtaining oxygen from water or air, as appropriate. The kidneys are important excretory and water-regulating organs. The sexes are generally separate, and reproduction is usually sexual. The evolution of the amnion allowed reproduction to take place on land. Many species of reptiles, and a few species of mammals, lay a shelled egg. In placental mammals, development takes place within the uterus of the female. Although use of vertebrates, as well as other animals in laboratory research, has been very beneficial in many ways, it also generates controversy for some. The Health feature, “Vertebrates and Human Medicine,” discusses some ways that products from vertebrates are used in medicine. A strong, jointed endoskeleton, a vertebral column composed of vertebrae, a closed circulatory system, efficient respiration and excretion, and a high degree of cephalization are characteristics demonstrating vertebrates are adapted to an active lifestyle. ­Figure 32.5 shows major milestones in the history of vertebrates: the evolution of jaws, limbs, and the amnion, an extraembryonic membrane that is first seen in the shelled amniotic egg of reptiles.

pectoral fin

a. Tiger shark, Galeocerdo cuvier

b. Blue-spotted salamander, Ambystoma

c. Rhinoceros iguanas, Cyclura

Figure 32.5  Milestones in vertebrate evolution.  a. The

evolution of jaws in fishes allows animals to be predators and to feed off other animals. b. The evolution of limbs in most amphibians is adaptive for locomotion on land. c. The evolution of an amnion and a shelled egg in reptiles is adaptive for reproduction on land.



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UNIT 6  Evolution and Diversity

SCIENCE IN YOUR LIFE  ►

HEALTH

Vertebrates and Human Medicine Hundreds of pharmaceutical products come from other vertebrates, and even those that produce poisons and toxins provide medicines that benefit us.

Natural Products with Medical Applications The black-and-white spitting cobra of Southeast Asia paralyzes its victims with a potent venom, which eventually leads to respiratory arrest. However, that venom is also the source of the drug Immunokine, which can inhibit some harmful effects of an overactive immune system. It is approved in Thailand for use in combating the side effects of cancer therapy, and it is being studied for use in treating AIDS, autoimmune diseases, and other disorders.  Although snakebites can be very painful, certain components found in venom actually relieve pain. The black mamba, found mainly in sub-Saharan Africa, is one of the most lethal snakes on Earth. Compounds in its venom called mambalgins, however, block pain signals by inhibiting the flow of certain ions through nerves that carry pain messages. When tested in mice, these compounds were as effective as morphine, with fewer side effects.  Another compound, known as epibatidine, derived from the skin of an endangered Ecuadorian poison-dart frog, is 50–200 times more powerful than morphine in relieving chronic and acute pain, without the addictive properties. Unfortunately, it can also have serious side effects, so companies have synthesized compounds with a similar structure, hoping to improve its safety profile. Other venoms mainly affect blood clotting. Eptifibatide is derived from the venom of the pigmy rattlesnake, which lives in the southeastern United States. Because it binds to blood

a. Snake

platelets and reduces their tendency to clump together, this drug is used to reduce the risk of clot formation in patients at risk for heart attacks. Alternatively, the venom of several pit vipers, such as the copperhead, contains “clotbusting” (thrombolytic) substances, which can be used to dissolve abnormal clots that have already formed.  Sharks produce a variety of chemicals with potentially medicinal properties. Squalamine is a steroidlike molecule that was first isolated from the liver of dogfish sharks. It has broad antimicrobial properties, and it can inhibit the abnormal growth of new blood vessels, which is a factor in cancer and a variety of other diseases. Squalamine is also safe enough to be used in the eye and is currently being tested as a potential treatment for macular degeneration, an eye disease that will affect 3 million Americans by 2020. 

Animal Pharming Some of the most powerful applications of genetic engineering can be found in the development of drugs and therapies for human diseases. In fact, this technology has led to a new industry: animal pharming, which uses genetically altered vertebrates, such as mice, sheep, goats, cows, pigs, and chickens, to produce medically useful pharmaceutical products.  To accomplish this, the human gene for some useful product is inserted into the embryo of a vertebrate. That embryo is implanted into a foster mother, which gives birth to the transgenic animal, which contains genes from the two sources. An adult transgenic vertebrate produces large quantities of the pharmed product in its blood, eggs, or milk, from which the product can be easily harvested and purified. 

b. Goats

An example of a pharmed product used in human medicine is human antithrombin. This medication is important in the treatment of individuals who have a hereditary deficiency of this protein and so are at high risk for lifethreatening blood clots. Approved by the FDA in 2009, the bioengineered drug, known by the brand name ATryn®, is purified from the milk of transgenic goats.

Xenotransplantation There is an alarming shortage of human donor organs to fill the need for hearts, kidneys, and livers. One solution is xenotransplantation, the transplantation of nonhuman vertebrate tissues and organs into humans. The first such transplant occurred in 1984 when a team of surgeons implanted a baboon heart into an infant, who, unfortunately, lived only 20 days.  Although apes are more closely related to humans, pigs are considered to be the best source for xenotransplants. Pig organs are similar to human organs in size, anatomy, and physiology, and large numbers of pigs can be produced quickly. Most infectious microbes of pigs are unlikely to infect a human recipient. Currently, pig heart valves and skin are routinely used for treatment of humans. Miniature pigs, whose heart size is similar to that of humans, are being genetically engineered to make their tissues less foreign to the human immune system, to minimize rejection. 

Questions to Consider 1. Is it ethical to change the genetic makeup of vertebrates in order to use them as drug or organ factories? 2. What are some of the health concerns that may arise due to xenotransplantation?

c. Pig heart

Figure 32A  Medical uses of animals.  a. Snake venom may be used to create pain medications. b. Mammals, such as these goats, may express pharmaceutical compounds in their milk. c. Pigs are now being genetically altered to provide a supply of hearts for heart transplant operations.



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Chapter 32  Animals: Chordates and Vertebrates tail

Fishes: Evolution of the Jaw

eye

The first vertebrates were jawless fishes that wiggled through the water and sucked up food from the ocean floor. Today, there are three living groups of fishes: jawless fishes, cartilaginous fishes, and bony fishes. The two latter groups have jaws, tooth-bearing structures in the head. Jaws evolved from the first pair of gill arches, structures that ordinarily support gills (Fig. 32.6). The second pair of gill arches became support structures for the jaws, instead of gills. The presence of jaws permits a predatory way of life in many species of fishes. Fishes are adapted to life in the water. Usually, they shed their sperm and eggs into the water, where fertilization occurs. The zygote develops into a swimming larva, which must fend for itself until it develops into the adult form.

Jawless Fishes Living representatives of the jawless fishes (about 65 species) are cylindrical and up to a meter long. They have smooth, scaleless skin and no jaws or paired fins. The two groups of living jawless fishes are hagfishes (class Myxini) and lampreys (class Cephalaspidomorphi). Although both are jawless, there are distinct differences. Hagfishes are an ancient group of fish. They possess a skull, but lack the vertebrae found in the other classes of vertebrates. However, molecular evidence suggests that these were once present in these fish, so they are traditionally classified with the vertebrates. Hagfishes are scavengers, feeding mainly on dead fishes.  Lampreys possess a true vertebral column. Most are parasites which use their round mouth as a sucker to attach itself to another fish and tap into its circulatory system. Unlike other fishes, the lamprey cannot take in water through its mouth. Instead, water moves in and out through the gill openings.

Cartilaginous Fishes

spiracle

swim bladder

stomach muscle bony vertebra

lateral line

brain nostril

scales mandible kidney

gonad

intestine gallbladder

b.

Cartilaginous fishes (class Chondrichthyes, about 750 species) are the sharks (see Fig. 32.5a), the rays (Fig. 32.7a), and the skates. This group of fishes is so named because they have skeletons of cartilage instead of bone. The small dogfish shark is often dissected

gills heart liver

lobed fins

skull

gill slits

2nd gill arch

flattened pectoral fin a. Blue-spotted stingray, Taeniura lymma

1st gill arch c. Coelacanth, Latimeria chalumnae

Figure 32.7  Jawed fishes.  a. Rays, such as this blue-spotted jaws

Figure 32.6  Evolution of the jaw.  The first jaw evolved from the

first and second gill arches of fishes.

stingray, are cartilaginous fishes. b. A soldierfish has the typical appearance and anatomy of a ray-finned fish, the most common type of bony fish. c. A coelacanth is a lobe-finned fish once thought to be extinct.



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UNIT 6  Evolution and Diversity

in biology laboratories. One of the most dangerous sharks, and one that is capable of living in both fresh and salt water, is the bullshark, although, in general, people are rarely attacked by sharks. Each year more people are injured or killed by dog attacks than shark attacks. The largest sharks, the whale sharks, feed on small fishes and marine invertebrates and do not attack humans. Skates and rays are rather flat fishes that live partly buried in the sand and feed on mussels, clams, and various crustaceans. Three well-developed senses enable sharks and rays to detect their prey: (1) They have the ability to sense electric currents in water—even those generated by the muscle movements of animals. (2) They have a lateral line system, a series of pressure-­ sensitive cells along both sides of the body that can sense pressure caused by nearby movement in the water. (3) They have a keen sense of smell. The part of the brain associated with this sense is enlarged relative to the other parts. Sharks can detect some scents from about a third of a mile away in the open ocean.

Bony Fishes Bony fishes (about 30,000 species) are by far the most numerous and diverse of all the vertebrates. Most of the bony fishes we eat, such as cod, trout, salmon, and haddock, are ray-finned fishes (class Actinopterygii). Their paired fins, which they use to balance and propel the body, are thin and supported by bony rays. Ray-finned fishes have various ways of life. Some, such as herring, are filter feeders; others, such as trout, are opportunists; and still others are predaceous carnivores, such as piranhas and barracudas. Ray-finned fishes have a swim bladder, which usually regulates buoyancy (Fig. 32.7b). By secreting gases into or absorbing gases from the bladder, these fishes can change their density, allowing them to go up or down in the water. The streamlined shape, fins, and muscle action of ray-finned fishes are all suited for locomotion in the water. Their skin is covered by bony scales that protect the body but do not prevent water loss. The gills of rayfinned fish are kept continuously moist by the passage of water through the mouth and out the gill slits during respiration. As the water passes over the gills, oxygen is absorbed and carbon dioxide is given off. Ray-finned fishes have a single-circuit circulatory system. The heart is a simple pump, and the blood flows through the chambers, including a nondivided atrium and ventricle, to the gills. Oxygenated blood leaves the gills and is circulated throughout the body by the heart (Fig. 32.8a). Another type of bony fish, called the lobe-finned fishes (class Sarcopterygii), are the ancestors of the amphibians. These fishes not only had fleshy appendages that could be adapted to land locomotion, but most also had a lung that was used for respiration. One type of lobe-finned fish, the coelacanth, was thought to have gone extinct about 20,000 years ago. However, some coelacanths have been discovered off the coasts of Eastern Africa and Indonesia, making them the only “living fossil” among these fishes (see Fig. 32.7c).

Amphibians: Transition to Land Amphibians (class Amphibia, about 6,000 species), whose class name means living on both land and in the water, are represented today by frogs, toads, newts, and salamanders. Most members of

this group lead a lifestyle that includes a larval stage that lives in the water, and an adult stage that lives on land. Caecilians are also classified as amphibians even though they are fossorial, wormlike amphibians that spend most of their life underground.  Amphibians evolved from the lobe-finned fishes with lungs by way of transitional forms. Two hypotheses have been suggested to account for the evolution of amphibians from lobe-finned fishes. Perhaps lobe-finned fishes had an advantage over others because they could use their lobed fins to move from pond to pond. Or perhaps the supply of food on land in the form of plants and insects— and the absence of predators—promoted further adaptations to the land environment. Paleontologists have recently found a well-­ preserved transitional fossil from the late Devonian period in Arctic Canada that represents an intermediate between lobe-finned fishes and tetrapods with limbs. This fossil, named Tiktaalik roseae, provides unique insights into how the legs of tetrapods arose (Fig. 32.9)  Aside from jointed appendages (which have been lost in the limbless caecilians), amphibians have other features not seen in bony fishes: they usually have four limbs, eyelids for keeping their eyes moist, ears (a tympanum) for picking up sound waves, and a voice-producing larynx. Relative to body size, the brain is larger than that of a fish. Adult amphibians usually have small lungs, but some species respire entirely through their skin. Air enters the mouth by way of nostrils, and when the floor of the mouth is raised, air is forced into the relatively small lungs. Respiration is

gill capillaries

lung and skin capillaries

lung capillaries

ventricle atrium

right atrium

left ventricle

other capillaries

other capillaries

other capillaries

a. Fishes

b. Amphibians and most reptiles

c. Some reptiles; birds and mammals

O2-rich blood

O2-poor blood

mixed blood

Figure 32.8  Vertebrate circulatory pathways.  a. The single-

loop pathway of fishes has a two-chambered heart. b. The double-loop pathway of other vertebrates sends blood to the lungs and to the body. In amphibians and most reptiles, limited mixing of oxygen-rich and oxygen-poor blood takes place in the single ventricle of their three-chambered heart. c. The four-chambered heart of some reptiles (crocodilians and birds) and mammals sends only oxygen-poor blood to the lungs and oxygen-rich blood to the body. 



Chapter 32  Animals: Chordates and Vertebrates

Transitional form

Ancestral amphibian

shoulder

pelvis

shoulder

pelvis

femur

humerus radius

659

femur

humerus

ulna fins

radius

tibia-fibula

tibia

ulna

fibula

limbs

Figure 32.9  Lobe-finned fishes to amphibians.  This transitional form links the lobes of lobe-finned fishes to the limbs of ancestral amphibians. Compare the fins of the transitional form (left) to the limbs of the ancestral amphibian (right). 

supplemented by gas exchange through the smooth, moist, and glandular skin. The amphibian heart has a divided atrium but only a single ventricle. The right atrium receives deoxygenated blood from the body, and the left atrium receives oxygenated blood from the lungs. These two types of blood are partially mixed in the single ventricle (see Fig. 32.8b). Mixed blood is then sent to all parts of the body. Some is sent to the skin, where it is further oxygenated. Currently, there is widespread concern about the future of amphibians because over 50% of all amphibian species are threatened with extinction. Because of their permeable skin and a life cycle that often depends on both water and land, they are thought to be “indicator species” of environmental quality. That is, they are the first to respond to environmental degradation, and disappearances of populations and species thus generate concern for other species—even humans.

Check Your Progress  32.2 1. Recognize characteristics that distinguish fish from other vertebrates.

2. List the features that distinguish amphibians from fish. 3. Describe the features that allow most amphibians to have life stages in water and on land.

32.3  Vertebrates: Reptiles and Mammals Learning Outcomes Upon completion of this section, you should be able to 1. Compare the major evolutionary innovations that distinguish the reptiles and mammals. 2. List the features that distinguish birds from the rest of the reptile lineage. 3. Identify the unique features that define the three living lineages of mammals.

Reptiles: Amniotic Egg Reptiles (class Reptilia, about 17,000 species, including birds) diversified and were most abundant between 245 and 65 million years ago. These reptiles included the mammal-like reptiles (now extinct), the ancestors of today’s living reptiles, and the dinosaurs (now extinct). Some dinosaurs are remembered for their great size. Brachiosaurus, an herbivore, was about 23 m (75 ft) long and about 17 m (56 ft) tall. Tyrannosaurus rex was 5 m (16 ft) tall when standing on its hind legs. The bipedal stance of some reptiles preceded the evolution of wings in birds. In fact, molecular and morphological evidence suggests that the birds are the descendants of the dinosaurs.  The reptiles living today are mainly turtles, alligators, snakes, lizards, and birds. The body is covered with hard, keratinized scales, which protect the animal from desiccation and from predators. Another adaptation for a land existence is the manner in which snakes typically use their tongue as a sense organ. Reptiles have well-developed lungs enclosed by a protective rib cage. When the rib cage expands, a partial vacuum establishes a negative pressure, which causes air to rush into the lungs. The atrium of the heart is always separated into right and left chambers, but division of the ventricle varies. An interventricular septum is incomplete in certain species. Therefore, some oxygenated and deoxygenated blood is mixed between the ventricles (see Fig. 32.8c). Perhaps the most outstanding evolutionary innovation that first appeared in the reptiles is their means of reproduction that is suitable to a land existence. The penis of the male passes sperm directly to the female. Fertilization is internal, and the female typically possesses leathery, flexible, shelled eggs. Some snakes lay eggs, while other snakes actually give birth to live young. This amniotic egg made development on land possible and eliminated the need for a swimming-larval stage during development. This egg provides the developing embryo with atmospheric oxygen, food, and water; it removes nitrogenous wastes; and it protects the embryo from drying out and from mechanical injury. This is accomplished by the presence of extraembryonic membranes such as the chorion (Fig. 32.10).



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UNIT 6  Evolution and Diversity egg shell shell

jaws

yolk sac albumin amnion embryo chorion allantois

air space a. Nile crocodile, Crocodylus niloticus

b.

Figure 32.10  The reptilian egg allows reproduction on land.  a. Baby American crocodile, Crocodylus, hatching out of its shell. Note that the shell is leathery and flexible, not brittle like birds’ eggs. b. Inside the egg, the embryo is surrounded by extraembryonic membranes. The chorion aids gas exchange, the yolk sac provides nutrients, the allantois stores waste, and the amnion encloses a fluid that prevents drying out and provides protection.

Fishes, amphibians, and living reptiles other than birds are ectothermic, meaning that their body temperature matches the temperature of the external environment. If it is cold externally, they are cold internally; if it is hot externally, they are hot internally. Reptiles try to regulate their body temperatures by basking in the sun if they need warmth or by hiding in the shadows if they need cooling off.

Feathered Reptiles To many people, the birds (class Aves) are the most conspicuous, melodic, beautiful, and fascinating group of vertebrates. Over 9,000 species of birds have been described. Birds range in size from the tiny “bee” hummingbird at 1.8 g to the ostrich at a maximum weight of 160 kg and a height of 2.7 m. The fossil record of birds, such as Archaeopteryx and newly described fossils from Mongolia and Spain, provides evidence that birds evolved from reptiles. Combined with molecular studies, birds are now considered part of the Reptilia. Their legs have scales and their feathers are modified reptilian scales. However, birds typically lay a hardshelled amniotic egg, rather than the leathery egg of reptiles.

Anatomy and Physiology of Birds Nearly every anatomical feature of a bird can be related to its ability to fly (Fig. 32.11a). The forelimbs are modified as wings. The hollow, very light bones are laced with air cavities. A horny beak has replaced jaws equipped with teeth, and a slender neck connects the head to a rounded, compact torso. The sternum is enlarged and has a keel, to which strong muscles are attached for flying. Respiration is efficient because the lobular lungs form anterior and posterior air sacs. The presence of these sacs means that the air circulates one way through the lungs and that gases are continuously exchanged

across respiratory tissues (Fig. 32.11b). Another benefit of air sacs is that they lighten the body for more efficient flying. Birds have a four-chambered heart that completely separates O2-rich blood from O2-poor blood. The left ventricle pumps O2-rich blood under pressure to the muscles (see Fig. 32.8c). Birds are endothermic. Like mammals, their internal temperature is constant because they generate and maintain metabolic heat. This may be associated with their efficient nervous, respiratory, and circulatory systems. Also, their feathers provide insulation. Birds have no bladder and excrete uric acid in a semi-dry state. Flight requires acute sense organs and an intricate nervous system. Birds have particularly good vision and well-developed brains. Their muscle reflexes are excellent. An enlarged portion of the brain seems to be the area responsible for instinctive behavior. A ritualized courtship often precedes mating. Many newly hatched birds require parental care before they are able to fly away and seek food for themselves. A remarkable aspect of bird behavior is the seasonal migration of many species over very long distances. Birds navigate by day and night, whether it’s sunny or cloudy, by using the sun and stars and even Earth’s magnetic field to guide them.

Diversity of Birds The majority of birds, including eagles, geese, and mockingbirds, have the ability to fly. However, some birds, such as emus, penguins, and ostriches, are flightless. Traditionally, birds have been classified based on beak and foot type (Fig. 32.12) and, to some extent, on their habitat and behavior. The various orders include birds of prey with notched beaks and sharp talons; shorebirds with long, slender, probing beaks and long, stiltlike legs; woodpeckers with sharp, chisel-like beaks and grasping feet; waterfowl with broad beaks and webbed toes; penguins with wings modified as paddles; and songbirds with perching feet.



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Feather anatomy barbule barb shaft inhalation

nostril

ear opening

trachea

trachea lung

anterior air sacs

esophagus

testis kidney gizzard

lung

exhalation posterior air sacs

1. Inhalation: Air enters posterior air sacs.

2. Exhalation begins: Air enters lung.

crop

vas deferens

inhaled air exhaled air

heart

ureter

liver sternum

exhalation

pancreas rectum cloaca 4. Exhalation ends: Air exits anterior air sacs. a. Bird and feather anatomy

3. Exhalation continues: Air enters anterior air sacs.

b. Respiratory system

Figure 32.11  Bird anatomy and physiology.  a. The anatomy of a pigeon is representative of bird anatomy. b. In birds, air passes one way through the lungs. 

a. Cardinal, Cardinalis cardinalis

b. Bald eagle, Haliaetus leucocephalus

c. Flamingo, Phoenicopterus ruber

Figure 32.12  Bird beaks.  a. A cardinal’s beak allows it to crack tough seeds. b. A bald eagle’s beak allows it to tear prey apart. c. A flamingo’s beak strains food from the water with bristles that fringe the mandibles.

Mammals: Hair and Mammary Glands Mammals (class Mammalia, about 4,600 species) also evolved from the reptiles. The first mammals were small, about the size of mice. During all the time the dinosaurs flourished (165–65 mya), mammals remained small in size and changed little evolutionarily. Some of the earliest mammalian groups are still represented today by the monotremes and marsupials. The placental mammals that evolved later went on to live in many habitats, including air, land, and sea.

The chief characteristics of mammals are body hair and milkproducing mammary glands. Almost all mammals are endothermic and maintain a constant internal temperature. Many of the adaptations of mammals are related to temperature control. Hair, for example, provides insulation against heat loss and allows mammals to be active, even in cold weather. Like birds, mammals have efficient respiratory and circulatory systems, which ensure a ready supply of oxygen to muscles whose contraction produces body heat. Also, like birds, mammals have double-loop circulation and a four-chambered heart (see Fig. 32.8c).



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Mammary glands enable females to feed (nurse) their young a nutrient-rich source of food. Nursing also creates a bond between mother and offspring that helps ensure parental care while the young are helpless. In all mammals (except monotremes), the young are born alive after a period of development in the uterus, a part of the female reproductive tract. Internal development shelters the young and allows the female to move actively about while the young are maturing. Mammals are classified as monotremes, marsupials, or placental mammals, according to how they reproduce.

Monotremes Monotremes are mammals that, like birds, have a cloaca, a terminal region of the digestive tract that serves as a common chamber for feces, excretory wastes, and sex cells. They also lay hardshelled amniotic eggs. They are represented by only five species: four species of spiny anteater and the duckbill platypus. All are found in Australia and New Guinea (Fig. 32.13a). The female duckbill platypus lays her eggs in a burrow in the ground. She incubates the eggs, and after hatching, the young lick up milk that seeps from modified sweat glands on the abdomens of both males and females. The spiny anteater has a pouch on the belly side formed by swollen mammary glands and longitudinal muscle. The egg moves from the cloaca to this pouch, where hatching takes place and the young remain for about 53 days. Then they stay in a burrow, where the mother periodically visits and nurses them.

a. Duckbill platypus, Ornithorhynchus anatinus

b. Koala, Phascolarctos cinereus

c. Virginia opossum, Didelphis virginianus

Figure 32.13  Monotremes and marsupials.  a. The duckbill

platypus is a monotreme that inhabits Australian streams. b. The koala is an Australian marsupial that lives in trees. c. The opossum is the only marsupial in the Americas. The Virginia opossum is found in a variety of habitats.

Marsupials The young of marsupials begin their development inside the female’s body, but they are born in a very immature condition. Newborns crawl up into a pouch on their mother’s abdomen. Inside the pouch, they attach to the nipples of mammary glands and continue to develop. Frequently, more are born than can be accommodated by the number of nipples, and it’s “first come, first served.” Marsupial mammals are found mainly in Australia, where they underwent adaptive radiation for several million years without competition from placental mammals. Among the herbivorous marsupials in Australia, koalas are tree-climbing browsers (Fig. 32.13b), and kangaroos are grazers. The Virginia opossum (Didelphis virginiana) is the only marsupial that occurs north of Mexico (Fig. 32.13c). Other species of marsupials live in Central and South America. The Tasmanian devil, a carnivorous marsupial about the size of a small to mediumsized dog, is now threatened with extinction due to a transmissible tumor disease.

Placental Mammals The vast majority of living mammals are placental mammals (Fig. 32.14). In these mammals, the extraembryonic membranes of the reptilian egg (see Fig. 32.10) have been modified for internal development within the uterus of the female. The chorion contributes to the fetal portion of the placenta, while a part of the uterine wall contributes to the maternal portion. Here, nutrients, oxygen, and waste are exchanged between fetal and maternal blood. Mammals, with the exception of marine mammals, are adapted to life on land and have limbs that allow them to move rapidly. In fact, an evaluation of mammalian features leads us to the conclusion that they lead active lives. The lungs are expanded not only by the action of the rib cage, but also by the contraction of the diaphragm, a horizontal muscle that separates the thoracic cavity from the abdominal cavity. The heart has four chambers. The internal temperature is constant, and hair, when present, helps insulate the body. The mammalian brain is well developed and enlarged due to the expansion of the cerebral hemispheres, which control the rest of the brain. The brain is not fully developed until after birth, and the young learn to take care of themselves during a period of dependency on their parents. Mammals have differentiated teeth. Typically, in the front, the incisors and canine teeth have cutting edges for capturing and killing prey. On the sides, the premolars and molars chew food. The specific shape and size of the teeth may be associated with whether the mammal is an herbivore (eats vegetation), a carnivore (eats meat), or an omnivore (eats both meat and vegetation). For example, mice (order Rodentia) have continuously growing incisors; horses have large, grinding molars; and dogs (order Carnivora) have long canine teeth. Placental mammals are classified based on their methods of obtaining food and their mode of locomotion. For example, bats (order Chiroptera) have membranous wings supported by digits; horses (order Perissodactyla) have long, hoofed legs; and whales (order Cetacea) have paddlelike forelimbs.



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Figure 32.14 

Placental mammals.  Placental mammals have adapted to various ways of life. a. Deer are herbivores that live in forests. b. Lions are carnivores on the African plain. c. Monkeys inhabit tropical forests. d. Whales are sea-dwelling placental mammals.

a. White-tailed deer, Odocoileus virginianus

c. White-faced monkey, Cebus capucinus

b. African lioness, Panthera leo

d. Killer whale, Orcinus orca

Primates Primata are members of the order Primates. In contrast to the other orders of placental mammals, most primates are adapted to an arboreal life—that is, for living in trees. Primate limbs are mobile, and the hands and feet both have five digits each. Many primates have a big toe and a thumb that are both opposable, meaning the big toe or thumb can touch each of the other toes or fingers. Humans don’t have an opposable big toe, but the thumb is opposable, and this results in a powerful and precise grip. The opposable thumb allows a primate to easily reach out and bring food to the mouth. When locomoting, primates grasp and release tree limbs freely because they have nails instead of the claws of their evolutionary ancestors. In primates, the snout is shortened considerably, allowing the eyes to move to the front of the head. The stereoscopic vision (or depth perception) that results permits primates to make accurate judgments about the distance and position of adjoining tree limbs. Some primates that are active during the day, such as humans, have color vision and strong visual acuity because the retina contains cone cells in addition to rod cells. Cone cells require bright light, but the image is sharp and in color. The lens of the eye focuses light directly on the fovea, a region of the retina where cone cells are concentrated. The evolutionary trend among primates is toward a larger and more complex brain. The brain size is smallest in prosimians and largest in modern humans. Increases in size of the cerebral cortex,

the brain region associated with learning, memory, thought, and awareness, is primarily responsible for the trend toward increased brain size. The portion of the brain devoted to smell gets smaller, and the portions devoted to sight increase in size and complexity during primate evolution. Also, more and more of the brain is involved in controlling and processing information received from the hands and the thumb. The result is good hand-eye coordination in humans. It is difficult to care for several offspring while moving from limb to limb, so one offspring per birth interval tends to be the norm in most primates. The juvenile period of dependency on parental care is extended, and there is an emphasis on learned behavior and complex social interactions. The order Primates has two suborders: the strepsirhini (lemurs, aye ayes, bush babies, and lorises) and the haplorhini (monkeys, apes, and humans). Thus, humans are more closely related to the monkeys and apes than they are to the strepsirhini.

Check Your Progress  32.3 1. List three features that distinguish reptiles from the mammals.

2. Recognize the features that make placental mammals unique.

3. Identify the characteristics that separate primates from the other mammals.



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32.4 Evolution of the Hominins Learning Outcomes 1. Explain the significance of bipedalism in hominin evolution.

2. Summarize the importance of both ardipithecines and australopithecines in hominin evolution.

3. Arrange the early species of Homo in evolutionary order. The evolutionary tree in Figure 32.15 shows that all primates share a common ancestor and that over time they diverged, producing the various lines of descent, or lineage, we see today. Notice that prosimians (also called strepsirhini), represented by lemurs, aye ayes, bush babies, and lorises, were the first types of primates to diverge from the primate line of descent, and African apes were the last group to diverge. Also note that the tree suggests that humans are most closely related to African apes. One of the most unfortunate misconceptions concerning human evolution is the belief that Darwin and others suggested that humans evolved from apes. On the contrary, humans and apes shared a common apelike ancestor. Today’s apes are our distant cousins, and we couldn’t have evolved from our cousins because we are contemporaries— living on Earth at the same time. Molecular data have been used to estimate the date of the split between the human lineage and that of apes. When two lines of descent first diverge from a common ancestor, the genes of the two lineages are nearly identical. But as time goes by, each lineage accumulates genetic changes. Many genetic changes are neutral (not tied to adaptation) and accumulate at a fairly constant rate. Such changes can be used as a kind of molecular clock to infer how closely two groups are related and the point at which they diverged. Molecular data suggest that the split between the ape and human lineages occurred about 6 to 8 mya. Currently, humans and chimpanzees are probably the most closely related, sharing more than 90% of our DNA.  Hominins (the designation that includes humans and species very closely related to humans) first evolved about 5 mya. Molecular data show that hominins, chimpanzees, and gorillas are all closely related and these groups must have shared a common ancestor sometime during the Miocene. Hominins, chimpanzees, and gorillas are now grouped together as hominines. The hominids include the hominines and the orangutan. The hominoids include the gibbon and the hominids. The hominoid common ancestor first evolved at the beginning of the Miocene about 23 mya.

The First Hominins There have been many recent advances in the study of the hominins, and recent discoveries of fossils in Africa are challenging our view of how early hominins evolved. Paleontologists use certain anatomical features when they try to determine if a fossil is a hominin. These features include bipedalism (walking on two feet), the shape of the face, and brain size. Today’s humans have a flatter face and a more pronounced chin than do the apes, because the human jaw is shorter than that of the apes. Then, too, our teeth are generally smaller and less specialized. We don’t have the sharp

canines of an ape, for example. Chimpanzees have a brain size of about 400 cubic centimeters (cc), and modern humans have a brain size of about 1,360 cc. It’s hard to decide which fossils are hominins, because human features evolved gradually and at different rates. Most investigators rely first and foremost on bipedalism as the hallmark of a hominin, regardless of the size of the brain.  There have been a number of recent fossil discoveries in Africa that have the potential to reshape our understanding of early hominin evolution, specifically the occurrence of the split between the ape human lineages. One of the oldest of these fossils, called Sahelanthropus tchadensis, dated at 7 mya, was found in Chad, in central Africa, far from eastern and southern Africa, where other hominid fossils were excavated. The only find, a skull, appears to be that of a hominin, because it has smaller canines and thicker tooth enamel than an ape. The braincase, however, is very apelike. It is impossible to tell if this hominin walked upright. Some suggest this fossil is ancestral to the gorilla. Orrorin tugenensis, dated at 6 mya and found in eastern Africa, is thought to be another early hominin, especially because the limb anatomy suggests a bipedal posture. However, the canine teeth are large and pointed, and the arm and finger bones retain adaptations for climbing. Some suggest this fossil is ancestral to the chimpanzee. Two species of ardipithecines have been uncovered, Ardipithecus kadabba and A. ramidus. Only teeth and a few bone bits have been found for A. kadabba, and these have been dated to around 5.6 mya. A more extensive collection of fossils has been collected for A. ramidus. To date, over 100 skeletons, all dated to 4.4 mya, have been identified from this species; all were collected near a small town in Ethiopia, East Africa. These fossils have been reconstructed to form a female fossil specimen, affectionately called Ardi. Some of Ardi’s features are primitive, like that of an ape, but others are like that of a human. Ardi was about the size of a chimpanzee, standing about 120 cm (4 ft) tall and weighing about 55 kg (110 lb). It appears that males and females were about the same size. Ardi had a small head compared to the size of her body. The skull had the same features as Sahelanthropus tchadensis,  but was smaller. Ardi’s brain size was around 300 to 350 cc, slightly less than that of a chimpanzee brain (around 400 cc), and much smaller than that of a modern human (1,360 cc). The muzzle (area of the nose and mouth) projects forward, and the forehead is low with heavy eyebrow ridges, a combination that makes the face more primitive than that of the australopithecines (discussed next). However, the projection of the face is less than that of a chimpanzee, because Ardi’s teeth were small and like those of an omnivore. She lacked the strong, sharp canines of a chimpanzee, and her diet probably consisted mostly of soft, rather than tough, plant material. Ardi could walk erect, but she spent a lot of time in trees. Ardi’s feet had a bone, missing in apes, that kept her feet squarely on the ground, a sure sign that she was bipedal and not a quadruped like the apes. Nevertheless, like the apes, she had an opposable big toe. Opposable toes allow an animal’s feet to grab hold of a tree limb. The wrists of Ardi’s hands were flexible, and most likely she moved along tree limbs on all fours, as ancient apes did.



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Chapter 32  Animals: Chordates and Vertebrates

Humans

common chimpanzee Gorillas western lowland gorilla

Hominoids

Hominines

Chimpanzees

Hominids

hominin

Bornean orangutan Gibbons

Anthropoids

Orangutans

white-handed gibbon rhesus monkey Old World Monkeys

capuchin monkey New World Monkeys

Mammalian ancestor enters trees.

Philippine tarsier

ring-tailed lemur

Prosimians

Tarsiers

Lemurs

Figure 32.15  Evolution of primates.  Primates are descended from an ancestor that may have resembled a tree shrew. The descendants of this ancestor adapted to the new way of life and developed traits such as a shortened snout and nails instead of claws. The time when each type of primate diverged from the main line of descent is known from the fossil record. A common ancestor was living at each point of divergence—for example, there was a common ancestor for hominines about 7 mya, for the hominoids about 15 mya, and one for anthropoids about 45 mya.



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Until recently, it was suggested that bipedalism evolved when a dramatic change in climate caused the forests of East Africa to be replaced by grassland. However, evidence suggests that Ardi lived in the woods, which questions the advantage that walking erect would have afforded her. Bipedalism does provide an advantage in caring for a helpless infant by allowing it to be carried by hand from one location to another. It is also possible that bipedalism benefited the males of the species as they foraged for food on the floor of the forests. More evidence is needed to better understand this mystery, but one thing is clear—the ardipithecines represent a link between our quadruped ancestors and the bipedal hominins.

Australopithecines The hominin line of descent begins in earnest with the australopithecines, a group of species that evolved and diversified in Africa. Originally, some australopithecines were classified according to their frame. Some were termed gracile (“slender”) types. Some were robust (“powerful”) and tended to have strong upper bodies and especially massive jaws. Recent changes in the classification of these groups has separated the gracile types into genus Australopithecus and the robust types into the genus Paranthropus. The genus Australopithecus gave rise to genus Homo. The first australopithecine to be discovered was unearthed in southern Africa by Raymond Dart in the 1920s. This hominin, named Australopithecus africanus, dates to about 2.9 mya and had a brain size of about 500 cc. Limb anatomy suggests these hominids walked upright. However, the proportions of the limbs were apelike. The forelimbs were longer than the hindlimbs. Some argue that A. africanus, with its relatively large brain, is a possible ancestral candidate for early Homo, whose limb proportions are similar to those of this fossil. In the 1970s, a team led by Donald Johanson unearthed nearly 250 fossils of a hominin called A. afarensis. A now-famous female skeleton dated at 3.18 mya is known worldwide by its field name, Lucy. Although her brain was small (400 cc), the shapes and relative proportions of her limbs indicate that Lucy stood upright and walked bipedally (Fig. 32.16a). Even better evidence of bipedal locomotion comes from a trail of footprints in Laetoli, Tanzania, dated about 3.7 mya. The larger prints are double, as though a smaller being was stepping in the footfalls of another. There are additional small prints off to the side, within hand-holding distance (Fig. 32.16b). The fact that the australopithecines were apelike with regards to brain size and walked erect like more modern humans, shows that human characteristics did not evolve all at one time. The term mosaic evolution is applied when different body parts change at different rates and, therefore, at different times. Australopithecus afarensis is most likely ancestral to the Paranthropus genus found in eastern Africa:  P. robustus and P. boisei. Paranthropus boisei had a powerful upper body and the largest molars of any hominin. This genus died out; therefore, it is possible that A. afarensis is ancestral to both A. africanus and early Homo.

Evolution of Early Homo The study of our genus, Homo, has undergone considerable scrutiny in the past several years. Increasingly, there is evidence that what were once considered separate species are, in fact, variations of a

b.

Figure 32.16  Australopithecus afarensis.  a. A reconstruction of Lucy on display at the St. Louis Zoo. b. These fossilized footprints occur in ash from a volcanic eruption some 3.7 mya. The larger footprints are double, and a third, smaller individual was walking a. to the side. (A female holding the hand of a youngster may have been walking in the footprints of a male.) The footprints suggest that A. afarensis walked bipedally.

single species. Notice in Figure 32.17 that a number of the members of the genus Homo  are indicated by hash marks to indicate areas under debate. Fossils are assigned to the genus Homo if (1) the brain size is 600 cc or greater, (2) the jaw and teeth resemble those of humans, and (3) tool use is evident. For our purposes, we will focus on the more traditional classification of the members of our genus.

Homo habilis  Homo habilis, dated between 2.0 and 1.9 mya, may be ancestral to modern humans. Some of these fossils have a brain size as large as 775 cc, about 45% larger than that of A. afarensis. The cheek teeth are smaller than even those of the gracile australopithecines. Therefore, it is likely that these early members of the genus Homo were omnivores who ate meat in addition to plant material. Bones at their campsites bear cut marks, indicating that they used tools to strip them of meat. The stone tools made by H. habilis, whose name means “handyman,” are rather crude. It’s possible that these are the cores from which they took flakes sharp enough to scrape away hide, cut tendons, and easily remove meat from bones. Early Homo skulls suggest that the portions of the brain associated with speech were enlarged. We can speculate that the ability to speak may have led to hunting cooperatively. Other members of the group may have remained plant gatherers. If so, both hunters and gatherers most likely ate together and shared their food. In this way, society and culture could have begun.



Chapter 32  Animals: Chordates and Vertebrates

Ardipithecus ramidus

Australopithecus afarensis

Australopithecus africanus

Homo habilis

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Homo sapiens Homo sapiens Homo neandertalensis Homo heidelbergensis Homo erectus Homo ergaster Homo habilis

Australopithecus africanus

Australopithecus sediba

Australopithecus afarensis

Paranthropus boisei

Ardipithecus ramidus

Paranthropus robustus

Sahelanthropus tchadensis 7.5

7

6.5

6

5.5

5

4.5

4 3.5 Million Years Ago (MYA)

3

2.5

2

1.5

1

0.5

0

Figure 32.17  Human evolution.  Several groups of extinct hominins preceded the evolution of modern humans. The groups have been divided into the early humanlike hominins (orange), later humanlike hominins (green), early Homo species (lavender), and finally the later Homo species (blue). The cross marks indicate areas where current research is focusing on combining groups into single species.

Homo ergaster and Homo erectus Homo erectus  fossils are found in Africa, Asia, and Europe and have been dated to between 1.9 and 0.3 mya. A Dutch anatomist named Eugene Dubois was the first to unearth H. erectus bones in Java in 1891. Since that time, many other fossils have been found in the same area. Although all fossils assigned the name H. erectus are similar in appearance, enough discrepancy exists to suggest that several different species have been included in this group. In particular, some experts suggest that the Asian form is Homo erectus and the African form is Homo ergaster (Fig. 32.18). Compared with H. habilis, H. erectus had a larger brain (about 1,000 cc) and a flatter face. The nose projected, however. This type of nose is adaptive for a hot, dry climate, because it permits water to be removed before air leaves the body. The recovery of an almost complete skeleton of a 10-year-old boy indicates that this species was much taller than the hominids discussed thus far. Males were 1.8 m tall (about 6 ft), and females were 1.55 m (approaching 5 ft). Indeed, these hominids were erect and most likely had a striding gait like ours. The robust, and most likely heavily muscled, skeleton still retained some australopithecine features. Even so, the size of the birth canal indicates that infants were born in an immature state that required an extended period of care. Homo erectus may have first appeared in Africa and then migrated into Asia and Europe (Fig. 32.19). At one time, the migration was thought to have occurred about 1 mya. Recently, H. erectus fossil remains in Java and the Republic of Georgia have

been dated at 1.9 and 1.6 mya, respectively. These remains push the evolution of H. erectus in Africa to an earlier date than has yet been determined. In any case, such an extensive population movement is a first in the history of humankind and a tribute to the intellectual and physical skills of the species. Homo erectus was the first hominid to use fire, and they fashioned more advanced tools than early Homos. These hominids used heavy, teardrop-shaped axes and cleavers. Flake tools were probably used for cutting and scraping. It could be that this species was a systematic hunter and brought kills to the same site over and over. In one location, researchers have found over 40,000 bones and 2,647 stones. These sites could have been “home bases,” where social interaction occurred and a prolonged childhood allowed time for learning. Perhaps a language evolved and a culture more like our own developed.

Homo floresiensis  In 2004, scientists announced the discovery of the fossil remains of Homo floresiensis. The 18,000-year-old fossil of a 1-m tall, 25-kg adult female was discovered on the island of Flores in the South Pacific. This important finding suggests that a species of Homo coexisted with modern Homo sapiens much more recently than Neandertals, which went extinct about 28,000 years ago. The specimen was the size of a three-year-old Homo sapiens sapiens but possessed a braincase only one-third the size of that of a modern human. Apparently, H. floresiensis used tools and fire. A 2007 study supports the hypothesis that this diminutive hominin and her peers evolved from normal-sized, island-hopping H. erectus populations

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that reached Flores about 840,000 years ago. Some scientists think that its small size is due to island dwarfing, the reduction in size of large animals when their gene pool is limited to a small environment. This phenomenon is seen in other island populations.

Check Your Progress  32.4 1. Explain when and why bipedalism might have evolved. 2. Describe the feature that distinguishes the first hominids from other hominines.

3. Discuss the general evolutionary trends in early species of Homo.

32.5  Evolution of Modern Humans Learning Outcomes Upon completion of this section, you should be able to 1. Explain how the replacement model relates to evolution of later species of the genus Homo. 2. Discuss the significance of increased tool use, language, and agriculture in Cro-Magnons.

Figure 32.18  Homo ergaster.  This skeleton of a 10-year-old boy

who lived 1.6 mya in eastern Africa shows femurs that are angled because the neck of the femur is quite long.

Eurasia

1.85 MYA

1.66 MYA

China

1.71 MYA East Africa

1.90– 2.36 MYA

1.66 MYA

Indonesia

Figure 32.19  Migration out of Africa.  The dates indicate the migration of early Homo erectus from Africa.1

Derived from “Evolution of Early Homo: An Integrated Biological Perspective,” S. Antón et al., Science 4 July 2014: 345 (6192). 1

Many early Homo species in Europe are now classified as Homo ­heidelbergensis. Just as H. erectus is believed to have evolved from H. ergaster in Asia, so H. heidelbergensis is believed to have evolved from H. ergaster in Europe. However, this is an active area of research, and new findings are constantly changing our understanding of human evolution during this time period. For the sake of discussion, we will group together the H. ergaster in Africa, H. erectus in Asia, and H.  heidelbergensis in Europe as a collective group of early Homo ­species who lived between 1.5 and 0.25 mya (see Fig. 32.17). The most widely accepted hypothesis for the evolution of modern humans from earlier Homo species is referred to as the replacement model, or out-of-Africa hypothesis (Fig. 32.20). The replacement model proposes that modern humans evolved from earlier Homo species only in Africa, and then modern humans migrated to Asia and Europe, where they replaced the existing species about 100,000 years bp (before the present). However, even this hypothesis is being challenged as new genomic information becomes available on the Neandertals and Denisovans.

Neandertals The Neandertals, which are classified as Homo neandertalensis, are a species that lived between 200,000 and 28,000 years ago. Neandertal fossils have been found from the Middle East throughout Europe. Neandertals take their name from Germany’s Neander Valley, where one of the first Neandertal skeletons, estimated to be about 200,000 years old, was discovered. According to the replacement model, Neandertals were eventually supplanted by modern humans. The exact mechanism of how this occurred is still being researched, with some studies suggesting that Homo sapiens outcompeted the Neandertals for resources. However, this view is being challenged by studies of the Neandertal genome (completed in 2010), which suggests that not only did



Chapter 32  Animals: Chordates and Vertebrates

MYA

0 (present day) 0.1

AFRICA

ASIA

EUROPE

0.5

1

2

migration of Homo erectus modern humans early Homo species Homo erectus

Figure 32.20  The replacement model.  According to the replacement model, modern humans evolved in Africa and then replaced early Homo species in Asia and Europe. Neandertals interbreed with Homo sapiens, but between 1 and 4% of the genomes of non-African Homo sapiens contain remnants of the Neandertal genome. Some scientists are suggesting that Neandertals were not a separate species, but simply a race of Homo sapiens that was eventually absorbed into the larger population. Research continues into these and other hypotheses that explain these similarities. Physiologically, the Neandertal brain was, on the average, slightly larger than that of H. sapiens  (1,400 cc, compared with 1,360 cc in most modern humans). The Neandertals were heavily muscled, especially in the shoulders and neck. The bones of the limbs were shorter and thicker than those of modern humans. It is hypothesized that a larger brain than that of modern humans was required to control the extra musculature. The Neandertals lived in Europe and Asia during the last Ice Age, and their sturdy build could have helped conserve heat. The Neandertals give evidence of being culturally advanced. Most lived in caves, but those living in the open may have built houses. They manufactured a variety of stone tools, including spear points, which could have been used for hunting. Scrapers and knives could have helped in food preparation. They most likely successfully hunted bears, woolly mammoths, rhinoceroses, reindeer, and other contemporary animals. They used and could control fire, which probably helped them cook meat and keep themselves warm. They even buried their dead with flowers and tools and may have had a religion. Perhaps they believed in life after death. If so, they were capable of thinking symbolically.

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might be the remains of a species of early Homo, possibly related to Homo erectus. However, mitochondrial DNA studies indicated that the fossil belonged to a species that existed around 1 million years ago, around the same time as Neandertals. The analyses suggest that the Denisovans and Neandertals shared a common ancestor but did not interbreed with one another, possibly because of their geographic locations. However, what is interesting is the fact that Homo sapiens in the Oceania region (New Guinea and nearby islands) share around 5% of their genomes with the Denisovans. In  2014, researchers reported that an allele that allows for highelevation living in Tibetans originated with the Denisovans (see the Scientific Inquiry feature, “Adapting to Life at High Elevations,” in section 1.1). When coupled with the Neandertal data, this suggests that modern Homo sapiens did not simply replace groups of early Homo species but, rather, may have assimilated them by inbreeding. Scientists are just beginning to unravel the implications of these Denisovan discoveries.

Cro-Magnons Cro-Magnons are the oldest fossils to be designated Homo sapiens. In keeping with the replacement model, the Cro-Magnons are named after a fossil location in France, where modern humans entered Asia and Europe from Africa approximately 100,000 years  ago. They probably reached western Europe about 40,000 years ago. Cro-Magnons had a thoroughly modern appearance. They had lighter bones, flat high foreheads, domed skulls housing brains of 1,350 cc, small teeth, and a distinct chin. They made advanced stone tools, including compound tools, such as by fitting stone flakes to a wooden handle. They may have been the first to make knifelike blades and to throw spears, enabling them to kill animals from a distance. They were such accomplished hunters that some researchers hypothesize they may have been responsible for the extinction of many larger mammals, such as the giant sloth, the mammoth, the saber-toothed tiger, and the giant ox, during the late Pleistocene epoch. This event is known as the Pleistocene overkill. Cro-Magnons hunted cooperatively, and perhaps they were the first to have language. They are believed to have lived in small groups, with the men hunting by day while the women remained at home with the children. It’s quite possible that this hunting way of life among prehistoric people influences our behavior even today. The Cro-Magnon culture included art. They sculpted small figurines out of reindeer bones and antlers. They also painted beautiful drawings of animals on cave walls in Spain and France. Cro-Magnons have been widely distributed about the globe ever since they evolved. As with any other species that has a wide geographic distribution, phenotypic and genotypic variations are noticeable between these populations. The Scientific Inquiry feature, “Human Ethnic Groups,” explores some of these differences.

Check Your Progress  32.5

Denisovans

1. Discuss the importance of the replacement model in the

In 2008, a fragment of a finger bone was discovered in Denisova Cave in southern Siberia. Initially, scientists thought that this

2. Compare and contrast Neandertals and Cro-Magnons.

evolution of the genus Homo.



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UNIT 6  Evolution and Diversity

SCIENCE IN YOUR LIFE  ►

SCIENTIFIC INQUIRY

Human Ethnic Groups Evolutionists have hypothesized that human variations evolved as adaptations to local environmental conditions. One obvious difference among people is skin color. A darker skin is protective against the high UV intensity of bright sunlight. On the other hand, a white skin ensures vitamin D production in the skin when the UV intensity is low. Harvard University geneticist Richard Lewontin points out, however, that this hypothesis concerning the survival value of dark and light skin has never been tested.  Two correlations between body shape and environmental conditions have been noted since the nineteenth century. The first, known as Bergmann’s rule, states that animals in colder regions of their range have a bulkier body build. The second, known as Allen’s rule, states that animals in colder regions of their

range have shorter limbs, digits, and ears. Both of these effects help regulate body temperature by increasing the surface-area-to-volume ratio in hot climates and decreasing the ratio in cold climates. For example, Figure 32B, shows that the Maasai of East Africa tend to be slightly built with elongated limbs, while the Eskimos, who live in northern regions, are bulky and have short limbs. Other anatomical differences among ethnic groups, such as hair texture, a fold on the upper eyelid (common in Asian peoples), and the shape of lips, cannot be explained as adaptations to the environment. Perhaps these features became fixed in different populations due simply to genetic drift. As far as intelligence is concerned, no significant disparities have been found among different ethnic groups. 

Genetic Evidence for Common Ancestry The replacement model for the evolution of humans, discussed earlier in this section, pertains to the origin of ethnic groups. This hypothesis proposes that all modern humans have a relatively recent common ancestor—that is, Cro-Magnon—who evolved in Africa and then spread into other regions. Paleontologists tell us that the variation among modern populations is considerably less than existed among archaic human populations some 250,000 years ago. If so, all ethnic groups evolved from the same single ancestral population. A comparative study of mitochondrial DNA shows that the differences among human populations are consistent with their having a common ancestor no more than a million years ago. Lewontin has also found that the genotypes of different modern populations are extremely similar. He examined variations in 17 genes, including blood groups and various enzymes, among seven major geographic groups: Europeans (caucasians), black Africans, mongoloids, South Asian Aborigines, Amerinds, Oceanians, and Australian Aborigines. He found that the great majority of genetic variation— 85%—occurs within ethnic groups, not between them. In other words, the amount of genetic variation between individuals of the same ethnic group is greater than the variation between any two ethnic groups. 

Questions to Consider

a.

b.

Figure 32B  Human ethnic groups.  a. The Maasai live in East Africa. b. Eskimos live

near the Arctic Circle. 

1. Why might a person’s outward appearance not necessarily be an indication of their ethnic background? 2. Some ethnic groups are more highly disposed to certain diseases than others, such as hypertension and cardiovascular disease. Explain what factors may have contributed to this in this ethnic population.

Conclusion Like all vertebrates, and all life, canines have an evolutionary history that may be traced back not only through the fossil record, but by the record contained within their genomes. The development of new genetic techniques has expanded our ability to analyze the genomes of a wide variety of organisms, from dogs to whales to humans. Evolutionary geneticists are using this information to redefine some of the previously established relationships—such as between humans and Neandertals.

Often, these studies reveal interesting information on our evolutionary past. For example, studies of the canine and human genomes reveal that changes in the neurological processes, diet, and digestion of these two species evolved together over time, suggesting that our interactions with man’s best friend may have not only played an important role in our behavior, but in how we evolved as a species.



Chapter 32  Animals: Chordates and Vertebrates

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MEDIA STUDY TOOLS http://connect.mheducation.com Maximize your study time with McGraw-Hill SmartBook®, the first adaptive textbook.

  Tutorials 32.4  Primate Classification

SUMMARIZE 32.1  Chordates ■ Organisms that are classified as chordates (tunicates, lancelets, and

■ Primates are mammals adapted to living in trees. The order Primates

includes the strepsirhini (lemurs, aye ayes, bush babies and lorises) and the haplorhini (monkeys, apes, and humans). ■ Strepsirhini diverged first, followed by the monkeys and then the apes.

vertebrates) have a dorsal supporting rod known as a notochord, a dorsal tubular nerve cord, pharyngeal pouches, and a postanal tail at some time in their life history. ■ Lancelets and tunicates are the nonvertebrate chordates. Adult tunicates lack chordate characteristics except gill slits, but adult lancelets have the four chordate characteristics.

32.4  Evolution of the Hominins

32.2  Vertebrates: Fish and Amphibians

■

■ The hominin lineage diverged from the same primate line as the rest of

■ Vertebrates have the four chordate characteristics as embryos, but the

notochord is replaced by the vertebral column.

■ Internal organs are well developed, and cephalization is apparent. The

amniotic egg was a major evolutionary milestone as well.

■ The vertebrate classes trace their evolutionary history as follows:

∙ Jawless fishes were the first vertebrates and are represented by hagfishes and lampreys that lacked jaws and fins. ∙ Cartilaginous fishes include the sharks, rays, and skates. Their skeleton is composed of cartilage. They possess jaws and fins. ∙ Bony fishes have jaws and two pairs of fins. The bony fishes include the ray-finned fishes and the lobe-finned fishes (some of which have lungs). ∙ Amphibians (e.g., frogs and salamanders) evolved from lobefinned fishes and have two pairs of limbs. Most amphibian species return to the water to reproduce, and their tadpoles or larvae then metamorphose into terrestrial adults.

32.3  Vertebrates: Reptiles and Mammals ■ Reptiles most often possess shelled eggs, which contain extraembry-

onic membranes, including an amnion that allows them to reproduce on land. They are ectothermic, meaning their body temperature matches the environment. ■ Birds are feathered reptiles; feathers help maintain a constant body temperature. They are endothermic, meaning that they can maintain an internal temperature. Hollow bones, lungs with air sacs that penetrate bones and allow one-way ventilation, and a keeled breastbone adapt birds for flight. ■ Mammals have hair to help them maintain a constant body temperature, and mammary glands for nursing of young. Monotremes lay eggs; marsupials have a pouch in which the newborn matures; and placental mammals, which are far more varied and numerous, retain offspring inside the uterus until birth.

■ ■ ■

the apes. A molecular clock can be used to infer the relatedness between the two groups. Molecular evidence suggests humans are most closely related to African apes, whose ancestry split from ours about 6 to 10 mya. Hominin evolution began in eastern Africa with the rise of ardipithecines and the australopithecines. Bipedalism, or walking upright, is one of the main features that separates modern humans. The most famous australopithecine is Lucy (3.2 mya), who walked bipedally and had a small brain.  Homo habilis, present about 2 mya, is certain to have made tools. Homo erectus, with a brain capacity of 1,000 cc and a striding gait, was the first to migrate out of Africa. The recent discovery of the fossil, Homo floresiensis, however, suggests a species of Homo coexisted with modern humans as recently as 18,000 years ago.

32.5  Evolution of Modern Humans ■ The replacement model suggests that modern humans, called  Cro-

Magnons, originated in Africa and, after migrating into Europe and Asia, replaced the other Homo species (including Neandertals and Denisovans) found there. Various lines of evidence support this hypothesis. ■ The Neandertals lived approximately 200,000 to 28,000 years ago. They were physically adapted to the last Ice Age. Cro-Magnons first appeared approximately 130,000 years ago. They had advanced culture and technology.

ASSESS Testing Yourself Choose the best answer for each question.

32.1  Chordates 1. Which of these is not a chordate characteristic? a. dorsal supporting rod, the notochord d.  postanal tail b. dorsal tubular nerve cord e.  vertebral column c. pharyngeal pouches



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UNIT 6  Evolution and Diversity

2. Label the diagram of a chordate below:

32.5  Evolution of Modern Humans

d.

c.

b.

a.

32.2  Vertebrates: Fish and Amphibians 3. Which of these is not characteristic of all vertebrates? a. complete digestive system b. closed circulatory system c. skin with either scales or feathers d. endoskeleton made of bone or cartilage e. vertebral column 4. The first vertebrates to evolve were a. amphibians. d. amniotes. b. jawed fishes. e. reptiles. c. jawless fishes. 5. Bony fishes are divided into which two groups? a. hagfishes and lampreys b. sharks and ray-finned fishes c. ray-finned fishes and lobe-finned fishes d. jawless fishes and cartilaginous fishes 6. Amphibians evolved from what type of ancestral fish? a. sea squirts and lancelets d. lobe-finned fishes b. ray-finned fishes e. jawless fishes c. cartilaginous fishes

32.3  Vertebrates: Reptiles and Mammals 7. The amniotes include all but the a. birds. c. mammals. b. reptiles. d. amphibians. 8. Which of the following groups has a heart with four distinct chambers? a. birds c. reptiles other than birds b. lampreys d. amphibians 9. The first mammals to evolve were a. aquatic. d. primates. b. placental. e. monotremes. c. marsupials.

32.4  Evolution of the Hominins 10. Choose the correct order of primate evolution, from the oldest to the most recent group. a. prosimians—anthropoids—hominoids—hominids—hominines b. hominines—hominids—hominoids—anthropoids—prosimians c. prosimians—anthropoids—hominines—hominids—hominoids d. anthropoids—hominines—hominids—hominoids—prosimians e. None of the above are correct. 11. The fossil nicknamed Lucy was a(n) a. early Homo. c. ardipithecine. b. australopith. d. modern human. 12. This genus are the direct ancestors of the genus Homo. a. Ardipithecus d. Denisovans b. Sahelanthropus e. None of these are correct. c. Australopithecus

13. Which species was probably the first of the genus Homo to migrate from Africa? a. Homo erectus d. Homo florensiensis b. Homo habilis e. None of these are correct. c. Denisovans 14. According to the fossil record, which of these species is the earliest of the Homo genus? a. H. sapiens d. H. heidelbergensis b. H. erectus e. H. ergaster c. H. habilis 15. According to the replacement model, Homo sapiens a. was the first to migrate from Africa. b. is the earliest species of the genus Homo. c. followed earlier Homo species out of Africa. d. originated in the New World. e. None of these are correct.

ENGAGE Thinking Critically 1. Researchers have recently sequenced the genome of several vertebrate species, including the North American green anole and the African clawed frog. What information might these studies give us that could be applied to the development of new medical treatments for humans? 2. Bipedalism has many selective advantages, including the increased ability to spot predators and prey. However, bipedalism has one particular disadvantage—upright posture leads to a smaller pelvic opening, which makes giving birth to an offspring with a large head very difficult. This situation results in a higher percentage of deaths (of both mother and child) during birth in humans compared to other primates. How can you explain the selection for a trait, such as bipedalism, that has both positive and negative consequences for fitness? 3. In studying recent fossils of the genus Homo, such as Cro-Magnon, biologists have determined that modern humans have not undergone much biological evolution in the past 50,000 years. Rather, cultural anthropologists argue that cultural evolution has been far more important than biological evolution in the recent history of modern humans. What do they mean by this? Support your argument with some examples. 4. Some modern ethnic groups (white Europeans, Asians) have apparently inherited genes from Neandertals that may influence resistance to cold temperatures. What types of genes might these be? 

PHOTO CREDITS Opener: © FLPA/SuperStock; 32.3: © Heather Angel/Natural Visions; 32.4: © Corbis RF; 32Aa: © Marie Holding/iStock/Getty Images Plus RF; 32Ab: © tomophotography/Moment/ Getty RF; 32Ac: © Victoria Dolidovich/Hemera/Getty Images Plus RF; 32.5a: © Alastair Pollock Photography/Getty RF; 32.5b: © Marvin Dembinsky Photo Associates/Alamy; 32.5c: © DEA Picture Library/Getty Images; 32.7a: © Comstock Images/PictureQuest; 32.7c: © Peter Scoones/SPL/Science Source; 32.10a: © Heinrich van den Berg/Gallo Images/Getty Images; 32.12a: © Gary W. Carter/Corbis; 32.12b: © Dale DeGabriele/Getty Images; 32.12c: © Medford Taylor/Getty Images; 32.13a: © D. Parer & E. Parer-Cook/Ardea; 32.13b: © John White Photos/Getty RF; 32.13c: © John Macgregor/Photolibrary/Getty Images; 32.14a: © Paul E. Tessier/Getty RF; 32.14b: © John Downer/Getty Images; 32.14c: © Paul Souders/ Corbis; 32.14d: © 2011 Tory Kallman/Getty RF; 32.16a: © Dan Dreyfus and Associates; 32.16b: © John Reader/Science Source; 32.17(A. ramidus): © Richard T. Nowitz/Science Source; 32.17(A. afarensis): © Scott Camazine/Alamy; 32.17(A. africanus): © Philippe Plailly/Science Source; 32.17(H. habilis): © Kike Calvo VWPics/Superstock; 32.17(H. sapiens): © Kenneth Garrett/Getty Images; 32.18: © Associated Press; 32Ba: © Sylvia S. Mader; 32Bb: © B & C Alexander/Science Source.

UNIT 7  Behavior and Ecology

33

CASE STUDY The Benefits of Living in a Society African lions often live in a group, or pride, containing between 2 and 18 closely related females. Living in a social unit increases the females’ chances of survival. As a group they can overcome large prey, which would not be possible by an individual. However, although the pride increases the success of hunting large animals, the food must be shared by the entire pride. The advantages and disadvantages of living in a society was a focus of the documentary African Cats, which helped to reveal the secrets of the lives of lions. Within a pride, all of the females tend to be genetically related (mothers, daughters, sisters, aunts). They work together to protect their territory while helping raise each other’s young. Female cubs will stay with the pride, while male cubs leave when they reach two to four years of age. These young males often form coalitions with related males. Once they reach maturity, the male coalition establishes a territory that overlaps with a pride. Sometimes a pride contains young cubs that are not related to the local male coalition. In this case, the cubs are often killed. This triggers the females to enter estrus, allowing each of the new males to have mating opportunities with the females of the pride. Killing the original cubs and mating with the females increases the likelihood that the males will be able to produce cubs with which they will share their genes. In this chapter, you will learn about animal behavior in general and gain a better understanding of the complexity of living in a social unit. We will explore whether behavior has a genetic or environmental basis. Then we will examine how behavior and communication are necessary components of the struggle to survive. As you read through the chapter, think about the following questions:

1. Which lion behaviors appear to have a genetic basis to them and which ones appear to have an environmental basis?

Behavioral Ecology

CHAPTER OUTLINE 33.1 Nature Versus Nurture: Genetic Influences

33.2 Nature Versus Nurture: Environmental Influences 33.3 Animal Communication 33.4 Behaviors That Affect Fitness

BEFORE YOU BEGIN Before beginning this chapter, take a few moments to review the following discussions: Section 1.1  What is the relationship between a stimuli and behavior? Section 23.3  How can the environment influence gene expression? Section 27.3  What role does sexual selection and male competition play in the evolution of a species?

2. Which methods of communication are most likely employed by the lions to defend their territory?

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UNIT 7  Behavior and Ecology

33.1  Nature Versus Nurture: Genetic Influences Learning Outcomes Upon completion of this section, you should be able to 1. Understand what is meant by the term nurture versus nature. 2. Describe an experiment that shows behavior can be genetically based.

Behavior represents the way that an organism reacts to a stimulus or situation. This reaction may take many forms, including movement or changes in thought patterns. Recall from Chapter 1 that the ability to react to a stimulus is an important characteristic of life. Therefore, the nature of behavior plays an important role in our understanding of life. The “nature versus nurture” question asks to what degree our genes (nature) and environmental influences (nurture) affect our behavior. Historically, scientists believed that behavior was either due to genetics or nature, but that pattern of thought has changed over the past few decades as new techniques and experiments have studied the interaction of genes and the environment. Behavior is now recognized as being multifactorial—meaning that there are interactions between genes (or multiple genes) and environmental factors. In humans, the science of behavioral genetics often uses twin studies (see the Scientific Inquiry feature, “Behavioral Genetics and Twin Studies”) to explore these interactions. While we usually discuss behavior in regards to animals since they respond more quickly to stimuli, behavior is now recognized to occur in any organism that responds to stimuli. In this section, we will examine a variety of experiments that have been conducted to help determine the degree to which genetics controls behavior.

Nurturing Behavior in Mice In several scientific studies, investigators have found that maternal behavior in mice was dependent on the presence of a gene called fosB. Normally, when mothers first inspect their newborns, various sensory information from their eyes, ears, nose, and touch receptors travel to the hypothalamus. This incoming information causes fosB alleles to be activated, and a particular protein is produced. The protein begins a process during which cellular enzymes and other genes are activated. The end result is a change in the neural circuitry within the hypothalamus, which manifests itself in maternal nurturing behavior toward the young. Mice that do not engage in nurturing behavior were found to lack fosB alleles, and the hypothalamus failed to make any of the products or to activate any of the enzymes and other genes that lead to maternal nurturing behavior. Female mice with fosB alleles tended to retrieve their young and bring them back to them after they became separated. (Fig. 33.1).

Food Choice in Garter Snakes Several experiments have been conducted using the garter snake, Thamnophis elegans, to determine if food preference has a genetic

Proportion of Pups Retrieved

674

0.8 0.6

fosB alleles present fosB alleles not present

0.4 0.2 0

a.

b. fosB alleles present.

c. fosB alleles not present.

Figure 33.1  Maternal care in mice.  a. A mother mouse with fosB

alleles spends time retrieving and crouching over its young, whereas mice that lack these alleles do not display these maternal behaviors. b. This typical mother mouse retrieves her young and crouches over them. c. This mouse, lacking fosB, does not retrieve her young and does not crouch over them.

basis. In California, inland garter snake populations are aquatic and commonly feed underwater on frogs and fish, whereas coastal populations are terrestrial and feed mainly on slugs. In the laboratory, inland adult snakes refused to eat slugs, while coastal snakes readily did so. Newborns resulting from matings between snakes from the two populations (inland and coastal) have an intermediate preference for slugs, suggesting a genetic basis for food choice. Differences between slug acceptors and slug rejecters appear to be inherited, but what physiological difference is there between the two populations? A clever experiment answered this question. Snakes use tongue flicks to recognize their prey, and their tongues carry chemicals to an odor receptor in their mouth. Newborns will even flick their tongues at cotton swabs dipped in fluids of their prey. Swabs were dipped in slug extract, and the number of tongue flicks were counted for newborn inland and coastal snakes. Coastal snakes had a higher number of tongue flicks (indicating more



Chapter 33  Behavioral Ecology

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25 inland coastal

Inland garter snake does not eat slugs.

Percentage of Snakes

20

15

10

5

0 Coastal garter snake eats slugs.

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

Tongue Flicks per Minute

Figure 33.2  Feeding behavior in garter snakes.  The number of tongue flicks by inland and coastal garter snakes is measured in terms of their

response to slug extract on cotton swabs. Coastal snakes tongue-flicked more than inland snakes, indicating that coastal snakes are more sensitive to the smell of slugs than inland snakes.

interest in the smell) than inland snakes (Fig. 33.2). Thus, inland snakes apparently do not eat slugs because they are not sensitive to their smell. A genetic difference between the two populations of snakes has resulted in a physiological difference in their nervous systems. Although hybrids showed a great deal of variation in the number of tongue flicks, they were generally intermediate, as predicted by the genetic hypothesis.

Egg-Laying Behavior of Marine Snails The nervous and endocrine systems are both responsible for the coordination of body systems. Is the endocrine system also involved in behavior? Experiments support endocrine system involvement. For example, the egg-laying behavior in the marine snail Aplysia involves a set sequence of movements. Following copulation, the animal extrudes long strings of more than a million eggs per egg case. It then takes the egg case string in its mouth, covers it with mucus, waves its head back and forth to wind the string into an irregular mass, and attaches the mass to a solid object, such as a rock. Scientists have isolated and analyzed an egg-laying hormone (ELH) that causes the snail to lay eggs even if it has not mated. ELH was found to be a small protein of 36 amino acids that diffuses into the circulatory system and causes the smooth muscle cells of the reproductive duct to contract and expel the egg string. Using recombinant DNA techniques, the investigators isolated the entire ELH gene. The gene’s product turned out to be a protein with 271 amino acids. The protein can be cleaved into as many as 11 possible products, and the ELH hormone is one of these. The hormone alone, or in conjunction with the gene’s other

products, is thought to control all the components of egg-laying behavior in Aplysia.

Check Your Progress  33.1 1. Identify how studies of animals suggest that behavior can be genetically based.

2. Explain how you would determine if a behavior was based primarily on genetics or on the environment.

33.2  Nature Versus Nurture: Environmental Influences Learning Outcomes Upon completion of this section, you should be able to 1. Identify an experiment that shows how behavior can be environmentally influenced. 2. Compare three different types of learned behavior.

Even though genetic inheritance serves as a basis for behavior, environmental influences (nurture) also affect behavior. For example, behaviorists originally believed that some behaviors were fixed action patterns (FAPs) elicited by a sign stimulus. But then they found that many behaviors improve with practice. In this context, learning is defined as a durable change in behavior brought about by experience.



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SCIENCE IN YOUR LIFE  ►

SCIENTIFIC INQUIRY

Behavioral Genetics and Twin Studies There are two types of twins—those that come from a single zygote (monozygotic, identical or paternal) and those that are the result of separate fertilization events (dizygotic or fraternal). Behavioral genetics often uses twin studies to examine the similarities and differences between monozygotic twins, dizygotic twins, and twins that have been raised apart, to attempt to address the nurture or nature debate. Many scientists attribute a person’s outcome, and in our case, behavior, to two factors: nature and nurture. Nature, your genes, gives you traits for eye color, hair color, and blood type. Nurture is based on your lifestyle and environment, including diet, rearing, and education. The study of monozygotic twins (Fig. 33A) has demonstrated that even though the individuals share the same genetic information, they are not always completely identical with regards to their traits and behavior. These studies have led researchers to recognize that there is a bridge between nature and nurture in the form of epigenetics (Gk. epi-, “upon, over”). The specific chemical reactions, or epigenetic “tags,” can come in different forms but are often associated with DNA methylation, in which a methyl group attaches to the cytosine base of DNA (Fig. 33B). With a methyl group attached, transcription cannot occur. The methyl group interferes with transcription factors and other proteins in the transcription machinery, thereby silencing or weakening a gene. Over time, the differences in these tags

accumulate, making twins increasingly differ- anxious rat with a different drug that removes methyl groups creates a calm rat. In drug develent from each other (Fig 33C). Epigenetics are heritable changes in gene opment, epigenetic medicines could be used to expression without changing the DNA correct or reverse the particular effect of a tag. sequence. Chemical reactions due to environmental exposure influence how genes are regu- Questions to Consider lated or turned off or on. This, in turn, may 1. What would a study of both monozygotic twins raised together and monozygotic affect many aspects of an individual, including twins raised apart tell you about environtheir behavior. Monozygotic twins present a mental influences? unique opportunity to study epigenetics because they are clones resulting from a split in 2. Epigenetics may be involved in the fine tuning of gene expression. How might this a single fertilized egg. Assuming a similar affect the behavior of an individual? upbringing, their gradual differences over time can therefore be attributed to their disparate control of genes. NH2 M M Epigenetics has important implicaT G C G A C T G tions for behavior. The appearance of tags C CH3 C N on genes helps scientists discover the C cause of some conditions and illnesses C C G A C T G A C that cannot be explained by DNA or O N M M M H genetic mutations alone. Identical twins DNA methylation is the addition of a methyl discordant (different) for autism, psychiatgroup (M) to the DNA base cytosine (C). ric disorders, and dyslexia have been shown to have different DNA methylation Figure 33B  DNA methylation.  DNA is on certain genes. methylated when a methyl group attaches to the In addition, the epigenetic changes cytosine nucleotide. are reversible. A study using rats showed that rat pups that are licked and nurtured by their mothers become calm adults. Rat pups that are not nurtured are anxious. Injecting a calm rat with a drug that adds Yellow shows where methyl groups creates an anxious the twins have epigenetic tags in rat. Conversely, injecting an the same place.

3-year-old twins

Red and green show where the twins have epigenetic tags in different places. 50-year-old twins

Figure 33C  Epigenetic tags on chromosomes.  Figure 33A  Identical twins.  Identical twins come from a single fertilized egg that splits in two. Their genes are the same.

One twin’s epigenetic tags are dyed green, and the other twin’s tags are dyed red. An overlap in green and red shows up as yellow. The 50-year-old twins have more epigenetic tags in different places than do the 3-year-old twins



Chapter 33  Behavioral Ecology

Learning in Birds Laughing gull (Leucophaeus atricilla) chicks’ begging behavior appears to be a FAP, because it is always performed the same way in response to the parent’s red bill (the sign stimulus). A chick directs a pecking motion toward the parent’s bill, grasps it, and strokes it downward (Fig. 33.3a). Parents bring about the begging behavior by swinging their bill gently from side to side. After the chick responds, the parent regurgitates food onto the nest floor. If need be, the parent then encourages the chick to eat. This interaction between the chicks and their parents suggests that the begging behavior involves learning. To test this hypothesis, diagrammatic pictures of gull heads were painted on small cards. Then, eggs collected in the field were hatched in a dark incubator to eliminate visual recognition before the test. On the day of hatching, each chick was allowed to make about a dozen pecks at the model. The chicks were returned to the nest, and then each was retested. The tests showed that on the average, only one-third of the pecks by a newly hatched chick strike the model. But one day after hatching, more than half of the pecks are accurate, and two days after hatching, the accuracy reaches a level of more than 75% (Fig. 33.3b, c). Investigators concluded that improvement in motor skills, as well as visual experience, strongly affect the development of chick begging behavior—evidence that the behavior is, in part, learned.

Imprinting Imprinting is considered a very simple form of learning, although it has a strong genetic component as well. Imprinting was first observed in birds when chicks, ducklings, and goslings

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followed the first moving object they saw after hatching. This object is ordinarily their mother, but investigators found that birds can imprint on any object, as long as it is the first moving object they see during a sensitive period of two to three days after hatching. The term sensitive period means that the behavior develops only during this time. A chick imprinted on a red ball follows it around and chirps whenever the ball is moved out of sight. Social interactions between parent and offspring during the sensitive period seem key to normal imprinting. For example, female mallards cluck during the entire time imprinting is occurring, and it could be that vocalization before and after hatching is necessary for normal imprinting.

Social Interactions and Learning White-crowned sparrows sing a species-specific song, but males of a particular region have their own dialect. Birds were caged to test the hypothesis that young white-crowned sparrows learn how to sing from older members of their species. Three groups of birds were tested. Birds in the first group heard no songs at all. When grown, these birds sang a song, but it was not fully developed. Birds in the second group heard tapes of white-crowns singing. When grown, they sang in that dialect, as long as the tapes had been played during a sensitive period from about age 10 to 50 days. White-crowned sparrows’ dialects (or other species’ songs) played before or after this sensitive period had no effect on their song. Birds in a third group did not hear tapes and instead were given an adult tutor of a different species. These birds sang the song of a still different species—no matter when the tutoring began—showing that social interactions apparently assist learning in birds.

Pecking accuracy of newborn

Pecking accuracy of two-day old

b.

Hits (percent)

Mean accuracy of pecking model for all chicks tested 100

a. Laughing gull adult and chick, Leucophaeus atricilla

50 25 0

Figure 33.3  Begging behavior in laughing gull chicks.  a. At about three days

of age, a laughing gull chick grasps the red bill of a parent, stroking it downward, and the parent then regurgitates food. b. The accuracy of a chick striking a test probe, painted red. c. Chick-pecking accuracy illustrated graphically. Note from these diagrams that a chick markedly improves its ability (within only two days) to peck a bill, a behavior that normally causes a parent to regurgitate food.

75

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1 2 Days in Nest

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Associative Learning A change in behavior that involves an association between two events is termed associative learning. For example, birds that get sick after eating a monarch butterfly no longer prey on monarch butterflies, even though they may be readily available. Or the smell of fresh-baked bread may entice you to eat even though you may have just eaten. Because you really enjoyed bread in the past, you associate the smell with those past experiences, which makes you hungry. Both classical conditioning and operant conditioning are examples of associative learning.

saliva at sight of food (unconditioned response)

saliva at sound of bell only (conditioned response)

Classical Conditioning In classical conditioning, the presentation of two different types of stimuli at the same time causes an animal to form an association between them. The best-known laboratory example of classical conditioning is an experiment done by the Russian psychologist Ivan Pavlov. First, Pavlov observed that dogs salivate when presented with food. Then he rang a bell whenever the dogs were fed. Eventually, the dogs would salivate whenever the bell was rung, regardless of whether food was present (Fig. 33.4). Classical conditioning suggests that an organism can be trained or conditioned to associate a specific response to a specific stimulus. Unconditioned responses are those that occur naturally, as when salivation follows the presentation of food. Conditioned responses are those that are learned, as when a dog learns to salivate when it hears a bell. Advertisements attempt to use classical conditioning. For example, commercials pair attractive people with a particular product in the hope that viewers will associate attractiveness with that product. This pleasant association may cause them to buy the product. In a similar way, it’s been suggested that holding children on your lap when reading to them facilitates an interest in reading. The belief is that they will associate a pleasant feeling with reading.

Operant Conditioning During operant conditioning, a stimulus-response connection is strengthened. Most people know that it is helpful to give an animal a reward, such as food or affection, when teaching them a trick. When we go to an animal show, it is quite obvious that trainers use operant conditioning. They present a stimulus, such as a hoop, and then give a reward (food) for the proper response (jumping through the hoop). B. F. Skinner is well known for studying this type of learning in the laboratory. In Skinner’s simplest type of experiment, a caged rat happens to press a lever and is rewarded with sugar pellets. Thereafter, the rat regularly presses the lever whenever it is hungry. In more sophisticated experiments, Skinner even taught pigeons to play ping-pong by rewarding desired responses to stimuli. When operant conditioning is applied to child rearing, it’s been suggested that parents who give a positive reinforcement for good behavior will be more successful than parents who punish behaviors they believe are undesirable.

sound of bell (conditioned stimulus)

food (unconditioned stimulus)

apparatus to measure saliva

Figure 33.4  Classical conditioning.  Ivan Pavlov discovered

classical conditioning by performing this experiment with dogs. A bell is rung when a dog is fed. Salivation is noted. Eventually, the dog salivates when the bell is rung even though no food is presented. Food is an unconditioned stimulus, and the sound of the bell is a conditioned stimulus that brings about the response—that is, salivation.

Check Your Progress  33.2 1. Identify examples of various types of learning. 2. Contrast the different types of associative learning.

33.3  Animal Communication Learning Outcomes Upon completion of this section, you should be able to 1. Summarize the various ways in which animals communicate. 2. Describe the advantages and disadvantages of chemical, auditory, visual, and tactile communication.

Animals exhibit a wide diversity of social behaviors. Some animal species are largely solitary and join with a member of the opposite sex only for the purpose of reproduction. Others pair, bond, and cooperate in raising offspring. Still others form a society in which members of the species are organized in a cooperative manner, extending beyond sexual and parental behavior. Social behavior among animals requires that they communicate with one another. Communication is a signal by a sender that influences the behavior of a receiver. The communication can be purposeful, but does not have to be. Bats send out a series of sound pulses and



Chapter 33  Behavioral Ecology

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listen for the corresponding echoes in order to find their way through dark caves and locate food at night. Some moths have an ability to hear these sound pulses, and they begin evasive tactics when they sense that a bat is near. The bats are not purposefully communicating with the moths. The bat sounds are simply a cue to the moths that danger is near. The types of communication signals used by animals include chemical, auditory, visual, and tactile.

Chemical Communication

Auditory Communication

urine onto a tree to mark its territory.

pattern, duration, and repetition. In an experiment with rats, a researcher discovered that an intruder can avoid attack by increasing the frequency with which it makes an appeasement sound. Male birds have songs for a number of different occasions, such as one song for distress, another for courting, and still another for marking territories. Sailors have long heard the songs of humpback whales transmitted through the hull of a ship. However, only recently has it been shown that the song has six basic themes, each with its own phrases, which can vary in length, be interspersed with sundry cries and chirps, and be heard for many miles. Interestingly, humpbacks’ songs change from year to year, but all the humpbacks in an area learn and converge on singing the same song. The purpose of the song is probably sexual, serving to advertise the availability of the singer. Language is the ultimate auditory communication. Only humans have the biological ability to produce a large number of different sounds and to put them together in many different ways. Nonhuman primates have different vocalizations, each having a definite meaning, such as when vervet monkeys give alarm calls (Fig. 33.6). Although chimpanzees can be taught to use an

Frequency (kilocycles per second)

Auditory (sound) communication has various advantages over other kinds of communication. It is faster than chemical communication, and is effective both night and day. Further, auditory communication can be modified not only by loudness but also by

Figure 33.5  Use of a pheromone.  This male cheetah is spraying

8 7 6 5 4 3 2 1 0

Frequency (kilocycles per second)

Chemical signals have the advantage of being effective both night and day. The term pheromone designates chemical signals in low concentration that are generally passed between members of the same species. For example, female moths secrete chemicals from special abdominal glands, which are detected by receptors on male antennae. The antennae are especially sensitive, and this ensures that only male moths of the same species will be able to detect them. Cheetahs and other cats mark their territories by depositing urine, feces, and anal gland secretions at the boundaries (Fig. 33.5). Klipspringers (small antelope) use secretions from a gland below the eye to mark twigs and grasses of their territory. Researchers are studying to what degree pheromones and hormones affect the behavior of mammals. Some researchers maintain that human behavior is influenced by undetectable pheromones wafting through the air. They have discovered that like the mouse, humans have an organ in the nose, called the vomeronasal organ (VNO), which can detect not only odors, but also pheromones. The neurons from this organ lead to the hypothalamus, the part of the brain that controls the release of various hormones in the body. Perhaps this organ is even involved in how people choose their mates (see the Health feature, “Mate Choice and Smelly T-Shirts,” in section 33.4).

0.5 Seconds Eagle a.

8 7 6 5 4 3 2 1 0 0.5 Seconds Leopard

b.

Figure 33.6  Auditory communication.  a. Vervet monkeys, Cercopithecus aethiops, are responding to an alarm call. Vervet monkeys can give different alarm calls according to whether a troop member sights an eagle or a leopard, for example. b. The frequency per second of the sound differs for each type of call.



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artificial language, they never progress beyond the capability level of a two-year-old child. It also has been difficult to prove that chimps understand the concept of grammar or can use their language to reason. As such, most anthropologists agree that humans possess a communication ability unparalleled by most animals.

6. 1.

Visual Communication Visual signals are most often used by species that are active during the day. Contests between males make use of threat postures to help prevent outright fighting, a behavior that might result in reduced fitness. A male baboon displaying full threat is an awesome sight that establishes his dominance and keeps peace within the baboon troop (Fig. 33.7). Many animals use complex courtship behaviors and displays. The plumage of a male Raggiana Bird of Paradise allows him to put on a spectacular courtship dance to attract a female, giving her a basis on which to select a mate. Defense and courtship displays are exaggerated and always performed in the same way so that their meaning is clear. Male fireflies use a flash pattern to signal females of the same species (Fig. 33.8). Visual communication allows animals to signal others of their intentions without the need to provide any auditory or chemical messages. The body language of students during a lecture provides an example. Some students lean forward in their seats and make eye contact with the instructor. They want the instructor to know they are interested and find the material of value. Others lean back in their chairs, look at the floor, or send text messages on their cell phones. These students indicate they are not interested in the material. Teachers can use students’ body language to determine if they are effectively presenting the material and make changes accordingly. Other human behaviors also send visual clues. The hairstyle and dress of a person or the way he or she walks and talks are ways to send messages to others. Psychologists have long tried to

7. 2.

8. 3. 9. 4.

5.

Figure 33.8  Fireflies use visual communication.  Each number represents the male flash pattern of a different species. The patterns are a behavioral reproductive isolation mechanism. understand how visual clues can be used to better understand human emotions and behavior. Some studies have suggested that women are apt to dress in an appealing manner and be sexually inviting when they are ovulating. People who dress in black, move slowly, fail to make eye contact, and sit alone may be telling others that they are unhappy. Similarly, body language in animals is being used to suggest that they too have emotions, as discussed in the Bioethical feature, “Do Animals Have Emotions?”

Tactile Communication

Figure 33.7  A male olive baboon displaying full threat.  In olive baboons, males are larger than females and have enlarged canines. Competition between males establishes a dominance hierarchy for the distribution of resources.

Tactile communication occurs when one animal touches another. For example, laughing gull chicks peck at the parent’s beak to induce the parent to feed them (see Fig. 33.3). A male leopard nuzzles the female’s neck to calm her and stimulate her willingness to mate. In primates, grooming—one animal cleaning the coat and skin of another—helps cement social bonds within a group. Honeybees use a combination of communication methods, including tactile ones, to convey information about their environment. When a foraging bee returns to the hive, it performs a waggle dance that communicates the distance and direction to the food



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SCIENCE IN YOUR LIFE  ►

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BIOETHICAL

Do Animals Have Emotions? In the light of recent YouTube video postings portraying the obvious expression of joy by dogs when their owners have returned from long absences, investigators have become more interested in determining to what extent animals have emotions. The body language of animals can be interpreted to suggest that they have feelings. When wolves reunite, they wag their tails to and fro, whine, and jump up and down. Many young animals play with one another or even by themselves, as when dogs chase their own tails. On the other end of the spectrum, on the death of a friend or parent, chimps are apt to sulk, stop eating, and even die. It seems reasonable to hypothesize that animals are “happy” when they reunite, “enjoy” themselves when they play, and are “depressed” over the loss of a close friend or relative. Most people would agree that an animal is feeling some type of emotion when it exhibits certain behaviors (Fig. 33D). In the past, scientists found it expedient to collect data only about observable behavior and to ignore the possible mental state of an animal. B. F. Skinner, whose research method is described in this chapter, was able to condition animals to respond automatically to a particular stimulus. He and others never considered

Is the dog happy to see his owner?

that animals might have feelings. But now, some scientists believe they have sufficient data to suggest that some animals express emotions such as fear, joy, embarrassment, jealousy, anger, love, sadness, and grief. One possible definition describes emotion as a psychological phenomenon that helps animals direct and manage their behavior. Multiple instances of soldiers returning from combat have shown how dogs react to seeing their owners after they have been absent for extended periods of time. On many occasions, the dogs run in circles, jump up and down, and excitedly lick their owner’s face. Cries and squeals of excitement can be heard coming from them continuously as they greet their owners. The Gabriela Cowperthwaite documentary Blackfish, about captive killer whales at SeaWorld, shows a scene in which keepers remove a young killer whale from its mother in order to relocate the young whale to another facility. When the mother cannot locate her offspring within the enclosure, she begins to shake violently and scream a long-rangefrequency call in an attempt to find her child. Perhaps it would be reasonable to consider the suggestion of Charles Darwin, who said that animals are different in degree rather

than in kind. This means that animals can feel love but perhaps not to the degree that humans can. When you think about it, it is unlikely that emotions first appeared in humans with no evolutionary homologies in animals. Dopamine controls the reward and pleasure center in the brain, as well as regulating emotional responses. High levels of dopamine are found in the brain of rats when they play, with the levels increasing when the rats anticipate the opportunity to play. Field research is providing information that is helping researchers learn how animal emotions correlate with their behavior, just as emotions influence human behavior.

Questions to Consider 1. Do you believe animals have emotions? Why or why not? If not, what experiment(s) would convince you that animals do have emotions? 2. Pet psychology, an emerging field, is based on the premise that pets have feelings and emotions. Would you spend $50–100 per hour to take your pet to a psychologist? 3. If it can be shown that animals have emotions, should animals still be kept in captivity?

Does the mother feel love for her calf?

Figure 33D  Emotions in animals.



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source (Fig. 33.9). Inside the hive, other foragers crowd around her and touch the forager with their antennae, and as the bee moves between the two loops of a figure 8, it buzzes noisily and shakes its entire body in so-called waggles. The angle of the straight run of the figure 8 to that of the direction of gravity is the same as that of the angle between the sun and the food source. In other words, a 40-degree angle to the left of vertical means that food is 40 degrees

to the left of the sun. Bees can use the sun as a compass because they have a biological clock that allows them to compensate for the movement of the sun in the sky. (A biological clock is an internal means of telling time—for example, darkness outside stimulates many animals to sleep.) Outside the hive, the dance is done on a horizontal surface, and the straight-run part of the figure 8 indicates the direction of the food.

Check Your Progress  33.3 1. Summarize the difference between tactile, visual, and auditory communication.

2. Identify ways in which communication is meant to affect the behavior of a receiver. 3. Describe the types of communication that would be the most effective in a dense forest.

33.4  Behaviors That Affect Fitness Learning Outcomes Upon completion of this section, you should be able to 1. Discuss why territoriality evolved in certain animal groups. 2. Explain why females should be choosier than males in selecting a mate. 3. Summarize the costs and benefits of living in a society. a.

xo xo

Behavioral ecology assumes that most behavior is subject to natural selection. We have established that behaviors can have a genetic basis, and we would expect that certain behaviors more than others will lead to increased survival and number of offspring. Therefore, much of the behavior of organisms we observe today must have adaptive value.

Territoriality and Fitness

b.

Figure 33.9  Communication among bees.  a. Honeybees do a

waggle dance to indicate the direction of food. b. If the dance is done outside the hive on a horizontal surface, the straight run of the dance will point to the food source. If the dance is done inside the hive on a vertical surface, the angle of the straightaway to that of the direction of gravity is the same as the angle of the food source to the sun.

In order to gather food, animals often have a particular home range where they can be found during the course of the day. Animals may actively defend a portion of their home range for their exclusive use as a food source or as a mating area. This portion of the home range is called their territory and the behavior is called territoriality. Territoriality is more likely to occur during times of reproduction. For example, gibbons live in the tropical rain forest of South and Southeast Asia, and territories are maintained by loud singing (Fig. 33.10). Males sing just before sunrise, and mated pairs sing duets during the morning. Males, but not females, show evidence of fighting to defend their territory in the form of broken teeth and scars. Obviously, defense of a territory has a certain cost; it takes energy to sing and fight off others. Also, you might get hurt. In order for territoriality to persist, it must have an adaptive value. Chief among the benefits of territoriality are to ensure a source of food, exclusive rights to one or more females, and to have a place to rear young and possibly to protect yourself from predators. The territory has to be the right size for the animal. Too large a territory cannot be defended, and too small a territory may not

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Energy Gain (J/s)

6.0

6 5

4.0

4 3

2.0

2 1

0 0

10

20 30 40 Length of Mussel (mm)

50

Number of Mussels Eaten per Day



Figure 33.11  Foraging for food.  When offered a choice of an equal number of each size of mussel, the shore crab, Carcinus maenas, prefers the intermediate size. This size provides the highest rate of net energy return. Net energy is determined by the energetic yield of the crab minus the energetic costs of breaking open the shell and digesting the mussel. eaten, it has no chance at all of having offspring in the future. Thus, animals may have to avoid foraging when the threat of predation is high. Animals often face these types of trade-offs that cause them to modify their behavior. As another example, many animal species stop eating during peak reproductive periods because competition for mates is often fierce.

Reproductive Strategies and Fitness Figure 33.10  Male and female gibbons.  Siamang gibbons, Hylobates syndactylus, are monogamous, and they both share the task of raising offspring. They also share the task of marking their territory by singing. As is often the case in monogamous relationships, the sexes are similar in appearance. Male is above and female is below. contain enough food. Cheetahs require a large territory to hunt for their prey and, therefore, they use urine to mark their territory (see Fig. 33.5). Hummingbirds are known to defend a very small territory because they depend on only a small patch of flowers as their food source.

Foraging for Food Animals need to ingest food that will provide more energy than the effort expended acquiring the food. In one study, it was shown that shore crabs ate intermediate-sized mussels because the net energy gain was more than if they ate larger-sized mussels (Fig. 33.11). The large mussels take too much energy to open per the amount of energy they provide. The optimal foraging model states that it is adaptive for foraging behavior (i.e., searching for food) and food choice to be as energetically efficient as possible. Even though it can be shown that animals that take in more energy are more likely to have more offspring, animals often have to consider other factors, such as escaping from predation. If an animal is killed and

Usually, primates are polygamous, and males monopolize multiple females. Because of gestation and lactation, females invest more in their offspring than do males and may not always be available for mating. Under these circumstances, it is adaptive for females to be concerned with a good food source. When food sources are clumped, females congregate in small groups. Because only a few females are expected to be receptive at a time, males will likely be able to defend these few from other males. Males are expected to compete with other males for the limited number of receptive females available (Fig. 33.12). A limited number of primates are polyandrous. Tamarins are squirrel-sized New World monkeys that live in Central or South America. Tamarins live together in groups of one or more families in which one female mates with more than one male. The female normally gives birth to twins of such a large size that the father, and not the mother, carry them about. This may be the reason these animals are polyandrous. Polyandry also occurs when the environment does not have sufficient resources to support several young at a time. Gibbons, as previously mentioned, are monogamous, which means that they pair bond. Subsequently, both the male and female help with the rearing of the young. Males are active fathers, frequently grooming and handling infants. Monogamy is relatively rare in primates, which includes prosimians, monkeys, and apes (only about 18% are monogamous). In primates, monogamy occurs when males have limited mating opportunities, territoriality exists,



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Figure 33.12  Hamadryas baboons.  Among Hamadryas baboons,

Papio hamadryas, a male, which is silver-white and twice the size of a female, keeps and guards a harem of females with whom he mates exclusively.

and the male is fairly certain the offspring are his. In gibbons, females are evenly distributed in the environment, most likely because they are aggressive to one another. Studies have shown that females do attack a speaker when it plays female sounds in their territory.

Sexual Selection Sexual selection is a form of natural selection that favors features that increase an animal’s chances of mating. These features are adaptive in the sense that they lead to increased fitness. Sexual selection often results in female choice and male competition. Because females produce a limited number of eggs in their lifetime and generally provide the majority of the parental care, it is adaptive for them to be choosy about their mate. If they choose a mate that passes on features to a male offspring that will cause him to be chosen by females, their fitness has increased. Whether females choose features that are adaptive to the environment is in question. For example, peahens are likely to choose peacocks that have the most elaborate tails. Such a fancy tail could otherwise be detrimental to the male and make him more likely to be captured by a predator. In one study, an extra ornament was attached to a father zebra finch and the daughters of this bird underwent the process of imprinting. Now, these females were more likely to choose a mate that also had the same artificial ornament. While females can always be sure an offspring is theirs, males do not have this certainty. However, males produce a plentiful supply of sperm. The best strategy for males to increase their fitness, therefore, is to have as many offspring as possible. Competition may be required for them to gain access to females, and ornaments, such as antlers, can enhance a male’s ability to fight (Fig. 33.13). When bull elk compete, they issue a loud number of screams that gives way to a series of grunts. If still necessary, the two bulls walk in parallel to show each other their physique. If this doesn’t convince

Figure 33.13  Competition.  During the mating season, elk males of the species Cervus elaphus, often find it necessary to engage in antler wrestling in order to have sole access to females in a territory.

one or the other to back off, the pair resorts to ramming each other with their antlers. Rarely is either bull actually hurt. Whereas a peacock cannot shed his tail, elk shed their antlers as soon as mating season is over.

Mating in Humans A study of human mating behavior shows that the concepts of female choice and male competition apply to humans as well as to other animals. That is, mate choice behavior in men and women in most human cultures seems to be influenced by its fitness consequences. Of course, applying the principles of evolutionary biology to human behavior is not without controversy because we can’t be reduced to being “preprogrammed” and still have the ability to make conscious choices. That said, understanding the evolutionary basis of animal behavior can provide some interesting insights into why people behave the way they do.

Human Males Compete Consider that women, by nature, must invest more in having a child than men. After all, it takes almost ten months to have a child, and pregnancy is followed by lactation when a woman may nurse her infant. Men, on the other hand, need only contribute sperm during a sex act that may require only a few minutes. The result is that men are generally more available for mating than are women. Because more men are available, they necessarily have to compete with others for the opportunity to mate. Like many other animals, humans are dimorphic. Males tend to be larger and more aggressive than females, perhaps as a result of past sexual selection by females. As in other animals, males may pay a price due to high levels of testosterone, the energetic costs of male-male competition, and increased stress associated with finding mates. Human men tend to live on average four to seven years less than females.



Chapter 33  Behavioral Ecology

Females Choose David Buss, an evolutionary psychologist at the University of Texas, conducted studies of female preference of a male mate across cultures in over 20 countries. His research, although somewhat controversial, suggests that the number-one trait females prefer in a male mate is his ability to obtain (financial) resources. Financial success means that men are more likely to provide females with the resources they need to raise their children, and thus increase the fitness (reproductive success) of the female. Other recent studies have shown that symmetry in facial and body features is also important in female mate choice, possibly related to the “good genes” hypothesis. That is, females may choose males with “good genes” so that their offspring have good genes and a better chance at survival. As an example, symmetry in other animals is often a sign of good health and a strong immune system (e.g., animals with high parasite loads are often asymmetrical). Females may thus choose symmetrical males to pass these traits on to their offspring.

Men Also Have a Choice Just as women choose men who can provide resources, men prefer youthfulness and attractiveness in females, signs that their partner can provide them with children. In one controversial study, men

SCIENCE IN YOUR LIFE  ►

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ages 8 to 80 across four continents preferred women with a waistto-hip ratio (WHR) of 0.7, regardless of weight. Research showed that a WHR of 0.7 is optimal for conception; that is, for each increase of 10% in WHR, the odds of conception during each ovulation event decrease by 30%. Men responding to questionnaires prefer physical attributes in their female mates that biologists associate with a strong immune system and good health (e.g., symmetry), high estrogen levels (indicating fertility), and especially youthfulness. On average, men marry women 2.5 years younger than they are, but as men age, they tend to prefer women who are many years younger. Men are capable of reproduction for many more years than women. Therefore, by choosing younger women, older men can increase their fitness. Other factors are also involved in human mate choice, as explained in the Health feature, “Mate Choice and Smelly T-Shirts.”

Societies and Fitness The principles of evolutionary biology can be applied to the study of social behavior in animals. Sociobiologists hypothesize that societies form when living in a society has a greater reproductive benefit than reproductive cost. An analysis of cost and benefit can help determine if this hypothesis is supported. Group living does have its benefits. It can help an animal avoid predators, rear offspring, and find food. A group of impalas is more likely to hear an

HEALTH

Mate Choice and Smelly T-Shirts Mate choice has been studied extensively in humans, largely because of our curiosity about how evolutionary biology and behavior in other animals may give us insight into human behavior. And, of course, mate choice is one of the more fascinating human behaviors. In one unusual and recent study, scientists had several men wear T-shirts for a few nights while they slept, thereby infusing the T-shirts with their unique smell. Then they asked women to rate the attractiveness of the male without seeing the person—that is, based only on the smell of the T-shirt. It turns out that women tended to choose men whose major histocompatibility complex (MHC) alleles were different from their own. Recall from Chapter 13 that the MHC is associated with recognition of foreign antigens (e.g., viruses or bacteria) and enhances the immune system’s ability to fight off infections. In choosing men with different (complementary) MHC alleles, females are maximizing the chance that their offspring have highly heterozygous MHC and potentially a more robust immune system. A more heterozygous MHC is associated with a better innate ability to recognize a more diverse group of antigens. In fact,

females in this study said unattractive shirt smells reminded them of their fathers, who, of course, have very similar MHC alleles. Where did scientists get the idea for such a strange experiment? It has already been well documented that mice preferentially choose mates with complementary MHC alleles based on smell. Of course, while evolutionary and behavioral biology can provide insights into human mate choice, the process is clearly complex. Although males prefer young faithful females with physical attributes that indicate fertility, they also look for symmetry, intelligence, and a sense of humor. And while females prefer males with financial resources, they also list such qualities as dependability and emotional stability (indicators of good parenting), physical attractiveness (indicated by symmetry and testosterone markers, such as above-average strength and height, possibly to defend resources), sense of humor, and intelligence. The way a person smells also could play a factor, as indicated by the T-shirt study. Then there are unknowns—the context under which people meet, their professions, their likes/ dislikes, etc.

Questions to Consider 1. Scientists have found a biological basis for infanticide. In lions, for example, males who kill the offspring of other males when they take over a pride then induce females to come into estrus, so they can mate with them. This increases male fitness. Should a biological basis for infanticide be used as a defense in human court cases where, say, a stepfather kills a stepchild? 2. As discussed in section 33.4, some studies suggest that men prefer women with a waistto-hip ratio of 0.7. Women of different weights can have this ratio because it is based on bone structure, rather than weight. Nonetheless, women may misinterpret this information, which can contribute to the epidemic of eating disorders, such as bulimia and anorexia. How can we better educate people about the scientific information on mate choice and the dangers of eating disorders? 3. Studies of human mate choice are generally complex, and often controversial. Should federal agencies fund these studies? Why or why not?



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approaching predator than a solitary one. Many fish moving rapidly in different directions might distract a would-be predator. Weaver birds form giant colonies that help protect them from predators, but the birds may also share information about food sources. Primate members of the same baboon troop signal to one another when they have found an especially bountiful fruit tree. Having a larger number of individuals looking for food increases the chances of finding it. Group living does have its disadvantages. When animals are crowded together into a small area, disputes can arise over access to the best feeding places and sleeping sites. Dominance hierarchies are one way to divide resources, but this puts subordinates at a disadvantage. Among red deer, sons increase the potential for the harem master to have a greater number of grandchildren. However, sons, being larger than daughters, need to be nursed more frequently and for a longer period of time. Subordinate females do not have access to enough food resources to adequately nurse sons and, therefore, they tend to rear daughters, not sons. Still, like the subordinate males in a baboon troop, subordinate females in a red deer harem may be better off in terms of fitness if they stay with a group instead of trying to survive on their own. Living in close quarters exposes individuals to illness and parasites that can easily pass from one animal to another. Social behavior helps to offset some of the proximity disadvantages. For example, baboons and other types of social primates invest much time in grooming one another, and this most likely helps them remain healthy. Humans use extensive medical care to help offset the health problems that arise from living in the densely populated cities around the world.

Sociobiology and Human Culture Humans today rely on living in organized societies. Clearly, the benefits of social living must outweigh the costs because we have organized governments, with laws that tend to increase the potential for human survival. The culture of a human society involves a wide spectrum of customs, ranging from how to dress to forms of entertainment, marriage rituals, and types of food. Language and the use of tools are essential to human culture. Language is used to socialize children, teach them how to use tools, educate them, and train them in skills that will increase their chances of success. Technological skills, such as knowing how to use a computer or how to fix plumbing, are taught from an early age. Certainly cultural evolution has surpassed biological evolution in the past few centuries. For example, medical advances, which are passed on through language, have increased the human life expectancy from 40 to 45 years a century ago to nearly 80 years today. Why did human societies originate? Perhaps the earliest organized societies were composed of “hunter-gatherers.” Which of our ancestors first became hunters is debated among paleontologists (biologists who study fossils) and sociologists. Nonetheless, scientists speculate that a predatory lifestyle may have encouraged the evolution of intelligence and the development of language. That is, cooperation, communication, and tools (such as weapons) are necessary for hunting large animals. In huntergatherer societies, men tended to hunt, while women utilized wild

and cultivated plants as a source of food. Sometimes, when animal food was scarce, hunter-gatherer societies relied on plant food cultivated by women. Cooperation increased their chances for survival.

Altruism Versus Self-Interest Altruism can be generally described as a self-sacrificing behavior for the good of another member of the society. In an evolutionary sense, altruism may compromise the fitness of the altruist, while benefiting the fitness of the recipient. Are animals truly altruists, given that natural selection should eventually rid populations of altruists due to their decreased reproductive success? In humans, we can clearly think of examples—a volunteer firefighter dying while trying to save a stranger, a soldier losing his life for his country, a woman diving in front of a car to save an elderly person from being hit. But what about other animals? In general, altruistic behavior in animals is explained by the concept of kin selection. Because close relatives share many of your genes, it may make sense to self-sacrifice to save them. For example, your siblings share 50% of your genes, so sacrificing your life for two brothers or sisters makes sense evolutionarily. Inclusive fitness refers to an individual’s personal reproductive success, as well as that of his or her relatives, and thus to an individual’s total genetic contribution to the next generation. The concepts of kin selection and inclusive fitness are well supported by research in complex animal societies. For example, in a colony of army ants (Fig. 33.14), the queen is inseminated only during her nuptial flight. Thereafter, she spends the rest of her life reproducing constantly, laying up to 30,000 eggs per day! The eggs hatch into three different sizes of sterile female workers whose jobs contribute to the benefit of the entire colony. The smallest workers (3 mm), called nurses, take care of the queen and the larvae by ­feeding and cleaning them. The intermediate-sized workers (3–12 mm),

Figure 33.14  Altruism and army ants.  A queen army ant has a large abdomen for egg production and is cared for by small ants, called nurses. The idea of inclusive fitness suggests that relatives, in addition to offspring, increase an individual’s reproductive success. Therefore, sterile nurses are being altruistic when they help the queen produce offspring to whom they are closely related.



Chapter 33  Behavioral Ecology

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which constitute most of the population, go out on raids to collect food. The soldiers (14 mm), which have relatively huge heads and powerful jaws, surround raiding parties to protect them and the colony from attack by intruders. The worker ants engage in this apparently altruistic behavior because it has a reproductive advantage. The queen ant is diploid (2n), but her mate is haploid. Thus, assuming the queen has had only one mate, her sister workers are more closely related to each other than normal siblings (75% versus 50%). Therefore, a worker can achieve higher inclusive fitness by helping her mother (the queen) produce additional sisters than by producing her own offspring, which would only share 50% of her genes. Thus, this behavior is not really altruistic. Rather, it is adaptive because it is likely due to the increase in inclusive fitness of the helpers relative to those that hypothetically might defect and breed on their own. Similar social patterns are observed in certain species of bees and wasps.

Reciprocal Altruism In some bird species, offspring from a previous clutch of eggs may stay at the nest to help parents rear the next batch of offspring. In a study of Florida scrub jays, the number of fledglings produced by an adult pair doubled when they had helpers. Mammalian offspring are also observed to help their parents (Fig. 33.15). Among jackals in Africa, solitary pairs managed to rear an average of 1.4 pups, whereas pairs with helpers reared 3.6 pups. What are the benefits of staying behind to help? First, a helper is contributing to the survival of its own kin. Therefore, the helper actually gains a fitness benefit. Second, a helper is more likely than a nonhelper to inherit a parental territory—including other helpers. Helping, then, involves making a minimal, short-term reproductive sacrifice in order to maximize future reproductive potential. Therefore, helpers at the nest are also practicing a form of reciprocal altruism. Reciprocal altruism also occurs in animals that are not necessarily closely related. In this event, an animal helps or cooperates with another animal with no immediate benefit. However, the animal that was helped will repay the debt at some later time. Reciprocal altruism usually occurs in groups of animals that are mutually dependent. Cheaters in reciprocal altruism are recognized and not reciprocated in future events. Reciprocal altruism occurs in vampire bats that live in the tropics. Bats returning to the roost after a feeding activity share their blood meal with other bats in the roost. If a bat fails to share blood with one that had previously shared blood with it, the cheater bat will be excluded from future blood sharing.

Check Your Progress  33.4 1. Explain how territoriality is related to foraging for food. 2. Compare and contrast reproductive strategies and forms of sexual selection.

3. Describe the advantages and disadvantages of social living.

Figure 33.15  Inclusive fitness.  A meerkat is acting as a babysitter for its younger sibling while their mother is away. Researchers point out that the helpful behavior of the older meerkat can lead to increased inclusive fitness.

Conclusion Female African lions form a society, called a pride, which provides benefits in foraging for food and protection of the young. The members of the pride are all closely related females (mothers, daughters, sisters, and aunts), which promotes altruism. However, this is not without a cost. While female cubs tend to stay in the pride their entire lives, the young males leave and as they mature they will form small groups of genetically related individuals. Upon reaching maturity, they will establish a territory of their own that will coincide with that of a pride or prides of female lions. The males will often kill cubs that are not of their genetic line in order to get the females to enter estrus. They will then mate with the females in an attempt to pass on their genetics. While group life does have some disadvantages, the benefits must outweigh them in order for it to persist.



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MEDIA STUDY TOOLS http://connect.mheducation.com Maximize your study time with McGraw-Hill SmartBook®, the first adaptive textbook.

SUMMARIZE

■ Males, who produce many sperm, are expected to compete to insemi-

33.1  Nature Versus Nurture: Genetic Influences

■ Females, who produce few eggs, are expected to be selective about

nate as many females as possible.

■ Investigators have long been interested in the degree to which nature

(genetics) or nurture (environment) influences behavior. ■ Hybrid studies with lovebirds and human twin studies are consistent with the hypothesis that behavior has a genetic basis. Garter snake experiments indicate that the nervous system controls behavior. Egglaying behavior in Aplysia has endocrine system involvement.

33.2  Nature Versus Nurture: Environmental Influences

■ ■ ■

■ Even some behaviors formerly thought to be fixed action patterns

(FAPs) can sometimes be modified by learning. A red spot on the bill of laughing gulls initiates chick begging behavior. However, with experience, chick begging accuracy improves. ■ Other studies also support the involvement of learning in behaviors. Imprinting during a sensitive period causes birds to follow the first moving object they see. ■ Song learning in birds involves various elements—including learning during the sensitive period, as well as the effect of social interactions outside the period. ■ Associative learning includes classical conditioning and operant conditioning. In classical conditioning, the pairing of two different types of stimuli causes an animal to form an association between them (e.g., dogs salivating at the sound of a bell). In operant conditioning, animals learn behaviors because they are rewarded when they perform them.

33.3  Animal Communication ■ Communication is an action by a sender that affects the behavior of a ■ ■ ■ ■

receiver. Pheromones are chemical signals passed between members of the same species. Auditory communication includes language, which occurs in humans and other animals. Visual communication allows signaling without auditory or chemical messages. Tactile communication is especially associated with social behavior.

33.4  Behaviors That Affect Fitness ■ Traits that promote reproductive success are expected to have fitness

advantages that outweigh their disadvantages. ■ Some animals are territorial and defend a territory that may have highquality food and/or nesting sites. ■ Sexual selection is a form of natural selection that selects for traits that increase an animal’s fitness.

■

■

their mates. Females may choose mates based on increased survival of offspring or on traits that make their sons more attractive to other females. Culture in human society has enabled us to dramatically increase the success of our species. It is likely that mating behavior and mate choice in humans have also been shaped by evolutionary forces. Living in a social group can have its advantages (e.g., avoiding predators, raising young, and finding food). It also has disadvantages (e.g., competition between members, spread of illness and parasites, and reduced reproductive potential). When animals live in groups, the fitness (number of fertile offspring produced) benefits must outweigh the costs, or social behavior would not exist. Sometimes animals exhibit altruism in order to increase their reproductive success. However, it is necessary to consider inclusive fitness, which includes personal reproductive success, as well as the reproductive success of relatives. Kin selection is important in social species because it increases the number of individuals who survive that share your genes. For example, social insects help their mother reproduce, but this behavior seems reasonable when we consider that siblings share 75% of their genes. Parental helpers in mammals and birds often inherit the parent’s territory.

ASSESS Testing Yourself Choose the best answer for each question.

33.1  Nature Versus Nurture: Genetic Influences 1. An organism’s response to a stimulus or situation is the definition of: a. adaptation. b. learning. c. memory. d. behavior. e. epigenetics. 2. Which of the following is not an example of a genetically based behavior? a. Inland garter snakes do not eat slugs, whereas coastal populations do. b. Maternal caring instinct in mice c. Learning begging behavior for food in some bird species. d. Snails lay eggs in response to an egg-laying hormone. e. All of the above are genetic.



Chapter 33  Behavioral Ecology

33.2  Nature Versus Nurture: Environmental Influences 3. How would the following graph differ if begging behavior in laughing gulls was a fixed action pattern? a. It would be a diagonal line with an upward incline. b. It would be a diagonal line with a downward incline. c. It would be a horizontal line. d. It would be a vertical line. e. None of these are correct.

Hits (percent)

100 75 50 25 0

0

1 2 Days in Nest

3

4

4. Which of the following best describes classical conditioning? a. the gradual strengthening of stimulus-response connections that seemingly are unrelated b. a type of associative learning in which there is no contingency between response and reinforcer c. learning behavior in which an organism follows the first moving object it encounters d. learning behavior in which an organism exhibits a fixed action pattern from the time of birth 5. In white-crowned sparrows, social experience exhibits a very strong influence over the development of singing patterns. What observation led to this conclusion? a. Birds learned to sing only when they were trained by other birds. b. The window in which birds learn from other birds is wider than that in which birds learn from tape recordings. c. Birds could learn different dialects only from other birds. d. Birds that learned to sing from a tape recorder could change their song when they listened to another bird.

33.3  Animal Communication For questions 6–9, match the type of communication in the key with its description. Answers may be used more than once or not at all. Key: a. chemical communication b. auditory communication c. visual communication d. tactile communication 6. Aphids (insects) release an alarm pheromone when they sense they are in danger.

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7. Male peacocks exhibit an elaborate display of feathers to attract females. 8. Ground squirrels give an alarm call to warn others of the approach of a predator. 9. Male silk moths are attracted to females by a sex attractant released by the female moth. 10. Sage grouses perform an elaborate courtship dance.

33.4  Behaviors That Affect Fitness 11. Which reproductive strategy is characterized by a female mating with multiple males? a. polygamy b. polyandry c. monogamy d. kin selection e. altruism 12. All of the following are benefits obtained through territoriality except a. access to mates. b. access to more food. c. access to more places to hide. d. access to more predators. e. increased space to raise more offspring.

ENGAGE Thinking Critically 1. You are studying two populations of rats—one in New York and the other in Florida. Adult New York rats seem to prefer Swiss cheese, whereas Florida rats seem to prefer cheddar. Design an experiment to determine the extent to which this behavior may be genetically controlled versus environmentally influenced. 2. If altruistic behavior has usually evolved to improve the altruist’s inclusive fitness by helping relatives, then why do you think humans perform altruistic acts for completely unrelated individuals (e.g., jumping into a pool to save a drowning person)?

PHOTO CREDITS Opener: © Frederic Coubet/Gallo Images/Getty Images; 33A: © Hero/Corbis/Glow Images RF; 33.1b: © Tom McHugh/Science Source; 33.1c: © Biosphoto/SuperStock; 33.2(inland): © John Serrao/Science Source; 33.2(coastal): © Chris Mattison/Alamy; 33.5: © Gregory G. Dimijian/Science Source; 33.6a(main): © Arco Images/GmbH/Alamy; 33.6a(inset): © Flavio Vallenari/E+/Getty RF; 32.7: © Raul Arboleda/AFP/Getty Images; 33.8(trees): © PhotoLink/ Getty RF; 33.8(firefly): © James Jordan Photography/Getty RF; 33D(right): © Bryce Flynn/ Moment/Getty RF; 33D(left): © The Forum, Jay Pickthorn/AP Images; 33.9a: © Scott Camazine/Alamy; 33.10: © Nicole Duplaix/Getty Images; 33.12: © Thomas Dobner 2006/ Alamy RF; 33.13: © D. Robert & Lorri Franz/Corbis; 33.14: © Alexander Wild/www. alexanderwild.com; 33.15: © Biosphoto/SuperStock.



34

Population and Community Ecology CHAPTER OUTLINE 34.1  The Scope of Ecology 34.2 Patterns of Population Growth 34.3 Interactions Between Populations 34.4 Ecological Succession BEFORE YOU BEGIN Before beginning this chapter, take a few moments to review the following discussions: Section 6.1  What are the forms of energy? Section 8.1  How does photosynthesis capture solar energy? Section 33.4  How does a species’ behavior determine the type of relationship it has with other species?

CASE STUDY Exotic Species In the early 1970s, several species of Asian carp were imported into Arkansas for the biocontrol of algal blooms in aquaculture facilities. Shortly thereafter, they escaped into the middle and lower Mississippi drainage. Over time, in some areas they have become the most abundant species and have now spread throughout the majority of the Mississippi River system. Asian carp mainly eat the microscopic algae and zooplankton in freshwater ecosystems. They can achieve weights up to a hundred pounds, grow to a length of more than 4 ft, and live up to 30 years. Because of their voracious appetite, they have the potential to reduce the population of native fish species such as gizzard shad, bigmouth buffalo, and native mussels due to competition for the same food sources. The main fear is that the Asian carp will put extreme pressure on the zooplankton populations, which can lead to a dense planktonic algal bloom. Ultimately, they can produce a significant disruption of the freshwater ecosystems that they invade. These fish also pose an economic threat by fouling the nets of commercial fishermen. Another major concern is the impact these fish may have upon the Great Lake’s annual $7 billion fishing industry. Our knowledge of population growth and regulation has led to controls on fishing, including size limits and catch limits, to try and sustain the world’s commercially valuable fish population. This knowledge can also be applied to management of species that can be considered harmful to a specific ecosystem. As you read through the chapter, think about the following questions:

1. Does the old saying “90% of the fish are found in 10% of the lake” have any truth to it?

2. Do population growth models apply to humans the same way they apply to other species?

3. Do more-developed or less-developed countries have a greater impact upon the Earth’s resources?

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Chapter 34  Population and Community Ecology

34.1  The Scope of Ecology Learning Outcomes Upon completion of this section, you should be able to 1. Identify what aspects of biology the study of ecology encompasses. 2. Recognize the different levels of ecological study.

Ecology is the study of the interactions of organisms with each other and with their physical environment. Ecology, like so many biological disciplines, is wide-ranging and involves several levels of study (Fig. 34.1). At the most basic level, ecologists study how organisms are adapted to their environment. For example, they might study why a particular species of fish in a coral reef lives only within a narrow temperature range in warm tropical waters. Most organisms do not exist singly. Rather, they are part of a population, which is defined as all the organisms of the same species interacting with the environment in a particular area. At the population level of study, ecologists describe the size of populations over time. For example, an ecologist might study the relative sizes of parrotfish populations within a coral reef over time. A community consists of all the various populations at a particular locale. A coral reef community contains numerous populations of fish species, crustaceans, corals, and so forth. At the community level, ecologists study how various extrinsic factors (e.g., weather) and intrinsic

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factors (e.g., species’ competition for resources) affect the size of these populations. An ecosystem encompasses a community of populations, as well as the nonliving environment. For example, energy flow and chemical cycling in a coral reef can affect the success of the organisms that inhabit it. Finally, the biosphere is that portion of the entire Earth’s surface—air, water, land—where living organisms exist. Knowing the composition and diversity of an ecosystem, such as a coral reef, is important to the dynamics of the biosphere.  Modern ecology is not just descriptive; it primarily focuses on developing testable hypotheses. A central goal of modern ecology is to develop models that explain and predict the distribution and abundance of populations and species. Ultimately, ecology considers not just one particular area, but species’ distributions throughout the biosphere. In this chapter, we particularly concentrate on the patterns of population growth, and how population growth is regulated. Then, at the community level, we examine the interactions between populations and how they change through time during the process of ecological succession.

Check Your Progress  34.1 1. Distinguish between a population and a community. 2. Describe the central goal of modern ecology.

Figure 34.1  Levels of ecological organization.  The study of ecology encompasses levels of organization that proceed from the individual organism to the population, to the community, and to an ecosystem.

Individual

Population

Community

Ecosystem

Coral reef community



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34.2  Patterns of Population Growth Learning Outcomes Upon completion of this section, you should be able to 1. Recognize how the rate of natural increase in a population is calculated. 2. Compare exponential and logistic population growth curves. 3. Recognize how the proportion of individuals at varying reproductive stages determines a population’s age distribution.

Each population has a particular pattern of growth. The population size can change according to the rate of natural  increase (r) or growth rate. Suppose, for example, a human population presently has a size of 1,000 individuals, the birthrate is 30 per year, and the death rate is 10 per year. The growth rate per year will be as follows: 30 − 10 / 1,000 = 0.02 = 2.0% per year

a.

Note that this per capita rate of increase disregards both immigration and emigration, which for this example can be assumed to be equal and thus to cancel each other out. The highest possible per capita rate of increase for a population is called its biotic potential (Fig. 34.2). Whether the biotic potential is high or low depends on a variety of factors such as (1) reproductive potential of the current population, (2) availability of food, (3) presence or absence of disease, and (4) presence or absence of predators. Suppose we are studying the growth of an insect population that is capable of infesting and taking over an area. Under these circumstances, exponential growth is expected. An exponential pattern of population growth results in a J-shaped curve (Fig. 34.3a). This pattern of population growth can be likened to compound interest at the bank: The more your money increases, the more interest you earn. If the insect population has 2,000 individuals and the per capita rate of increase is 20% per month, there will be 2,400 insects after one month, 2,880 after two months, 3,456 after three months, and so forth.

b.

Figure 34.2  Biotic potential.  A population’s maximum growth rate under ideal conditions—that is, its biotic potential—is greatly influenced by the number of offspring produced in each reproductive event. a. Mice, which produce many offspring that quickly mature to produce more offspring, have a much higher biotic potential than (b) the rhinoceros, which produces only one or two offspring during infrequent reproductive events.

exponential growth

lag

Number of Organisms

Number of Organisms

carrying capacity

stable equilibrium logistic growth

exponential growth lag Time

Time a.

environmental resistance

b.

Figure 34.3  Patterns of population growth.  a. Exponential growth results in a J-shaped growth curve because the growth rate is positive, and

increasing, as in insects. b. Logistic growth results in an S-shaped growth curve because environmental resistance causes the population size to level off and be in a steady state at the carrying capacity of the environment, as in fish.



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Notice that in exponential growth that there are two distinct phases (Fig. 34.3a):

Usually, exponential growth cannot continue for long because of environmental resistance. Environmental resistance encompasses all those environmental conditions—such as limited food supply, accumulation of waste products, increased competition, or predation—that prevent populations from achieving their biotic potential. The amount of environmental resistance will increase as the population grows larger. This eventually causes the population growth to level off, resulting in an S-shaped pattern called logistic growth (Fig. 34.3b). The characteristics of logistic growth include: ■■ ■■ ■■ ■■

1,000

Lag phase. Growth is slow because the population is small. Exponential growth phase. Growth is accelerating, and the population is exhibiting its biotic potential.

Lag phase. Growth is slow because the population is small. Exponential growth phase. Growth is accelerating, due to biotic potential. Logistic growth phase. The rate of population growth slows down. Stable equilibrium phase. Little if any growth takes place because births and deaths are about equal.

The stable equilibrium phase is said to occur at the carrying capacity of the environment. The carrying capacity is the number of individuals of a species that a particular environment can support. Our knowledge of logistic growth has practical implications. The model predicts that exponential growth occurs only when population size is much lower than the carrying capacity. So, for example, if humans are using a fish population as a continuous food source, it would be best to maintain that population size in the exponential phase of growth when the birthrate is the highest. If we overharvest, the fish population will sink into the lag phase, and it will be years before exponential growth recurs. On the other hand, if we are trying to limit the growth of a pest, it is best to reduce the carrying capacity. Simply reducing the population size only encourages exponential growth to begin again. Farmers can reduce the carrying capacity for a pest by alternating rows of different crops within their fields instead of growing one type of crop throughout the entire field.

Survivorship Population growth patterns assume that populations are made up of identically aged individuals. In real populations, however, individuals are in different stages of their life spans. Let us consider how many members of an original group of individuals born at the same time, called a cohort, are still alive after certain intervals of time. The result is a survivorship curve. For the sake of discussion, three types of idealized survivorship curves are recognized (Fig. 34.4). The type I curve is characteristic of a population in which most individuals survive well past the midpoint, and death comes near the end of the maximum life span. On the other hand, the type III curve is typical for a population in which most individuals die very young. In the type II curve,

100 Number of Survivors

■■ ■■

I human songbird

II

10

III oyster 0

50 Percent of Life Span

100

Figure 34.4  Survivorship curves.  Humans have a type I survivorship curve: the individual usually lives a normal life span, and then death is increasingly expected. Songbirds have a type II curve: the chances of surviving are the same for any particular age. Oysters have a type III curve: most deaths occur during the free-swimming larva stage, but oysters that survive to adulthood usually live a normal life span. survivorship decreases at a constant rate throughout the life span. Some species, however, do not fit any of these curves exactly. Much can be learned about the life history of a species by studying its survivorship curve. Only a few members of a population with a type III survivorship curve will actually contribute offspring to the next generation. Because death comes early for most members, only a few live long enough to reproduce.

Human Population Growth Figure 34.5a illustrates human population growth. Growth in lessdeveloped countries is still in the exponential phase and is not expected to level off until about 2040, while growth in more-­ developed countries has leveled off. The equivalent of a mediumsized city (roughly 220,000) is added to the world’s population every day, and 80 million (equivalent to the combined populations of Argentina, Ecuador, and Peru) are added every year. It has been increasingly difficult to predict patterns of future population growth, not only because the growth rate of the human population is decreasing, but also because economic factors and advances in medicine have an influence on the birthrate. Historically, growth rate was described by describing the doubling time, the length of time it takes for the population size to double. This was considered to be in the range of 35–60 years for humans, but most population experts recognize that it will probably take longer to reach 12–14 billion. Regardless, any increase in population size



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12 highest growth

10

lowest growth

Billions of People

8

6

4

less-developed countries

2

b. more-developed countries

0 1750

1800

1850

1900

1950

2008

2150

Year a.

Figure 34.5  World population growth.  a. The graph shows world population size in the past, with estimates to 2150. People in (b) moredeveloped countries have a high standard of living and will contribute the least to world population growth, whereas people in (c) less-developed countries have a low standard of living and will contribute the most to world population growth.

will put increased demands on our ability to produce and distribute resources. In the relatively near future, the world will need to increase the amount of food, jobs, water, energy, and so on just to maintain the present standard of living. Many people are gravely concerned that the amount of time it takes to add an additional billion people to the world population has become shorter. The first billion was reached in 1800; the second billion in 1930; the third billion in 1960; and today there are over 7.4 billion. By contrast, zero population growth, in which the birthrate equals the death rate and population size remains steady, can be achieved only if the per capita rate of increase declines. Recent estimates by the United Nations suggest that the world's population may level off around 10 billion sometime around 2062. 

More-Developed Versus Less-Developed Countries The more-developed countries (MDCs), typified by countries in North America and Europe, are those in which population growth is low and people enjoy a good standard of living (Fig. 34.5b). The less-developed countries (LDCs), such as countries in Latin America, Africa, and Asia, are those in which population growth is expanding rapidly and the majority of people live in poverty (Fig. 34.5c).

c.

The MDCs doubled their populations between 1850 and 1950 due to a decline in the death rate, the development of modern medicine, and improved socioeconomic conditions. However, the transition from an agricultural society to an urban society reduced the incentive for people to have large families (e.g., more people to help on the farm) because maintaining large families in urban areas is more costly than in rural areas, causing birthrates to decline. These factors, combined with the decreased death rate, caused populations in MDCs to experience only modest growth between 1950 and 1975. This sequence of events (i.e., decreased death rate followed by decreased birthrate) is termed a demographic transition. Yearly growth of the MDCs as a whole has now stabilized at about 0.1%. The populations of several of the MDCs, including Germany, Greece, Italy, Hungary, and Japan, are not growing or are actually decreasing in size. In contrast, there is no leveling off in U.S. population growth due to the number of individuals that immigrate to the United States each year. In addition, an unusually large number of babies were born between 1947 and 1964 (called a baby boom). This means a large number of women are still of reproductive age. Although death rates began to decline steeply in the LDCs following World War II with the importation of modern medicine from the MDCs, birthrates remained high. The yearly growth of

Chapter 34  Population and Community Ecology

1. Establish and/or strengthen family planning programs. A decline in growth is seen in countries with good family planning programs supported by community leaders. Currently, 25% of women in sub-Saharan Africa say they would like to delay or stop childbearing, and yet they lack access to birth control. Similarly, 15% of women in Asia and Latin America have an unmet need for birth control. 2. Use social progress to reduce the desire for large families. Many couples in LDCs presently desire as many as four to six children. But providing education, raising the status of women, reducing child mortality, and improving economic stability are desirable social improvements that could help decrease population growth. 3. Delay the onset of childbearing. A delay in the onset of childbearing and wider spacing of births could cause a temporary decline in the birthrate and reduce the present reproductive rate.

Age Distributions The age-structure diagram divides the population into three age  groups: prereproductive, reproductive, and postreproductive (Fig. 34.6). The LDCs are experiencing a population momentum because they have more women entering the reproductive years than older women leaving them. It is a misconception that if each couple has two children, referred to as replacement reproduction, that zero population growth will occur. The population will continue to grow as long as there are more young women entering their reproductive years than there are older women leaving them, which is common among most of the LDCs. Living many years after reproduction also contributes to an increase in the population. This type of population growth results in unstable age structure (Fig. 34.6b). In LDCs, the more quickly replacement reproduction is achieved, the sooner zero population growth will occur. In MDCs, on the other hand, it is more typical that the number of people in the prereproductive class roughly equals the number in the reproductive age class. This causes a stable age structure, and population numbers will remain about the same for the foreseeable future (Fig. 34.6a).

Check Your Progress  34.2 1. Explain why populations don’t grow to their biotic potential. 2. Describe the three types of idealized population survivorship curves.

3. Recognize how differences in age structure explain

differences in population growth rates between LDCs and MDCs.

80+ 75–79 70–74 65–69 60–64 55–59 50–54 45–49 40–44 35–39 30–34 25–29 20–24 15–19 10–14 5–9 0–4

695

postreproductive reproductive prereproductive

Millions a. More-developed countries (MDCs)

Age (in years)

the LDCs peaked at 2.5% between 1960 and 1965. Since that time, a demographic transition has occurred: the decline in the death rates slowed, and birthrates fell. Yearly growth now averages 1.9%. Due to exponential growth, the population of LDCs may jump from around 5 billion to around 8 billion by 2050. Most of this growth will occur in Africa, Asia, and Latin America. Suggestions for greatly reducing the expected increase include the following:

Age (in years)



80+ 75–79 70–74 65–69 60–64 55–59 50–54 45–49 40–44 35–39 30–34 25–29 20–24 15–19 10–14 5–9 0–4

postreproductive reproductive prereproductive

300 250 200 150 100 50 0 50 100 150 200 250 300 Millions b. Less-developed countries (LDCs)

Figure 34.6  Age structure diagrams.  The diagrams illustrate that (a) the MDCs are approaching stabilization, whereas (b) the LDCs will expand rapidly, resulting in unstable age structure.

34.3  Interactions Between Populations Learning Outcomes Upon completion of this section, you should be able to 1. Identify the difference between density dependence and density independence. 2. Describe how competition and predation can impact population growth. 3. List the three types of symbiosis, and discuss the characteristics of each.

Ecologists wish to determine the factors that regulate population growth. Life history patterns are characterized by how long it takes to reach reproductive maturity and the level of reproductive output



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UNIT 7  Behavior and Ecology

Opportunistic Species (r-strategist)

Equilibrium Species (K-strategist) • Large individuals • Long lifespan • Slow to mature • Few and large offspring • Much care of offspring • Most young survive to reproductive age • Adapted to stable environment

• Small individuals • Short lifespan • Fast to mature • Many offspring • Little or no care of offspring • Many offspring die before reproducing • Early reproductive age

Figure 34.7  Life history patterns.  Dandelions are an opportunistic species (r-strategist) with the characteristics noted, and bears are an equilibrium species (K-strategist) with the characteristics noted. Often the distinctions between these two possible life history patterns are not as clear-cut as they may seem. (Fig. 34.7). Populations that are small in size, mature early, and have a short life span exhibit an opportunistic pattern. They are also known as r-strategist species, which tend to produce many relatively small offspring and forego parental care in favor of a greater number of offspring. The more offspring, the more likely it is that some of them will survive to a reproductive age. Classic examples of opportunistic species are many insects and weeds. In contrast, a population that remains pretty much at the carrying capacity and allocates energy to their growth and survival as well as to the growth and survival of their offspring follows an equilibrium pattern. These populations are known as K-strategist species, which tend to be fairly large, are slow to mature, and have a fairly long life span. They are specialists rather than generalists and tend to become extinct when their normal way of life is altered. The best examples of equilibrium species are found among birds and mammals. For example, the Florida panther, the largest mammal in the Florida Everglades, requires a very large range, and produces few offspring, which must be cared for. Currently, the Florida panther is unable to compensate for a reduction in its range due to human destruction of its habitat and is therefore on the verge of extinction. It is recognized that the environment contains both abiotic and biotic components. Abiotic factors, such as weather and natural disasters, are density-independent factors, meaning that the effects of the factor are the same for all sizes of populations. For example, fires don’t necessarily kill a larger percentage of individuals as the population increases in size. On the other hand, biotic factors, such as competition, predation, and parasitism, are

called density-dependent factors. The effects of a density-­ dependent factor depend on the size of the population. The denser a population is, the faster a disease might spread, for example. Populations that have the opportunistic life history pattern tend to be regulated by density-independent factors, and those that have the equilibrium life history pattern tend to be regulated by densitydependent factors.

Competition Competition, a density-dependent factor, occurs when members of different species try to utilize a resource (such as light, space, or nutrients) that is in limited supply. Understanding the effects of competition requires a grasp of the concept of ecological niche. The ecological niche is the role a species plays in the community, including the habitat it requires and its interactions with other organisms. The niche is essentially a species’ total way of life, and thus includes the resources needed to meet its energy, nutrient, survival, and reproductive demands. According to the competitive exclusion principle, no two species can occupy the same ecological niche at the same time if resources are limited. This principle is exemplified by an experiment involving two species of paramecia. When grown together in one test tube with limited resources, only one species will survive (Fig. 34.8). In another laboratory experiment, two species of paramecia can occupy the same tube if one species feeds on bacteria at the bottom of the tube and the other feeds on bacteria suspended in solution. The division of feeding niches, called resource

Population Density

Chapter 34  Population and Community Ecology

P. aurelia grown separately

Population Density



P. caudatum grown separately

697

high tide

Chthamalus

area of competition

Population Density

Balanus Both species grown together

low tide

Time

Figure 34.8  Competition between two laboratory populations of paramecia.  When grown alone in pure culture (top two graphs),

Paramecium aurelia and Paramecium caudatum exhibit logistic growth. When the two species are grown together in mixed culture (bottom graph), P. aurelia is the better competitor, and P. caudatum dies out. This experiment illustrates the competitive exclusion principle.

partitioning, decreases competition between the two species and allows occupancy of different niches and therefore survival. On the Scottish coast, a small barnacle (Chthamalus stellatus) lives on the high part of the intertidal zone, and a large barnacle (Balanus balanoides) lives on the lower part (Fig. 34.9). Freeswimming larvae of both species attach themselves to rocks at any point in the intertidal zone, where they develop into the sessile adult forms. In the lower zone, the large Balanus barnacles seem to either force the smaller Chthamalus individuals off the rocks or grow over them. Competition is therefore restricting the range of Chthamalus on the rocks. Chthamalus is more resistant to drying out than is Balanus. Therefore, it has an advantage that permits it to grow in the upper intertidal zone. It should be noted, however, that the greatest competition for resources will always come from within your own species, an occurrence known as intraspecific competition. Two individuals of the same species will have nearly identical requirements for survival, thus producing competition when resources are limited.

Predation Predation occurs when one organism, called the predator, feeds on another, called the prey. In the broadest sense, predators include not only animals such as lions that kill zebras, but also filter-feeding blue whales that strain krill from ocean waters, and even whitetailed deer that feed upon a farmer’s cornfield.

Figure 34.9  Competition between two species of barnacles. 

Competition prevents two species of barnacles from occupying as much of the intertidal zone as possible. Both exist in the area of competition between Chthamalus and Balanus. Above this area, only Chthamalus survives, and below it only Balanus survives.

Predator-Prey Population Dynamics Predators reduce the population density of prey, as shown by a laboratory study in which the protozoans Paramecium caudatum (prey) and Didinium nasutum (predator) were grown together in a culture medium. Didinium ate all the Paramecium and then died of starvation. In nature, we can find a similar example. When a gardener brought prickly-pear cactus to Australia from South America, the cactus spread out of control until millions of acres were covered with nothing but cacti. The cacti were brought under control when a moth from South America, whose caterpillar feeds only on the cactus, was introduced. Now, both cactus and moth are found at greatly reduced densities in Australia. Mathematical formulas predict that predator and prey populations tend to cycle instead of maintaining a steady state. Cycling can occur when (1) the predator population overkills the prey, causing the predator population to decline in number, or (2) the prey population overshoots the carrying capacity and suffers a crash, causing the predator population to crash due to the lack of food. In either case, the result would be a series of peaks and valleys in population density of both species, with the predator population lagging slightly behind the prey. A famous case of predator-prey cycles occurs between the snowshoe hare and the Canadian lynx, a type of small, predatory cat (Fig. 34.10). The snowshoe hare is a common herbivore in the coniferous forests of North America, where it feeds on terminal



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UNIT 7  Behavior and Ecology

twigs of various shrubs and small trees. The Canadian lynx feeds on snowshoe hares but also on ruffed grouse and spruce grouse, two types of birds. Investigators at first assumed that the lynx had brought about the decline of the hare population. But others noted that the decline in snowshoe hare abundance was accompanied by low growth and reproductive rates that could be signs of a food shortage. It appears that both explanations apply to the data. In other words, both a predator-hare cycle and a hare-food cycle have combined to produce an overall effect, which is observed in Figure 34.10. Although not shown on the graph, densities of the grouse populations also cycle, perhaps because the lynx switches to this food source when the hare population declines. Predators and prey do not normally exist as simple, two-species systems, and therefore, abundance patterns should be viewed with the complete community in mind.

In plants, the sharp spines of the cactus, the pointed leaves of holly, and the tough, leathery leaves of the oak tree all discourage predation by insects. Plants even produce poisonous chemical compounds to deter predation. Animals have varied antipredator defenses such as poisonous secretions, concealment, fright, flocking together, and warning coloration (Fig. 34.11), as well as mimicry. A good example of concealment occurs with caddisfly larvae, which build protective cases out of sticks and sand, making themselves look like the stream bottom where they reside.

Mimicry  Mimicry occurs when one species resembles another species that has evolved to defend against predators or resembles an object in the environment. Mimicry can help a predator capture food or a prey avoid capture. For example, angler fishes have lures that resemble worms for the purpose of bringing fish within reach. To avoid capture, some inchworms resemble twigs, and some caterpillars can transform themselves into shapes resembling snakes. Batesian mimicry (named for Henry Bates, who discovered it) occurs when a prey species that is not harmful mimics another species that has a successful antipredator defense. Many examples of Batesian mimicry involve warning coloration. For example, stinging insects all tend to have black and yellow bands, like those of the wasp in Figure 34.12a. Once a predator has experienced the defense of the wasp, it remembers the coloration and tends to avoid animals that look similar, such as the nonstinging flower fly and

Antipredator Defenses While predators have evolved strategies to secure the maximum amount of food with minimal expenditure of energy, prey organisms have evolved strategies to escape predation. As is discussed in the Scientific Inquiry feature, “Interactions and Coevolution,” the process of Coevolution occurs when two species adapt in response to selective pressure imposed by the other. This phenomenon applies to predation, as well as to symbiotic interactions.

Figure 34.10  Predator-prey cycling of a lynx and a

Snowshoe hare and lynx cycles, boreal forest, Kluane, Yukon 5

100

snowshoe hares per ha lynx tracks per 100 km

4

80

3

60

2

40

1

20

0 1987 1990

1995

2000

2005

snowshoe hare.  Research indicates that the snowshoe hare population reaches a peak abundance before that of the lynx by a year or more. A study conducted from 1987 to 2009 shows a cycling of the lynx and snowshoe hare populations.

0 2009

Source: Krebs 2010 lynx snowshoe hare



Chapter 34  Population and Community Ecology

SCIENCE IN YOUR LIFE  ►

699

SCIENTIFIC INQUIRY

Interactions and Coevolution Coevolution is present when two species adapt in response to selective pressure imposed by the other. Symbiosis (close association between two species), which includes parasitism, commensalism, and mutualism, is especially prone to the process of coevolution.  Coevolution also occurs between predators and prey. For example, a cheetah sprints forward to catch its prey, which selects for the gazelles that are fast enough to avoid capture. Over generations, the adaptation of the prey may put selective pressure on the predator for an adaptation to the prey’s defense mechanism. In this way, an evolutionary “arms race” can develop.  The process of coevolution has been studied in the brown-headed cowbird, a social parasite that reproduces at the expense of other birds by laying its eggs in their nests. It is a

strange sight to see a small bird feeding a cowbird nestling several times its size. Investigators discovered that in order to “trick” a host bird, the female cowbird has to quickly lay an egg that mimics the host’s egg while the host is away from the nest. The cowbird will leave most of the host’s eggs in the nest to prevent the host from deserting a nest with only one egg. The cowbird chick hatches first and is behaviorally adapted to pushing other eggs out of the nest (Fig. 34A)  At this stage in the “arms race,” the cowbird appears to have the upper hand; however, selection may favor host birds that are able to distinguish the cowbird eggs from their own. In the case of the yellow warbler, the adults have evolved the mechanism of building a new nest on top of the cowbird eggs in order to avoid being brood parasitized. 

The relationship between parasite and host can even include the ability of parasites to seemingly manipulate the behavior of their hosts in self-serving ways. Ants infected with the lance fluke mysteriously cling to blades of grass with their mouthparts. There, the infected ants are eaten by grazing sheep, transmitting the flukes to the next host in their life cycle. Similarly, snails of the genus Succinea are parasitized by worms of the genus Leucochloridium. As the worms mature, they invade the snail’s eyestalks, making them resemble edible caterpillars. Birds eat the infected snails, allowing the parasites to complete their development inside the urinary tracts of birds.  The traditional view was that as host and parasite coevolved, each would become more tolerant of the other. Eventually, parasites could become commensal, or harmless to the host. Then over time, the parasite and host might even become mutualists. In fact, the evolution of the eukaryotic cell by endosymbiosis (see section 3.5) is based on the supposition that some early bacteria took up residence inside a larger cell, and then the parasite and cell became mutualists.  However, this argument also raises questions because no organism is capable of “looking ahead” at its evolutionary fate. Rather, if an aggressive parasite could transmit more of itself in less time than a benign one, aggressiveness would be favored by natural selection, because the most aggressive would reproduce. Other factors, such as the life cycle of the host, can determine whether aggressiveness is beneficial or not. 

Questions to Consider a.

b.

Figure 34A  Social parasitism.  The brown-headed cowbird, Molothrus ater, is a social parasite of more than 220 species of birds. a. The blue eggs of the eastern bluebird and the speckled egg of the cowbird in a nest. b. A cowbird chick that is outcompeting its nestmates for food 

longhorn beetle in Figure 34.12b, c, which can be considered Batesian mimics. Another type of mimicry occurs when species that resemble each other all have successful defenses. For example, many coral snake species have brilliant red, black, and yellow body rings and are venomous. Mimics that share the same protective defense are called Müllerian mimics, named after Fritz Müller, who

1. What happens to a species if it cannot coevolve along with the species it is interacting with?  2. Why is it necessary for a parasite to avoid killing its host? 

discovered the phenomenon. The bumblebee in Figure 34.12d is a Müllerian mimic of the wasp in Figure 34.12a—that is, both insects have the same appearance and mode of defense. Just as with other antipredator defenses, behavior plays a role in mimicry. Mimicry works better if the mimic acts like the model (the species it is mimicking) in addition to looking like it. For example, beetles that phenotypically resemble a wasp actively fly



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eye

false eyespots

false head

a.

b.

c.

Figure 34.11  Antipredator defenses.  a. The skin secretions of poison-dart frogs are so poisonous that they were used by natives to make their arrows

instant lethal weapons. The bright coloration of these frogs is called “warning coloration” because brightly colored species are generally toxic. b. The caterpillar of the eastern tiger swallowtail butterfly has false eyespots used to confuse a predator. c. The South American lantern fly has a large false head that resembles that of an alligator. This may frighten a predator into thinking it is facing a dangerous animal.

a.

b.

c.

d.

Figure 34.12  Mimicry among insects.  a. A yellow jacket wasp uses stinging as its defense. b. A flower fly and (c) a longhorn beetle are Batesian

mimics because they are incapable of stinging another animal, and yet they have the same appearance as the yellow jacket wasp. d. A bumblebee and the yellow jacket wasp are Müllerian mimics because they have a similar appearance, and both use stinging as a defense.

from place to place and spend most of their time in the same habitat as the wasp model.

Symbiosis Symbiosis refers to close interactions between members of different species. Three types of symbiotic relationships have traditionally been defined—parasitism, commensalism, and mutualism. Table 34.1 lists these three categories in terms of their benefits to one species or the other. Of the three types, only mutualism may increase the population size of both species. However, some ecologists now consider it difficult to try to classify symbiotic relationships into these categories, because the amount of harm or good the individuals of two species do to one another depends on what the investigator chooses to measure. Although the following discussion describes the traditional classification system, bear in mind that symbiotic relationships do not always fall neatly into these three categories.

Parasitism Parasitism is a symbiotic relationship in which the parasite derives nourishment from another organism, called the host.

Therefore, the parasite benefits and the host is harmed. Parasites occur in all kingdoms of life. Bacteria (e.g., strep infection), protists (e.g., malaria), fungi (e.g., athlete’s foot), plants (e.g., mistletoe), and animals (e.g., tapeworm) all include parasitic species. The effects of parasites on the health of the host can range from slightly weakening them to eventually killing them over time. In addition to providing nourishment, some host organisms also provide their parasites with a place to live and reproduce, as well as a mechanism for dispersing offspring to new hosts. Many parasites have both a primary and secondary host. The secondary host may be a vector that transmits the parasite to the next primary host. As an example, consider the deer ticks Ixodes dammini and I. ricinus in the eastern and western United States, respectively. Deer ticks are arthropods that go through a number of stages (egg, larva, nymph, adult). They are so named because adults feed and mate on white-tailed deer in the fall. The female lays her eggs on the ground, and when the eggs hatch in the spring, they become larvae that feed primarily on white-footed mice. If a mouse is infected with the bacterium Borrelia burgdorferi, the larvae become infected also. The larvae overwinter and molt the next spring to become nymphs that can, by chance, take a blood meal from a human. At this time, the tick may pass the bacterium on to a



Chapter 34  Population and Community Ecology

701

TABLE 34.1  Symbiotic Relationships Species 1

Species 2

Parasitism

Benefited

Harmed

Commensalism

Benefited

No effect

Mutualism

Benefited

Benefited

*

*Can be considered a type of predation.

human, who subsequently comes down with Lyme disease, characterized by arthritic-like symptoms and often a “bull’s-eye” rash around the site of the tick bite. The nymphs develop into adults, and the cycle begins again.

Commensalism Commensalism is a symbiotic relationship between two species in which one species is benefited and the other is neither benefited nor harmed. Often one species provides a home and/or transportation for the other species, as when barnacles attach themselves to whales. Clownfishes live within the waving mass of tentacles of sea anemones. Because most fishes avoid the venomous tentacles of the anemones, clownfishes are protected from predators. If clownfishes attract other fishes on which the anemone can feed, this relationship borders on mutualism. Other examples of commensalism may also be considered mutualistic. For example, birds called cattle egrets benefit from feeding near cattle because the cattle flush insects and other animals from the vegetation as they graze (Fig. 34.13).

Figure 34.13  Egret symbiosis.  Cattle egrets eat insects off and around various animals, such as this African cape buffalo.

Mutualism Mutualism is a symbiotic relationship in which both members of the association benefit, though not necessarily equally. An example is the relationship between plants and their animal pollinators. When herbivores, such as insects, feed on pollen they gain a meal while the plants increase their reproductive chances. Ants form mutualistic relationships with both plants and insects. In tropical America, the bullhorn acacia tree is adapted to provide a home for ants of the species Pseudomyrmex ferruginea. Unlike other acacias, this species has swollen thorns with a hollow interior, where ant larvae can grow and develop. In addition to housing the ants, the acacias provide them with food. The ants constantly protect the plant from the caterpillars of moths and butterflies by swarming and stinging them, and from other plants that might shade the plant. When the ants on experimental trees were poisoned, the trees died. Cleaning symbiosis is a relationship in which the individuals being cleaned are often vertebrates. Crustaceans, fish, and birds act as cleaners and are associated with a variety of vertebrate clients. Large fish in coral reefs line up at cleaning stations and wait their turn to be cleaned by small fish that often enter the mouths of the large fish (Fig. 34.14). It’s been suggested that cleaners may be exploiting the relationship by feeding on host tissues as well as on ectoparasites. On the other hand, cleaning could ultimately lead to net gains in client fitness by ridding them of parasites.

Figure 34.14  Cleaning symbiosis.  A cleaner wrasse, Labroides

dimidiatus, in the mouth of a spotted sweetlip, Plectorhincus chaetodonoides, is feeding off parasites. Does this association improve the health of the sweetlip, or is the sweetlip being exploited? Investigation is under way.



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UNIT 7  Behavior and Ecology

Check Your Progress  34.3 1. Describe an example of mutualism. 2. Identify examples of populations that would exhibit opportunistic growth.

3. Explain how resource partitioning can allow for the coexistence of species that have similar niches.

4. Recognize the various interactions that will exist between various species within a community.

through a series of stages, as shown in Figure 34.15. Note that, in  this case, the process began with grasses followed by shrubs, and then a mixture of shrubs and trees, until finally there were only trees. Succession can also occur in aquatic communities, as when lakes and ponds sometimes undergo stages whereby they become filled in and eventually disappear.

Models of Succession

34.4  Ecological Succession Learning Outcomes Upon completion of this section, you should be able to 1. Compare the various types of succession. 2. Choose the correct sequence of events that occur during ecological succession.

Communities are composed of all of the interacting populations in an area that are regulated by biotic interactions and ecological disturbances. Ecological succession is a change in a community’s composition that is directional and follows a continuous pattern of colonization by new species. On land, primary succession is the establishment of a plant community in a newly formed area where there is no soil formation. Primary succession typically follows a major disturbance such as a volcanic eruption or a glacial retreat. Secondary succession is the return of a community to its natural vegetation following a disturbance, as when a cultivated cornfield returns to its natural state. Pioneer species are the first species to begin the process of secondary succession of disturbed habitats. Succession progresses

Ecologists have developed various hypotheses to explain succession and predict future events. The climax-pattern model of succession says that particular areas will always lead to the same type of community, called a climax community. This model is based on the observation that climate plays a significant role in determining whether a desert, a grassland, or a particular type of forest results. Therefore, coniferous forests are expected to occur in northern latitudes, deciduous forests in temperate zones, and tropical rain forests in the tropics. The climax-pattern model of succession now recognizes that the exact composition of a community need not always be the same. That is, while we might expect to see a coniferous forest as opposed to a tropical rain forest in northern latitudes, the exact mix of plants and animals after each stage of succession can vary. The facilitation model of succession can be observed as shown in the example in Figure 34.15. It’s possible that each successive community prepares the way for the next, and this is why grassshrub-forest development occurs sequentially. On the other hand, the inhibition model says that colonists hold on to their space and inhibit the growth of other plants until the colonists die or are damaged. Still another possible model, the tolerance model, predicts that different types of plants can colonize an area at the same time. Sheer chance determines which

Figure 34.15  Secondary succession in a forest.  Secondary

succession is a process that begins in areas that already have soil (not on bare rock as primary succession does). This example of secondary succession occurred in a large conifer plantation in central New York State. Notice that certain species are common to the particular stages. However, the process of regrowth shows approximately the same series of stages as secondary succession would in a former cornfield. (Arrows indicate the passage of time.)

grasses

low shrubs

high shrubs

shrub/tree mix

low trees

high trees

climax community



Chapter 34  Population and Community Ecology

seeds arrive first, and successional stages may simply reflect the length of time it takes individuals to mature. This alone could account for the grass-shrub-forest development seen in Figure 34.15. In reality, the models we have mentioned are not mutually exclusive, and succession is recognized as a complex process.

703

Check Your Progress  34.4 1. Identify the events that would lead to succession. 2. Compare and contrast the various models used to explain succession.

Conclusion Asian carp were initially brought into the United States to help with algae control of aquaculture facilities, but soon thereafter they escaped and have spread through the ­majority of the Mississippi River system. These fish are voracious eaters of zooplankton, which is in direct competition with many of the native fish and mussels. Due to the extreme size, weight, and life span of the Asian carp, they have

the  potential to completely disrupt the Mississippi River ecosystem. Other concerns include their fouling the nets of commercial fishermen as well as the impact they may have upon the $7 billion fishing industry of the Great Lakes. Understanding population ecology is critical to determining the impact these fish may have upon the other species that live in the same community.

MEDIA STUDY TOOLS http://connect.mheducation.com Maximize your study time with McGraw-Hill SmartBook®, the first adaptive textbook.

 

Animations

34.3  Life History Patterns • Endosymbiosis

 

3D Animations

34.2  Population Ecology: Exponential Population Growth • Population Ecology: Logistic Growth 34.3  Population Ecology: Densityindependent Factors • Population Ecology: Density-dependent Factors

SUMMARIZE 34.1  The Scope of Ecology ■ Ecologists study biotic and abiotic interactions of organisms at several

levels: individual, population, community, ecosystem, and biosphere. Ecology is the study of the interactions between organisms and their physical environment.

34.2  Patterns of Population Growth ■ The rate of natural increase (r) is calculated by subtracting the num-

ber of deaths from the number of births and dividing by the number of individuals in the population. ■ Two patterns of population growth are possible. Exponential growth results in a J-shaped curve because, as the population increases in size, so does the number of new members.

  Tutorials 34.2  Patterns of Population Growth

■ Under ideal circumstances, exponential growth occurs due to a popula-

tion’s biotic potential. However, exponential growth cannot continue indefinitely because environmental resistance opposes biotic potential, and logistic population growth occurs. ■ Under logistic growth, an S-shaped growth curve results when the population stops growing near the carrying capacity. ■ Populations tend to have one of three types of survivorship curves, depending on whether most individuals live out the normal life span (type I), die at a constant rate regardless of age (type II), or die early (type III). To best follow population growth a cohort, or group of individuals born at the same time, is tracked throughout their life. ■ Human population can be appreciated by looking at the doubling time, or the length of time it takes for the population size to double. Zero population growth can be achieved when the birthrate equals the death rate.



UNIT 7  Behavior and Ecology

■ More-developed countries (MDCs), such as the United States and

most of western Europe, are those in which the population growth is low and people have a good standard of living. A demographic transition occurs when a decreased death rate is followed by a decreased birthrate, causing the population growth of countries to slow. ■ The human population is expanding exponentially, mostly within lessdeveloped countries (LDCs) in Africa, Asia, and Latin America. Support for family planning, human development, and delayed childbearing could help lessen the expected increase. ■ Age-structure diagrams help divide the population into three age groups: prereproductive, reproductive, and postreproductive. Replacement reproduction occurs when each couple has two children, but it does not lead to zero population growth.

34.3  Interactions Between Populations ■ Opportunistic species rapidly produce many young, generally lack

■

■

■ ■

■

■

parental care, and rely on rapid dispersal to new, unoccupied environments. Their population size is regulated by density-independent factors. Equilibrium species produce few young, which often require parental care, and their population size is typically regulated by density-­ dependent factors, such as competition and predation. The competitive exclusion principle states that no two species can occupy the same ecological niche at the same time when resources are limiting. With limited resources, either resource partitioning occurs or one species goes locally extinct. Each species will require a specific habitat in order to meet its basic needs. Coevolution occurs when two species adapt in response to selective pressure imposed by the other. Predator-prey interactions can cause prey populations to decline and  remain at relatively low densities, cause decline of predator populations, or cycling of both predator and prey population ­ densities. Prey defenses take many forms, such as concealment, use of fright, and warning coloration. Batesian mimicry occurs when one species has the warning coloration but lacks the defense of another species. ­Müllerian mimicry occurs when two species with the same warning coloration have a similar defense. In parasitism, the parasite benefits and the host is harmed. Parasites often utilize more than one host. In commensalism, neither party is harmed, and one species often provides a home and/or transportation for another species. In mutualism, both partners benefit. Examples of mutualistic relationships include flowers and their pollinators, ants and acacia trees, and cleaning symbiosis.

34.4  Ecological Succession ■ A change in community composition over time is called ecological

succession.

ASSESS Testing Yourself Choose the best answer for each question.

34.1  The Scope of Ecology 1. Place the following levels of organization in order, from lowest to highest. a. population, organism, community, ecosystem b. community, ecosystem, population, organism c. organism, community, population, ecosystem d. population, ecosystem, organism, community e. organism, population, community, ecosystem 2. Which of these levels of ecological study involves an interaction between a community of organisms and their environment? a. organisms b. populations c. communities d. ecosystem e. All of these are correct. 

34.2  Patterns of Population Growth 3. Assume that a deer population contains 500 animals. The birthrate is 105 animals per year, and the death rate is 100 animals per year. The rate of natural increase per year is a. 5%. b. 4%. c. 3%. d. 2%. e. 1%. 4. What type of survivorship curve would you expect for a plant species in which most seedlings die? a. type I b. type II c. type III 5. Label this logistic growth curve:

c.

Number of Organisms

704

b.

d. e.

f. a. Time

■ The climax-pattern model of succession says that particular areas will

always lead to the same type of community.

■ The facilitation model implies that some community members prepare

the landscape for new species during succession. The inhibition model suggests that some species prevent colonization, and the tolerance model suggests that species colonize simultaneously. ■ A climax community is stable and associated with a particular geographic area.

6. If an age-structure diagram indicates that there are more people in the prereproductive stage of life than in the reproductive stage, then replacement reproduction, over the long run, will result in a. zero population growth. b. an increase in population size. c. a decrease in population size.



Chapter 34  Population and Community Ecology

34.3  Interactions Between Populations

ENGAGE

7. Which of the following defines the role of a species in a community, including its habitat and interaction with other species? a. ecological niche d. mimicry b. competitive exclusion e. None of these are correct. c. competition level 8. A colorful, nonpoisonous frog that mimics a poison-dart frog is exhibiting a. Batesian mimicry. b. Müllerian mimicry. For problems 9–11, indicate the type of symbiosis illustrated by each example. Some answers may be used more than once.

Thinking Critically

Key: a. parasitism b. commensalism c. mutualism 9. Single-celled algae live in the tissues of coral animals. The algae provide food for the coral, while the coral provides a stable home for the algae. 10. Flowering plants reward visiting insects with nectar. The insects, in turn, carry pollen to other flowers. 11. Small wasps lay eggs on other insects. The eggs hatch into larvae that feed on the insects and kill them.

705

1. You are a farmer fighting off a crop insect pest. You know from evolutionary biology and natural selection that using the same pesticide causes resistance to evolve in the insect population. From what you have learned about population growth and regulation, what are some strategies you might use to control the insect population? 2. You find two species of insects, both using bright coloration and similar color patterns as an antipredator defense. Design an experiment to tell whether this is an example of Müllerian or Batesian mimicry. 3. Upland game hunters in Illinois have been noticing a decrease in the rabbit and pheasant populations over the past five years. They have lobbied the Department of Natural Resources and the state legislature to allow them to shoot red-tailed hawks, one of the rabbit’s and pheasant’s main predators. Explain why shooting hawks will not ultimately solve the problem of a decreased rabbit and pheasant population  4. If predators tend to reduce the population densities of prey, what prevents predators from reducing prey populations to such low levels that they drive themselves extinct?

34.4  Ecological Succession 12. Mosses growing on bare rock will eventually help to create soil. These mosses are involved in _____ succession. a. primary c. tertiary b. secondary 13. Assume that a farm field is allowed to return to its natural state. By chance, the field is first colonized by native grasses, which begin the succession process. This is an example of which model of succession? a. climax pattern c. facilitation b. tolerance d. inhibition

PHOTO CREDITS Opener: © AP Photo/Illinois River Biological Station via the Detroit Free Press, Nerissa Michaels; 34.1: © David Hall/Science Source; 34.2a: © Daniel Heuclin/NHPA/Photoshot; 34.2b: © Tracey Thompson/Corbis RF; 34.5b: © Image State/Alamy; 34.5c: © Michael Coyne/Getty Images; 34.7(dandelions): © Elena Elisseeva/Alamy RF; 34.7(bears): © Winfried Wisniewski/The Image Bank/Getty Images; 34.10: © Alan Carey/Science Source; 34Aa: © Daniel Dempsster Photography/Alamy; 34Ab: © Rolf Nussbaumer/Alamy; 34.11a: © MedioImages/SuperStock RF; 34.11b: © Scott Camazine/Science Source; 34.11c: © National Audubon Society/A. Cosmos Blank/Science Source; 34.12a: © Rami Aapasuo/ Alamy; 34.12b: © Pauline S. Mills/Getty RF; 34.12c: © blickwinkel/Alamy; 34.12d: © James H. Robinson/Science Source; 34.13: © James Hager/Getty Images; 34.14: © Bill Wood/ Photoshot.



35

Nature of Ecosystems CHAPTER OUTLINE 35.1 The Biotic Components of Ecosystems 35.2 Energy Flow 35.3 Global Biogeochemical Cycles BEFORE YOU BEGIN Before beginning this chapter, take a few moments to review the following discussions: Section 2.1  Which elements are basic to all life? Section 2.3  How do the properties of water influence the biogeochemical cycles? Section 34.3  What role does the predator-prey cycle play in the energy flow of an ecosystem?

CASE STUDY The Wolves of Yellowstone By 1926, the last known wolf pack had been eliminated from Yellowstone National Park. The extinction of the wolves produced a multitude of ecological effects that would ripple through the entire park for the next 70 years. Fewer wolves meant less predation upon the elk and other large herbivores. An increase in herbivores meant more pressure upon their food sources. Cottonwoods, willows, and streamside vegetation soon became depleted due to the increasing elk population.  Overbrowsing of the streamside vegetation produced a ripple effect throughout the stream ecosystem. This decreased the stability of the stream bank, which in turn caused increased erosion and sediment load in the streams. Fish, insects, birds, and all the wildlife associated with the streams were negatively impacted.  The reintroduction of wolves into Yellowstone began in 1995. Since then, biologists have seen signs of the stream bank stabilizing due to the decreased browsing by elk and other herbivores. This decrease in browsing is partly due to the wolves’ preying upon the elk and other large herbivores in the park. Many of the problems that were caused by the eradication of wolves from Yellowstone have begun to be repaired as a direct result of their reintroduction. This chapter examines various types of interactions among the populations of a community. It also looks at the interactions with the nonliving component of an ecosystem.  In this chapter, you will learn about how energy flows through the members of biotic communities and how abiotic chemicals and nutrients cycle through ecosystems.  As you read through the chapter, think about the following questions:

1. How do trophic levels shape the nature of an ecosystem? 2. How does energy move from producers to consumers in an ecosystem?

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The populations within an ecosystem are categorized according to their food source. Autotrophs (i.e., “self-feeders”) require only inorganic nutrients and an outside energy source to produce organic nutrients for their own use as food. Because they produce food for themselves and other members of the community, they are called producers (Fig. 35.1a). Photosynthetic organisms such as

algae and plants produce most of the organic nutrients for the biosphere. Some bacteria are chemoautotrophs. They reduce carbon dioxide to an organic compound by using energetic electrons derived from inorganic compounds, such as hydrogen sulfides, hydrogen gas, or ammonium ions. Chemoautotrophs have been found to support communities at hydrothermal vents along deepsea oceanic ridges. Heterotrophs (i.e., “other feeders”) need an outside source of organic nutrients. Because they consume food, they are called consumers. Herbivores are animals that feed directly on plants or algae (Fig. 35.1b). Carnivores feed on other animals. Snakes that feed on rabbits are carnivores, and so are the hawks that feed on these snakes (Fig. 35.1c). In this example, herbivorous rabbits are primary consumers, snakes are secondary consumers, and the hawks are tertiary consumers. Sometimes tertiary consumers are called top predators. Omnivores are animals (including humans) that feed on both plants and animals. Decomposers are heterotrophic bacteria, some species of protists and fungi, such as molds and mushrooms, that break down nonliving organic matter (Fig. 35.1d). They perform a very valuable service because they release inorganic nutrients that are taken up by plants back into the environment. Detritus is partially decomposed matter in the water or soil. Detritivores are found in all ecosystems. Earthworms and some beetles, termites, and maggots are soil detritivores.

a. Producers

c. Carnivores

b. Herbivores

d. Decomposers

35.1  The Biotic Components of Ecosystems Learning Outcomes Upon completion of this section, you should be able to 1. Identify the ways that autotrophs and heterotrophs obtain nutrients. 2. Describe the energy flow and biogeochemical cycling within and among ecosystems.

An ecosystem possesses both abiotic and biotic components. The abiotic components include the nonliving aspects, such as sunlight, inorganic nutrients, and water availability; and conditions, such as type of soil, average temperature, and wind speed. The biotic components of an ecosystem are the various populations of species that form the community.

Populations Within an Ecosystem

Figure 35.1  Biotic components.  a. Diatoms, a type of algae, and green plants are producers. b. Caterpillars and rabbits are herbivores. c. Snakes and hawks are carnivores. d. Bacteria and mushrooms are decomposers.



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UNIT 7  Behavior and Ecology

Energy Flow and Chemical Cycling

at io n

Heat to environment

lu cel

la

r pi es r r

growth and reproduction de at

h

def ec

Energy to carnivores

on

exc re ti ion at

Every ecosystem is characterized by two fundamental phenomena: energy flow and chemical cycling. Energy flow begins when producers absorb solar energy, and chemical cycling begins when producers take in inorganic nutrients from the physical environment (Fig. 35.2). Thereafter, producers make organic nutrients (food) directly for themselves and indirectly for the other populations of the ecosystem. Most ecosystems cannot exist without a continual supply of solar energy. Chemicals cycle when inorganic nutrients are returned to the producers from the atmosphere or soil. Only a portion of the organic nutrients made by autotrophs is passed on to heterotrophs because plants use some of the organic molecules to fuel their own cellular respiration. Similarly, only a small percentage of nutrients taken in by heterotrophs is available to higher-level consumers. Figure 35.3 shows why. A certain amount of the food eaten by a herbivore is eliminated as feces that are recycled by decomposers. Metabolic wastes are excreted as urine. Of the assimilated energy, a large portion is utilized during cellular respiration to provide energy to the organism and thereafter becomes heat. Only the remaining food, which is converted into increased body weight (or additional offspring), becomes available to carnivores. In Chapter 6, you learned that energy flow is described by the laws of thermodynamics: (1) energy cannot be created or destroyed; and (2) in every energy transformation, some energy is lost as heat. These principles explain why ecosystems are dependent on a continual outside source of energy, such as sunlight, and why only a part of the original energy from producers is available to consumers.

Energy to detritus feeders

Figure 35.3  Energy balances.  Only about 10% of the food energy taken in by a herbivore is passed on to carnivores and much of the rest is lost as heat to the environment. A large portion goes to detritivores via defecation, excretion, and death, and another large portion is used for cellular respiration. Check Your Progress  35.1 1. Identify the nutritional differences between producers, consumers, and decomposers.

solar energy

2. Explain why most energy fails to be converted to a usable

heat

form when one organism eats another.

35.2  Energy Flow

producers

Learning Outcomes consumers

inorganic nutrient pool

heat

heat

decomposers

energy nutrients

Figure 35.2  Energy flow and nutrient cycling.  Nutrients cycle,

but energy flows through an ecosystem. As energy transformations repeatedly occur, all the energy derived from the sun eventually dissipates as heat.

Upon completion of this section, you should be able to 1. Recognize the difference between a food chain and a food web. 2. Explain the energy flow among populations through food webs and ecological pyramids.

The interconnecting paths of energy flow within ecosystems are represented by diagrams called food webs. Figure 35.4a is a ­grazing food web. Grazing food webs do not begin with grazers; instead they begin with producers like the trees in a forest. Caterpillars feed on leaves in the trees; and mice, rabbits, and deer feed on leaves at or near the ground. Birds, chipmunks, and mice eat fruits and nuts, but are omnivores because they also feed on caterpillars. These herbivores and omnivores all provide food for a number of different carnivores. Figure 35.4b is a detrital food web, which begins with detritus. Detritus is food for soil organisms such as earthworms and beetles. These animals are, in turn, fed on by salamanders and



Chapter 35  Nature of Ecosystems

Autotrophs

Herbivores/Omnivores

709

Carnivores owls

nuts birds

hawks

leaf-eating insects

deer foxes

leaves

rabbits

chipmunks skunks

snakes

detritus mice mice a.

death death

fungi and bacteria in detritus

death

invertebrates

carnivorous invertebrates

salamanders

shrews

b.

Figure 35.4  Grazing and detrital food webs.  Food webs are descriptions of who eats whom. a. Tan arrows illustrate possible grazing food webs. For example, birds, which feed on nuts, may be eaten by a hawk. Autotrophs such as the tree are producers (first trophic level); the first series of animals are primary consumers (second trophic level); and the next group of animals are secondary consumers (third trophic level). b. Green arrows illustrate possible detrital food webs, which begin with detritus, the waste products and remains of dead organisms. A large portion of these remains are from the grazing food web illustrated in (a). The organisms in the detrital food web are sometimes consumed by animals in the grazing food web, as when robins feed on earthworms. Thus, the grazing food web and the detrital food web are connected.

shrews. Because the members of the detrital food web may become food for aboveground carnivores, the detrital and grazing food webs are joined. In this particular forest, the organic matter lying on the forest floor and mixed into the soil contains over twice as much energy as the leaves of living trees. Therefore, more energy in a forest may be funneling through the detrital food web than through the grazing food web.

Trophic Levels You can see that Figure 35.4a would allow us to link organisms one to another in a straight line, according to who eats whom. ­Diagrams that show a single path of energy flow such as this are called food chains. For example, in the grazing food web, we could find this grazing food chain: leaves ⟶ caterpillars ⟶ sparrows ⟶ hawks



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UNIT 7  Behavior and Ecology

And in the detrital food web (Fig. 35.4b), we could find this detrital food chain: detritus ⟶ earthworms ⟶ shrews A trophic level is composed of all the organisms that feed at a particular link in a food chain. In the grazing food web in Figure 35.4a, going from left to right, the trees are primary producers (first trophic level). The first series of animals, the herbivores, are primary consumers (second trophic level). The group of animals at the far right, the carnivores, are secondary consumers (third trophic level).

number of organisms can be useful, because in terms of only numbers, one oak tree may have hundreds of caterpillars. Although it appears that there are more herbivores than autotrophs, the biomass of autotrophs is much greater. Similarly, the biomass of herbivores is greater than that of carnivores. However, in aquatic ecosystems, such as lakes and open seas where algae are the main producers, the herbivores may have a greater biomass than the producers. The reason is that, over time, the algae reproduce rapidly, but they are also consumed at a high rate. Any pyramids like this one, which have more herbivores than producers, are called inverted pyramids:

Ecological Pyramids The shortness of food chains can be attributed to the loss of energy between trophic levels. In general, only about 10% of the energy of one trophic level is available to the next trophic level. Therefore, if a herbivore population consumes 1,000 kg of plant material, only about 100 kg is converted to herbivore tissue, 10 kg to first-level carnivores, and 1 kg to second-level carnivores. This 10% rule explains why few carnivores can be supported in a food web. The large energy losses that occur between successive trophic levels are sometimes depicted as an ecological pyramid (Fig. 35.5). Biomass is the number of organisms multiplied by their weight. Thinking in terms of biomass rather than simply the

herbivores producers (algae)

relative dry weight

Ecologists are hesitant to use pyramids to describe ecological relationships for all situations. Detrital food chains are rarely included in pyramids, but they may represent a large portion of energy in many ecosystems, making conclusions based on aboveground pyramids potentially inaccurate.

Check Your Progress  35.2 1. Explain why ecosystems generally support few carnivores. 2. Explain how biomass relates to the structure of ecological pyramids.

top carnivores

35.3  Global Biogeochemical Cycles Learning Outcomes

carnivores

herbivores

producers

Figure 35.5  Ecological pyramid.  An ecological pyramid reflects the loss of energy from one trophic level to the next. Energy is lost as herbivores feed on producers, carnivores feed on herbivores, and top carnivores feed on carnivores. This loss of energy results in decreasing biomass and numbers of organisms at higher trophic levels relative to lower trophic levels (levels not to scale).

Upon completion of this section, you should be able to 1. Define what is meant by a biogeochemical cycle. 2. Identify the steps of the water cycle, the phosphorus cycle, the nitrogen cycle, and the carbon cycle. 3. Identify how human activities can alter each of the biogeochemical cycles.

Because the pathways by which chemicals circulate through ecosystems involve both biotic and geological components, they are known as biogeochemical cycles. For each element, chemical cycling may involve (1) a reservoir—a source normally unavailable to producers, such as fossilized remains, rocks, and deep-sea sediments; (2) an exchange pool—a source from which organisms generally take chemicals, such as the atmosphere or soil; and (3) the biotic community—through which chemicals move along food chains, perhaps never entering an exchange pool (Fig. 35.6). With the exception of water, which exists in gas, liquid, and solid forms (and sometimes as plasma), there are two types of biogeochemical cycles. In a gaseous cycle, exemplified by the carbon and nitrogen cycles, the element is withdrawn from and returns to the atmosphere as a gas. In a sedimentary cycle, exemplified by the phosphorus cycle, the element is absorbed from soil by plant roots,



Chapter 35  Nature of Ecosystems

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Figure 35.6  Model for chemical cycling.  Chemical nutrients cycle between these components of ecosystems. Reservoirs, which include fossil fuels, minerals in rocks, and sediments in oceans, are normally relatively unavailable sources. However, exchange pools, such as those in the atmosphere, soil, and water, are available sources of chemicals for the biotic community. When human activities (purple arrows) remove chemicals from reservoirs and pools and make them available to the biotic community, pollution can result.

human activities

Reservoir • fossil fuels • mineral in rocks • sediment in oceans

Exchange Pool • atmosphere • soil • water Community

passed to heterotrophs, and eventually returned to the soil by decomposers. The diagrams in the next few sections show how nutrients flow between terrestrial and aquatic ecosystems. In the nitrogen and phosphorus cycles, these nutrients run off from a terrestrial to an aquatic ecosystem and in that way enrich the aquatic ecosystem. However, too much of these nutrients can lead to excessive growth of algae. Decaying organic matter in aquatic ecosystems can be a source of nutrients for intertidal inhabitants such as fiddler crabs. Seabirds feed on fish but deposit guano (droppings) on land, and in that way phosphorus from the water is deposited on land. Anything put into the environment in one ecosystem can find its way to another ecosystem. As evidence of this, scientists find the soot from urban areas and pesticides from agricultural fields in the snow and animals of the Arctic.

umers

po decom

se rs

ducers

on s

pro

c

The Water Cycle The water (hydrologic) cycle is described in Figure 35.7. During the water cycle, fresh water is distilled from salt water. First, evaporation occurs. During evaporation, a liquid (in this case, water) changes from a liquid state to a gaseous state. The sun’s rays cause fresh water to evaporate from seawater, and the salts are left behind. Next, condensation occurs. During condensation, a gas is changed to a liquid. The amount of water evaporating from the oceans exceeds the amount of precipitation that falls back into the ocean; often this excess moves over and falls on land. Water also evaporates from land, from plants (transpiration), and from bodies of fresh water. Because land lies above sea level, gravity eventually returns all fresh water to the sea. In the meantime, water is contained within standing waters (lakes and ponds), flowing water (streams and rivers), and groundwater.



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UNIT 7  Behavior and Ecology

SCIENCE IN YOUR LIFE  ►

ECOLOGY

The California Drought Water is quickly becoming the “blue gold” of the American southwest. Every day, new groundwater wells are being drilled, some to depths of over 1,000 feet and costing over $300,000 each, to

access an ever-decreasing supply of water. Nowhere is this more apparent than California, which is experiencing one of the worst multiple year droughts in its history (Fig. 35Aa). There are two sources of water for ­California—surface water in rivers and streams U.S. Drought Monitor and groundwater pumped from aquifers. The California majority of California’s water (over 80%) comes from surface water Intensity: sources, which originates as D0 Abnormally Dry spring snow melts in the D1 Moderate Drought inland mountain ranges. D2 Severe Drought However, for the past D3 Extreme Drought several years, snowfall D4 Exceptional Drought amounts have been considerably below average for the mountain ranges nearest to California. The aquifers under California do not have the reserves to compensate for long periods of drought. Furthermore, the use of water from aquifers may only provide a temporary solution. This is because, unlike ­ groundwater, aquifers take a long time to recharge. While some aquifers lie close to the surface, and may be partially regenerated by groundwater over time, the aquifers in a. California are deep underground, and may take thousands, or even millions of years, to ­ replenish. What worries scientists more are computer models which suggest that the worse is yet to come. While droughts, which are characterized as a prolonged period of low rainfall, have occurred in 11 of the past 14 years in the southwest, the worst b. is probably yet to come. When predictions of Figure 35A  The water crisis in California.  a. The extent of the current levels of clithe drought. b. A desalinization plant in southern California. mate change are facThe U.S. Drought Monitor is jointly produced by the National Drought Mitigation Center at tored into the models, the University of Nebraska-Lincoln, the United States Department of Agriculture, and the the data suggest that National Oceanic and Atmospheric Administration. Map courtesy of NDMC-UNL.

there is an 80% chance that California and the remainder of the southwest may experience a “megadrought” (an event lasting more than 35 years) before 2080. Scientists believe that an event such as this may have led to the decline of the Pueblo people over 1,400 years ago.

Solutions for the Future There are only two solutions—decrease water demand or increase the water supply. The population of California has increased by almost 20  million people in the past 40 years, and is predicted to continue to grow in the future. Mandatory water restrictions often help, but these measures often specifically target residential use. As is the case in many parts of the country, most of the water is used by agriculture and industry. In California, the major user is agriculture. California grows the majority of the United State’s fruits and vegetables, and thus any disruption has a ripple effect in the food supply. Increasing the water supply is also difficult. Aquaducts, canals, and pipelines are expensive, and take a considerable time to build. Desalinization plants (Fig. 35Ab), which remove the salt from ocean water, require a significant amount of energy, and often only supply water for a small percent of the population. For example, the new desalinization plant in San Diego, the largest in the world, will only supply around 10% of the local population with water. So what then is the solution? Most would agree that it is a complex problem, and that a single solution will not adequately address the entire problem. New technologies in the areas of water desalinization may help, as will more research in how reservoirs are recharged in the water cycle. However, most scientists agree that the lessons learned from the California crisis may help us better address some of the global challenges that we will face as our climate changes.

Questions to Consider 1. Suppose that you were in charge of managing California’s water supply. What changes would you make and how would you allocate water resources? 2. How might both industry and agriculture be encouraged to change their procedures to reduce their freshwater footprint?



Chapter 35  Nature of Ecosystems

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Figure 35.7  The water cycle.  Evaporation from the ocean exceeds precipitation, so there is a net movement of water vapor onto land, where precipitation results in the eventual flow of surface water and groundwater back to the sea. On land, transpiration by plants contributes to evaporation.

transpiration from plants and evaporation from soil H2O in Atmosphere

precipitation over land

net transport of water vapor by wind

lake

evaporation from ocean

precipitation to ocean

freshwater runoff

Ocean

aquifer

Ice

Groundwaters

Some of the water from precipitation (e.g., rain, snow, sleet, hail, and fog) sinks, or percolates, into the ground and saturates the earth to a certain level. The top of the saturation zone is called the groundwater table, or simply, the water table. Because water infiltrates through the soil and rock layers, sometimes groundwater is also located in aquifers, rock layers that contain water and release it in appreciable quantities to wells or springs. Aquifers are recharged when rainfall and melted snow percolate into the soil.

Human Activities In some parts of the United States, especially the arid West and southern Florida, withdrawals from aquifers exceed any possibility of recharge. This is called “groundwater mining.” In these locations, the groundwater level is dropping, and residents may run out of groundwater, at least for irrigation purposes, within a few years.  Fresh water, which makes up only about 3% of the world’s supply of water, is called a renewable resource because a new supply is always being produced as a result of the water cycle. But it is

possible to run out of fresh water when the available supply is not adequate or has become polluted so that it is not usable. As the Ecology feature, “The California Drought,” explains, the consequences of these droughts may extend well past the regional level.

The Phosphorus Cycle In the phosphorus cycle, phosphorus moves from rocks on land to the oceans, where it gets trapped in sediments. Then phosphorus moves back onto land following a geological upheaval. You can verify this by following the appropriate arrows in Figure 35.8. However, on land, the very slow weathering of rocks makes phosphate ions (PO43– and HPO42–) available to plants, which take up phosphate from the soil. Producers use phosphate to form a variety of molecules, including phospholipids, ATP, and the nucleotides that become a part of DNA and RNA. Animals incorporate some of the phosphate into teeth, bones, and shells that take many years to decompose. Eventually, however, phosphate ions become available to



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UNIT 7  Behavior and Ecology

producers once again. Because the available amount of phosphate is already being utilized within food chains, phosphate is usually a limiting inorganic nutrient for plants—that is, their growth is limited by the amount of available phosphorus. Some phosphate runs off into aquatic ecosystems, where algae acquire phosphate before the excess becomes trapped in sediments. Phosphate in marine sediments does not become available to producers on land again until a geological upheaval exposes sedimentary rocks to weathering once more. Phosphorus does not enter the atmosphere, therefore, the phosphorus cycle is called a sedimentary cycle.

Human Activities Humans boost the amount of available phosphate by mining phosphate ores and using them to make fertilizers, animal feed

supplements, and detergents. Fertilizers usually contain three basic ingredients: nitrogen, phosphorus, and potassium. Some laundry detergents still contain approximately 35–75% sodium triphosphate, but many companies are phasing out the phosphates altogether. The amount of phosphate in animal feed varies. Animal wastes from livestock feedlots, fertilizers from lawns and cropland, and untreated and treated sewage discharged from cities all add excess phosphate to nearby waters. The end result is cultural eutrophication (over-enrichment), which can lead to an algal bloom, as indicated by green scum floating on the water or excessive mats of filamentous algae. When the algae die off, decomposers use up all available oxygen for cellular respiration. The result can be a massive fish kill.

The Nitrogen Cycle Even though nitrogen gas (N2) makes up about 78% of the atmosphere, it is unavailable for plants to use. Therefore, nitrogen is also a limiting inorganic nutrient for plants.

Nitrogen Fixation In the nitrogen cycle, nitrogen fixation occurs when nitrogen gas (N2) is converted to ammonium ions (NH4+), a form plants can use (Fig. 35.9). Some cyanobacteria in aquatic ecosystems and some free-living bacteria in soil are able to fix atmospheric nitrogen in this way. Other nitrogen-fixing bacteria live in

mineable rock

weathering

phosphate mining

geological uplift sewage treatment plants

fertilizer plants runoff

animals and animal wastes

Biotic Community

phosphate in solution

phosphate in soil biota

decomposers detritus

sedimentation

Figure 35.8  The phosphorus cycle. 

Purple arrows represent human activities; gray arrows represent natural events.



Chapter 35  Nature of Ecosystems

nodules on the roots of legumes (see Chapter 9). They make organic compounds containing nitrogen available to the host plants so that the plant can form proteins and nucleic acids.

Nitrification Plants can also use nitrates as a source of nitrogen. The production of nitrates during the nitrogen cycle is called nitrification. Nitrification can occur in two ways: (1) Nitrogen gas is converted to nitrate (NO3–) in the atmosphere when cosmic radiation, meteor trails, and lightning provide the high energy needed for nitrogen to react with oxygen. (2) Ammonium ions in the soil from decomposition are converted to nitrate by soil bacteria in a two-step process.

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First, nitrite-producing bacteria convert ammonium to nitrite (NO2–), and then nitrate-producing bacteria convert nitrite to nitrate. These two groups of bacteria, called the nitrifying bacteria, are chemoautotrophs.

Denitrification Denitrification is the conversion of nitrate back to nitrogen gas, which will enter the atmosphere. Denitrifying bacteria living in the anaerobic mud of lakes, bogs, and estuaries carry out this process as a part of their own metabolism. In the nitrogen cycle, denitrification would counterbalance nitrogen fixation except for human activities.

N2 (nitrogen gas) in Atmosphere

N2 fixation

N2 fixation

nitrogen-fixing bacteria in nodules and soil

denitrification

runoff human activities

plants nitrification dead organisms and animal waste

denitrifying bacteria

decomposers

NO3– cyanobacteria

NH4+

Biotic Community

Biotic Community

(ammonium)

denitrification

NH4+

phytoplankton

nitrifying bacteria

NO2– (nitrite)

NO3– (nitrate)

decomposers

denitrifying bacteria

sedimentation

Figure 35.9  The nitrogen cycle.  Purple arrows represent human activities; gray arrows represent natural events.



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SCIENCE IN YOUR LIFE  ►

ECOLOGY

Photochemical Smog Smog is well known to most people, especially those who live in large cities, as a type of air pollution that is a mixture of smoke and fog. While breathing any type of air pollution can be damaging to the respiratory system, a particular type of smog that may be even more harmful is called photochemical smog. Photochemical smog arises when primary pollutants react with one another under the influence of sunlight to form a more deadly combination of chemicals. For example, two primary pollutants, nitrogen oxides (NOx) and volatile organic compounds (VOCs), including hydrocarbons from fossil fuel use, react with one another in the presence of sunlight to produce nitrogen dioxide (NO2), ozone (O3), and PAN (peroxyacetylnitrate). Exposure to these chemicals can result in respiratory distress, eye irritation, headache, and certain types of cancer. Large, industrialized cities with warm, sunny climates—such as Los Angeles; Sydney, Australia; Mexico City; and Buenos Aires, Argentina—are particularly susceptible to photochemical smog. If the city is surrounded by hills, a thermal inversion may aggravate the situation. Normally, warm air near the ground rises, so that pollutants are dispersed and carried away

by air currents. But sometimes during a thermal inversion, smog gets trapped near the Earth by a blanket of warm air (Fig. 35B). This may occur when a cold front brings in cold air, which settles beneath a warm layer. The trapped pollutants cannot disperse, and the results are dangerous to a person’s respiratory health. Even healthy adults experience a reduction in lung capacity when exposed to photochemical smog for long periods or during vigorous outdoor activities. Repeated exposures to high concentrations of ozone are associated with respiratory problems, such as an increased rate of lung infections and permanent lung damage. Children, the elderly, asthmatics, and individuals with emphysema or other similar disorders are particularly at risk. Even though federal legislation is in place to bring air pollution under control, more than half the people in the United States live in cities polluted by too much smog. In the long run, pollution prevention is usually easier and cheaper than pollution cleanup. Some prevention suggestions are as follows: ∙ Encourage use of public transportation and burn fuels that do not produce pollutants.

∙ Increase recycling in order to reduce the amount of waste that is incinerated. ∙ Reduce energy use so that power plants need to provide less. ∙ Use renewable energy sources, such as solar, wind, or water power. ∙ Require industries to meet clean-air standards.

Questions to Consider 1. One of the most significant sources of the pollutants that contribute to photochemical smog is exhaust from automobiles. Hydrogen cell cars run on hydrogen and electricity and emit only water.  Do you think that the government should provide strong incentives, such as tax breaks, for people who pay extra for these cars? 2. Why do you think exposure to ozone causes more serious respiratory problems in the young and the elderly than in healthy adults? 3. How often do you use public transportation? Whether you live in a big city or not, would you be willing to use public transportation if it added, for example, an hour to the total time you spend each day traveling to school or work? Why or why not?

cooler air sunlight ssun su ligh htt h

cool air

+

+ from cars cars r and rs and an factories orries es es

nitrogen n ittrrog itr og o gen n oxides ox o xid xxi id des de ess ((NO e N X)

warm air

vo vol vvolati olati ol atile at e org o or rg gan an niic ic volatile organic com cco omp po pou ound ou nds dss d compounds ((VOCs) VO VOC V OC O Cs)

=

ozone oz o z ((O O3)

b. Normal pattern cool air warm inversion layer cool air

a. Ground-level ozone formation

c. Thermal inversion

Figure 35B  Thermal inversion.  a. The chemical reactions that produce phytochemical smog. b. Normally, pollutants escape into the

atmosphere when warm air rises. c. During a thermal inversion, a layer of warm air (warm inversion layer) overlies and traps pollutants in cool air below.



Chapter 35  Nature of Ecosystems

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Human Activities

The Carbon Cycle

Human activities nearly double the nitrogen fixation rate when fertilizers are produced from N2. Fertilizer, which also contains phosphate, runs off into lakes and rivers and results in algal overgrowth and fish kill. Fertilizer use also results in the release of nitrous oxide (N2O), a greenhouse gas, component of acid rain, and contributor to ozone shield depletion.

In the carbon cycle, plants in both terrestrial and aquatic ecosystems take up carbon dioxide (CO2) from the air through photosynthesis. They incorporate carbon into food that is used by autotrophs and heterotrophs alike (Fig. 35.10). When all organisms, including plants, respire, a portion of this carbon is returned to the atmosphere as carbon dioxide. In aquatic ecosystems, the exchange of carbon dioxide with the atmosphere is primarily indirect. However, there is a small quantity of free carbon dioxide in the water. Carbon dioxide from the air combines with water to produce bicarbonate ions (HCO3–), a source of carbon for algae that make up the base of most aquatic ecosystems. Similarly, when aquatic organisms respire, the carbon dioxide they give off becomes bicarbonate ion. The amount of bicarbonate in the water is in equilibrium with the amount of carbon dioxide in the air. Thus far, the ocean has acted as a buffer, absorbing much of the excess CO2, but scientists are concerned about how much the ocean can actually hold.

combustion

CO2 in Atmosphere

photosynthesis

destruction of vegetation

respiration decay

Land plants

diffusion Ocean runoff

bicarbonate (HCO3– )

Soils sedimentation coal natural gas

oil

dead organisms and animal waste

Figure 35.10  The carbon cycle.  Carbon cycles between

atmospheric and geological reservoirs. Purple arrows represent human activities; gray arrows represent natural events.

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UNIT 7  Behavior and Ecology

Reservoirs Hold Carbon Living and dead organisms contain organic carbon and serve as one of the reservoirs for the carbon cycle. The world’s biotic components, particularly trees, contain 800 billion tons of organic carbon, and an additional 1,000–3,000 billion metric tons are estimated to be held in the remains of plants and animals in the soil. Before decomposition can occur, some of these remains are subjected to physical processes that transform them into coal, oil, and natural gas. We call these materials the fossil fuels. Most of the fossil fuels were formed during the Carboniferous period, 286 to 360 MYA, when an exceptionally large amount of organic matter was buried before decomposing. Another reservoir is the inorganic carbonate that accumulates in limestone and in calcium carbonate shells of many marine organisms.

Human Activities The transfer rates of carbon dioxide due to photosynthesis and cellular respiration are just about even. However, due to human activities, more carbon dioxide is being released into the atmosphere than is being removed. In 1850, atmospheric CO2 was at about 280 parts per million (ppm); today, it is over 400 ppm. This increase is largely due to the burning of fossil fuels and the destruction of forests to make way for farmland and pasture. Today, the amount of carbon dioxide released into the atmosphere is about twice the amount that remains in the atmosphere. It’s believed that most of this dissolves in the ocean. The increased amount of carbon dioxide (and other gases) in the atmosphere is causing a rise in temperature called global warming. These gases allow the sun’s rays to pass through, but they absorb and radiate heat back to Earth, a phenomenon called the greenhouse effect. Global warming is contributing to climate change, which is causing significant changes to the Earth, as discussed in Chapter 36. The Ecology feature, “Climate Change and

Carbon Emissions,” discusses worldwide efforts to cut back on greenhouse gases. Carbon dioxide is not the only greenhouse gas,  these gases also play a role in the greenhouse effect: ■■

■■

■■

Methane (CH4): A single molecule of methane has 21 times the warming potential of a molecule of carbon dioxide, making it a powerful greenhouse gas. Methane is a natural byproduct of the decay of organic material but is also released by landfills and the production of coal, natural gas, and oil. Nitrous oxide (N2O): Nitrous oxide is released from the combustion of fossil fuels and as gaseous waste from many industrial activities. Hydrofluorocarbons: These were initially produced to reduce the levels of ozone-depleting compounds in the upper atmosphere. Unfortunately, although present in very small quantities, they are potent greenhouse gases.

Figure 35.11 shows how the average temperature in the United States has steadily increased over the past two centuries. If the Earth’s temperature continues to rise, more water will evaporate, forming more clouds. This sets up a positive feedback effect that could increase global warming still more. The global climate has already warmed about 0.6°C (1.1°F) since the Industrial Revolution. Enhancements in computer science are allowing scientists to explore the majority of the variables that influence the global climate. Most climate scientists agree that the Earth’s temperature may rise 1.5–4.5°C (2.0–8.1°F) by 2100 if greenhouse emissions continue at the current rates.

Check Your Progress  35.3 1. Recognize the steps of each of the biogeochemical cycles. 2. Provide examples of how human activities can disrupt the biogeochemical cycles.

Figure 35.11  Climate change in the

United States.  The average temperature in the United States has steadily increased over the past two centuries, leading to more severe droughts and more erratic periods of precipitation that are altering the composition of many communities.

Temperature change (°F per century):

-4

-3

-2

-1

0

1

2

Gray interval: -0.1 to 0.1°

3

4



Chapter 35  Nature of Ecosystems

SCIENCE IN YOUR LIFE  ►

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ECOLOGY

Climate Change and Carbon Emissions 

Emissions in billion metric tons

Scientists around the world are working on collecting and interpreting environmental indicators that will help us understand how and why the Earth’s climate is changing. Changes in the average temperature of a region, precipitation patterns, sea levels, and greenhouse gas concentrations are all indicators that our climate is changing. Since 1901, the average temperature across the United States has risen, with 2000– 2009 being the warmest decade on record worldwide with 30–60% of the United States experiencing drought conditions. Average precipitation rates have also increased by 6% over the past century. Since 1990, the United States has experienced eight of the top ten years of extreme precipitation events. Average precipitation rates have also increased by 6% over the past century. Since 1990, the United States has experienced 8 of the top 10 years of extreme precipitation events. Increases in sea surface temperatures

produce a more active hurricane season. Since  the mid-1990s, the Atlantic Ocean, the ­Caribbean, and the Gulf of Mexico have seen 6 of the 10 most active hurricane seasons. Sea levels worldwide have risen an average of 1 inch per decade due to the overall increase in the surface temperature of the world’s oceans. Over half of the human population lives within 60 miles of the coast. Climate models suggest that we will see a rise in sea levels of 3 to 4 feet over the next century. New York City ranges from 5 feet to 16 feet above sea level, while the Florida Keys are an average of 3 to 4 feet above sea level. Even if the rising waters don’t produce flooding, many coastal areas will be exposed to increasingly severe storms and storm surges that could lead to significant economic losses.  The amount of carbon dioxide produced increased annually from 1995–2013 (Fig. 35Ca). Production of electricity is the largest producer of greenhouse gas emissions in the United States,

37.5 35 32.5 30 27.5 25 22.5 1995 1997 1999 2001 2003 2005 2007 2009 2011 2013

a.

Other 34%

Iran 2% Canada 2% Korea 2% Germany 2% Indonesia 2% b.

China 23%

U.S. 15%

Japan 4%

Brazil 4%

India 6% Russian Federation 5%

Figure 35C  Global warming and climate change.  a. CO2 emissions have increased significantly in the past hundred years, causing changes in global climate patterns, including more severe droughts and erratic patterns of precipitation. b. Although China is the largest producer of CO2, the United States produces more CO2 per person (or capita). 

followed by transportation. Carbon dioxide concentrations in the atmosphere now exceed over 400 parts per million (ppm). The Kyoto Protocol was initially adopted on December 11, 1997, and entered into force on February 16, 2005. The goal of the protocol was to achieve stabilization and reduction of the greenhouse gas concentrations in the atmosphere. As of November 2009, 187 countries had ratified the protocol with the goal of reducing emissions by an average of 5.2% by the year 2012. Unfortunately, the United States never ratified the agreement, even though it is responsible for approximately 19% of the global CO2 emissions from fossil fuel combustion and is the largest per-capita emission producer in the world (Fig 35Cb).  The Copenhagen conference in 2009 ended without any type of binding agreement for long-term action against climate change. It did produce a collective commitment by many developed nations to raise 30 billion dollars to be used to help poor nations cope with the effects of and combat climate change.  The 2010 Cancun summit on climate change helped solidify this agreement. Because deforestation produces about 15% of the global carbon emissions, many developing countries will be able to receive incentives to prevent the destruction of their rain forests.  The 2013 UN Climate Change Conference in Warsaw was successful in establishing ways to help developing nations reduce greenhouse emissions as a result of deforestation as well as establishing finance commitments to assist developing nations. The conference also helped keep governments on track toward a universal climate agreement that will be presented in 2015, which will be implemented in 2020.  Interestingly, there is some evidence that change is occurring. In 2014, for the first time in 40 years, there was no increase in the growth of carbon dioxide emissions. However, no growth does not indicate a reduction in carbon dioxide entering the atmosphere, nor does it lessen the potential problems associated with global climate change. But it may indicate that these conferences and agreements are having an effect.

Questions to Consider 1. Should the United States and other developed nations pay developing nations to preserve their forests? 2. Do individuals have a personal responsibility to help prevent climate change, or is it a governmental responsibility?



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UNIT 7  Behavior and Ecology

Conclusion In order to understand the role of the wolves in ecosystems such as Yellowstone, scientists had to piece together information on the evolutionary history of the organisms in the community with an understanding of how the ecosystem functions as a biological system. Coupled with this was some fascinating detective work, with experimental results suggesting that it was the demise of the wolves, not other ecological factors, that was responsible.

The story of the wolves is a positive one for science. By reintroducing wolves into Yellowstone in the late 1990s and closely monitoring not only the health of the wolf population but also the size and composition of the elk herds, conservation ecologists have been able to document that the Yellowstone ecosystem is beginning to return to a healthy status.

MEDIA STUDY TOOLS http://connect.mheducation.com Maximize your study time with McGraw-Hill SmartBook®, the first adaptive textbook.

  Tutorials 35.1  Cycling of Energy and Nutrients in an Ecosystem 35.3  Carbon Cycle

SUMMARIZE 35.1  The Biotic Components of Ecosystems ■ An ecosystem is composed of populations of organisms (biotic com-

ponent) plus their physical environment (abiotic component).

■ Producers are autotrophs that transform solar energy into food for

themselves and all consumers. Consumers are heterotrophs that take in organic food. As herbivores feed on plants or algae and carnivores feed on herbivores, energy is converted to heat. Omnivores are organisms that feed on both plants and animals. Feces, urine, and dead bodies become food for decomposers. Decomposers return some proportion of inorganic nutrients to autotrophs, and other portions are imported or exported among ecosystems in global cycles. Detritus is the partially decomposed matter in the soil and water. ■ Ecosystems are characterized by energy flow and chemical cycling. Energy is lost from the biosphere, but inorganic nutrients are not. They recycle within and among ecosystems. Eventually, all the solar energy that enters an ecosystem is converted to heat, and thus ecosystems require a continual supply of solar energy.

35.2  Energy Flow ■ Ecosystems contain food webs in which the various organisms are

connected by trophic relationships. In a grazing food web, food chains begin with a producer. In a detrital food web, food chains begin with detritus. The two food webs are joined when the same consumer links both a grazing food chain and a detrital food chain. ■ A trophic level contains all the organisms that feed at a particular link in a food chain. Ecological pyramids show trophic levels stacked one on top of the other like building blocks. They are shaped like pyramids

because most energy is lost from one trophic level to the next, and thus the number of species that can be sustained decreases. Biomass is the number of organisms multiplied by their weight.

35.3  Global Biogeochemical Cycles ■ Biogeochemical cycles consist of reservoirs, exchange pools, and

biotic communities. A reservoir pool contains elements available on a limited basis to living organisms, such as fossil fuels, sediments, and rocks. An exchange pool, such as the atmosphere, soil, and water, is a ready source of nutrients for living organisms. ■ In the water (hydrologic) cycle, evaporation over the ocean is not compensated for by rainfall. Evaporation from terrestrial ecosystems includes transpiration from plants. Condensation occurs when a gas is changed into a liquid. Precipitation returns water to the land in the form of rain, snow, sleet, and fog. Rainfall over land results in bodies of fresh water plus groundwater, including aquifers. Eventually, all water returns to the oceans. ■ In the phosphorus cycle, the biotic community recycles phosphorus back to the producers, and only limited quantities are made available by the weathering of rocks. Phosphates are mined for fertilizer production. When phosphates and nitrates enter lakes, ponds, and eventually the ocean, pollution and cultural eutrophication, or over-enrichment, occurs. ■ In the nitrogen cycle, certain bacteria in water, soil, and within root nodules undergo nitrogen fixation. Denitrification is when various bacteria return nitrogen to the atmosphere. Human activities convert atmospheric nitrogen to fertilizer, which is broken down by soil bacteria. Humans also burn fossil fuels. In this way, nitrogen oxides can contribute to the formation of smog and acid rain, both of which are detrimental to animal and plant life.



Chapter 35  Nature of Ecosystems

■ In the carbon cycle, organisms add as much carbon dioxide to the

atmosphere as they remove. Shells in ocean sediments, organic compounds in living and dead organisms, and fossil fuels are reservoirs for carbon. Human activities, such as burning fossil fuels and trees, add carbon dioxide and other gases to the atmosphere. A buildup of these “greenhouse gases” allows the sun’s rays to pass through but they radiate the heat back to the Earth, creating a greenhouse effect. This is leading to global warming and causing climate change. A rise in sea level and a change in climate patterns is expected to follow.

ASSESS Testing Yourself Choose the best answer for each question.

35.1  The Biotic Components of Ecosystems 1. Label the populations in the ecosystem diagram below. solar energy

heat a. b.

6. The ______ of an ecosystem is equal to the weight of an organism times the number of this organism in the ecosystem. a. niche d. biomass b. trophic level e. biodiversity c. food web

35.3  Global Biogeochemical Cycles For questions 7–12, match each characteristic to the cycles listed in the key. More than one answer can be used, and answers can be used more than once. Key: a. water cycle b. carbon cycle c. nitrogen cycle d. phosphorus cycle e. none of the cycles f. all of the cycles 7. Involves transpiration and precipitation 8. Utilizes bacteria to make the compounds usable to plants 9. Involves the participation of decomposers 10. The atmosphere acts as a reservoir. 11. Rocks are the reservoir in this cycle. 12. Imbalances are contributing to global climate change.

ENGAGE

c. d. heat

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heat energy nutrients

BioNOW Want to know how this science is relevant to your life? Check out the BioNow videos below. ■ Deer Autopsy

2. In an ecosystem, these organisms are responsible for converting solar energy to the stored energy found in organic compounds. a. herbivores b. decomposers c. producers d. carnivores e. None of these are correct. 3. Generally, energy ________  an ecosystem and nutrients ________ an ecosystem. a. cycles within; cycle within b. cycles within; flow through c. flows through; cycle within d. flows through; flow through

35.2  Energy Flow 4. Of the total amount of energy that passes from one trophic level to another, about 10% is a. respired and becomes heat. b. passed out as feces or urine. c. stored as body tissue. d. recycled to autotrophs. e. All of these are correct. 5. This form of a food web begins with waste materials and the remains of dead organisms. a. aquatic d. atmospheric b. detrital e. geologic c. grazing

■ Energy Part II: Photosynthesis

Thinking Critically 1. A large forest has been removed by timber harvest (clear-cutting) and the land has not been replanted. After several years, humidity and rainfall in the area seem to have decreased. Use your knowledge of the water cycle to explain these observations. 2. What impact might fungicides and pesticides have on detrital food webs? How do you think the nutrient cycles would be affected by the use of fungicides and pesticides? 3. Why would an agricultural extension agent recommend planting a legume, such as clover, in a pasture? 4. What types of things can you do at home to facilitate the cycling of nutrients such as carbon and nitrogen? What types of things can you do at home to conserve water or improve the quality of a nearby body of water?

PHOTO CREDITS Opener: © DLILLC/Corbis RF; 35.1a(diatom): © Ed Reschke; 35.1a(tree): © Hermann Eisenbeiss/Science Source; 35.1b(caterpillar): © Corbis RF; 35.1b(rabbit): © Gerald C. Kelley/Science Source; 35.1c(snake eating rabbit): © Derrick Hamrick/imagebroker/Corbis; 35.1c(hawk and snake): © Tze-hsin Woo/Getty RF; 35.1d(bacteria): © Image Source/Getty RF; 35.1d(mushroom): © Denise McCullough; 35.3: © George D. Lepp/Science Source; 35Aa: The U.S. Drought Monitor is jointly produced by the National Drought Mitigation Center at the University of Nebraska-Lincoln, the United States Department of Agriculture, and the National Oceanic and Atmospheric Administration. Map courtesy of NDMC-UNL.; 35Ab: © Mike Blake/Reuters/Corbis; 35Ba: © Bill Aron/PhotoEdit.



36

Major Ecosystems of the Biosphere CHAPTER OUTLINE 36.1  Climate and the Biosphere 36.2 Terrestrial Ecosystems 36.3 Aquatic Ecosystems BEFORE YOU BEGIN Before beginning this chapter, take a few moments to review the following discussions: Section 8.4  How do cellular processes like photosynthesis adapt to climate? Section 34.3  What types of symbiotic relationships may allow species to survive in harsh environments? Section 35.3  What is the difference between global warming and climate change?

CASE STUDY DDT in the Water DDT was originally developed as a broad-spectrum pesticide to help control insect populations on South Pacific islands during World War II. Due to concerns about environmental effects, DDT has been banned in the United States since 1972, although it is still used in other parts of the world in agricultural and disease-control programs. However, we are still finding traces of it in various locations across the country. DDT’s persistence in the environment causes it to be a global environmental and health problem. Due to its stability (it takes 15 years to break down), and its ability to accumulate in fatty tissue, it has been found in organisms ranging from Adelie penguins in Antarctica to bald eagles in the United States, and even in humans. The harmful effects of DDT include liver damage, a potential link to various cancers, decreased reproductive success, and temporary damage to the nervous system. Human exposure to DDT tends to come in the form of eating plants and animals that are contaminated. The Great Lakes and many other waterways have fish consumption advisories due to the presence of DDT in these systems. Once DDT enters an aquatic ecosystem, it can increase in concentration as it moves up the food chain. The use of DDT in many developing nations around the world enables it to become a part of the global ecosystem. Soil and sediment runoff provides a means for DDT to enter into the aquatic ecosystems. As the water evaporates, the DDT may enter into the atmosphere. When the water reaches cooler latitudes, it condenses, forming rain. The rain carries the DDT particles back to the surface. The global wind circulation patterns allow DDT to be carried to every region of the planet. As you read through the chapter, think about the following questions:

1. Should DDT use be permitted in any part of the world? 2. Is there any biome in which DDT can be used that would not impact another biome?

3. Is it possible to isolate the actions in one biome from the rest of the planet?

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Chapter 36  Major Ecosystems of the Biosphere

36.1  Climate and the Biosphere Learning Outcomes Upon completion of this section, you should be able to 1. Describe how solar radiation produces variations in the Earth’s climate. 2. Explain how global air circulation patterns and physical geographic features are associated with the Earth’s temperature and rainfall.

hemisphere. The direction in which the air rises and cools determines the direction of the wind (Fig. 36.2). At the equator, the sun heats the air and evaporates water. The warm, moist air rises and loses most of its moisture as rain, resulting in the greatest amounts of rainfall occurring nearest to the equator. The rising air flows

Ascending moist air cools and loses moisture.

60°N ies westerl

Climate refers to the prevailing weather conditions in a particular region. Climate is dictated by temperature and rainfall, which in turn are influenced by the following factors: (1) variations in the distribution of solar radiation due to the tilt of the Earth as it orbits about the sun; and (2) other effects, such as topography and proximity to water bodies.

30°N ds e win nor theast trad e qu

Effect of Solar Radiation Because the Earth is spherically shaped, the sun’s rays are more direct at the equator and more spread out progressing toward the poles. Therefore, the tropical regions nearest the equator are warmer than the temperate regions farther away from the equator. In addition, Earth does not face the sun directly. Rather, it is on a slight tilt (about 23°) away from the sun. As the Earth orbits around the sun throughout the year, different parts of the planet are tilted toward or away from the sun, which determines the seasons. For example, during winter, the Northern Hemisphere is tilted away from the sun (Fig. 36.1). At the same time, the Southern Hemisphere is tilted toward the sun and it experiences summer. Because the Earth completes one rotation on its axis per day and its surface consists of continents and oceans, the flow of warm and cold air form three large circulation patterns in each

atoria l doldrums equ atoria l doldrums sou theas t tra

30°S de winds

weste rlies

Figure 36.2  Global wind circulation.  Air ascends and descends as

shown because Earth rotates on its axis. Also, the trade winds blow from the northeast to the west in the Northern Hemisphere, and blow from the southeast to the west in the Southern Hemisphere. The westerlies blow toward the east. 23°

Figure 36.1 

Distribution of solar energy. 

60

Winter solstice Northern Hemisphere tilts away from sun, December

30 equ

ator

30

equator

60

South Pole

30

60

sun

equ

ator

30

30

equ

ator

30

equator

30 ator

b.

a. Because Earth is a sphere, beams of solar energy striking the planet near one of the poles are spread over a wider area than similar beams striking Earth at the equator. b. The seasons of the Northern and Southern Hemispheres are due to the tilt of Earth on its axis as it rotates around the sun.

60

Summer solstice Northern Hemisphere tilts toward sun, June equ

a.

60°S

Descending dry air warms and retains moisture.

Vernal equinox Sun aims directly at equator, March

North Pole

0°

30

Autumnal equinox Sun aims directly at equator, September



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UNIT 7  Behavior and Ecology

toward the poles, but at about 30° north and south latitude, it sinks toward Earth’s surface and reheats. As the dry air descends and warms once again, areas of high pressure are generated, which results in low rainfall. As a result, the great deserts of Africa, Australia, and the Americas occur at these latitudes. At about 60° north and south latitude, the warm air rises and cools as it does near the equator, producing a low-pressure area. Low pressure results in zones of high rainfall. Between 30° and 60° latitude, the strong wind patterns known as the westerlies occur in both the Northern and Southern Hemispheres. The westerlies move from west to east. The west coasts of the continents at these latitudes are wet, as in the U.S. Pacific Northwest, where a temperate (evergreen) rain forest is located. Weaker winds, called the polar easterlies, blow from east to west at latitudes higher than 60° in both hemispheres. The direction of wind patterns, such as the easterlies and westerlies, is affected by the spinning of the Earth about its axis. That is, in the Northern Hemisphere, large-scale winds generally move clockwise, while in the Southern Hemisphere, they move counterclockwise. This explains, for example, why the northeast trade winds blow from the northeast toward the southwest and the southeast trade winds blow from the southeast toward the northwest (Fig. 36.2). Trade winds are so called because early sailors depended on them to power the movement of sailing ships.

Other Effects The term topography refers to the physical features, or “the lay,” of the land. One physical feature that affects climate is the presence of mountains. As air blows up and over a mountain range, it rises and cools, causing condensation to occur. One side of the mountain, called the windward side, receives more rainfall than the other side, called the leeward side. On the leeward side, the dry air descends, often producing a dry arid environment (Fig. 36.3). The difference between the windward side and the leeward side can be quite dramatic. In the Hawaiian Islands, for example, the windward side of the mountains typically receives more than 750 cm of rain a year while the leeward side, which is in a rain shadow, gets an average of only 50 cm of rain and is generally sunny. In the United States, the western side of the Sierra Nevada mountain range is lush, while the eastern side is a semidesert. In contrast to landmasses, the oceans are slower to change temperature. This causes coasts to have a unique weather pattern that is not seen farther inland. During the day, the land warms more quickly than the ocean, and the air above the land rises. Then a cool sea breeze blows in from the ocean. At night, the reverse happens and the breeze blows from the land to the sea. In India and some other countries in southern Asia, the land heats more rapidly than the waters of the Indian Ocean during spring. The difference in temperature between the land and the ocean causes a gigantic circulation of air: warm air rises over the land, and cooler air comes in off the ocean to replace it. As the warm air rises, it loses its moisture, creating a monsoon climate in which wet ocean winds blow onshore for almost half the year. During the monsoon season, rainfall is particularly heavy on the windward side of hills. Cherrapunji in northern India receives an annual average of 1,090 cm of rain a year because of its high altitude. The chief crop of India is rice, which starts to grow when the monsoon rains begin.

condensation dry air

moist air rain shadow

windward side

leeward side

Figure 36.3  Formation of a rain shadow.  When winds from the sea cross a coastal mountain range, they rise and release their moisture as they cool the windward side of the mountain. The leeward side of a mountain is warmer and receives relatively little rain. Therefore, it is said to lie in a “rain shadow.” The weather pattern has reversed by November, when the land has become cooler than the ocean. Therefore, dry winds blow from the Asian continent across the Indian Ocean. In the winter, the air over the land is dry, the skies cloudless, and temperatures pleasant. Other large bodies of water create major weather patterns. For example, in the United States, people often speak of the “lake effect,” meaning that in the winter, arctic winds blowing over the Great Lakes become warm and moisture-laden. When these winds rise and lose their moisture, snow begins to fall. Places such as Buffalo, New York, get heavy snowfalls due to the lake effect, and snow is on the ground there for an average of 90 to 140 days every year.

Check Your Progress  36.1 1. Identify the conditions that account for the different seasons.

2. Recognize the various features that will affect rainfall.

36.2  Terrestrial Ecosystems Learning Outcomes Upon completion of this section, you should be able to 1. Identify the geographical distribution of the major terrestrial ecosystems. 2. Summarize the key characteristics of the major terrestrial biomes.



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Chapter 36  Major Ecosystems of the Biosphere

A major type of terrestrial ecosystem is called a biome. Biomes are characterized by particular climatic conditions, as well as the plants and animals living there. When terrestrial biomes are plotted according to their mean annual temperature and mean annual precipitation, a particular pattern results (Fig. 36.4a). The distribution of biomes is shown in Figure 36.4b. Even though this figure shows what appear

−15 −10

Arcticalpine tundra

−5 taiga

0

10 15

30

desert

25

cold temperate

temperate temper e perat per e d eciduo du u s deciduous fforest orest

temperate rain forest

grassland

rub thorn sc

20

d lan od wo nd shrubla

5

semideser t

Mean Annual Temperature (°C)

Figure 36.4  Pattern of biome distribution.  a. Pattern of world biomes in relation to temperature and moisture. The dashed line encloses a wide range of environments in which either grasses or woody plants can dominate the area, depending on the soil type. b. The same type of biome can occur in different regions of the world, as shown on this global map.

to be clear demarcations, note that biomes gradually change from one type to another. To simplify our discussion in this chapter, we will group the biomes into general categories. Although we will be discussing each type of biome separately, keep in mind that all biomes are linked to form the biosphere, with each biome having connections to all the other terrestrial and aquatic ecosystems.

savanna savann a a a

50

100

tropical seasonal forest

warm temperate tropical rain forest

150 200 250 300 350 Mean Annual Precipitation (cm)

tropical

400

450

a. Biome pattern of temperature and precipitation

polar ice tundra taiga mountain zone temperate deciduous forest temperate rain forest tropical deciduous forest tropical seasonal forest tropical rain forest shrubland temperate grassland savanna semidesert desert

b. Distribution of biomes



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The distribution of the biomes and their corresponding communities of organisms is determined principally by differences in climate due to solar radiation, water, and defining topographical features. Both latitude and altitude are responsible for temperature gradients. When traveling from the equator to the North Pole, it would be possible to observe first a tropical rain forest, followed by a temperate deciduous forest, a coniferous forest, and tundra. A similar type of sequence can also be seen when ascending a mountain (Fig. 36.5). The coniferous forest of a mountain is called a montane coniferous forest, and the tundra near the peak of a mountain is called an alpine tundra. However, when going from the equator to the South Pole, one would not reach a region corresponding to the coniferous forest and tundra of the Northern Hemisphere because of the absence of large landmasses in the Southern Hemisphere.

only the topmost layer of earth thaws. The permafrost beneath this layer is always frozen, and therefore, drainage is minimal. Trees are not found in the tundra because the growing season is too short, their roots cannot penetrate the permafrost, and they cannot become anchored in the shallow, boggy soil during the brief summer. Instead, the ground is covered with short grasses and sedges, as well as numerous patches of lichens and mosses in summer (Fig. 36.6a). Dwarf woody shrubs, such as dwarf birch, flower and seed quickly during the short growing season. Few animals live in the tundra year-round, but nearly all species have adaptations for living in extreme cold and short growing seasons. In winter, for example, the ptarmigan (a grouse) burrows in the snow during storms, and the musk ox conserves heat because of its thick coat and short, squat body. Other species, such as caribou (Fig. 36.6b) and reindeer, migrate to and from the tundra, as do the wolves that prey upon them. In the summer, the tundra is alive with numerous insects and birds, particularly shore-birds and waterfowl that migrate inland.

Tundra The tundra biome, which encircles the arctic region just south of the ice-covered polar seas in the Northern Hemisphere, covers about 20% of Earth’s land surface. As previously mentioned, a similar ecosystem, called the alpine tundra, occurs above the timberline on mountain ranges. The arctic tundra is cold and dark much of the year. Because rainfall amounts to only about 20 cm a year, the tundra could possibly be considered a desert. However, melting snow creates a landscape of pools and bogs in the summer. This is because

Coniferous Forests Coniferous forests are found in three locations: in the taiga, which extends around the world in the northern part of North America and Eurasia; near mountaintops (where it is called a ­montane coniferous forest); and along the Pacific coast of North ­America, as far south as northern California.

ice

alpine tundra

Increasing Altitude

montane coniferous forest

deciduous forest tropical forest temperate deciduous forest

coniferous forest

tundra

ice

Increasing Latitude

Figure 36.5  Climate and terrestrial biomes.  Biomes change with altitude just as they do with latitude because vegetation is partly determined by

temperature. Precipitation also plays a significant role, which is one reason grasslands, instead of tropical or deciduous forests, are sometimes found at the base of mountains.



Chapter 36  Major Ecosystems of the Biosphere

a. Tundra

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b. Bull caribou

Figure 36.6  Tundra.  a. Vegetation consists principally of lichens, mosses, grasses, and low-growing shrubs. b. Caribou feed on the vegetation in

the summer.

Figure 36.7  Coniferous forest.  a. The taiga, which means swampland, is a coniferous forest that spans northern Europe, Asia, and North America. b. The appellation “spruce-moose” refers to the dominant presence of spruce trees and moose, which frequent the ponds.

a. Spruce a. Spruce treestrees in the in taiga the taiga biome biome

Taiga typifies the coniferous forest with its cone-bearing trees, such as spruce, fir, and pine (Fig. 36.7a). These trees are well adapted to the cold because both the leaves (reduced to needles) and bark have thick coverings. Also, the needlelike leaves can withstand the weight of heavy snow. There is a limited understory of plants, but the floor is covered by low-lying mosses and lichens. Birds

b. Bull b. Bull moose, moose, Alces Alces americanus, americanus, a large a large mammal mammal

harvest the seeds of the conifers, and bears, deer, moose, beaver, and muskrat live around the cool lakes and along the streams (Fig.  36.7b). Wolves prey on these larger mammals. A montane coniferous forest also harbors the wolverine and the mountain lion. The coniferous forest that runs along the west coast of Canada and the United States is sometimes called a temperate rain forest.



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The plentiful rainfall and rich soil have produced some of the tallest conifer trees ever known, including the coastal redwoods. Small sections of this forest are considered old-growth forest because trees average over 150 years old, with some trees even as old as 800 years. It truly is an evergreen forest because mosses, ferns, and other plants often grow on tree trunks. Whether the limited portion of old-growth forest that remains should be protected from logging is an important and controversial conservation issue.

Temperate Deciduous Forests Temperate deciduous forests are found south of the taiga in eastern North America, eastern Asia, and much of Europe. The climate in these areas is moderate, with relatively high rainfall (75–150 cm per year). The seasons are well defined, and the growing season ranges between 140 and 300 days. The trees, such as oak, beech, and maple, have broad leaves and are deciduous because they lose their leaves in fall and regrow them in spring. The tallest trees form a canopy, an upper layer of leaves that are the first to receive sunlight and thereby create shade below (Fig. 36.8). Even so, enough sunlight penetrates to provide energy for another layer of trees, called understory trees. Beneath these trees are shrubs and herbaceous plants that may flower in the spring

before the trees have put forth their leaves. Mosses, lichens, and ferns can reside beneath the shrub layer. This stratification provides a variety of habitats for insects and birds. Ground life is also plentiful. Squirrels, cottontail rabbits, shrews, skunks, woodchucks, and chipmunks are small herbivores. These and ground birds such as turkeys, pheasants, and grouse are preyed on by red foxes. White-tail deer and black bears have recently increased in number. In contrast to the taiga, a greater diversity of amphibians and reptiles occur in this biome because the winters are not as cold. Frogs and turtles generally prefer an aquatic existence, as do the beaver and muskrat. Abundant fruits, nuts, and berries provide a supply of food for the winter. The leaves, after turning brilliant colors and falling to the ground, contribute to the rich layer of humus after decomposition. The minerals within the rich soil are washed far into the ground by spring rains, but the deep tree roots capture these nutrients and cycle them back through the forest system.

Tropical Forests The most common type of tropical forest is the tropical rain ­forest, which is found in areas of South America, Africa, and the Indo-Malayan region near the equator. The temperature in a tropical rain forest is always warm (between 20° and 25°C), and rainfall

Figure 36.8  Temperate deciduous forest.  a. The Shawnee National Forest in Illinois is home to many varied plants and animals. b. Marsh marigolds may be found in wetland areas, chipmunks feed on acorns, and bobcats prey on these and other small mammals.

marsh marigolds

a.

b. bobcat

eastern chipmunk



Chapter 36  Major Ecosystems of the Biosphere

is plentiful (a minimum of 190 cm per year). As a result of these favorable climate conditions, this is the richest land biome in terms of species diversity. The diversity of species is enormous—a 10-km2 area of tropical rain forest may contain 750 species of trees and 1,500 species of flowering plants. A tropical rain forest has a complex structure, with many levels of life. Some broadleaf evergreen trees grow to heights of 15–50 m or more. These tall trees often have trunks buttressed at ground level to prevent them from toppling over. Lianas, or woody vines, often encircle rainforest trees as they grow. Although some animals live on the ground (e.g., pacas, agoutis, peccaries, and armadillos), many also live in the trees (Fig. 36.9). Insect life is so abundant that the majority of species have not yet been identified. Termites play a vital role in the decomposition of woody plant material, and ants are found everywhere. The various birds, such as hummingbirds, parakeets, parrots, and toucans, are often beautifully colored. Amphibians and reptiles are well represented by many types of frogs, snakes, and lizards. Lemurs, sloths, and monkeys are well-known primates that feed on the fruits of the trees. The largest carnivores are the big cats—the jaguars in South America and the leopards in Africa and Asia. Many animals spend their entire life in the canopy, as do some plants. Epiphytes are plants that grow on other plants but usually

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have roots of their own that absorb moisture and minerals leached from the canopy. Others catch rain and debris in hollows produced by overlapping leaf bases. The most common epiphytes are related to pineapples, orchids, and ferns. Whereas the soil of a temperate deciduous forest biome is rich enough for agricultural purposes, the soil of a tropical rain forest biome is not. Nutrients are cycled directly from the litter to the plants again. Productivity is high because of warm and consistent temperatures, a year-long growing season, and the rapid recycling of nutrients from litter decomposition. To make up for the soil’s low fertility, trees are sometimes cut and burned, so that the resulting ashes can provide enough nutrients for several harvests. This practice, called swidden agriculture, or slash-and-burn agriculture, can be successful if done on a small scale. Once the crops are harvested, the soil nutrients become depleted, consequently removing nutrients from the system. Erosion is high due to lack of tree roots and the heavy rainfall, causing additional nutrients to be washed away. While we usually think of tropical forests as nonseasonal rain forests, tropical deciduous forests that have wet and dry seasons are found in India, Southeast Asia, West Africa, South and Central ­America, the West Indies, and northern Australia. They also have deciduous trees, and some of these forests also contain elephants, tigers, and hippopotamuses in addition to the animals mentioned previously.

Figure 36.9  Representative animals and plants of tropical

rain forests.  Some plants and animals of the rain forest are shown here.

poison-dart frog

blue and gold macaw

spike-headed katydid

blue morpho butterfly

jaguar

orchid

chameleon



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a. Shrubland overview

b. Greater Roadrunner

Figure 36.10  Shrublands.  a. Shrublands, such as chaparral in California, are subject to raging fires, but the shrubs are adapted to quickly regrow. b. Greater roadrunners find a home in the chaparral. 

Shrublands Shrublands tend to occur along coasts that have dry summers and receive most of their rainfall during their winter (Fig. 36.10a). They are characterized by shrubs that have small but thick evergreen leaves, often coated with a waxy material that prevents loss of moisture. These shrubs are adapted to withstand arid conditions and can also quickly regrow after a fire. In fact, the seeds of many species require the heat and scarring action of fire to induce germination. Other shrubs sprout from the roots after a fire. Typical shrubland species include coyotes, jackrabbits, gophers, and other rodents, as well as fire-adapted plant species such as chemise.  The dense shrubland that occurs in California is known as chaparral. This type of shrubland, called Mediterranean, lacks an understory and ground litter, and is highly flammable. Chaparrals are also found in South Africa and Australia. Typical animals of the chaparral include mule deer, rodents, lizards, and greater roadrunners (Fig 36.10b). 

a. Tall-grass prairie

Grasslands Grasslands occur where rainfall is greater than 25 cm but generally insufficient to support trees. For example, it is too dry for forests and too wet for deserts to form in temperate areas where rainfall is between 25 to 75 cm. Natural grasslands once covered more than 40% of Earth’s land surface, but most areas that were once grasslands are now used to grow crops such as wheat, corn, and soybeans. Grasses are well adapted to a changing environment and can tolerate some grazing, as well as flooding, drought, and sometimes fire. Where rainfall is high, large tall grasses that reach more than 2 m in height (e.g., pampas grass) can flourish. In drier areas, shorter grasses between 5 and 10 cm are dominant (e.g., grama grass). The growth of grasses is also seasonal. As a result, some grassland animals such as bison migrate, whereas ground squirrels hibernate, when there is little grass for them to eat.

b. American bison

Figure 36.11  The prairie.  a. Tall-grass prairies are seas of grasses dotted by pines and junipers. b. Bison, once abundant, are now being reintroduced into certain areas.

Temperate Grasslands The temperate grasslands include the Russian steppes, the South American pampas, and the North American prairies (Fig. 36.11). Large herds of bison—estimated at hundreds of thousands—once



Chapter 36  Major Ecosystems of the Biosphere

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Figure 36.12  The savanna. 

The African savanna varies from grassland to widely spaced shrubs and trees. This biome supports a large assemblage of herbivores, including zebras (a), wildebeests (b), and giraffes (c). Carnivores, such as the cheetah (d), prey on these.

a. Herbivores of the savanna biome

d. Cheetah

b. Wildebeest

c. Giraffe

roamed the prairies, as did herds of pronghorn antelope. Now, small mammals, such as mice, prairie dogs, and rabbits, typically live below ground but usually feed above ground. Hawks, snakes, badgers, coyotes, and foxes feed on these mammals.

allows the sun’s rays to penetrate easily, but nights are cold because heat escapes easily into the atmosphere. The Sahara—which stretches all the way from the Atlantic coast of Africa to the Arabian Peninsula—and a few other deserts have little or no vegetation. But most deserts have a variety of plants (Fig. 36.13). The best-known desert perennials in North America are the succulent, spiny-leafed cacti, which have stems that store water and carry on photosynthesis. Also common are nonsucculent shrubs, such as the many-branched sagebrush and the spiny-branched ocotillo that produces leaves during wet periods and sheds them during dry periods. Some animals are adapted to the desert environment. For example, many desert animals are nocturnal. They are active at night when it is cooler. Reptiles and insects have waterproof outer coverings that conserve water. A desert has numerous insects, which pass through the stages of development from pupa to adult while there is rain. Reptiles, especially lizards and snakes, are a characteristic group of vertebrates found in deserts, but running birds (e.g., the roadrunner) and rodents (e.g., the kangaroo rat) are also well known (Fig. 36.13). Coyotes and hawks prey on the rodents.

Savannas Savannas, which are grasslands that contain some trees, occur in regions where a relatively cool, dry season is followed by a hot, rainy season. One tree that can survive the severe dry season is the flattopped acacia, which sheds its leaves during a drought. The African savanna supports a tremendous variety and number of large herbivores (Fig. 36.12). Elephants and giraffes are browsers that feed on tree vegetation. Antelopes, zebras, wildebeests, water buffalo, and rhinoceroses are grazers that feed on grasses. Any plant litter that is not consumed by grazers is attacked by a variety of smaller organisms, among them termites. Termites build towering mounds in which they tend fungal gardens that they use for food. The herbivores support a large population of carnivores. Lions and hyenas sometimes hunt in packs, cheetahs hunt singly by day, and leopards hunt singly by night.

Deserts As discussed in section 36.1, deserts are usually found at latitudes of about 30°, in both the Northern and Southern Hemispheres. The winds that descend in these regions lack moisture, and annual rainfall is less than 25 cm. Days are hot because a lack of cloud cover

Check Your Progress  36.2 1. Identify the features that would enable one biome to have a greater biodiversity than another.

2. Compare and contrast the biodiversity of the various land biomes.



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Figure 36.13  Deserts.  Plants and animals that live in a desert are adapted to arid conditions. a. The plants are either succulents, which retain moisture, or shrubs with woody stems and small leaves, which lose little moisture. b. The kangaroo rat feeds on seeds and other vegetation. c. Burrowing owls feed on rodents, reptiles, and insects.  d. The kit fox is a desert carnivore.

b. Bannertail kangaroo rat

c. Burrowing owls

d. Kit fox

a.

36.3  Aquatic Ecosystems Learning Outcomes Upon completion of this section, you should be able to 1. Compare the characteristics of freshwater and saltwater ecosystems. 2. Recognize the differences between the pelagic and benthic divisions of the ocean.

Aquatic ecosystems are generally classified as freshwater (inland) or saltwater (usually marine). Brackish water, however, is a mixture of fresh and salt water. In this section, we describe lakes as examples of freshwater ecosystems and oceans as examples of saltwater ecosystems. Coastal ecosystems will represent areas of brackish water.

Lakes Lakes are bodies of fresh water often classified by their nutrient abundance. Oligotrophic (nutrient-poor) lakes are characterized by a small amount of organic matter and low productivity

(Fig. 36.14a). Eutrophic (nutrient-rich) lakes typically have plentiful organic matter and high productivity (Fig. 36.14b). Such lakes are usually situated in naturally nutrient-rich regions or are enriched by agricultural or urban and suburban runoff. Oligotrophic lakes can become eutrophic through large inputs of nutrients, a process called eutrophication. Excess nutrient inputs, such as nitrogen fertilizer, can cause eutrophication, which can lead to fish kills. In temperate environments, deep lakes are stratified during the summer and winter. In summer, lakes in the temperate zone have three layers of water that differ in temperature (Fig. 36.15). The surface layer, the epilimnion, is warm due to solar radiation; the thermocline is the middle layer that decreases 1°C per meter of depth; and the lowest layer, the hypolimnion, is cold. These differences in temperature prevent mixing. The warmer, less dense water of the epilimnion “floats” on top of the colder, more dense water of the hypolimnion. As the season progresses, the epilimnion becomes nutrientpoor, while the hypolimnion begins to be depleted of oxygen. ­Phytoplankton (see Chapter 29) found in the sunlit epilimnion use up nutrients during photosynthesis and in turn release oxygen. Detritus naturally falls by gravity to the bottom of the lake where



Chapter 36  Major Ecosystems of the Biosphere

a. Oligotrophic lake

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b. Eutrophic lake

Figure 36.14  Types of lakes.  Lakes can be classified according to whether they are (a) oligotrophic (nutrient-poor) or (b) eutrophic (nutrient-rich).

Eutrophic lakes tend to have large populations of algae and rooted plants, resulting in a large population of decomposers that use up much of the oxygen and leave little oxygen for fishes.

Summer stratification wind epilimnion 24–25°C thermocline hypolimnion 5–8°C

Spring overturn

Fall overturn wind

wind

Winter stratification wind

ice

2–3°C most of lake 4°C

Figure 36.15  Lake stratification in a temperate region. 

Temperature profiles of a large oligotrophic lake in a temperate region vary with the season. During the spring and fall overturns, the deep waters receive oxygen from surface waters, and surface waters receive inorganic nutrients from deep waters.

oxygen is used up as decomposition occurs. Decomposition in turn releases nutrients. In the fall, as the epilimnion cools, and in the spring, as it warms, an overturn occurs. In the fall, the upper epilimnion

waters become cooler than the hypolimnion waters. This causes the surface water to sink and the deep water to rise. The fall overturn continues until the temperature is uniform throughout the lake. At this point, wind helps circulate the water so that mixing occurs. As winter approaches, the water cools further. Ice formation begins at the top, and the ice floats because it is less dense than cold water. Ice has an insulating effect, preventing further cooling of the water below. This permits aquatic organisms to live through the winter in the water beneath the surface of the ice. In the spring, as the ice melts, the cooler water on top sinks below the warmer water below it. The spring overturn continues until the temperature is uniform through the lake. At this point, wind aids in the circulation of water as before. When the surface waters absorb solar radiation, thermal stratification occurs once more. This vertical stratification and seasonal change of temperatures in a lake basin influence the seasonal distribution of fish and other aquatic life. For example, cold-water fish move to the deeper water in summer and inhabit the upper water in winter. In the fall and spring just after mixing occurs, phytoplankton growth at the surface is most abundant.

Life Zones In both fresh and salt water, microscopic plankton, which includes phytoplankton and also zooplankton (see Chapter 29), play important roles in aquatic ecosystems. Lakes and ponds can be divided into several life zones, as shown in Figure 36.16. Aquatic plants are rooted in the shallow littoral zone of a lake, and various microscopic organisms cling to these plants and to rocks. Some organisms, such as the water strider, live at the water-air interface and can literally walk on water. In the limnetic zone, small fishes, such as minnows and killifish, feed on plankton and



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also serve as food for large fishes. In the profundal zone, zooplankton and fishes, such as whitefish, feed on debris that falls from above. Pike species are an example of “lurking predators.” They wait among vegetation around the margins of lakes and surge out to catch passing prey. A few insect larvae are in the limnetic zone, but they are far more abundant in both the littoral and profundal zones. Midge larvae and ghost worms are common members of the benthos, animals that live on the bottom in the benthic zone. In a lake, benthos organisms include crayfishes, snails, clams, and various types of worms and insect larvae.

Coastal Ecosystems Estuaries An estuary is where fresh water and salt water meet and mix (Fig. 36.17). Coastal bays, tidal marshes, fjords (an inlet of water

between high cliffs), some deltas (a triangular-shaped area of land at the mouth of a river), and lagoons (a body of water separated from the sea by a narrow strip of land) are all examples of estuaries (Fig. 36.18). Organisms living in an estuary must be able to withstand constant mixing of waters and rapid changes in salinity. Not many organisms are adapted to this environment, but those that can survive here find an abundance of nutrients. An estuary acts as a nutrient trap because the sea prevents the rapid escape of nutrients brought by a river. It has been estimated that well over half of all marine fishes develop in the protective environment of an estuary, which explains why estuaries are called the nurseries of the sea. Estuaries are also feeding grounds for many birds, fish, and shellfish because they offer a ready supply of food. Salt marshes, dominated by salt marsh cordgrass, are often associated with estuaries. So are mudflats and mangrove swamps, where sediment and nutrients from the land collect.

Littoral Zone

surface organisms Water strider, Gerris sp. clinging organisms

fishes

Benthic Zone

phytoplankton zooplankton

insect larvae Limnetic Zone

Northern pike, Esox lucius

Figure 36.16  Zones of a lake.  Rooted plants and clinging organisms live in the littoral zone. Phytoplankton, zooplankton, and fishes are in the sunlit limnetic zone. Water striders stand on the surface film of water with water-repellent feet. Crayfishes and molluscs are in the profundal zone as well as the littoral zone. Pike are top carnivores prized by fishermen.

bottom-dwelling organisms

Profundal Zone



Chapter 36  Major Ecosystems of the Biosphere

freshwater input

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white shrimp

Marine snails, at the base of salt marsh cordgrass, feed on algae.

freshwater input

spawning and nursery

phytoplankton

spawning and nursery leas

t sal

t

mos

t sal

t

ocean water

Figure 36.17 

Estuary structure and function.  Because an estuary is

located where a river flows into the ocean, it receives nutrients from the land. Estuaries serve as a nursery for the spawning and rearing of the young for many species of fishes, shrimp and other crustaceans, and molluscs.

a. Mudflat

a. Mudflat

b. Mangrove swamp

Figure 36.18  Types of estuaries.  Types of estuaries include (a) mudflats, which form at the mouth of rivers in temperate zones and are frequented by migrant birds, and (b) mangrove swamps, which skirt the coastlines of many tropical and subtropical lands. The tangled roots of mangrove trees trap sediments and nutrients that sustain many immature forms of sea life. The salt marsh depicted in Figure 36.17 is yet another type of estuary-associated system.

Seashores Seashores, which may be rocky or sandy, are constantly bombarded by the sea as the tides roll in and out (Fig. 36.19). The ­littoral zone lies between the high- and low-tide marks. The littoral zone of a rocky beach is divided into subzones. In the upper portion of the littoral zone, barnacles are attached so tightly to

b. Mangrove swamp

stone by their own secretions that their calcareous outer plates remain in place even after the enclosed shrimplike animal dies. In the midportion of the littoral zone, brown algae known as rockweed may overlie the barnacles. In the lower portions of the littoral zone, oysters and mussels attach themselves to the rocks by filaments called byssal threads. Also present are snails called limpets



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SCIENCE IN YOUR LIFE  ►

ECOLOGY

The Fate of Prescription Medicine What happens to prescriptions when they are flushed down the drain? Multiple studies have found detectable concentrations of pharmaceuticals and organic wastewater chemicals in rivers and streams across the United States (Fig. 36A). U.S. Geological Survey scientists found 12 of 22 pharmaceuticals and 32 of the 47 organic wastewater chemicals looked for in the streams of the Boulder Creek Watershed, Colorado. Some of these chemicals included antibiotics used to treat bacterial infections, antimicrobial agents used in soaps, endocrine-active chemicals (such as those found in birth control pills), as well as caffeine. While few of the detectable chemicals exceeded water-quality standards, the scientists did note that no standards exist for a number of these compounds, raising concerns about what represents acceptable levels in the water supply. While difficult to fully assess the complete ecological impact of these chemicals, a wide range of problems have been attributed to them. Native fish populations were found to exhibit endocrine disruption due to exposure to  these contaminants. Endocrine disruptions can produce adverse effects on the development of the nervous and reproductive systems, as well as changes in the fish’s response to stressors in the environment. A 2008 study showed fish populations with altered sex ratios, reduced gonad size, and disrupted ovarian and testicular development in streams containing a mixture of endocrine-active chemicals. Fish in contaminated streams were also found to

a. Sea stars at low tide

Figure 36A  Effects of pharmaceuticals on fish.  In some

cases male fish have developed ovaries as a result of exposure to endocrine-active chemicals.

contain both male and female reproductive organs (referred to as intersex). These results indicate that the reproductive potential of fish populations living in streams in which various pharmaceuticals and organic wastewater chemicals are present may be compromised. In an attempt to decrease the contamination of the nation’s waterways, the Office of National Drug Control Policy suggests working with local agencies such as pharmacies or hazardous waste collection sites that hold “take-back days” in which they collect unused medicines and dispose of them properly. Some

states are also exploring a mail/ship back program to collect unused prescriptions. Proper disposal of prescriptions is the first step in keeping our nation’s waterways safe.

Questions to Consider 1. Should each community be held liable for the water quality of the aquatic ecosystems to which it is connected? 2. What do you think should be the highest priority chemicals that should be banned from entering aquatic ecosystems?

b. Shorebirds

Figure 36.19  Seashores.  a. Some organisms of a rocky coast live in tidal pools. b. A sandy shore looks devoid of life except for the birds that feed there.



It is customary to place the organisms of the oceans into either the pelagic division (open waters) or the benthic division (ocean

Figure 36.20  Marine

environment.  Organisms reside in the pelagic

division (blue) where waters are divided as indicated. Organisms also reside in the benthic division (brown) in the surfaces and zones indicated.

Pelagic Division

high-tide mark

Neritic Province

littoral zone

low-tide mark

Oceanic Province

f

el

h ls

e a on ent lz n i a nt or itt co bl

su epipelagic zone

Benthic Division

bathyal zone

mesopelagic zone 1,200 m

bathypelagic zone

continental slop e

120 m

3,000 m

abyssal zone 4,000 m

ne

Pelagic Division

Neritic Province  The abundant sunlight and inorganic nutrients provide for a large concentration of organisms in the neritic province. Phytoplankton, consisting of suspended algae, is food not only for zooplankton but also for small fishes. These small fishes, in turn, are food for larger commercially valuable fishes. Coral reefs are areas of high biological abundance and productivity found in shallow, nutrient-poor, tropical waters. Their chief constituents are stony corals, animals that have a calcium carbonate (limestone) exoskeleton, and calcareous red and green algae. Corals tend to form colonies derived from individual corals that have reproduced by means of budding. Corals provide a home for a microscopic alga called zooxanthella. The corals, which feed

zo

Oceans cover approximately three-quarters of our planet. Climate is driven by the sun, but the oceans play a major role in redistributing heat in the biosphere. Water tends to be warm at the equator and much cooler at the poles because of the distribution of the sun’s rays, as discussed earlier in this chapter (see Fig. 36.1a). Air takes on the temperature of the water below, and then warm air moves from the equator to the poles. In other words, the oceans have a major influence on wind patterns. (The landmasses also play a role, but water holds heat longer and remains cool longer during periods of changing temperature than land.) When the wind blows strongly and steadily across a great expanse of ocean for a long time, friction from the moving air begins to drag the water along with it. Once the water has been set in motion, its momentum, aided by the wind, keeps it moving in a steady flow called a current. Because the ocean currents eventually strike land, they move in a circular path—clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. As the currents flow, they move warm water from the equator to the poles. One such current, called the Gulf Stream, brings tropical Caribbean water to the east coast of North America and the higher latitudes of western Europe. Without the Gulf Stream, Great ­Britain, which has a relatively warm temperature, would be as cold as Greenland. In the Southern Hemisphere, another major ocean current warms the eastern coast of South America. Also, in the Southern Hemisphere, the Humboldt Current flows toward the equator. The Humboldt Current carries phosphorusrich cold water northward along the west coast of South A ­ merica. During a process called upwelling, cold offshore winds cause cold nutrient-rich waters to rise and take the place of warm nutrient-poor waters. In South America, the enriched waters nourish an abundance of marine life that supports the fisheries of Peru and northern Chile. When the Humboldt Current is not as cool as usual, upwelling does not occur, stagnation results, the fisheries decline, and climate patterns change globally. This phenomenon is discussed in the Ecology feature, “El Niño–Southern Oscillation.”

floor). The geographic areas and zones of an ocean are shown in Figure 36.20, and the animals that inhabit the different zones are shown in Figure 36.21. The pelagic division includes the neritic province and the oceanic province.

al

Oceans

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depth (meters)

abyssal plain

ab ys s

and periwinkles. Periwinkles have a coiled shell and secure themselves by hiding in crevices or under seaweeds, while limpets press their single, flattened cone tightly to a rock. Below the littoral zone, macroscopic seaweeds, which are the main photosynthesizers, anchor themselves to the rocks by holdfasts. Organisms cannot attach themselves to shifting, unstable sands on a sandy beach. Therefore, nearly all the permanent beach residents dwell underground. They either burrow during the day and surface to feed at night, or they remain permanently within their burrows and tubes. Ghost crabs and sandhoppers (amphipods) burrow themselves above the high-tide mark and feed at night when the tide is out. Sandworms and sand (ghost) shrimp remain within their burrows in the littoral zone and feed on detritus whenever possible. Still lower in the sand, clams, cockles, and sand dollars can be found.

Chapter 36  Major Ecosystems of the Biosphere



SCIENCE IN YOUR LIFE  ►

ECOLOGY

El Niño–Southern Oscillation Climate largely determines the distribution of La Niña life on Earth. Short-term variations in climate, Pacific Ocean which we call weather, also have a pronounced effect on living organisms. There is no better • Upwelling off the west example than an El Niño. Originally, El Niño coast of South America brings cold waters to the referred to a warming of the seas off the coast strong surface. warm of Peru around Christmas—hence, the name El trade wate winds r Niño, “the boy child,” after Jesus. • Barometric pressure is high over the southNow scientists prefer the term El Niño– prev ailing eastern Pacific. Southern Oscillation (ENSO) to denote a ocea n cu rrent severe weather change brought on by an inter• Monsoons associated with s the Indian Ocean occur. action between the atmosphere and ocean currents. Ordinarily, the southeast trade winds • Hurricanes occur off the cold move along the coast of South America and east coast of the United wate r States. turn west because of Earth’s daily rotation on its axis. As the winds drag warm ocean waters from east to west, there is an upwelling of nutrient-rich cold water from the ocean’s El Niño equator depths, resulting in a bountiful Peruvian harSouth America vest of anchovies. When the warm ocean Australia waters reach their western destination, the • Great ocean warming monsoons bring rain to India and Indonesia. occurs off the west coast weak of the Americas. Scientists have noted that these events correwarm trade wate winds late with a difference in the barometric presr • Barometric pressure is prev sure over the Indian Ocean and the southeastern low over the southailing ocea eastern Pacific. Pacific—that is, the barometric pressure is low n cu rrent s over the Indian Ocean and high over the south• Monsoons associated eastern Pacific. But when a “southern oscillawith the Indian Ocean fail. tion” occurs and the barometric pressures • Hurricanes occur off the cold switch, an El Niño begins. west coast of the United wate r During an El Niño, both the northeast and States. the southeast trade winds slacken. Upwelling no longer occurs, and the anchovy catch off the coast of Peru plummets. During a severe El Figure 36B  La Niña and El Niño. Niño, waters from the east never reach the west, and the winds lose their moisture in the coast so that flooding occurs in the spring. Some these gases allow the sun’s rays to pass through, middle of the Pacific instead of over the Indian parts of the United States, however, benefit from but they trap the heat. Ocean. The monsoons fail, and drought occurs an El Niño. The Northeast is warmer than usual, in India, Indonesia, Africa, and Australia. Har- few if any hurricanes hit the east coast, and there Questions to Consider vests decline, cattle must be slaughtered, and is a lull in tornadoes throughout the Midwest. 1. Recent research has suggested that global warming is contributing to the increased frefamine is likely in highly populated India and Altogether, a severe El Niño affects the weather quency and severity of El Niño events. This Africa, where funds to import replacement over three-quarters of the globe. Eventually, an El Niño dies out, and norcan cause severe drought in India and Africa, supplies of food are limited. In 2009, severe as well as greatly reduce fish catch in parts drought caused fires that killed hundreds of mal conditions return. The normal cold-water of the Southern Hemisphere. Should the people and caused billions of dollars in damage state off the coast of Peru is known as La Niña (the girl). Figure 36B contrasts the weather conresponsibility for decreasing greenhouse gas in the state of Victoria, Australia. emissions be greater for more-developed A backward movement of winds and ocean ditions of a La Niña with those of an El Niño. countries (MDCs), because they produce currents may even occur so that the waters warm Since 1991, the sea surface has been almost cona  greater amount of greenhouse gases to more than 14°F above normal along the west tinuously warm, and two record-breaking El per  capita than less-developed countries coast of the Americas. This is a sign that a severe Niños have occurred. What could be causing (LDCs), even though El Niños have more El Niño has occurred, and the weather changes more of the El Niño state than the La Niña state? severe consequences for LDCs? are dramatic in the Americas also. Southern Cal- Some scientists argue that this environmental ifornia is hit by storms and even hurricanes, and change is related to global warming, a rise in 2. Aside from the recent increases in gasoline prices, would you consider buying a more the deserts of Peru and Chile receive so much environmental temperature due to greenhouse fuel-efficient car or spend more on plane rain that flooding occurs. A jet stream (strong gases in the atmosphere, as discussed in Chapflights to purchase “carbon credits” to do wind currents) can carry moisture into Texas, ter  35. Greenhouse gases, including carbon your part in decreasing the emission of Louisiana, and Florida, with flooding a near cer- dioxide, are released by humans in mass quantigreenhouse gases such as carbon dioxide? tainty. Or the winds can turn northward and ties into the atmosphere by, for example, burnWhy or why not? deposit snow in the mountains along the west ing fossil fuels. Like the glass of a greenhouse,



Chapter 36  Major Ecosystems of the Biosphere

are characteristic of the epipelagic, mesopelagic, and bathypelagic zones of the pelagic division compared to the abyssal zone of the benthic division.

jellyfish phytoplankton

squid

zooplankton

dolphin

barracuda sea turtle

tuna lantern fish

prawn

mackerel

giant squid

Mesopelagic Zone (120 – 1,200 m)

ocean bonito

midshipman

sperm whale viperfish

hagfish

anglerfish

Bathypelagic Zone (1,200 – 3,000 m)

Oceanic Province The oceanic province lacks the inorganic nutrients of the neritic province and, therefore, does not have as high a concentration of phytoplankton, even though it is sunlit. Photosynthesizers are food for a large assembly of zooplankton, which then become food for herrings and bluefishes. These, in turn, are eaten by larger mackerels, tunas, and sharks. Flying fishes, which glide above the surface, are preyed upon by dolphins. Whales are other mammals found in the epipelagic zone (Fig. 36.21). Baleen whales strain krill (small crustaceans) from the water, and toothed sperm whales feed primarily on the common squid. Animals in the mesopelagic zone are carnivores that are adapted to the absence of light, and tend to be translucent, red, or luminescent. There are luminescent shrimps, squids, and fishes, such as lantern and hatchet fishes. The bathypelagic zone is in complete darkness except for an occasional flash of bioluminescent light. Carnivores and scavengers are found in this zone. Strange-looking fishes with distensible mouths and abdomens and small, tubular eyes feed on infrequent prey.

Figure 36.21  Ocean inhabitants.  Different organisms

Epipelagic Zone (0 – 120 m)

at night, and the algae, which photosynthesize during the day, are mutualistic and share materials and nutrients. The close relationship of corals and zooxanthellae is likely the reason coral reefs form only in shallow sunlit water—the algae need sunlight for photosynthesis. A reef is densely populated with life. The baleen whale large number of crevices and caves provide shelter for filter feeders (e.g., sponges, sea squirts, and fan worms) and for scavengers (e.g., crabs and sea urchins). There are many types of small, beautifully colored fishes. Parrotfishes feed directly on corals, and others feed on plankton or detritus. Small fishes become shark food for larger fishes, including snappers and other fish that are caught for human consumption. The barracuda, moray eel, and various shark species are top predators in coral reefs.

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Benthic Division deep-sea shrimp gulper

tripod fish

brittle stars

glass sponges

Abyssal Zone (3,000 m – bottom)

The benthic division includes organisms that live on or in the soil of the continental shelf (sublittoral zone), the continental slope (bathyal zone), and the abyssal plain (abyssal zone) (see Fig. 36.20). Seaweed grows in the sublittoral zone, and can be found in batches on outcroppings as the water

sea cucumber

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UNIT 7  Behavior and Ecology

gets deeper. More diversity of life exists in the sublittoral and bathyal zones than in the abyssal zone. In the first two zones, clams, worms, and sea urchins are preyed upon by starfishes, lobsters, crabs, and brittle stars. Photosynthesizing algae occur in the sunlit sublittoral zone, but benthic organisms in the bathyal zone are dependent on the detritus that falls from the waters above. The abyssal zone is inhabited by animals that live at the soilwater interface of the abyssal plain (Fig. 36.21). It once was thought that few animals exist in this zone because of the intense pressure and the extreme cold. Yet many invertebrates live there by feeding on debris floating down from the mesopelagic zone. Sea lilies rise above the seafloor; sea cucumbers and sea urchins crawl around on the sea bottom; and tube worms burrow in the mud. The flat abyssal plain is interrupted by enormous underwater mountain chains called oceanic ridges. Along the axes of the ridges, crustal plates spread apart, and molten magma from Earth’s core rises to fill the gap. At hydrothermal vents, seawater percolates through cracks and is heated to about 350°C, causing sulfate to react with water and form hydrogen sulfide (H2S). Free-living or mutualistic chemoautotrophic bacteria use energetic electrons derived from hydrogen sulfide to reduce bicarbonate to organic compounds. These compounds support an ecosystem that includes huge tube worms and clams. It was a surprise to find such communities of organisms living so deep in the ocean, where light never penetrates. Life is possible so deep because unlike photosynthesizers, chemoautotrophs are producers that do not require solar energy.

Check Your Progress  36.3 1. Compare and contrast the various freshwater ecosystems. 2. Explain the major characteristics of the two oceanic divisions.

Conclusion DDT was developed in the 1940s to be used as a broad-­ spectrum pesticide. Due to the ecological damage it can lead to, DDT has been banned in the United States. Various countries throughout the world still use it to help combat insects. DDT can persist in the environment for up to 15 years, enabling it to find its way into a variety of ecosystems. When released, it will eventually run off into nearby aquatic ecosystems. This can lead to an accumulation of DDT in the organisms that live in these biomes. It can also evaporate into the atmosphere and get carried across the planet by global air currents. DDT can then be released into communities thousands of miles away from the original source. Health officials are concerned with the harmful effects it can have upon people, including reproductive issues and damage to the liver and nervous ­system. Because of its persistence, DDT has been able to reach every corner of the planet.

MEDIA STUDY TOOLS http://connect.mheducation.com Maximize your study time with McGraw-Hill SmartBook®, the first adaptive textbook.

 

Animations

36.2 Biomes

  Tutorial 36.2  Factors Influencing the Distribution of Biomes

SUMMARIZE

■ Because Earth rotates on its axis daily, the winds blow in opposite

36.1  Climate and the Biosphere

■ Air rising over coastal regions loses its moisture on the windward side

■ Climate refers to the prevailing weather conditions in a particular region. ■ The sun’s rays are spread out over a larger area at the poles than the

vertical rays at the equator because the Earth is spherical. Thus, surface temperatures decrease from the equator toward each pole. ■ The planet is tilted on its axis, and the seasons change depending on which hemisphere is closest to the sun as Earth completes its annual rotation. ■ Warm air rises near the equator, loses its moisture, and then descends at about 30° north and south latitude, and continuing toward the poles. When the air descends, it retains moisture, and therefore, the great deserts of the world are formed at 30° latitudes.

directions above and below the equator.

as the elevation increases. This may cause monsoons, or extended periods of rain. Less rain falls on the leeward side, creating a rain shadow.

36.2  Terrestrial Ecosystems ■ A biome is a major type of terrestrial ecosystem. ■ Temperature and rainfall influence the distribution of biomes around

the world.

■ The tundra is the northernmost biome and consists largely of short grasses

and sedges and dwarf woody plants. Most of the water in the soil is frozen year-round (i.e., permafrost) because of cold winters and short summers.



Chapter 36  Major Ecosystems of the Biosphere

■ The taiga, a coniferous forest, has less rainfall than other types of ■ ■

■ ■

■

forests. The temperate deciduous forest has trees that gain and lose their leaves during spring and fall, respectively. Tropical rain forests are the most complex and productive of all biomes. Many of the plants in the tropics are epiphytes, meaning that they grow on other plants. Shrublands usually occur along coasts that have dry summers, and receive most of their rainfall in the winter. Among grasslands, the savanna, a tropical grassland, supports the greatest number of different types of large herbivores. Prairies, such as those in the mid United States, have a limited variety of vegetation and animal life. Deserts are characterized by rainfall less than 25 cm a year. Plants, such as cacti, are succulents, and others are shrubs with thick leaves— both of which are adaptations to conserve water.

36.3  Aquatic Ecosystems ■ In deep temperate zone lakes, water depth determines the temperature

■

■

■ ■

■

and the concentrations of nutrients and gases. A large input of nutrients into a body of water is called eutrophication. During spring overturn and fall overturn, nutrients from the bottom lake layers are redistributed while the entire body of water is cycled. Lakes and ponds have three life zones. Rooted plants and clinging organisms live in the littoral zone; plankton and fishes live in the ­sunlit limnetic zone; and bottom-dwelling organisms, such as crayfishes and molluscs, live in the profundal zone. Oceans play a major role in determining the climate of the land biomes. Ocean currents help move warm water from the equator toward the poles. During an upwelling, cold offshore winds cause cold nutrientrich waters to rise and take the place of warm nutrient-poor waters. Marine ecosystems are divided into coastal ecosystems and the oceans. The coastal ecosystems, especially estuaries, are more productive than the oceans. Estuaries (e.g., salt marshes, mudflats, and mangrove forests) are near the mouth of a river and are called the nurseries of the sea. Coral reefs contain a tremendous diversity of species and play important roles in the health of the ocean. An ocean is divided into the pelagic division and the benthic division. The pelagic division (open waters) has three zones. The epipelagic zone receives adequate sunlight and supports the most life. The mesopelagic and bathypelagic zones contain organisms adapted to minimum light and no light, respectively. The benthic division (ocean floor) includes organisms living on the continental shelf in the sublittoral zone, the continental slope in the bathyal zone, and the abyssal plain in the abyssal zone. Underwater hydrothermal vents are formed when seawater percolates through cracks and is heated by molten magma from the Earth’s core. Life here is based upon the sulfate reacting with water to form hydrogen sulfide.

ASSESS Testing Yourself Choose the best answer for each question.

36.1  Climate and the Biosphere 1. The seasons are best explained by a. the distribution of temperature and rainfall in biomes. b. the tilt of the Earth as it orbits about the sun.  c. the daily rotation of the Earth on its axis. d. the fact that the equator is warm and the poles are cold. 

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2. Which region on Earth receives the largest amount of direct solar radiation throughout the year? a. Northern Hemisphere above 60° b. Northern Hemisphere between 30° and 60° c. Southern Hemisphere below 60° d. Southern Hemisphere between 30° and 60° e. the equator, between 30° north and 30° south  3. Which biome is associated with the rain shadow of a mountain? a. desert d. coniferous forest b. tropical rain forest e. chaparral c. taiga

36.2  Terrestrial Ecosystems 4. Which of these influences the location of a particular biome? a. latitude b. average annual rainfall c. average annual temperature d. altitude e. All of these are correct. 5. What is the geographic distribution of a temperate deciduous forest? a. the northern part of North America and Eurasia b. just south of the polar caps in the Northern Hemisphere c. eastern North America and eastern Asia d. near the equator in South America and Africa e. central United States and central Asia  6. Which of these pairs is mismatched? a. tundra—permafrost b. savanna—Acacia trees c. prairie—epiphytes d. coniferous forest—evergreen trees 7. The forest with a multilevel canopy is the a. tropical rain forest. b. coniferous forest. c. tundra. d. temperate deciduous forest. 

36.3  Aquatic Ecosystems 8. An estuary acts as a nutrient trap because of the a. action of rivers and tides. b. depth at which photosynthesis can occur. c. amount of rainfall received. d. height of the water table.  9. Phytoplankton are more likely to be found in which life zone of a lake? a. limnetic zone c. benthic zone b. profundal zone d. All of these are correct.  10. El Niño  a. prevents the ocean current from upwelling. b. causes stagnation of the nutrients. c. changes global climate patterns. d. decreases the local fisheries. e. All of these are correct.

ENGAGE Thinking Critically 1. Climate change is predicted to have the least severe consequences in terms of temperature change nearest the equator and more severe consequences progressing into the more temperate zones. Why do you think this is?



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UNIT 7  Behavior and Ecology

2. Scientists have documented a phenomenon called “biodiversity gradients,” which shows that the greatest amounts of species diversity exist nearest the equator and that diversity tends to diminish moving through increasing latitudes. Based on your knowledge of climate, topography, and general characteristics of terrestrial biomes, explain the phenomenon of biodiversity gradients. 3. Based on what you know about tropical forests versus temperate forests, why do you think temperate forest habitats are more suitable for long-term agriculture, despite the fact that they have a shorter growing season than tropical forests? 4. Pharmaceutical companies are “bioprospecting” the tropical rain forests. These companies are looking for naturally occurring compounds in plants or animals that can be used as drugs for a variety of diseases. The most promising compounds act as antibacterial or antifungal agents. Even discounting the fact that the higher density of species in tropical rain forests would produce a wider array of compounds than another biome, why would antibacterial and antifungal compounds be more likely to evolve in this biome? 

PHOTO CREDITS Openers: (Earth): © Ingram Publishing/Alamy RF; (eagle): ©Frank Leung/Getty RF; (penguins): © Brand X Pictures/Getty RF; 36.6a: © U.S. Fish & Wildlife Service; 36.6b: © Roberta Olenick/All Canada Photos/Getty Images; 36.7a: © Creatas/Jupiterimages RF; 36.7b: © Jupiterimages/liquidlibrary/360/Getty RF; 36.8a(Shawnee National Forest): © David S. Rice/Science Source; 36.8b(bobcat): © Tom McHugh/Science Source; 36.8b(marigolds): © 4FR/iStock/360/Getty RF; 36.8b(chipmunk): © Elena Elisseeva/Tetra Images/Getty RF; 36.9(frog): © kikkerdick/Getty RF; 36.9(katydid): © G. Fischer/Blickwinkel/age fotostock; 36.9(jaguar): © Sandy Windelspecht/Ricochet Creative Productions LLC; 36.9(macaw): © PhotoLink/Getty RF; 36.9(butterfly): © Kevin Schafer/Getty Images; 36.9(orchids): © Professor David F. Cox, Lincoln Land Community College; 36.9(lizard): © J.H. Pete Carmichael/Getty Images; 36.10a: © Steven P. Lynch; 36.10b: © Gary Kramer/U.S. Fish & Wildlife Service; 36.11a: © Jim Steinberg/Science Source; 36.11b: © Fuse/Getty RF; 36.12a: © Danita Delimont/Alamy; 36.12b: © Tim Graham/Alamy; 36.12c: © Robert Muckley/Getty RF; 36.12d: © Digital Vision/Getty RF; 36.13a: © Corbis RF; 36.13b: © Bob Calhoun/ Photoshot; 36.13c: © Lee Karney/USFWS; 36.13d: © Kevin Schafer/Getty Images; 36.14a: © Roger Evans/Science Source; 36.14b: © Pat Watson/McGraw-Hill Education; 36.16(pike): © abadonian/iStock/360/Getty RF; 36.16(water strider): © Matti Suopajarvi/Getty RF; 36.17(shrimp): © Ken Lucas/Ardea London Limited; 36.17(snails): © Jack Glisson/Alamy; 36.18a: © Marc Lester/Getty Images; 36.18b: © shakzu/iStock/360/Getty RF; 36A(left): © Mark Dierker/McGraw Hill Education; 36A(right): © Pat Clayton/Image Bank/Getty Images; 36.19a: © NOAA Central Library; 36.19b: © Jeff Greenburg/Science Source.

CASE STUDY Gills Onions’ Waste-to-Energy Project Gills Onions is one of the largest onion producers/processors in the United States. Over 1 million pounds (lb) of onions are processed every day. During processing, Gills Onions generates approximately 300,000 lb of onion waste daily. Previously, the waste was composted and then hauled to local farm fields and spread as fertilizer. The hauling and spreading of the onion compost led to a multitude of problems, including odor, runoff, acidification of the soil, and a significantly large carbon footprint. Nearly $400,000 was spent annually on the disposal of the onion waste. Gills Onions’ Advanced Energy Recovery System (AERS) has enabled them to become the first food-processing facility in the world to produce ultra-clean energy from their own waste. They are able to convert 100% of their daily onion waste into renewable energy and cattle feed. The AERS extracts the juice from the onion peels and uses an anaerobic reactor to produce methane-rich biogas that powers two fuel cells. This electricity is then used to power the onion-processing plant, saving Gills Onions an estimated $700,000 in annual electrical cost. The remaining onion pulp is further processed into cattle feed and sold to local ranchers. Additional savings come from the elimination of $400,000 in annual cost associated with the disposal of the onion waste. Greenhouse emissions have also been reduced by the elimination of the truck traffic required to haul the compost to local fields, thus reducing Gills Onions’ carbon footprint by an estimated 14,500 metric tons (mt) of CO2e (equivalent) emissions per year. The AERS is expected to produce a full payback of the $10.8 million spent in less than six years. As you read through the chapter, think about the following questions:

1. Should all major food processing plants be required to develop and use

37

Conservation Biology

CHAPTER OUTLINE 37.1  Conservation Biology and Biodiversity 37.2 Value of Biodiversity 37.3 Threats to Biodiversity 37.4 Habitat Conservation and Restoration 37.5 Working Toward a Sustainable Society BEFORE YOU BEGIN Before beginning this chapter, take a few moments to review the following discussions: Table 27.1  What is the cause of the current extinction crisis? Section 34.2  What factors are influencing human population growth? Section 35.2  What role does biodiversity play in the energy flow of ecosystems?

systems similar to Gills Onions’ AERS?

2. What are the potential negative consequences associated with Gills Onions’ development of the AERS?

3. Can the indirect values associated with the AERS actually be measured in direct value terms?

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UNIT 7  Behavior and Ecology

37.1  Conservation Biology and Biodiversity Learning Outcomes Upon completion of this section, you should be able to 1. Summarize the goals of conservation biology. 2. Recognize the spectrum of biodiversity.

Conservation biology is an interdisciplinary science with the explicit goal of protecting biodiversity and the Earth’s natural resources. Conservation biology is considered interdisciplinary because it relies on many subdisciplines of biology for the development of basic scientific concepts to describe biodiversity. The application of these concepts, so that biodiversity can be sustainably managed and conserved for future generations, is critical to the future of our planet. Basic Biology

systematics ecology

behavior physiology

field biology

genetics Conservation Biology biopark management

wildlife management

forestry

agronomy

veterinary science Applied Biology

evolutionary biology

Conservation biology has often been called a “crisis discipline” or a “discipline with a deadline” because we are facing a sixth mass extinction. Unlike the five previous mass extinctions that have occurred naturally throughout Earth’s history, the current mass extinction is much more rapid. It is estimated that as many as 10–20% of all species may be extinct in the next 30 to 50 years, and some researchers suggest that we may lose as many as 50% of all species by the year 2100. Thus, it is urgent that we take steps to ensure that everyone understands the concept of biodiversity, the value of biodiversity, the causes of biodiversity loss, and the potential consequences of reduced biodiversity.

Biodiversity At its simplest level, biodiversity is the variety of life on Earth. Biodiversity is commonly described in terms of the number of species found in a given area or ecosystem. To date, approximately 1.6–1.7 million species have been described and catalogued across the globe (Fig. 37.1). This may only be a small fraction of Earth’s species, however. Recent scientific studies of biodiversity suggest that there are probably around 8.7 million species (not counting bacteria or viruses) on the planet, but some think that this number is much higher. Obviously, most species have yet to be discovered and described. According to the U.S. Fish and Wildlife Service (FWS), there are over 470 animal species and 720 plant species in the United States that are in danger of extinction.  Globally, over 30,000 identified species are threatened with extinction. An endangered species is one that faces immediate extinction

range management

fisheries biology

The application of basic scientific principles for conservation and management means that it is often necessary for conservation biologists to work with government officials at both the local and federal levels. Social scientists and economists are also involved because many conservation decisions have socioeconomic impacts in addition to biological ones. Public education is also critical for the success of conservation biology—educating people facilitates better-informed consumer decisions, such as product choices that minimize environmental impact. Conservation biology is unique among the life sciences in that it embodies the following ethical principles: (1) biodiversity is desirable for the biosphere and therefore for humans; (2) humaninduced extinctions are therefore undesirable; (3) complex interactions within ecosystems and communities support biodiversity and the maintenance of such interactions is therefore desirable; and (4)  biodiversity generated by evolutionary change has intrinsic value, regardless of any practical benefit. Because species within communities often interact in subtle and complex ways, disruption to the ecosystems in which they live can have unpredictable and potentially dire consequences. For example, loss of a single, ecologically important species could result in large numbers of secondary extinctions. Thus, disruption of ecosystems or ecosystem functioning needs to be avoided.

plants 346,000

animals 1.1 million

fungi 128,000 bacteria and archaea 10,500 protists 22,200*

viruses-2,900** * includes protozoans and chromists ** not considered by some to be living

Figure 37.1  Number of catalogued species.  Efforts are

underway to catalogue all of the described species on the planet. Currently, between 1.5 to 1.6 million species have been catalogued.

Source: www.catalogueoflife.org/



Chapter 37  Conservation Biology

throughout all or most of its range. Examples of endangered species include the hawksbill sea turtle, California condor, giant panda, and the snow leopard. Threatened species are likely to become endangered in the foreseeable future. Examples of threatened species include the Navaho sedge, northern spotted owl, and coho salmon. To develop a meaningful understanding of life on Earth, we need to know more about species than just their total number. Although we most often think at the species level, ecologists and conservation biologists necessarily describe biodiversity at three other levels of biological organization: genetic diversity, ecosystem diversity, and landscape diversity. Genetic diversity includes the number of different alleles, as well as the relative frequencies of those alleles in populations and species. It is genetic diversity that underlies the evolutionary potential of populations, or their capacity to adapt to future environmental change. Thus, populations with high genetic diversity are more likely to have some individuals that can survive changes in their community or ecosystem. Low genetic diversity made the 1846 potato blight in Ireland and the 1984 outbreak of citrus canker in Florida worse than they would have been if the populations were more genetically diverse. Higher genetic diversity would have likely meant that there would have been a higher proportion of individuals that, by chance, were resistant to the pathogens that decimated these crops. Ecosystem diversity is dependent on the interactions between species and their abiotic environment in a particular area. An ecosystem may have one or several types of communities within it.

bald eagle

kokanee salmon (3 1,000) bald eagles (3 7) opossum shrimp (per m2)

150

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The composition of species within a community may differ dramatically among different community types. Consider the species you would find in a temperate rain forest versus a savannah. Variation in community composition increases the levels of biodiversity in the biosphere. Although past conservation efforts frequently focused on saving particular charismatic species, such as the ­California condor, the black-footed ferret, or the spotted owl, such species-specific methods may not be the most beneficial for all species in the ecosystem. Instead, a more effective approach is to conserve species that play critical roles in their ecosystem, such as those whose loss may result in many secondary extinctions. The reintroduction of wolves in Yellowstone National Park has had a dramatic effect upon the health of the park. Without wolves the elk population significantly increased, which then caused overgrazing of the aspen groves. Wolves have helped reduce the number of elk, thereby increasing the number of aspen and other trees found in floodplains that, in turn, increase the habitat for songbirds and beavers. Beavers have then built dams that have flooded areas, making them suitable for muskrats, ducks, otters, and amphibians. Further, elk carcasses that are left by wolves become food for grizzly bears and eagles. Disrupting a community can threaten the existence of more than one species. Opossum shrimp, Mysis relicta, were introduced into Flathead Lake in Montana and its tributaries as food for salmon. The shrimp ate so much zooplankton that, in the end, there was far less food for the fish and, ultimately, for the grizzly bears and bald eagles (Fig. 37.2).

Figure 37.2  Eagles and bears feed on spawning salmon. 

Humans introduced the opossum shrimp as prey for salmon. Instead, the shrimp competed with salmon for zooplankton as a food source. The salmon, eagle, and bear populations subsequently declined.

Number

100

50

0

grizzly bear 1979 1981 1983 1985 1987 1989 Year Introduction of Opossum Shrimp

zooplankton

kokanee salmon

opossum shrimp (Mysis relicta)



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Landscape diversity involves a group of interacting ecosystems within a single landscape. For example, plains, mountains, and rivers may be fragmented to the point that they are connected only by patches (or corridors) that allow organisms to move from one ecosystem to another. Fragmentation of the landscape may reduce reproductive capacity by increasing predation risk during dispersal between suitable habitat patches or creating difficulty in finding mates.

Check Your Progress  37.1 1. Describe how conservation biology is supported by a variety of disciplines.

2. Explain why ecosystem-level conservation may be more important than species-level conservation.

Wild species, like the rosy periwinkle, Catharanthus roseus, are sources of many medicines.

37.2  Value of Biodiversity Learning Outcomes Upon completion of this section, you should be able to 1. Compare the direct and indirect values of biodiversity. 2. Describe the role that biodiversity plays in a natural ecosystem.

Conservation biology strives to reverse the trend toward the extinction of thousands of plants and animals. To achieve this goal, it is necessary to make all people aware that biodiversity is a resource of immense value.

Direct Value

Wild species, like the nine-banded armadillo, Dasypus novemcinctus, play a role in medical research.

Various individual species perform services for humans and ­contribute greatly to the value we should place on biodiversity. Figure 37.3 gives examples of the direct value of wildlife.

Medicinal Value Most of the prescription drugs used in the United States, valued at over $200 billion, were originally derived from living organisms. The rosy periwinkle from Madagascar is an excellent example of a tropical plant that has provided us with useful medicines. Potent chemicals from this plant are now used to treat two forms of cancer: leukemia and Hodgkin disease. Because of these drugs, the survival rate for childhood leukemia has improved from 10 to 90%, and Hodgkin disease is usually curable. Although the value of saving a life cannot be calculated, it is still sometimes easier for us to appreciate the worth of a resource if it is explained in monetary terms. Thus, researchers estimate that, judging from the success rate in the past, hundreds of additional types of drugs are yet to be found in tropical rain forests, and the value of this resource to society is probably $147 billion. You may already know that the antibiotic penicillin is derived from a fungus, and that certain species of bacteria produce the antibiotics tetracycline and streptomycin. These drugs have proven to be indispensable in the treatment of various sexually transmitted diseases (STDs). Leprosy is among those diseases for which, at

Wild species, like ladybugs, play a role in biological control of agricultural pests.

Figure 37.3  Direct value of wildlife.  The direct services of wild

species benefit humans immensely. It is sometimes possible to calculate the monetary value, which is always surprisingly large.

this time, there is no cure. The bacterium that causes leprosy will not grow in the laboratory, but scientists discovered that it grows naturally in the nine-banded armadillo. Having a source of the bacterium may make it possible to find a cure for leprosy. The blood of horseshoe crabs contains a substance called limulus amoebocyte lysate, which is used to ensure that medical devices, such as pacemakers, surgical implants, and prosthetic devices, are



Chapter 37  Conservation Biology

free of bacteria. Blood is taken from 250,000 crabs a year, which are then returned to the sea unharmed.

Agricultural Value Crops such as wheat, corn, and rice are derived from wild plants that have been modified to be high producers. The same highyield, genetically similar strains tend to be grown worldwide. When rice crops in Africa were being devastated by a virus, researchers grew wild rice plants from thousands of seed samples until they found one that contained a gene for resistance to the virus. These wild plants were then used in a breeding program to transfer the gene into high-yield rice plants. If this variety of wild rice had become extinct before it could be discovered, rice cultivation in Africa might have collapsed.

SCIENCE IN YOUR LIFE   ►

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Biological pest controls—natural predators and parasites— are often preferable to using chemical pesticides. When a rice pest called the brown planthopper became resistant to pesticides, farmers began to use natural brown planthopper enemies instead. The economic savings were calculated at well over $1 billion. Similarly, cotton growers in Cañete Valley, Peru, found that pesticides were no longer working against the cotton aphid because they had evolved resistance. Research identified natural predators that are now being used to an ever-greater degree by cotton farmers. The savings have been enormous. Most flowering plants are pollinated by animals such as bees, wasps, butterflies, beetles, birds, and bats. Honeybees play a major role in this process. As is explored in the Ecology feature, “Colony Collapse Disorder,” honeybee populations worldwide are decreasing, which presents a serious threat to modern agriculture.

ECOLOGY

Colony Collapse Disorder Imagine standing in the produce section of your supermarket. You’re shocked to see that there are no apples, cucumbers, broccoli, onions, pumpkins, squash, carrots, blueberries, avocados, almonds, or cherries. This could happen at grocery stores in the future. All the crops mentioned, as well as many others, are dependent on honeybees for pollination. Your diet and $15 billion worth of crops might suffer if the honeybees aren’t available to perform their important job of moving pollen. Although there are wild bee populations that pollinate crops, domestic honeybees are easily managed and transported from place to place when their pollination services are needed. Colonies of honeybees have experienced a number of health problems since the 1980s. Mites—animals similar to ticks—have always been a danger for bees. Varroa mites and tracheal mites were early causes of colony stress and bee deaths (Fig. 37A). However, beekeepers were very alarmed in 2006 when entire colonies of bees began to vanish. Researchers started referring to the phenomenon as colony collapse disorder (CCD). There doesn’t appear to be one factor that causes seemingly healthy bees to vanish from their hives. Scientists now believe that multiple factors may stress bees, causing them to be vulnerable to infection by a parasite or pathogen. The indiscriminate use of pesticides (specifically, neonicotinoids), the strain of being moved from place to place to pollinate crops, and/or poor nutrition (because genetically engineered plants don’t provide as much food for the bees) may contribute to CCD.

mites

Figure 37A  Our primary pollinator.  Honeybee populations are decreasing as a result

of colony collapse disorder (CCD).

CCD is also occurring in the honeybee populations in other countries. Ideally, a causeand-effect treatment for CCD will be found soon, thanks to worldwide research dedicated to solving the problem, as well as improved funding from agricultural agencies. Until then, there are things you can do to keep bees in your area healthy. Research and then plant native plants in your yard and garden. These typically require less fertilizer and water than other plants, and they will provide more pollen and nectar for the bees. In the southern and midwestern regions of the United States, bees enjoy red clover, foxglove, bee balm, and ­joe-pye weed. Desert willow and manzanita

will attract desert bees. Choose palms for tropical areas. In addition, native plants that flower at different times of the year will provide a constant food source. Midday is typically when bees are out foraging, so if you have to use pesticides, apply them late in the day. Plants that rely on the honeybees for pollination—as well as your body—will thank you!

Questions to Consider 1. What other environmental factors may be contributing to CCD? 2. What other species might be suffering ­similar problems?



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Consumptive Use Value We have experienced much success in cultivating crops, keeping domesticated animals, growing trees on plantations, and so forth. But so far, aquaculture, the growing of fish and shellfish for human consumption, has contributed only minimally to human welfare. Instead, most freshwater and marine harvests depend on wild animals, such as fishes (e.g., trout, cod, and tuna) and crustaceans (e.g., lobsters, shrimps, and crabs). Obviously, these aquatic organisms are an invaluable biodiversity resource. The environment provides all sorts of other products that are sold in the marketplace worldwide, including wild fruits and vegetables, skins, fibers, beeswax, and seaweed. Also, some people obtain their meat directly from the wild. In one study, researchers calculated that the economic value of wild pig in the diet of native hunters in Sarawak, East Malaysia, was approximately $40 million per year. Similarly, many trees are still felled for their wood. Researchers have calculated that a species-rich forest in the Peruvian Amazon region is worth far more if the forest is used for fruit and rubber production than for timber production. Whereas fruit and the latex needed to produce rubber can be brought to market indefinitely, no more timber or other resulting products can be produced when trees are cut down.

Indirect Value The wild species we have been discussing live in ecosystems. If we want to preserve them, many scientists argue it is more economical to save the ecosystems than the individual species. Ecosystems perform many services for modern humans. These services are said to be indirect because they are pervasive and it is not easy to associate a direct dollar value to them. Human survival depends on the functions that natural ecosystems perform for us. Studies have suggested that the indirect value of ecosystem services surpasses that of the global gross national product—about $33 trillion per year!

Biogeochemical Cycles You’ll recall from Chapter 35 that ecosystems are characterized by energy flow and chemical cycling. The biodiversity within ecosystems contributes to the workings of the water, carbon, nitrogen, phosphorus, and other biogeochemical cycles. We are dependent on these cycles for fresh water, removal of carbon dioxide from the atmosphere, uptake of excess soil nitrogen, and provision of phosphate. When human activities upset the usual workings of biogeochemical cycles, the dire environmental consequences include the release of excess pollutants that are harmful to us. Unfortunately, technology is unable to artificially replicate any of the biogeochemical cycles.

Waste Disposal Decomposers break down dead organic matter and other types of wastes to inorganic nutrients that are used by the producers within ecosystems. This function aids humans immensely because we dump millions of tons of waste material into natural ecosystems each year. If it were not for decomposition, waste would soon cover the entire surface of our planet. We can build sewage treatment

plants, but they are expensive. In addition, few of them break down solid wastes completely to inorganic nutrients. It is less expensive and more efficient to water plants and trees with partially treated wastewater and let soil bacteria cleanse it completely. Biological communities are also capable of breaking down and immobilizing pollutants like heavy metals and pesticides that humans release into the environment. A review of wetland functions in Canada assigned a value of $50,000 per hectare (2.5 acres or 10,000 m2) per year to the ability of natural areas to purify water and take up pollutants.

Provision of Fresh Water Few terrestrial organisms are adapted to living in a salty ­environment—they need fresh water. The water cycle continually supplies fresh water to terrestrial ecosystems. Humans use fresh water in innumerable ways, including drinking and irrigation of their crops. Freshwater ecosystems, such as rivers and lakes, also provide us with fish and other types of edible organisms. Unlike other commodities, there is no substitute for fresh water. We can remove salt from seawater to obtain fresh water, but the cost of desalinization is about four to eight times the average cost of fresh water acquired via the water cycle.

Flood Prevention Forests and other natural ecosystems exert a “sponge effect,” thereby reducing flooding. They soak up water and then release it at a regular rate. When rain falls in a natural area, plant foliage and dead leaves lessen its impact, and the soil slowly absorbs it, especially if the soil has been aerated by organisms. The water-holding capacity of forests reduces the possibility of flooding. The value of a marshland outside Boston, Massachusetts, has been estimated at $72,000 per hectare per year based solely on its ability to reduce floods. Flooding in New Orleans by Hurricane Katrina in 2005 would likely have been much less severe if the natural wetlands and marshlands around the Gulf of Mexico had still been intact.

Prevention of Soil Erosion Intact ecosystems naturally retain soil and prevent soil erosion. The importance of this ecosystem attribute is especially observed following deforestation. In Pakistan, the world’s largest dam, the Tarbela Dam, is losing its storage capacity of 12 billion cubic meters (m3) many years sooner than expected because silt is building up behind the dam due to deforestation. At one time, the ­Philippines exported $100 million worth of oysters, mussels, and clams each year. Now, silt carried down rivers following deforestation is smothering the mangrove ecosystem that serves as a nursery for the sea. In general, most coastal ecosystems are not as bountiful as they once were because of deforestation, coastal development, and aquatic pollution.

Regulation of Climate At the local level, trees provide shade, reduce the need for fans and air conditioners during the summer, and provide buffers from noise. In tropical rain forests, trees maintain regional rainfall, and without them the forests become arid.



Chapter 37  Conservation Biology

Globally, forests ameliorate the climate because they take up carbon dioxide and release oxygen. The leaves of trees use carbon dioxide when they photosynthesize, and the bodies of the trees store carbon. When trees are cut and burned, carbon dioxide is released into the atmosphere. Carbon dioxide makes a significant contribution to global warming, which is expected to be stressful for many plants and animals.

Ecotourism

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37.3  Threats to Biodiversity Learning Outcomes Upon completion of this section, you should be able to 1. Classify the five causes of extinction. 2. Compare natural and human influenced causes of extinction.

1. Identify the differences between consumptive use value

We are presently in a biodiversity crisis—the number of extinctions (loss of species) expected to occur in the near future is unparalleled in Earth’s history. To identify the role that humans are playing in modern extinctions, researchers examined the records of 1,880 threatened and endangered wild species in the United States. Habitat loss is the most significant factor that was involved in 85% of the cases (Fig. 37.4a). Exotic species had a hand in nearly 50%, pollution was a factor in 24%, overexploitation in 17%, and disease in 3%. Note that the percentages add up to more than 100% because most of these species are imperiled for more than one reason. These five causes reflect the relative importance of the five leading causes for species’ extinctions worldwide. Macaws illustrate that a combination of factors can lead to a species decline (Fig. 37.4b). Not only has their habitat been reduced by encroaching timber and mining companies, but macaws are also hunted for food and collected for the pet trade.  DNA technology may now help enforce laws to protect wildlife collected for the pet trade, as described in the Scientific Inquiry feature, “Wildlife Conservation and DNA.”

2. Explain the difference between direct and indirect values of

Habitat Loss

Many people prefer to vacation in natural areas. In the United States, nearly 100 million people spend nearly $5 billion each year on fees, travel, lodging, and food associated with ecotourism. Tourists are often interested in activities such as sport fishing, whale watching, boat riding, hiking, birdwatching, and the like. Some merely want to immerse themselves in the beauty and serenity of nature. Many underdeveloped countries in tropical regions, such as Belize and Costa Rica, are taking advantage of this by offering “ecotours” of the local biodiversity. Providing guided tours of natural ecosystems, such as tropical rain forests, is often more profitable than destroying them. Ecotourism also provides indirect value for wildlife. As an example, the tusks of an elephant are worth around $10,000 for the ivory, but the Kenyan government estimates that a single elephant is worth as much as $1.2 million in tourist dollars.

Check Your Progress  37.2 and agricultural value of biodiversity. biodiversity.

Habitat loss has occurred in all ecosystems, and human disruption of natural habitats is the most influential factor in biodiversity loss. Concern has now centered on tropical rain forests and coral reefs because they are particularly rich in species diversity. A sequence of events in Brazil offers a fairly typical example of the manner in which rain forest is converted to land uninhabitable for wildlife. The construction of a major highway into the forest first provided a way to reach the interior of the forest. Habitat Loss Small towns and industries sprang up along the highway, and roads branching Alien Species off the main highway gave rise to even Pollution more roads. The result was fragmentaOverexploitation tion of the once immense forest. The government offered subsidies to anyone Disease willing to take up residence in the forest, and the people who came cut and burned 0 20 40 60 80 100 trees in patches to facilitate grazing. % Species Affected by Threat Tropical soils contain limited nutrients, and burning of trees releases additional a. Threats to wildlife nutrients that support cattle grazing for b. Macaws only about three years. Once the land was degraded, the farmers moved on to Figure 37.4  Habitat loss.  a. In a study that examined records of imperiled U.S. plants and animals, another portion of the forest to start over habitat loss emerged as the greatest threat to wildlife. b. Macaws that reside in South American tropical rain again. forests are endangered for some of the reasons listed in the graph (a). 



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UNIT 7  Behavior and Ecology

SCIENCE IN YOUR LIFE  ►

SCIENTIFIC INQUIRY

Wildlife Conservation and DNA After DNA analysis, scientists were amazed to find that some 60% of loggerhead turtles found drowned in Mediterranean fishing nets were from beaches in the southeastern United States. Because the unlucky creatures were a good representative sample of the turtles in the area, that meant more than half of the young turtles living in the Mediterranean Sea had hatched from nests on beaches in Florida, Georgia, and South Carolina (Fig. 37Ba). Some 20,000 to 50,000 loggerheads die each year due to the Mediterranean fisheries, which may partly explain the decline in loggerheads nesting on southeastern U.S. beaches for the last 25 years. Jaguars (Panthera onca) (Fig. 37Bb) are the third largest cats in the world behind lions and tigers. They are the largest cats in the Western Hemisphere. Their natural range extends from as far north as Mexico to as far south as Argentina. Currently, they are listed as “Near Threatened” by the International Union for the Conservation of Nature (IUCN). Conservation of this top-level predator, which has an extensive range, requires support from all the countries that are home to jaguars. Detailed genetic analysis of jaguar DNA has indicated that whether they live in Mexico, Argentina, or anywhere in-between, they are all the same species. They are the only wideranging carnivore in the world that shows genetic continuity across their entire range. This genetic information led to the formation of the Jaguar Corridor Initiative (JCI), whose goal is to create a genetic corridor (a habitat corridor that allows gene flow) that links ­jaguar populations in all of the 18 countries in Latin America, from Mexico to Argentina, hopefully ensuring the survival of this species. In what will become a classic example of how DNA analysis might be used to protect endangered species from future ruin, scientists from the United States and New Zealand carried out discreet experiments in a Japanese hotel room on whale sushi bought in local markets. Sushi, a staple of the Japanese diet, is rice and meat wrapped in seaweed. Armed

a.

b.

Figure 37B  DNA studies.  Conservation of the  (a) loggerhead turtle and (b) jaguar is being aided by DNA analysis.

with a miniature DNA sampling machine, the scientists found that, of the 16 pieces of whale sushi they examined, many were from whales that are endangered or protected under an international moratorium on whaling. “Their findings demonstrated the true power of DNA studies,” says David Woodruff, a conservation biologist at the University of California, San Diego. One sample was from an endangered humpback, four were from fin whales, one was from a northern minke, and another from a beaked whale. Stephen Palumbi, of Harvard University, says the technique could be used for monitoring and verifying catches. Until then, he says, “no species of whale can be considered safe.” Meanwhile, the U.S. Fish and Wildlife Service Forensics Laboratory in Ashland, Oregon, is already on the watch for wildlife crimes in the United States and 122 other countries that send samples to them for analysis. The lab has blood samples, for example, for all the

Loss of habitat also affects freshwater and marine biodiversity. Coastal degradation is mainly due to the large concentration of people living on or near the coast. At least 40% of the world’s population lives within 100 km (60 mi) of a coastline, and this number is expected to increase. In the United States, over one-half of the population lives within 80 km (50 mi) of the coasts (including the Great

wolves being released into Yellowstone National Park. The lab has many cases currently pending in court that cannot be discussed. However, one story tells of the lab’s first DNA-matching case. Shortly after the lab opened in 1989, California wildlife authorities contacted the director of the lab. They had seized the carcass of a trophy-sized deer from a hunter. They believed the deer had been shot illegally on a 3,000-acre preserve. The agents found a gut pile on the property but had no way to match it to the carcass. The hunter had two witnesses to deny the deer had been shot on the preserve. However, lab analysis made a perfect match between tissue from the gut pile and tissue from the carcass.

Questions to Consider 1. How might DNA analysis be used to prevent the poaching of endangered species? 2. What benefits would occur if there was an  international database of DNA from endangered and threatened species?

Lakes). Coastal habitation leads to beach erosion and direct inputs of pollutants into freshwater or marine systems. Already, 60% of coral reefs have been destroyed or nearly so, with siltation resulting from beach erosion as a leading cause. It is possible that all coral reefs may disappear during the next 40 years unless our behaviors drastically change. Mangrove forest destruction is also a problem.



Indonesia, with the most mangrove acreage, has lost 45% of its mangroves, and the percentage is even higher for other tropical countries. Wetland areas, estuaries, and seagrass beds are also being rapidly destroyed by the actions of humans.

Alien Species Alien species, sometimes also called exotic species, are nonnative members of a community. Communities around the world are characterized by unique assemblages of species that have evolved together in an area. Introduction of exotic species can disrupt this balance by changing the interactions between species in a food web, as shown in Figure 37.2. In this example, opossum shrimp introduced in a lake in Montana added a trophic level that, in the end, meant less food for bald eagles and grizzly bears. Often, exotic species can directly compete with or prey upon native species, thereby reducing their abundance or causing localized extinction (Fig 37.5). For these reasons, exotics are the second most important reason for biodiversity loss. Humans have introduced exotic species into new ecosystems by the following means: Colonization. Europeans, in particular, brought various familiar species with them when they colonized new places. For example, the pilgrims brought the dandelion to the United States as a familiar salad green. The British brought foxes and stoats (a type of weasel) to New Zealand, and these predators have resulted in the loss of nearly 40% of all of New Zealand’s bird species. These birds are easy prey because they evolved without mammalian predators, which were completely absent from New Zealand. Horticulture and agriculture. Some exotics now taking over vast tracts of land have escaped from cultivated areas. Kudzu is a vine from Japan that the U.S. Department of Agriculture thought would help prevent soil erosion. The plant now covers much of the landscape in the South, including walnut, magnolia, and gum trees. Similarly, the water hyacinth was introduced to the United States from South America because of its

a. Kudzu

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beautiful flowers. Today, it clogs up waterways and diminishes natural diversity. Accidental transport. Global trade and travel accidentally bring many new species from one country to another. The zebra mussel from the Caspian Sea was accidentally introduced into the Great Lakes in 1988. It now forms dense beds that reduce biodiversity by outcompeting native mussels. Zebra mussels also decrease the amount of food available for higher levels in the food chain because they are such effective filter feeders. Zebra mussels cause millions in damage per year by clogging sewage and other pipes. Other exotics introduced into the United States include the Argentinian fire ant and the nutria, a large rodent found throughout the Southeast.

Exotics on Islands Islands are particularly susceptible to environmental discord caused by the introduction of exotic species. Islands have unique assemblages of native species that are closely adapted to one another and cannot compete well against exotics. Myrtle trees, Myrica faya, introduced into the Hawaiian Islands from the Canary Islands, are symbiotic with a type of bacterium that is capable of nitrogen fixation. This feature allows the species to establish itself on nutrient-poor volcanic soil such as that found in Hawaii. Once established, myrtle trees halt normal ecological succession by outcompeting native plants on volcanic soil. The brown tree snake has been introduced onto a number of islands in the Pacific Ocean. The snake eats eggs, nestlings, and adult birds. On Guam, it has reduced ten native bird species to the point of extinction. Mongooses introduced into the Hawaiian Islands to control rats have increased dramatically in abundance and also prey on native birds, causing some population declines (Fig. 37.5b).

Pollution In the present context, pollution can be defined as any environmental change that adversely affects the lives and health of living

b. Mongoose

Figure 37.5  Exotic species.  a. Kudzu, a vine from Japan, was introduced in several southern states to control erosion. Today, kudzu has taken over and displaced many native plants. b. Mongooses were introduced into Hawaii to control rats, but they also prey on native birds.



UNIT 7  Behavior and Ecology

organisms. Pollution has been identified as the third main cause of extinction. Pollution can also weaken organisms and make them more susceptible to disease. Biodiversity is particularly threatened by the following types of environmental pollution: Acid deposition. Sulfur dioxide from power plants and nitrogen oxides from auto exhaust are converted to acids when combined with atmospheric water vapor. These acids return to Earth during precipitation (rain or snow) or via dry deposition. This acid deposition decimates forests because it weakens trees and increases their susceptibility to disease and insects. In addition, because acid deposition can make the pH of water too low for organisms to survive, many lakes in northern states are now lifeless. Eutrophication. Lakes are also under stress due to overenrichment. When lakes receive excess nutrients due to runoff from agricultural fields and wastewater from sewage treatment, algae begin to grow in abundance. Death of algae leads to an abundance of decomposers that decreases the available oxygen and often leads to fish kills. Ozone depletion. The ozone shield protects Earth from harmful ultraviolet (UV) radiation. The release of chlorofluorocarbons (CFCs), in particular, into the atmosphere causes the shield to break down, leading to impaired growth of crops and trees and the death of plankton that sustain oceanic life. The immune systems and ability of all organisms to resist disease will most likely be weakened. Ozone depletion has also led to dramatic increases in skin cancer. Organic chemicals. Our modern society uses organic chemicals in all sorts of ways. Organic chemicals called nonylphenols are used in products ranging from pesticides to dishwashing detergents, cosmetics, plastics, and spermicides. Many of these chemicals can mimic the effects of hormones, and if so, they are called endocrine-disruptors. For example, investigators exposed young salmon to nonylphenol. Although these fish are born in fresh water and mature in salt water, 20–30% of fish exposed to nonylphenol were unable to make the transition from fresh to salt water. Nonylphenols cause the pituitary to produce prolactin, a hormone that may prevent saltwater adaptation. Other endocrine-disrupting contaminants can possibly affect the endocrine system and thereby the reproductive potential of other animals, including humans. Climate change. The term climate change refers to recent changes in the Earth’s climate. The major contributor to climate change is the phenomenon of global warming, which is an increase in Earth’s temperature due to the increase of greenhouse gases, such as carbon dioxide and methane, in the atmosphere. Climate change is expected to have many detrimental effects, including the destruction of coastal wetlands due to a rise in sea levels (Fig. 37.6), a shift in suitable temperatures in locations where various species cannot live, and the death of coral reefs. An upward shift in temperatures could influence everything from growing seasons in plants to migratory ­patterns in animals. As temperatures rise, regions of suitable climate for various terrestrial species may shift toward the poles and higher elevations. Extinctions are expected because the present assemblages of species in ecosystems will be

Mean Global Temperature Change (°C)

752

5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 –0.5

maximum likely increase most probable temperature for a twofold increase in CO2

minimum likely increase

1860

1900

1940

1980 Year

2020

2060

2100

a.

b.

Figure 37.6  Climate change and global warming.  a. Mean

global temperature is expected to rise due to the introduction of greenhouse gases into the atmosphere. b. An increase in the global temperature causes glaciers to melt, thus increasing sea level and threatening coastal ecosystems.

disrupted as some species migrate northward (or southward in the Southern Hemisphere) following the environmental changes. Other species with limited mobility may not be able to migrate quickly enough. To remain in a favorable habitat, it’s been calculated that the rate of beech tree migration would have to be 40 times faster than has ever been observed.

Overexploitation Overexploitation occurs when the number of individuals taken from a wild population is so great that the population becomes severely reduced in numbers. A positive feedback cycle explains overexploitation: the smaller the population, the more commercially valuable its members, and the greater the incentive to capture the few remaining organisms. Poachers are very active in the collecting and sale of endangered and threatened species because it has become so lucrative. The overall international value of trading wildlife species is $20 billion, of which $8 billion is attributed to the illegal sale of rare species.



Markets for rare plants and exotic pets support both legal and illegal trade in wild species. Rustlers dig up rare cacti, such as the crested saguaros, and sell them to gardeners for as much as $15,000 each. Parrots are among birds taken from the wild for sale to pet owners. For every bird delivered alive, many more have died in the process. The same holds true for tropical fish, which often come from the coral reefs of Indonesia and the ­Philippines. Divers dynamite reefs or use plastic squeeze-bottles of cyanide to stun the fish within them. In the process, many fish and corals die. The Convention on International Trade in Endangered Species (CITES) was an agreement established in 1973 to ensure that international trade of species does not threaten their survival. Today, over 35,600 species of plants and animals receive some level of protection from over 172 countries worldwide. Poachers still hunt for hides, claws, tusks, horns, or bones of many endangered mammals. Because of its rarity, a single Siberian tiger is now worth more than $500,000—its bones are pulverized and used as a medicinal powder. The horns of rhinoceroses become ornate carved daggers, and their bones are also ground up to sell as a medicine. The ivory of an elephant’s tusk is used to make art objects, jewelry, or piano keys. The fur of a Bengal tiger sells for as much as $100,000 in Tokyo. The Food and Agricultural Organization of the United Nations tells us that we have now overexploited 11 of 15 major oceanic fishing areas. Fish are a renewable resource if harvesting does not exceed the ability of the population to reproduce. Our society uses larger and more efficient fishing fleets to decimate fishing stocks. Pelagic species such as tuna are captured by purse seine fishing, in which a very large net surrounds a school of fish, and then the net is closed in the same manner as a drawstring purse. Up to thousands of dolphins that swim above schools of tuna are often captured and then killed in this type of net. However, many tuna suppliers advertise their product as “dolphin safe.” Other fishing boats drag huge trawling nets, large enough to accommodate 12 jumbo jets, along the seafloor to capture bottom-dwelling fish (Fig. 37.7a). Only large fish are kept; undesirable small fish and sea turtles are discarded, dying, back into the ocean. Trawling has been called the marine equivalent of clear-cutting trees because after the net goes by, the sea bottom is devastated (Fig. 37.7b). Today’s fishing practices don’t allow fisheries to recover. Cod and haddock, once the most abundant bottom-dwelling fish along the northeast coast of the United States, are now often outnumbered by dogfish and skate. Entire marine ecosystems can be disrupted by overfishing, as exemplified on the U.S. west coast. When sea otters began to decline in numbers, investigators found that they were being eaten by orcas (killer whales). Usually orcas prefer seals and sea lions to sea otters, but they began eating sea otters when few seals and sea lions could be found. The decline in the seal and sea lion population was due to the decline in the perch and herring populations as a result of overfishing. Ordinarily, sea otters keep the population of sea urchins, which feed on kelp, under control. But with fewer sea otters around, the sea urchin population exploded and decimated the kelp beds. Thus, overfishing set in motion a chain of events that detrimentally altered the food web of an ecosystem.

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a. Fishing by use of a drag net a

b. Result of drag net fishing

Figure 37.7  Trawling.  a. These fish were caught by dragging a net along the seafloor. b. Appearance of the seafloor after the net passed.

Disease Wildlife is subject to emerging diseases just as humans are. Due to the encroachment of humans on their habitat, wildlife have been exposed to new pathogens from domestic animals living nearby. For example, canine distemper was spread from domesticated dogs to lions in the African Serengeti, causing population declines. Avian influenza likely emerged from domesticated fowl (e.g., chicken) populations and has led to the deaths of millions of wild birds. The significant effect of diseases on biodiversity is underscored by National Wildlife Health Center findings that almost half of sea otter deaths along the coast of California are due to infectious diseases. Scientists tell us the number of pathogens that cause disease is on the rise, and just as human health is threatened, so is that of wildlife. Extinctions due simply to disease may occur, and are currently implicated in the worldwide decline of amphibians (Fig. 37.8).



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1. Discuss two ways in which exotic species are introduced

contain large numbers of endemic species, or those not found anywhere else. An example of such a hotspot is the tropical forests of Madagascar, within which 93% of the primate species, 99% of the amphibian species, and over 80% of the plant species are endemic. The Cape region of South Africa, Indonesia, the coast of California, and the Great Barrier Reef of Australia are also considered biodiversity hotspots. We can also focus our efforts on conserving habitat for ­keystone species, or those whose loss would result in a great number of secondary extinctions. Keystone species need not be the most abundant. Although they are relatively low in numbers, wolves can be considered a keystone species in Yellowstone National Park (see chapter opener). Bats have also been designated keystone species in Old World tropical forests. Bats are extremely important pollinators and dispersers of tree seeds. The loss of bats results in failure of many tree species to reproduce and can lead to a loss of biodiversity. Other keystone species include grizzly bears (Fig. 37.9a), beavers in wetlands, and elephants in grasslands and forests. Keystone species should not be confused with flagship ­species, or those that evoke an emotional response from humans. Flagship species are considered charismatic and are valued for their beauty, regal nature, or similarity to people’s pets. Flagship species such as lions, tigers, dolphins, and the giant panda can motivate the public to conserve biodiversity.

2. Identify the five main causes of extinction. 3. Recognize various reasons why pollution can impact

Landscape Conservation and Reserve Design

Figure 37.8  Amphibian at risk.  The harlequin toad is nearly extinct due to infections by a fungal pathogen. Diseases are implicated in global population declines and extinctions of amphibians.

Check Your Progress  37.3 into new areas.

biodiversity.

37.4  Habitat Conservation and Restoration Learning Outcomes Upon completion of this section, you should be able to 1. Describe the value of preserving biodiversity hotspots. 2. Distinguish between keystone species and flagship species. 3. Summarize the goals of habitat restoration.

Habitat Conservation Because habitat loss is the leading cause of species’ extinctions, conservation of habitat is of primary concern. One way to prioritize which habitats to conserve is to focus on those that contain the highest levels of biodiversity. Generally, biodiversity is highest at the tropics, and it declines toward each pole. Tropical rain forests and coral reefs are examples of ecosystems known for possessing high levels of biodiversity. Some regions of the world are called biodiversity hotspots because they contain unusually large concentrations of species. Biodiversity in these hotspots accounts for about 44% of all known higher plant species and 35% of all terrestrial vertebrate species but covers only about 1.4% of Earth’s land area. Hotspots may also

Conservation often has to occur at the landscape level because sufficient habitat may not be available in a single place to sustain a viable population of a particular species. Grizzly bears, once numbering between 50,000 and 100,000 south of Canada, are now estimated at about 1,000 individuals in six small, subdivided populations. Grizzly bear conservation entails maintenance of a number of different types of ecosystems, including plains, mountains, and rivers. Other species also rely on multiple ecosystem types, such as amphibians, that rely on wetlands and terrestrial habitat. Conserving a single one of these ecosystems would not be sufficient, nor would conservation of several ecosystems in isolation of one another. It is thus necessary to establish conservation corridors that allow animals to move safely between habitats that would otherwise be isolated. Corridors are often necessary because landscapes are subdivided due to urbanization, agriculture, and other aspects of human development. A corridor may be as small as an overpass for wildlife to cross a highway or a strip of forested habitat that is maintained along a stream after timber harvest. They can be as large as a patch of land several kilometers wide to allow for animals to follow migration routes. Landscape conservation for a single species is often beneficial for other species that share the same habitats. For example, conservation of the northern spotted owl (Fig. 37.9b) helps protect many other species that inhabit temperate old growth forests. The last of the contiguous 48 states’ harlequin ducks, westslope cutthroat trout, lynx, pine martins, wolverines, and great gray owls are found in areas occupied by grizzly bears. The geographic range of grizzly bears also overlaps with 40% of Montana’s vascular plants of conservation concern.



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30.55% increasing percentage of patch influenced by edge effects 43.75%

64% habitat patch a.

88.8% area subject to edge effect

a. Grizzly bear, Ursus arctos horribilis

brown-headed cowbird chick yellow warbler chick

b. Old-growth forest; northern spotted owl, Strix occidentalis caurina (inset)

Figure 37.9  Habitat

preservation.  When particular species are protected, other wildlife benefits. a. The Greater Yellowstone Ecosystem has been delineated in an effort to save grizzly bears, which need a very large habitat. b. Currently, the remaining portions of old-growth forests in the Pacific Northwest are not being logged in order to save the northern spotted owl (inset).

Edge Effects When conserving landscapes, it is necessary to consider edge effects. An edge reduces the amount of habitat typical of an ecosystem because the edges around a patch have a habitat slightly different from the interior of the patch. For example, forest edges are brighter, warmer, drier, and windier, with more  vines, shrubs, and weeds than the forest interior. Also, Figure 37.10a shows that a small and a large patch of habitat have the same amount of edge. Therefore, the effective habitat shrinks as a patch gets smaller.

b.

Figure 37.10  Edge effect.  a. The smaller the patch, the greater the proportion that is subject to the edge effect. b. Cowbirds lay their eggs in the nests of songbirds (yellow warblers). A cowbird is bigger than a warbler nestling and will be able to acquire most of the food brought by the warbler parent.

Many popular game animals, such as turkeys and white-tail deer, are more plentiful in the edge region of a particular area. However, today it is known that creating edges can be detrimental to wildlife because of habitat fragmentation. Edge effects can also have a serious impact on population size. Songbird populations west of the Mississippi have been declining of late, and ornithologists have noticed that the nesting success of songbirds is quite low at the edge of a forest. The cause turns out to be the brown-headed cowbird, a social parasite of songbirds. Adult cowbirds prefer to feed in open agricultural areas, and they only briefly enter the forest when searching for a host nest in which to lay their eggs (Fig. 37.10b). Cowbirds are therefore benefited, while songbirds are disadvantaged, by the edge effect.



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tourism, and limited sustainable use for cultural purposes. Next, there is a buffer zone that surrounds the core where only lowimpact human activities are allowed. A transition area then surrounds the buffer zone. This area is meant to support sustainable human development, tourism, and agriculture. Although there has been widespread cooperation with the establishment of biosphere reserves, there are few that strictly follow the United Nations’ model. People are often unwilling to modify their lifestyles to the extent necessary to achieve sustainability, and funding for biosphere maintenance is often limited. For example, funding levels may be insufficient for compensating landowners for adhering to sustainable development practices in buffer zones or transition areas.

Reserve Design Conservation reserves are those areas that are set aside with the primary goal of protecting biodiversity within them. As such, reserves should be largely protected from human activities, except perhaps ecotourism or limited scientific research. When considering the arguments made earlier in this section, reserves should contain sufficient amounts of habitat to sustain the biodiversity within them. This space should include multiple ecosystems that are connected by corridors to allow movement of animals and dispersal of plant species between habitat types. This space should also include enough area to account for edge effects, meaning that the absolute amount of land included in each reserve is likely higher than the usable habitat within it. Globally, a network of more than 650 biosphere reserves has been designated by the United Nations. The maintenance of biosphere reserves is in the hands of the countries and territories in which they are located, and compliance is voluntary. Because the goals of biosphere reserves include preservation of local cultural values, as well as maintenance of biodiversity and the ability to foster sustainable human development, there has been more cooperation than was initially expected. Each biosphere reserve is divided into three areas. First, there is the central core reserve, which allows only research, light

SCIENCE IN YOUR LIFE   ►

Habitat Restoration In cases where habitat has already been modified in an area to the extent that conservation and reserve formation may not be viable, or to reverse existing damage, habitat restoration is an alternative. Restoration ecology is a subdiscipline of conservation biology that seeks scientific ways to return ecosystems to their state prior to habitat degradation. Although habitat restoration is perceived as beneficial, there is some concern that the restored areas may not be

ECOLOGY

ois

Therkildsen Field Station

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Chautauqua National Wildlife Refuge

Conservancy office

Emiquon National Wildlife Refuge Sp

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Dickson Mounds Museum

Emiquon Preserve

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Emiquon Complex 78 US Fish and Wildlife Service The Nature Conservancy IL Dept. of 97 Natural Resources 78 University of Illinois Springfield

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Emiquon was once a vital component of the Illinois River system, which is a major tributary of the Mississippi River. It included a complex system of backwater wetlands and lakes that originally covered an area of approximately 14,000 acres (Fig. 37C). The vast diversity of native plants and animals found within Emiquon have supported human civilizations for over 10,000 years. Early cultures accessed the abundant inland fisheries and a variety of mussel species that could be found there. River otters, mink, beaver, and migrating birds, as well as a variety of prairie grasses on the floodplains would have been used by native peoples (Fig. 37D). In the early 1900s, the Illinois River was one of the most economically significant river ecosystems in the United States. It boasted the highest mussel abundance and productive commercial fisheries per mile of any stream in the United States. Each year during the seasonal flooding, the river would deposit nutrient-laden sediment onto the floodplain. This created a highly productive floodplain ecosystem. With the abundance of nutrients, a wide diversity of plant life would have been supported. This broad producer base would have been able to support a

er

Emiquon Floodplain Restoration

Ri ve r

Throughout the twentieth century, modern cultures altered the natural ebb and flow of the Illinois River to better suit their needs. Levees were constructed, resulting in the isolation of nearly half of the original floodplain from the river. Native habitats were converted to agricultural land. Corn and soybeans soon replaced the variety of prairie and wetland vegetation that were once a hallmark of the Emiquon floodplain. The river itself was also altered to better facilitate the export and import of a variety of commercial goods by boat. With the isolation of these areas from the seasonal flooding, a decrease in the amount of nutrients being deposited also occurred. This resulted in a continuous decline in the natural productivity of the floodplain.

Havana

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97

136

Figure 37C  The Emiquon restoration project.  Restoration plans call for allowing the river to return to some of its former range.

number of trophic levels as well as a wide diversity of food webs.

Restoration Plan The National Research Council has identified Emiquon as one of three large-floodplain river ecosystems in the United States that can be restored to some semblance of its original diversity and function. The floodplain restoration work at Emiquon is a key part of The Nature Conservancy’s efforts to conserve the Illinois River.



Chapter 37  Conservation Biology

as functionally equivalent to the natural regions that were once there. Nonetheless, habitat restoration is increasing, and in the process, three principles have emerged thus far. First, it is best to begin as soon as possible before remaining fragments of the original habitat are lost. These fragments are sources of wildlife and seeds from which to restock the restored habitat. Second, once the natural histories of the species in the habitat are understood, it is best to use biological techniques that mimic natural processes to bring about restoration. This might take the form of using controlled burns to bring back grassland habitats, biological pest controls to rid the area of exotic species, or bioremediation techniques to clean up pollutants. Third, the goal is sustainable development, the ability of an ecosystem to maintain itself while providing services to humans. The Ecology feature, “Emiquon Floodplain Restoration,” presents an example of a successful conservation project in west-central Illinois.

Check Your Progress  37.4 1. Identify ways in which landscape preservation is more valuable than ecosystem preservation.

2. Explain how edge effects influence nature reserve design. 3. List and explain the three principles of habitat restoration.

37.5  Working Toward a Sustainable Society Learning Outcomes Upon completion of this section, you should be able to 1. Compare renewable versus nonrenewable resources. 2. List two ways to transition to renewable energy resources. 3. Discuss how modern agriculture can be changed to minimize environmental impacts.

The majority of biodiversity loss is the result of human consumption of resources (Fig. 37.11). Some resources are nonrenewable, and some are renewable. Nonrenewable resources are limited in supply. For example, the amount of land, fossil fuels, and minerals is finite and can be exhausted. Even though better extraction methods can make more fossil fuels and minerals available, and efficient use and recycling can make the supply last longer, eventually these resources will run out. Renewable resources are not limited in supply. For example, certain forms of energy, such as solar and wind energy, can be replenished.

Figure 37D  Emiquon restoration. 

Forty-nine black-crowned night-heron nests have been recorded at Emiquon since the restoration project began. Research at Emiquon is providing critical information about wetland restoration techniques.

The Emiquon Science Advisory Council is composed of over 40 scientists and managers who provide guidance for the restoration and managed connection between Emiquon’s restored floodplain and the Illinois River. Once established, this connection would allow for the seasonal flooding that it previously experienced. This seasonal flooding

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would help naturalize the flow of the river and restore the cyclical process of flooding and drying out. Flooding would provide more nutrients for wetland plants as well as help to improve the water quality. Access between the river and floodplain would once again be restored for various aquatic species, such as paddlefish and gar. Both species need a

variety of habitats for reproduction and survival. The Conservancy is continuing to work with the Illinois Natural History Survey, the University of Illinois, Springfield, and others to survey the community diversity. Since 2007, they have documented over 250 species of birds, including over 90% of Illinois endangered and threatened wetland bird species.  In 2009, the normal operation of Emiquon was estimated to have contributed 1.1 million dollars to the local economy. As usage continues to increase, so does the economic impact Emiquon is having on local economies. To date, scientists, students, and decision makers from five continents have visited Emiquon. The knowledge gained from the research at Emiquon and The Nature Conservancy’s work on the Illinois River and within the Upper Mississippi River system is helping influence floodplain restoration projects around the world.

Questions to Consider 1. In what ways might scientists assess whether the restoration plan was successful in its goal? 2. How might the Emiquon Floodplain Restoration plan be used as a model for other ­restoration projects?



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UNIT 7  Behavior and Ecology Human population land

water

food

energy

minerals

nonrenewable energy

materials

minimal recycling processing and manufacturing

Figure 37.11  Resources.  Humans use land, water, food, energy, and minerals to meet their basic needs, such as a place to live, food to eat, and products that make their lives easier.

waste energy (heat)

The following characteristics indicate that human society in its current form is not sustainable (Fig. 37.12a): 1. A considerable proportion of land, and therefore of natural ecosystems, is being altered for human purposes. 2. Our society primarily utilizes nonrenewable fossil fuel energy, which leads to global warming, acid precipitation, and smog. 3. Even though fresh water is a renewable resource, we are using it faster than it can be replenished in aquifers and other sources. 4. Agriculture requires large inputs of nonrenewable fossil fuel energy, fertilizer, and pesticides, which create much pollution. 5. At least half of the agricultural yield in the United States goes toward feeding animals; it takes 10 lb of grain to produce 1 lb of meat. 6. Minerals are nonrenewable, and the mining, manufacture, and use of mineral products is responsible for a significant amount of environmental pollution. To move toward a more sustainable society, we should move away from the use of nonrenewable resources to renewable ones. A sustainable society, like a sustainable ecosystem, should be able to provide the same goods and services for future generations that it provides for the current one. At the same time, biodiversity would be conserved. A natural ecosystem can offer clues as to what a sustainable human society would be like. A natural ecosystem uses only renewable solar energy, and its materials cycle through herbivores, carnivores, and detritivores, and back to producers once again. It is clear that if we want to develop a sustainable society, we too should use renewable energy sources and recycle materials (Fig. 37.12b). Section 37.4 discusses ways to conserve land and habitat for species. In addition to recycling and reuse of many of Earth’s minerals (e.g., aluminum, copper, iron, and gold), which are nonrenewable, we should also consider alternative energy use, water conservation, and modifications to the way we conduct modern agriculture.

Energy Presently, about 14% of the world’s energy supply comes from nuclear power, and 61% comes from fossil fuels; both of these are finite, nonrenewable sources. Fossil fuels (oil, natural gas, and coal) are so named because they are derived from the compressed remains of plants and animals that died millions of years ago. Currently, shortage of fossil fuels such as oil has contributed to the dramatic increases in heating and gasoline costs. Comparatively

waste materials

products

minimal recycling a. Human society at present

renewable energy

materials

processing and manufacturing

waste energy (heat)

waste materials

maximal recycling

products

maximal recycling b. Sustainable society

Figure 37.12  Current human society versus a sustainable

society.  a. At present, our “throwaway” society is characterized by high input of energy and raw materials, large output of waste materials and energy in the form of heat (red arrows), and minimal recycling (purple arrows). b. A sustainable society would be characterized by the use of only renewable energy sources, reuse of heat and waste materials, and maximal recycling of products (purple arrows).

speaking, each person in a more-developed country (MDC) uses approximately as much energy in one day as a person in a lessdeveloped country (LDC) uses in one year. Increasing our reliance on renewable energy resources is a major step toward becoming a sustainable society.

Renewable Energy Sources Renewable types of energy include hydropower, geothermal energy, wind power, and solar energy. Hydroelectric plants convert the energy of falling water into electricity (Fig. 37.13). Worldwide, hydropower presently generates approximately 17% of



Figure 37.13  Hydropower.  Hydropower dams provide a clean form of energy but can be ecologically disastrous in other ways.

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required by a coal-fired power plant or a solar thermal energy system. A community that generates its own electricity by using wind power can solve the problem of uneven energy production by selling electricity to a local public utility when an excess is available and buying electricity from the same facility when wind power is in short supply. Solar energy is diffuse energy that must be (1) collected, (2)  converted to another form, and (3) stored if it is to compete with other available forms of energy. Passive solar heating of a house is successful when the windows of the house face the sun, the building is well insulated, and heat can be stored in water tanks, rocks, bricks, or some other suitable material. In a photovoltaic (solar) cell, a wafer of an electron-emitting metal is in contact with another metal that collects the electrons and passes them along into wires in a steady stream. Spurred by the oil shocks of the 1970s and in 2008, the U.S. government has been supporting the development of photovoltaic cells. As a result, the price of buying one has dropped from about $100 per watt (in the early 1970s) to around $0.74 (2013). The photovoltaic cells placed on roofs, for example, generate electricity that can be used inside a building and/or sold back to a power company (Fig. 37.14b). Traditional cars have internal combustion engines that run on gasoline, but hybrid cars that are increasing in popularity due to rising fuel costs, run on both gasoline and electricity, increasing mileage per gallon. Now that we have better ways to capture solar energy, scientists are investigating the possibility of using solar energy to extract hydrogen from water via electrolysis. The hydrogen can then be used as a clean-burning fuel that produces water as a waste product.

all electricity utilized. In the United States, hydropower accounts for approximately 7% of total energy produced and 51% of the renewable energy used. Brazil, New Zealand, and Switzerland produce at least 75% of their electricity with hydropower, but Canada is the Western world’s leading hydropower producer, accounting for 97% of its renewable energy generation. One way to reduce our reliance on nonrenewable resources such as fossil fuels is to increase our reliance on hydropower. However, much of the recent hydropower development has been due to construction of enormous dams that have detrimental environmental effects. Small-scale dams that generate less power per dam, but do not have the same environmental impact, are believed to be the more environmentally responsible choice. Geothermal energy is produced because Earth has an internal source of heat. Elements such as uranium, thorium, radium, and plutonium undergo radioactive decay underground and then heat the surrounding rocks to hundreds of degrees Celsius. When the rocks are in contact with underground streams or lakes, huge b. Solar amounts of steam and hot water are produced. This steam can be piped up to the surface to supply hot water for home heating or to run steam-driven turbogenerators. The California Geysers Project, for example, is one of the world’s largest geothermal electricity generating complexes. Wind power is expected to account for a significant percentage of our energy needs in the future (Fig. 37.14a). Despite the common belief that a huge amount of c. Hydrogen land is required for “wind farms” that pro- a. Wind duce commercial electricity, the actual Figure 37.14  Other renewable energy sources.  a. Wind power requires land on which to amount of space for a wind farm com- place enough windmills to generate energy. b. Photovoltaic cells on rooftops and (c) the use of hydrogen cars pares favorably to the amount of land will reduce air pollution and dependence on fossil fuels.



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Increasingly, vehicles are powered by fuel cells that use hydrogen to produce electricity. Fuel cells are now powering buses in Vancouver and Chicago. Hydrogen cars are already in limited production (Fig. 37.14c). Hydrogen fuel can be produced locally or in central locations, using energy from photovoltaic cells. If in central locations, hydrogen can be piped to filling stations using the natural gas pipes already in place in the United States. The advantages of such a solar-hydrogen revolution would be at least twofold: (1)  the world would no longer be dependent on its limited oil reserves, and (2) environmental problems, such as global warming, acid rain, and smog, would begin to lessen.

Water In some areas of the world, people do not have ready access to clean and safe drinking water. It’s considered a human right for people to have clean drinking water, but actually, most fresh water is utilized by industry and agriculture. Worldwide, 60% of all fresh water is used to irrigate crops! Domestically in MDCs, more water is usually used for bathing, flushing toilets, and watering lawns than for drinking and cooking. Although the needs of the human population overall do not exceed the renewable supply of water, this is not the case in certain regions of the United States and the world. When needed, humans increase the supply of fresh water by damming rivers and withdrawing water from aquifers. Dams have drawbacks: (1) Reservoirs behind the dam lose water due to evaporation and seepage into underlying rock beds. The amount of water lost sometimes equals the amount dams make available! (2) The salt left behind by evaporation and agricultural runoff increases salinity and can make a river’s water unusable farther downstream. (3) Over time, dams hold back less water because of sediment buildup. Sometimes a reservoir becomes so full of silt that it is no longer useful for storing water. (4) The alteration of a river or stream has a negative impact on the native wildlife. To meet their freshwater needs, people are pumping vast amounts of water from underground reservoirs called aquifers. Aquifers hold about 1,000 times the amount of water that falls on land as precipitation each year. In the past 50 years, groundwater depletion has become a problem in many areas of the world. Removal of water is causing land subsidence, settling of the soil as it dries out. In California’s San Joaquin valley, an area of more than 13,000 km2 has subsided at least 30 cm due to groundwater depletion, and in the worst spot, the surface of the ground has dropped more than 9 m! Subsidence damages canals, buildings, and underground pipes. For more on the California aquifers, see the Ecology feature, “The California Drought,” in section 35.3.

Conservation of Water By 2025, two-thirds of the world’s population may be facing serious water shortages. Some solutions for expanding water supplies have been suggested. Planting drought- and salt-tolerant crops would help a lot. Development of many such crops is already underway due to genetic engineering, as discussed in Chapter 26. Using drip irrigation delivers more water to crops and saves about 50% over traditional methods while increasing crop yields. Although the first drip systems were developed in 1960, they are currently used on less than 1% of irrigated land. Most governments

subsidize irrigation so heavily that farmers have little incentive to invest in drip systems or other water-saving methods. Reusing water and adopting conservation measures could help the world’s industries cut their water demands by more than half. For example, recycling washing machine water, shower water, or water used to wash cars through a filter before it is discarded as sewage could significantly reduce domestic water usage. Home yard irrigation should occur during dusk and dawn hours, as opposed to in the middle of the day when evaporation is at its highest. Purchasing and using dual-flush toilets can also save millions of gallons of water per year.

Agriculture In 1950, the global human population numbered 2.5 billion, and at  that time, there was only enough food to provide less than 2,000 calories per person per day. Today, current agricultural practices provide enough food to provide everyone on Earth a healthy diet consisting of 2,500 calories per day. However, one-sixth of the world’s population (over 1 billion people) are currently considered malnourished due to lack of proper distribution and the redirection of grain to feed livestock. In addition, modern farming methods are environmentally destructive in the following ways. First, planting only a few genetic varieties, or monocultures (a genetically identical crop), means that a single destructive parasite or pathogen can cause huge crop losses. Second, modern farming relies on heavy use of fertilizers, pesticides, and herbicides. Pesticides reduce soil fertility because they kill off beneficial soil organisms as well as pests. Pesticides also select for artificial resistance in insects, increasing eradication costs via increased pesticide use. In addition, fertilizers, pesticides, and herbicides all contribute to pollution. Third, modern agriculture uses significant amounts of fresh water through irrigation. Fourth, modern agriculture uses large amounts of fuels. Fertilizer production is energy intensive, irrigation pumps require energy to remove water from aquifers, large tractors and even airplanes are used to spread fertilizers, pesticides, and herbicides, and large machines are often used to harvest crops. Several alternatives exist to employing modern agricultural methods. Polyculture, or planting of several varieties of crop in the field simultaneously can reduce the susceptibility of crops to pests or diseases (Fig. 37.15a). Polyculture also reduces the amount of herbicides necessary to kill competing weeds and can be used to replenish nutrients to topsoil. Crop rotation—where, for example, nitrogen-fixing crops, such as legumes, are alternated across harvest years with crops that utilize soil nitrogen, such as corn—can help reduce the use of nitrogen-containing fertilizers. Such multiuse farming techniques generally help increase the amount of organic matter and nutrients in the soil. Organic farms are also increasing in number. Organic farms, as mandated by the U.S. Department of Agriculture, are those in which synthetic pesticides and herbicides are avoided or largely excluded from use. Organic farming has become increasingly profitable in recent years because people are more willing to purchase organic produce, despite the increased cost relative to nonorganic produce. Health concerns surrounding pesticide content in nonorganic produce, as well as better tasting food, have helped this trend. One way that organic farmers have eliminated the need for pesticides is by using integrated pest management. This technique



a. Polyculture

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b. Contour farming

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c. Biological pest control

Figure 37.15  Methods that make farming more friendly to the environment.  a. Polyculture reduces the ability of one parasite to wipe out an

entire crop and reduces the need to use an herbicide to kill weeds. This farmer has planted alfalfa in between strips of corn, which also replenishes the nitrogen content of the soil (instead of adding fertilizers). Alfalfa, a legume, has root nodules that contain nitrogen-fixing bacteria. b. Contour farming with no-till conserves topsoil, because water has less tendency to run off. c. Instead of pesticides, it is possible to use natural predators. Here, ladybugs are feeding on aphids, an insect pest species.

encourages the growth of competitive beneficial insects and uses biological pest control methods (also called “biocontrol”). Biocontrol has also helped reduce pesticide use in traditional farms as well. Such methods include using natural predators, such as spiders and ladybird beetles, to reduce the numbers of pests (Fig. 37.15c). Use of natural or engineered pathogens has also been suggested for biocontrol of pests, but there is concern that such diseases will escape crop areas and affect natural plants or wildlife. Several types of farming now exist that reduce erosion and help to minimize topsoil loss, such as contour farming (Fig.  37.15b). Terrace farming involves converting steep slopes into steplike hills to minimize erosion, and some farmers plant “natural fences,” such as rows of trees, around crops to prevent topsoil loss due to wind or other factors. These trees can also be used as other products; mature rubber trees provide us with ­rubber, and tagua nuts are an excellent substitute for ivory, for example. Cover crops, which are often a mixture of legumes and grasses, also help stabilize soil between rows of cash crops. Finally, avoiding farming on steep slopes also helps reduce ­erosion. Soil nutrients can be increased through composting, organic farming techniques, or other self-renewable methods. In general, we should consider using precision farming (PF) techniques that rely on accumulated knowledge to reduce habitat destruction, while improving crop yields.

Urban Growth

toll costs for cars occupied by more than one person. Portland, Oregon, even has short-term electric car rental stations around the city, whereby people can rent cars for an evening and easily drop them off when they are finished. Maintaining a network of safe bicycle lanes also encourages people to ride their bikes to work. A way to curtail urban sprawl, or extensive expansion of cities outward, is to build them upward. Several cities, such as Vancouver, Canada, are building many high-rise apartment buildings to accommodate more people in a smaller area. As new buildings are built, they should be as “green” as possible—by using solar or geothermal energy for heating, as well as being constructed out of sustainable or recycled materials. Space on top of buildings could be used to make “green roofs,” whereby a garden of grasses, herbs, and vegetables is planted to assist temperature control, supply food, reduce rainwater runoff, and be visually appealing. As more people move into cities, city officials can focus on renovating older parts of the city, as opposed to spreading out further. Modern cities can also have more planned green spaces, including plentiful walking and bicycle paths. In city parks, native species that attract bees and butterflies and require less water and fertilizers could be planted, as opposed to traditional grasses. Sustainable cities can also improve storm-water management by using sediment traps for storm drains, artificial wetlands, and holding ponds. As new development occurs, cities can increase the use of porous surfaces for walking paths, parking lots, and roads. These surfaces reflect less heat, while soaking up rainwater runoff.

More and more people are moving to cities. Growth of cities increases pollution via many sources, including automobile Check Your Progress  37.5 exhaust, runoff of pollutants on impervious surfaces (e.g., roads) and into waterways, noise pollution, and industrial and domestic 1. Identify which nonrenewable energy use is potentially wastes. Thus, the growth of cities involves careful planning to environmentally harmful. serve the needs of new arrivals, without overexpansion of the city. 2. List ways in which we can conserve water to minimize Energy use in cities can be curtailed by providing a good pubnegative environmental impacts. lic transportation system, preferably one that is energy efficient. 3. Discuss two alternative agricultural practices that are environmentally friendly. Many U.S. cities are now encouraging carpooling by having HOV (high-occupancy vehicle) lanes on highways, as well as reduced

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Conclusion to the processing plant, saving an estimated $700,000 in electricity costs per year. The remaining onion waste is then sold as livestock feed, eliminating the $400,000 in expenses associated with the waste disposal. Gills Onions has become the first food-processing facility in the world to produce ultra-clean energy from their own waste. They have also reduced their carbon footprint by an estimated 14,500 metric tons (mt) of CO2e (equivalent) emissions per year.

Gills Onions processes over 1 million lb of onions every day, producing approximately 300,000 lb of waste. Previously, the waste was composted and hauled to local farm fields and spread as fertilizer. This technique resulted in a variety of environmental concerns as well as a significant amount of financial loss. To help resolve the problem, Gills Onions developed an Advanced Energy Recovery System that would turn the onion waste into a methane-rich biogas that powers two fuel cells. The fuel cells in turn supply energy

MEDIA STUDY TOOLS http://connect.mheducation.com Maximize your study time with McGraw-Hill SmartBook®, the first adaptive textbook.

  Tutorials 37.3  Global Climate Change

SUMMARIZE 37.1  Conservation Biology and Biodiversity ■ Conservation biology is an interdisciplinary science, with the goal of

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conserving Earth’s biodiversity for the good of all species, that requires input from a variety of subdisciplines. Biodiversity is the variety of life that exists on Earth. Of the estimated 8 million species that live on Earth, nearly 75,000 of the described species are classified as endangered species. This means they face immediate extinction throughout all or most of their range. Thousands of species are listed as threatened species, meaning that they may become endangered in the foreseeable future. Although biodiversity is often described as the total number of species, it is also considered at the genetic, ecosystem, and landscape levels. Genetic diversity helps ensure that populations can evolve in the face of future environmental change. Ecosystem diversity considers the functioning of communities within ecosystems and the interconnectedness of species within such communities. Landscape diversity includes all of the ecosystems that may interact in a particular area.

37.2  Value of Biodiversity ■ A direct value of biodiversity includes the use of wild species for our

medical needs, agricultural value, and consumptive purposes.

■ The indirect value of biodiversity includes the workings of biogeo-

chemical cycles, waste disposal, provision of fresh water, prevention of soil erosion, and regulation of climate.

■ Ecotourism is an indirect value of biodiversity that is increasing as an

important source of income for many less-developed countries (LDCs).

37.3  Threats to Biodiversity ■ The five major causes of extinction, in decreasing order of importance,

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are habitat loss, introduction of alien species, pollution, overexploitation, and disease. Habitat loss has occurred in all parts of the biosphere, but concern has now centered on tropical rain forests and coral reefs, where biodiversity is especially high. Exotic species have been introduced into foreign ecosystems because of colonization, horticulture or agriculture, and accidental transport. Among the various causes of pollution (acid rain, eutrophication, ozone depletion, and organic chemicals), climate change and global warming are expected to cause the most instances of extinction. Overexploitation is exemplified by commercial fishing, which is so efficient that fisheries of the world are collapsing. Wildlife species are increasingly threatened by disease-causing pathogens.

37.4  Habitat Conservation and Restoration ■ Biodiversity hotspots, such as tropical rain forests and coral reefs, are

often high-priority conservation areas because they contain unusually large concentrations of species. ■ Keystone species are often a target for conservation because their loss would result in many secondary extinctions. Flagship species are those that elicit an emotional response and often motivate the public to support conservation.



Chapter 37  Conservation Biology

■ Habitat conservation should be considered at the landscape level, with

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the formation of conservation corridors that allow species to travel safely between ecosystem types. Edge effects reduce the amount of suitable habitat and should be considered in reserve design. Conservation reserves are areas set aside with the primary goal of protecting the biodiversity within them. Biosphere reserves should consist of a core area with nearly no disturbance, a surrounding buffer area where only low-impact human activities are allowed, and an outer transition area meant to support sustainable human development, tourism, and agriculture. Restoration ecology is the field of biology that is used to help return ecosystems to their state prior to habitat degradation. Restoration ecology has three basic principles: (1) begin as soon as possible, (2) use biological techniques that mimic natural processes to bring about restoration, and (3) maintain ability of an ecosystem to maintain itself while providing services to humans.

37.5  Working Toward a Sustainable Society ■ The current biodiversity crisis is ultimately caused by human actions. ■ Today’s society is considered unsustainable because we rely heavily on

nonrenewable resources to maintain our existence.

■ A shift of focus toward renewable resources (wind, solar, geothermal,

and hydropower) and sustainable use of water and agricultural practices will help reduce human impact. The use of a photovoltaic (solar) cell is a form of renewable energy. ■ Globally, many people do not have access to clean drinking water. In the southwestern United States, people are rapidly draining the aquifers to meet the growing demand for water. The removal of the water from the aquifers has led to land subsidence, settling of the soil as it dries out. In order to prevent problems, people need to become more aware of water conservation methods. ■ These methods include drip irrigation, planting drought- and salttolerant crops, and household water reuse and recycling methods. ■ Modern-day agriculture can employ new methods, such as biocontrol to reduce reliance on pesticides, and new forms of farming, such as contour farming, to reduce topsoil erosion.

ASSESS Testing Yourself Choose the best answer for each question.

37.1  Conservation Biology and Biodiversity 1. Eagles and bears feed on spawning salmon. If shrimp are introduced that compete with salmon for food, a. the salmon population will decline. b. the eagle and bear populations will decline. c. only the shrimp population will decline. d. all populations will increase in size. e. Both a and b are correct. 2. Which of the following is based on the interactions of species with their abiotic environment? a. genetic diversity b. landscape diversity c. ecosystem diversity d. biocultural diversity e. None of these are correct.

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3. A species that has an immediate threat to extinction is said to be: a. threatened. c. alien. b. endangered. d. extinct.

37.2  Value of Biodiversity 4. The most significant cause of the loss of biodiversity is a. habitat loss. b. pollution. c. exotic species. d. disease. e. overexploitation. 5. Which of these is not an indirect value of species? a. participates in biogeochemical cycles b. participates in waste disposal c. helps provide fresh water d. prevents soil erosion e. All of these are correct.

37.3  Threats to Biodiversity 6. Which of these is a true statement? a. Habitat loss is the most frequent cause of extinctions today. b. Exotic species are often introduced into ecosystems by accidental transport. c. Climate change may cause many extinctions but also expands the ranges of other species. d. Overexploitation of fisheries could lead to a complete collapse of the fishing industry. e. All of these statements are true.  7. Which of these is expected if the average global temperature increases? a. the inability of some species to migrate to cooler climates as environmental temperatures rise b. the possible destruction of coral reefs c. an increase in the number of parasites in the temperate zone d. a population decline for some species but an increase for others e. All of these are correct.  8. Complete the following graph by labeling each bar (a–e) with a cause of extinction, from the most influential to the least influential. a. b. c. d. e. 0

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60 80 100

% Species Affected by Threat

37.4  Habitat Conservation and Restoration 9. Edge effects a. result from species crowding at edge of habitats. b. mean that no plants can grow well at the edge of a mountain. c. may increase wind and temperatures around the outside of habitats. d. increase competition between species at the edge of their territories. e. All of these are correct.



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10. Biodiversity hotspots a. have few populations because the temperature is too hot. b. contain about 20% of Earth’s species even though their area is small. c. are always found in tropical rain forests and coral reefs. d. are sources of species for the ecosystems of the world. e. All except a are correct. 11. Which of the following should not be considered when designing nature reserves? a. edge effects that may require an increase in size b. corridors to facilitate wildlife movement among habitat patches c. primarily the needs of a single, important species d. inclusion of multiple ecosystem types

37.5  Working Toward a Sustainable Socity 12. Which feature is part of a sustainable society? a. recycling and composting b. use of mass transit in urban environments c. integrated pest management to control crop damage d. preservation of wetlands e. All of these are correct. 13. Which feature is associated with the sustainability of a rural society? a. planting cover crops when farming b. using green roofs c. designing energy-efficient transportation systems d. making areas more pedestrian- and bike-friendly e. All of these are correct.

ENGAGE BioNOW Want to know how this science is relevant to your life? Check out the BioNow video below. ■ Biodiversity

Thinking Critically 1. Visit www.myfootprint.org to determine the size of your ecological footprint and then discuss ways that you may reduce that value. 2. What are some ways we can reduce our reliance on nonrenewable resources and move toward increased reliance on renewable resources? 3. Why should people be generally more concerned about pollutants in our meat than in our vegetables? 4. Discuss how removal of a keystone species can disrupt ecosystem functioning. Give an example.

PHOTO CREDITS Opener: © J. Emilio Flores/La Opinion/Newscom; 37.3(periwinkle): © Steven P. Lynch; 37.3(armadillo): © Photodisc/Getty RF; 37.3(ladybug): © D. Hurst/Alamy RF; 37A: © USDA/Lila De Guzman, photographer; 37.4b: © IT Stock/PunchStock RF; 37Ba: © hotshotsworldwide/Getty RF; 37Bb: © Photodisc RF; 37.5a: © Chuck Pratt/Photoshot; 37.5b: © Chris Johns/National Geographic Image Collection; 37.6b: © Secret Sea Visions/ Photolibrary/Getty Images; 37.7a: © StrahilDimitrov/iStock/360/Getty RF; 37.7b: © Peter Auster/University of Connecticut; 37.8: © Dr. Paul A. Zahl/Science Source; 37.9a: © Deb Garside/Design Pics RF; 37.9b(forest): © Buddy Mays/Corbis RF; 37.9b(owl): © Michael Sewell/Getty Images; 37.10b: © Jeff Foott/Getty Images; 37D(bird): © NHPA/SuperStock; 37D(researcher): © Marilyn Kok, University of Illinois Springfield; 37.11(land): © Vol. 39 PhotoDisc/Getty RF; 37.11(water): © Evelyn Jo Johnson; 37.11(food): © John Thoeming/ McGraw-Hill Education; 37.11(energy): © Photo Link/Getty RF; 37.11(minerals): © T. O’Keefe/PhotoLink/Getty RF; 37.12a: © Kent Knudson/PhotoLink/Getty Images RF; 37.12b: © Scenics of America/PhotoLink/Getty RF; 37.13: © Corbis RF; 37.14a: © Glen Allison/ Getty RF; 37.14b: © Danita Delimont/Getty Images; 37.14c: © 2009 The Associated Press; 37.15a: © David R. Frazier/Science Source; 37.15b: © Inga Spence/Alamy; 37.15c: © Perennou Nuridsany/Science Source.

APPENDIX A Answer Key CHAPTER 1 The Search For Life 1. Characterist ics are: (1): organization; (2) ways to acquire materials and energy; (3) reproduction; (4) responses to stimuli; (5) homeostasis; (6) growth and development; (7) capacity for adaptation to their environment. 2. You would look for evidence of water and the chemicals that make up biomolecules. 3. If life is similar, then the processes involved in living organisms is universal throughout the solar system. If life is different, then other processes must be considered.

Check Your Progress 1.1: 1. Characteristics are: (1): organization; (2) ways to acquire materials and energy; (3) reproduction; (4) responses to stimuli; (5) homeostasis; (6) growth and development; (7) capacity for adaptation to their environment. 2. Levels of organization are: cell, tissue, organ, organ system, organism, population, community, ecosystem, biosphere. 3. As an organism acquires an adaptation that makes it better able to live and reproduce, these traits are passed on to the next generation and the population evolves. 1.2: 1. Domains are the largest, most inclusive classification category. The three domains are Archaea, Bacteria, and Eukarya. The first two are composed of species with prokaryotic cells (no membranebound nucleus), and the third has organisms with eukaryotic cells. Archaea can live in extreme aquatic environments, and Eukarya and Bacteria live almost everywhere else. Four kingdoms have been classified within the Eukarya: Protista, Fungi, Plantae, and Animalia. 2. Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species. 3. A hierarchical classification system allows increasing specificity about the organisms under study, which helps define their evolutionary relationships. 1.3: 1. An experimental variable is the one factor that is being tested in an experiment to determine what it contributes. 2. The control group in an experiment is included to determine if the experiment is sensitive to the effect of the variable of interest. The test group experiences the variable and the control group does not. 3. An observation is made and a hypothesis is constructed to possibly explain the observation. An experiment is designed to test the hypothesis, involving deductive reasoning and a prediction. The experiment is conducted, data collected and analyzed, and conclusions made about whether the hypothesis is supported by the data or not. 1.4: 1. New technology applies scientific knowledge to issues important to humans and can be used in investigations that lead to scientific discoveries. 2. Biodiversity contributes to the health of ecosystems on which we depend for food, medicines, and raw materials. Ecosystems that are destroyed can no longer function well in water cycling, soil conservation, and chemical cycling. 3. Emerging diseases appear in a population that has no immunity against them. Often, little is known about the causative agent and how to control it. Techniques for containing the emerging disease are often undeveloped. As Earth’s climate warms, ecosystems are challenged by changes in water availability, temperature extremes, and altered habitats, all of which can reduce biodiversity.

Science in Your Life Adapting to Life at High Elevations 1. Extremely cold or hot environments; environments with different lengths of sunlight; humid versus dry environments 2. Perhaps they have larger lung capacities, faster heart rates, or an increased ability to deliver oxygen to the tissues.

Testing Yourself 1. c; 2. b; 3. b; 4. c; 5. c; 6. a; 7. b; 8. d; 9. c; 10. d; 11. c; 12. d.

Thinking Critically 1. Model organisms have been characterized and standardized. They are identical in their make up and physiology. When analyzing the data from an experiment testing an experimental

variable in a model organism, one can be more confident that the dependent variable is related to the experimental variable rather than some other factor. 2. If life on Earth is unique in its characteristics, then the processes by which life emerged and evolved by natural selection are not universal. 3. The experimental chemical should be studied in a model species. The experiment should have both a control group (that receives no drug, or the inert ingredients used in the drug’s formulation) and a treatment group (that receives the drug). Both groups should experience identical conditions and have the same type of cancer. Data is collected on tumor development in the treatment group relative to the control group. If the treatment group shows greater tumor reduction than the control group, the drug may then be approved for trials on humans.

CHAPTER 2 Sports Drinks and Exercise 1. High heat capacity, high heat of vaporization, effective solvent, cohesive, adhesive, high surface tension, less dense in frozen state compared to liquid state. 2. Carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur.

Check Your Progress 2.1: 1. Earth’s crust has a different function and is constructed differently than living organisms. It is not a carbon-based life form that exhibits the characteristics of life. 2. See Figure 2.3. Ca has twenty protons and twenty neutrons in the nucleus, two electrons in the first orbital, eight electrons in the second orbital, eight electrons in the third orbital, and two electrons in the fourth orbital. 3. High levels of radiation can cause burns, harm cells, and change DNA structure, with cancer being a possible result of exposure. However, radiation can be used beneficially to kill cancer cells. Also, radioisotopes are used extensively in medicine to image the body through X-rays, positron emission tomography, and computerized tomography. Bacteria and fungi present on medical equipment, in mail, and in food can be killed by exposure to radiation. 4. Oxygen 16 has eight protons, eight electrons, and eight neutrons. Oxygen 18 differs by having ten neutrons (two more). 2.2: 1. Nitrogen gas (N2) is a molecule because two atoms of nitrogen share a covalent bond. Carbon dioxide (CO2) is a compound because two different elements are bonded together. However, CO2 is also a molecule because if you broke apart the carbon and oxygen atoms, the properties of carbon dioxide would be lost. 2. Carbon has six electrons, two in the first orbital, and four in the second orbital. Its incomplete outer orbital can accept or share four electrons, forming four covalent bonds. 3. In nonpolar covalent bonds there is equal sharing of electrons between the two atoms. In polar covalent bonds the electrons are shared unequally due in part to differences in the sizes of the two atoms involved. 2.3: 1. Heat capacity of water is the amount of energy needed to raise the temperature of one gram one degree Celsius, while heat of vaporization is the amount of energy needed to convert one gram of the hottest water to a gas. 2. Cohesion describes the ability of water molecules to cling to each other. Adhesion is a measure of water’s ability to cling to other polar surfaces, such as the sides of a vessel. 3. A solution at pH 6 contains 1 × 10−6 moles per liter H+ ions, while a solution at pH 8 contains 1 × 10−8 moles per liter H+ ions. 1 × 10−6 is a hundred times larger than 1 × 10−8. 2.4: 1. Because they contain carbon and hydrogen and are present in living organisms. 2. Hydrolysis reactions involve the addition of a molecule of water across a chemical bond, splitting the molecule in two. Dehydration reactions remove a molecule of water, joining two molecules. 2.5: 1. A ring structure of carbons with each bonded to a hydrogen and an OH group. 2. Humans are able to break the bonds between the glucose molecules in starch, but not between the glucose molecules in cellulose. 2.6: 1. Phospholipids and cholesterol, which is a steroid. 2. A double bond tends to make a fatty acid liquid

since it introduces a “kink” in the chain that prevents the fatty acids from packing as tightly together, producing a solid. 3. Triglycerides are used for storage of energy, insulation, and protection of organs. The presence of the steroid, cholesterol, in plasma membranes influences their fluidity. It is a precursor for the hormones testosterone and estrogen as well as bile salts. 2.7: 1. Enzymes, structural proteins, hormones, transport molecules, and membrane channels. 2. Each one has an amino group, an acidic group, and an R group bound to a central carbon atom. The R group varies in structure between one amino acid and another. 3. primary— linear sequence of amino acids; secondary—orientation of polypeptide chain, coiled or folded determined by hydrogen bonding; tertiary—three dimensional structure maintained by covalent, ionic, or hydrogen bonding; quaternary—structure of two or more polypeptides joined in a protein. 2.8:1. In the sequence of the nucleotide bases A, C, T, and G. 2. In the chemical bonds of the last two phosphate groups.

Science in Your Life Japan’s Nuclear Crisis 1. 137Cs emits beta particles and gamma radiation which can lead to DNA damage, burns, and increased risk of cancer. Its half life is 30 years so it remains in the environment for a long time. 131I emits lesser amounts of beta particles and gamma radiation than 137Cs but can also cause tissue damage. It has a short half life of 8 days and is used in the diagnosis and treatment of thyroid disease. 133Xe can cause tissue damage through the emission of beta particles, but its half life is just 5 days, which limits its impact. It is a Noble gas and has a low reactivity. It can be used in detecting and treating lung disorders. 2. 137Cs remains in the environment for a long time and the amount of radiation given off leading to DNA damage, burns, and cancer. A Balanced Diet 1. 50% fruits and vegetables. Answers will vary with students. 2. Trans fats and unsaturated fats are bad for you because they are associated with cardiovascular disease. Monounsaturated and polyunsaturated oils are important foods for health because they protect against cardiovascular disease. 3. Wholegrain carbohydrates contain more fiber and vitamins and minerals. The fiber, when in the gut, can bind up cholesterol and lead to its elimination in the feces. Simple carbohydrates are absorbed quickly and lead to peaks in blood sugar, which upsets blood glucose regulation.

Testing Yourself 1. b; 2. c; 3. d; 4. c; 5. a; 6. b; 7. d; 8. e; 9. a; 10. c; 11. a; 12. c; 13. c; 14. b; 15. b; 16. d; 17. d; 18. c; 19. d; 20. a; 21. b; 22. c.

Thinking Critically 1. The surface of the water is smooth because of the cohesion between water molecules. Water droplets cling to skin because water is cohesive. You are cooler because the water is evaporating and this removes a lot of heat from your skin because water has a high heat of evaporation. 2. Protein function is dependent on the three-dimensional shape a protein molecule assumes. How a polypeptide chain folds into sheets or coils, how parts of the polypeptide chains interact with other parts, and how multiple chains of proteins interact among themselves all define the shape and ultimately the function of protein molecules. 3. How reactive an element is depends on the number of electrons in its outer shell. Isotopes do not differ in their number of electrons.

CHAPTER 3 Tay-Sachs: When Lysosomes Fail to Function 1. Peroxisomes, present in animals and plants, have enzymes that break down fats. In plant leaves, peroxisomes can counteract photosynthesis by generating carbon dioxide and

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Appendix A

using up oxygen. 2. The cell relies on lysosomes to dispose of faulty organelles. The cell lacks a mechanism for disposing of faulty lysosomes.

Check Your Progress 3.1: 1. If humans were just one cell the surface-area-tovolume ratio would be too small to allow for efficient movement of nutrients and oxygen into the cell and wastes, including carbon dioxide, out of the cell. 2. The cell is the basic unit of life because all living organisms are made up of cells and all cells come from preexisting cells. 3. A large cell has a reduced surface-area-to-volume ratio and the amount of diffusion of nutrients, wastes, and gases across the cell membrane is reduced. 3.2: 1. To form a barrier between the inside and outside of the cell and regulate what crosses that barrier. 2. See Figure 3.3. 3. See Table 3.1. 3.3: 1. Protecting the cell while remaining permeable. 2. The endomembrane system transports molecules throughout the cell because the organelles that compose it are connected directly or by transport vesicles. 3. Plant cells need to make their food. Chloroplasts allow them to capture solar energy to produce organic molecules. Plant cells need mitochondria to break down the organic molecules to produce ATP which is used for energy in its metabolism. 3.4: 1. All three are part of the cytoskeleton and are dynamic. Intermediate filaments play a structural role while both actin filaments and microtubules have a structural and a movement role. Actin filaments are composed of actin monomers, microtubules are composed of tubulin monomers, and intermediate filaments are composed of various types of fibrous polypeptides. 2. Cilia and flagella have a 9 + 2 pattern of microtubule doublets while centrioles have a 9 + 0 pattern of microtubule triplets. 3. The dynein side arms on the microtubule doublets slide past each other using the energy of ATP. 3.5: 1. They are similar to bacteria in size and structure, are bounded by a double membrane, contain a limited amount of genetic material, divide by splitting, have their own ribosomes (similar to prokaryotic ribosomes) and do produce some proteins, and have ribosomal RNA sequences similar to prokaryotic rRNA sequences. 2. Mitochondria were originally aerobic heterotrophic bacteria taken up by a eukaryotic cell and chloroplasts were cyanobacteria similarly taken up. Neither the bacteria nor cyanobacteria were destroyed, but instead successfully lived inside the host cell, contributing their metabolic functions to the whole.

Science in Your Life Modern Microscopy 1. The image becomes blurred and detail is lost. 2. A stream of electrons is used to create the image rather than visible light which humans see as color.

Testing Yourself 1. c; 2. a; 3. d; 4. d; 5. c; 6. e; 7. d; 8. c; 9. b; 10. c; 11. a; 12. b; 13. d; 14. b.

Thinking Critically 1. Locate a cell line that has the mechanism to secrete proteins and can be grown under laboratory conditions. Modify the genome of that cell line so that it manufactures and secretes the drug protein so that it can be harvested. 2. Cells, whether naturally occurring or synthesized, depend on diffusion and transport processes whose efficiency varies with cell size. 3. Knowing the basic structure and function of cells on Earth helps in recognizing similarities and differences in life forms from other planets.

CHAPTER 4 Red Hot Chili Peppers 1. The roles of membrane proteins include: stabilize and shape the membrane; create channels through which molecules pass; interact with molecules and carry them across the membrane; identify cells as being foreign or self; bind to specific molecules at the cell surface and bring about a cellular response; serve as enzymes, catalyzing biological reactions. 2. facilitated diffusion.

Check Your Progress 4.1: 1. Phospholipids compose a bilayer that separates the inside from the outside of the cell. Steroids in the bilayer regulate the fluidity of the membrane. Proteins present in the membrane contribute to its structure, the passage of molecules across the membrane, signaling pathways, cell

recognition, and enzyme reactions. 2. See Figure 4.2. 4.2: 1. During diffusion, molecules move from an area of high concentration to an area of low concentration. Facilitated transport promotes the movement of these molecules down a concentration gradient across a plasma membrane. Carrier proteins reversibly bind to the molecule and speed up their passage. 2. A hypertonic environment has a lower concentration of water and a higher concentration of solutes than a hypotonic environment. Water will move from the hypotonic environment where it is present at a higher concentration, to the hypertonic environment across a semipermeable membrane by osmosis. 3. Both move molecules across the plasma membrane and require a carrier molecule. Facilitated transport does not use energy while active transport does. Facilitated transport moves items down their concentration gradient, while active transport moves against the concentration gradient. 4. It is specific for a particular solute. 4.3: 1. Collagen and elastin fibers provide structure to the ECM. Fibronectin binds to integrin in the membrane and can signal the cell’s cytoskeleton. Proteoglycans, shaped as bottle brushes, assist in cell signaling by regulating the passage of molecules through the ECM. 2. Adhesion junction—mechanically attaches adjacent cells; gap junction—allows cells to communicate through channels; tight junction—connects plasma membranes, creating a tight barrier. 3. The ECM of an animal cell is a meshwork of proteins and polysaccharides that can vary in amount and flexibility depending on the cell type. The plant cell wall, external to the plasma membrane, is composed of cellulose fibrils and pectin that allows flexibility during growth and eventually hardens into a rigid structure.

Science in Your Life How Cells Talk to One Another: 1. The signaling molecule will have no effect on the cell. 2. Because the transduction pathway is a series of proteins, each of which can bring about a response in another protein. 3. That part of the signal transduction pathway involved with changes in the movement of the cell. This might control the spread of the cancer to other parts of the body.

Testing Yourself 1. b; 2. e; 3. d; 4. c; 5. c; 6. b; 7. e; 8. d; 9. c; 10. c; 11. c.

Thinking Critically 1. The receptor molecule changes its shape and that activates an internal signaling pathway, usually involving peripheral proteins and secondary messengers. 2. Lower concentrations of chloride outside of the cell results in a lower ionic concentration. Less water is attracted to the mucus outside the cells.

CHAPTER 5 Genetics of Breast Cancer 1. G1 checkpoint—determines if DNA is damaged and if growth signals and nutrients necessary for cell division are available. G2 checkpoint—determines if DNA has not finished replicating or if it is damaged, thus preventing start of M stage of cell cycle. M checkpoint—determines if chromosomes are properly aligned by the spindle assembly, assuring proper distribution to the daughter cells. 2. G1 checkpoint 3. If the checkpoints fail, the cell cycle will proceed even if the DNA is damaged, not replicated completely, or not distributed evenly between the daughter cells. Cancer, which is unregulated cell growth, can result.

Check Your Progress 5.1: 1. See Figure 5.1. 2. It removes unwanted tissue and abnormal cells. 5.2: 1. Cells stop at the G1 checkpoint if conditions are not favorable for cell division and/or there is DNA damage. Cells stop at the G2 checkpoint if the DNA has not finished replicating and/or there is DNA damage. Cells stop at the M checkpoint if the chromosomes are not lined up correctly at the metaphase plate. 2. Oncogenes encode proteins that continuously promote the cell cycle, leading to unregulated cell division. Tumor suppressor genes function to inhibit the cell cycle. If mutated the proteins that they express would not be active and the cell cycle would continue, possibly leading to cancer. 5.3: 1. One chromatid before replication; two chromatids after. 2. See Figure 5.7. 3. Both result in two daughter cells with identical genetic material. Animal cells form a cleavage furrow between the daughter nuclei, which is constricted by the action of a band of actin filaments. Plant cells build a new cell wall between the daughter cells by fusing

together vesicles produced by the Golgi apparatus. 5.4: 1. In order to divide the DNA content in half. Because the DNA was already replicated before meiosis began. 2. See Figures 5.11 and 5.14. 3. During crossing-over genetic material is exchanged between nonsister chromatids. This produces a different combination of genes. Independent assortment mixes the whole chromosomes donated by both parents. The end result of both processes is cells that do not have genetic material identical to either parent. 5.5: 1. See Table 5.1. 2. See Table 5.2. 5.6: 1. Sperm and egg cells are haploid. They function in sexual reproduction. 2. Four sperm; one egg.

Science in Your Life Genetic Testing for Cancer Genes: 1. Proponents for genetic testing cite that it can lead to early detection and a greater chance for successful treatment. Opponents feel that since genetic testing does not prevent the disease it is of less value. Fears exist that the results of genetic testing will be misused and that negative test results for a genetic mutation would lead to increased risky health behaviors. 2. The answer depends on one’s perception of their risk for cancer and the consequences, both negative and positive, of obtaining genetic test results.

Testing Yourself 1. See Fig. 5.1; 2. d; 3. b; 4. a; 5. d; 6. e; 7. b; 8. c; 9. c; 10. c; 11. d; 12. b; 13. d; 14. d; 15. c.

Thinking Critically 1. a. BPA could increase or decrease the production of cyclins, which would affect cell cycle progress through the check points. b. Because BPA acts like a hormone its affects are amplified within the cell. 2. If a cell cycle checkpoint is nonfunctional, then the cell proceeds through the cell cycle even if it is damaged. It can replicate unstopped, leading to cancer. 3. Sexual reproduction introduces genetic variation that may be an adaptive advantage to organisms responding to changing environments.

CHAPTER 6 Vampire Bats 1. To act as a catalyst, increasing the rate of the conversion of reactant to product in a series of linked reactions. 2. Temperature, pH, concentration of substrate, presence of activators, inhibitors, and cofactors.

Check Your Progress 6.1: 1. Once energy is released as heat it cannot be recaptured for recycling. 2. With digestion you are converting chemical energy to kinetic energy with a loss of heat according to the 1st law, which says that energy cannot be created or destroyed but can be changed from one form to another, and the 2nd law, which says that there will be a loss of usable energy every time energy is converted from one type to another. 6.2: 1. Endergonic. 2. ATP, with its three phosphate groups, is analogous to a charged battery which can provide energy via a coupled reaction when one phosphate is cleaved off. ADP that is produced is like a discharged battery, which requires an input of energy to become ATP again. 6.3: 1. Enzymes are needed to reduce that activation energy of the reactions present in a biochemical reaction, thus allowing reactions to occur under conditions of the cell. A cell can convert an enzyme from an inactive form to an active one by the addition or removal of phosphate groups or by cleaving off parts of the protein. Cellular enzymes are subject to feedback inhibition. The presence of cofactors and coenzymes in the cell regulates enzyme activity. 2. Enzymes lower the activation energy of a reaction, allowing it to occur with lower inputs of energy. 3. How well the enzyme interacts with the reactants and the rate at which product is formed is determined by the threedimensional shape of the protein molecule and its active site. 6.4: 1. They both involve the gain and loss of hydrogen atoms. In photosynthesis, water loses hydrogen atoms and carbon dioxide gains them. In cellular respiration, glucose loses hydrogen atoms and oxygen gains them. 2. See Figure 6.11.

Science in Your Life Enzyme Inhibitors Can Spell Death: 1. The arguments for using chemicals in medicine, even if they are dangerous, are based on the benefit they can provide versus the risk they represent. 2. The search for and harvesting of species can be detrimental to the existence of

the organism and its environment. However, the chemicals may benefit the health and well being of humans and other organisms without causing side effects.

Appendix A

CHAPTER 8 Colors of Fall

1. c; 2. e; 3. b; 4. d; 5. b; 6. c; 7. d; 8. d; 9. b; 10. a; 11. d; 12. c.

1. Chlorophylls. 2. Green chlorophyll is present in the spring and summer, but it degrades in the fall and the red or yellow carotenoids become visible.

Thinking Critically

Check Your Progress

Testing Yourself

1. The energy of activation must still be overcome. Enzymes enable the reactions to occur at the conditions present within the body. 2. Glycogen must be synthesized from individual glucose molecules. This type of reaction (anabolism) requires the input of energy. 3. The chemical reactions of photosynthesis and cellular respiration are very different. Mitochondria do not have the appropriate enzymes, structure, or light harvesting pigments to carry out photosynthesis.

CHAPTER 7 Metabolic Demands on Athletes 1. Aerobic metabolism in the cell uses oxygen to break down glucose to form ATP. Without oxygen under anaerobic conditions, glucose is broken down just to pyruvate, with much less ATP production. 2. As glucose is broken down in a series of controlled reactions in the cytoplasm and mitochondria, some of its chemical energy is released and used to make ATP. 3. Proteins are deaminated by the liver and their carbon skeletons can enter glycolysis, be converted to acetyl groups, or enter the citric acid cycle. Fats are broken down to glycerol and fatty acids. The glycerol can be converted to pyruvate to be converted to acetyl CoA, and the fatty acids are broken down to acetyl CoA, which enters the citric acid cycle.

Check Your Progress 7.1: 1. NAD+ and FAD serve as coenzymes for the enzymes involved in cellular respiration. Both molecules receive electrons and become reduced (NADH and FADH2) and transport the electrons to inside the mitochondria, where they function in the electron transport chain. 2. Glycolysis is an anaerobic process. The preparatory reaction, citric acid cycle, and electron transport chain are dependent on the presence of oxygen. 3. See Figure 7.2. 7.2: 1. Glucose must be activated by the investment of 2 ATPs before the cascade of reactions oxidizing glyceraldehyde 3-phosphate, forming 4 ATPs, can happen. 2. For each molecule of glucose, the inputs are 2 ATPs and 2 NAD+s. The outputs are 2 molecules of pyruvate, 2 NADHs, 2 ADPs, and 4 ATPs. There is a net gain of 2 ATPs. 7.3: 1. When muscles are working hard in a burst of activity, using up available oxygen. 2. During fermentation, pyruvate accepts electrons from NADH formed during glycolysis, and lactate is formed. The oxidized NAD+ is available to pick up more hydrogen atoms from glycolysis. 7.4: 1. See Figure 7.2. 2. See Figure 7.9. 3. The NADH formed outside the mitochondria during glycolysis sometimes cannot cross into the mitochondria. Instead NADH delivers its electrons to the electron transport chain using a process that costs one ATP/2 electrons. Since 2 NADH are formed during glycolysis, the total ATP count is reduced to 4 from 6.

Science in Your Life Metabolic Fate of Pizza: 1. The cheeseburger and fries would be digested into molecules of fat, protein, and carbohydrate. Each of these molecules would enter the cellular respiration pathways as depicted in Figure 7A. 2. Water is used in the hydrolysis reactions involved in the breakdown of carbohydrates, proteins, and lipids.

Testing Yourself 1. b; 2. d; 3. a; 4. c; 5. e; 6. c; 7. d; 8. d; 9. d; 10. e; 11. a; 12. b; 13. c.

Thinking Critically 1. Plasma membrane. 2. If the electron transport chain is the same in insects and humans, then Rotenone would inhibit the human chain also. In fact, rotenone is very toxic to humans. 3. The faster fatty acids are converted to acetyl CoA and enter the citric acid cycle, the greater the rate of respiration. These compounds may convert fatty acids to acetyl CoA more rapidly.

8.1: 1. The sun. 2. See Figure 8.2. 3. 6CO2 + 6H2O → C6H12O6 + 6O2 with the input of solar energy and presence of pigments. 4. Light reactions require the input of light energy. They result is the generation of NADPH and ATP, which function in the Calvin cycle. The Calvin cycle reactions are independent of the input of light energy. They reduce CO2 to produce carbohydrates. 8.2: 1. They reflect and do not absorb wavelengths of light that humans see as green. 2. In the noncyclic electron pathway, the excited electron from photosystem II passes down an electron transport chain to photosystem I, producing ATP. The excited electron from photosystem I is passed to NADP+, making NADPH. Both ATP and NADPH are used in the Calvin cycle reactions. 3. See Figure 8.8. 4. H+ ions present in the thylakoid space are at a higher concentration than in the stroma. The tendency is for the ions to move down this electrochemical gradient, releasing the stored energy to fuel ATP synthesis. 8.3: 1. Carbon dioxide is attached to RuBP by the enzyme RuBP carboxylase, and then converted to G3P in a two-step reaction involving ATP as the energy source and NADPH + H+ as the source of the electrons for the reduction step. 2. It takes 3 turns to make one G3P but for every 3 turns, 5 molecules of G3P are used to re-form 3 molecules of RuBP. 3. G3P can be converted into glucose, sucrose, starch, cellulose, fatty acids, or amino acids. 8.4: 1. Sugarcane, corn, succulent plants such as cacti. 2. It allows the stomata to close during the day so there is less loss of water without affecting the plant’s ability to fix carbon dioxide. 8.5: 1. See Figure 8.12. 2. Because the chemical pathways are not the reverse of each other. Different enzymes, cellular structures, and pigments are involved in the two processes.

Science in Your Life Diesel Power from Algae: 1. Producing biofuels from algae would reduce the demand for fossil fuels, thus reducing the carbon footprint. 2. Biodiesel production could benefit from upscaling the processes involved and increasing the efficiency of the distribution of the product. The New Rice: 1. Reducing meat and fuel consumption frees up disproportionately large amounts of resources that could be directed toward grain production. 2. No. Sustainable farming practices could help but GMOs solve some of the challenges of flooding and climate change directly.

Testing Yourself 1. a. thylakoid, b. oxygen, c. stroma, d. Calvin cycle, e. granum; 2. c; 3. e; 4. d; 5. a; 6. d; 7. e; 8. d; 9. c; 10. e; 11. c; 12. c.

Thinking Critically 1. Leaves that are broad allow maximum surface area for exposure to sunlight. Leaves that are thin allow sunlight to penetrate and reach the chloroplasts. 2. Artificial leaves need a large surface area for absorbing light energy and for allowing carbon dioxide and oxygen exchange. The light absorbing mechanisms need to be aligned with the mechanisms for generating carbohydrates and ATP. 3. The fixation of carbon dioxide.

CHAPTER 9 The Amazing Neem Tree 1. Roots stabilize plants in the soil and provide support. They absorb water and minerals from the soil. The stems are usually upright and support the leaves. Stems house the vascular tissue used in the transport of water, minerals, and organic molecules. Leaves are the major site of photosynthesis. 2. Variations in roots, stems, and leaves make it possible for plants to live in environments that differ in soil types, moisture, sunlight, and wind.

Check Your Progress 9.1: 1. Because all stems have nodes and internodes, while not all stems are above ground. 2. See Figure 9.1 for structure comparison. Roots: the root hairs, with their large surface

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area, allow for uptake of water and minerals. The branching of the roots allows them to stabilize the aboveground portion of the plant. Stems allow for continued growth and contain xylem and phloem for transportation of water and nutrients through the plant. Leaves are thin so that sunlight can reach the chloroplasts and gases can diffuse easily. The petiole allows for maximum exposure to the sun. 9.2: 1. Meristematic tissue. 2. Epidermal tissue: protection, conservation of water; Ground tissue: photosynthesis, carbohydrate storage; Vascular tissue: transport of water and nutrients. 3. Ground tissue: parenchyma (progenitor cells), collenchyma (flexible support), sclerenchyma (support, some transport water); Vascular tissue: xylem (transports water and minerals from roots to leaves), phloem (transports sugar and other organic compounds throughout the plant). 9.3: 1. See Figure 9.7. 2. Monocots: corn, grass, palms; Dicots: dandelions, oak trees, potatoes, kale. 9.4: 1. Via a Casparian strip, which requires water and minerals to pass through the cells to enter the xylem. 2. The monocot root has a vascular cylinder surrounding a central pith, while the eudicot root has the phloem and xylem in the center of the root. 3. Adventitious roots (stabilizing the shoot system), epiphytes (absorption of water), haustoria (absorption of water and nutrients), root nodules (obtaining nutrients), mycorrhizae (absorption of water and nutrients). 9.5: 1. See Figures 9.15 and 9.16. 2. In the spring when water is abundant, secondary xylem contains wide vessel elements with thin walls. When water becomes restricted in the summer, the wood has fewer vessels and contains thick-walled vessels. 3. Stolons in aboveground horizontal stems produce new plants when touching the soil. Cacti have aboveground vertical stems that store water. Many vines have stems with tendrils that twine around structures. Rhizomes are underground horizontal stems. Corms are bulbous underground stems which are dormant in winter. 9.6: 1. See Figure 9.21. 2. Shade plants have broad, wide leaves; desert plants have reduced leaves with sunken stomata. Leaves tend to be thin and flat to allow maximum sunlight absorption. 9.7: 1. Cohesion-water molecules tend to cling together; adhesion-water molecules interact with the walls of the xylem vessels; both are required to keep the column of water together as it ascends the xylem channel. 2. Evaporation of water from a leaf causes the whole channel of water to move upward, drawing in more water from the roots. 3. After sugar is actively transported into the phloem sieve tubes, water follows by osmosis. This creates a pressure that causes the phloem contents to flow from the source to the sink.

Science in Your Life The Many Uses of Bamboo: 1. Whether bamboo products are worth the extra cost depends on the price, how much money the buyer has, and the value the buyer puts on the product and its lesser environmental impact. 2. Farmers could be offered economic incentives and subsidies to switch to growing bamboo. 3. Competition with other markets that are established and employ many, and displacement of other habitats to grow bamboo. Using Plants to Clean up Toxic Messes: 1. If the plant has converted the pollutant into a form that is not a hazard, then the dead plants can be treated as usual. If the plant has collected and concentrated the pollutant, then the dead plant material must be collected and treated as hazardous waste. 2. Each plant has a unique physiology which allows it to handle toxic materials differently. 3. Pollutants would move into a plant and be distributed throughout the plant just like water and minerals are absorbed and distributed, via the cohesion-tension, and organic molecules via the pressure-flow, models.

Testing Yourself 1. b; 2. d; 3. a; 4. c; 5. c; 6. c; 7. c; 8. c; 9. c; 10. b; 11. b; 12. b; 13. a; 14. d; 15. d.

Thinking Critically 1. Stems have nodes and internodes. The xylem and phloem in roots are arranged centrally while xylem and phloem in stems are found in bundles, either dispersed throughout the stem or arranged in a ring toward the outside of the stem. 2. Woody plants can increase in girth and grow taller to reach sunlight, but this costs energy. 3. Both use active transport and osmosis for transporting substances into the vessels. Plants use passive means of transport, whereas humans use a pump. Plants separate water and minerals from organic nutrients, while humans do not. Plants have two separate sets of vessels (xylem and phloem) with the xylem being arranged in a one way pathway for transport, while humans have two



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Appendix A

connecting sets of vessels (arteries and veins) arranged in a circular pathway. Some of the vessels in plants are dead, while all of the vessels are alive in humans. 4. At night the large number of stomata open and are able to absorb moisture from coastal fog. During the day the stomata close to prevent loss of moisture.

CHAPTER 10 Seedless Plants 1. Auxins, gibberellins, cytokinins, abscisic acid, and ethylene. 2. Selective breeding; creation of transgenic organisms, often called genetically modified organisms; introduction of genetic traits that offer resistance to insects, herbicides, salty environments, or potato blight, or lead to production of monounsaturated fatty acids in soybeans, or increase productivity by altering C3 plants to C4 plants.

Check Your Progress 10.1:1. The sporophyte generation produces microspores and megaspores. A microspore undergoes mitosis and becomes a pollen grain (male gametophyte), and the megaspore undergoes mitosis to become a microscopic embryo sac (female gametophyte). 2. Anther: development of pollen grains. Filament: holds up anther. Stamen: male part of flower consisting of anther and filament. Pistil (or carpel): female part of flower, consisting of stigma, style, ovary, and ovule. Stigma: sticky knob for holding pollen. Style: holds up stigma. Ovary: enlarged base that holds one or more ovules. Ovule: houses megaspores and, when fertilized, is chamber where seed develops. 3. Heterosporous means that a plant produces two types of spores: microspores which become a pollen grain (male gametophyte) and megaspores which become the embryo sac (female gametophyte). 4. See Figure 10.5. 10.2: 1. In monocots food molecules are absorbed from the endosperm. In eudicots the nutrients are stored in the cotyledons and the endosperm disappears. 2. Adaptions include: wooly hairs; plumes; wings, like on maple seeds; small size, as with orchids; parachutes of dandelions; and the bursting seed pods of touch-me-nots. 3. Regulation of the timing of the germination process assures that seeds are in the best environment for survival. 4. Compare Figure 10.9 with Figure 10.10. 10.3: 1. Vegetative propagation and tissue culture. 2. See Figure 10.11. 3. Corn, cotton, soybean, and potato plants all have been genetically engineered to be resistant to herbicides. Tomatoes have been engineered to be salt tolerant. 10.4: 1. Auxins—promote shoot growth from the apical meristem or from the ancillary buds when the terminal bud is removed. Gibberellins cause elongation of cells. Cytokinins promote cell division. Abscisic acid initiates and maintains seed and bud dormancy, while controlling the opening of stomata under water stress. Ethylene functions in the process of abscission. 2. Plant growth tropisms include growth toward or away from a stimulus, such as light, gravity, and touch. 3. Short day plants flower during shorter daylight periods, while long day plants flower during longer-light days. 4. Since phytochrome conversion from inactive to active forms is the first step in a signaling pathway that results in flowering, regulating the conversion could lead to controlling plant flowering commercially.

Science in Your Life The Coevolution of Plants and Their Pollinators: 1. They have a symbiotic relationship in which both partners benefit. 2. Without bees, the plants they pollinate would not reproduce. The organization of the ecosystems in which the plants live would be altered, threatening all those organisms who depend on the ecosystem for food, habitats, and products. 3. Mutualistic relationships in which both members benefit have coevolved. These include bees and sweet and fragrant flowers; wasps and orchids; moths and fragrant pale flowers; butterflies with bright, odorless flowers; hummingbirds and red flowers with curled margins; and bats and flowers that open at night and are light colored. Are Genetically Engineered Foods Safe?: 1. Proponents of labeling GMO organisms, specifically GMO crops, feel that there should be transparency and that the consumer has the right to know what they are buying so that they can make their own decisions about a purchase. Opponents to labeling GMOs feel it is too costly to agricultural companies and subject to the consumer’s unsubstantiated fears. The complexity of even defining a GMO makes labeling GMO products difficult. 2. If the safety of golden rice can be demonstrated and endorsed by those knowledgeable about public health, not just the scientists and businesses who

developed the organism, then it should be planted and distributed widely to help prevent blindness and promote better health.

Testing Yourself 1. b; 2. a. diploid, b. anther. c. ovule, d. ovary, e. haploid, f. megaspore, g. pollen grain, h. embryo sac, i. sperm, j. seed; 3. a. stigma, b. style, c. ovary, d. ovule, e. receptacle, f. peduncle, g. sepal, h. petal, i. filament, j. anther; 4. b; 5. c; 6. e; 7. d; 8. c; 9. d; 10. b; 11. e; 12. a; 13. c; 14. d.

Thinking Critically 1. Meiosis is used to produce gametes (sperm and eggs) in animals and to produce microspores and megaspores in flowering plants. Mitosis in animals leads to growth and development of the zygote. In plants, microspores and megaspores undergo mitosis to produce male and female gametophytes. 2. A single, specialized pollinator species will ensure a greater probability of pollen transfer and fertilization to the correct species. 3. You could measure the presence and distribution of abscisic acid in the stems of sunflowers as they bend throughout the day and look for correlations. 4. Seeds are a product of sexual reproduction and contain a certain amount of genetic variability. While asexual propagation by tubers is cheaper, faster, and easier than by seeds, there is an inherent risk of not having genetic variability if the environment changes.

CHAPTER 11 The Biology of Performing 1. More engaged systems: respiratory, cardiovascular, skeletal, muscular, nervous, integumentary. Less engaged: lymphatic and immune, digestive, urinary, endocrine, reproductive. 2. Epithelial, connective, muscular, nervous. 3. Organ systems interact within the body to maintain homeostasis, transport substances, support function, and control disease.

Check Your Progress 11.1: 1. See Figure 11.1. 2. See Figure 11.2. 3. See Figure 11.4. 4. A neuron has a cell body, dendrites, and an axon. Neurons function in sensory input, integration of information, and motor output. Neuroglia support and nourish neurons and participate in brain function. 11.2: 1. Thoracic, abdominal. 2. Mucus protects the digestive, respiratory, urinary, and reproductive systems from bacteria and viruses. Serous fluid lubricates membranes that support internal organs and divide body cavities. Synovial fluid lubricates cartilage at the end of bones. 11.3: 1. Integumentary—skin; cardiovascular—heart; lymphatic and immune—lymph nodes; digestive—small intestine; respiratory—lungs; urinary—kidneys; nervous— spinal cord;—musculoskeletal—skeleton; endocrine— pancreas; reproductive—ovaries. 2. Integumentary, and lymphatic and immune. 11.4: 1. See Figure 11.8. 2. They might not absorb enough UVB rays to produce adequate levels of vitamin D which helps to keep bones strong. 3. Nails—protect the distal parts of digits; hair follicles— produce hair; oil glands secrete sebum. 11.5: 1. In negative feedback, a sensor detects a change in internal conditions, resulting in a response that brings the conditions back to normal. Positive feedback involves an ever-greater change in the same direction until the initial stimulus stops. 2. The respiratory, digestive, and urinary systems interact to take in O2 and nutrients needed by the body, and eliminate unused or waste products such as CO2 and nitrates. They assure that blood pH, blood volume, and glucose levels are maintained within strict ranges. 3. Failure of the immune system results in infectious diseases such as shingles and candidiasis. Failure of the cardiovascular system results in heart failure or strokes. Failure of endocrine organs can result in diabetes or hypothyroidism.

Science in Your Life UV Rays: Too Much Exposure or Too Little?: 1. Skin cancer interrupts the functions of skin which contribute to homeostasis, such as regulating body temperature, controlling water loss, and protecting against pathogens. 2. Being unaware of what melanoma looks like or that it is a serious health threat; not being able to see the skin on your back; having darkly pigmented skin that makes moles harder to see. Barriers to accessing health care services such as poverty, lack of insurance, and adequate transportation also contribute. 3. There is disagreement as to what is a desirable

level of vitamin D in the bloodstream and uncertainty about the effect of maintaining any specific level. There are benefits for having enough vitamin D and risks with ingesting too much. Just a Snip, Please: Testing Hair for Drugs: 1. Procedures and policies for testing for drugs must comply with the Fourth Amendment. However, an employer has the right to deny an applicant on the basis of drug evidence. 2. Anytime drug use interferes with job performance or the safety of others, drug testing is warranted. In competitive sports, performance enhancing drugs alter the balance between players. An organization certifying athletes’ performance has the right to set the policies covering testing for drugs.

Testing Yourself 1. c; 2. a; 3. e; 4. a. ventral cavity, b. thoracic cavity, c. abdominal cavity, d. pelvic cavity, e. dorsal cavity, f. cranial cavity, g. vertebral canal, h. diaphragm; 5. d; 6. b; 7. a. hair shaft, b. epidermis, c. dermis, d. subcutaneous layer, e. adipose tissue, f. sweat gland, g. hair follicle, h. arrector pili muscle, i. oil gland, j. sensory receptor, k. basal cells, l. sweat pore; 8. c; 9. b; 10. b.

Thinking Critically 1. Blood is like a tissue because it contains similarly specialized cells that perform a common body function. Blood cells are specialized to circulate throughout the body, bringing O2 to the tissues (red blood cells), preventing the blood from being lost from the body (platelets), and protecting the body from infection (white blood cells). 2. a. Negative feedback, since secretion of the pituitary gland hormone is inhibited by epinephrine. b. Positive feedback, because the stronger the signal becomes, the stronger the stimulus to respond, until urination occurs. c. Negative feedback, because the various sensory receptors are detecting a change in a set point (blood volume), and the kidney is responding by increasing urine production. 3. Through homeostatic mechanisms, relatively constant internal environments are maintained. If internal conditions vary too much from expected ranges, metabolic functions are disrupted and death can occur.

CHAPTER 12 A Silent Killer 1. Systolic pressure results from the contraction of the ventricles and diastolic pressure results when the ventricles relax. 2. Sensory receptors in the carotid arteries and aorta detect changes in blood pressures and communicate that information to the cardiovascular center in the brain. Nerve impulses are sent to the blood vessels to either constrict or relax. 3. Risk factors for hypertension include obesity, smoking, chronic stress, and high salt intake.

Check Your Progress 12.1: 1. Blood flow is controlled in arteries by contraction of smooth muscle in artery walls. Capillary blood flow is affected by the pressure of arterial supply plus the contraction of pre-capillary sphincters. Venous blood flow is affected by arterial and capillary blood flow, and valves that prevent blood from flowing backward. 2. O2, CO2, glucose, amino acids. 12.2: 1. See Figure 12.3. 2. See Figure 12.6. 3. Diabetes; heart disease,; liver disease; Alzheimer and Parkinson diseases. 4. If the protein content of the blood is reduced, then the osmotic pressure of the blood is lowered and less water moves from the interstitial fluid into the blood. 12.3: 1. Blood enters the right atrium, then the right ventricle, then travels through the pulmonary arteries to the lungs, and through the pulmonary veins back to the left atrium and then left ventricle. 2. If the left ventricle was not contracting strongly, blood would back up into the lung, increasing blood pressure there, resulting in increased fluid leakage from blood vessels in the lung. 3. The heartbeat “lub” sound is caused by the closure of the atrioventricular valves and the “dub” sound is caused by the semilunar valves closing. 4. Intrinsic control of the heartbeat is needed to maintain regular coordinated contractions, while extrinsic control allows the heartbeat to change in response to different physiological demands. 12.4: 1. Pulmonary arteries. 2. Blood flows to the lungs via the pulmonary arteries, and returns to the heart via the pulmonary veins. Blood flows to the body via the aorta, and returns via the superior and inferior venae cavae. 3. Blood flows in one direction in arteries due to the pressure created from the pumping of the heart, and in veins mainly due to muscle contraction and the presence of valves.

12.5: 1. Atherosclerosis occurs when plaque builds up in the linings of arteries. The plaque can restrict blood flow through vessels, or break off and form an embolus. Hypertension can lead to stroke and heart attack. Heart valve disease results in the malfunction of the pumping of the heart. Strokes result when brain blood vessels burst or are blocked. Heart attacks occur when coronary arteries are blocked. Aneurysms happen when the artery wall bulges out and sometimes bursts. 2. See Table 12.2. 3. Failing hearts can be replaced with a transplant from a human donor, or with an artificial heart. A left ventricular assist device can help a failing heart function until it can be replaced. Potential problems include rejection of a transplanted heart by the recipient’s immune system, and the mechanical failure of an artificial pump.

Science in Your Life Medicinal Leeches: Medicine Meets Fear Factor: 1. The response to this question depends on one’s discomfort with leeches as a medical device. 2. Diseases were once thought to occur because of an imbalance between the “humors” in the body. It was thought that blood letting would reestablish the correct balance. 3. The introduction of yeast and bacteria to reestablish a healthful gut flora; therapy pets to help people cope with physical and mental challenges. Prevention of Cardiovascular Disease: 1. The response to this will depend on the individual. 2. Infection with Group B Streptococcus can impact heart valve health. Infections in general increase inflammation, which can lead to increased plaque formation. Infections of the gut can lead to decreased absorption of nutrients and lead to lipid imbalances and vitamin deficiencies, both of which can influence cardiovascular health. 3. The French paradox refers to the observed low rates of cardiovascular disease in the French people, who characteristically consume diets rich in saturated fats and cholesterol. Explanations for the paradox include increased red wine consumption, cultural differences in attitudes about food, consumption of more fruits and vegetables, and increased levels of physical activity.

Testing Yourself 1. a; 2. d; 3. c; 4. b; 5. e; 6. d; 7 b; 8. e; 9. a; 10. d; 11. a; 12. a. carotid artery, b. pulmonary vein, c. aorta, d. renal artery, e. pulmonary artery, f. superior vena cava, g. inferior vena cava, h. hepatic vein, i. hepatic portal vein, j. renal vein; 13. d; 14. c.

Thinking Critically 1. Tissues that do not contain blood vessels include the corneas of the eyes, the outermost layer of the skin, and the cartilage that lines joints. These tissues presumably either receive their required nutrients by diffusion from nearby vessels, and/or have lower metabolic requirements than more highly vascularized tissues. 2. The answer will vary depending on age, but for someone with an average heart rate of 70 beats per minute, on their 20th birthday their heart will have beaten 70 × 60 minutes/hour × 24 hours/day × 365 days/year × 20 years, or 735,840,000 times. In terms of volume, 5.25 liters/minute × 60 minutes/hour × 24 hours/day × 365 days/year × 20 years = 55,188,000 liters (over 14 million gallons). 3. Tracing “a” shows too many QRS complexes, indicating atrial fibrillation. The complexes in tracing “b” are missing the “T” waves, and the heartbeat is too slow, suggesting a problem with electrical transmission in the heart. A pacemaker is an electrical device that can be implanted in a patient’s heart muscle, which takes the place of a malfunctioning SA node, to deliver a regular electrical impulse to the heart muscle. 4. A total artificial heart (TAH) would have to be as incredibly durable as the heart is, to last even 20 years. The TAH also needs to be made of a material that does not stimulate the body to react against it in any way, including the immune and clotting systems.

CHAPTER 13 When Antibodies Are Lacking 1. Up until 6 months, children benefit from the passive immunity received from their mothers. 2. Without sufficient levels of antibodies, XLA patients cannot effectively remove bacteria which are outside of the cells in the body. NK cells and cytotoxic T cells function to remove virus infected cells.

Check Your Progress 13.1: 1. Excess interstitial fluid moves into the lymph vessels and is returned to the bloodstream. Fats from the small

Appendix A intestine move into the lymph vessels and are transported into the bloodstream. White blood cells in the lymph system function in protecting the body from disease. 2. Lymph is the excess interstitial fluid that has been absorbed into lymphatic vessels. It is mostly water but also contains nutrients, electrolytes, oxygen, hormones, enzymes, and wastes. Blood contains erythrocytes, leukocytes, platelets, waste solutes, and products of cells. 3. In the red bone marrow and the thymus, which are primary lymphoid organs, lymphocytes develop and mature. In the lymph nodes and the spleen, lymphocytes become activated. 13.2: 1. The skin and mucous membranes are physical barriers; a chemical barrier is the acid of the stomach. 2. See Figure 13.3. 3. Macrophages kill pathogens by engulfing them into a vesicle that has an acid pH, hydrolytic enzymes, and reactive oxygen compounds. NK cells induce cells that lack self-MHC-I molecules to undergo apoptosis (cell suicide). 4. The complement system enhances phagocytosis of pathogens, activates inflammation, and kills pathogens by forming a membrane attack complex. 13.3: 1. Diversity in antigen receptor occurs because: 1. Genes for T and B cell receptors contain segments that code for many parts of the antigen receptor. 2. Enzymes in the T and B cells cut out these segments and combine them differently. 3. Mutations may be introduced as these segments are combined. 4. The two protein chains, containing the variation of steps 1–3, combine to produce a new receptor. 2. The clonal selection theory states that B cells and T cells have cell surface receptors for only one specific antigen. When the cell contacts that specific antigen, it is selected to undergo clonal expansion (divide) and differentiate into either memory cells or cells that actively fight infection. 3. The five classes are IgG (activates complement, crosses the placenta), IgM (early response, activates complement), IgA (found in body secretions), IgD (found only on immature B cells), and IgE (protects against parasites). 4. Helper T cells recognize antigen fragments in combination with MCH molecules presented by antigen-presenting cells that have MHC class II proteins on their surface. Cytotoxic T cells recognize antigenpresenting cells with MHC class I proteins. Suppressor T cells inhibit these responses and act to control adaptive immunity. 13.4: 1. Three types include: attenuated (live but nonvirulent), genetically engineered (also called subunit vaccines), and DNA vaccines. 2. The immune system of the newborn is immature, plus the newborn has not been exposed to any infectious agents, so it would be very susceptible to infectious diseases if it did not receive antibodies from its mother. 3. The interaction of cytokines with their receptors could be blocked using antibodies that react with the cytokine, or possibly with the receptor. 4. Monoclonal antibodies are extremely specific because they react with only one antigen. Because of their specificity and purity, reaction against the preparation is avoided. 13.5: 1. Immediate allergic responses are caused by IgE antibodies that have already been produced against the offending allergen; delayed-type responses take longer because they require sensitized T cells to secrete cytokines that cause inflammation. 2. If a mother is Rh-positive, she will not produce anti-Rh antibodies that would react with the antigens of the fetus. 3. Drugs that inhibit cytokine production may inhibit certain desirable immune responses that protect the individual. 13.6: 1. Muscular weakness occurs in patients with myasthenia gravis because antibodies attach to and disrupt neuromuscular junctions. In multiple sclerosis, T cells attack the myelin surrounding nerves and hamper nerve conduction. Antibody–antigen complexes form in people with systemic lupus erythematosus and these get deposited in organs, including the kidneys. Antibody–antigen complexes form in rheumatoid arthritis and these primarily affect the joints. 2. A congenital immunodeficiency disease is severe combined immunodeficiency. AIDS is an acquired immunodeficiency disease.

Science in Your Life Should Parents Be Required to Have Their Kids Vaccinated? 1. Because unvaccinated children pose a risk to the population, restrictions in attending public school are justified. 2. The importance of religious beliefs must be weighed against good practices of health care for the whole population. It is reasonable that society would restrict where unvaccinated children could interact with the general population. 3. Pediatricians have belief sets that are implemented in their practice of medicine. In choosing not to see unvaccinated children they are following their conclusion that it is dangerous to the community to not vaccinate

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children. Perhaps a better approach would be to see unvaccinated children in a separate setting and attempt to educate them and their parents on the benefits of vaccination. Immediate Allergic Responses: 1. People previously exposed to an antigen can develop hypersensitivity, leading to immediate allergic responses. 2. One explanation is that children in developed countries are exposed to reduced kinds and levels of allergens. Their ability to cope with the allergens when encountered later in life is compromised. 3. Epinephrine constricts blood vessels, increasing blood pressure, and reducing the possibility of anaphylactic shock. Opportunistic Infections and HIV: 1. Deficiencies in immune function can occur in healthy people undergoing medical challenges. Once over the challenge, immune function is restored and the yeast infection is resolved. With HIV-infected people, the virus remains, constantly compromising their immune function and ability to clear the yeast. 2. The number of helper T cells which would help to combat herpesvirus 8 is reduced in HIV positive people. This increases their susceptibility to herpesvirus 8. 3. Almost all helper T cells are gone. The individual has little defense against the virus.

Testing Yourself 1. c; 2. b; 3. a; 4. c; 5. c; 6. c; 7. a; 8. a; 9. e; 10. a. antigen binding sites, b. light chain, c. heavy chain, V = variable, C = constant; 11. d; 12. d; 13. d; 14. d; 15. c; 16. b.

Thinking Critically 1. Innate immune mechanisms are valuable mainly because they can react very quickly to a wide variety of pathogens. However, organisms lacking adaptive immunity are not able to mount specific antibody and T cell responses targeted at one specific organism, nor are they able to develop memory cells that are able to respond faster to subsequent exposures to the same organism. 2. The administration of antivenom antibodies is an example of passive immunity. The antibodies will go away over time. If a person is bitten again they need another injection of antivenom. 3. ABO incompatibility between mother and fetus usually causes no problems because the antibodies the mother has against the nonself A or B antigen are normally of the IgM type, which is too large to cross the placenta. IgG antibodies can cross the placenta. Because around 85% of the population is Rh-positive, most females do not produce anti-Rh antibodies that could damage or kill the fetus; therefore, there is not a strong evolutionary selective pressure to eliminate the Rh molecule.

CHAPTER 14 Gastric By-pass Surgery 1. The mouth and stomach mechanically and chemically break down food. The stomach enzymatically breaks down some proteins. With the exception of alcohol and glucose, nutrients are not absorbed until food reaches the small intestine. Enzymatic digestion occurs in the small intestine. Once absorbed into the bloodstream, nutrients are distributed to the liver or tissues for processing or utilization. 2. Genetic factors influencing the breakdown and absorption of food, as well as the rate at which it is metabolized, influence a person’s weight. Also, one’s disposition toward physical activity or other behaviors requiring energy can impact weight.

Check Your Progress 14.1: 1. Food passes through the mouth, pharynx, esophagus, stomach, small intestine, and large intestine. Mechanical digestion occurs in the mouth and stomach. Chemical digestion occurs in the stomach and small intestine. 2. The small intestine processes food for digestion and absorbs nutrients. Its lower part, the ileum, is involved in immune responses. The large intestine absorbs water, salts, and some vitamins, and eliminates indigestible material as feces. 3. Hormones that increase digestive activity include gastrin, secretin, and CCK. Inhibiting these might regulate weight but they can cause difficulties in digesting food, leading to excess gas or constipation. 14.2: 1. The pancreas secretes hormones (endocrine) such as insulin and glucagon into the blood, and also secretes digestive enzymes (exocrine) into the small intestine by way of ducts. 2. Salivary glands produce saliva, which moistens food and contains amylase, which begins the digestion of starch. The liver performs functions summarized in Table 14.2. The main function of the gallbladder is to store bile. The pancreas secretes bicarbonate, digestive enzymes, and the hormones insulin and glucagon. 3. The gallbladder stores bile and people can live without this function. The liver



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has essential functions of detoxification, storage, synthesis, lipid regulation, and hemoglobin breakdown, all of which are necessary for survival. 14.3: 1. Carbohydrate digestion begins in the mouth, with salivary amylase, and continues in the small intestine, due to maltase and pancreatic amylase. Protein digestion begins in the stomach, where pepsin breaks proteins into peptides, and continues with trypsin and peptidases in the small intestine. Fats are mainly digested by pancreatic lipase in the small intestine. 2. Carbohydrates are broken down to glucose; proteins to amino acids; fats to glycerol and fatty acids. 3. The digestive system is divided into many compartments, each of which has a distinctive internal environment and pH. These differences contribute to regulation of digestive enzymes. 14.4: 1. The glycemic index (GI) of foods is an indicator of whether the blood glucose response to their ingestion is high or low. The advantage of consuming foods with a high GI is that they can provide a quick burst of energy, but that energy is usually short-lived and over time, consuming too many high-GI foods can lead to type 2 diabetes and heart disease. 2. A diet deficient in protein does not supply adequate amounts of essential amino acids used to synthesize necessary proteins. Too much protein must be deaminated in the liver, which requires abundant water for excretion and calcium loss in urine. 3. Vitamins are organic molecules (other than carbohydrates, fats, and glucose) that act as coenzymes and cannot be synthesized by the body. Minerals are inorganic elements that serve a variety of functions. Major minerals such as calcium are in cells and body fluids and also can serve as structural components of tissues. Trace minerals are needed in smaller amounts and are parts of larger molecules. 4. Dietary supplements can correct deficiencies in vitamin and mineral intake. However, in developed countries such deficiencies do not exist. In that case, the supplements are in excess of what is needed and can be present at toxic levels. The body eliminates water soluble nutrients in the urine but lipid-soluble vitamins A and D are stored in body fat and can reach high concentrations. 14.5: 1. People with anorexia nervosa restrict their food intake, thinking of themselves as overweight no matter what they weigh. Those with bulimia nervosa eat more than they think is appropriate and rid themselves of the food by vomiting or the use of laxatives. Binge eating involves ingesting more food than needed, usually for emotional reasons. This most often leads to obesity. People with pica eat unusual substances for reasons that are not understood. 2. Abnormally increased or decreased levels of hormones that influence appetite, such as gastrin, gastric inhibitory peptide (GIP), or leptin, can predispose a person to eating disorders. 14.6: 1. Most stomach ulcers are caused by the bacterium Helicobacter pylori. 2. The pancreas is essential in regulating serum glucose levels by the secretion of insulin and glucagon. The pancreatic juice it secretes neutralizes stomach acid and provides digestive enzymes. The liver synthesizes bile and essential proteins, regulates glucose and cholesterol levels, metabolizes many waste products, and stores iron and many vitamins. Both organs are essential to life.

Science in Your Life Vegetarians: Where Do You Get Your Protein? 1. If these animals have grown big then it indicates that a vegetarian diet is adequate. However, their digestive systems are structured differently and the way they digest food varies from humans. What works for cattle, horses, and elephants may not work for humans. 2. If amino acids are not stored, it seems logical that you must eat a full complement of all 20 essential amino acids at one time. Eating different foods that supply the full complement of amino acids is desirable. 3. If a specific amino acid is not available, then elongation of a peptide chain on the ribosome during translation will not occur. A Bacterial Culprit for Stomach Ulcers: 1. To survive, H. pylori must either have enzymes that operate with acidic optimal pHs or it must be able to isolate itself from the acidic environment. 2. People are reluctant to change their beliefs and thinking about scientific principles. 3. The argument could be made that it was ethical but not prudent behavior. Better to have used animal models to test the impact of H. pylori.

Testing Yourself 1. c; 2. c; 3. d; 4. c; 5. a; 6. d; 7. e; 8. c; 9. a; 10. c; 11. b; 12. b; 13. e; 14. b; 15. a; 16. d; 17. b.

Thinking Critically 1. A loss of mechanical digestion could be replaced by eating cooked and pureed foods. A lack of chemical digestion,

however, would prevent the breakdown of food into smaller, absorbable molecules, which would necessitate direct introduction of nutrients into the bloodstream. 2. Digestion of complex carbohydrates begins in the mouth with salivary amylase, and continues in the small intestine with pancreatic amylase. Ultimately, monosaccharides such as glucose are absorbed from the small intestine into the hepatic portal vein, where some travel to the liver and some enter the general circulation via the hepatic veins and inferior vena cava. Cells transport glucose molecules across their plasma membranes and utilize them to produce ATP. 3. Excess amino acids are transported to the liver where they are deaminated and urea is produced. This process uses water and can result in dehydration. Eating too much protein can also result in calcium loss in the urine. 4. Only tube 4 containing pepsin, HCl, and water should result in the breakdown of the egg white. The other tubes lack a necessary component of this reaction mix.

CHAPTER 15 Living With Asthma 1. Narrowing of the airways due to swelling would restrict how much air could pass through. 2. Treatments for asthma often dilate bronchi. They address the symptoms rather than the causes of asthma. 3. Asthma is a response to the body’s exposure to allergens. Regulating the immune system’s response to allergens is extremely difficult.

Check Your Progress 15.1: 1. The upper respiratory tract consists of the nasal cavities, pharynx, and larynx. The lower respiratory tract consists of the trachea, bronchial tree, and lungs. 2. The trachea lies ventral to the esophagus. When food is swallowed, the epiglottis covers the tracheal opening (glottis) so the food slides over the epiglottis and into the esophagus. 3. O2 travels through the nasal cavity, pharynx, larynx, trachea, bronchial tree, and lungs, where it diffuses across the alveolar and capillary endothelial cells to the bloodstream. 4. Pulmonary surfactants lower the surface tension of the coating of the alveoli, preventing their sides from collapsing upon themselves. 15.2: 1. Tidal volume is the amount of air that normally moves in and out of the lungs with each breath. Vital capacity is the maximum volume of air that can be moved in and out during a single breath. Expiratory reserve volume is the air that can be forcibly exhaled beyond the tidal volume. Residual volume is the air left in the lungs after a forced exhalation. 2. Inspiration requires the contraction of the diaphragm and external intercostal muscles; expiration is passive because it requires no muscle contractions, just the elastic recoil of the thoracic wall and lungs. 3. The respiratory center in the brain automatically sends nerve impulses to the diaphragm and intercostal muscles. The vagus nerve carries inhibitory impulses from the lungs to the brain to stop the lungs from overstretching. The carotid bodies have chemoreceptors that monitor levels of O2 in the blood. 15.3: 1. Hemoglobin’s most essential function is to carry O2 (as oxyhemoglobin) to the tissues, but it also carries a small amount of CO2 (as carbaminohemoglobin) back to the lungs. Excess H+ ions can be taken up by hemoglobin, which then becomes reduced hemoglobin. 2. Arterial blood is brighter red than venous blood because the red blood cells in arterial blood contain oxyhemoglobin, which is bright red, compared to the red blood cells in venous blood that contain reduced hemoglobin, which is darker. Blood from a cut appears bright red because hemoglobin becomes oxyhemoglobin upon exposure to O2 in the air. 3. The exchange of gases in respiration is by diffusion. No energy is required, which indicates a passive process. 15.4: 1. Pharyngitis is an inflammation of the throat, while tonsillitis occurs when the tonsils become inflamed and enlarged. 2. Smoking damages the tissues of the respiratory track directly. Infections from bacteria, fungi, protozoans, and viruses become more likely if the lung tissue is damaged. 3. Cystic fibrosis.

Science in Your Life Artificial Lung Technology: 1. Cost of the procedure as well as expanded time frames for development would restrict its distribution to the general public. 2. Lungs have a complex structure, which includes surfaces for gas exchange and an association with a circulatory system for transportation. 3. Tissues grown in the lab can be engineered to not display surface antigens which would identify the tissue as nonself. This would reduce tissue rejection. Timing of the transplant could be controlled rather

than waiting for a donor to appear. Are E-cigs Safe?: 1. With nicotine at low levels one can feel stimulated and, at high levels, relaxed. Both effects could be pleasurable and addictive. 2. E-cigs could be interfering with oxygen and carbon dioxide exchange and balances, pH levels, cardiac output, and cardiovascular health. Questions about Smoking and Health: 1. Avoid being near people who are smoking, especially in closed areas. Also do not occupy areas where smoking has occurred. 2. The toxins in cigarette smoke can be transmitted throughout the body and damage many organ systems.

Testing Yourself 1. a. nasal cavity, b. nostril, c. pharynx, d. epiglottis, e. glottis, f. larynx, g. trachea, h. bronchus, i. bronchiole; 2. a. sinus, b. hard palate, c. epiglottis, d. larynx, e. uvula, f. glottis; 3. b; 4. b; 5. b; 6. a; 7. a; 8. d; 9. d; 10. d; 11. d.

Thinking Critically 1. The respiratory system of birds is more efficient than that of mammals where inspired air passes into a “dead end” (the alveoli) and must be moved back out along the same path. Some amount of air always remains in the lungs. In the bird system, air passes in one direction through the air sacs. 2. Enlarged tonsils and adenoids partially block the airway, restricting flow. 3. Since CO binds to hemoglobin much more strongly than O2, much less O2 is delivered to the tissues and respiration is compromised. The specific cause of death would be asphyxiation. 4. When a person breathes through a tracheostomy, the air does not pass through the upper respiratory tract, which normally warms and humidifies the air, as well as removing many impurities. Therefore, tracheostomy patients are prone to lower respiratory tract infections.

CHAPTER 16 Born With Bad Kidneys 1. Excretion of metabolic wastes, osmoregulation, maintaining acid-base balance, secretion of hormones. 2. If collecting ducts are diseased or obstructed, then urine cannot reach the renal pelvis and will back up. 3. Sickled red blood cells are not as flexible as normal red blood cells and get clogged up in capillary beds, obstructing the flow of blood. Proper kidney function is dependent on many capillary beds allowing the exchange of gases, ions, and nutrients.

Check Your Progress 16.1: 1. Excretion of metabolic wastes, osmoregulation, maintaining acid-base balance, secretion of hormones. 2. See Figure 16.1. 3. With kidney disease, wastes will build up, salt and pH balance will be disrupted, and hormone secretion of the kidneys will be changed, all of which would interfere with homeostasis. 16.2: 1. The renal cortex contains the glomeruli, proximal convoluted tubules (PCT), and distal convoluted tubules (DCT). The renal medulla contains the loops of the nephron. 2. The epithelium of the PCT has many microvilli which increase the surface area for reabsorption. The DCT is composed of cuboidal epithelial cells that are not designed for reabsorption. 3. All these processes involve the movement of a substance across biological membranes. Diffusion and passive transport are dependent on concentration; active transport is not, but it requires energy. Active and passive transport usually require carrier molecules. 4. Glucose is normally returned from the glomerular filtrate to the blood by reabsorption via carrier proteins at the proximal convoluted tubule. 16.3: 1. See Figure 16.7. Reabsorption of salt, establishment of a solute gradient, reabsorption of water. 2. When blood volume and pressure is low, the juxtaglomerular apparatus secretes renin, which converts angiotensin I, which is then converted to angiotensin II that causes aldosterone secretion. 3. Diuretics can inhibit the secretion of ADH, increase the glomerular filtration rate, decrease the tubular reabsorption of sodium, or inhibit active transport of sodium at the loop of the nephron or at the DCT. 4. Kidneys regulate H+, HCO3−OH− and NH4+ to maintain blood pH 7.4. 16.4: 1. If kidneys cannot produce urine, then excretion of metabolic wastes is blocked, regulation of blood volume is compromised, maintenance of blood pH at 7.4 is impaired, and hormone secretion including renin, aldosterone, and erythropoietin is hindered. The lack of any of these functions is life threatening. 2. Smoking increases the risk of bladder cancer 5-fold due to the toxic byproducts of cigarettes being secreted into the bladder. 3. Females have a shorter, broader urethra that is closer to the anal opening and is more easily

contaminated with bacteria. An enlarged prostate in males may cause difficult urination.

Science in Your Life Matching Organs for Transplantation: 1. If kidneys for transplantation could be grown from a person’s own stem cells, then problems with mismatched blood groups, HLAs, or MHC proteins would be avoided. The need to take immunosupressive drugs would be avoided. The challenges for doing this include engineering the production of a kidney that functions well and is safe. 2. There are more antigens than the MHC molecules that are displayed on our cells which define one person versus another. 3. Since available organs are rare and the costs involved are considerable, consideration must be made of the probable success of a transplant. Factors could include the age and health of the recipient as well as their capacity to cope with the process of receiving a transplant and the long-term health maintenance.

Testing Yourself 1. a. renal cortex, b. renal vein, c. ureter, d. renal pelvis; 2. b; 3. d; 4. c; 5. b; 6. a; 7. a. renal cortex, b. renal vein, c. ureter, d. renal pelvis; 8. b; 9. b; 10. a; 11. b; 12. a.

Thinking Critically 1. If one is not eating large amounts of salt, then the regulation of sodium levels in the blood and the symptom of high blood pressure would be dependent on kidney function rather than diet. If the release of aldosterone is being successfully regulated by the kidney’s release of renin, then blood pressure should be under control. 2. Angiotensin II is a powerful vasoconstrictor in addition to its stimulating the adrenal cortex to release aldosterone, which leads to the reabsorption of sodium ions. Blocking angiotensin II’s production would lower blood pressure. 3. Frequent urination with diabetes mellitus is due to the presence of excess glucose in the urine filtrate, which draws water into the urine by osmosis. With diabetes insipidus, the increased urine output is due to the absence of ADH.

CHAPTER 17 An Autoimmune Attack 1. Protection and insulation of nerves; nerve conduction will be disrupted. 2. In the PNS, Schwann cells wrap themselves around axons, forming myelin. In the CNS, oligodentrocytes, a type of neuroglial cell, form the myelin. With MS, oligodendrocytes are targeted by the immune system. 3. White matter, which contains myelinated axons, is disrupted by MS.

Check Your Progress 17.1: 1. Three classes of neurons are sensory neurons, which take messages to the CNS; interneurons, which sum up messages from sensory neurons and other interneurons and communicate with motor neurons; and motor neurons, which take messages away from the CNS to effector organs, muscles, or glands. 2. Most neurons contain dendrites, a cell body, and an axon. See Figure 17.2. 3. Myelin is formed by Schwann cells in the PNS, and by oligodendroglial cells in the CNS. 4. Gray matter contains nonmyelinated nerve fibers; in white matter, the fibers are myelinated. The brain has gray matter on the surface and white matter in deeper tissue. That pattern is reversed in the spinal cord. 17.2: 1. The sodiumpotassium pumps in neurons are always transporting Na+ to the outside, and K+ to the inside of the cell. 2. During an action potential, the Na+ gates open and Na+ enters the cell, causing a depolarization. Then the Na+ gates close and the K+ gates open, causing a repolarization (even a slight hyperpolarization). See Figure 17.4. 3. See Figure 17.5. 4. After the action potential passes one part of an axon, the sodium gates in that part are unable to open for a period of time, called the refractory period. This prevents action potentials from moving backward. A node of Ranvier is a gap between Schwann cells that make up the myelin sheath of an axon of a PNS neuron. Saltatory conduction is the “jumping” of action potentials as they spread from node to node. Synaptic integration is the summing up of all incoming excitatory and inhibitory messages by a neuron. 17.3: 1. The CNS is composed of the spinal cord and the brain. The PNS, which lies outside of the CNS, is composed of nerves and ganglia. Sensory fibers send information to the CNS and motor fibers conduct information away from the CNS to

Appendix A tissues and organs. Ganglia are areas of nerves that are collections of cell bodies. 2. The spinal cord is structured so that the brain can communicate with the peripheral nerves. This communication involves sensory and motor functions. 3. Cerebrum: receives sensory information, integrates it, and commands voluntary motor responses. Diencephalon has two parts: hypothalamus serves as a link between the nervous and endocrine system, maintaining homeostasis; thalamus receives sensory inputs except for smell. Cerebellum: processes information about body position and maintains posture and balance. Brain stem: acts as a relay between the cerebrum and spinal cord or cerebellum; regulates breathing through the pons; regulates vital functions through the medulla oblongata; receives and sends signals between the higher brain centers and the spinal cord. 17.4: 1. The hippocampus within the limbic system is involved in learning and memory. It communicates with the prefrontal area of the brain in these functions. 2. Short-term memory is useful in managing operations within the present by holding a thought in mind. Long-term memory involves semantic and episodic memory as well as skill memory. It allows recall of the past and use of that information to shape future actions. 3. Mice that lack a glutamate receptor in their hippocampus were unable to learn to run mazes. 4. Wernicke’s area is involved in the comprehension of speech; Broca’s area is involved in the actual motor function of speech. 17.5: 1. See Figure 17.15. 2. When you touch a hot stove, your hand withdraws before you feel the pain because the nerve pathway for a reflex arc travels directly through the spinal cord, without input from the brain. 3. See Table 17.1. 4. The neurological explanation for a stomachache after jogging might be that your brain directed more activity to the sympathetic system, and less to the parasympathetic. This could decrease peristalsis and direct blood away from the digestive system, leading to some discomfort. 17.6: 1. Continued exposure to a drug over time might lead to compensatory responses at synaptic clefts that result in a decreased sensitivity to the drug. For example, if the drug prevents release of the NT (see Fig. 17.18 part 2) the cell might synthesize more of the NT. 2. Cocaine prevents synaptic uptake of dopamine. Heroin binds to endorphin receptors. Methamphetamine results in high amounts of dopamine released in the brain. Bath Salts inhibit the reuptake of several neurotransmitters. 17.7: 1. Major symptoms of AD include loss of short-term memory, progressing to an inability to carry out basic functions. These symptoms can begin before age 50, but usually not before 65. 2. With Parkinson disease, the dopamine-releasing neurons in the brain degenerate. With lower levels of dopamine, motor control decreases and involuntary tremors and muscular rigidity develop. 3. Autoimmune diseases occur when the immune system attacks self-cells, tissues, or organs. In MS, the immune system attacks myelin, oligodendrocytes, and neurons. 4. H. influenza can cause disease if the immune status of the carrier is compromised by fatigue, poor health, viral infection, lack of exercise, or risky behaviors.

Science in Your Life Why Do We Sleep? 1. Sleep is a period of inactivity involving a suspension of consciousness, lack of responsiveness, and changes in brain wave activity. It is essential to health. 2. The difficulty in measuring the processes and actions of sleep contribute to the mystery. 3. Sleep is a time for the brain to organize, and integrate through new connections, the information accumulated during the day. It is also a time for housekeeping functions performed by supportive cells in the brain. Caffeine: Good or Bad for You? 1. Individual experiences with caffeine will be reported here. 2. Because caffeine is a drug it should be reported, just like other ingredients are listed in the Nutrition Facts labels of foods. 3. Arguments for or against this should recognize the large effects caffeine can have on human physiology, and the age of the population to which the marketing of the drinks is directed.

Testing Yourself 1. d; 2. a; 3. a; 4. b; 5. b; 6. c; 7. c; 8. d; 9. d; 10. b; 11. a; 12. a. central canal, b. dorsal horn, c. white mater, d. cell body of interneuron, e. cell body of sensory neuron; 13. e; 14. d; 15. d; 16. d; 17. d.

Thinking Critically 1. A way scientists study the functions of specific brain areas is to note which brain functions are disrupted by brain tumors or other disorders like strokes that affect only a specific part

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of the brain. Other data has been obtained from brain surgeries in which the patient is kept awake, and different areas of the brain can be electrically stimulated to see what parts of the body (or higher functions) they control. 2. The first reason that nicotine, cocaine, and methamphetamine would be poor choices as therapeutic drugs is the high incidence of addiction and other undesirable side effects. A second problem would be the adaptation of the body over time, so that less dopamine is produced in the brain, which would counteract any temporary beneficial effects for Parkinson disease. 3. In order to develop rational treatments for any disease it is very helpful, if not essential, to understand what causes the disease. In many brain diseases these factors are not completely understood. In addition, the brain is less accessible to investigation than other organs, due to its protected location inside the skull, the bloodbrain barrier, and its essential function for life. 4. When acetylcholinesterase is inhibited, the effects of acetylcholine, which include muscle contraction, salivation, elevated heart rate and increased blood pressure, are prolonged.

CHAPTER 18 Improving Your Eyesight 1. The cornea is a transparent layer that is a continuation of the sclera and is the first layer through which light passes into the eye. It assists in focusing the light. 2. The ciliary muscles contract or relax to change the shape of the lens. This allows the lens to focus the image correctly on the retina.

Check Your Progress 18.1: 1. Interoceptors are involved in regulating critical body functions like blood pressure, blood volume, and blood pH. 2. Chemoreceptors—taste and smell; photoreceptors—vision; mechanoreceptors—hearing and balance; thermoreceptors— heat and cold. 3. Without sensory adaptation, we would constantly be stimulated by all sorts of inputs, which could overwhelm the brain’s ability to interpret. 18.2: 1. Somatic sensory receptors that are interoceptors include proprioceptors (such as muscle spindles and Golgi tendon organs), and pain receptors in the skin and internal organs. Somatic sensory receptors that are exteroceptors include cutaneous receptors like Meissner corpuscles, Krause end bulbs, Merkel disks, Pacinian corpuscles, Ruffini endings, and free nerve endings sensitive to temperature. 2. See Figure 18.3. 3. Someone who lacks muscle spindles would be susceptible to injuring their muscles and joints due to a lack of awareness of limb position and degree of muscle contraction. A person who lacked pain receptors would be without the benefit of the protective function of pain and would be very prone to injury. 18.3: 1. The tongue has taste cells and the nasal cavity has olfactory cells. Both are chemoreceptors to which molecules bind and nerve impulses are sent to the brain. The tongue has five types of taste cells distributed over the tongue, while there are about 1,000 different types of olfactory cells in the olfactory epithelium. 2. Adaptations that allow some animals to have more sensitive sense of smell would include having a greater number of olfactory receptors (and increased space in the nasal cavity for these receptors) and having more sensory nerve axons to receive signals from these receptors. 3. The olfactory bulbs in the brain have direct connections to centers for memory in the limbic system. 18.4: 1. Conjunctiva, cornea, aqueous humor in the anterior compartment, through the iris, lens, vitreous humor in the posterior compartment, retina including the ganglion cell layer, bipolar cell layer, and finally the rod cell and cone cell layer where light is converted to nerve impulses. 2. When the ciliary muscle is relaxed, the suspensory ligaments that attach to the lens are taut, which causes the lens to become flatter. When the ciliary muscle contracts, the suspensory ligaments are relaxed, allowing the lens to become more round. 3. As many as 150 rod cells, but perhaps only one cone cell, synapse(s) on an individual ganglion cell, so that more information can be sent to the brain for an individual cone cell versus a rod cell. 4. The optic chiasma is X-shaped, formed by the optic nerve fibers crossing over. The image sent to the brain is split because one side of the optic tract carries information about the opposite side of the visual field. The two sides of the visual field must communicate with each other in the brain for perception of the entire visual field. 18.5: 1. See Figure 18.11. Sound waves travel down the auditory canal to the tympanic membrane, and then to the bones of the middle ear, which are connected to the cochlea in the inner ear. It is in the cochlea that the sound waves are converted to neural impulses that are transmitted to



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the brain. 2. The mechanoreceptors responsible for transducing sound waves into nerve impulses are located on hair cells on the basilar membrane of the spiral organ in the cochlea of the inner ear. 3. Sound waves transmitted via the oval window cause the basilar membrane to vibrate. Near the tip of the spiral organ, the basilar membrane vibrates more in response to higher pitched sound, and at the base it responds to lower pitches. Different sensory nerves supply each part of the basilar membrane, and take information to different parts of the auditory cortex, where the pitch is interpreted. 4. Loud sounds cause the basilar membrane to vibrate more rapidly, which is interpreted by the brain as volume. 18.6: 1. See Figure 18.13. 2. The ampulla and cupula are associated with the semicircular canals; the otoliths, saccule, and utricle are associated with the vestibule. 3. Having two systems for equilibrium allows the brain to receive information about rotational movement from the semicircular canals, and about the body’s position at rest from the vestibule. Although it might be possible to distinguish between these with just one system, it would not be as efficient or sensitive. 18.7: 1. The sense of smell can protect us from dangerous foods, chemicals, and situations. Without it, one is more vulnerable to injury from such sources. 2. Nearsightedness results when the eyeball is elongated; hence the lens focuses in front of the retina. Farsightedness results when the eyeball is too short. Astigmatism is caused by a misshapen cornea or lens. 3. In macular degeneration, the choroid vessels thicken and no longer function, which leads to the destruction of cone cells, resulting in blindness. The drainage system in the eyes fails and fluid builds up in patients with glaucoma. The increased pressure in the eye destroys nerves first in the peripheral vision but can lead eventually to complete blindness. With cataracts, the lens of the eye becomes cloudy and eventually opaque. No light can get through and total lack of vision can result.

Science in Your Life Preventing Hearing Loss: 1. A regulation that music players restrict output to 100 db seems like an issue of consumer protection. Many people do not recognize the danger of loud noises and would benefit from industry standards that prevent harm. 2. Answers will vary between people and likely would correlate with age.

Testing Yourself 1. a; 2. c; 3. d; 4. a; 5. b; 6. e; 7. b; 8. a. retina, b. choroid, c. sclera, d. optic nerve, e. fovea centralis, f. ciliary body, g. lens, h. iris, i. pupil, j. cornea; 9. d; 10. c; 11. a. stapes, b. incus, c. malleus, d. tympanic membrane, e. semicircular canals, f. cochlea, g. auditory tube; 12. d; 13. c; 14. a; 15. a; 16. d; 17. a.

Thinking Critically 1. Without sensory adaptation to constant everyday stimuli, we would constantly be bombarded with and distracted by a huge amount of sensory data (about touch, temperature, pressure, etc.). In contrast, there are large risks for not responding to pain, as shown by the damage that occurs to people who lack normal pain perception. 2. Once it reaches the retina, light must pass through layers of axons, ganglion cells, and bipolar cells before it reaches the rod or cone cells. From the right side of the left eyeball, nerve impulses travel via the optic nerve, through the optic chiasma, then through the right optic tract, until they synapse with neurons in the thalamus. Axons from thalamic neurons transmit this information to the visual area of the occipital cortex. 3. The null hypothesis would be that there is no association between the density of taste buds on the tongue and obesity. The prediction based on this hypothesis is that obese people will have the same density of taste buds as normal weight people. 4. smell, chemoreceptors in olfactory epithelium; sight, rods and cones in the retina; hearing, hair cells in spiral organ; gravity, hair cells in vestibule; motion, hair cells in semicircular canals.

and lymph. Spongy bone appears at the end of the long bones and is composed of thin plates arranged to form spaces. 2. Hyaline cartilage is found at the ends of long bones (in joints), in the nose, ribs, larynx, and trachea. The disks between the vertebrae and in the knee are made of fibrocartilage and elastic cartilage is found in the ear and epiglottis. 3. Both ligaments and tendons are composed of dense fibrous connective tissue, but ligaments connect bone to bone, while tendons connect muscle to bone. 4. During bone remodeling, osteoclasts break down bone, and osteoblasts rebuild it. 19.2: 1. See Figure 19.3. 2. True ribs are attached to the sternum by costal cartilage. False ribs connect to the sternum via a common cartilage. Floating ribs do not attach to the sternum. 3. The knee is a typical hinge joint, the elbow is a pivot joint, and the hip is a ball-andsocket joint. 19.3: 1. Smooth, cardiac, and skeletal. 2. The origin of a muscle is its bone attachment that remains stationary, while the insertion is on the bone that moves. 3. See Tables 19.1 and 19.2. 4. Flexion involves movement that decreases the angle between bones; extension is the opposite of flexion; abduction moves a bone away from the body; adduction brings a limb toward the midline; rotation moves the part around an axis. 19.4: 1. Three unique components of muscle cells are the T tubules, the myofibrils, and myoglobin. 2. During muscle contraction, myosin filaments use energy from ATP to pull actin filaments toward the center of the sarcomere. 3. Neurons control muscle contraction by releasing neurotransmitters at the neuromuscular junction. These chemicals bind to receptors on the sarcolemma, stimulating impulses that spread via the T tubules. 4. Creatine phosphate breakdown is the fastest way to provide ATP for muscle contraction but it only supplies enough energy for a few seconds of intense activity. Cellular respiration provides most ATP used by a muscle cell by breaking down glycogen and fatty acids. Fermentation can supply ATP without requiring oxygen. 19.5: 1. Three stages of a muscle twitch are the latent, contraction, and relaxation periods. 2. The index finger requires fine motor control and would have a higher ratio of motor nerve axons per muscle fibers than a muscle in your back. 3. Domesticated birds no longer migrate by flying long distances, so they have no need for the endurance associated with slow-twitch fibers. 19.6: 1. Untreated bone fractures can heal, especially if the individual is able to prevent the bone from moving, or if it occurs in a bone that does not move a lot (i.e., a rib). However, if two broken ends of a bone continue to move, an exaggerated bone callus usually forms, and the fracture may not heal well. 2. Astronauts experience bone loss because the bones are not bearing weight. They would be remodeled over time by overactivity of the osteoclasts. 3. Rheumatoid arthritis is an autoimmune disease in which the immune system attacks and damages the joints; osteoarthritis is usually secondary to traumatic damage to a joint (either from an injury or over the lifetime of an older person).

Science in Your Life Dead on the “Farm”: 1. Concerns include whether bodies were being treated ethically, whether bodies were secure from predators, whether the site would attract thrill seekers, and contamination of ground water. 2. Osteoarthritis, rheumatoid arthritis, rickets, scurvy, tuberculosis. 3. Botanists might study the age and variety of plant growth over a burial site to determine how long the remains had been there. Chemists might study the composition of the remains, searching for toxins. Geologists could assess the impact of local soil and rock conditions on the deterioration rate of the remains. Psychologists could interpret evidence to determine the motivation and habits of a criminal. The Discovery of Botox: 1. Botulism could be treated by administering antibodies to the botulinum toxin. 2. Patenting a naturally occurring molecule should not be allowed, but patenting the delivery mechanism of the molecule should be allowed.

CHAPTER 19

Testing Yourself

Replacing Joints 1. She is relatively young, in good health, and remains active. 2. It provides a smooth surface for the end of the bones to move against each other and gives support. 3. The knee is a simpler joint that involves hinge movement, not rotational.

1. b; 2. d; 3. b; 4. d; 5. a. coxal bones, b. patella, c. metatarsals, d. phalanges, e. humerus, f. ulna, g. radius, h. femur, i. fibula, j. tibia, k. tarsals; 6. a; 7. c; 8 e; 9 a. cross bridge, b. myosin, c. actin, d. Z line, e. H zone, f. A band, g. I band; 10. d; 11. d; 12. a; 13. e; 14. b; 15. c.

Check Your Progress

Thinking Critically

19.1: 1. Compact bone is composed of tubular units and is highly organized, with central canals providing blood, nerves,

1. a. With a loss of dystrophin, the force of muscle contraction cannot be conducted efficiently or in a

coordinated manner to the connective tissue of the muscle. b. With a loss of dystrophin, the muscle is not exercised as much and atrophies over time. 2. Aerobic respiration is preferable to fermentation because it generates much more ATP per molecule of glucose. Oxygen serves as a final electron acceptor in the electron transport chain present in mitochondria. 3. Animal tissues contain high levels of amino acids, and the consumption of excess amino acids causes the urine to become more acidic, which may cause increased loss of calcium in the urine. The level of daily exercise may be another important factor, if Chinese people are more active than Americans.

CHAPTER 20 Grade School Diabetic 1. Insulin and glucagon. 2. See Figure 20.9. 3. The brain requires a constant supply of glucose and without it unconsciousness can result.

Check Your Progress 20.1: 1. Hypothalamus, pituitary gland, thyroid gland, adrenal glands, thymus, pancreas. 2. When an endocrine gland is controlled by negative feedback, it is sensitive to the condition it is regulating or the blood level of the hormone it is producing. 3. Peptide hormones bind to receptors in the plasma membrane and do not enter the cell. A second messenger inside the cell is needed to initiate an enzyme cascade. 4. It has been found that women prefer axillary odors from men with different MHC than themselves. 20.2: 1. Neurons in the hypothalamus produce hormones that pass through axons into the posterior pituitary where they are stored. The hypothalamus controls the anterior pituitary by producing hormones that travel to the anterior pituitary through a portal system. 2. See Table 20.1. 3. The release of oxytocin from the posterior pituitary is controlled by positive feedback in which the amount of oxytocin released increases as the stimulus continues. 4. Diagnosis is difficult because of the many hormones controlled by the pituitary and the many organs and tissues affected. 20.3: 1. Levels of T3 and T4 are controlled by negative feedback. When levels of T3 and T4 rise, the anterior pituitary stops producing thyroid-stimulating hormone. 2. Calcitonin decreases blood calcium by increasing calcium deposition in bones. A person with a calcitoninproducing thyroid tumor would be expected to have low blood calcium levels, leading to disrupted nerve conduction, muscle contraction, and blood clotting, as well as increased bone density. 3. Parathyroid hormone increases the activity of osteoclasts, promotes reabsorption of calcium by the kidneys, causes blood phosphate to decrease, and activates vitamin D, so removal of the parathyroid glands would tend to cause decreased blood calcium and increased blood phosphate. 20.4: 1. Secretion of hormones by the adrenal medulla is under direct nervous control; secretions of the adrenal cortex are controlled either by release of ACTH from the anterior pituitary (for glucocorticoids) or by the renin-angiotensin system (for mineralcorticoids). 2. Epinephrine from the adrenal medulla helps the body deal with stress by activating the fight-or-flight response (increased heart rate, blood pressure, and energy level); glucocorticoids from the adrenal cortex raise blood glucose levels and inhibit the body’s inflammatory response. 3. Aldosterone causes the kidneys to absorb sodium and excrete potassium, which generally raises blood pressure. 20.5: 1. The exocrine tissue of the pancreas produces and secretes juices that help in digestion in the small intestine. The endocrine cells of the pancreas produce the hormones glucagon, insulin, and somatostatin. 2. See Figure 20.9. 3. Somatostatin inhibits the release of growth hormone and suppresses glucagon and insulin secretion by negative feedback mechanism. 20.6: 1. Testosterone and estrogen stimulate the development of male and female sex characteristics and the production of gametes. 2. Some of the worst side effects from taking anabolic steroids are liver dysfunction and cancer, kidney disease, heart damage, stunted growth, infertility, and impotency. 3. Leptin signals satiety, a sense of not being hungry. 4. Prostaglandins are not distributed in the blood, but instead act close to where they are produced within cells. 20.7: 1. ADH causes more water to be reabsorbed in the kidneys, decreasing urine volume. Inhibition of ADH secretion by alcohol results in increased amounts of urine and frequency of urination. 2. In iodine deficiency, the thyroid gland cannot synthesize enough T3 and T4. The anterior pituitary responds by releasing large amounts of TSH, which stimulates the thyroid gland to enlarge. 3. In Addison disease, antibodies play a role in destruction of the

adrenal cortex by the immune system. In Graves disease, antibodies bind to the TSH receptor, stimulating production of T3 and T4 rather than destroying the gland. 4. Type 1 diabetes is due to an insulin shortage and is treated by the administration of insulin. Type 2 results from an insensitivity to insulin, often specifically due to insufficient numbers of insulin receptors on cells. It is treated with medications, weight loss, and increased exercise.

Science in Your Life Human Growth Hormone: 1, 2, and 3. Here people are asked to present their opinions about HGH, supplements, and the responsibility of celebrities in our society. They should be able to defend their position with logical arguments on these issues. Melatonin: 1. Depression, whatever the cause, is often viewed as a weakness that people should just get over. Explaining SAD as a chemical imbalance can evoke more sympathy for those suffering from it. 2. If melatonin levels are peeking within your body at a time that you need to be awake, it will be more difficult to function during that period. 3. Responses to this question should address the potential harm that might result from an unregulated market of melatonin, which can be classified as a drug.

Testing Yourself 1. a. pineal gland, b. hypothalamus, c. pituitary gland, d. thymus, e. pancreas, f. adrenal glands, g. thyroid gland; 2. a; 3. b; 4. b; 5. c; 6. c; 7. d; 8. c; 9. d; 10. b; 11. e; 12. c; 13. b; 14. c; 15. d.

Thinking Critically 1. The nervous system allows for quick, almost immediate responses to stress or danger, and fine-tuning of all other bodily functions (plus all the other benefits of higher brain functions), while the endocrine system provides longerlasting responses. 2. If Cushing syndrome is caused by a problem (such as cancer) of the adrenal gland itself, ACTH levels would be very low, because of negative feedback action of cortisol on the hypothalamus/pituitary. 3. Pheromones perceived as odors may affect sexual attraction, aggression, love, or almost any emotion, and we would not necessarily be aware of these effects.

CHAPTER 21 Three Parents — One Baby 1. The mother by way of the egg. 2. With IVF, an egg is fertilized with sperm outside of the body and allowed to develop into an embryo, which is then implanted into the uterus. With artificial insemination, sperm are placed in the vagina by a medical provider. Gamete intrafallopian transfer is like IVF except that oocytes and sperm are placed in the uterine tubes right after they have been mixed together. With intracytoplasmic sperm injection, a single sperm is injected into an oocyte, which is allowed to develop and is then placed in the uterus.

Check Your Progress 21.1: 1. The seminiferous tubules in the testes produce sperm. The epididymis stores the sperm, and the vasa deferentia carry the sperm to the female via the urethra. The urethra also functions in urination. Interstitial cells, which lie between the seminiferous tubules, secrete androgens, most importantly testosterone. 2. See Figure 21.3. 3. GnRH stimulates the anterior pituitary to secrete the two gonadotropic hormones, FSH and LSH. FSH promotes the production of sperm. ICSH (LH hormone in males) controls the production of testosterone by interstitial cells. Testosterone controls the development and function of the testes and associated organs. It also brings about male secondary sex characteristics. 21.2: 1. The uterine tubes are severed and tied off. Movement of eggs down the uterine tubes is prevented. 2. In males, the urethra and external genitalia (the penis) function in both reproduction and urination. In contrast, these two systems are separate in females. 3. Contractions of the uterus may help to move sperm from the uterus to the uterine tubes. 21.3: 1. During the follicular phase, FSH from the anterior pituitary promotes the development of an ovarian follicle. Ovulation signals the end of the follicular phase. The major feature of the luteal phase is secretion of progesterone by the corpus luteum, which causes the uterine lining to thicken in order to be ready to receive a fertilized oocyte. 2. Estrogen from an ovarian follicle stimulates the proliferative phase of

Appendix A the uterine cycle, during which the endometrium begins to thicken. Progesterone from the corpus luteum leads to the secretory phase of the uterine cycle, in which the endometrium thickens and uterine glands produce thick mucus. 3. In pregnancy, the placenta produces human chorionic gonadotropin, which maintains progesterone production by the corpus luteum until the placenta can produce progesterone and estrogen, which inhibit the ovarian cycle and maintain the endometrium. See Figure 21.9 for an explanation of the cycle of a nonpregnant woman. During menopause no eggs develop and the ovarian and uterine cycles cease. 21.4: 1. Methods that prevent ovulation: oral contraception, contraceptive implants, contraceptive injections/patch, vaginal ring. Methods that prevent fertilization: diaphragm, cervical cap, male condom, female condom, coitus interruptus, spermicidal jellies, creams or foams, natural family planning, vaginal film. 2. The following methods physically block sperm from entering the uterus: condoms, diaphragm, and cervical cap. 3. Birth control methods that prevent the contact of male ejaculate with female tissues reduce the transmission of STDs. 4. Emergency contraception includes methods of birth control that prevent pregnancy after unprotected sex. 21.5: 1. STDs caused by bacteria: Chlamydia, gonorrhea, syphilis; STDs caused by viruses: genital herpes, HPV, hepatitis. 2. HIV can destroy CD4+ T cells by directly infecting them, by inducing apoptosis in infected and uninfected cells, and by causing cytotoxic T cells to kill HIV-infected cells. 3. Reverse transcriptase inhibitors block production of viral DNA from RNA; protease inhibitors prevent cleavage of long viral proteins; fusion inhibitors interfere with HIV entry into cells; integrase inhibitors prevent insertion of viral DNA into host DNA. 4. Viruses are not affected by antibiotics. They can remain inside cells, protected from antibodies produced by the immune system. 21.6: 1. Viagra, Levitra, and Cialis increase the blood flow to the penis during sexual intercourse. 2. Endometriosis is the presence of endometrial-like tissue outside of the uterine cavity. It can induce inflammation of the uterine lining, which is painful, and can result in infertility. 3. Ovarian cancer is more frequently fatal than testicular cancer mainly because the ovaries are inside a woman’s body, hidden from detection until the cancer has spread to other organs. 4. Artificial insemination by donor requires the least medical intervention, since sperm is simply collected, concentrated, and placed into the vagina (or uterus) by a physician. In vitro fertilization mixes oocytes and sperm outside the body and the embryos are transferred to the uterus of the female. Gamete intrafallopian transfer requires more intervention, since the oocytes and sperm are brought together in vitro, then introduced into one of the woman’s uterine tubes. Intracytoplasmic sperm injection requires the most intervention, as a single sperm must be injected into an oocyte.

Science in Your Life Preventing Transmission of STDs: 1. You cannot be totally certain that your partner does not have a STD, even if they have been tested multiple times. Many people are unaware that they are carriers of STDs because of the absence of symptoms. 2. A negative HIV test results from no or low levels of antibodies to the virus being present in the blood. This may result during the earliest stages of infection. 3. Abstinence or barrier methods of birth control are most effective at preventing the transmission of HIV. Least effective methods are those that allow the ejaculate to enter the female reproductive system. The Challenges of Developing a Vaccine Against HIV: 1. Those receiving the HIV/AIDS vaccine might believe that it was 100% effective at preventing infection by HIV and would pursue unsafe sexual habits. 2. Beliefs that those who are infected by HIV somehow deserved it because of their behaviors might prevent a country from investing enough into the development and implementation of a vaccine program. EndocrineDisrupting Contaminants: 1. Responses to this question will be based on one’s own experiences and perceptions of risk and benefits of endocrine-disrupting chemicals in food. 2. Mechanisms of disruption of endocrine pathways include blocking binding sites at the plasma membrane, interference of the movement of the hormone through the plasma membrane, and interruption of transmission of the signal by a second messenger. 3. Agriculture and manufacturing practices today are dependent on many chemicals, some of which are potential endocrine disruptors. Without the chemicals, the cost of production would rise and those costs would be passed on to the consumer. Whether you would be willing to pay the extra costs is dependent on the individual.

A-9

Testing Yourself 1. a. seminal vesicle, b. ejaculatory duct, c. prostate gland, d. bulbourethral gland, e. anus, f. vas deferens, g. epididymis, h. testis, i. scrotum, j. foreskin, k. glans penis, l. penis, m. urethra, n. vas deferens, o. urinary bladder; 2. c; 3. d; 4. a. uterine tube, b. ovary, c. uterus, d. urethra, e. clitoris, f. anus, g. vagina, h. cervix; 5. b; 6. b; 7. c; 8. b; 9. c; 10. d; 11. e; 12. c; 13. c; 14. b; 15. a; 16. d.

Thinking Critically 1. Current exposure levels to chemicals that potentially could reduce sperm production in males could be measured and compared to previous levels. 2. The main advantage of a monthly menstrual cycle is that the human female ovulates every month, which increases the number of opportunities for becoming pregnant. 3. Although BPH may cause frequent or difficult urination, it usually does not block urination completely. Untreated prostate cancer, however, often spreads to other tissues, such as the bone marrow, where the cancer cells may interfere with normal functions. 4. A potential mother who has low body fat is likely less well nourished and less able to nurse or otherwise provide for her offspring successfully.

CHAPTER 22 How Long Will You Live? 1. Human development requires cellular differentiation and morphogenesis. Both processes are controlled by the specific genes that are expressed and in what order they are expressed. 2. On a cellular level, changes in hormone receptor activity, the size of telomers, and DNA damage due to free radicals are some of the major causes of aging. On a physiological level, development of cardiovascular disease, and decreased immune function contribute to aging, as well as destructive habits involving diet, exercise, and exposure to environmental risks and pathogens.

Check Your Progress 22.1: 1. See Figure 22.1. 2. The oocyte’s membrane depolarizes and lifts off of the oocyte, preventing any further sperm from binding. Enzymes released by the cortical granules lead to the formation of the fertilization membrane, which results in the lifting of the zona pellucida from the surface of the oocyte. This prevents other sperm from binding to the zona pellucida. 3. Cellular: All three have cleavage resulting in the formation of a multicellular embryo and the formation of a blastula. In the lancelet, the cleavage produces a ball of cells of equal size. In the frog, the cells are not of equal size, and in the chicken, cleavage is restricted to a layer of cells over the yolk. Tissue: All develop three germ layers: ectoderm, mesoderm, and endoderm. In the lancelet, mesoderm forms by outpocketing of the archenteron, while in the frog and chicken, mesoderm forms by migration of cells between the ectoderm and endoderm. 4. See Table 22.1. 22.2: 1. Cellular differentiation results in cells becoming specialized in structure and function due to differential gene expression. Morphogenesis produces the shape and form of the body first through the migration of cell types to produce the germ layers. 2. A morphogen is the product of a morphogen gene and is present usually as a gradient within a developing organism. The gradients determine the shape of the organism. 3. The homeobox sequence in a homeotic gene codes for a homeodomain composed of 60 amino acids. The homeodomain protein binds to DNA and controls gene expression. 22.3. 1. Umbilical blood vessels from the allantois; first blood cells from the yolk sac; fetal half of the placenta from the chorion. 2. Months 1 and 2: week 1— fertilization, cleavage, blastocyst formation; week 2— implantation, yolk sac and amnion form, gastrulation occurs; week 3—the nervous system appears, development of the heart begins; weeks 4–5—development of the chorion and allantois, umbilical cord forms, limb buds appear, head enlarges, developing eyes, ears, and nose; weeks 6–8—head assumes normal position relative to body, all organ systems are developed. Months 3 and 4: cartilage replaces bone, head growth slows, eyelashes, eyebrows, head hair, fingernails, and nipples appear. Months 5 through 7: lanugo covered by vernix caseosa appears, eyelids are open. 3. See Figure 22.15. The unique fetal structures are: foramen ovale, ductus arteriosus, umbilical arteries, umbilical vein, ductus venosus. 4. The placenta is a structure of the developing embryo that is attached to the uterine wall, that allows the exchange of gases, nutrients, and wastes between the maternal and embryo



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Appendix A

circulatory systems. 22.4: 1. Progesterone causes smooth muscle to relax and arteries to expand, leading to low blood pressure. The renin-angiotensin-aldosterone mechanism leads to sodium and water retention. Blood volume, number of RBCs, and cardiac output increase. Blood flow to the kidneys, placenta, skin, and breasts rises. 2. Stage 2. 3. Colostrum is a thin, yellow, milky fluid, rich in proteins including antibodies, that is produced by breasts during the first two days after birth, before milk production begins. 22.5: 1. Telomeres have a genetic influence on aging. The number of times a cell divides is controlled by the length of telomeres. The older the cell, the shorter the telomere. 2. A high calorie diet results in the generation of more free radicals, which can damage DNA and lead to cellular aging. Low calorie diets have been associated with longer lifespans. Also, the consumption of foods high in antioxidants counteracts the effects of free radicals and likely slows aging. 3. atherosclerosis and elevated blood pressure, cardiovascular system; increased susceptibility to infections, immune system; pneumonia, respiratory system; enlarged prostate, excretory system; presbyopia and cataracts, sensory systems; osteoporosis, musculoskeletal system; insulin insensitivity, depression due to low testosterone, and increased heart disease in postmenopausal women, reproductive systems.

Science in Your Life Preventing and Testing for Birth Defects: 1. Environmental exposure to toxins, unbalanced diet, lack of exercise, stress, depression, long hours standing on your feet at work, disease pathogens. Alzheimer Disease: 1. Low concentrations of ACh in the hippocampus is one characteristic of Alzheimer disease. The hippocampus is a structure in the limbic system that is involved in learning and memory. The loss of memory is a symptom of Alzheimer disease. 2. Amyloid plaques which envelop branches of axons in Alzheimer disease patients can lead to inflammation and result in neuron death. Controlling inflammation could prevent neuron death.

Testing Yourself 1. c; 2. c; 3. d; 4. d; 5. a; 6. c; 7. a. ovulation, b. fertilization, c. cleavage, d. morula formation; 8. e; 9. e; 10. e; 11. d; 12. c; 13. e.

Thinking Critically 1. The egg is what contributes the cytoplasm and the organelles to the developing embryo. Therefore only the mother’s mitochondria are present in the embryo. 2. HCG is produced by the placenta and is excreted by the kidneys. 3. The presence of FSH, which promotes the development of a follicle in the ovary, and a surge in LH, which causes ovulation, are measured in these tests. The tests are based on color-coded antibodies to the hormones.

CHAPTER 23 Phenylketonuria 1. Traits such as the ability to break down phenylalanine are encoded in our DNA. In the absence of this trait, the environment can be controlled by changing the diet to eliminate phenylalanine. 2. See Figures 23.9 and 23.11. 3. The sperm or oocytes produced during meiosis carry just one copy of each gene, which can then combine to create a variety of diploid zygotes, each unique in its genetic makeup and phenotype.

Check Your Progress 23.1: 1. The phenotype of an individual is its appearance, which is determined by the genes it carries for traits carried in its genome. The genes present in the genome comprise its genotype. 2. A testcross is conducted to determine if an individual that is expressing a dominant allele is heterozygous or homozygous for that gene. 3. The Law of Segregation states that each individual has two factors for each trait and these factors separate during gamete formation such that each gamete contains only one factor. At fertilization, the zygote receives two factors for each trait. The Law of Independent Assortment states that during gamete formation, each pair of factors separates independently of other factors and that all possible combinations of factors can occur. 4. A phenotypic ratio of 3:1 results from a one-trait cross and a ratio of 9:3:3:1 results from a two-trait cross. 23.2: 1. See Figures 23.9 and 23.11. 2. Autosomal recessive disorders result from the inheritance of two recessive alleles. The child is affected and

is homozygous recessive, but neither parent is because they are heterozygous. In autosomal dominant disorders, both parents are affected but the child can be unaffected. 23.3: 1. With incomplete dominance, the heterozygote exhibits a phenotype that is intermediate between the homozygous recessive and the homozygous dominant. 2. The mother has to be heterozygous for type A blood and the second allele for type O blood would have come from the father. He could have been heterozygous for type A, type B, or type O blood. 23.4: 1. Environmental factors include: nutritional status, diet, temperature, exposure to sunlight. 2. Identical twins have identical genomes and have developed from identical oocytes containing the same cytoplasmic composition. However, the expression of their genes is influenced by environmental inputs. Variation between identical twins is attributed to the inputs from the environment on their development.

Science in Your Life Investigations of Gregor Mendel: 1. Large numbers are needed to determine if the event has happened by chance alone or as a result of the variable being studied. 2. There are serious ethical considerations that must be addressed in human studies that are not pertinent to plant studies. 3. Incomplete dominance, codominance, multiple allele inheritance, polygenic inheritance. Genetics of Taste: 1. Organisms in the environment that sense bitterness by taste will avoid bitter tasting food. Those bitter plants or animals will survive to propagate and their numbers and the frequency of the alleles of their genes will increase in the population. 2. If plants taste bitter, they will be more resistant to being eaten. More plants will survive to reproduce.

Testing Yourself 1. d; 2. e; 3. b; 4. a; 5. a; 6. b; 7. b; 8. b; 9. d; 10. d; 11. d; 12. d.

Thinking Critically 1. Cross the fruit fly with the trait to a true breeding fruit fly without the trait and determine the phenotypic ratios. If the trait does not appear in the offspring or appears 25% of the time then it is recessive. If the trait appears 75% of the time then it is dominant. 2. Track the appearance of that trait in identical twins raised in different environments. 3. Even identical twins have differences in environmental influences. Beginning in the womb, each twin experiences their environment in different ways.

CHAPTER 24 Down Syndrome 1. Nondisjunction during meiosis I or during meiosis II. 2. Inheritance of three copies of chromosome 21. 3. By amniocentesis or chorionic villus sampling.

Check Your Progress 24.1: 1. Linked alleles are all on one chromosome and they tend to be inherited all together, thus they do not exhibit independent assortment. 2. Two genes that are ten map units apart exhibit a 10% chance of crossing-over. 24.2: 1. Sex linked alleles appear on the X chromosome and more rarely on the Y chromosome. Their inheritance is dependent on the distribution of the X and Y chromosomes in males and females. Recessive alleles on the X chromosome are expressed in males, while they are not in heterozygous females. 2. 50% of female offspring and 50% of male offspring would express the recessive allele. 3. Color blindness: 8% of Caucasians perceive red and green colors differently. Duchenne muscular dystrophy: caused by the absence of the protein dystrophin which leads to calcium entering the cell, which promotes an enzyme that dissolves muscle fibers. Fragile X syndrome: an abnormally high number of repeat sequences of CGG on the X chromosome. Hemophilia: either due to the absence or low level of clotting factor VIII or factor IX, resulting in a reduced ability of the blood to clot. 24.3: 1. In normal XX females, one of the X chromosomes becomes inactive and forms a Barr body, which indicates that only one functional X chromosome is needed in females just like in males. Extra X chromosomes all become Barr bodies. In males, extra X chromosomes also become Barr bodies. Extra Y chromosomes do not seem to interfere with normal development, perhaps because there are so few genes on the Y chromosome. 2. Turner syndrome and Kleinefelter syndrome can be caused by nondisjunction during oogenesis at meiosis I or at meiosis II. 24.4: 1. When chromosomes break and do not rejoin, the deletion from one

chromosome can be added to the corresponding homologous chromosome, creating a duplication. 2. If in the process of translocation the allele of interest remains whole, then it can be that there is no loss of function. If after an inversion, the alignment of the two homologous chromosomes can be achieved by forming a loop, then it is possible that the individual can be phenotypically normal.

Science in Your Life TREDS: 1. Polymerase slippage along unstable triplet expansions during meiosis is greater than in mitotic divisions. 2. Mother, because the syndrome is X-linked. 3. Huntington disease results in the alteration of the huntingtin protein by the inclusion of many glutamines. The disease is apparent by the degeneration of brain cells. With fragile X syndrome, there are a range of symptoms, the severity of which depends on the number of CGG repeats present.

Testing Yourself 1. c; 2. c; 3. b; 4. a; 5. b; 6. d; 7. c; 8. a; 9. e; 10. d; 11. c; 12. a; 13. b.

Thinking Critically 1. Mosaicism in men with Klinfelter syndrome can happen during cell division after conception. A person with mosaicism might have tissues with 2 different phenotypes, depending on whether the cell contains XY or XXY chromosomes, and which of the X chromosomes is inactivated in the XXY cells. Parents will need to prepare for the necessary added responsibilities of a child with a handicap. They may need to arrange the finances for additional caregivers in the home and extensive medical care. 2. Chromosome 1 is so large and carries so many genes that three copies are lethal. 3. As long as the translocation is balanced, meaning that the entire amount of DNA is still present, and no alleles are disrupted, the person can express all of the genes normally and be phenotypically normal. However, when the translocated chromosomes go through meiosis and crossing-over, gametes without the entire amount of DNA present will be produced. This would result in infertility. 4. The father must be color blind unless there is a genetic abnormality in the daughter (i.e., she has only one X chromosome). The husband is not the father.

CHAPTER 25 Xeroderma Pigmentosum 1. The information in DNA is transcribed into RNA molecules, which through their various functions translate that information into proteins. 2. An alteration in the DNA leads to a change in the information transcribed to RNA and translated to proteins. The function of those proteins is altered, allowing unregulated cell growth and reduced apoptosis.

Check Your Progress 25.1: 1. See Figure 25.1. Griffith concluded that some substance was passed from the dead S strain bacteria to the living R strain and this substance had the capacity to transform the R strain. This was indicated to be genetic material. Later Avery and others identified the genetic material as DNA. 2. See Figure 25.2. 3. See Figure 25.3. 25.2: 1. A new double strand of DNA is composed of one old strand and one newly synthesized strand. 2. See Figure 25.4 and Figure 25.5. 25.3: 1. mRNA carries the genetic information encoded in DNA from the nucleus to the ribosomes in the cytoplasm. At the ribosome composed of rRNA and proteins, mRNA assembles amino acids carried by tRNA into chains, which are eventually released to form functional proteins. 2. The information stored in DNA is transcribed into mRNA which then, with the help of tRNA at the ribosomes made of rRNA, translates the information into protein structure. 3. The genetic code contains more than one triplet codon for each amino acid. 25.4: 1. Different cells are subject to different controls of gene expression such as X-inactivation and pretranscriptional, transcriptional, posttranscriptional, translational, and posttranslational control. 2. In the absence of lactose, a repressor is bound to the promoter/operator complex and no lactase is produced. Lactose will bind to the repressor and prevent it from binding to the operator. The gene for lactase can then be expressed. 3. See Figure 25.18. 25.5: 1. Gene mutations can happen when DNA replication produces errors, when a mutagen causes

physical damage to DNA, and when transposons jump into a new location, disrupting the neighboring genes. 2. A point mutation occurs when one DNA nucleotide is changed. A frameshift mutation occurs when a nucleotide is inserted or deleted. 3. Cancer cells have high levels of mutagenesis and disregulated cell cycles. They avoid apoptosis, and can metastasize.

Science in Your Life Finding the Structure of DNA: 1. 30%. 2. Knowing the structure of DNA has elucidated its function, allowing humans to regulate gene expression, analyze genetic material’s presence, and recognize its relatedness from species to species. The advances have been enormous. 3. DNA has two strands that are composed of specific pairs of nucleotides. Separate the strands and you have two templates for replication. Benign Versus Malignant Tumors: 1. The presence of abnormal cells in the blood would indicate that a cancer was malignant. 2. Cells in a malignant tumor would keep dividing, passing all the check points of the cell cycle in spite of DNA damage, faulty DNA replication, or misaligned chromosomes during mitosis. Prevention of Cancer: 1. The responses to this question will reflect people’s personal habits. 2. Tobacco smoke contains harmful chemicals that can be transmitted throughout the body to cause cancer at multiple sites. 3. Tanning beds increase exposure to UV radiation—a cause of skin cancer. Genetic Testing for Cancer Genes: 1. Genetic testing is already becoming increasingly available based on people’s ability to pay for it. Whether it should be financed by insurance will be determined by the limits of health care plans to supply services. 2. The use of this type of service would depend on one’s commitment to its value and the ability to pay for it. 3. Maintaining one’s health is a personal responsibility because not maintaining it affects others negatively in our society, both emotionally and physically, as well as increasing the cost of health care delivery. It is likely that the role of personal freedom and personal responsibility will be addressed in this question.

Testing Yourself 1. a; 2. b; 3. c; 4. c; 5. a; 6. c; 7. b; 8. c; 9. a; 10. c; 11. c; 12. a; 13. e.

Thinking Critically 1. Housekeeping genes are those that all cells require such as those that encode for rRNA, genes encoding enzymes needed for energy metabolism, and genes for membrane transport proteins. 2. Cancer cells express telomerase, allowing the cells that have damaged DNA to continue to divide. The cell loses the mechanism for cell death. 3. In cases where a copy of a gene inherited from a parent is defective, the second gene inherited from the other parent must be altered before cancer develops. Multiple environmental exposures and habits can influence whether cancer develops or not.

CHAPTER 26 Biotechnology and Diabetes 1. Treat protoplasts with electric current while they are suspended in a solution containing foreign DNA. 2. A GMO has had its genetic material altered by techniques used in recombinant DNA technology. It contains various DNA molecules that have been combined to create a new genome. Transgenic organisms are a subset of GMOs. They have a gene from another species inserted into their genome. 3. Examples of animals that have been genetically modified include: 1—fish, cows, pigs, rabbits, and sheep carrying bovine growth hormone, which leads to larger animals; 2—goats which produce human growth hormone in their milk; 3—animals to produce drugs for cystic fibrosis in their milk.

Check Your Progress 26.1: 1. Using a restriction enzyme to cut open plasmid or other DNA, followed by use of DNA ligase to seal the foreign gene into the plasmid. 2. See Figure 26.2. 3. Each person has a unique number of short tandem repeats (STRs) at multiple loci. Separating these STRs by gel electrophoresis or by the technique of STR profiling allows the identification of each individual. 26.2: 1. Improved agriculture, production of pharmaceuticals, production of hormones, clean-up of environmental pollution. 2. Cloned organism has the identical genetic makeup. Transgenic organisms carry a gene that has

Appendix A been inserted from another organism. 26.3: 1. In vivo and ex vivo methods using viral vectors are used to infect cells and introduce corrected genes. They also infect people with viruses that help fight off cancer. 2. An example of in vivo gene therapy is use of an adenovirus vector as an inhalant to treat cystic fibrosis patients. An example of ex vivo gene therapy is removal of dysfunctional genes from an SCID patient, using a virus to “inject” the correct gene into these cells, and re-inoculating the patient with the corrected cells. 26.4: 1. Genomics is the study of the complete genomic sequences of organisms. Proteomics studies the proteins that are produced from the genome within a cell. 2. Comparative genomics can allow us to better understand gene function because unknown function in a focal organism that is close in genetic sequence to known genes in related organisms can often have a similar function. 3. Bioinformatics, which involves computer technologies, special software, and statistics, is used to study the enormous amount of information obtained from genomics and proteomics.

Science in Your Life Forms of Cloning: 1. The techniques used in therapeutic cloning that direct either ESCs or adult stem cells to develop into specific cell types overlap with the techniques involved in reproductive cloning of an entire animal. 2. Diseases that involve organ systems containing different organs made up of various tissues, such as atherosclerosis or congestive pulmonary heart disease, might not be treatable using therapeutic cloning. DNA Fingerprinting and the Criminal Justice System: 1. A valid fear for contributing your DNA information to a national databank is that at some point the release of that information will cause harm through some discriminatory process. 2. The legal system is always striving to achieve their goals. The balance of personal freedoms versus society’s need of a legal system should be discussed. Regulation of the DNA databank to insure proper sample collection, storage, processing, and interpretation must be insured. 3. Access of defendants to DNA fingerprinting data for their own defense seems like a personal right. Funding the cost of this is a societal concern. Are Genetically Engineered Foods Safe? 1. Labeling is an effective way to be transparent about production of food and a way to communicate to the consumer what is in their food. However it represents an added cost that could be difficult to implement especially if each state had their own regulations. 2. Opinion one way or the other on this issue will depend on the perceived risk of GMOs versus their measured benefit to the world. Whatever changes are made should be through established legal routes. 3. Respondents would need to get more information confirming the safety of golden rice and its benefit to people eating it. Unfortunately this type of information may take many years to accumulate. Testing for Genetic Disorders: 1. DNA microarrays allow for the detection of what DNA is being expressed in a cell or organism. Previous techniques just determined that an aberrant sequence was present in the genome. 2. Comparison between diseased tissue gene expression and normal tissue can be made. 3. Cells from an individual with heart disease would have chromosomal variations that could be detected using genomic microarrays, when they were compared to cells from healthy individuals. Drugs could then be developed to target these chromosomal variations.

Testing Yourself 1. e; 2. c; 3. c; 4. d; 5. d; 6. c; 7. b; 8. c; 9. c; 10. e.

Thinking Critically 1. Pros of viral gene therapy include treating or correcting potentially fatal or severely debilitating genetic disorders such as SCID. Gene therapy can also be used to treat difficult cancers. Cons include safety of using viruses and the fact that many patients have developed leukemia as a result of gene therapy. Many consequences of gene therapy are unknown. 2. Genes for basic function, such as DNA replication, protein synthesis, and basic cellular processes such as respiration, should be similar.

CHAPTER 27

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Check Your Progress 27.1: 1. The four elements of Darwin’s theory of natural selection are: 1) Individuals within a species exhibit genetically inheritable variations. 2) Organisms compete for limited resources. 3) Individual organisms have different reproductive success. 4) Organisms adapt to their environment over generations through natural selection. 2. Changes to an organism’s phenotype acquired during its lifetime are not inheritable and therefore are not passed on to the next generation. 3. Adaptations are evolved traits that result in an organism being better suited to its environment. 27.2: 1. Fossils found in the strata of sedimentary rocks of different geological eras, periods, and epochs show evolutionary links between modern organisms and ancient organisms. The range and distribution of plants and animals is studied in the field of biogeography and this information helps to understand how different communities begin whenever landmasses or water environments are separated. 2. Breeders use artificial selection to increase the frequency of desired traits in a breed of dogs. The breed is evolving over time due to this. 3. Chimpanzees, because they are more closely related evolutionarily. 27.3: 1. There must be no mutations, selection, genetic drift, or gene flow. The population must also have random mating. 2. Allele frequencies will change if the HardyWeinberg conditions are not met. As an example, directional selection will decrease the frequency of the allele that is not favored. 27.4: 1. Mutations create genetic variation in a population. Genetic drift is the change in allelic frequency due to the random meeting of gametes during fertilization. Small populations are particularly susceptible to this process. Gene flow is the movement of alleles between populations and results in increased genetic diversity. Nonrandom mating, in which individuals select a mate based on desired traits, results in increased allelic frequencies of those traits. Natural selection results in individuals that are better adapted to the environment having more offspring than those that are less well-adapted. 2. With natural selection, there must be variation in phenotypic traits. This variation must be heritable. There must be over-reproduction and a struggle for survival due to limited resources. There must be differential reproductive success, where some individuals produce more offspring than others. 3. The parasite that causes malaria cannot live within the red blood cells of the heterozygote because upon infection the cell sickles and loses K+, resulting in the death of the parasite. Carrying the trait costs the individual. They are less fit, and in areas of the world without the malaria parasite this would represent a disadvantage. 27.5: 1. See Figure 27.20. 2. Punctuated equilibrium. 27.6: 1. Taxonomy involves identifying, naming, and classifying organisms into taxa. Phylogenetics studies the evolutionary relatedness of organisms and classifies organisms into clades which reflect their evolutionary descent. 2. A phylogenetic tree is an estimation and visual representation of the evolutionary history of biodiversity. A cladogram is composed of branches which are clades. Each clade contains the most recent ancestor and its descendants who share a derived trait. 3. Domain Bacteria, domain Archaea, domain Eukarya.

Science in Your Life Inbreeding in the Pingelapese: 1. The trait would have been removed from the population once Mwanenised died. If he had only five children the distribution of the trait would have been smaller and the impact on the population would have been less. 2. The Pingelapese population is not in Hardy-Weinberg equilibrium because the measured frequency of the allele for achromatopsia is changing in the small and isolated population. Evolution of Antibiotic Resistance: 1. Responses to this question will be based on the person’s view of individual rights versus the good of the community. The rights of the individual are being compromised for the greater good of all the individuals in the city. 2. Tuberculosis is a world-wide disease that can move from country to country. Efforts that the USA takes to control tuberculosis will ultimately benefit all humanity as well as the U.S. population.

Testing Yourself 1. b; 2. b; 3. b; 4. c; 5. c; 6. e; 7. d; 8. c; 9. f; 10. b; 11. e; 12. b; 13. b; 14. b; 15. b; 16. b.

Evolution of Antibiotic Resistance

Thinking Critically

1. Directional selection. 2. Resistance would evolve more quickly in a species that reproduces rapidly and has a high mutation rate.

1. To solve this, q2 = 0.0064, therefore q = 0.08 and p = 0.92. 2pq = 2(0.08)(0.92) = 0.147 = 14.7%. 2. HIV is hard to treat because it evolves resistance to drugs used to treat it very



A-12

Appendix A

quickly. HIV’s mutation rates are so high that millions of new strains evolve daily, some of which are resistant to drugs that an infected patient may be being treated with. Eventually these strains will increase in frequency within a patient such that a drug is no longer effective. The somewhat good news is that this process often takes years and, with proper and early treatment, HIV-infected patients can live up to 15 to 20 years or more before the virus becomes fatal. 3. The disease may be in higher allele frequency in South America because there is balancing selection. That is, heterozygote carriers of the disease may be resistant to some pathogen that is present in South America but not North America. 4. 16%. 5. 10%.

CHAPTER 28 The West Africa Ebola Outbreak 1. Viruses infect cells by first attaching to receptor molecules of the host cell surface. The viral envelope fuses with the host’s plasma membrane and the capsid and viral genome then enters the cell. 2. No because viruses are not cells, they can only reproduce within other living cells, and they exhibit no metabolic activity when outside of those cells. 3. Viruscaused diseases include: common cold, flu, avian flu, measles, mumps, rubella, cold sores, genital warts, chickenpox, mononucleosis, AIDS.

Check Your Progress 28.1: 1. Pasteur demonstrated that sterile broth enclosed in a flask that was not open to the air did not spontaneously generate microbes. 2. Microbes include bacteria, archaea, protists, fungi, and viruses. 3. Your microbiota are all the microbes, the beneficial ones and the harmful ones, that live on or within you. 28.2: 1. The chemical evolution of biomolecules from inorganic compounds was the first step in biological evolution. 2. Biomolecules, such as amino acids and nucleotides, had to be generated before polymers of these (DNA, RNA, proteins) could be constructed. Once polymers appeared, protobionts could evolve, leading eventually to the appearance of living cells. 3. The “primordial soup” hypothesis explains the evolution of organic molecules from inorganic molecules by simulating the atmospheric environment containing CH4, NH3, H2, and H2O exposed to heat and electrical sparks. The “iron-sulfur world” hypothesis explains the appearance of organic molecules at thermal vents at the bottom of the oceans. Ammonia, CO, and hydrogen sulfide pass over iron and nickel sulfide minerals which act as catalysts to form organic molecules. 4. Which comes first, the proteins or the genetic material? In the protein-first hypothesis, polypeptides formed and some of them had enzymatic activity which eventually led to DNA formation. In the RNA-first hypothesis, RNA appeared first and could act as a substrate and an enzyme. DNA and proteins appeared later. Another hypothesis is that polypeptides and RNA evolved simultaneously, thus eliminating the paradox. 28.3: 1. Archaea and Bacteria lack a nucleus and membranebound organelles in the cytoplasm. Archaea shares features with Eukarya listed in Table 28.1. 2. The plasma membranes of Archaea have a monolayer of lipids with branched side chains, compared to the lipid bilayer of bacterial and eukaryote membranes. This contributes to the ability of many archaea to grow at very high temperatures. 3. In addition to the membrane structural variations described above, archaeal enzymes must be able to function at high temperatures and/or salt concentrations that would inactivate the enzymes of most other organisms. 4. Archaea living in the digestive tracts of cattle produce methane that contributes to global warming. Giving up beef in your diet would help to reduce this cause of global warming. 28.4: 1. The three basic bacterial shapes are bacillus (rod), coccus (spherical), and spirillum (spiralshaped). 2. In bacterial conjugation, actual physical contact (via a sex pilus) is required for two bacteria to share DNA. Transformation is the uptake of DNA from the environment by a bacterial cell, and in transduction DNA is transferred from one bacterium to another by viruses. 3. Heterotrophic bacteria use organic compounds from their environment for energy. Chemoautotrophs reduce CO2 to an organic compound using electrons derived from ammonia, hydrogen gas, and hydrogen sulfide, and sometimes minerals such as iron. 4. Antibiotics work by interfering with normal bacterial metabolism such as protein synthesis and cell wall synthesis. Bacteria can become resistant to antibiotics if a mutation occurs that alters the target of the drug (often an enzyme). Alternatively, some bacteria may acquire the ability to alter the antibiotic itself, so the drug is destroyed or inactivated. 28.5: 1. A typical enveloped virus has a protein capsid that contains its nucleic acid (DNA or RNA) and is surrounded by

a lipid envelope in which protein spikes are embedded. 2. Attachment: viral spikes bind to receptor molecules on host cell. Entry: viral envelope fuses with plasma membrane and capsid and viral genome enters cell. Replication: viral enzyme makes copies of its genome. Biosynthesis: more capsid and spike proteins are made on host ribosomes. Assembly: capsid forms around viral genome. Budding: new viruses containing some host plasma membrane and spikes are released from cell. 3. Vaccines are available for measles, mumps, and rubella. Vaccines are not available for HIV, hepatitis C, and mononucleosis. 4. Viruses, viroids, and prions are all acellular pathogens. Viroids’ genomes are a single strand of circular RNA that is 1/10 the size of a viral genome. It codes for no proteins. Viroids cause disease in plants and not animals. Prions are proteinaceous molecules that are derived from normal proteins that have changed shape. They lead to neurological wasting diseases in animals, including humans.

Science in Your Life DIY Bio: 1. Maybe, if caution is exerted to not create pathogens or other organisms that cause harm. 2. Not necessarily. A great deal of creativity comes out of DIY projects and those who conduct them. 3. Other applications include transferring genes that improve pest resistance, productivity, nutritional content, heat tolerance, and drought resistance. Antibiotics and Probiotics: 1. Without antibiotics, a tool to fight infections of pathogenic bacteria would be removed. Each of us would be more vulnerable to bacterial infections. 2. Microflora can be influenced by the host’s exposure to chemicals, diet, health, exercise, and the functioning of the immune system. 3. Just as our immune system can discern between self and nonself cells, it has the potential to recognize one type of bacteria from another through cell surface receptors.

Testing Yourself 1. a; 2. d; 3. a; 4. d; 5. d; 6. d; 7. b; 8. d; 9. b; 10. b; 11. a; 12. b; 13 b.

Thinking Critically 1. Three reasons why bacteria are useful as a model genetic organism include: fast generation time, small size, and ease of generating genetic mutations. 2. Viruses that use RNA as their genetic material would be more likely to contain their own enzyme than those that use DNA, because DNA viruses can use the enzymes that their host cells normally use to copy their cellular DNA. Since host cells do not have enzymes to make copies of RNA, RNA viruses must produce their own RNA-replicating enzymes. 3. Antiviral drugs could interfere with any phase of viral replication including attachment, entry, replication, biosynthesis, assembly, and budding. For instance the drug might mimic a molecule that normally binds to receptor proteins on the membrane and prevent the virus from binding. 4. Cell wall synthesis; endospore production; mechanisms of protein synthesis unique to prokaryotes.

CHAPTER 29 Dangerous Protists 1. No. 2. Protists, which are eukaryotes and for the most part unicellular and microscopic, are likely related to the first eukaryotic cell to have evolved. They represent a bridge between the first eukaryotic cells and multicellular organisms. 3. Protists and fungi both benefit human health and welfare and also severely impact it negatively. Some provide useful products while others are parasitic or disease causing.

Check Your Progress 29.1: 1. The endosymbiotic theory proposes that mitochondria were the result of a nucleated cell engulfing an aerobic bacterial cell. Under the theory, chloroplasts originated when a nucleated cell with a mitochondria engulfed a cyanobacteria. 2. Protists carry out sexual reproduction when experiencing unfavorable conditions. This often results in cysts or spores which can survive in the environment until conditions improve, thus promoting dispersion to environments better suited for growth. 3. Algae are typically photosynthetic, converting CO2 into carbohydrates which are used as a food source. Protozoa are heterotrophs. Water molds and cellular slime molds are saprotrophs that feed on dead plant material. Cellular slime molds can also eat bacteria. 4. Plasmodium causes malaria. Trypanosoma causes African sleeping sickness. Members

of the genus Entamoeba cause amoebic dysentery. 29.2: 1. Fungi secrete enzymes and digest available food externally. The broken down food is then absorbed. Algae are primarily photosynthetic and require carbon dioxide, water, and a source of energy. Amoeboids surround algae, bacteria, or other protists, which they use as food, with their pseudopods, and digest the food in a food vacuole. 2. Hyphae—individual filaments of a fungus. Mycelium—mass of hyphae composing the body of a fungus. Chitin—a component of the fungal cell wall. 3. See Figure 29.20 for zygospore fungi reproduction; Figure 29.21 for sac fungi; Figure 29.22 for yeast; Figure 29.23 for club fungi. 4. Superficial mycoses include athlete’s foot and ringworm. Systemic mycoses include histoplasmosis and coccidioidomycosis.

Science in Your Life African Sleeping Sickness: 1. Poor nations have fewer resources that can be directed to controlling tsetse fly populations and providing clean running water. Thus, their populations experience increased exposure to fly bites. Also, the access to health care in poor nations is limited, so treatment of the disease is not assured. 2. African sleeping sickness is most prevalent in poor nations where tsetse flies thrive and livestock live in close connection with humans. The number of flies is large and the routes of transmission between unaffected domestic animals and humans are well established. Deadly Fungi: 1. Toxins help fungi avoid predation by animals, including humans who eat them. They serve as a defense mechanism. 2. Understanding the mechanism of action of fungal toxins could lead to modeling new drugs with similar but modified effects.

Testing Yourself 1. a; 2. b; 3. c; 4. b; 5. a; 6. d; 7. b; 8. d; 9. b; 10. c; 11. c; 12. b; 13. e; 14. a; 15. d.

Thinking Critically 1. The Plasmodium parasites complete part of their life cycle inside red blood cells. Two basic mechanisms have been proposed to explain malaria resistance in people who carry the sickle cell gene: 1) the abnormal hemoglobin inhibits the parasite from completing some part of its life cycle, or 2) infected RBCs from sickle cell carriers are more susceptible to being destroyed by the host immune system than infected RBCs from normal individuals. If an effective malaria vaccine becomes widely implemented, the natural resistance to malaria provided by the sickle cell gene would become a less significant factor. Over time, the sickle cell gene would become less prevalent in the population as the advantage of having only one copy of the gene diminished. 2. A chemical to prevent termite infestations could be targeted at killing the bacteria like an antibiotic works, or it could prevent the bacteria from producing the enzyme, or it could interfere with the action of the enzyme. 3. The outer layer of skin is composed of dead epithelial cells which are constantly being shed. Human skin is often exposed to soap and water. In order to attach and grow on the skin, the fungi need to be able to penetrate to the lower layers of the dermis and be able to withstand the usual methods of washing. The lungs are more hospitable to fungi, due to the lining being thin, moist, and protected.

CHAPTER 30 The Source of Coal 1. Lycophytes were the first to have vascular tissue. Ferns have large leaves called megaphylls. Gymnosperms developed seeds. 2. Angiosperms developed flowers and fruits.The development of flowers and their coevolution with insect pollinators afforded angiosperms high reproductive success which promoted their adaptive radiation to most parts of the world.

Check Your Progress 30.1: 1. Protection of the embryo from desiccation developed in mosses. Vascular tissue appeared in the lycophytes. Ferns evolved megaphylls with their large surface to capture light energy. Seeds first appeared in the gymnosperms. Angiosperms have flowers and fruits. 2. The sporophyte produces haploid spores by meiosis. Spores are reproductive cells that can produce gametophytes by mitosis. The gametophytes then produce gametes by mitosis that can fuse to form the diploid zygote which develops into the sporophyte by mitosis. 30.2: 1. Nonvascular plants are limited by not having vascular tissue to transport water and nutrients. They are restricted to moist environments to grow because the gametophyte requires

a film of water for the flagellated sperm to swim in to reach the egg. 2. Nonvascular plants have developed rootlike, stemlike, and leaflike structures which help them to live on land. Because they are small and simple they can inhabit microhabitats that other plants cannot. 3. A liverwort has a flat-lobed thallus, whereas a moss has leafy green shoots. 30.3: 1. Lycophytes were the first plants to have vascular tissue and true roots, stems, and leaves. 2. Microphylls are small leaves with a single vein containing xylem and phloem. Megaphylls are larger and their vascular tissue is branched. 3. See Figure 30.10. 30.4: 1. Microspores develop in pollen sacs and they become pollen grains (male gametophyes). Megaspores develop in the ovule and one out of four becomes an embryo sac (female gametophyte). 2. Double fertilization in flowering plants results in presence of endosperm in addition to a zygote. Presence of an ovary leads to production of seeds enclosed by a fruit. Animals are often used as pollinators. 3. See Table 30.1. Monocots have one cotyledon (seed leaves that nourish the embryo) and eudicots have two.

Science in Your Life Plants and Humans: 1. Plants contain chemicals that they use for defense against viruses and organisms that invade and harm them. Isolating and understanding the mechanism of action of these chemicals could lead to the development of potentially beneficial drugs for humans. 2. Wheat, corn, and rice have been selectively bred for many years to increase ease of cultivation and crop yields. The standardization of plant types that are successfully grown commercially has led to limitations on the types of plants used for food. 3. Other uses include fuel, soil stabilization and enrichment, control of wetlands, windbreaks, and shade providers.

Testing Yourself 1. a; 2. c; 3. a. sporophyte, b. meiosis, c. gametophyte, d. fertilization; 4. c; 5. a; 6. e; 7. e; 8. b; 9. b; 10. b; 11. c; 12. a. stamen, b. carpel, c. receptacle, d. petals, e. sepals.

Thinking Critically 1. Angiosperms have protected seeds that can lay dormant before germination. They also have double fertilization, which provides endosperm as nourishment for the embryo. Angiosperms have flowers, which attract animal pollinators and allow for dispersal of pollen and fertilization of other plants. Finally, angiosperms have fruits, which are also attractive to animals and can allow long-distance dispersal of seeds to new habitats. 2. Humans do not reach reproductive maturity until puberty. All cells, except eggs and sperm, are diploid. Haploid eggs or sperm are produced by meiosis in ovaries or testes. Fertilization occurs when a sperm cell meets an egg cell, forming a diploid zygote which grows into a fetus. In contrast, plants have an alternation between a diploid sporophyte generation and a haploid gametophyte generation. The sporophyte produces haploid, reproductive spores by meiosis that can develop into a gametophyte by mitosis. The gametophyte produces gametes by mitosis. Sperm and egg fuse, forming a diploid zygote that undergoes mitosis and becomes the sporophyte.

CHAPTER 31 Neglected Tropical Diseases 1. Invertebrates evolved from a choanoflagellate ancestor and outnumber the vertebrates by a large margin. See Figure 31.2 for the phylogenetic tree of the major animal phyla and notice the placement of the Chordates which include the vertebrates. 2. With a variety of developmental stages, body plans, and lifestyles, invertebrates have been able to radiate to occupy innumerable habitats on Earth.

Check Your Progress 31.1: 1. Unlike plants, animals are heterotrophs and must digest their food internally. Fungi secrete enzymes to break down food externally and then absorb the nutrients. 2. Five characteristics animals have in common are: 1. movement or locomotion via muscle fibers; 2. multicellularity; 3. typically diploid adults; 4. usually undergo sexual reproduction and experience developmental stages; 5. usually heterotrophic. 3. A mollusc is multicellular, has three tissue layers and a body cavity, undergoes protostome development, and has or had a trocophore larva. 31.2: 1. Sponges are multicellular, sessile filter feeders. They have an outer layer of flattened epidermal cells, a middle semifluid matrix with amoeboid cells, and an inner layer composed of collar cells. They are capable of reassembling from single cells into a complete organism.

Appendix A 2. Cnidarians have true tissues and have an ectoderm and endoderm as embryonic germ layers. They exhibit radial symmetry as adults and demonstrate two body forms, a polyp and a medusa. 3. See Figure 31.7. 31.3: 1. Flatworms, molluscs, and annelids all are protostomes that are bilaterally symmetrical, have three tissue layers, and are multicellular. 2. Flatworms have no body cavity, an incomplete digestive tract, and no circulatory system. Molluscs have a true coelom, a complete digestive tract, and an open circulatory system. Annelids have a true coelom, a complete digestive tract, and a closed circulatory system. 3. Molluscs have a visceral mass, a strong muscular foot, and a mantle. 31.4: 1. Both roundworms and arthropods periodically shed their outer covering and they both have a complete digestive tract. 2. Rigid, but jointed exoskeleton, segmentation, well developed nervous system, wide variety of respiratory organs, and reduced competition through metamorphosis. 3. Crustaceans have hard, calcified exoskeletons and typically have heads with compound eyes and five pairs of appendages. 31.5: 1. Echinoderms and chordates are closely related because they are both deuterostomes. They share similar embryological development and the coelom forms from an outpocketing of the primitive gut. 2. Echinoderms have endoskeletons composed of calcium, containing plates which have spines that protrude through their skin. They are most often radially symmetrical and have free-swimming larvae.

Science in Your Life Destruction of the Coral Reefs: 1. Coral reefs are found throughout the world in shallow, warm, clear water through which light can pass. They provide a variety of types of shelter for many different organisms. 2. Deforestation, increased sedimentation, global warming, emergence of pathogens, and increased levels of nutrients are all reversible causes of coral reef decline if there is sufficient political capital to make the needed changes. Maggots: A Surprising Tool for Crime Scene Investigation: 1. Forensic entomologists can correlate the growth rate of larvae they observe with the temperature records for the area where the body was found and can pinpoint the time of death accurately. 2. Responses will vary but entomology evidence has been shown to be quite accurate. 3. When a body has been exposed to insects and has been dead for a day or more.

Testing Yourself 1. d; 2. e; 3. b; 4. a; 5. c; 6. b; 7. c; 8. d; 9. e; 10. d; 11. d; 12. c; 13. a. head, b. antenna, c. simple eye, d. compound eye, e. thorax, f. tympanum, g. abdomen, h. forewing, i. hindwing, j. ovipositor, k. spiracles; 14. e; 15. d; 16. e.

Thinking Critically 1. Cnidarians are aquatic and usually have a sessile polyp and motile medusa life stage. Cnidarians can reproduce both asexually and sexually. Bilaterally symmetrical species are typically motile and most often reproduce sexually. 2. Multicellularity allows for specialization of cells, development of tissues, and the formation of organs. Separate functions are assumed by each which increases specificity and efficiency of function. 3. A parasite gains some benefit from the host such as nutrients, housing, or protection. In return a parasite must compromise its structure, functions, or behaviors to comply with the host’s body.

CHAPTER 32 Evolution of Man’s Best Friend 1. Vertebrates evolved from an ancestral chordate. 2. Vertebrates have all four chordate characteristics but the notochord is replaced by a column of vertebrae. Usually there are two pairs of appendages along with a jointed endoskeleton. A skull and a high degree of cephalization as well as jaws, eyes, and ears are present. 3. Coelacanths and crocodiles.

Check Your Progress 32.1: 1. Chordates have a notochord, nerve cord, pharyngeal pouches, and a postanal tail. 2. Lancelets are shaped like a two-edged knife while tunicates look like a sac with thick walls. 3. Mammals have a bony skeleton, lungs, four limbs, an amniotic egg, and mammary glands. 32.2: 1. Fish are adapted to living in water, undergo external fertilization, and have a zygote that is a swimming larva. 2. Amphibians have jointed appendages, four limbs, eyelids, ears, and a larynx. 3. The amphibian larval stage requires a water environment for survival. Metamorphosis of the larvae to the tetrapod adult that can breathe through lungs and its skin allows survival on

A-13

land. 32.3: 1. Reptiles compared to mammals are covered by keratinized scales, do not have a diaphragm, and have an interventricular septum that varies as to its completeness. 2. Placental mammals are unique because the extraembryonic membranes have been modified for internal development within the uterus. 3. Primates are adapted to an arboreal life, have mobile limbs, and the hands and feet have five digits. 32.4: 1. Bipedalism may have evolved because a dramatic change in climate caused forests to be replaced by grassland about 4 mya. It is thought that hominids began to stand upright to keep the sun off their backs and to have better vision for detecting predators or prey in grasslands. However, recent fossil findings suggest bipedalism might have evolved as early as 7 mya, while hominids still lived in forests. The first hominids may have walked upright on large branches as they collected fruit from overhead. The upright stance later made it easier for traveling on the ground and foraging among bushes. 2. Bipedalism. 3. Early species of Homo had a smaller brain size, had humanlike teeth, and could use tools. 32.5: 1. The replacement model or out-of-Africa hypothesis, proposes that modern humans evolved from archaic humans only in Africa, and then modern humans migrated to Asia and Europe, where they replaced the archaic species about 100,000 years bp. In contrast, the multiregional continuity hypothesis proposes that modern humans arose from archaic humans in Africa, Asia, and Europe at roughly the same time. 2. Neandertals lived between 200,000 and 28,000 years ago and were likely supplanted by modern humans (Cro-Magnons at the time). Neandertal brains were slightly larger (1,400 cc). They had massive brow ridges, a forward-sloping forehead, a receding lower jaw, and wide, flat noses. The bones of Neandertals were shorter and thicker than those of CroMagnons. Cro-Magnons had lighter bones, flat high foreheads, domed skulls housing brains of 1,350 cc, small teeth, and a distinct chin.

Science in Your Life Vertebrates and Human Medicine: 1. How this question is answered will depend on people’s opinions of the status of humans within the whole of the Earth’s organisms. Whether animals are here to serve humans should be addressed. 2. Receiving an organ from another species demands that drugs to prevent organ rejection be taken forever. Other species may harbor viruses to which humans are susceptible. Organs from other species can vary as to size and function from humans and challenge compatibility within the human body. Human Ethnic Groups: 1. A person’s ethnic background is ultimately reflected in their genotype and not in their phenotype. 2. As different ethnic groups evolved from a common ancestor their exposure to selective pressures from the environment, mutagens, genetic drift, and gene flow varied. Different diseases could develop at different levels in the groups as a result.

Testing Yourself 1. e; 2. a. pharyngeal pouch, b. dorsal tubular nerve cord, c. notocord, d. postanal tail; 3. c; 4. c; 5. c; 6. d; 7. d; 8. a; 9. e; 10. a; 11. b; 12. c; 13. a; 14. c; 15. d.

Thinking Critically 1. Because we have a distant common ancestry with all vertebrates, we share many genes in common with them. Continued study of processes such as regeneration or immune response in amphibians could give us clues as to which human genes may be similar and potentially “turned on” or stimulated in such a way to benefit people by curing diseases or regenerating limbs. In general, genomic studies benefit humans because we are better understanding genes associated with developmental processes and disease resistance. 2. Bipedalism must have provided more of a fitness advantage than the cost of a smaller pelvis for birthing. Upright stance may have given hominids advantages when forests shifted to grasslands about 4 mya. These advantages included: keeping the sun off their backs and better vision to detect predators and prey. These advantages, which included better avoidance of starvation and/or being eaten, outweigh the cost of a more difficult birthing process. 3. Cultural evolution encompasses human behavior and products, including technology. Language is also critical for cultural evolution because products and ideas are passed on from generation to generation not by genetic inheritance, but by verbal transmission. We learn about history from others, which plants are good for us and which ones are poisonous, and most recently, about medicines that can save our lives. In the past 100 years, the human life span has increased by more than 50% due to technology and culture. 4. Genes that control



A-14

Appendix A

insulating body fat, oxygen usage and metabolic rate, shorter stature, and more boxlike body form.

Testing Yourself

CHAPTER 33

Thinking Critically

The Benefits of Living in a Society 1. Genetic based lion behaviors include living in a pride of females, female cubs staying with the pride, male cubs forming coalitions with related males, and local males killing cubs they are not related to. Environmentally based behaviors include hunting patterns that reflect the available resources and defined territories determined by the habitat. 2. Chemical and visual.

Check Your Progress 33.1: 1. Identical twins that are separated at birth and exposed to different environments growing up will show similarities, simply because they are genetically identical. Similar food preferences or other behaviors suggest that there is a genetic basis, since their environments were mostly different. 2. A study comparing the variability of specific behaviors of monozygotic twins raised in the same environment with those behaviors of monozygotic twins raised in different environments would help to determine if a behavior was based on environmental factors. 33.2: 1. Examples of learning include laughing gull chicks learning begging behaviors, ducklings imprinting on their mother, and white-crowned sparrows learning a species specific song. 2. Classical conditioning presents a subject with two different stimuli and leads them to make an association between them (e.g., dogs and food and the sound of a bell). Operant conditioning involves strengthening a stimulus-response connection through such methods as rewards. 33.3: 1. Auditory communication is fast, and is not dependent on daylight. It can vary in loudness, pattern, duration, and repetition. Visual communication is most often used during the day though fireflies who emit their own light use this at night. It is a mechanism of expression of status, happiness, and intention without making physical contact. Tactile communication occurs when one organism touches another to provide information about the environment, express their emotional status or receptivity, or strengthen social bonds. 2. Communication could warn a receiver, show receptivity to mating, mark a territory, tell a receiver where food is, and help in navigation. 3. Auditory communication would be most effective in a forest. Visual would be least effective. 33.4: 1. Animals may defend a portion of their home range that has particularly good food resources. 2. Females produce a limited number of eggs in their lifetime, whereas males produce essentially limitless amounts of sperm. Females are also usually the sex that provides more parental care than males, so they are choosy to ensure the highest quality males in terms of fitness or parental care that the males may provide for the offspring. 3. Advantages: avoiding predators, finding food, cooperative offspring rearing. Disadvantages: higher competition for mates and food, spread of diseases and parasites.

Science in Your Life Behavioral Genetics and Twin Studies: 1. Monozygotic twins raised apart would differ in those traits that are influenced by the environment through epigenetics. 2. Through epigenetics an individual could respond to environmental pressures by changing behaviors. This contrasts with populations responding over generations. Do Animals Have Emotions? 1. It appears that animals can be happy, can enjoy themselves, and even get sad at times. Those who respond should be able to support their position about animal emotions by specific examples and experimental evidence that is convincing. 2. The answer to this question will be based on a person’s perception of cost and return relative to one’s available resources and value system. 3. Whether animals having emotions determines their status as research subjects kept in captivity could be argued either way, especially if they differ in the degree of their emotional response. Mate Choice and Smelly T-shirts: 1. Definitely not. Humans have societal roles that are more defined than those operating in a lion pride. We as a group work to insure the survival of offspring. 2. It is necessary to communicate that health is not defined by absolute weight but instead by one’s ability to live well and reproduce. 3. Research into the behaviors of humans in their choice of mates could have a positive impact on our society’s ability to understand and manage human societies. If money is available, then research in mate choice is warranted.

1. d; 2. c; 3. c; 4. a; 5. b; 6. a; 7. c; 8. b; 9. a; 10. d; 11. b; 12. d. 1. One experiment would be to catch rats from New York and Florida, breed them in the lab, and see if hand-raised newborns (without their parents) have the same preferences as the adults in natural populations. If offspring have no preference, an environmental or learned component would be suggested. This could further be tested by rearing one offspring group (composed of both New York and Florida rats) on one type of cheese and another offspring group on another type of cheese. A cheese preference test could be conducted later in life, and if rats that showed no preference at birth prefer the cheese they were raised on later in life, an environmental (“nurture”) component is suggested. 2. It is not exactly clear why altruistic behavior exists in humans. In the context of culture, someone might save someone else’s life because it makes them feel good to perform such an act. Perhaps people do it so others would be indebted to them or for monetary gain. Perhaps some people perform altruistic behavior so that their societal status would increase (e.g., to be a “hero”)—this may afford them more opportunities (e.g., jobs) that may allow them to improve their ability to obtain resources. Nonetheless, altruism in humans is a complicated issue, but since we are a social species, there may be some benefit in the context of living in a society.

CHAPTER 34 Exotic Species 1. The distribution of fish within a lake will vary according to the availability of suitable habitats and food for the species in the community. That may be only 10% of the lake. 2. To some extent, population growth models apply to humans. However, humans have been able to increase the carrying capacity of the Earth for their species through technology, science, and use of the Earth’s resources. They also use methods of birth control. 3. More developed countries use more resources per capita and therefore have a greater impact.

Check Your Progress 34.1: 1. A population is all of the organisms of the same species within a specific area. A community is all of the populations present in one location. 2. Ecology seeks to better understand how organisms interact with other organisms within their physical environment. 34.2: 1. To reach their biotic potential, a population would need to experience no environmental resistance. It would have no limitations on its reproductive potential, have unlimited food, be disease free, and have no predators. This does not happen. 2. Type I individuals have a normal life span and then die at an increasing rate after that. Type II individuals experience a constant rate of survival throughout their lives. Type III individuals have a decreased rate of survival early in their life and the rate of survival increases during their life. 3. LDCs tend to have a larger percentage of their population in the prereproductive group than MDCs. This produces an unstable age structure because that group will reach reproductive age and have babies, increasing the population. 34.3: 1. Mutualism is exemplified by insect pollinators that receive nectar from flowers and spread pollen to other flowers. 2. Opportunistic growth patterns are exhibited by organisms such as insects and weeds that produce many small offspring with little parental input. 3. By partitioning resources such as food into different niches, competition between species is reduced and survival is increased. 4. Species within a community can experience many types of interactions, including predator-prey, parasitism, commensalism, and mutualism. 34.4: 1. The directional changes in a community’s composition that is called succession occurs when the community is newly established and there is no soil, or when a major disturbance has disrupted the structure of the community. 2. Climaxpattern model—areas will always lead to the climax community. Facilitation model—each successive community helps the development of the next community. Inhibition model—each species in a community resists being replaced by a successive species. Tolerance model—chance determines which species colonize an area. The order of succession may reflect the time it takes the species to mature.

Science in Your Life Interactions and Coevolution: 1. Without coevolution and the changes it brings about, one species would eventually be eliminated by the other.

2. A parasite that kills its host is removing all the benefits it obtains from the host. It would likely not survive without the host.

Testing Yourself 1. e; 2. d; 3. e; 4. c; 5. a. lag phase, b. environmental resistance, c. carrying capacity, d. stable equilibrium, e. deceleration, f. exponential growth; 6. b; 7. a; 8. a; 9. c; 10. c; 11. a; 12. a; 13. b.

Thinking Critically 1. Rotate the use of different pesticides. Use biological controls. Grow different crops year to year. 2. You would need to test if the two species have the same mode of defense. If so it would be Mullerian mimicry. If only one species was harmful then it would be Batesian mimicry. 3. Preditor-prey systems do not exist as simple two species systems. The decline in pheasant and rabbit populations could in part be due to limited food supplies and resulting lower fertility rates. Eliminating the red-tailed hawk will not change food availability. Likely the hawk would shift what it eats from rabbits and pheasants as they become scarce. 4. As a predator reduces the population of prey, it will become increasingly difficult to survive on the limited resource of that prey and the predator will seek out different sources of food to survive. This will happen before the prey becomes extinct.

CHAPTER 35 The Wolves of Yellowstone 1. Trophic levels vary in the types and numbers of organisms that feed at a particular place in a food chain. Their biomass and behaviors influence the way nutrients are cycled and energy flows through an ecosystem. Some pollutants, such as mercury, become increasingly concentrated in animals moving from lower to higher trophic levels. 2. Biogeochemical cycles include the reservoir and exchange pools of the chemicals involved. Human activity can alter the balance between these and the communities of producers, consumers, and decomposers.

Check Your Progress 35.1: 1. Producers are at the base of the food chain because they photosynthesize. They are autotrophs. Consumers need to eat other organisms for nutrients to survive and are heterotrophs. Decomposers consume dead organic matter and break it down. 2. Most energy (90%) is lost as heat to the environment due to inefficiencies in consumption and digestion. 35.2: 1. Little biomass, in the form of carnivores, remains at the top trophic levels in an ecosystem because most energy is lost as heat when one organism eats another. 2. Usually the biomass of autotrophs is greater than that of herbivores, which is greater than the carnivores. But in aquatic ecosystems, herbivores can have higher biomass than producers which can be consumed at a high rate. 35.3: 1. See Figures 35.7, 35.8, 35.9, and 35.10. 2. Humans burn fossil fuels, releasing carbon dioxide into the atmosphere, thus causing rises in global temperatures. Humans mine phosphorus and distribute it into the environment as fertilizers, animal feed supplements, and detergents, where it becomes available to the biotic community. Humans apply nitrogen as fertilizer, which runs off into lakes and rivers and is released as nitrous oxide into the atmosphere. Humans deplete sources of fresh water, creating scarcities.

Science in Your Life The California Drought: 1. Restrictions on water usage must be imposed and enforced on residents and on agriculture and industry. Financial and logistical support should be given to developing the technology and skills involved in generating and managing water supplies. 2. Both should be required by guidelines and legislation to develop water saving measures in their procedures. Initial incentives and financial support should be offered to make the needed changes. Photochemical Smog: 1. How much extra people are willing to pay for a fuel efficient car is a combination of how important a person feels it is and a person’s financial capacity. The government’s role should be to support the development of reasonably priced fuel efficient vehicles and also the development of public transportation systems that reduce or eliminate the need for cars. 2. The young and elderly are less able to fight off lung infections as a result of ozone-related lung damage. 3. The willingness to use public transportation depends on the

balance between the level of inconvenience of using the system versus the personal benefits received. Such benefits include satisfaction at conserving energy. Climate Change and Carbon Emissions: 1. Developing nations often have limited resources that can be devoted to conservation measures. Financial support from developed nations is one possible strategy that would help to preserve forests, which would benefit all countries. 2. Yes, individuals have a personal responsibility to help prevent climate change.

Testing Yourself 1. a. producers, b. consumers, c. inorganic nutrient pool, d. decomposers; 2. c; 3. c; 4. c; 5. b; 6. d; 7. a; 8. c; 9. c, d; 10. a, b, c; 11. d; 12. f.

Thinking Critically 1. Removal of trees changes the balance of evaporation, condensation, and precipitation in an ecosystem. Removal of trees removes shade, causing increases in evaporation relative to condensation and precipitation. In addition, removal of trees causes erosion (due to reduction of tree roots that hold the soil together), which causes more water to run off into the ocean. This causes precipitation patterns to shift from overland, where the forest was, to over the ocean. 2. Fungicides and pesticides would decrease the numbers of organisms involved in the detrital food web. When they are used, the amount of phosphate, ammonium, nitrite, and nitrate available in the soil and sediments would be reduced. 3. Legumes contain nitrogen-fixing bacteria in the nodules on their roots. Growing them increases the amount of nitrogen available to plants grown in that soil. 4. Composting would recycle both carbon and nitrogen. Water can be conserved by shorter showers, less laundry washing, no lawn watering, and more efficient dishwashing. Decreasing the use of fertilizers or detergents would decrease the runoff into nearby bodies of water.

CHAPTER 36 DDT in the Water 1. The use of DDT should be controlled and restricted to extreme situations that warrant it. 2. DDT moves throughout all biomes and impacts them all eventually. 3. All biomes are interconnected and cannot be isolated from each other.

Check Your Progress 36.1: 1. The proximity of the portion of the Earth to the sun determines the different seasons. Because the Earth is tilted on its axis, during part of the year the southern hemisphere is farther from the sun than the northern hemisphere and vice versa. 2. Solar energy heats up air and causes water to evaporate. As this air rises, it cools and the moisture condenses. It falls out as rain. Mountains affect rainfall. As air blows up and over a mountain range, it rises and cools and the moisture condenses as rain. 36.2: 1. Features in a biome that promote biodiversity include warm temperatures, plenty of rain, and complex structure with many levels of living organisms. 2. The tropical rain forest has the highest species diversity, while the tundra has the lowest. 36.3: 1. Lakes can range from nutrient poor (oligotrophic) to nutrient rich (eutrophic). Deep lakes are stratified during summer and winter into epilimnion, thermocline, and hypolimnion layers. Estuaries are where fresh water and saltwater meet and mix. 2. The pelagic division, which is divided into the neritic and oceanic province, is characterized by organisms that live in the open waters under varied levels of sunlight and nutrient availability. The benthic division is characterized by organisms that live on or in the ocean floor.

Science in Your Life The Fate of Prescription Medicine: 1. Yes because freshwater supplies must be preserved for the benefit of everyone. Protection of freshwater supplies is not an individual decision but instead a responsibility of all. 2. The highest priority chemicals that should be banned are those that cause or potentially will cause the greatest harm to the most people. El Niño—Southern Oscillation: 1. Yes MDCs have the responsibility to reduce their emissions because their habits have a worldwide impact and they should be accountable to all people. 2. Yes, if one believed reducing greenhouse gases was important, that buying a fuel-efficient car or paying “carbon credits” would be an effective measure to take, and if one had enough money to support such efforts.

Testing Yourself 1. b; 2. e; 3. a; 4. e; 5. c; 6. c; 7. d; 8. a; 9. a; 10. e.

Appendix A

Thinking Critically 1. Because the sun aims directly at the equator, it is already relatively warm and temperatures are relatively constant in tropical areas near the equator. As you move into the temperate zones (both north and south) away from the equator, the sun’s rays get dispersed. Rising air flows toward the poles, sinks to the Earth’s surface and reheats. With greenhouse gases trapping solar radiation, the warmer air will move toward the poles and away from the equator, making temperature increases in temperate areas larger than those in tropical areas nearest to the equator. 2. Equatorial climates have the highest amount of rainfall, which supports high primary productivity. They also have the most stable climate in terms of temperature, so they can support many plant and animal species that have narrow temperature tolerances. As one moves more north or south from the equator, temperatures get hotter in the deserts, with large temperature fluctuations and little rainfall. This requires plant and animal species adapted for water conservation and those that can deal with large temperature fluctuations. Farther from the equator are taiga and tundra. Taiga has a very short growing season, as it is cold and the ground only thaws for a few months of the year. This makes primary productivity lower and thus, fewer animals can be supported in the food web. Tundra is extremely cold and has permafrost, making ground cover ice and plant growth virtually impossible. Thus, there is very low diversity in areas of tundra—for example, marine life such as penguins that are aquatic for part of the year where they can feed. 3. Although tropical forests have basically a year-round growing season, versus a growing season that ranges from 140–300 days in temperate forest habitats, temperate forests have more nutrient-rich soil. Decomposition rates are slower in temperate areas because of colder temperatures and thus, nutrients are recycled more slowly. In tropical forests, nutrients are cycled rapidly from the soil directly back into the plants. When tropical forests are slashed and burned to initiate agriculture, nutrients are released into the soil, providing nutrients for several harvests. However, the soils become nutrient-poor rather quickly, requiring forest regrowth to occur before further harvesting can occur. 4. The rich diversity and high density of species within a tropical rain forest increases the amount of competition between species. The evolution of antibacterial and antifungal compounds within a species might improve their success at survival and reproduction.

CHAPTER 37

A-15

corridors. This supports the populations of species such as grizzly bears, that require different types of ecosystems for survival. Maintaining just one ecosystem often cannot provide adequate habitat for species. 2. Because edge effects essentially remove the amount of suitable habitat in an area, nature reserves should account for this and generally be designed to be larger. 3. Three principles of habitat restoration are: 1) begin as soon as possible to protect remaining habitat; 2) use biological techniques to mimic natural processes; and, 3) focus on sustainable development, allowing the ecosystem to maintain itself while providing services to humans. 37.5: 1. Most nonrenewable energy use is concentrated on fossil fuel burning, which releases greenhouse gases into the environment and contributes to global warming. 2. We can plant salt and drought tolerant crops, use drip irrigation, reduce and recycle home use water, limit yard irrigation, and use water efficient toilets. 3. Crop rotation helps maintain nutrients in the soil (e.g., by alternating nitrogen-fixing crops, such as legumes). Both organic farming and biological pest control remove or reduce the use of pesticides and herbicides. Contour farming, terrace farming, planting cover crops, and natural fences all help reduce erosion.

Science in Your Life Colony Collapse Disorder: 1. Climate change which results in imbalance between the bees and their environments due to temperature increases and moisture decreases could lead to CCD. Also, the ever increasing human population places further restraints on habitats and the growth of flowers upon which bees depend. 2. Other insects and animals susceptible to mite predators, pesticides, stress, and habitat changes could also suffer. Wildlife Conservation and DNA: 1. DNA analysis allows for the specific identification of a confiscated carcass. If that DNA profile matches previously obtained DNA analyses of animals in a specific environment, then a match can be made and the poacher prosecuted. 2. The power of the ability to connect all endangered or threatened animals with their source would be increased tremendously. Prosecutors would have a stronger tool of evidence. Emiquon Floodplain Restoration: 1. Scientists could determine the number of species present, the population size of each species, and how the species are distributed throughout the floodplain. If increasing biodiversity is demonstrated, then the restoration plan is successful. 2. The successful techniques used to restore the Emiquon floodplain could be adapted to other restricted floodplains and applied there to increase biodiversity.

Gills Onion’s Waste-To-Energy Project

Testing Yourself

1. Establishing systems such as AERS should be strongly encouraged through technical support, economic incentives, and backing for the capital outlay necessary for implementation. 2. While the AERS system appears to be a win-win situation it is somewhat vulnerable to shifts in market pricing. It represents a very large capital investment that needs to be flexible enough to survive. 3. To some degree, indirect costs can be estimated and translated into direct value terms.

1. e; 2. c; 3. b; 4. a; 5. e; 6. e; 7. e; 8. a. habitat loss, b. exotic species, c. pollution, d. overexploitation, e. disease; 9. c; 10. d; 11. c; 12. e; 13. a.

Check Your Progress 37.1: 1. Conservation biology relies on multiple disciplines in basic (e.g., physiology, genetics, ecology) and applied (e.g., forestry, fisheries biology, agronomy) biology to achieve its goal of protecting biodiversity and Earth’s natural resources. 2. Ecosystem-level conservation will protect habitats for many communities and the species they contain, whereas species-level conservation may protect one species while not accounting for the needs of the whole community. 37.2: 1. Consumptive use value of biodiversity describes the direct value of a natural product, such as timber. Agricultural use value is the value of those products derived from wild plants and developed by humans for agriculture, such as crops. 2. Direct value is the dollar value of product in the market. Indirect values, which are difficult to measure, are those benefits incurred as a result of the existence and functioning of ecosystems. 37.3: 1. Exotic species can be introduced by humans when they colonize a new area, by accidental transport, or by horticulture and agriculture. 2. The five main causes of extinction are: habitat loss, introduction of exotic species, pollution, over exploitation, and diseases. 3. Pollution can weaken immunity and lead to increased susceptibility to disease, as well as directly poison organisms. Pollution leads to global warming, which can have negative impacts on species’ geographic ranges. 37.4: 1. Landscape preservation maintains multiple ecosystem types that are connected by

Thinking Critically 1. Each person’s answer will depend on the lifestyle they lead, but reducing use of fossil fuels, living in and maintaining a smaller home, reducing water consumption, and reducing meat consumption are ways to reduce ecological footprints. 2. One simple way to reduce our reliance on nonrenewable resources is to reduce consumption by conserving electricity, water, and gas for example. Another way is through recycling, which reuses nonrenewable resources. We can also switch from using nonrenewable energy resources, such as fossil fuels, to renewable energy sources, such as hydropower, geothermal energy, wind power, solar power, and hydrogen fuel. 3. Biological magnification is the process by which pollutants increase in their concentration up the food chain. While an herbivore may eat a certain amount of biomass of plants, a carnivore eats several herbivores, thereby magnifying the concentration of pesticides in the plants. The farther an organism is up the food chain, the more the chemicals are magnified in the food. Thus, people who often are tertiary or even quaternary consumers should be more concerned about pesticide concentrations in their food than herbivores, since they eat plants directly. 3. Extinction of a keystone species may cause secondary extinctions or change the food web dynamics of an ecosystem, as seen with wolves in Yellowstone National Park. Without wolves, elk are numerous and alter aspen groves, their favorite food. Wolves have helped reduce the number of elk, and their carcasses become food for grizzly bears and eagles. Reduction of elk also results in increases in the number of aspen and other trees found in floodplains that, in turn, increases habitat for songbirds and beavers. Beavers’ dams flood areas, making them suitable for muskrats, ducks, otters, and amphibians.



APPENDIX B Metric System

A-16



A-17

Appendix B

The Periodic Table of the Elements Atomic number

1

H

Group Ia 1 1

Solid

IIa

3

7

Li

4

Gas

9

Be

lithium

beryllium

11

23

Na

12

sodium

magnesium

19

39

K

potassium

37

85

20

40

Ca 38

88

rubidium

strontium

56

cesium

137

Ba barium

223

francium

88

226

Ra radium

IVb 45

scandium

55

Cs

IIIb

21

Sc

calcium

Rb Sr 133

Unknown

24

Mg

39

89

Y

yttrium

57

139

La

lanthanum

89

227

Ac

actinium

Metalloids

C

Other nonmetals Halogens

Nonmetals Liquid

hydrogen

Fr

Atomic mass Atomic symbol

hydrogen

H

87

1

22

Vb 48

Ti

VIb 51

V

titanium

40

23

vanadium

91

Zr

41

93

zirconium

niobium

73

178

Hf

hafnium

261

Rf

rutherfordium

58

140

Ce cerium

90

232

Th thorium

Cr

chromium

42

96

25

181

Ta

tantalum

105

260

molybdenum

74

184

W

tungsten

106

Alkaline earth metals

H

43

98

263

59

seaborgium

141

Pr

praseodymium

91

231

Pa

protactinium

60

144

technetium

cobalt

101

45

186

ruthenium

76

190

59

rhenium

osmium

261

108

bohrium

101

46

106

62

109

meitnerium

63

195

platinum

Mt

150

palladium

Pt

266

152

110

281

Ds

darmstadtium

64

IIb

29

64

copper

FPO 78

192

iridium

265

hassium

147

157

47

108

92

U

238

uranium

promethium

93

237

samarium

94

242

Np Pu neptunium

plutonium

europium

95

243

silver

79

80

gold

gallium

112

201

111

mercury

272

112

32

73

49

115

113

82

207

Pb 114 flerovium

65

66

67

68

163

Dy

dysprosium

98

berkelium

249

californium

165

erbium

99

100

Es

254

einsteinium

Te

167

53 iodine

85

115

210

polonium

astatine

116

117

293

ununpentium

livermorium

69

70

169

173

36

84

Kr

krypton

127

54

131

Xe

xenon

210

86

222

Rn

Po At 288

40

argon

80

I

tellurium

Uup Lv

Ho Er holmium

128

18

Ar

bromine

84

209

35

Br

antimony

bismuth

289

52

35

20

Ne

chlorine

79

83

Bi

lead

284

122

17

10

neon

Cl

selenium

Sb

tin

204

thalium

285

119

Sn

indium

81

51

ununtrium

97

sulfur

50

copernicum

96

32

S

34

75

helium

fluorine

16

phosphorus

arsenic

roentgenium

247

31

33

germanium

Rg Cn Uut Fl 159

oxygen

15

9

19

F

O

P

silicon

70

Au Hg Tl

terbium

curium

31

28

8

4

He

VIIa 16

Ga Ge As Se

cadmium

197

Am Cm Bk Cf americium

65

14

7

nitrogen

Si

Ag Cd In

gadolinium

247

48

27

aluminum

30 zinc

Nd Pm Sm Eu Gd Tb neodymium

13

VIa 14

N

carbon

Al

Cu Zn

nickel

rhodium

77

Re Os Ir

61

28

Ru Rh Pd

75

107

Ib 59

Co Ni

iron

Db Sg Bh Hs dubnium

27

6

Va 12

C

boron

Post-transition metals

56

11

B

Transition metals

26

IVa

5

Actinoids

Rf

44

IIIa

Lanthanoids

Metals

Mn Fe manganese

2

Alkali metals

VIIIb 55

VIIIa

Noble gases

VIIb 52

Nb Mo Tc

72

104

24

Hg

radon

294

118

294

Uus Uuo ununseptium

71

ununoctium

175

Tm Yb Lu thulium

253

101

256

ytterbium

lutetium

102

103

254

257

Fm Md No Lr fermium

mendelevium

nobelium

lawrencium



GLOSSARY A

abiotic  Nonliving; examples are water, gases, and minerals. abscisic acid (ABA)  Plant hormone that causes stomata to close and initiates and maintains dormancy. abscission  Dropping of leaves, fruits, or flowers from a land plant. acid  Molecules tending to raise the hydrogen ion concentration in a solution and thus to lower its pH numerically. acquired immunodeficiency syndrome (AIDS)  Disease caused by the HIV virus that destroys helper T cells and macrophages of the immune system, thus preventing an immune response to pathogens; caused by sexual contact with an infected person, intravenous drug use, and transfusions of contaminated blood (rare). acromegaly  Condition caused by an overproduction of growth hormones as an adult. actin  One of two major proteins of muscle; makes up thin filaments in myofibrils of muscle fibers. See also myosin. actin filament Component of the cytoskeleton; plays a role in the movement of the cell and its organelles; a protein filament in a sarcomere of a muscle, its movement shortens the sarcomere, yielding muscle contraction. action potential  Electrochemical changes that take place across the axon membrane; the nerve impulse. active immunity  Ability to produce antibodies due to the immune system’s response to a microorganism or a vaccine. active site  Region of an enzyme where the substrate binds and where the chemical reaction occurs. active transport  Use of a plasma membrane carrier protein to move a molecule or ion from a region of lower concentration to one of higher concentration; it opposes equilibrium and requires energy. acute bronchitis  Inflammation of the bronchi in the lungs; often caused by a viral or bacterial infection. adaptation  Species’ modification in structure, function, or behavior that makes a species more suitable to its environment. adaptive immunity  Type of immunity that is characterized by the response of lymphocytes to specific antigens. adaptive radiation  Rapid evolution of several species from a common ancestor into new ecological or geographical zones. Addison disease Condition caused by an insufficient production of cortisol by the adrenal glands. adenine (A) One of four nitrogen-containing bases in nucleotides composing the structure of  DNA and RNA. Pairs with uracil (U) and ­thymine (T). adenosine  Portion of ATP and ADP that is composed of the base adenine and the sugar ribose.

adhesion  The ability of water molecules to cling to, or be attracted to, a surface, such as a transport vessel in a plant or animal. adhesion junction  Junction between cells in which the adjacent plasma membranes do not touch but are held together by intercellular filaments attached to buttonlike thickenings. adipose tissue Connective tissue in which fat is stored. ADP (adenosine diphosphate) Nucleotide with two phosphate groups that can accept another phosphate group and become ATP. adrenal cortex  Outer portion of the adrenal gland; secretes mineralocorticoids, such as aldosterone, and glucocorticoids, such as cortisol. adrenal gland Gland that lies atop a kidney; the adrenal medulla produces the hormones epinephrine and norepinephrine, and the adrenal cortex produces the glucocorticoid and mineralocorticoid hormones. adrenal medulla  Inner portion of the adrenal gland; secretes the hormones epinephrine and norepinephrine. adrenocorticotropic hormone (ACTH)  Hormone secreted by the anterior lobe of the pituitary gland that stimulates activity in the adrenal cortex. aerobic  A chemical process that requires air (oxygen); phase of cellular respiration that requires oxygen. African sleeping sickness Disease caused by the protist Trypanosoma brucei, which is transmitted by the tsetse fly. afterbirth  Another term for the placenta; the expulsion of the afterbirth represents the third stage of parturition. age structure diagram  In demographics, a display of the age groups of a population; a growing population has a pyramid-shaped diagram. aging  General term for the progressive physiological changes that occur to the human body over time. aldosterone  Hormone secreted by the adrenal cortex that regulates the sodium and potassium ion balance of the blood. algae  Photosynthetic protists that may be unicellular or live in colonies or filaments; historically have been classified according to color. alien species  Nonnative species that migrate or are introduced by humans into a new ecosystem; also called exotic species. allantois  Extraembryonic membrane that accumulates nitrogenous wastes in the eggs of reptiles, including birds; contributes to the formation of umbilical blood vessels in mammals. allele  Alternative form of a gene; alleles occur at the same locus on homologous chromosomes. allergy  Immune response to substances that usually are not recognized as foreign. allopatric speciation  Model that proposes that new species arise due to an interruption of gene flow between populations that are separated geographically.

alternation of generations Life cycle, typical of land plants, in which a diploid sporophyte alternates with a haploid gametophyte. altruism  Social interaction that has the potential to decrease the lifetime reproductive success of the member exhibiting the behavior. alveoli  (sing., alveolus) In humans, terminal, microscopic, grapelike air sac found in lungs. Alzheimer disease  Disease of the central nervous system (brain) that is characterized by an accumulation of beta amyloid protein and neurofibrillary tangles in the hippocampus and amygdala. AM fungi Fungi with branching invaginations (arbuscular mycorrhizal or AM) used to invade plant roots. amino acid Organic molecule composed of an amino group and an acid group; covalently bonds to produce peptide molecules. ammonia  Nitrogenous end product that takes a limited amount of energy to produce but requires much water to excrete because it is toxic. amnion  Extraembryonic membrane of birds, reptiles, and mammals that forms an enclosing, fluid-filled sac. amniotic egg  Egg that has an amnion, as seen during the development of reptiles (including birds) and mammals. amoebic dysentery  Condition caused by protistans belonging to the genus Entamoeba; characterized by severe diarrhea and fluid loss. amoeboid  Cell that moves and engulfs debris with pseudopods. amphibian  Member of vertebrate class Amphibia that includes frogs, toads, and salamanders; they are tied to a watery environment for reproduction. amygdala  Part of the limbic system of the brain; associated with emotional experiences. amyotrophic lateral sclerosis (ALS)  Neurodegenerative disease affecting the motor neurons in the brain and spinal cord, resulting in paralysis and death; also known as Lou Gehrig’s disease. anabolic steroid  Synthetic steroid that mimics the effect of testosterone. anabolism  Chemical reaction in which smaller molecules (monomers) are combined to form larger molecules (polymers); anabolic metabolism. anaerobic  A chemical reaction that occurs in the absence of oxygen; an example is the fermentation reactions. analogous structure Structure that has a similar function in separate lineages but differs in anatomy and ancestry. anaphase  Fourth phase of mitosis; chromosomes move toward the poles of the spindle. anaphylactic shock  Severe systemic form of anaphylaxis involving bronchiolar constriction, impaired breathing, vasodilation, and a rapid drop in blood pressure with a threat of circulatory failure. androgen  Male sex hormone (e.g., testosterone). anemia  Disease that is characterized by a decrease in the number of red blood cells (erythrocytes);

G-1

G-2

Glossary

may be genetic in origin or the result of another disease or condition in the body. aneurysm  Expansion in the walls of the blood vessel; often caused by atherosclerosis or ­ hypertension. angina pectoris Condition characterized by thoracic pain resulting from occluded coronary arteries; may precede a heart attack. angiogenesis  Formation of new blood vessels; rapid angiogenesis is a characteristic of cancer cells. angiosperm  Flowering land plant; the seeds are borne within a fruit. animal  Multicellular, heterotrophic eukaryote that undergoes development to achieve its final form. In general, animals are mobile organisms, characterized by the presence of muscular and nervous tissue. annelid  The segmented worms, such as the earthworm and the clam worm. annual ring  Layer of wood (secondary xylem) usually produced during one growing season. anorexia nervosa  Eating disorder characterized by a severe psychological fear of gaining weight. anterior pituitary Portion of the pituitary gland that is controlled by the hypothalamus and produces six types of hormones, some of which control other endocrine glands. anther  Male structure of a flowering plant where pollen is produced; attached to the flower by the filament. antheridium  (pl., antheridia) Sperm-producing structures, as in the moss life cycle. anthropoid  Group of primates that includes ­monkeys, apes, and humans. antibiotics  Chemicals that interfere with the physiological activities, or structure, of bacteria. antibody  Protein produced in response to the presence of an antigen; each antibody combines with a specific antigen. antibody-mediated immunity  Specific mechanism of defense in which plasma cells derived from B cells produce antibodies that combine with antigens. anticodon  Three-base sequence in a transfer RNA molecule base that pairs with a complementary codon in mRNA. antidiuretic hormone (ADH) Hormone, secreted by the posterior pituitary, that increases the permeability of the collecting ducts in a kidney. antigen  Foreign substance, usually a protein or a polysaccharide, that stimulates the immune system to react, such as to produce antibodies. antigen-presenting cell (APC)  Cell that displays an antigen to certain cells of the immune system so they can defend the body against that particular antigen. anus  Outlet of the digestive tube. aorta  In humans, the major systemic artery that takes blood from the heart to the tissues. apical dominance  Influence of a terminal bud in suppressing the growth of axillary buds. apical meristem  In vascular land plants, masses of cells in the root and shoot that reproduce and elongate as primary growth occurs. apoptosis  Programmed cell death; involves a cascade of specific cellular events leading to death and destruction of the cell.

appendicular skeleton  Part of the vertebrate skeleton comprising the appendages, shoulder girdle, and hip girdle. appendix (vermiform appendix)  In humans, small, tubular appendage that extends outward from the cecum of the large intestine. aquaporin  Channel protein through which water can diffuse across a membrane. aquifer  Rock layers that contain water and release it in appreciable quantities to wells or springs. arachnid  Group of arthropods that includes spiders and scorpions. arboreal  Living in trees. archaeans, archaea  Prokaryotic organisms that are members of the domain Archaea. archegonium  Egg-producing structures, as in the moss life cycle. ardipithecines  Common name for species of the genus Ardipithecus, an early, and now extinct, hominin. arteriole  Vessel that takes blood from an artery to capillaries. artery  Blood vessel that transports blood away from the heart. arthritis  Condition characterized by an inflammation of the joints; two common forms are osteoarthritis and rheumatoid arthritis arthropod  Invertebrates, with an exoskeleton and jointed appendages, such as crustaceans and insects. articular cartilage  Form of hyaline cartilage that occurs at joints where two bones meet. artificial selection  Intentional breeding of certain traits, or combinations of traits, over others to produce a desirable outcome. ascocarp  Reproductive structure of the sac fungi (phylum Ascomycota). associative learning  Acquired ability to associate two stimuli or between a stimulus and a response. asthma  Condition in which bronchioles constrict and cause difficulty in breathing. astigmatism  Uneven shape of the cornea or lens of the eye; causes a distortion in the light reaching the retina of the eye. asymmetrical  Lack of any symmetrical relationship in the morphology of an organism. atherosclerosis  Form of cardiovascular disease characterized by the accumulation of fatty materials (usually cholesterol) in the arteries. atom  Smallest particle of an element that displays the properties of the element. atomic mass  Average of atom mass units for all the isotopes of an atom. atomic number Number of protons within the nucleus of an atom. ATP (adenosine triphosphate) Nucleotide with three phosphate groups. The breakdown of ATP into ADP + P  makes energy available for energyrequiring processes in cells. ATP synthase  Complex of proteins in the cristae of mitochondria and thylakoid membrane of chloroplasts that produces ATP from the diffusion of hydrogen ions across a membrane. atrial natriuretic hormone (ANH) Hormone secreted by the heart that increases sodium excretion by inhibitng the secretion of aldosterone. atrioventricular valve  Heart valve located between an atrium and a ventricle.

atrium  Chamber; particularly an upper chamber of the heart, lying above a ventricle. auditory canal  Structure that funnels sounds from the outer ear to the middle ear. auditory tube  Also called the eustachian tube; connects the middle ear to the nasopharynx for the equalization of pressure. australopithecine  One of several species of Australopithecus, a genus that contains the first generally recognized humanlike hominins. autoimmune disease  Disease that results when the immune system mistakenly attacks the body’s own tissues. autonomic system Portion of the peripheral nervous system that regulates internal organs. autosome  Chromosome pairs that are the same between the sexes; in humans, all but the X and Y chromosomes. autotroph  Organism that can capture energy and synthesize organic molecules from inorganic nutrients. auxin  Plant hormone regulating growth, particularly cell elongation; also called indoleacetic acid (IAA). axial skeleton  Part of the vertebrate skeleton forming the vertical support or axis, including the skull, the rib cage, and the vertebral column. axillary bud  Bud located in the axil of a leaf. axon  Elongated portion of a neuron that conducts nerve impulses, typically from the cell body to the synapse.

B

B cell  Lymphocyte that matures in the bone marrow and, when stimulated by the presence of a specific antigen, gives rise to antibody-producing plasma cells. B-cell receptor (BCR)  Molecule on the surface of a B cell that binds to a specific antigen. bacteriophage  Virus that infects bacteria. bacterium  (pl., bacteria) Single-celled prokaryotic organisms that are members of the domain ­Bacteria. Lack a nucleus and membrane-bound organelles. bark  External part of a tree, containing cork, cork cambium, and phloem. Barr body  Dark-staining body in the cell nuclei of female mammals that contains a condensed, inactive X chromosome; named after its discoverer, Murray Barr. basal nuclei Subcortical nuclei deep within the white matter that serve as relay stations for motor impulses and produce dopamine to help control skeletal muscle activities. base  Molecules tending to lower the hydrogen ion concentration in a solution and thus raise the pH numerically. basidium  Clublike structure in which nuclear fusion, meiosis, and basidiospore production occur during sexual reproduction of club fungi. basophil  White blood cell with a granular cytoplasm; able to be stained with a basic dye. behavior  Observable, coordinated responses to environmental stimuli. benign  Mass of cells derived from a single mutated cell that has repeatedly undergone cell division but has remained at the site of origin.

Glossary G-3

benign prostatic hyperplasia (BPH)  Enlargement of the prostate gland that may cause irritation of the bladder and an urge to urinate more frequently. benthic division Ocean or lake floor, extending from the high-tide mark to the deepest depths. bicarbonate ion  Ion that participates in buffering the blood, and the form in which carbon dioxide is transported in the bloodstream. bilateral symmetry  Body plan having two corresponding or complementary halves. bile  Secretion of the liver that is temporarily stored and concentrated in the gallbladder before being released into the small intestine, where it emulsifies fat. binary fission Splitting of a parent cell into two daughter cells; serves as an asexual form of reproduction in bacteria. biodiversity  Total number of species, the variability of their genes, and the communities in which they live. biodiversity hotspot  Region of the world that contains unusually large concentrations of species. biogeochemical cycle  Circulating pathway of elements, such as carbon and nitrogen, involving exchange pools, storage areas, and biotic communities. biogeography  Study of the geographical distribution of organisms. bioinformatics  Area of scientific study that utilizes computer technologies to analyze large sets of data, typically in the study of genomics and proteomics. biology  The branch of science that is concerned with the study of life and living organisms. biomagnification  The accumulation of pollutants as they move up the food web. biomass  The number of organisms multiplied by their weight. biome  One of the biosphere’s major communities, characterized in particular by certain climatic conditions and particular types of plants. biomolecule  Organic molecules such as proteins, nucleic acids, carbohydrates, and fats. biosphere  Zone of air, land, and water at the surface of the Earth, in which living organisms are found. biotechnology  Scientific research that involves researching and producing commercial or agricultural products that are made with or derived from transgenic organisms. biotic  Term that indicates life; a cell is an example of a biotic factor in the environment. biotic potential  Maximum population growth rate under ideal conditions. bipedalism  Walking erect on two feet; distinguishing characteristic of primates. bird  Endothermic reptile that has feathers and wings, is often adapted for flight, and lays hardshelled eggs. birth control pill  Form of oral contraception that typically contains a combination of estrogen and progesterone. bivalve  Type of mollusc with a shell composed of two valves; includes clams, oysters, and scallops. bladder stones  Condition that causes inflammation of the bladder; may be caused by kidney stones, infections, or other conditions that restrict the flow of urine from the bladder.

blade  Broad, expanded portion of a land plant leaf that may be single or compound leaflets. blastocoel  Fluid-filled cavity of a blastula. blastocyst  Early stage of human embryonic development that consists of a hollow, fluid-filled ball of cells. blastopore  Opening into the primitive gut formed at gastrulation. blastula  Hollow, fluid-filled ball of cells occurring during animal development prior to gastrula formation. blind spot Region of the retina, lacking rods or cones, where the optic nerve leaves the eye. blood  Fluid circulated by the heart through a closed system of vessels; type of connective tissue. blood pressure  Force of blood pushing against the inside wall of blood vessels. body cavity  In vertebrates, defined regions of the body in which organs reside. bone  Connective tissue having protein fibers and a hard matrix of inorganic salts, notably calcium salts. bony fishes Vertebrates belonging to the class of fish called Osteichthyes that have a bony, rather than cartilaginous, skeleton. bottleneck effect  Type of genetic drift; occurs when a majority of genotypes are prevented from participating in the production of the next generation as a result of a natural disaster or human interference. brachiopod  Invertebrate animal (lophotrochozoan) that possesses hard shells (or valves) on the upper and lower surfaces of the organism. brain  Ganglionic mass at the anterior end of the nerve cord; in vertebrates, the brain is located in the cranial cavity of the skull. brain stem  In mammals, portion of the brain consisting of the medulla oblongata, pons, and midbrain. bronchi (sing., bronchus)  In terrestrial vertebrates, branch of the trachea that leads to the lungs. bronchiole  In terrestrial vertebrates, small tube that conducts air from a bronchus to the alveoli. brown algae  Marine photosynthetic protists with a notable abundance of xanthophyll pigments; this group includes well-known seaweeds of northern rocky shores. bryophytes  Term used to identify plants that do not have vascular tissue. Inlcudes the liverworts, hornworts, and mosses. bryozoan  Invertebrate animal (lophotrochozoan) that lives as a mass of interconnected individuals. buffer  Substance or group of substances that tend to resist pH changes of a solution, thus stabilizing its relative acidity and basicity. bulbourethral glands  Male sex glands that produce pre-ejaculate fluid that neutralizes acid in the urethra. bulimia nervosa  Eating disorder characterized by binge eating followed by purging of food (vomiting). bundle sheath  Layer of cells surrounding the vascular tissue in the leaf of a plant. bursae  (sing., bursa) Fluid-filled sacs that reduce friction between tendons and ligaments; often found near synovial joints such as the elbow and knee. bursitis  Inflammation of the bursae.

C

C3 Plant  Plant that fixes carbon dioxide via the Calvin cycle; the first stable product of C3 photosynthesis is a 3-carbon compound. C4 plant  Plant that fixes carbon dioxide to produce a C4 molecule that releases carbon dioxide to the Calvin cycle. calcitonin  Hormone secreted by the thyroid gland that increases the blood calcium level. calorie  Amount of heat energy required to raise the temperature of one gram of water 1°C. Calvin cycle reaction Portion of photosynthesis that takes place in the stroma of chloroplasts and can occur in the dark; it uses the products of the light reactions to reduce CO2 to a carbohydrate. calyx  The sepals collectively; the outermost flower whorl. CAM  Crassulacean-acid metabolism; a form of photosynthesis in succulent plants that separates the light-dependent and Calvin reactions by time. cancer  Malignant tumor whose nondifferentiated cells exhibit loss of contact inhibition, uncontrolled growth, and the ability to invade tissue and metastasize. capillary  Microscopic blood vessel; gases and other substances are exchanged across the walls of a capillary between blood and tissue fluid. capsid  Protective protein containing the genetic material of a virus. capsule  A form of glycocalyx that consists of a gelatinous layer; found in blue-green algae and certain bacteria. carbaminohemoglobin  Hemoglobin carrying carbon dioxide. carbohydrate  Class of organic compounds that typically contain carbon, hydrogen, and oxygen in a 1:2:1 ratio; includes the monosaccharides, disaccharides, and polysaccharides. carbon dioxide fixation  Process by which carbon dioxide gas is attached to an organic compound; in photosynthesis, this occurs in the Calvin cycle reactions. carbonic anhydrase  Enzyme in red blood cells that speeds the formation of carbonic acid from water and carbon dioxide. carcinogen  Environmental agent that causes mutations leading to the development of cancer. cardiac cycle One complete cycle of systole and diastole for all heart chambers. cardiac muscle  Striated, involuntary muscle tissue found only in the heart. cardiac output  Blood volume pumped by each ventricle per minute (not total output pumped by both ventricles). cardiovascular system  Organ system of humans that consists of the heart and blood vessels; transports blood, nutrients, gases, and wastes; assists in the defense against disease; helps control homeostasis. carnivore  Consumer in a food chain that eats other animals. carotenoid  An accessory photosynthetic pigment of plants and algae that are often yellow or orange in color; consists of two classes—the xanthophylls and the carotenes. carotid body  Structure located at the branching of the carotid arteries; contains chemoreceptors sensitive to the O2, CO2, and H+ content in blood.



G-4

Glossary

carpel  Ovule-bearing unit that is a part of a pistil. carrier protein Protein in the plasma membrane that combines with and transports a molecule or ion across the plasma membrane. carrying capacity (K) Largest number of organisms of a particular species that can be maintained indefinitely by a given environment. cartilage  Connective tissue in which the cells lie within lacunae embedded in a flexible, proteinaceous matrix. cartilaginous fishes  Vertebrates that belong to the class of fish called the Chondrichthyes: possess a cartilaginous, rather than bony, skeleton; include sharks, rays, and skates. cartilaginous joint  Form of joint that is connected by hyaline cartilage. Casparian strip  Layer of impermeable lignin and suberin bordering four sides of root endodermal cells; prevents water and solute transport between adjacent cells. catabolism  Metabolic process that breaks down large molecules into smaller ones; catabolic metabolism. cataracts  Condition where the lens of the eye becomes opaque, preventing the transmission of light to the retina. cecum  Region of the large intestine just below the small intestine to which the appendix is attached. cell  The smallest unit of life that displays all the properties of life; composed of cytoplasm surrounded by a plasma membrane. cell body  Portion of a neuron that contains a nucleus and from which dendrites and an axon extend. cell cycle  An ordered sequence of events in eukaryotes that involves cell growth and nuclear division; consists of the stages G1, S, G2, and M. cell plate  Structure across a dividing plant cell that signals the location of new plasma membranes and cell walls. cell recognition protein Glycoproteins in the plasma membrane that identify self and help the body defend itself against pathogens. cell theory  One of the major theories of biology, which states that all organisms are made up of cells; cells are capable of self-reproduction and come only from preexisting cells. cell wall Cellular structure that surrounds a plant, protistan, fungal, or bacterial cell and maintains the cell’s shape and rigidity; composed of polysaccharides. cell-mediated immunity Specific mechanism of defense in which T cells destroy antigen-bearing cells. cellular differentiation  Process and developmental stages by which a cell becomes specialized for a particular function. cellular respiration  Metabolic reactions that use the energy from carbohydrate, fatty acid, or amino acid breakdown to produce ATP molecules. cellular slime mold  Free-living amoeboid cells that feed on bacteria and yeasts by phagocytosis and aggregate to form a plasmodium that produces spores. cellulose  Polysaccharide that is the major complex carbohydrate in plant cell walls. central dogma  Term used to describe the process by which information flows from DNA to RNA to protein in a cell.

central nervous system (CNS)  Portion of the nervous system consisting of the brain and spinal cord. centriole  Cell structure, existing in pairs, that occurs in the centrosome and may help organize a mitotic spindle for chromosome movement during animal cell division. centromere  Constriction where sister chromatids of a chromosome are held together. centrosome  Central microtubule organizing center of cells. In animal cells, it contains two centrioles. cephalization  Having a well-recognized anterior head with a brain and sensory receptors. cephalopod  Type of mollusc in which the head is prominent and the foot is modified to form two arms and several tentacles; includes squids, cuttlefish, octopuses, and nautiluses. cerebellum  In terrestrial vertebrates, portion of the brain that coordinates skeletal muscles to produce smooth, graceful motions. cerebral cortex Outer layer of cerebral hemispheres; receives sensory information and controls motor activities. cerebral hemisphere  Either of the two lobes of the cerebrum in vertebrates. cerebrospinal fluid  Fluid found in the ventricles of the brain, in the central canal of the spinal cord, and in association with the meninges. cerebrum  Largest part of the brain in mammals. cervical cancer  Form of cancer that is often caused by the human papillomavirus (HPV); vaccinations are available to reduce risk. cervix  Narrow end of the uterus, which leads into the vagina. channel protein Protein that forms a channel to allow a particular molecule or ion to cross the plasma membrane. chemical energy  Energy associated with the interaction of atoms in a molecule. chemiosmosis  Process by which mitochondria and chloroplasts use the energy of an electron transport chain to create a hydrogen ion gradient that drives ATP formation. chemoautotroph  Organism able to synthesize organic molecules by using carbon dioxide as the carbon source and the oxidation of an inorganic substance (such as hydrogen sulfide) as the energy source. chemoreceptor  Sensory receptor that is sensitive to chemical stimulation—for example, receptors for taste and smell. chitin  Strong but flexible nitrogenous polysaccharide found in the exoskeleton of arthropods and in the cell walls of fungi. chlamydia  Sexually-transmitted disease caused by Chlamydia trachomatis; one of the more common forms of STDs. chlorophyll  Green photosynthetic pigment of algae and plants that absorbs solar energy; occurs as chlorophyll a and chlorophyll b. chloroplast  Membrane-bounded organelle in algae and plants with chlorophyll-containing membranous thylakoids; where photosynthesis takes place. choanoflagellate  Single-celled and colonial protists that are most closely related to the animals. cholesterol  A steroid found in the plasma membrane of animal cells and from which other types of steroids are derived.

chordate  Animals that have a dorsal tubular nerve cord, a notochord, pharyngeal gill pouches, and a postanal tail at some point in their life cycle; includes a few types of invertebrates (e.g., sea squirts and lancelets) and the vertebrates. chorion  Extraembryonic membrane functioning for respiratory exchange in birds and reptiles; contributes to placenta formation in mammals. choroid  Vascular, pigmented middle layer of the eyeball. chromalveolate  Supergroup of eukaryotes that includes alveolates and stramenopiles. chromatid  Following replication, a chromosome consists of a pair of sister chromatids, held together at the centromere; each chromatid is comprised of a single DNA helix. chromatin  Network of DNA strands and associated proteins observed within a nucleus of a cell. chromoplast  Plastid in land plants responsible for orange, yellow, and red color of plants, including the autumn colors in leaves. chromosomal mutations  Changes in the physical structure of a chromosome; includes deletions, duplications, inversions, and translocations. chromosome  The structure that transmits the genetic material from one generation to the next; composed of condensed chromatin; each species has a particular number of chromosomes that is passed on to the next generation. chronic bronchitis  Inflammation of the bronchi in the lungs that is usually caused by smoking or exposure to environmental contaminants. chyme  Thick, semiliquid food material that passes from the stomach to the small intestine. chytrid  Mostly aquatic fungi with flagellated spores that may represent the most ancestral fungal lineage. cilia  (sing., cilium) Short, hairlike projections from the plasma membrane, occurring usually in larger numbers. ciliary muscle  Within the ciliary body of the vertebrate eye, the ciliary muscle controls the shape of the lens. ciliate  Complex unicellular protist that moves by means of cilia and digests food in food vacuoles. circadian rhythm  Biological rhythm with a 24-hour cycle. circulatory system  In animals, an organ system that moves substances to and from cells, usually via a heart, blood, and blood vessels. circumcision  Surgical removal of the foreskin from the penis; female circumcision involves the partial or total removal of some part of the female genitalia. cirrhosis  Chronic, irreversible injury to liver tissue; commonly caused by frequent alcohol consumption. citric acid cycle  Cycle of reactions in mitochondria that begins with citric acid. This cycle breaks down an acetyl group and produces CO2, ATP, NADH, and FADH2; also called the Krebs cycle. cladistics  Method of systematics that uses derived characters to determine monophyletic groups and construct cladograms. cladogram  In cladistics, a branching diagram that shows the relationship among species in regard to their shared derived characters.

Glossary G-5

class  One of the categories, or taxa, used to group species; the taxon above the order level. classical conditioning  Type of learning whereby an unconditioned stimulus that elicits a specific response is paired with a neutral stimulus so that the response becomes conditioned. classification  Process of naming organisms and assigning them to taxonomic groups (taxa). cleavage  Cell division without cytoplasmic addition or enlargement; occurs during the first stage of animal development. cleavage furrow Indentation in the plasma membrane of animal cells during cell division; formation marks the start of cytokinesis. climate  Generalized weather patterns of an area, primarily determined by temperature and average rainfall. climate change  Recent changes in the Earth’s climate; evidence suggests that this is primarily due to human influence, including the increased release of greenhouse gases. climax community In ecology, community that results when succession has come to an end. clitoris  Organ of sexual arousal in a female. clonal selection  States that the antigen selects which lymphocyte will undergo clonal expansion and produce more lymphocytes bearing the same type of receptor. cloning  Production of identical copies. In organisms, the production of organisms with the same genes; in genetic engineering, the production of many identical copies of a gene. closed circulatory system A type of circulatory system where blood is confined to vessels and is kept separate from the interstitial fluid. clotting  Also called coagulation, the response of the body to an injury in the vessels of the circulatory system; involves platelets and clotting proteins. club fungi  Fungi that produce spores in club-shaped basidia within a fruiting body; includes mushrooms, shelf fungi, and puffballs. club moss Type of seedless vascular plant; also called ground pines. cnidarian  Invertebrates existing as either a polyp or medusa, with two tissue layers and radial symmetry. cnidocytes  Specialized stinging cells of a cnidarian; contain a toxin-filled capsule called a nematocyst. cochlea  Spiral-shaped structure of the vertebrate inner ear containing the sensory receptors for hearing. codominance  Inheritance pattern in which both alleles of a gene are equally expressed in a heterozygote. codon  Three-base sequence in messenger RNA that during translation directs the addition of a particular amino acid into a protein or directs termination of the process. coelom  Body cavity of an animal; the method by which the coelom is formed (or lack of formation) is an identifying characteristic in animal classification. coenzyme  Nonprotein organic molecule that aids the action of the enzyme to which it is loosely bound. coevolution  Mutual evolution in which two species exert selective pressures on the other species. cofactor  Nonprotein assistant required by an enzyme in order to function; many cofactors are metal ions, others are coenzymes.

cohesion  The ability of water molecules to cling to each other due to the process of hydrogen bonding. cohesion-tension model  Explanation for upward trans­ port of water in xylem based upon transpirationcreated tension and the cohesive properties of water molecules. cohort  Group of individuals having a statistical factor in common, such as year of birth, in a population study. coleoptile  Protective sheath that covers the young leaves of a seedling. collagen fiber  White fiber in the matrix of connective tissue giving flexibility and strength. collar cells Choanocytes of poriferans (sponges); inner layer of flagellated cells. collecting duct  Duct within the kidney that receives fluid from several nephrons; the reabsorption of water occurs here. collenchyma  Plant tissue composed of cells with unevenly thickened walls; supports growth of stems and petioles. color blindness Inability to detect specific wavelengths of light associated with color; red-green color blindness is the most common type. color vision Ability of the eye to detect specific wavelengths of light; involves specialized cone cells that detect blue, green, and red wavelengths. colostrum  Substance produced during the early days of lactation; rich in proteins and antibodies. columnar epithelium  Type of epithelial tissue with cylindrical cells. comb jelly Invertebrates that resemble jelly fishes and are the largest animals to be propelled by beating cilia. commensalism  Symbiotic relationship in which one species is benefited, and the other is neither harmed nor benefited. common ancestor  Ancestor common to at least two lines of descent. communication  Signal by a sender that influences the behavior of a receiver. community  Assemblage of species interacting with one another within the same environment. compact bone  Type of bone that contains osteons consisting of concentric layers of matrix and osteocytes in lacunae. companion cell  Plant cell in the vascular tissue of plants that metabolically supports the conducting cells of the phloem. comparative genomics  Study of genomes through the direct comparison of the genes and DNA sequences from multiple species. competition  Results when members of a species attempt to use a resource that is in limited supply. competitive exclusion principle  Theory that no two species can occupy the same niche in the same place and at the same time. complement  Collective name for a series of enzymes and activators in the blood, some of which may bind to antibody and may lead to rupture of a foreign cell. complementary base pairing Hydrogen bonding between particular purines and pyrimidines; responsible for the structure of DNA, and some RNA, molecules. compound  Substance having two or more different elements in a fixed ratio.

compound eye  Type of eye found in arthropods; it is composed of many independent visual units. concentration gradient  Gradual change in chemical concentration between two areas of differing concentrations. conclusion  Statement made following an experiment as to whether or not the results support the hypothesis. condensation  The process by which water vapor turns into liquid; at the cellular level, the removal of water between two molecules for the purpose of forming a chemical bond. cone cell Photoreceptor in vertebrate eyes that responds to bright light and makes color vision possible. conifer  Member of a group of cone-bearing gymnosperm land plants that includes pine, cedar, and spruce trees. coniferous forest  Forested area generally found in areas with temperate climate and characterized by cone-bearing trees that are mostly evergreens with needle-shaped or scalelike leaves. conjugation  Transfer of genetic material from one cell to another. conjunctiva  Delicate membrane that lines the eyelid, protecting the sclera. connective tissue  Type of animal tissue that binds structures together, provides support and protection, fills spaces, stores fat, and forms blood cells; adipose tissue, cartilage, bone, and blood are types of connective tissue; living cells in a nonliving matrix. conservation biology Discipline that seeks to understand the effects of human activities on species, communities, and ecosystems, and to develop practical approaches to preventing the extinction of species and the destruction of ecosystems. conservation corridors Strips of land that allow animals to move safely between habitats that would otherwise be isolated. conservation reserves  Areas that are set aside with the primary goal of protecting biodiversity within them. constipation  Changes in the frequency of bowel movements that leads to the formation of hard, dry fecal material. consumer  Organism that feeds on another organism in a food chain generally; primary consumers eat plants, and secondary consumers eat animals. continental drift  The movement of the Earth’s crust by plate tectonics resulting in the movement of continents with respect to one another. contraceptive  Medications and devices that reduce the chance of pregnancy. contraceptive implants  Synthetic forms of progesterone that prevent ovulation by disrupting the ovarian cycle. contraceptive injection  This form of contraception contains synthetic forms of progesterone (or a combination of progesterone and estrogen) that prevent ovulation by disrupting the ovarian cycle. contraceptive patch  Applied to the skin, this form of contraception contains synthetic forms of progesterone that prevent ovulation by disrupting the ovarian cycle. contraceptive vaccine Under development, this birth control method immunizes against the



G-6

Glossary

hormone HCG, crucial to maintaining implantation of the embryo. contractile vacuoles  Structure found in some protistan cells, usually freshwater species, that are involved in osmoregulation. control  Sample that goes through all the steps of an experiment but does not contain the variable being tested; a standard against which the results of an experiment are checked. convergent evolution  Similarity in structure in distantly related groups, generally due to similar selective pressures in like environments. coral reef Coral formations in shallow tropical waters that support an abundance of diversity. cork  Outer covering of the bark of trees; made of dead cells that may be sloughed off. cork cambium Lateral meristem that produces cork. cornea  Transparent, anterior portion of the outer layer of the eyeball. corolla  The petals, collectively; usually the conspicuously colored flower whorl. corpus luteum  Follicle that has released an egg and increases its secretion of progesterone. cortex  In plants, ground tissue bounded by the epidermis and vascular tissue in stems and roots; in animals, outer layer of an organ, such as the cortex of the kidney or adrenal gland. cortisol  Glucocorticoid secreted by the adrenal cortex that responds to stress on a long-term basis; reduces inflammation and promotes protein and fat metabolism. cotyledon  Seed leaf for embryo of a flowering plant; provides nutrient molecules for the developing plant before photosynthesis begins. coupled reactions  Reactions that occur simultaneously; one is an exergonic reaction that releases energy, and the other is an endergonic reaction that requires an input of energy in order to occur. covalent bond  Chemical bond in which atoms share one pair of electrons. cranial nerve  Nerve that arises from the brain. creatine phosphate  High energy compound found in muscular tissue; represents the fastest source of energy for muscle contraction. cristae  (sing., crista) Short, fingerlike projections formed by the folding of the inner membrane of mitochondria. Cro-Magnon  Common name for the first fossils to be designated Homo sapiens. crossing-over  Exchange of segments between nonsister chromatids of a bivalent during meiosis. crustacean  Member of a group of aquatic arthropods that contains, among others, shrimps, crabs, crayfish, and lobsters. cuboidal epithelium  Type of epithelial tissue with cube-shaped cells. cultural eutrophication Over-enrichment caused by human activities leading to excessive bacterial growth and oxygen depletion of a body of water. culture  Total pattern of human behavior; includes technology and the arts, and is dependent upon the capacity to speak and transmit knowledge. Cushing syndrome  Condition caused by the overproduction of ACTH, resulting in too much cortisol in the body.

cutaneous receptor  Sensory receptors of the dermis that are activated by touch, pain, pressure, and temperature. cuticle  In plants, a waxy layer covering the epidermis of plants that protects the plant against water loss and disease-causing organisms. In animals, an outer covering that protects and supports the organism. cyanobacterium  (pl., cyanobacteria) Photosynthetic bacterium that contains chlorophyll and releases oxygen; formerly called a blue-green alga. cycad  Type of gymnosperm with palmate leaves and massive cones; cycads are most often found in the tropics and subtropics. cyclic adenosine monophosphate (cAMP) ATPrelated compound that acts as the second messenger in peptide hormone transduction; it initiates activity of the metabolic machinery. cyclic electron pathway  Alternate form of the lightdependent reactions in photosynthesis that does not produce NADPH. cyclin  Protein that cycles in quantity as the cell cycle progresses; combines with and activates the kinases that function to promote the events of the cycle. cyst  In protists and invertebrates, resting structure that contains reproductive bodies or embryos. cystic fibrosis (CF) Genetic disease caused by a defect in the CFTR gene, which is responsible for the formation of a transmembrane chloride ion transporter; causes the mucus of the body to be viscous. cystitis  Inflammation of the bladder. cytokine  Type of protein secreted by a T lymphocyte that attacks viruses, virally infected cells, and cancer cells. cytokinesis  Division of the cytoplasm following mitosis or meiosis. cytokinin  Plant hormone that promotes cell division; often works in combination with auxin during organ development in plant embryos. cytoplasm  Region of a cell between the nucleus, or the nucleoid region of a bacterium, and the plasma membrane; contains the organelles of the cell. cytoplasmic segregation Process that parcels out the maternal determinants, which play a role in development, during mitosis. cytosine (C)  One of four nitrogen-containing bases in nucleotides composing the structure of DNA and RNA; pairs with guanine. cytoskeleton  Internal framework of the cell, consisting of microtubules, actin filaments, and intermediate filaments. cytotoxic T cell  T lymphocyte that attacks and kills antigen-bearing cells.

D

data  Facts or information collected through observation and/or experimentation. daughter chromosomes  Following anaphase in cell division, the structures produced by the separation of the sister chromatids. day-neutral plant Plant whose flowering is not dependent on day length—e.g., tomato and cucumber. deciduous  Land plant that sheds its leaves annually.

decomposer  Organism, usually a bacterium or fungus, that breaks down organic matter into inorganic nutrients that can be recycled in the environment. deductive reasoning  The use of general principles to predict specific outcomes. Often uses “if . . . then” statements. dehydration reaction  Chemical reaction in which a water molecule is released during the formation of a covalent bond. delayed allergic response Allergic response initiated at the site of the allergen by sensitized T cells, involving macrophages and regulated by cytokines. deletion  Change in chromosome structure in which the end of a chromosome breaks off or two simultaneous breaks lead to the loss of an internal segment; often causes abnormalities—e.g., cri du chat syndrome. demographic transition  Due to industrialization, a decline in the birthrate following a reduction in the death rate so that the population growth rate is lowered. denatured  Loss of a protein’s or enzyme’s normal shape so that it no longer functions; usually caused by a less than optimal pH and temperature. dendrite  Part of a neuron that sends signals toward the cell body. dendritic cell  Antigen-presenting cell of the epidermis and mucous membranes. Denisovans  A recently discovered member of the genus Homo that lived in Asia, and possibly Europe, around the same time as the Neandertals. denitrification  Conversion of nitrate or nitrite to nitrogen gas by bacteria in soil. dense fibrous connective tissue  Type of connective tissue containing many collagen fibers packed together; found in tendons and ligaments, for example. density-dependent factor  Biotic factor, such as disease or competition, that affects population size in a direct relationship to the population’s density. density-independent factor  Abiotic factor, such as fire or flood, that affects population size independent of the population’s density. dermis  In mammals, thick layer of the skin underlying the epidermis. desert  Ecological biome characterized by a limited amount of rainfall; deserts have hot days and cool nights. detrital food chain  Straight-line linking of organisms according to who eats whom, beginning with detritus. detrital food web  Complex pattern of interlocking and crisscrossing food chains, beginning with detritus. detritivore  Any organism that obtains most of its nutrients from the detritus in an ecosystem. detritus  Partially decomposed material; may be found in either soil or water. deuterostome  Group of coelomate animals in which the second embryonic opening is associated with the mouth; the first embryonic opening, the blastopore, is associated with the anus. development  Process of regulated growth and differentiation of cells and tissues.

Glossary G-7

diabetes insipidus  Condition caused by a lack of antidiuretic hormone (ADH); characterized by excessive thirst and over-production of urine. diabetes mellitus Condition caused by an insulin imbalance in the body; Type I diabetes results from not enough insulin being produced; Type II diabetes is caused by the body (specifically adipose tissue) not responding to insulin in the blood. diabetic retinopathy  Complication of diabetes that causes the capillaries in the retina to become damaged, potentially causing blindness. diaphragm  In mammals, dome-shaped muscularized sheet separating the thoracic cavity from the abdominal cavity; contraceptive device that prevents sperm from reaching the egg. diarrhea  Excessively frequent and watery bowel movements. diastole  Relaxation period of a heart chamber during the cardiac cycle. diatom  Golden-brown alga with a cell wall in two parts, or valves; significant component of phytoplankton. diffusion  Movement of molecules or ions from a region of higher to lower concentration; it requires no energy and tends to lead to an equal distribution (equilibrium). digestive system Organ system that consists of digestive organs (stomach, intestine, etc.) and accessory organs (liver, etc.); ingests and digests food; absorbs nutrients and eliminates wastes. dihybrid cross  Cross between parents that differ in two traits. dinoflagellate  Photosynthetic unicellular protist with two flagella, one whiplash and the other located within a groove between protective cellulose plates; significant part of phytoplankton. diploid (2n)  Cell condition in which two of each type of chromosome are present. directional selection  Outcome of natural selection in which an extreme phenotype is favored, usually in a changing environment. disaccharide  Sugar that contains two monosaccharide units; e.g., maltose. disease  Abnormality in the body’s physiology that typically upsets homeostasis. disruptive selection  Outcome of natural selection in which the two extreme phenotypes are favored over the average phenotype, leading to more than one distinct form. distal convoluted tubule  Final portion of a nephron that joins with a collecting duct; associated with tubular secretion. diuretic  Chemical that increases the flow of urine; an example is caffeine. DNA (deoxyribonucleic acid)  Nucleic acid polymer produced from covalent bonding of nucleotide monomers that contain the sugar deoxyribose; the genetic material of nearly all organisms. DNA fingerprint  Process that examines differences in DNA patterns to identify the source of a DNA sample; also called DNA profiling. DNA ligase Enzyme that links DNA fragments; used during production of recombinant DNA to join foreign DNA to vector DNA. DNA polymerase During replication, an enzyme that joins the nucleotides complementary to a DNA template.

DNA replication  Synthesis of a new DNA double helix prior to mitosis and meiosis in eukaryotic cells and during prokaryotic fission in prokaryotic cells. domain  Largest of the categories, or taxa, used to group species; the three domains are Archaea, Bacteria, and Eukarya. domain Archaea  One of the three domains of life; contains prokaryotic cells that often live in extreme habitats and have unique genetic, biochemical, and physiological characteristics; its members are sometimes referred to as archaea. domain Bacteria  One of the three domains of life; contains prokaryotic cells that differ from archaea because they have their own unique genetic, biochemical, and physiological characteristics. domain Eukarya  One of the three domains of life, consisting of organisms with eukaryotic cells; includes protists, fungi, plants, and animals. dominant allele Allele that exerts its phenotypic effect in the heterozygote; it masks the expression of the recessive allele. dormancy  In plants, a cessation of growth under conditions that seem appropriate for growth. dorsal cavity One of two main body cavities in humans; contains the cranial cavity and vertebral canal. dorsal root ganglion  Mass of sensory neuron cell bodies located in the dorsal root of a spinal nerve. double fertilization  In flowering plants, one sperm nucleus unites with the egg nucleus, and a second sperm nucleus unites with the polar nuclei of an embryo sac. double helix Double spiral; describes the threedimensional shape of DNA. doubling time  Number of years it takes for a population to double in size. dryopithecine  Tree dwelling primate existing 12–9 mya; ancestral to apes. Duchenne muscular dystrophy Genetic disorder that is characterized by a wasting of muscle tissue; displays an X-linked recessive pattern of inheritance. ductus arteriosus  Structure of the heart associated with fetal circulation; provides a pathway for returning any blood that enters the right ventricle back to the aorta. duodenum  First part of the small intestine, where chyme enters from the stomach. duplication  Change in chromosome structure in which a particular segment is present more than once in the same chromosome.

E

ecdysozoa  Protostome characterized by periodic molting of the exoskeleton. Includes the roundworms and arthropods. echinoderm  Invertebrates such as sea stars, sea urchins, and sand dollars; characterized by radial symmetry and a water vascular system. ecological niche  Role an organism plays in its community, including its habitat and its interactions with other organisms. ecological pyramid Visual depiction of the biomass, number of organisms, or energy content of various trophic levels in a food web—from the producer to the final consumer populations.

ecological succession  The gradual replacement of communities in an area following a disturbance (secondary succession) or the creation of new soil (primary succession). ecology  Study of the interactions of organisms with other organisms and with the physical and chemical environment. ecosystem  Biological community together with the associated abiotic environment; characterized by a flow of energy and a cycling of inorganic nutrients. ecosystem diversity  Variety of species in a particular locale, dependent on the species’ interactions. ectoderm  Outermost primary tissue layer of an animal embryo; gives rise to the nervous system and the outer layer of the integument. ectothermic  Body temperature that varies according to the environmental temperature. edge effect  Phenomenon in which the edges around a landscape patch provide a slightly different habitat than the favorable habitat in the interior of the patch. egg  Also called an ovum; haploid cell that is usually fertilized by a sperm to form a diploid zygote. elastic cartilage Type of cartilage composed of elastic fibers, allowing greater flexibility. elastic fiber  Yellow fiber in the matrix of connective tissue, providing flexibility. electrocardiogram (ECG)  Recording of the electrical activity associated with the heartbeat. electron  Negative subatomic particle, moving about in an energy level around the nucleus of an atom. electron shell  The average location, or energy level, of an electron in an atom. Often drawn as concentric circles around the nucleus. electron transport chain (ETC)  Process in a cell that involves the passage of electrons along a series of membrane-bound electron carrier molecules from a higher to lower energy level; the energy released is used for the synthesis of ATP. electronegativity  The ability of an atom to attract electrons toward itself in a chemical bond. elements  Substances that cannot be broken down into substances with different properties; composed of only one type atom. elephantiasis  Disease caused by a roundworm called the filarial worm. elongation  Middle stage of translation in which additional amino acids specified by the mRNA are added to the growing polypeptide. embryo  Stage of a multicellular organism that develops from a zygote before it becomes freeliving; in seed plants, the embryo is part of the seed. embryo sac Female gametophyte (megagametophyte) of flowering plants. embryonic development  In humans, the first two months of development following fertilization, during which the major organs are formed. embryonic disk  During human development, flattened area during gastrulation from which the embryo arises. emergency contraception Form of contraception that is taken following unprotected intercourse. emerging disease  Disease, such as SARS or MERS, that has not previously been detected in humans. emphysema  Disorder of the respiratory system, specifically the lungs, that is characterized by



G-8

Glossary

damage to the alveoli, thus reducing the ability to exchange gases with the external environment. endangered species A species that is in peril of immediate extinction throughout all or most of its range (e.g., California condor, snow leopard). endergonic reaction Chemical reaction that requires an input of energy; opposite of exergonic reaction. endocrine gland Ductless organ that secretes hormone(s) into the bloodstream. endocrine system Organ system involved in the coordination of body activities; uses hormones as chemical signals secreted into the bloodstream. endocytosis  Process by which substances are moved into the cell from the environment; includes phagocytosis, pinocytosis, and receptor-mediated endocytosis. endoderm  Innermost primary tissue layer of an animal embryo that gives rise to the linings of the digestive tract and associated structures. endodermis  Internal plant root tissue forming a boundary between the cortex and the vascular cylinder. endometrioisis  Condition in which endometrial tissue is located outside of the uterine cavity, causing pain and discomfort. endometrium  Mucous membrane lining the interior surface of the uterus. endoplasmic reticulum (ER)  System of membranous saccules and channels in the cytoplasm, often with attached ribosomes. endoskeleton  Protective internal skeleton, as in vertebrates. endosperm  In flowering plants, nutritive storage tissue that is derived from the union of a sperm nucleus and polar nuclei in the embryo sac. endospore  Spore formed within prokaryotic cell that protects the DNA within the cell from environmental stress. endosymbiosis  See also endosymbiotic theory. endosymbiotic theory  Explanation of the evolution of eukaryotic organelles by phagocytosis of prokaryotes. endothermic  Maintenance of a constant body temperature independent of the environmental temperature. energy  Capacity to do work and bring about change; occurs in a variety of forms. energy of activation  Energy that must be added in order for molecules to react with one another. entropy  Measure of disorder or randomness in a system. envelope  Lipid covering of some viruses; located outside of the capsid. environmental resistance Total of factors in the environment that limit the numerical increase of a population in a particular region. enzymatic protein  Protein that catalyzes a specific reaction; may be found in the plasma membrane or the cytoplasm of the cell. enzyme  Organic catalyst, usually a protein, that speeds a reaction in cells due to its particular shape. enzyme inhibition Means by which cells regulate enzyme activity; may be competitive or non-­ competitive inhibition. eosinophil  White blood cell containing cytoplasmic granules that stain with acidic dye.

epidermal tissue  Exterior tissue, usually one cell thick, of leaves, young stems, roots, and other parts of plants. epidermis  In mammals, the outer, protective layer of the skin; in plants, tissue that covers roots, leaves, and stems of nonwoody organisms. epididymis  Location of sperm maturation in an adult human male; located in the testis. epigenetic inheritance An inheritance pattern in which a nuclear gene has been modified but the changed expression of the gene is not permanent over many generations; the transmission of genetic information by means that are not based on the coding sequences of a gene. epiglottis  Structure that covers the glottis, the airtract opening, during the process of swallowing. epinephrine  Hormone secreted by the adrenal medulla in times of stress; also referred to as adrenaline. epiphyte  Plant that takes its nourishment from the air because its placement in other plants gives it an aerial position. episiotomy  Surgical procedure during the process of birth (parturition) that involves a small incision to enlarge the vagina; allows for passage of the baby’s head. epithelial tissue  Tissue that lines hollow organs and covers surfaces. erectile dysfunction (ED) Also called impotence, this represents an inability to produce or maintain an erection. erythrocyte  Red blood cell; contains hemoglobin and carries oxygen from the lungs or gills to the tissues in vertebrates. erythropoietin (EPO) Hormone produced by the kidneys that speeds red blood cell formation. esophagus  Muscular tube for moving swallowed food from the pharynx to the stomach. essential amino acid  One of eight amino acids that are unable to be produced by the human body and therefore must be obtained in the diet. essential fatty acids Forms of fatty acids that are unable to be produced by the human body and therefore must be obtained in the diet. estrogen  Female sex hormone that helps maintain sexual organs and secondary sex characteristics. estuary  Portion of the ocean located where a river enters and fresh water mixes with salt water. ethylene  Plant hormone that causes ripening of fruit and is also involved in abscission. euchromatin  Chromatin with a lower level of compaction and therefore accessible for transcription. eudicot (Eudicotyledone) Flowering plant group; members have two embryonic leaves (cotyledons), net-veined leaves, vascular bundles in a ring, flower parts in fours or fives and their multiples, and other characteristics. euglenid  Flagellated and flexible freshwater singlecelled protist that usually contains chloroplasts and has a semirigid cell wall. eukaryotic cell (eukaryote)  Type of cell that has a membrane-bound nucleus and membranous organelles; found in organisms within the domain Eukarya. eutrophication  Enrichment of water by inorganic nutrients used by phytoplankton. Often, overenrichment caused by human activities leads to

excessive bacterial growth and oxygen depletion. evaporation  The process by which liquid water turns into gaseous form (water vapor). evergreen  Land plant that sheds leaves over a long period, so some leaves are always present. evolution  Genetic change in a species over time, resulting in the development of genetic and phenotypic differences that are the basis of natural selection; descent of organisms from a common ancestor. ex vivo gene therapy  Gene therapy in which cells are removed from an organism, and DNA is injected to correct a genetic defect; the cells are returned to the organism to treat a disease or disorder. exchange pool  Location in a biogeochemical cycle from which organisms generally take a specific chemical (i.e., phosphorus). excretion  Elimination of metabolic wastes by an organism at exchange boundaries such as the plasma membrane of unicellular organisms and excretory tubules of multicellular animals. exergonic reaction  Chemical reaction that releases energy; opposite of endergonic reaction. exocrine gland  Gland that secretes its product to an epithelial surface directly or through ducts. exocytosis  Process in which an intracellular vesicle fuses with the plasma membrane so that the vesicle’s contents are released outside the cell. exon  Segment of mRNA containing the proteincoding portion of a gene that remains within the mRNA after splicing has occurred. experiment  A test of a hypothesis that examines the influence of a single variable. Often involves both control and test groups. experimental design Methodology by which an experiment will seek to support the hypothesis. experimental variable Factor of the experiment being tested. expiration  Act of expelling air from the lungs; exhalation. exponential growth  Growth, particularly of a population, in which the increase occurs in the same manner as compound interest. external respiration  Exchange of oxygen and carbon dioxide between alveoli of the lungs and blood. exteroceptor  Sensory receptors of the peripheral nervous system that detect stimuli from outside the body. extinct; extinction  Total disappearance of a species or higher group. extracellular matrix (ECM)  Nonliving substance secreted by some animal cells; is composed of protein and polysaccharides. extraembryonic membrane  Membrane that is not a part of the embryo but is necessary to the continued existence and health of the embryo. eyespot apparatus  Structure found in some species of protists that is used to sense light and light intensity.

F

facilitated transport  Passive transfer of a substance into or out of a cell along a concentration gradient by a process that requires a protein carrier.

Glossary G-9

FAD (flavin adenine dinucleotide)  A coenzyme of oxidation-reduction that becomes FADH2 as oxidation of substrates occurs in the mitochondria during cellular respiration, FAD then delivers electrons to the electron transport chain. fall overturn  Mixing process that occurs in fall in stratified lakes, whereby oxygen-rich top waters mix with nutrient-rich bottom waters. familial hypercholesterolemia Genetic disorder that causes an accumulation of cholesterol in the blood due to defects in the LDL-receptors on the cell surface. family  One of the categories, or taxa, used to group species; the taxon located above the genus level. farsighted  Condition in which an individual cannot focus on objects closely; caused by the focusing of the image behind the retina of the eye. fat  Organic molecule that contains glycerol and three fatty acids; energy storage molecule. fate maps  Diagram used by developmental geneticists that tracks the differentiation of a cell in an embryo. fatty acid Molecule that contains a hydrocarbon chain and ends with an acid group. female condom  Contraceptive device, usually made from polyurethane, that fits onto the female’s cervix. fermentation  Anaerobic breakdown of glucose that results in a gain of two ATP and end products such as alcohol and lactate; occurs in the cytoplasm of cells. fern  Member of a group of land plants that have large fronds; in the sexual life cycle, the independent gametophyte produces flagellated sperm, and the vascular sporophyte produces windblown spores. fertilization  Fusion of sperm and egg nuclei, producing a zygote that develops into a new individual. fetal development Period in human development from three to nine months after conception that involves growth of the fetus and development of the internal organs. fiber  General term for indigestible plant material, typically complex carbohydrates such as cellulose. fibrin  Protein involved in blood clotting; acts to trap cells to seal wounds in the blood vessels. fibroblast  Cell found in loose connective tissue that synthesizes collagen and elastic fibers in the matrix. fibrocartilage  Cartilage with a matrix of strong collagenous fibers. fibromyalgia  Disorder associated with chronic, widespread pain; cause is not currently known. fibrous joint  Type of joint between two bones that is immovable; sutures are examples of fibrous joints. fibrous root system  In most monocots, a mass of similarly sized roots that cling to the soil. filament  End-to-end chains of cells that form as cell division occurs in only one plane; in plants, the elongated stalk of a stamen. fimbria  (pl., fimbriae) Small, bristlelike fiber on the surface of a bacterial cell, which attaches bacteria to a surface; also fingerlike extension from the oviduct near the ovary. first messenger  Chemical signal, such as a peptide hormone, that binds to a plasma membrane receptor protein and alters the metabolism of a cell because a second messenger is activated.

fish  Aquatic, gill-breathing vertebrate that usually has fins and skin covered with scales; fishes were among the earliest vertebrates that evolved. fitness  Ability of an organism to reproduce and pass its genes to the next fertile generation; measured against the ability of other organisms to reproduce in the same environment. fixed action pattern (FAP)  Innate behavior pattern that is stereotyped, spontaneous, independent of immediate control, genetically encoded, and independent of individual learning. flagellates  Heterotrophic protozoans that propel themselves using one or more flagella. flagellum  (pl., flagella) Long, slender extension used for locomotion by some bacteria, protozoans, and sperm. flagship species  Species that evoke a strong emotional response in humans; charismatic, cute, regal (e.g., lions, tigers, dolphins, pandas). flatworms  Invertebrates such as planarians and tapeworms with extremely thin bodies, a three-branched gastrovascular cavity, and a ladder type nervous system. flower  Reproductive organ of a flowering plant, consisting of several kinds of modified leaves arranged in concentric rings and attached to a modified stem called the receptacle. fluid-mosaic model Model for the plasma membrane based on the changing location and pattern of protein molecules in a fluid phospholipid bilayer. follicle  Structure in the ovary of animals that contains an oocyte; site of oocyte production. follicle-stimulating hormone (FSH) Hormone released by the anterior pituitary; in males it ­promotes the production of sperm; in females it promotes the development of the follicle in the ovary. follicular phase  First half of the ovarian cycle, during which the follicle matures and much estrogen (and some progesterone) is produced. fontanels  Membranous regions found in the skulls of infants; permit the skull to move through the birth canal. food chain  The order in which one population feeds on another in an ecosystem, thereby showing the flow of energy from a detritivore (detrital food chain) or a producer (grazing food chain) to the final consumer. food web  In ecosystems, a complex pattern of interlocking and crisscrossing food chains. foot  Animal structure involved in locomotion. foramen magnum  Large opening at the base of the skull that allows for a connection between the spinal cord and brain stem. foramen ovale  Structural feature of the fetal heart that allows for blood to move directly between the right and left atrium. foraminiferan  A protist bearing a calcium carbonate test with many openings through which pseudopods extend. formed elements  Portion of the blood that consists of erythrocytes, leukocytes, and platelets (thrombocytes). formula  A group of symbols and numbers used to express the composition of a compound. fossil  Any past evidence of an organism that has been preserved in the Earth’s crust.

fossil fuel Remains of once-living organisms that are burned to release energy, such as coal, oil, and natural gas. founder effect  Cause of genetic drift due to colonization by a limited number of individuals who, by chance, have different genotype and allele frequencies than the parent population. fovea centralis  Region of the retina consisting of densely packed cones; responsible for the greatest visual acuity. fragile X syndrome  Genetic condition caused by an abnormal number of nucleotide repeats; named after the appearance, and not physical characteristics, of the X chromosome. frameshift mutation  Insertion or deletion of at least one base so that the reading frame of the corresponding mRNA changes. free energy  Energy in a system that is capable of performing work. fruit  Flowering plant structure consisting of one or more ripened ovaries that usually contain seeds. functional genomics  Study of gene function at the genome level. It involves the study of many genes simultaneously and the use of DNA microarrays. functional group  Specific cluster of atoms attached to the carbon skeleton of organic molecules that enters into reactions and behaves in a predictable way. fungi  (sing., fungus) Eukaryotic saprotrophic decomposer; the body is made up of filaments called hyphae that form a mass called a mycelium.

G

gallbladder  Organ attached to the liver that serves to store and concentrate bile. gallstones  Hardened form of bile that may block the bile ducts and cause pancreatitis. gamete  Haploid sex cell; e.g., egg or sperm. gametogenesis  Development of the male and female sex gametes. gametophyte  Haploid generation of the alternationof-generations life cycle of a plant; produces gametes that unite to form a diploid zygote. ganglia  (sing., ganglion) Collection or bundle of neuron cell bodies usually outside the central nervous system. gap junction  Junction between cells formed by the joining of two adjacent plasma membranes; it lends strength and allows ions, sugars, and small molecules to pass between cells. gastropod  Mollusc with a broad, flat foot for crawling (e.g., snails and slugs). gastrovascular cavity  Blind digestive cavity in animals that have a sac body plan. gastrula  Stage of animal development during which the germ layers form, at least in part, by invagination. gastrulation  Formation of a gastrula from a blastula; characterized by an invagination to form cell layers of a caplike structure. gene  Unit of heredity existing as alleles on the chromosomes; in diploid organisms, typically two alleles are inherited—one from each parent. gene cloning  DNA cloning to produce many identical copies of the same gene. gene flow Sharing of genes between two populations through interbreeding.



G-10

Glossary

gene mutation Altered gene whose sequence of bases differs from the original sequence. gene pool  Total of the alleles of all the individuals in a population. gene therapy  Correction of a detrimental mutation by the insertion of DNA sequences into the genome of a cell. genetic code Universal code that has existed for eons and allows for conversion of DNA and RNA’s chemical code to a sequence of amino acids in a protein. Each codon consists of three bases that stand for one of the 20 amino acids found in proteins or directs the termination of translation. genetic diversity Variety among members of a population. genetic drift  Mechanism of evolution due to random changes in the allelic frequencies of a population; more likely to occur in small populations or when only a few individuals of a large population reproduce. genetic engineering  Human-induced changes in the genome of an organism, often performed for the benefit of humans. genetic equilibrium  Description for a population in which the frequency of alleles for a given trait is not changing over time. genetic profile  An individual’s genome, including any possible mutations. genetic recombination  Process in which chromosomes are broken and rejoined to form novel combinations; in this way offspring receive alleles in combinations different from their parents. genetic variation Differences in the sequences of nucleotides in the genes of an individual. Introduced during the process of meiosis, or by mutation. genetically modified organisms (GMOs) Organisms whose genetic material has been altered or enhanced using DNA technology. genital herpes  Sexually-transmitted disease caused by the herpes simplex virus (both type 1 and type 2). genital warts  Sexually–transmitted disease caused by the human papillomavirus (HPV). genome  Sum of all of the genetic information in a cell or organism. genomics  Area of study that examines the genome of a species or group of species. genotype  Genes of an organism for a particular trait or traits; often designated by letters—for example, BB or Aa. genus  One of the categories, or taxa, used to group species; contains those species that are most closely related through evolution. geologic timescale  History of the Earth based on the fossil record and divided into eras, periods, and epochs. germ cell  During zygote development, cells that are set aside from the somatic cells and that will eventually undergo meiosis to produce gametes. germ layer Primary tissue layer of a vertebrate embryo—namely, ectoderm, mesoderm, or endoderm. germinate  Beginning of growth of a seed, spore, or zygote, especially after a period of dormancy. gerontology  Study of aging and the aging processes.

gibberellin  Plant hormone promoting increased stem growth; also involved in flowering and seed germination. gigantism  Condition caused by the overproduction of growth hormone during childhood; individuals often also have diabetes mellitus and other health problems. gills  Respiratory organ in most aquatic animals; in fish, an outward extension of the pharynx. ginkgo  Member of phylum Ginkgophyta; maidenhair tree. girdling  Removing a strip of bark from around a tree. gland  Epithelial cell or group of epithelial cells that are specialized to secrete a substance. glaucoma  Condition in which the fluid in the eye (aqueous humor) accumulates, increasing pressure in the eye and damaging nerve fibers. global warming  Predicted increase in the Earth’s temperature due to human activities that promote the greenhouse effect. glomerular capsule Cuplike structure that is the initial portion of a nephron. glomerular filtration Movement of small molecules from the glomerulus into the glomerular capsule due to the action of blood pressure. glomerulus  Capillary network within the glomerular capsule of a nephron. glottis  Opening for airflow in the larynx. glucagon  Hormone, produced by the pancreas, that stimulates the liver to break down glycogen, thus raising blood glucose levels. glucocorticoid  Type of hormone secreted by the adrenal cortex that influences carbohydrate, fat, and protein metabolism; see also cortisol. glucose  Six-carbon monosaccharide; used as an energy source during cellular respiration and as a monomer of the structural polysaccharides. glycemic index  Nutritional measurement of how the body responds to the glucose level of a food. Foods with a high glycemic index result in the release of high amounts of insulin. glycogen  Storage polysaccharide found in animals; composed of glucose molecules joined in a linear fashion but having numerous branches. glycolipid  Lipid in plasma membranes that contains an attached carbohydrate chain; assembled in the Golgi apparatus. glycolysis  Anaerobic breakdown of glucose that results in a gain of two ATP and the production of pyruvate; occurs in the cytoplasm of cells. glycoprotein  Protein in plasma membranes that has an attached carbohydrate chain; assembled in the Golgi apparatus. gnetophyte  Member of one of the four phyla of gymnosperms; Gnetophyta has only three living genera, which differ greatly from one another— e.g., Welwitschia and Ephedra. Golgi apparatus  Organelle consisting of sacs and vesicles that processes, packages, and distributes molecules about or from the cell. gonad  Organ that produces gametes; the ovary produces eggs, and the testis produces sperm. gonadotropic hormone  Substance, secreted by the anterior pituitary, that regulates the activity of the ovaries and testes; principally, follicle-­ stimulating hormone (FSH) and luteinizing hormone (LH).

gonadotropin-releasing hormone (GnRH) Hormone released by the hypothalamus that influences the secretion of hormones by the anterior pituitary, thus regulating the activity of the testes. gonorrhea  Sexually-transmitted disease caused by the bacterium Neisseria gonorrhoeae. gout  Condition caused by the inability of the urinary system to remove uric acid from the body. gradualistic model  Model of genetic change in a species which suggests that change occurs at a slow, steady pace over time. Gram stain Method of determining whether a culture of bacteria possess a layer of exposed peptidoglycan outside of their plasma membrane; historically used to classify bacteria as Gram-positive or Gram-negative, now used to determine the most effective antibiotic treatment. granum  (pl., grana) Stack of chlorophyll-­ containing thylakoids in a chloroplast. grassland  Biome characterized by rainfall greater than 25 cm/yr, grazing animals, and warm summers; includes the prairie of the U.S. midwest and the African savanna. Graves disease Disease associated with the overproduction of T3 and T4 by the thyroid; caused by antibodies that interact with TSH receptors in the thyroid. gravitational equilibrium  Maintenance of balance when the head and body are motionless. gravitropism  Growth response of roots and stems of plants to the Earth’s gravity; roots demonstrate positive gravitropism, and stems demonstrate negative gravitropism. gray crescent  Region that forms during early cellular differentiation of the embryo; believed to contain unique chemical signals that are important during development. gray matter  Nonmyelinated axons and cell bodies in the central nervous system. grazing food chain  A flow of energy to a straightline linking of organisms according to who eats whom. grazing food web  Complex pattern of interlocking and crisscrossing food chains that begins with populations of autotrophs serving as producers. green algae  Members of a diverse group of photosynthetic protists; contain chlorophylls a and b and have other biochemical characteristics like those of plants. greenhouse effect  Reradiation of solar heat toward the Earth, caused by an atmosphere that allows the sun’s rays to pass through, but traps the heat in the same manner as the glass of a greenhouse. greenhouse gases  Gases in the atmosphere such as carbon dioxide, methane, water vapor, ozone, and nitrous oxide that are involved in the greenhouse effect. ground tissue Tissue that constitutes most of the body of a plant; consists of parenchyma, collenchyma, and sclerenchyma cells that function in storage, basic metabolism, and support. growth factor  A hormone or chemical, secreted by one cell, that may stimulate or inhibit growth of another cell or cells. growth hormone (GH)  Substance secreted by the anterior pituitary; controls size of an individual

Glossary G-11

by promoting cell division, protein synthesis, and bone growth. growth plate  Area of cartilage within a long bone that permits the bone to increase in length. guanine (G)  One of four nitrogen-containing bases in nucleotides composing the structure of DNA and RNA; pairs with cytosine. guard cell One of two cells that surround a leaf stoma; changes in the turgor pressure of these cells cause the stoma to open or close. Guillain-Barré syndrome Disease caused by inflammation in the nervous system, resulting in demyelination of the nerve axons in the peripheral nervous system. gymnosperm  Type of woody seed plant in which the seeds are not enclosed by fruit and are usually borne in cones, such as those of the conifers.

H

habitat  Place where an organism lives and is able to survive and reproduce. hair cells  Cells located in the inner ear that function as mechanoreceptors for hearing. hair follicle Tubelike depression in the skin in which a hair develops. halophile  Type of archaean that lives in extremely salty habitats. haploid (n)  Cell condition in which only one of each type of chromosome is present. Hardy-Weinberg equilibrium Mathematical law stating that the gene frequencies in a population remain stable if evolution does not occur due to nonrandom mating, selection, migration, and genetic drift. heart  Muscular organ whose contraction causes blood to circulate in the body of an animal. heart attack Damage to the myocardium due to blocked circulation in the coronary arteries; myocardial infarction. heat  Type of kinetic energy associated with the random motion of molecules. helper T cell  Secretes lymphokines, which stimulate all kinds of immune cells. hemocoel  Residual coelom found in arthropods, which is filled with hemolymph. hemodialysis  Treatment for kidney failure in which the patient’s blood is passed through an artificial kidney to remove waste material and toxins. hemoglobin  Iron-containing respiratory pigment occurring in vertebrate red blood cells and in the blood plasma of some invertebrates. hemophilia  Genetic disorder that is caused by a deficiency of a clotting factor in the blood. hepatitis  Inflammation of the liver. Viral hepatitis occurs in several forms. herbivore  Primary consumer in a grazing food chain; a plant eater. hermaphroditic  Type of animal that has both male and female sex organs. heterochromatin  Highly compacted chromatin that is not accessible for transcription. heterotroph  Organism that cannot synthesize needed organic compounds from inorganic substances and therefore must take in organic food. heterozygote advantage  Situation in which individuals heterozygous for a trait have a selective

advantage over those who are homozygous dominant or recessive; an example is sickle cell disease. heterozygous  Possessing unlike alleles for a particular trait. hexose  Any monosaccharide that contains six carbons; examples are glucose and galactose. hippocampus  Region of the central nervous system associated with learning and memory; part of the limbic system. histamine  Substance, produced by basophils in blood and mast cells in connective tissue, that causes capillaries to dilate. histone  A group of proteins involved in forming the nucleosome structure of eukaryote chromatin. homeodomain  Conserved DNA-binding region of transcription factors encoded by the homeobox of homeotic genes. homeostasis  Maintenance of normal internal conditions in a cell or an organism by means of selfregulating mechanisms. homeotic genes  Genes that control the overall body plan by controlling the fate of groups of cells during development. hominid  Taxon that includes humans, chimpanzees, gorillas, and orangutans. hominin  Taxon that includes humans and species very closely related to humans plus chimpanzees. hominine  Taxon that includes humans, chimpanzees, and gorillas. hominoid  Taxon that includes the hominids plus the gibbons. homologous chromosome Member of a pair of chromosomes that are alike and come together in synapsis during prophase of the first meiotic division; a homologue. homologous gene  Gene that codes for the same protein, even if the base sequence may be different. homologous structure  A structure that is similar in different types of organisms because these organisms descended from a common ancestor. homologue  Member of a homologous pair of chromosomes. homology  Similarity of parts or organs of different organisms caused by evolutionary derivation from a corresponding part or organ in a remote ancestor, and usually having a similar embryonic origin. homozygous  Possessing two identical alleles for a particular trait. hormone  Chemical messenger, produced in one part of the body, that controls the activity of other parts. horsetail  A seedless vascular plant having only one genus (Equisetum) in existence today; characterized by rhizomes, scalelike leaves, strobili, and tough, rigid stems. host  Organism that provides nourishment and/or shelter for a parasite. human chorionic gonadotropin (HCG)  Gonadotropic hormone produced by the chorion that functions to maintain the uterine lining. Human Genome Project (HGP)  Initiative to determine the complete sequence of the human genome and to analyze this information. human immunodeficiency virus (HIV)  A retrovirus that is responsible for the disease AIDS.

hunter-gatherer  A hominin that hunted animals and gathered plants for food. Huntington disease Autosomal dominant genetic disorder that affects the nervous system; results in a progressive loss of neurons in the brain. hyaline cartilage  Cartilage whose cells lie in lacunae separated by a white translucent matrix containing very fine collagen fibers. hybridization  Interbreeding between two different species; typically prevented by prezygotic isolation mechanisms. hydrogen bond  Weak bond that arises between a slightly positive hydrogen atom of one molecule and a slightly negative atom of another molecule, or between parts of the same molecule. hydrolysis reaction  Splitting of a chemical bond by the addition of water, with the H+ going to one molecule and the OH− going to the other. hydrophilic  Type of molecule, often polar, that interacts with water by dissolving in water and/or by forming hydrogen bonds with water molecules. hydrophobic  Type of molecule that is typically nonpolar  and therefore does not interact easily with water. hydrostatic skeleton Fluid-filled body compartment that provides support for muscle contraction resulting in movement; seen in cnidarians, flatworms, roundworms, and segmented worms. hydrothermal vent Hot springs in the seafloor along ocean ridges where heated sea water and sulfate react to produce hydrogen sulfide; here, chemosynthetic bacteria support a community of varied organisms. hypertension  Form of cardiovascular disease characterized by blood pressure over 140/95 (over 45 years of age) or 130/90 (under 45 years of age). hyperthyroidism  Caused by the over-secretion of hormones from the thyroid gland; symptoms include hyperactivity, nervousness, and insomnia. hypertonic solution Higher solute concentration (less water) than the cytoplasm of a cell; causes cell to lose water by osmosis. hypha  (pl. hyphae). Filament of the vegetative body of a fungus. hypothalamic-inhibiting hormone One of many hormones produced by the hypothalamus, that inhibits the secretion of an anterior pituitary hormone. hypothalamic-releasing hormone One of many hormones, produced by the hypothalamus, that stimulates the secretion of an anterior pituitary hormone. hypothalamus  In vertebrates, part of the brain that helps regulate the internal environment of the body—for example, heart rate, body temperature, and water balance. hypothesis  Supposition established by reasoning after consideration of available evidence; it can be tested by obtaining more data, often by experimentation. hypothyroidism  Caused by the under-secretion of hormones from the thyroid gland; symptoms include weight gain, lethargic behavior, and depression. hypotonic solution  Solution that contains a lower solute (more water) concentration than the cytoplasm of a cell; causes cell to gain water by osmosis.



G-12

I

Glossary

ileum  Region of the small intestine; connects the jejunum and large intestine. immediate allergic response  Allergic response that occurs within seconds of contact with an allergen; caused by the attachment of the allergen to IgE antibodies. immune system  System associated with protection against pathogens, toxins, and some cancerous cells; in humans this is an organ system. immunity  Ability of the body to protect itself from foreign substances and cells, including diseasecausing agents. immunization  Strategy for achieving immunity to the effects of specific disease-causing agents. immunodeficiency disease Condition that is the result of the immune system’s inability to protect the body. immunoglobulin (Ig)  Globular plasma protein that functions as an antibody. implantation  In placental mammals, the embedding of an embryo at the blastocyst stage into the endometrium of the uterus. imprinting  Learning to make a particular response to only one type of animal or object. inbreeding  Mating between closely related individuals; influences the genotype ratios of the gene pool. inclusive fitness  Fitness that results from personal reproduction and from helping nondescendant relatives reproduce. incomplete dominance  Inheritance pattern in which an offspring has an intermediate phenotype, as when a red-flowered plant and a white-flowered plant produce pink-flowered offspring. incomplete penetrance Dominant alleles that are either not always, or partially expressed. incus  Bone found in the middle ear that assists in the transmission of sound to the inner ear; also called the anvil. independent assortment  Alleles of unlinked genes segregate independently of each other during meiosis so that the gametes can contain all possible combinations of alleles. induced fit model Change in the shape of an enzyme’s active site that enhances the fit between the active site and its substrate(s). induction  Ability of a chemical or a tissue to influence the development of another tissue. inductive reasoning Using specific observations and the process of logic and reasoning to arrive at general scientific principles. infertility  Failure of a couple to achieve pregnancy after one year of regular, unprotected intercourse inflammatory response  Tissue response to injury that is characterized by redness, swelling, pain, and heat. inheritance of acquired characteristics  Lamarckian belief that characteristics acquired during the lifetime of an organism can be passed on to offspring. initiation  First stage of translation in which the translational machinery binds an mRNA and assembles. innate immunity An immune response that does not require a previous exposure to the pathogen.

inner ear  Portion of the ear consisting of a vestibule, semicircular canals, and the cochlea where equilibrium is maintained and sound is transmitted. insect  Type of arthropod. The head has antennae, compound eyes, and simple eyes; the thorax has three pairs of legs and often wings; and the abdomen has internal organs. insertion  When referring to a muscle, the connection of the muscle to the bone that moves. inspiration  Act of taking air into the lungs; inhalation. insulin  Hormone, released by the pancreas, that serves to lower blood glucose levels by stimulating the uptake of glucose by cells, especially muscle and liver cells. integration  Summing up of excitatory and inhibitory signals by a neuron or by some part of the brain. integumentary system Organ system of humans that contains the skin, protects the body, synthesizes sensory input, assists in temperature regulation, and synthesizes vitamin D. interferon  Antiviral agent produced by an infected cell that blocks the infection of another cell. interkinesis  Period of time between meiosis I and meiosis II during which no DNA replication takes place. intermediate filament Ropelike assemblies of fibrous polypeptides in the cytoskeleton that provide support and strength to cells; so called because they are intermediate in size between actin filaments and microtubules. internal respiration  Exchange of oxygen and carbon dioxide between blood and tissue fluid. interneuron  Neuron located within the central nervous system that conveys messages between parts of the central nervous system. internode  In vascular plants, the region of a stem between two successive nodes. interoceptor  Sensory receptors of the peripheral nervous system that detect stimuli from inside the body. interphase  Stages of the cell cycle (G1, S, G2) during which growth and DNA synthesis occur when the nucleus is not actively dividing. interstitial cells  Cells of the male reproductive system that secrete the androgens (such as testosterone). interstitial fluid Fluid that surrounds the body’s cells; consists of dissolved substances that leave the blood capillaries by filtration and diffusion. intervertebral disks  Fibrocartilage located between the vertebrae that act as shock absorbers. intrauterine device (IUD) Contraceptive device that is inserted into the female’s uterus; believed to work by altering the environment of the uterus. intron  Intervening sequence found between exons in mRNA; removed by RNA processing before translation. inversion  Change in chromosome structure in which a segment of a chromosome is turned around 180°; this reversed sequence of genes can lead to altered gene activity and abnormalities. invertebrate  Animal without a vertebral column or backbone. ion  Charged particle that carries a negative or positive charge.

ionic bond Chemical bond in which ions are attracted to one another by opposite charges. iris  Muscular ring that surrounds the pupil and regulates the passage of light through this opening. isomer  Molecules with the same molecular formula but a different structure, and therefore a different shape. isotonic solution  Solution that is equal in solute concentration to that of the cytoplasm of a cell; causes cell to neither lose nor gain water by osmosis. isotope  Atoms of the same element having the same atomic number but a different mass number due to a variation in the number of neutrons.

J

jaundice  Yellowish tint to the skin caused by an abnormal amount of bilirubin (bile pigment) in the blood, indicating liver malfunction. jawless fishes  Type of fishes that lack jaws (agnathan); includes hagfishes and lampreys. jejunum  Region of the small intestine located between the duodenum and ileum. joint  Articulation between two bones of a skeleton. junction protein(s)  Proteins in the cell membrane that assist in cell-to-cell communication.

K

K-selection  Favorable life-history strategy under stable environmental conditions characterized by the production of a few offspring with much attention given to offspring survival. karyotype  Chromosomes arranged by pairs according to their size, shape, and general appearance in mitotic metaphase. keystone species  Species whose activities significantly affect community structure. kidney stones  Hardened granules that may form in the renal pelvis of the kidney; caused by factors such as diet, infections, or pH imbalance. kidneys  Paired organs of the vertebrate urinary system that regulate the chemical composition of the blood and produce a waste product called urine. kin selection Indirect selection; adaptation to the environment due to the reproductive success of an individual’s relatives. kinetic energy  Energy associated with motion. kinetochore  An assembly of proteins that attaches to the centromere of a chromosome during mitosis. kingdom  One of the categories, or taxa, used to group species; the taxon above phylum.

L

lactation  Secretion of milk by mammary glands, usually for the nourishment of an infant. lacteal  Lymphatic vessel in an intestinal villus; aids in the absorption of fats. lacuna  Small pit or hollow cavity, as in bone or cartilage, where a cell or cells are located. lake  Body of fresh water, often classified by nutrient status, such as oligotrophic (nutrient-poor) or eutrophic (nutrient-rich). lancelet  Invertebrate chordate with a body that resembles a lancet and has the four chordate characteristics as an adult.

Glossary G-13

landscape diversity Variety of habitat elements within an ecosystem (e.g., plains, mountains, and rivers). lanugo  Fine, downlike covering of the fetus. large intestine  In vertebrates, portion of the digestive tract that follows the small intestine; in humans, consists of the cecum, colon, rectum, and anal canal. laryngitis  Inflammation of the larynx, usually resulting in an inability or difficulty in speaking. larynx  Cartilaginous organ located between the pharynx and the trachea; in humans, contains the vocal cords; sometimes called the voice box. last universal common ancestor (LUCA)  The first living organism on the planet, from which all life evolved. law  Universal principle that describes the basic functions of the natural world. law of independent assortment  Mendelian principle that explains how combinations of traits appear in gametes; see also independent assortment. law of segregation Mendelian principle that explains how, in a diploid organism, alleles separate during the formation of the gametes. laws of thermodynamics Two laws explaining energy and its relationships and exchanges. The first, also called the “law of conservation,” says that energy cannot be created or destroyed but can only be changed from one form to another; the second says that energy cannot be changed from one form to another without a loss of usable energy. leaf  Lateral appendage of a stem, highly variable in structure, often containing cells that carry out photosynthesis. leaf vein  Vascular tissue within a leaf. learning  Relatively permanent change in an animal’s behavior that results from practice and experience. leech  Blood-sucking annelid, usually found in fresh water, with a sucker at each end of a segmented body. lens  Transparent, “disk” structure found in the vertebrate eye behind the iris; brings objects into focus on the retina. leptin  Hormone produced by adipose tissue that acts on the hypothalamus to signal satiety (fullness). less-developed country (LDC) Country that is becoming industrialized; typically, population growth is expanding rapidly, and the majority of people live in poverty. leukocyte  White blood cell, of which there are several types, each having a specific function in protecting the body from invasion by foreign substances and organisms. lichen  Symbiotic relationship between certain fungi and either cyanobacteria or algae, in which the fungi possibly provide inorganic food or water and the algae or cyanobacteria provide organic food. ligament  Tough cord or band of dense fibrous tissue that binds bone to bone at a joint. light reaction  Portion of photosynthesis that captures solar energy and takes place in thylakoid membranes of chloroplasts; it produces ATP and NADPH. lignin  Chemical that hardens the cell walls of land plants.

limbic system  In humans, functional association of various brain centers, including the amygdala and hippocampus; governs learning and memory and various emotions such as pleasure, fear, and happiness. limiting factor  Resource or environmental condition that restricts the abundance and distribution of an organism. lineage  Line of descent represented by a branch in a phylogenetic tree. linkage group Term used to identify groups of alleles on a chromosome that tend to be inherited together. lipase  Fat-digesting enzyme secreted by the pancreas. lipid  Class of organic compounds that tends to be soluble in nonpolar solvents; includes fats and oils. lipopolysaccharide  Lipid-sugar molecules that are found on the exterior of some species of bacteria; presence indicates that the species is Gram-negative. liposome  Droplet of phospholipid molecules formed in a liquid environment. littoral zone Shore zone between high-tide mark and low-tide mark; also, shallow water of a lake where light penetrates to the bottom. liver  Large, dark red internal organ that produces urea and bile, detoxifies the blood, stores glycogen, and produces the plasma proteins, among other functions. liverwort  Bryophyte with a lobed or leafy gametophyte and a sporophyte composed of a stalk and capsule. lobe-finned fishes  Type of fishes with limblike fins; also called the sarcopterygians. locus  Physical location of a trait (or gene) on a chromosome. logistic growth Population increase that results in an S-shaped curve; growth is slow at first, steepens, and then levels off due to environmental resistance. long-day plant  Plant that flowers when day length is longer than a critical length; e.g., wheat, barley, clover, and spinach. loop of the nephron  Portion of a nephron between the proximal and distal convoluted tubules; functions in water reabsorption. loose fibrous connective tissue  Tissue composed mainly of fibroblasts widely separated by a matrix containing collagen and elastic fibers. lophophoran  A general term to describe several groups of lophotrochozoans that have a feeding structure called a lophophore. lophotrochozoa  Main group of protostomes; widely diverse. Includes the flatworms, rotifers, annelids, and molluscs. lumen  Cavity inside any tubular structure, such as the lumen of the digestive tract. lung cancer  Uncontrolled cell growth that affects any component of the respiratory system. lungs  Internal respiratory organ containing moist surfaces for gas exchange. luteal phase  Second half of the ovarian cycle, during which the corpus luteum develops and much progesterone (and some estrogen) is produced. luteinizing hormone (LH) Hormone released by the anterior pituitary; in males it regulates the production of testosterone by the interstitial cells;

in females it promotes the development of the corpus luteum in the ovary; also called interstitial cell-stimulating hormone (ICSH) in males. lymph  Fluid, derived from interstitial fluid, that is carried in lymphatic vessels. lymph node  Mass of lymphatic tissue located along the course of a lymphatic vessel. lymphatic capillary Smallest vessels of the lymphatic system; closed-ended; responsible for the uptake of fluids from the surrounding tissues. lymphatic system  Organ system consisting of lymphatic vessels and lymphatic organs; transports lymph and lipids, and aids the immune system. lymphatic vessel Vessel of the lymphatic system that is responsible for transporting excess interstitial fluid, or lymph, from the tissues to the circulatory system. lymphocyte  Specialized white blood cell that functions in specific defense; occurs in two forms—T lymphocytes and B lymphocytes. lymphoid organ  Organ other than a lymphatic vessel that is part of the lymphatic system; the lymphatic organs are the lymph nodes, tonsils, spleen, thymus gland, and bone marrow. lysogenic cycle Bacteriophage life cycle in which the virus incorporates its DNA into that of a bacterium; occurs preliminary to the lytic cycle. lysosome  Membrane-bound vesicle that contains hydrolytic enzymes for digesting macromolecules and bacteria; used to recycle worn-out cellular organelles. lytic cycle Bacteriophage life cycle in which the virus takes over the operation of the bacterium immediately upon entering it and subsequently destroys the bacterium.

M

macroevolution  Large-scale evolutionary change, such as the formation of new species. macrophage  In vertebrates, large phagocytic cell derived from a monocyte, that ingests microbes and debris. macular degeneration  Condition in which the capillaries supplying the retina of the eye become damaged, resulting in reduced vision and blindness. malaria  Disease caused by the protozoan Plasmodium vivax;  transmitted by the Anopheles mosquito. male condom  Contraceptive device, usually made of latex, that fits over the male penis. malignant  Term that is used to describe cancer cells meaning that it has the ability to move to other areas of the body and threaten life. malleus  Bone found in the middle ear that assists in the transmission of sound to the inner ear; also called the hammer. Malpighian tubule Blind, threadlike excretory tubule attached to the gut of an insect. maltase  Enzyme produced in small intestine that breaks down maltose to two glucose molecules. mammal  Endothermic vertebrate characterized especially by the presence of hair and mammary glands. mantle  In molluscs, an extension of the body wall that covers the visceral mass and may secrete a shell.



G-14

Glossary

Marfan syndrome Autosomal dominant genetic disorder of the connective tissue, specifically the fibrillin protein. marsupial  Member of a group of mammals bearing immature young nursed in a marsupium, or pouch—for example, kangaroo and opossum. mass extinction  Episode of large-scale extinction in which large numbers of species disappear in a few million years or less. mass number  Mass of an atom equal to the number of protons plus the number of neutrons within the nucleus. mast cell  Connective tissue cell that releases histamine in allergic reactions. matrix  Unstructured semifluid substance that fills the space between cells in connective tissues or inside organelles. matter  Anything that takes up space and has mass. mechanical energy  A type of kinetic energy associated with the position, or motion (such as walking or running) of an object. mechanoreceptor  Sensory receptor that responds to mechanical stimuli, such as pressure, sound waves, or gravity. medulla oblongata  In vertebrates, part of the brain stem that is continuous with the spinal cord; controls heartbeat, blood pressure, breathing, and other vital functions. medusa  Among cnidarians, bell-shaped body form that is directed downward and contains much mesoglea. megaphyll  Large leaf with several to many veins. megaspore  One of the two types of spores produced by seed plants; develops into a female gametophyte (embryo sac). meiosis  Type of nuclear division that reduces the chromosome number from 2n to n; daughter cells receive the haploid number of chromosomes in varied combinations; also called reduction division. melanocyte  Specialized cell in the epidermis that produces melanin, the pigment responsible for skin color. melanocyte-stimulating hormone (MSH)  Substance that causes melanocytes to secrete melanin in most vertebrates. melatonin  Hormone, secreted by the pineal gland, that is involved in biorhythms. membrane-first hypothesis Proposes that the plasma membrane was the first component of the early cells to evolve. memory  Capacity of the brain to store and retrieve information about past sensations and perceptions; essential to learning. memory B cell Forms during a primary immune response but enters a resting phase until a secondary immune response occurs. memory T cell  T cell that differentiates during an initial infection and responds rapidly during subsequent exposure to the same antigen. meninges  Protective membranous coverings around the central nervous system. meningitis  A condition that refers to inflammation of the brain or spinal cord meninges (membranes). menopause  Termination of the ovarian and uterine cycles in older women. menstruation  Periodic shedding of tissue and blood from the inner lining of the uterus in primates.

meristematic tissue Undifferentiated embryonic tissue in the active growth regions of plants. mesoderm  Middle primary tissue layer of an animal embryo that gives rise to muscle, several internal organs, and connective tissue layers. mesoglea  Transparent jellylike substance located between the endoderm and ectoderm of some sponges and cnidarians. mesophyll  Inner, thickest layer of a leaf consisting of palisade and spongy mesophyll; the site of most of photosynthesis. messenger RNA (mRNA) Type of RNA formed from a DNA template and bearing coded information for the amino acid sequence of a polypeptide. metabolic pathway Series of linked reactions, beginning with a particular reactant and terminating with an end product. metabolic pool Metabolites that are the products of and/or the substrates for key reactions in cells, allowing one type of molecule to be changed into another type, such as carbohydrates converted to fats. metabolism  The sum of the chemical reactions that occur in a cell. metamorphosis  Change in shape and form that some animals, such as insects, undergo during development. metaphase  Third phase of mitosis; chromosomes are aligned at the metaphase plate. metaphase plate  A disk formed during metaphase in which all of a cell’s chromosomes lie in a single plane at right angles to the spindle fibers. metastasis  Spread of cancer from the place of origin throughout the body; caused by the ability of cancer cells to migrate and invade tissues. methanogen  Type of archaean that lives in oxygenfree habitats, such as swamps, and releases methane gas. MHC (major histocompatibility complex) protein  Protein marker that is a part of cell-surface markers anchored in the plasma membrane, which the immune system uses to identify “self.” micelle  Single layer of fatty acids (or phospholipids) that orientate themselves in an aqueous environment. microbiota  Typical complement of bacterial species found on the human body; includes species found both internally and externally. microevolution  Change in gene frequencies between populations of a species over time. micronutrient  Essential element needed in small amounts for plant growth, such as boron, copper, and zinc. microRNA (miRNA) Short sequences of RNA, usually less than 22 nucleotides, that are involved in posttranscriptional regulation of gene expression. These molecules either inhibit, or reduce, the expression of specific genes. microspore  One of the two types of spores produced by seed plants; develops into a male gametophyte (pollen grain). Microsporidia  Phyla of fungi that are characterized as intracellular parasites of animals. microtubule  Small, cylindrical organelle composed of tubulin protein around an empty central core; present in the cytoplasm, centrioles, cilia, and flagella. midbrain  In mammals, the part of the brain located below the thalamus and above the pons.

middle ear  Portion of the ear consisting of the tympanic membrane, the oval and round windows, and the ossicles, where sound is amplified. migration  Regular back-and-forth movement of animals between two geographic areas at particular times of the year. mimicry  Superficial resemblance of two or more species; a survival mechanism that avoids predation by appearing to be noxious. mineral  Naturally occurring inorganic substance containing two or more elements; certain minerals are needed in the diet. mineralocorticoid  Hormone secreted by the adrenal cortex that regulates salt and water balance, leading to increases in blood volume and blood pressure. mitochondria  (sing., mitochondrion) Membranebounded organelle in which ATP molecules are produced during the process of cellular respiration. mitosis  The stage of cellular reproduction in which nuclear division occurs; process in which a parent nucleus produces two daughter nuclei, each having the same number and kinds of chromosomes as the parent nucleus. model  Simulation of a process that aids conceptual understanding until the process can be studied firsthand; a hypothesis that describes how a particular process could possibly be carried out. mold  Various fungi whose body consists of a mass of hyphae (filaments) that grow on and receive nourishment from organic matter such as human food and clothing. mole  The molecular weight of a molecule expressed in grams; contains 6.023 X 1023 molecules. molecular clock  Idea that the rate at which mutational changes accumulate in certain genes is constant over time and is not involved in adaptation to the environment. molecule  Union of two or more atoms of the same element; also, the smallest part of a compound that retains the properties of the compound. molluscs  Invertebrates including squids, clams, snails, and chitons; characterized by a visceral mass, a mantle, and a foot. monoclonal antibody  One of many antibodies produced by a clone of hybridoma cells that all bind to the same antigen. monocot (Monocotyledone)  Flowering plant group; members have one embryonic leaf (­cotyledon), parallel-veined leaves, scattered vascular bundles, flower parts in threes or multiples of three, and other characteristics. monocyte  Type of agranular leukocyte that functions as a phagocyte, particularly after it becomes a macrophage, which is also an antigen-­presenting cell. monohybrid cross  Cross between parents that differ in only one trait. monomer  Small molecule that is a subunit of a polymer—e.g., glucose is a monomer of starch. monosaccharide  Simple sugar; a carbohydrate that cannot be broken down by hydrolysis—e.g., glucose; also, any monomer of the polysaccharides. monosomy  Chromosome condition in which a diploid cell has one less chromosome than normal; designated as 2n-1. monotreme  Egg-laying mammal—e.g., duckbill platypus or spiny anteater.

Glossary G-15

monsoon  Climate in India and southern Asia caused by wet ocean winds that blow onshore for almost half the year. more-developed country (MDC) Country that is industrialized; typically, population growth is low, and the people enjoy a good standard of living overall. morphogen genes  Specific sequences of DNA that determine the relationship of structures during development; often expressed in a gradient across the embryo. morphogenesis  Emergence of shape in tissues, organs, or the entire embryo during development. morphology  Physical characteristics that contribute to the appearance of an organism. morula  Spherical mass of cells resulting from cleavage during animal development prior to the blastula stage. mosaic evolution  Concept that human characteristics did not evolve at the same rate; for example, some body parts are more humanlike than others in early hominins. moss  Bryophyte that is typically found in moist habitats. motor (efferent) neuron  Nerve cell that conducts nerve impulses away from the central nervous system and innervates effectors (muscle and glands). motor molecule Protein that moves along either actin filaments or microtubules and translocates organelles. motor unit  Combination of nerve fibers and muscles fibers in a muscle. mouth  In humans, organ of the digestive tract where food is chewed and mixed with saliva. mRNA transcript  mRNA molecule formed during transcription that has a sequence of bases complementary to a gene. mucosa  Epithelial membrane containing cells that secrete mucus; found in the inner cell layers of the digestive (first layer) and respiratory tracts. mucous membrane  Body membrane that lines the tubes of several organ systems in humans; secretes mucus that helps protect the body from infection. multicellular  Organism composed of many cells; usually has organized tissues, organs, and organ systems. multiple alleles  Inheritance pattern in which there are more than two alleles for a particular trait; each individual has only two of all possible alleles. multiple sclerosis (MS)  Disease of the central nervous system characterized by the breakdown of myelin in the neurons; considered to be an autoimmune disease. muscle tone  Physiological condition by which, at any given time, some muscle fibers are always contracting in a muscle, even when the muscle is at rest. muscle twitch  Single contraction of a muscle fiber; typically lasts only a fraction of a second. muscular dystrophy (MD) Disease that causes a weakening of the muscles; has a strong genetic component. muscular tissue  Type of animal tissue composed of fibers that shorten and lengthen to produce movements.

muscularis  Smooth muscle layer found in the digestive tract. musculoskeletal system Name for the combined muscular and skeletal systems of humans; involved in movement and posture. mutagen  Chemical or physical agent that increases the chance of mutation. mutation  Change in the nucleotide structure of an organism’s DNA. mutualism  Symbiotic relationship in which both species benefit in terms of growth and reproduction. myasthenia gravis (MG)  Autoimmune disease in which antibodies interfere with neuromuscular junctions, causing muscle weakness. mycelium  Tangled mass of hyphal filaments composing the vegetative body of a fungus. mycorrhizae  (sing., mycorrhiza) Mutualistic relationship between fungal hyphae and roots of vascular plants. mycoses  General name for fungal diseases of animals. myelin sheath  White, fatty material—derived from the membrane of neurolemmocytes—that forms a covering for nerve fibers. myofibril  Specific muscle cell organelle containing a linear arrangement of sarcomeres, which shorten to produce muscle contraction. myosin  Muscle protein making up the thick filaments in a sarcomere; it pulls actin to shorten the sarcomere, yielding muscle contraction.

N

NAD+ (nicotinamide adenine dinucleotide)  Coenzyme in oxidation-reduction reactions that accepts electrons and hydrogen ions to become NADH + H+ as oxidation of substrates occurs. During cellular respiration, NADH carries electrons to the electron transport chain in mitochondria. NADP+ (nicotinamide adenine dinucleotide ­phosphate)  Coenzyme in oxidation-reduction reactions that accepts electrons and hydrogen ions to become NADPH + H+. During photosynthesis, NADPH participates in the reduction of carbon dioxide to a carbohydrate. nail  Flattened epithelial tissue from the stratum lucidum of the skin; located on the tips of fingers and toes. natural killer (NK) cell  Lymphocyte that causes an infected or cancerous cell to burst. natural selection Mechanism of evolutionary change caused by environmental selection of organisms most fit to reproduce; results in adaptation to the environment. Neandertal  Hominin with a sturdy build that lived during the last Ice Age in Europe and the Middle East; hunted large game and left evidence of being culturally advanced. nearsighted (myopic)  Condition in which an individual cannot focus on objects at a distance; caused by the focusing of the image in front of the retina of the eye. negative feedback Mechanism of homeostatic response by which the output of a system suppresses or inhibits activity of the system.

nematocyst  In cnidarians, a capsule that contains a threadlike fiber, the release of which aids in the capture of prey. nephridium  (pl., nephridia) Segmentally arranged, paired excretory tubule of many invertebrates, as in the earthworm. nephron  Microscopic kidney unit that regulates blood composition by glomerular filtration, tubular reabsorption, and tubular secretion. nerve  Bundle of long axons outside the central nervous system. nerve cord  Component of an invertebrate’s nervous system; usually located ventrally in invertebrates; commonly called the spinal cord in vertebrates. nerve fiber Axon; conducts nerve impulses away from the cell. Axons are classified as either myelinated or unmyelinated based on the presence or absence of a myelin sheath. nerve impulse  Electrical signal that conveys information along the length of a neuron. nerve net Diffuse, noncentralized arrangement of nerve cells in cnidarians. nervous system Organ system of humans that includes the brain, spinal cord, sense organs (eyes, ears, etc.) and associated nerves. Receives, integrates, and stores sensory input; coordinates activity of other organ systems. nervous tissue  Tissue that contains nerve cells (neurons), which conduct impulses, and neuroglia, which support, protect, and provide nutrients to neurons. neurodegenerative disease  Disease, usually caused by a prion, virus, or bacterium, that damages or impairs the function of nervous tissue. neuroglia  Nonconducting nerve cells that are intimately associated with neurons and function in a supportive capacity. neuromuscular junction Region where an axon bulb approaches a muscle fiber; contains a presynaptic membrane, a synaptic cleft, and a postsynaptic membrane. neuron  Nerve cell that characteristically has three parts: dendrites, cell body, and an axon. neurotransmitter  Chemical stored at the ends of axons that is responsible for transmission across a synapse. neutron  Neutral subatomic particle, located in the nucleus and assigned one atomic mass unit. neutrophil  Granular leukocyte that is the most abundant of the white blood cells; first to respond to infection. nitrification  Process by which nitrogen in ammonia and organic compounds is oxidized to nitrites and nitrates by soil bacteria. nitrogen fixation  Process whereby free atmospheric nitrogen is converted into compounds, such as ammonium and nitrates, usually by bacteria. node  In plants, the place where one or more leaves attach to a stem. nodes of Ranvier  Gaps in the myelin sheath around a nerve fiber. noncyclic electron pathway  Light-dependent photosynthetic pathway that is used to generate ATP and NADPH; because the pathway is noncyclic, the electrons must be replaced by the splitting of water (photolysis). nondisjunction  Failure of the homologous chromosomes or sister chromatids to separate during



G-16

Glossary

either mitosis or meiosis; produces cells with abnormal chromosome numbers. nonpolar covalent bond  Bond in which the sharing of electrons between atoms is fairly equal. nonrandom mating  Mating among individuals on the basis of their phenotypic similarities or differences, rather than mating on a random ­ basis. nonrenewable resource  Minerals, fossil fuels, and other materials present in essentially fixed amounts (within the human timescale) in our environment. nonvascular plants Bryophytes, such as mosses and liverworts, that have no vascular tissue and either occur in moist locations or have special adaptations for living in dry locations. norepinephrine  Neurotransmitter of the postganglionic fibers in the sympathetic division of the autonomic system; also, a hormone produced by the adrenal medulla. Sometimes referred to as noradrenaline. nose  External structure of the respiratory system, that is involved in the process of ventilation. notochord  Cartilage-like supportive dorsal rod in all chordates at some time in their life cycle; replaced by vertebrae in vertebrates. nuclear envelope  Double membrane that surrounds the nucleus in eukaryotic cells and is connected to the endoplasmic reticulum; has pores that allow substances to pass between the nucleus and the cytoplasm. nuclear pore  Opening in the nuclear envelope that permits the passage of proteins into the nucleus and ribosomal subunits out of the nucleus. nucleic acid  Polymer of nucleotides; both DNA and RNA are nucleic acids. nucleoid  Region of the cytoplasm in prokaryotic cells where DNA is located; it is not bound by a nuclear envelope. nucleolus  Dark-staining, spherical body in the nucleus that produces ribosomal subunits. nucleoplasm  Semifluid medium of the nucleus containing chromatin. nucleosome  In the nucleus of a eukaryotic cell, a unit composed of DNA wound around a core of eight histone proteins, giving the appearance of a string of beads. nucleotide  Monomer of DNA and RNA consisting of a 5-carbon sugar bonded to a nitrogenous base and a phosphate group. nucleus  Membrane-bound organelle within a eukaryotic cell that contains chromosomes and controls the structure and function of the cell.

O

obesity  Overweight condition characterized as having a body mass index (BMI) greater than 30. observation  Initial step in the scientific method that often involves the recording of data from an experiment or natural event. octet rule  The observation that an atom is most stable when its outer shell is complete and contains eight electrons; an exception is hydrogen, which requires only two electrons in its outer shell to have a completed shell. oil  Triglyceride, usually of plant origin, that is composed of glycerol and three fatty acids and is

liquid in consistency due to many unsaturated bonds in the hydrocarbon chains of the fatty acids. oil gland  Gland of the skin, associated with a hair follicle, that secretes sebum; sebaceous gland. olfactory cell Modified neuron that is a sensory receptor for the sense of smell. oligodendrocyte  Type of glial cell that forms myelin sheaths around neurons in the CNS. omnivore  Organism in a food chain that feeds on both plants and animals. oncogene  Cancer-causing gene formed by a mutation in a proto-oncogene; codes for proteins that stimulate the cell cycle and inhibit apoptosis. oocyte  Immature egg that is undergoing meiosis; upon completion of meiosis, the oocyte becomes an egg. oogenesis  Production of eggs in females by the process of meiosis and maturation. open circulatory system Arrangement of internal transport in which blood bathes the organs directly, and there is no distinction between blood and interstitial fluid. operant conditioning Learning that results from rewarding or reinforcing a particular behavior. operon  Group of structural and regulating genes that function as a single unit. opportunistic infections  Diseases caused by pathogens that are normally suppressed by the immune system but may become pathogenic if the immune system is compromised, as is the case with HIV/AIDS. order  One of the categories, or taxa, used to group species; the taxon located above the family level. organ  Combination of two or more different tissues performing a common function. organ of Corti  Structure in the vertebrate inner ear that contains auditory receptors (also called spiral organ). organ system Group of related organs working together; examples are the digestive and endocrine systems. organelle  Small, membranous structures in the cytoplasm having a specific structure and function. organic chemistry Branch of science that deals with organic molecules including those that are unique to living things. organic molecule Molecule that always contains carbon and hydrogen, and often contain oxygen as well; organic molecules are associated with living things. orientation  In birds, the ability to know present location by tracking stimuli in the environment. origin  When referring to a muscle, the connection on the stationary bone. osmoregulation  Regulation of the water–salt balance to maintain a normal balance within internal fluids. osmosis  Diffusion of water through a selectively permeable membrane. osmotic pressure  Measure of the tendency of water to move across a selectively permeable membrane; visible as an increase in liquid on the side of the membrane with higher solute concentration. ossicle  One of the small bones of the vertebrate middle ear—malleus, incus, and stapes.

osteoarthritis (OA)  Form of degenerative joint disease which is the result of the loss of cartilage in a synovial joint. osteoblast  Bone-forming cell. osteoclast  Cell that is responsible for bone resorption. osteocyte  Mature bone cell located within the lacunae of bone. osteoporosis  Condition characterized by a loss of bone density; associated with levels of sex hormones and diet. otitis media  Inflammation of the middle ear. otolith  Calcium carbonate granule associated with sensory receptors for detecting movement of the head; in vertebrates, located in the utricle and saccule. outer ear  Portion of the ear consisting of the pinna and the auditory canal. oval window  Structure of the middle ear that conducts sound from the middle ear to the inner ear. ovarian cancer Form of cancer that affects the uterus; often difficult to diagnose due to a lack of symptoms. ovarian cycle Monthly changes occurring in the ovary that determine the level of sex hormones in the blood. ovary  In flowering plants, the enlarged, ovule-bearing portion of the carpel that develops into a fruit; female gonad in animals that produces an egg and female sex hormones. overexploitation  When the number of individuals taken from a wild population is so great that the population becomes severely reduced in numbers. ovulation  Bursting of a follicle when a secondary oocyte is released from the ovary; if fertilization occurs, the secondary oocyte becomes an egg. ovule  In seed plants, a structure that contains the female gametophyte and has the potential to develop into a seed. oxidation  Loss of one or more electrons from an atom or molecule; in biological systems, generally the loss of hydrogen atoms. oxidation-reduction reaction  A paired set of chemical reactions in which one molecule gives up electrons (oxidized) while another molecule accepts electrons (reduced); commonly called a redox reaction. oxygen debt  Amount of oxygen required to oxidize lactic acid produced anaerobically during strenuous muscle activity. oxyhemoglobin  Compound formed when oxygen combines with hemoglobin. oxytocin  Hormone, released by the posterior pituitary, that causes contraction of the uterus and milk letdown. ozone shield  Accumulation of O3, formed from oxygen in the upper atmosphere; a filtering layer that protects the Earth from ultraviolet radiation.

P

p53  The protein produced from a tumor suppressor gene that (1) attempts to repair DNA damage, or (2) stops the cell cycle, or (3) initiates apoptosis. pacemaker  Cells of the sinoatrial node of the heart; electrical device designed to mimic the normal electrical patterns of the heart.

Glossary G-17

pain receptors  Sensory receptors that are sensitive to chemicals released by damaged cells; also called nociceptors. paleontology  Study of fossils that results in knowledge about the history of life. pancreas  Internal organ that produces digestive enzymes and the hormones insulin and glucagon. pancreatic amylase  Enzyme that digests starch to maltose. pancreatic cancer  Form of cancer that originates in the pancreas; one of the more fatal forms of cancer. pancreatic islet  Masses of cells that constitute the endocrine portion of the pancreas. pancreatitis  Inflammation of the pancreas; may be caused by infection, gallstones, or excessive use of alcohol. Pap test  Procedure that removes a few cells from the cervix to look for evidence of cervical cancer. parasite  Species that is dependent on a host species for survival, usually to the detriment of the host species. parasitism  Symbiotic relationship in which one species (the parasite) benefits in terms of growth and reproduction to the detriment of the other species (the host). parasympathetic division Division of the autonomic system that is active under normal c onditions; uses acetylcholine as a ­ neurotransmitter. parathyroid gland  Gland embedded in the posterior surface of the thyroid gland; it produces parathyroid hormone. parathyroid hormone (PTH)  Hormone secreted by the four parathyroid glands that increases the blood calcium level and decreases the phosphate level. parenchyma  Plant tissue composed of the leastspecialized of all plant cells; found in all organs of a plant. Parkinson disease (PD)  Progressive deterioration of the central nervous system due to a deficiency in the neurotransmitter dopamine. parsimony  In systematics, the simplest solution in the analysis of evolutionary relationships. partial pressure  Pressure exerted by each gas in a mixture of gases. parturition  Process of giving birth; divided into three stages. passive immunity Protection against infection acquired by transfer of antibodies to a susceptible individual. pathogen  Disease-causing agent such as viruses, parasitic bacteria, fungi, and animals. pattern formation Positioning of cells during development that determines the final shape of an organism. pectoral girdle  Portion of the vertebrate skeleton that provides support and attachment for the upper (fore) limbs; consists of the scapula and clavicle on each side of the body. pedigree  Chart of genetic relationship of family individuals across generations. pelagic division  Open portion of the ocean. pellicle  Structure that supports the cell membrane of some species of protists; used to maintain structure in a water environment.

pelvic girdle  Portion of the vertebrate skeleton to which the lower (hind) limbs are attached; consists of the coxal bones. pelvic inflammatory disease (PID) Condition in which the uterine tubes become inflamed; often caused by chlamydial infections. penis  Male copulatory organ; in humans, the male organ of sexual intercourse. pentose  Five-carbon monosaccharide. Examples are deoxyribose found in DNA and ribose found in RNA. pepsin  Enzyme secreted by gastric glands that digests proteins to peptides. peptidase  Intestinal enzyme that breaks down short chains of amino acids to individual amino acids that are absorbed across the intestinal wall. peptide  Two or more amino acids joined together by covalent bonding. peptide bond  Type of covalent bond that joins two amino acids. peptide hormone  Type of hormone that is a protein, a peptide, glycoprotein, or is derived from an amino acid. peptidoglycan  Polysaccharide that contains short chains of amino acids; found in bacterial cell walls. perception  Processing of sensory stimuli that occurs when the brain interprets information being received from the sensory receptors. perennial  Flowering plant that lives more than one growing season because the underground parts regrow each season. pericycle  Layer of cells surrounding the vascular tissue of roots; produces branch roots. periosteum  Fibrous connective tissue that covers the surface of a long bone; contains blood vessels that service the cells within the bone. peripheral nervous system (PNS)  Nerves and ganglia that lie outside the central nervous system. peristalsis  Wavelike contractions that propel substances along a tubular structure such as the esophagus. permafrost  Permanently frozen ground, usually occurring in the tundra, a biome of Arctic regions. peroxisome  Enzyme-filled vesicle in which fatty acids and amino acids are metabolized to hydrogen peroxide that is broken down to harmless products. petal  A flower part that occurs just inside the sepals; often conspicuously colored to attract pollinators. petiole  The part of a plant leaf that connects the blade to the stem. pH scale  Measurement scale for hydrogen ion concentration. Based on the formula –log[H+]. phagocytes  Type of white blood cells that destroy pathogens using phagocytosis. phagocytosis  Process by which cells engulf large substances, forming an intracellular vacuole. pharyngeal pouches Developmental characteristic of a chordate; these may develop into gills, or other structures such as auditory tubes and glands. pharyngitis  Inflammation of the pharynx; often caused by viruses or bacteria. pharynx  In vertebrates, common passageway for both food intake and air movement; located between the mouth and the esophagus. phenomena  Observable natural event or fact. phenotype  Visible expression of a genotype—e.g., brown eyes or attached earlobes.

phenylketonuria (PKU) Autosomal recessive genetic disorder that causes a lack of the enzyme that metabolizes phenylalanine; the accumulation of phenylalanine causes problems with nervous system development and function. pheromone  Chemical messenger that works at a distance and alters the behavior of another member of the same species. phloem  Vascular tissue that conducts organic solutes in plants; contains sieve-tube members and companion cells. phoronid  Invertebrate animal (lophophoran) that possesses a protective tube made of chitin. phospholipid  Molecule that forms the bilayer of the cell’s membranes; has a polar, hydrophilic head bonded to two nonpolar, hydrophobic tails. photoautotroph  Organism able to synthesize organic molecules by using carbon dioxide as the carbon source and sunlight as the energy source. photoperiod (photoperiodism)  Relative lengths of daylight and darkness that affect the physiology and behavior of an organism. photoreceptor  Sensory receptor that responds to light stimuli. photosynthesis  Process, usually occurring within chloroplasts, that uses solar energy to reduce carbon dioxide to carbohydrate. photosystem  Photosynthetic unit where solar energy is absorbed and high-energy electrons are generated; contains a pigment complex and an electron acceptor; occurs as PS (photosystem) I and PS II. phototropism  Growth response of plant stems to light; stems demonstrate positive phototropism. phylogenetic tree  Diagram that indicates common ancestors and lines of descent among a group of organisms. phylogenetics  Area of systematic biology that uses evidence to construct phylogeny, or a hypothesis of the evolutionary relationships between species. phylogeny  Evolutionary history of a group of organisms. phylum  One of the categories, or taxa, used to group species; the taxon located above the class level. phytochrome  Photoreversible plant pigment that is involved in photoperiodism and other responses of plants, such as etiolation. phytoplankton  Part of plankton containing organisms that photosynthesize, releasing oxygen to the atmosphere and serving as food producers in aquatic ecosystems. pineal gland  Gland—either at the skin surface (fish, amphibians) or in the third ventricle of the brain (mammals)—that produces melatonin. pinna  External flap of the ear; serves to collect and funnel sound toward the middle ear. pinocytosis  Process by which vesicle formation brings macromolecules into the cell. pith  Parenchyma tissue in the center of some stems and roots. pituitary dwarfism  Condition caused when too little growth hormone is produced during childhood; individuals with this condition have a small stature, but body parts in normal proportions. pituitary gland  Small gland that lies just inferior to the hypothalamus; consists of the anterior and posterior pituitary, both of which produce hormones.



G-18

Glossary

placenta  Organ formed during the development of placental mammals from the chorion and the uterine wall; allows the embryo, and then the fetus, to acquire nutrients and rid itself of wastes; produces hormones that regulate pregnancy. placental mammal  Also called the eutherians; species that rely on internal development whereby the fetus exchanges nutrients and wastes with its mother via a placenta. plankton  Freshwater and marine organisms that are suspended on or near the surface of the water; includes phytoplankton and zooplankton. plants  Multicellular, photosynthetic eukaryotes that increasingly became adapted to live on land. plasma  In vertebrates, the liquid portion of blood; contains nutrients, wastes, salts, and proteins. plasma cell Mature B cell that mass-produces antibodies. plasma membrane Membrane surrounding the cytoplasm that consists of a phospholipid bilayer with embedded proteins; functions to regulate the entrance and exit of molecules from cell. plasmid  Extrachromosomal ring of accessory DNA in the cytoplasm of prokaryotes. plasmodesmata  In plants, cytoplasmic connections in the cell wall that connect two adjacent cells. plasmodial slime mold  Free-living mass of cytoplasm that moves by pseudopods on a forest floor or in a field, feeding on decaying plant material by phagocytosis; reproduces by spore formation. plasmolysis  Contraction of the cell contents due to the loss of water. plate tectonics Concept that the Earth’s crust is divided into a number of fairly rigid plates whose movements account for continental drift. platelet  Thrombocyte; component of blood that is necessary to blood clotting. pleiotropy  Inheritance pattern in which one gene affects many phenotypic characteristics of the individual. pneumonia  Condition of the respiratory system characterized by the filling of the bronchi and alveoli with fluid; caused by a viral, fungal, or bacterial pathogen. point mutation Change of only one base in the sequence of bases in a gene. polar body Nonfunctional product of oogenesis produced by the unequal division of cytoplasm in females during meiosis; in humans three of the four cells produced by meiosis are polar bodies. polar covalent bond  Bond in which the sharing of electrons between atoms is unequal. pollen cone  Reproductive structure of a gymnosperm that produces windblown pollen; microstrobili. pollen grain  In seed plants, structure that is derived from a microspore and develops into a male gametophyte. pollen tube  In seed plants, a tube that forms when a pollen grain lands on the stigma and germinates. The tube grows, passing between the cells of the stigma and the style to reach the egg inside an ovule, where fertilization occurs. pollination  In gymnosperms, the transfer of pollen from pollen cone to seed cone; in angiosperms, the transfer of pollen from anther to stigma. pollinator  Animal, typically an insect or bird, that transfers pollen between plants.

pollution  Any environmental change that adversely affects the lives and health of living organisms. polyandrous  Female animals that have several male mates; found in the New World monkeys where the males help in rearing the offspring. polygamous  Male animals that have several female mates. polygenic inheritance Pattern of inheritance in which a trait is controlled by several allelic pairs. polygenic trait  Traits that are under the control of multiple genes as opposed to monogenic (singlegene) traits. polymer  Macromolecule consisting of covalently bonded monomers; for example, a polypeptide is a polymer of monomers called amino acids. polymerase chain reaction (PCR) Biotechnology technique that uses the enzyme DNA polymerase to produce billions of copies of a specific piece of DNA. polyp  Among cnidarians, body form that is directed upward and contains much mesoglea; in anatomy: small, abnormal growth that arises from the epithelial lining. polypeptide  Polymer of many amino acids linked by peptide bonds. polyribosome  String of ribosomes simultaneously translating regions of the same mRNA strand during protein synthesis. polysaccharide  Polymer made from carbohydrate monomers; the polysaccharides starch and glycogen are polymers of glucose monomers. pons  Portion of the brain stem above the medulla oblongata and below the midbrain; assists the medulla oblongata in regulating the breathing rate. population  Group of organisms of the same species occupying a certain area and sharing a common gene pool. population genetics  The study of gene frequencies and their changes within a population. Porifera  Invertebrates that are pore-bearing filter feeders whose inner body wall is lined by collar cells that resemble a unicellular choanoflagellate. portal system  Pathway of blood flow that begins and ends in capillaries, such as the portal system located between the small intestine and liver. positive feedback Mechanism of homeostatic response in which the output of the system intensifies and increases the activity of the system. postanal tail Characteristic of a chordate; tail extends past the anus of the digestive system. posterior pituitary Portion of the pituitary gland that stores and secretes oxytocin and antidiuretic hormone produced by the hypothalamus. posttranscriptional control Gene expression following transcription that regulates the way mRNA transcripts are processed. posttranslational control Alternation of gene expression by changing a protein’s activity after it is translated. potential energy Stored energy in a potentially usable form, as a result of location or spatial arrangement. precipitation  The process by which water leaves the atmosphere and falls to the ground as water, ice, or snow. predation  Interaction in which one organism (the predator) uses another (the prey) as a food source.

predator  Organism that practices predation. prediction  Step of the scientific process that follows the formulation of a hypothesis and assists in creating the experimental design. preparatory (prep) reaction  Reaction that oxidizes pyruvate with the release of carbon dioxide; results in acetyl CoA and connects glycolysis to the citric acid cycle. pressure-flow model  Explanation for phloem transport; osmotic pressure following active transport of sugar into phloem produces a flow of sap from a source to a sink. prey  Organism that provides nourishment for a predator. prezygotic isolating mechanism Anatomical, ­physiological, or behavioral difference between two species that prevents the possibility of mating. primate  Member of the order Primates; includes prosimians, monkeys, apes, and hominins, all of whom have adaptations for living in trees. principle  Theory that is generally accepted by an overwhelming number of scientists; also called a law. prion  Infectious particle consisting of protein only and no nucleic acid. producer  Photosynthetic organism at the start of a grazing food chain that makes its own food—e.g., green plants on land and algae in water. product  Substance that forms as a result of a reaction. profundal zone Underwater region of a lake (or ocean) that is just below where light penetrates; often very cold. progesterone  Female sex hormone that helps maintain sexual organs and secondary sex characteristics. proglottid  Segment of a tapeworm that contains both male and female sex organs and becomes a bag of eggs. prokaryote  Organism that lacks a nucleus and the membrane-bound organelles that are typically found in eukaryotes. prokaryotic cell  Cells that generally lack a membrane-bound nucleus and organelles; the cell type within the domains Bacteria and Archaea. prolactin  Hormone secreted by the anterior pituitary that stimulates the production of milk from the mammary glands. prometaphase  Phase of cell division that occurs between prophase and metaphase and is characterized by attachment of the spindle fibers to the kinetochores of each sister chromatid. promoter  In an operon, a sequence of DNA where RNA polymerase binds prior to transcription. prophase  First phase of mitosis; characterized by the condensation of the chromatin; chromosomes are visible, but scattered in the nucleus. proprioceptor  Class of mechanoreceptors responsible for maintaining the body’s equilibrium and posture; involved in reflex actions. prosimian  Group of primates that includes lemurs and tarsiers, and may resemble the first primates to have evolved. prostaglandin  Hormone that has various and powerful local effects. prostate cancer  Cancer of the prostate gland; may be detected using a PSA test.

Glossary G-19

prostate gland  Gland in male humans that secretes an alkaline, cloudy, fluid that increases the motility of sperm. protease  Enzyme that breaks the peptide bonds between amino acids in proteins, polypeptides, and peptides. protein  Polymer of amino acids; often consisting of one or more polypeptides and having a complex three-dimensional shape. protein-first hypothesis  In chemical evolution, the proposal that protein originated before other macromolecules and made possible the formation of protocells. proteome  Sum of the expressed proteins in a cell. proteomics  Study of the complete collection of proteins that a cell or organism expresses. protists  The group of eukaryotic organisms that are not a plant, fungus, or animal. Protists are generally a microscopic, complex single cell; they evolved before other types of eukaryotes in the history of Earth. proto-oncogene  Gene that promotes the cell cycle and prevents apoptosis; may become an oncogene through mutation. protobiont (protocell) In biological evolution, a possible cell forerunner that became a cell once it acquired genes. proton  Positive subatomic particle located in the nucleus and assigned one atomic mass unit. protostome  Group of coelomate animals in which the first embryonic opening (the blastopore) is associated with the mouth. protozoan  Heterotrophic, single-celled protist that moves by flagella, cilia, or pseudopodia. proximal convoluted tubule  Portion of a nephron following the glomerular capsule where tubular reabsorption of filtrate occurs. pseudocoelom  Body cavity lying between the digestive tract and body wall that is incompletely lined by mesoderm. pseudopod  Cytoplasmic extension of amoeboid protists; used for locomotion and engulfing food. pteridophyte  Ferns and their allies (horsetails and whisk ferns). pulmonary artery Blood vessel that transports ­oxygen-poor blood from the heart to the lungs. pulmonary circuit Circulatory pathway between the lungs and the heart. pulmonary fibrosis  Respiratory condition characterized by the buildup of connective tissue in the lungs; typically caused by inhalation of coal dust, silica, or asbestos. pulmonary tuberculosis Respiratory infection caused by the bacterium Mycobacterium tuberculosis. pulmonary vein Blood vessel that transports ­oxygen-rich blood from the lungs to the heart. pulse  Vibration felt in arterial walls due to expansion of the aorta following ventricle contraction. punctuated equilibrium Model of evolutionary change in a species which suggests that there are long periods of little or no change, followed by brief periods of rapid speciation. Punnett square  Visual representation developed by Reginald Punnett that is used to calculate the expected results of simple genetic crosses. pupil  Opening in the center of the iris of the vertebrate eye.

purine  Nucleotides with a double-ring structure; examples are adenine and guanine. pyelonephritis  Infection of the kidneys, often caused by an initial infection in one of the ureters. pyrimidine  Nucleotides with a single ring in their structure; examples are thymine, cytosine, and uracil.

R

r-selection  Favorable life history strategy under certain environmental conditions; characterized by a high reproductive rate with little or no attention given to offspring survival. radial symmetry  Body plan in which similar parts are arranged around a central axis, like spokes of a wheel. radiocarbon dating  Process of radiometric dating that measures the decay of 14C to 14N. rain shadow  Leeward side (side sheltered from the wind) of a mountainous barrier, which receives much less precipitation than the windward side. rate of natural increase (r)  Growth rate dependent on the number of individuals that are born each year and the number of individuals that die each year. ray-finned bony fishes  Group of bony fishes with fins supported by parallel bony rays connected by webs of thin tissue. RB  The protein of a tumor suppressor gene; interprets growth signals and nutrient availability before allowing the cell cycle to proceed. reactant  Substance that participates in a reaction. receptacle  Area where a flower attaches to a floral stalk. receptor  Type of membrane protein that binds to specific molecules in the environment, providing a mechanism for the cell to sense and adjust to its surroundings. receptor protein Protein located in the plasma membrane or within the cell; binds to a substance that alters some metabolic aspect of the cell. receptor-mediated endocytosis  Selective uptake of molecules into a cell by vacuole formation after they bind to specific receptor proteins in the plasma membrane. recessive allele Allele that exerts its phenotypic effect only in the homozygote; its expression is masked by a dominant allele. reciprocal altruism The trading of helpful or cooperative acts, such as helping at the nest, by individuals—the animal that was helped will repay the debt at some later time. recombinant DNA (rDNA) DNA that contains genes from more than one source. red algae Marine photosynthetic protists with a notable abundance of phycobilin pigments; includes coralline algae of coral reefs. red blood cell Erythrocyte; contains hemoglobin and carries oxygen from the lungs or gills to the tissues in vertebrates. red bone marrow  Vascularized, modified connective tissue that is sometimes found in the cavities of spongy bone; site of blood cell formation. red tide  A population bloom of dinoflagellates that causes coastal waters to turn red. Releases a toxin that can lead to paralytic shellfish poisoning.

redox reaction  A paired set of chemical reactions in which one molecule gives up electrons (oxidized) while another molecule accepts electrons (reduced); also called an oxidation-reduction reaction. reduced hemoglobin Globin chains within the hemoglobin molecule that have combined with hydrogen ions (H+). reduction  Gain of electrons by an atom or molecule with a concurrent storage of energy; in biological systems, the electrons are accompanied by hydrogen ions. reflex action  Automatic, involuntary response of an organism to a stimulus. refractory period  Time following an action potential when a neuron is unable to conduct another nerve impulse. renewable resource  Resource normally replaced or replenished by natural processes and not depleted by moderate use. renin  Enzyme released by the kidneys that leads to the secretion of aldosterone and a rise in blood pressure. replacement model  Hypothesis of the evolution of modern humans from archaic species; also called the out-of-Africa hypothesis. replacement reproduction Population in which each person is replaced by only one child. replication fork  In eukaryotic DNA replication, the location where the two parental DNA strands separate. repressor  In an operon, protein molecule that binds to an operator, preventing transcription of structural genes. reproduce  To produce a new individual of the same kind. reproduction  The process of producing a new individual of the same kind. reproductive cloning Used to create an organism that is genetically identical to the original individual. reproductive isolation  Model by which new species arise when gene flow is disrupted between two populations, genetic changes accumulate, and the populations are subsequently unable to mate and produce viable offspring. reproductive system  Organ system in humans that includes the sex-specific organs (testes, ovaries, etc.); produces and transports gametes; in females, nurtures and gives birth to offspring. reproductively isolated  Descriptive term that indicates that a population is incapable of interbreeding with another population. reptile  Terrestrial vertebrate with internal fertilization, scaly skin, and an egg with a leathery shell; includes snakes, lizards, turtles, crocodiles, and birds. reservoir  Location in a biogeochemical cycle where a chemical or resource is stored for long periods of time and is typically unavailable to living organisms. resource  Abiotic and biotic components of an environment that support or are needed by living organisms. resource partitioning Mechanism that increases the number of niches by apportioning the supply of a resource such as food or living space between species.



G-20

Glossary

respiration  Sequence of events that results in gas exchange between the cells of the body and the environment. respiratory center Group of nerve cells in the medulla oblongata that send out nerve impulses on a rhythmic basis, resulting in involuntary inspiration on an ongoing basis. respiratory system Organ system of humans that includes the lungs and associated structures; involved in the exchange of gases; helps control pH. responding variable Result or change that occurs when an experimental variable is utilized in an experiment. resting potential  Membrane potential of an inactive neuron. restoration ecology  Seeks scientific ways to return ecosystems to their state prior to habitat degradation. restriction enzyme Bacterial enzyme that stops viral reproduction by cleaving viral DNA; used to cut DNA at specific points during production of recombinant DNA. reticular activating system (RAS) Area of the brain that contains the reticular formation; acts as a relay for information to and from the peripheral nervous system and higher processing centers of the brain. reticular connective tissue  Form of connective tissue that supports the lymph nodes, spleen, thymus, and bone marrow. reticular fiber  Very thin collagen fiber in the matrix of connective tissue, highly branched and forming delicate supporting networks. retina  Innermost layer of the vertebrate eyeball containing the photoreceptors—rod cells and cone cells. retinal  Light-absorbing molecule found in the photoreceptors of the eye; usually combined with opsin to form rhodopsin. retinal detachment  Condition characterized by the separation of the retina from the choroid layer of the eye. retrovirus  RNA virus, containing the enzyme reverse transcriptase, that carries out RNA/DNA transcription. reverse transcriptase  Viral enzyme found in retroviruses that is capable of converting their RNA genome into a DNA copy. rheumatoid arthritis (RA) Autoimmune disease that causes inflammation of the joints. rhizome  Rootlike underground stem. rhodopsin  Light-absorbing molecule in rod cells and cone cells that contains a pigment and the protein opsin. ribose  Pentose sugar found in RNA. ribosomal RNA (rRNA)  Structural form of RNA found in the ribosomes. ribosome  Site of protein synthesis in a cell; composed of proteins and ribosomal RNA (rRNA). ribozyme  RNA molecule that functions as an enzyme that can catalyze chemical reactions. ringworm  Condition of the skin caused by a fungal infection. RNA (ribonucleic acid) Nucleic acid produced from covalent bonding of nucleotide monomers that contain the sugar ribose; occurs in many forms, including: messenger RNA, ribosomal RNA, and transfer RNA.

RNA interference Cellular process that utilizes miRNA and siRNA molecules to reduce, or inhibit, the expression of specific genes. RNA polymerase  During transcription, an enzyme that creates an mRNA transcript by joining nucleotides complementary to a DNA template. RNA-first hypothesis  In chemical evolution, the proposal that RNA originated before other macromolecules and allowed the formation of the first cell(s). rod cell Photoreceptor in vertebrate eyes that responds to dim light. root  Organ system of a plant that is responsible for anchoring the plant, absorbing water and minerals, and storing carbohydrates. root cap  Protective cover of the root tip, whose cells are constantly replaced as they are ground off when the root pushes through rough soil particles. root hair  Extension of a root epidermal cell that collectively increases the surface area for the absorption of water and minerals. root nodule Structure on plant root that contains nitrogen-fixing bacteria. root system  Includes the main root and all of its lateral (side) branches. rotational equilibrium Maintenance of balance when the head and body are suddenly moved or rotated. rotifer  Microscopic invertebrates characterized by ciliated corona that when beating looks like a rotating wheel. rough ER (endoplasmic reticulum) Membranous system of tubules, vesicles, and sacs in cells; has attached ribosomes. round window Structure of the middle ear that assists in the transmission of sound from the middle ear to the inner ear. roundworm  Invertebrates with nonsegmented cylindrical body covered by a cuticle that molts; some forms are free-living in water and soil, and many are parasitic. RuBP carboxylase  An enzyme that starts the Calvin cycle reactions by catalyzing attachment of the carbon atom from CO2 to RuBP.

S

sac fungi Fungi that produce spores in fingerlike sacs called asci within a fuiting body; includes morels, truffles, yeasts, and molds. saccule  Saclike cavity in the vestibule of the vertebrate inner ear; contains sensory receptors for gravitational equilibrium. salivary amylase  In humans, enzyme in saliva that digests starch to maltose. salivary gland  In humans, gland associated with the mouth, that secretes saliva. salt  Solid substances formed by ionic bonds that usually dissociate into individual ions in water. saltatory conduction  Movement of nerve impulses from one node to another along a myelinated axon. saprotroph  Organism that secretes digestive enzymes and absorbs the resulting nutrients back across the plasma membrane. sarcolemma  Plasma membrane of a muscle fiber; also forms the tubules of the T system involved in muscular contraction.

sarcomere  One of many units, arranged linearly within a myofibril, whose contraction produces muscle contraction. sarcoplasmic reticulum  Smooth endoplasmic reticulum of skeletal muscle cells; surrounds the myofibrils and stores calcium ions. saturated fatty acid  Fatty acid molecule that lacks double bonds between the carbons of its hydrocarbon chain. The chain bears the maximum number of hydrogens possible. savanna  Terrestrial biome that is a grassland in Africa, characterized by few trees and a severe dry season. Schwann cell  Cell that surrounds a fiber of a peripheral nerve and forms the myelin sheath. scientific method Process by which scientists ­formulate a hypothesis, gather data by observation and experimentation, and come to a conclusion. sclera  White, fibrous, outer layer of the eyeball. sclerenchyma  Plant tissue composed of cells with heavily lignified cell walls; functions in support. scolex  Tapeworm head region; contains hooks and suckers for attachment to host. scrotum  Saclike structures of a male that houses the testes. second messenger  Chemical signal, such as cyclic AMP, that causes the cell to respond to the first messenger—a hormone bound to a plasma membrane receptor protein. secretion  Release of a substance by exocytosis from a cell. sedimentation  Process by which particulate material accumulates and forms a stratum. seed  Mature ovule that contains an embryo, with stored food enclosed in a protective coat. seed cone  Reproductive structure of a gymnosperm that produces windblown seeds; megastrobili. seed plant  Vascular plant that disperses seeds; the gymnosperms and angiosperms. seedless vascular plant Collective name for club mosses and ferns; characterized by windblown spores. segmentation  Repetition of body units as seen in the earthworm. selectively permeable  Property of the plasma membrane that allows some substances to pass, but prohibits the movement of others. semen (seminal fluid)  Thick, whitish fluid consisting of sperm and secretions from several glands of the male reproductive tract. semicircular canal  One of three half-circle-shaped canals of the vertebrate inner ear; contains sensory receptors for rotational equilibrium. semiconservative replication  Process of DNA replication that results in two double helix molecules, each having one parental and one new strand. semilunar valve Valve resembling a half-moon, located between the ventricles and their attached vessels. seminal vesicles  Glands in male humans that secrete a viscous fluid that provides nutrition to the sperm cells. seminiferous tubule Long, coiled structure contained within chambers of the testis, where sperm are produced. senescence  Sum of the processes involving aging, decline, and eventual death of a plant or plant part.

Glossary G-21

sensation  Processing of sensory stimuli that involves detection of nerve impulses by the cerebral cortex of the brain. sensory (afferent) neuron  Nerve cell that transmits nerve impulses to the central nervous system after a sensory receptor has been stimulated. sensory adaptation  Change in the sensitivity of a receptor that usually makes the receptor less sensitive to its stimulus. sensory receptor Structure that receives either external or internal environmental stimuli and is a part of a sensory neuron or transmits signals to a sensory neuron. sensory transduction  Process by which a sensory receptor converts an input to a nerve impulse. sepal  Outermost, leaflike covering of the flower; usually green in color. septate  Having cell walls; some fungal species have hyphae that are septate. septum  Partition or wall that divides two areas; the septum in the heart separates the right half from the left half. serosa  Outer embryonic membrane of birds and reptiles; chorion. serous membrane  Body membrane in the thoracic and abdominal cavities that secretes a watery lubricant. Sertoli cell Male reproductive cells that support, nourish and regulate the sperm-producing cells. sessile  Organism that is permanently attached to a substrate, such as rock. seta  (pl., setae) A needlelike, chitinous bristle in annelids, arthropods, and others. severe combined immunodeficiency (SCID)  Immune disease characterized by the impairment, or lack of, T or B cells in the body. sex chromosome  Chromosomes that differ between the sexes; in humans, these represent the X and Y chromosomes. sex-linked  Trait controlled by a gene on a sex chromosome; often described as either X-linked or Y-linked. sexual reproduction  Reproduction involving meiosis, gamete formation, and fertilization; produces offspring with chromosomes inherited from each parent with a unique combination of genes. sexual selection Changes in males and females, often due to male competition and female selectivity, leading to increased fitness. shared derived traits Used in classification systems, such as cladistics, to determine the evolutionary relationship between two species. shoot system  Aboveground portion of a plant consisting of the stem, leaves, and flowers. short tandem repeat (STR) profiling  Procedure of analyzing DNA in which PCR and gel electrophoresis are used to create a banding pattern; these are usually unique for each individual; process used in DNA barcoding. short-day plant  Plant that flowers when day length is shorter than a critical length—e.g., cocklebur, poinsettia, and chrysanthemum. shrubland  Arid terrestrial biome characterized by shrubs and tending to occur along coasts that have dry summers and receive most of their rainfall in the winter. sickle cell disease  Autosomal recessive genetic disorder that causes a malformation of hemoglobin

molecules, causing red blood cells to form a sickle shape; also sometimes called sickle cell anemia due to the symptoms of the disease. sieve-tube member  Member that joins with others in the phloem tissue of plants as a means of transport for nutrient sap. signal transduction Process that occurs within a cell when a molecular signal (protein, hormone, etc.) initiates a response within the interior of the cell. sink  In the pressure-flow model of phloem transport, the location (roots) from which sugar is constantly being removed. Sugar will flow to the roots from the source. sinus  Spaces within the cranium that reduce the overall weight of the skull. sinusitis  Inflammation of the cranial sinuses in the head. sister chromatid  One of two genetically identical chromosomal units that are the result of DNA replication and are attached to each other at the centromere. skeletal muscle Striated, voluntary muscle tissue that comprises the majority of the muscles in the human body; also called striated muscle. skin  Outer covering of the body; can be called the integumentary system because it contains organs such as sense organs. skull  Common name for facial bones and bones of the cranium. sliding filament model  An explanation for muscle contraction based on the movement of actin filaments in relation to myosin filaments. small interfering RNAs (siRNA)  Short sequences of RNA, typically less than 25 nucleotides, that are involved in posttranscriptional control of gene expression through a process called RNA interference. small intestine In vertebrates, the portion of the digestive tract that precedes the large intestine; in humans, consists of the duodenum, jejunum, and ileum. smooth ER (endoplasmic reticulum)  Membranous system of tubules, vesicles, and sacs in eukaryotic cells; site of lipid synthesis; lacks attached ribosomes. smooth muscle Nonstriated, involuntary muscles found in the walls of internal organs. sodium-potassium pump Carrier protein in the plasma membrane that moves sodium ions out of and potassium ions into cells; important in the function of nerve and muscle cells in animals. soil  Accumulation of inorganic rock material and organic matter that is capable of supporting the growth of vegetation. solute  Substance that is dissolved in a solvent, forming a solution. solution  Fluid (the solvent) that contains a dissolved solid (the solute). solvent  Liquid portion of a solution that serves to dissolve a solute. somatic cell  Body cell; excludes cells that undergo meiosis and become sperm or eggs. somatic system  Portion of the peripheral nervous system containing motor neurons that control skeletal muscles. somatostatin  Hormone, produced by the pancreas, stomach, and small intestine, that inhibits the

effects of growth hormones; also suppresses activity of hormones such as insulin and glucagon. source  In the pressure-flow model of phloem transport, the location (leaves) of sugar production. Sugar will flow from the leaves to the sink. speciation  Origin of new species due to the evolutionary process of descent with modification. species  Group of similarly constructed organisms capable of interbreeding and producing fertile offspring; organisms that share a common gene pool; the taxon at the lowest level of classification. specific epithet In the binomial system of taxonomy, the second part of an organism’s name; it may be descriptive. sperm  Male gamete having a haploid number of chromosomes and the ability to fertilize an egg, the female gamete. spermatogenesis  Production of sperm in males by the process of meiosis and maturation. spicule  Skeletal structure of sponges composed of calcium carbonate or silicate. spinal cord In vertebrates, the nerve cord that is continuous with the base of the brain and housed within the vertebral column. spinal nerve  Nerve that arises from the spinal cord. spindle  Collection of microtubules that assist in the orderly distribution of chromosomes during cell division. spleen  Large, glandular organ located in the upper left region of the abdomen; stores and filters blood. spongy bone Type of bone that has an irregular, meshlike arrangement of thin plates of bone. sporangium  (pl., sporangia) Structure that produces spores. spore  Asexual reproductive or resting cell capable of developing into a new organism without fusion with another cell, in contrast to a gamete. sporophyte  Diploid generation of the alternationof-generations life cycle of a plant; produces haploid spores that develop into the haploid generation. sporozoan  Spore-forming protist that has no means of locomotion and is typically a parasite with a complex life cycle; usually has both sexual and asexual phases. spring overturn Mixing process that occurs in spring in stratified lakes, whereby oxygen-rich top waters mix with nutrient-rich bottom waters. squamous epithelium  Type of epithelial tissue that contains flat cells. stabilizing selection  Outcome of natural selection in which extreme phenotypes are eliminated and the average phenotype is conserved. stamen  In flowering plants, the portion of the flower that consists of a filament and an anther containing pollen sacs where pollen is produced. standard deviation A statistical analysis of data from an observation or experiment; measures how much the data varies. stapes  Bone found in the middle ear that assists in the transmission of sound to the inner ear; also called the stirrup. starch  Storage polysaccharide found in plants that is composed of glucose molecules joined in a linear fashion with few side chains.



G-22

Glossary

stem  Usually the upright, vertical portion of a plant that transports substances to and from the leaves. stem cell  Type of cell that acts as a source for other types of cells; capable of continuously dividing. stereoscopic vision  Vision characterized by depth perception and three-dimensionality. steroid  Type of lipid molecule having a complex of four carbon rings—e.g., cholesterol, estrogen, progesterone, and testosterone. steroid hormone  Type of hormone that has a complex of four carbon rings, but each one has different side chains. stigma  In flowering plants, portion of the carpel where pollen grains adhere and germinate before fertilization can occur. stolon  Stem that grows horizontally along the ground and may give rise to new plants where it contacts the soil—e.g., the runners of a strawberry plant. stomach  In vertebrates, muscular sac that mixes food with gastric juices to form chyme, which enters the small intestine. stomach ulcer  Open sore in the lining of the stomach; frequently caused by the bacteria Helicobacter pylori. stomata  (sing., stoma) Small openings between two guard cells on the underside of leaf epidermis through which gases pass. strata  (sing., stratum) Ancient layer of sedimentary rock; results from slow deposition of silt, volcanic ash, and other materials. striated  Having bands; in cardiac and skeletal muscle, alternating light and dark bands produced by the distribution of contractile proteins. stroke  Condition resulting when an arteriole in the brain bursts or becomes blocked by an embolism; cerebrovascular accident. stroma  Region within a chloroplast that surrounds the grana; contains enzymes involved in the synthesis of carbohydrates during the Calvin cycle of photosynthesis. style  Elongated, central portion of the carpel between the ovary and stigma. submucosa  Tissue layer just under the epithelial lining of the lumen of the digestive tract (second layer). subsidence  Occurs when a portion of Earth’s surface gradually settles downward. substrate  Reactant in an enzyme-controlled reaction. substrate-level ATP synthesis Process in which ATP is formed by transferring a phosphate from a metabolic substrate to ADP. supergroup  Systematic term that refers to the major groups of eukaryotes. surface tension  Force that holds moist membranes together due to the attraction of water molecules through hydrogen bonds. surface-area-to-volume ratio  Ratio of a cell’s outside area to its internal volume; the relationship limits the maximum size of a cell. survivorship  Probability of newborn individuals of a cohort surviving to particular ages. sustainable  Ability of a society or ecosystem to maintain itself while also providing services to humans. suture  Line of union between two nonarticulating bones, as in the skull.

sweat gland  Skin gland that secretes a fluid substance for evaporative cooling; sudoriferous gland. symbiosis  Relationship that occurs when two different species live together in a unique way; it may be beneficial, neutral, or detrimental to one or both species. symbiotic relationship  See also symbiosis. symmetry  Pattern of similarity in an object. sympathetic division Division of the autonomic system that is active when an organism is under stress; uses norepinephrine as a neurotransmitter. sympatric speciation  Origin of new species in populations that overlap geographically. synapse  Junction between neurons consisting of the presynaptic (axon) membrane, the synaptic cleft, and the postsynaptic (usually dendrite) membrane. synapsis  Pairing of homologous chromosomes during meiosis I. synaptic cleft  Small gap between presynaptic and postsynaptic cells of a synapse. synaptonemal complex  Protein structure that forms between the homologous chromosomes of prophase I of meiosis; promotes the process of crossing-over. synovial joint Freely moving joint in which two bones are separated by a cavity. synovial membrane Body membrane that lines synovial joints; secretes synovial fluid that lubricates the joint. syphilis  Sexually transmitted disease caused by the bacterium Treponema pallidum. systematic biology  Another name for the study of systematics, or the study of the evolutionary relationships between species. systematics  Study of the diversity of life for the purpose of understanding the evolutionary relationships between species. systemic circuit  Circulatory pathway of blood flow between the tissues and the heart. systemic lupus erythematosus (SLE)  An autoimmune disease that eventually causes death through kidney failure. systole  Contraction period of the heart during the cardiac cycle.

T

T (traverse) tubules Portions of the sarcolemma (plasma membrane) of a muscle cell that interact with the sarcoplasmic reticulum of the cell. T cell  Lymphocyte that matures in the thymus and exists in four varieties, one of which kills antigenbearing cells outright. T-cell receptor (TCR)  Molecule on the surface of a T cell that can bind to a specific antigen fragment in combination with an MHC molecule. taiga  Terrestrial biome that is a coniferous forest extending in a broad belt across northern Eurasia and North America. taproot  Main axis of a root that penetrates deeply and is used by certain plants (such as carrots) for food storage. taste bud  Structure in the vertebrate mouth containing sensory receptors for taste; in humans, most taste buds are on the tongue. taxon  (pl., taxa) Group of organisms that fills a particular classification category.

taxonomist  Scientist that investigates the identification and naming of new organisms. taxonomy  Branch of biology concerned with identifying, describing, and naming organisms. Tay-Sachs disease  Autosomal recessive genetic disorder that results in a deficiency in the enzyme hexosaminidase A; causes an accumulation of glycolipids in the lysosomes, resulting in a progressive loss of psychomotor functions. technology  Application of scientific information to solve or address a need of humans. telomere  Tip of the end of a chromosome that shortens with each cell division and may thereby regulate the number of times a cell can divide. telophase  Final phase of mitosis; daughter cells are located at each pole. temperate deciduous forest  Forest found south of the taiga; characterized by deciduous trees such as oak, beech, and maple, moderate climate, relatively high rainfall, stratified plant growth, and plentiful ground life. template  Parental strand of DNA that serves as a guide for the complementary daughter strand produced during DNA replication. tendon  Strap of fibrous connective tissue that connects skeletal muscle to bone. terminal bud  Bud that develops at the apex of a shoot. termination  End of translation that occurs when a ribosome reaches a stop codon on the mRNA that it is translating, causing release of the completed protein. testcross  Cross between an individual with a dominant phenotype and an individual with a recessive phenotype to determine whether the dominant individual is homozygous or heterozygous. testes  Male gonads that produce sperm and the male sex hormones. testicular cancer One of several forms of cancer that affect the testes of males; usually characterized by abnormal tenderness or lumps in one of the testicles. testosterone  Male sex hormone that helps maintain sexual organs and secondary sex characteristics. tetanus  Summation of muscle contractions to a level of maximum sustainability. tetany  Severe spasm caused by involuntary contraction of the skeletal muscles due to a calcium imbalance. tetrapod  Four-footed vertebrate; includes amphibians, reptiles, birds, and mammals. thalamus  In vertebrates, the portion of the diencephalon that passes on selected sensory information to the cerebrum. theory  Concept, or a collection of concepts, widely supported by a broad range of observations, experiments, and data. therapeutic cloning  Used to create mature cells of various cell types. Facilitates study of specialization of cells and provide cells and tissue to treat human illnesses. thermoacidophile  Type of archaean that lives in hot, acidic, aquatic habitats, such as hot springs or near hydrothermal vents. thermoreceptor  Sensory receptor that detects heat. threatened species  Species that is likely to become an endangered species in the foreseeable future (e.g., bald eagle, gray wolf, Louisiana black bear).

Glossary G-23

three-domain system Classification system that places all organisms into one of three large domains—Bacteria, Archaea, and Eukarya. thrombin  Enzyme that is involved in blood clotting; acts on fibrinogen molecules to produce fibrin. thrombocyte  Platelet; component of blood that is necessary to blood clotting. thrush  Condition caused by an infection of the oral cavity with the fungi Candida. thylakoid  Flattened sac within a granum of a chloroplast; membrane contains chlorophyll; location where the light reactions of photosynthesis occur. thymine (T)  One of four nitrogen-containing bases in nucleotides composing the structure of DNA; pairs with adenine. thymus  Lymphoid organ involved in the development and functioning of the immune system; T lymphocytes mature in the thymus. thyroid gland  Large gland in the neck that produces several important hormones, including thyroxine, triiodothyronine, and calcitonin. thyroid-stimulating hormone (TSH) Substance produced by the anterior pituitary that causes the  thyroid to secrete thyroxine and triiodothyronine. thyroxine (T4)  Hormone secreted from the thyroid gland that promotes growth and development; in general, it increases the metabolic rate in cells. tight junction  Junction between cells when adjacent plasma membrane proteins join to form an impermeable barrier. tissue  Group of similar cells combined to perform a common function. tissue culture  Process of growing tissue artificially, usually in a liquid medium in laboratory glassware. tone  Continuous, partial contraction of muscle. tonicity  The solute concentration (osmolarity) of a solution compared to that of a cell. If the solution is isotonic to the cell, there is no net movement of water; if the solution is hypotonic, the cell gains water; and if the solution is hypertonic, the cell loses water. tonsillitis  Inflammation of the tonsils—lymphoid tissue located in the pharynx. totipotent  Cell that has the full genetic potential of the organism, including the potential to develop into a complete organism. toxin  Poisonous substance produced by living cells or organisms. Toxins are often proteins that are capable of causing disease on contact with or absorption by body tissues. trachea  (pl., tracheae) In insects, air tube located between the spiracles and the tracheoles. In tetrapod vertebrates, air tube (windpipe) that runs between the larynx and the bronchi. tracheid  In vascular plants, type of cell in xylem that has tapered ends and pits through which water and minerals flow. tract  Bundle of myelinated axons in the central nervous system. trait  A characteristic of an organism; may be based on the physiology, morphology, or the genetics of the organism. trans-fats  Unsaturated fatty acid chains in which the configuration of the carbon–carbon double bonds is such that the hydrogen atoms are across from

each other, as opposed to being on the same side (cis). transcription  First stage of gene expression; process whereby a DNA strand serves as a template for the formation of mRNA. transcription factor  In eukaryotes, protein required for the initiation of transcription by RNA polymerase. transcriptional control  Control of gene expression by the use of transcription factors, and other proteins, that regulate either the initiation of transcription or the rate at which it occurs. transduction  Exchange of DNA between bacteria by means of a bacteriophage. transduction pathway Series of proteins or enzymes that change a signal to one understood by the cell. transfer RNA (tRNA)  Type of RNA that transfers a particular amino acid to a ribosome during protein synthesis; at one end, it binds to the amino acid, and at the other end it has an anticodon that binds to an mRNA codon. transformation  Taking up of extraneous genetic material from the environment by bacteria. transgenic organism  An organism whose genome has been altered by the insertion of genes from another species. transitional fossil  Fossil that bears a resemblance to two groups that in the present day are classified separately. transitional links  Evidence of evolution, typically fossils, that bear a resemblance to two groups that in the present day are classified separately. translation  During gene expression, the process whereby ribosomes use the sequence of codons in mRNA to produce a polypeptide with a particular sequence of amino acids. translational control  Gene expression regulated by influencing the interaction of the mRNA transcripts with the ribosome. translocation  Movement of a chromosomal segment from one chromosome to another nonhomologous chromosome, leading to abnormalities—e.g., Down syndrome. transpiration  Plant’s loss of water to the atmosphere, mainly through evaporation at leaf stomata. transposon  DNA sequence capable of randomly moving from one site to another in the genome. trichinosis  Infection caused by the roundworm Trichinella spiralis. triglyceride  Neutral fat composed of glycerol and three fatty acids; typically involved in energy storage. triplet code  During gene expression, each sequence of three nucleotide bases stands for a particular amino acid. trisomy  Chromosome condition in which a diploid cell has one more chromosome than normal; designated as 2n+1. trochozoan  Type of protostome that produces a trochophore larva; also has two bands of cilia around its middle. trophic level  Feeding level of one or more populations in a food web. trophoblast  Outer membrane surrounding the embryo in mammals; when thickened by a layer

of mesoderm, it becomes the chorion, an extraembryonic membrane. tropical rain forest  Biome found near the equator, characterized by warm weather, plentiful rainfall, high biodiversity, and mainly tree-living animal life. tropism  In plants, a growth response toward or away from a directional stimulus. trypsin  Protein-digesting enzyme secreted by the pancreas. tubal ligation  Surgical form of reproductive sterilization that cuts and seals the uterine tubes in a female. tubular reabsorption Movement of primarily nutrient molecules and water from the contents of the nephron into blood at the proximal convoluted tubule. tubular secretion Movement of certain molecules from blood into the distal convoluted tubule of a nephron so that they are added to urine. tumor  Cells derived from a single mutated cell that has repeatedly undergone cell division; benign tumors remain at the site of origin, while malignant tumors metastasize. tumor suppressor gene  Gene that codes for a protein that ordinarily suppresses the cell cycle; inactivity due to a mutation can lead to a tumor. tundra  Biome characterized by permanently frozen subsoil found between the ice cap and the tree line of regions of the Arctic, just south of the icecovered polar seas in the Northern Hemisphere; also known as the arctic tundra. Alpine tundra, having similar characteristics, is found near the peak of a mountain. tunicate  Type of primitive invertebrate chordate. turgor pressure  Pressure of the cell contents against the cell wall; in plant cells, determined by the water content of the vacuole; provides internal support. tympanic membrane Membranous region that receives air vibrations in an auditory organ; in humans, the eardrum.

U

umbilical cord  Cord connecting the fetus to the placenta, through which blood vessels pass. unsaturated fatty acid Fatty acid molecule that contains double bonds between some carbons of its hydrocarbon chain; thus contains fewer hydrogens than a saturated hydrocarbon chain. upwelling  Upward movement of deep, nutrient-rich water along coasts; it replaces surface waters that move away from shore when the direction of prevailing wind shifts. uracil (U) Pyrimidine base that occurs in RNA, replacing thymine. urea  Main nitrogenous waste of terrestrial amphibians and most mammals. ureter  Tubular structure conducting urine from the kidney to the urinary bladder. urethra  Tubular structure that receives urine from the bladder and carries it to the outside of the body. uric acid  Main nitrogenous waste of insects, reptiles, and birds.



G-24

Glossary

urinary bladder  Organ where urine is stored. urinary system Organ system of humans that includes the kidneys, urinary bladder, and associated structures; excretes metabolic wastes; maintains fluid balance; helps control pH. urine  Liquid waste product made by the nephrons of the vertebrate kidney through the processes of glomerular filtration, tubular reabsorption, and tubular secretion. uterine cycle  Cycle that runs concurrently with the ovarian cycle; it prepares the uterus to receive a developing zygote. uterine tubes  Transport the oocyte (egg) from the ovary to the uterus; location of fertilization; also called the oviducts or fallopian tubes. uterus  In mammals, expanded portion of the female reproductive tract through which eggs pass to the environment or in which an embryo develops and is nourished before birth. utricle  Cavity in the vestibule of the vertebrate inner ear; contains sensory receptors for gravitational equilibrium.

V

Vaccine  Preventative measure that uses antigens prepared in such a way so as to promote immunity without causing the disease. vacuole  Membrane-bound sac, larger than a vesicle; usually functions in storage and can contain a variety of substances. In plants, the central vacuole fills much of the interior of the cell. vagina  Component of the female reproductive system that serves as the birth canal; receives the penis during copulation. vaginal contraceptive ring  Form of contraceptive containing estrogen and progesterone that is inserted into the vagina following menstruation. valence shell  The outer electron shell of an atom. Contains the valence electrons, which determine the chemical reactivity of the atom. vas deferens  Also called the ductus deferens; storage location for mature sperm before they pass into the ejaculatory duct. vascular bundle  In plants, primary phloem and primary xylem enclosed by a bundle sheath. vascular cambium  In plants, lateral meristem that produces secondary phloem and secondary xylem. vascular cylinder In eudicots, the tissues in the middle of a root, consisting of the pericycle and vascular tissues. vascular plant  Plant that has xylem and phloem. vascular tissue  Transport tissue in plants, consisting of xylem and phloem. vasectomy  Surgical form of reproductive sterilization that cuts and seals the vas deferens in a male. vector  In genetic engineering, a means to transfer foreign genetic material into a cell—e.g., a plasmid. vein  Blood vessel that arises from venules and transports blood toward the heart.

vena cava  Large systemic vein that returns blood to the right atrium of the heart in tetrapods; either the superior or inferior vena cava. ventilation  Process of moving air into and out of the lungs; breathing. ventral cavity One of two main body cavities in humans; contains the thoracic, abdominal, and pelvic cavities. ventricle  Cavity in an organ, such as a lower chamber of the heart or the ventricles of the brain. venule  Vessel that takes blood from capillaries to a vein. vernix caseosa  White, greasy covering of the developing fetus; protects the fetus’ skin from the amniotic fluid. vertebral column  Portion of the vertebrate endoskeleton that houses the spinal cord; consists of many vertebrae separated by intervertebral disks. vertebrate  Chordate in which the notochord is replaced by a vertebral column. vertigo  Equilibrium disorder that is often associated with problems in the receptors of the semicircular canals in the ear. vesicle  Small, membrane-bound sac that stores substances within a cell. vessel element  Cell that joins with others to form a major conducting tube found in xylem. vestibule  Space or cavity at the entrance to a canal, such as the cavity that lies between the semicircular canals and the cochlea. vestigial structure  Remnant of a structure that was functional in some ancestor but is no longer functional in the organism in question. villus  Small, fingerlike projection of the inner small intestinal wall. viroid  Infectious strand of RNA devoid of a capsid and much smaller than a virus. virus  Acellular parasitic agent consisting of an outer capsid of protein and an inner core of nucleic acid (DNA or RNA). visceral mass  Internal organs of an organism; typically includes components of the digestive, reproductive and urinary systems (if present). visual accommodation  Ability of the eye to focus at different distances by changing the curvature of the lens. vitamin  Organic nutrient that is required in small amounts for metabolic functions. Vitamins are often part of coenzymes. viviparous  Animal that gives birth after partial development of offspring within mother. vocal cord  In humans, fold of tissue within the larynx; creates vocal sounds when it vibrates. vulva  Common name for the external genital organs of a female.

W

water (hydrologic) cycle  Interdependent and continuous circulation of water from the ocean, to the atmosphere, to the land, and back to the ocean. water mold Filamentous organisms having cell walls made of cellulose; typically decomposers

of dead freshwater organisms, but some are parasites of aquatic or terrestrial organisms. water vascular system  Series of canals that takes water to the tube feet of an echinoderm, allowing them to expand. wax  Sticky, solid, water-repellent lipid consisting of many long-chain fatty acids usually linked to long-chain alcohols. wetland  Area that is covered by water at some point in the year. See also bog, marsh, or swamp. white blood cell  Leukocyte, of which there are several types, each having a specific function in protecting the body from invasion by foreign substances and organisms. white matter  Myelinated axons in the central nervous system. whorl  Cluster of branches, or other plant structures, that occurs in a circular pattern. wobble hypothesis  Ability of the tRNAs to recognize more than one codon; the codons differ in their third nucleotide. wood  Secondary xylem that builds up year after year in woody plants and becomes the annual rings.

X

X-linked  Allele that is located on an X chromosome; not all X-linked genes code for sexual characteristics. xylem  Vascular tissue that transports water and mineral solutes upward through the plant body; it contains vessel elements and tracheids.

Y

yeast  Unicellular fungus that has a single nucleus and reproduces asexually by budding or fission, or sexually through spore formation. yolk  Dense nutrient material in the egg of a bird or reptile. yolk sac One of the extraembryonic membranes that, in shelled vertebrates, contains yolk for the nourishment of the embryo, and in placental mammals is the first site for blood cell formation.

Z

zero population growth  No growth in population size. zooplankton  Part of plankton containing protozoans and other types of microscopic animals. zoospore  Spore that is motile by means of one or more flagella. zygospore  Thick-walled resting cell formed during sexual reproduction of zygospore fungi. zygospore fungi  Fungi, such as black bread mold, that reproduce by forming windblown spores in sporangia; sexual reproduction involves a thickwalled zygospore. zygote  Diploid cell formed by the union of two gametes; the product of fertilization.

INDEX Note: Page numbers in italics indicate material presented in figures and tables.

A

ABA (abscisic acid), 184–185, 185 A bands, 378 Abdomen crayfish, 643 grasshopper, 645 Abdominal cavity, 197, 197 Abduction, of muscles, 375 Abiotic components of ecosystems, 707 ABO blood types, 476, 477 ABO system, 247 Abscisic acid (ABA), 184–185, 185 Abscission, 184 Abstinence, 422 Abyssal zone, 739, 739, 740 Accessory fruits, 176 Accessory organs of digestive system, 260–261, 260–261 of skin, 202, 202 Acetabulum, 371, 371 Acetylcholine (ACh), 318, 319, 329, 337, 378, 460 Acetylcholinesterase (AChE), 318, 319, 337 Acetyl groups, 119 Achenes, 176 Achromatopsia, 550, 550–551 Acid-base balance, 299, 306–307, 307 Acid-base buffer systems, 306 Acid deposition, 752 Acidosis, 306 Acids, 27–28, 28 Acne, 203 Acquired characteristics, inheritance of, 537 Acromegaly, 405, 405 Acromion process, 370 Acrosomal enzymes, 441 Acrosomes, 415, 441, 441 ACTH. See Adrenocorticotropic hormone Actin, 378 Actin filaments, 49–50, 56, 56–57 Action potential, 316, 316–317 Activation energy of, 104–105, 105 of enzymes, 106–107 Active immunity, 241, 241–242 Active sites, of enzymes, 105, 105 Active transport, 67, 71–72, 72 Acute bronchitis, 291, 292 Acute diseases, 207 AD (Alzheimer disease), 37, 335, 335, 460 Adaptation as characteristic of life, 5 of ferns, 614 of flowering plants to land environments, 170 to high elevations, 6 natural selection and, 5, 538 of nonvascular plants, 611 Adaptive immunity, 235, 237–241, 238–240 Adaptive radiation, 556 Addison disease, 406, 407 Adduction, of muscles, 375 Adductor longus, 376–377 Adductor muscles, 636 Adenine, 38, 38, 39 Adenoids, 255, 290 Adenosine diphosphate (ADP), 39, 39, 102 Adenosine monophosphate (AMP), 394 Adenosine triphosphate (ATP) coupled reactions and, 102 cycle of, 102, 103 energy for cells and, 102–103, 103 in muscle contraction, 380, 382–383, 382–383 production of, 120, 134

protobiont evolution and, 568 reaction process, 39, 39 structure of, 102 substrate-level synthesis of, 116, 119 ADH (antidiuretic hormone), 306, 393, 395, 405 Adhesion, 26, 162 Adhesion junctions, 75–76, 76, 191 Adipocytes, 193 Adipose tissue, 193, 193–194 ADP (adenosine diphosphate), 39, 39, 102 Adrenal cortex, 393, 399, 399, 400 Adrenal gland disorders, 406, 407 Adrenal glands, 393–394, 399–400, 399–401 Adrenal medulla, 393, 399, 399, 400 Adrenocorticotropic hormone (ACTH), 393, 396, 399, 400, 405 Adrenoleukodystrophy (ALD), 54 Adult stem cells, 217 Advanced Energy Recovery System (AERS), 743, 762 Adventitious roots, 152 AEDs (automatic external defibrillators), 222 Aerobic metabolism, 113, 114 AERS (Advanced Energy Recovery System), 743, 762 Aeshna, 644 AF (atrial fibrillation), 222 Afferent arterioles, 301 African sleeping sickness, 593–594, 594 Afterbirth, 453, 456 Agar, 587 Age distributions, 695 Age-structure diagrams, 695, 695 Agglutination, 247, 247 Aggregate fruits, 176 Aging cardiovascular system and, 458–459 damage accumulation hypothsis on, 458 defined, 457 digestive system and, 459 endocrine system and, 460–461 excretory system and, 459 factors influencing, 458 hormones and, 458 immune system and, 459 integumentary system and, 458 of leaves, 184 life expectancy and, 440, 458 musculoskeletal system and, 460 nervous system and, 459 reproductive system and, 461 respiratory system and, 459 sensory systems and, 459 Agranular leukocytes, 213, 215 Agricultural value of species, 747 Agriculture biological pest control for, 747 plants and, 180–181, 181 sustainable, 760–761, 761 transgenic crops, 180–181, 181 Agriculture Department, U.S. (USDA), 34, 263 Agrobacterium tumefaciens, 573 AHA (American Heart Association), 226 AID (artificial insemination by donor), 436 AIDS. See HIV/AIDS Alagille syndrome, 492, 492 Alanine, 36 Albumin, 37, 214, 660 Alcohol birth defects and, 454 cancer and, 515 cardiovascular disease and, 226 fermentation of, 118–119 nervous system and, 332 ALD (adrenoleukodystrophy), 54 Aldosterone, 305, 393, 400, 400

Algae blue-green algae, 572 brown, 588, 588 characteristics of, 572 diesel power from, 136 green, 585–587, 607 photosynthesis and, 128 red, 587–588, 588 Alien species, 751, 751 Alkalosis, 306 Allantois, 448, 448, 450, 660 Alleles defined, 465 dominant, 467, 468, 468 frequency of, 546–547, 547, 549, 549 multiple allele inheritance, 476 recessive, 467, 468 sex-linked, 484, 484–485 X-linked, 484, 484–485 Allen’s rule, 670 Allergens, 245 Allergies, 245–246, 245–246, 576 Allopatric speciation, 555–556, 556 All-or-none law, 384 Alpha cells, 401 Alpine tundra, 726 ALS (amyotrophic lateral sclerosis), 337, 337 Alternation of generations, 170, 607, 609, 609 Altman, Sidney, 567 Altruism, 686–687, 686–687 Alveoli, 282 Alzheimer disease (AD), 37, 335, 335, 460 Amanitas, 598, 600, 600 Amborella trichopoda, 618 Ambystoma, 655 American Heart Association (AHA), 226 AM fungi, 601 Amino acids, 35, 36, 266 Amino group, 29 Ammonia in evolution of monomers, 566 in urinary system, 299, 306 Amniocentesis, 455, 490 Amnion, 448, 448, 449, 660 Amniotic cavity, 450 Amniotic egg, 655, 659, 660 Amoebic dysentery, 595 Amoeboid-related diseases, 595 Amoeboids, 590 Amoebozoans, 585, 590–592, 591 AMP (adenosine monophosphate), 394 Amphibians circulation in, 658, 659 decline of, 753, 754 evolution of, 658–659, 658–659 in phylogenetic tree of chordates, 653 Ampulla, 354, 648 AMU (atomic mass unit), 19 Amygdala, 324, 324 Amyloplasts, 55 Amyotrophic lateral sclerosis (ALS), 337, 337 Anabolic steroids, 402, 402, 416 Anabolism, 102 Anaerobic metabolism, 113, 114 Analogous structures, 543 Anaphase, 85, 87, 93–94 Anaphase I, 90, 90–91, 93–94 Anaphase II, 91, 92, 94 Anaphylactic shock, 245 Anatomical data on animals, 625–628, 627–628 Anatomical evidence, for evolution, 543, 543–544 Anatomical terminology, 197 Anchor cells, 446 Androgen insensitivity, 513

Androgens, 393, 402 Anemia, 214 Aneurysm, 227 Angina pectoris, 227 Angiogenesis, 516 Angioplasty, 227–228, 228 Angiosperms, 144, 170, 607, 608, 618–621. See also Flowering plants Angiotensin, 305, 400 ANH (atrial natriuretic hormone), 305, 401 Animalcules, 634 Animal pole, 442 Animals. See also specific animals anatomical data on, 625–628, 627–628 cell anatomy of, 49 cell surfaces in, 75–76, 75–76 characteristics of, 7, 8, 626 communication in, 678–680, 679–680, 682, 682 cytokinesis in, 85, 88, 88 developmental stages of, 442–443, 442–444 embryonic development of, 626, 628, 628 emotions in, 681 evolutionary trends among, 625–628, 625–628 fear in, 681 joy in, 681 meiosis in, 90, 92 mitosis in, 85, 86–87 pharming, 656 phylogenetic tree of, 627 symmetry in, 625–626 transgenic, 526, 527 xenotransplantation from, 656 Annelids, 627, 638–640, 638–640 Annual rings, 156, 158, 158 Anorexia nervosa, 272, 272–273 Anosmia, 354, 459 Antagonistic hormones, 393 Antagonistic pairs, 375, 375 Antedon, 647 Antennae, crayfish, 643 Anterior, defined, 197 Anterior ganglion, in clam, 637 Anterior pituitary, 396 Anther cultures, 179 Antheridia, 610 Anthers, 171, 619 Antibiotics for bacterial diseases, 575–576 history of, 576 resistance to, 535, 553, 560, 575–576 Antibodies defined, 238 monoclonal, 244, 244–245 structure of, 238–239, 239 types of, 239, 239 Antibody-mediated immunity, 238 Antibody titer, 241 Anticodons, 504, 504 Antidiuresis, 306 Antidiuretic hormone (ADH), 306, 393, 395, 405 Antifungal drugs, 602 Antigen, defined, 237 Antigenic drift, 579, 579 Antigenic shift, 579, 579 Antigen-presenting cells (APCs), 239 Antigen receptors, 237–241, 238–240 Antimicrobial agents, 107 Antioxidants, 127, 140, 269 Antiparallel DNA strands, 499 Antipredator defenses, 698–700, 700 Antiretroviral drugs, 427 Antiviral drugs, 580 Ants, 686, 686

I-1

I-2

Index

Anus crayfish, 643 human, 259 sea star, 648 Anxiety, cardiovascular disease and, 226 Aorta function of, 211, 219, 224 grasshopper, 645 Aortic bodies, 286 Aortic semilunar valve, 219 APC gene, 83 APCs (antigen-presenting cells), 239 Aphids, 164, 165 Aphonopelma, 642 Apical dominance, 183, 183 Apical meristem, 144, 154, 154–155 Apis, 644 Aplysina fistularis, 629 Apoptosis cancer and, 514 cell cycle and, 80, 81, 81 in development, 448 in thymus, 235 Appendicular skeleton, 366, 366, 370–371, 370–371 Appendix, 259 Apple blossom, 618 Aquaporins, 67, 304 Aquatic ecosystems, 732–737, 733–737, 739, 739–740 Aqueous humor, 348 Aquifers, 713, 760 Arachidonate, 403 Arachnids, 645–647, 646 Archaea characteristics of, 6, 7, 8 comparison with Eukarya, 569, 569 as domain, 6–8, 7, 559, 560 halophilic, 569, 569–570 methanogenic, 570, 570 size and structure of, 569 structural features of, 47 thermoacidophilic, 570, 570 types of, 569, 569–570 Archaeplastids, 585–588, 587–588 Archegonia, 610 Archenteron, 442, 443 Arctic tundra, 726 Ardipithecines, 664 Armadillos, 746, 746 Army ants, 686, 686 Arrhythmias, 222 ART (assisted reproductive technology), 412, 436 Arteries, 211, 211 Arterioles, 211 Arteriovenous shunts, 212 Arthritis, 250, 386, 386, 400 Arthropods, 641–647 arachnids, 645–647, 646 characteristics, 641–643, 642 crustaceans, 643, 643 diversity of, 642 exoskeleton in, 641 eyes of, 641 insects, 643–645, 644–645 metamorphosis in, 643, 645 nervous system in, 641 in phylogenetic tree, 627 respiratory organs in, 641, 643 segmentation in, 641 Articular cartilage, 363 Artificial hearts, 228, 228 Artificial insemination by donor (AID), 436 Artificial kidney machines, 307, 307 Artificial lungs, 288 Artificial selection, 544–545, 545 Ascaris lumbricoides, 640, 641 Ascending colon, 259 Ascocarp, 597 Asexual reproduction, in plants, 178–181, 179–181 Asian carp, 690, 703 Assembly, viral, 578 Assisted reproductive technology (ART), 412, 436

Association areas, 321, 322 Associative learning, 678 Assortment, independent, 90, 91, 470–471 Asters, 85 Asthma, 245, 279, 292, 292, 294 Astigmatism, 355, 355 Astrocytes, 196 Asymmetrical animals, 625–626 Atherosclerosis, 225 Athletes metabolism and, 113 muscle contraction and, 384–385, 385 Athlete’s foot, 601, 602 Atlas, 368 Atomic mass, 19 Atomic mass unit (AMU), 19 Atomic number, 19 Atomic structure, 18–19, 19 Atoms, 2, 3, 18 ATP. See Adenosine triphosphate ATP synthase complex, 121, 133 Atria, 219 Atrial fibrillation (AF), 222 Atrial natriuretic hormone (ANH), 305, 401 Atrioventricular bundle, 221 Atrioventricular node, 221 Atrioventricular valves, 219 Atrophy, 385 Attachment, viral, 578 Auditory association area, 321 Auditory canal, 351, 351, 352 Auditory communication, 679, 679–680 Auditory cortex, 352 Auditory pathway, 352–353 Auditory tubes, 291, 351, 351 Australopithecines, 666, 666 Autism, 242, 285 Autodigestion, 53 Autoimmune disorders, 248, 250, 313 Automatic external defibrillators (AEDs), 222 Autonomic system, 328–329, 330–331 Autosomal disorders, 472–475, 473–475 Autosomes, 484 Autotrophs, 128, 128, 707 Autumn, colors of, 127, 140 Auxins, 183, 183–184 Avery, Oswald, 498 Avian influenza, 14, 580 Axial skeleton, 366, 366–369, 367–370 Axillary buds, 148, 153 Axillary nodes, 235 Axons, 314, 315, 327 Axon terminal, 317

B

Baboons, 680, 680, 684 Bacilli, 571, 571 Bacteria antibiotic resistance in, 535, 553, 560, 575–576 binary fission in, 572, 572 characteristics of, 6, 7, 8 chromosomes in, 572 diseases caused by, 573–575, 574–575 as domain, 6–8, 7, 559, 560 drugs for, 575–576 in early investigations of DNA, 497 flagella of, 46, 47, 572 “flesh-eating,” 574, 574 in food poisoning, 575 gene transfer in, 572 gram-negative, 571l gram-positive, 571 metabolism in, 572–573, 573 reproduction in, 572, 572 size and structure of, 46–47, 46–47, 571, 571–572 STDs caused by, 430–432 transgenic, 524–526, 526 Bacterial flora, 264 Bacterial septicemia, 553 Bacteriophages, 578 Bacteriorhodopsin, 570 Balance disorders, 356, 358 Balanced polymorphisms, 554

Balanus balanoides, 697, 697 Bald eagles, 661, 745, 745 Baleen whales, 739, 739 Ball-and-socket joints, 373, 374 Ball-and-stick models, 24, 24 Bamboo, 157, 166 Bangham, Alec, 568 Bannertail kangaroo rats, 732 Barbary fig cactus, 618 Bark, 156 Barnacles, 697, 697 Barr body, 490, 509 Basal cell carcinoma, 201, 201 Basal cells, 202 Basal nuclei, 323 Basement membranes, 191 Base pairing, 39, 499, 499 Bases, 27–28, 28 Base substitutions, 512, 512 Basic local alignment search tool (BLAST), 532 Basidia, 598 Basilar membrane, 352 Basophils, 213, 215 Batesian mimicry, 698–699 Bath salts, 334 Bathypelagic zone, 739, 739 Bats, 99, 110, 173, 662 B-cell receptors (BCRs), 238, 238 B cells, 215, 235, 237, 238–239 BCRs (B-cell receptors), 238, 238 Bears, 745, 745 Becquerel, Antoine-Henri, 20 Bees, 173, 619, 644, 680, 682, 682, 747 Begging behavior, 677 Behavior animal communication and, 678–680, 679–680, 682, 682 associative learning and, 678 classical conditioning and, 678, 678 defined, 674 environmental influences on, 675–678, 677–678 fitness and, 682–687, 683–684, 686–687 genetic influences on, 674–675, 674–675 nurturing, 674, 674 operant conditioning and, 678 response to stimuli and, 5 social interactions and, 677 twin studies and, 676 Benign cancer, 514, 514 Benign positional vertigo (BPV), 358 Benign prostatic hyperplasia (BPH), 432–433, 433 Benthic division, 734, 734, 739–740 Bergmann’s rule, 670 Beta cells, 401 Beta-hexosaminidase A, 43 Bicarbonate ions, 286 Biceps brachii, 376–377 Bicuspid valve, 219 Bifidobacterium, 576 Bilateral symmetry, 626 Bile, 261 Bilirubin, 214, 261 Binary fission, 572, 572 Binge eating disorder, 273 Biochemical evidence, for evolution, 544, 544 Biodiversity biogeochemical cycles and, 748 defined, 744 disease and, 753, 754 habitat loss and, 13, 13–14, 749, 749–751 number of catalogued species, 744, 744 preservation of, 166 threats to, 749, 749–754, 751–754 types of, 745–746 value of, 746, 746–749 Biodiversity hotspots, 754 Bioethical considerations cloning, 521 DNA fingerprinting, 524 emotions in animals, 681 genetic testing for cancer genes, 83 HIV/AIDS vaccine, 429 human growth hormone, 397

plants for cleaning up toxic messes, 163 vaccinations for children, 242 Biofuels, 136 Biogeochemical cycles, 710–719 biodiversity and, 748 carbon cycle, 717, 717–718 exchange pools and, 710 human activities and, 713, 714, 717, 718 models for, 710–711, 711 nitrogen cycle, 714–715, 715, 717 phosphorus cycle, 713, 714, 714 reservoirs and, 710, 718 water cycle, 711–713, 713 Biogeography, evolution and, 540, 542, 542 Bioinformatics, 532, 532 Biological organization, 2, 3 Bioluminescence, 630 Biomass, 710 Biomes, 725–726, 725–726 Biomolecules, 29, 566 Biosphere, 3, 4, 691 Biosynthesis, 578 Biotechnology bioinformatics, 532, 532 diabetes and, 519, 532 genetically modified organisms and, 169, 180–182, 187, 524–525 genomics, 530, 531 proteomics, 531–532 Biotic components of ecosystems, 707–708, 707–708 Biotic potential, 692, 692 Biotin, 269 Bipedalism, 664 Bird flu, 14, 580 Birds anatomy of, 660, 661 diversity of, 660, 661 learning in, 675, 677 pollination by, 173 as reptiles, 660 Birdwing butterfly, 644 Birth, 455–457, 456–457 Birth control, 422–424, 423 Birth control pills, 422, 423 Birth defects, 454, 455 Birth weight, stabilizing selection and, 551–552, 552 Bison, 730, 730–731 Bivalves, 636–638, 637 Black mold, 602 Bladder. See Gallbladder; Urinary bladder Bladder cancer, 309 Bladder stones, 309, 309 Blades, of plants, 148 BLAST (basic local alignment search tool), 532 Blastocoel, 442 Blastocysts, 449, 449 Blastopores, 442, 628 Blastula, 442, 628 Bleaching, coral, 630 Blending concept of inheritance, 466 Blindness causes of, 355–356 color, 354–355, 355, 485, 550, 550–551 Blind spot, of eye, 349–350, 350 Blood body fluids related to, 217–218, 218 in cardiovascular system, 212–218, 212–218 clotting, 215–216, 216 composition of, 212–214, 213 as connective tissue, 194, 194–195 formation in red bone marrow, 217, 217 formed elements in, 213, 214 path through heart, 219–220 pH of, 28 plasma, 194, 194–195, 212, 214 Blood clots, 6, 228 Blood fluke, 634 Blood glucose regulation, 401, 401 Blood pressure, 224, 224–225, 400, 400. See also Hypertension Blood type inheritance, 476, 477 Blood-type reactions, 247–248, 247–248

Index I-3 Blood vessels, 211, 211–212 Blowflies, 646 “Blue baby,” 453 Blue flag iris, 618 Blue-green algae, 572 BMI (body mass index), 263–264, 264 Bobcats, 728 Body cavities, 197, 197 Body mass index (BMI), 263–264, 264 Body membranes, 197, 197–198 Body stalk, 450 Body temperature regulation, 204–205, 205 Bohr, Niels, 22 Bohr model of atoms, 22, 22 Bonding, chemical, 23–25, 23–25 Bone marrow blood formation in, 217, 217 red, 217, 217, 234, 363 stem cells, 217, 217 yellow, 363, 364 Bones. See also specific bones of appendicular skeleton, 366, 366, 370–371, 370–371 of axial skeleton, 366, 366–369, 367–370 classification of, 363, 366, 366 compact, 193, 194, 363, 364 defined, 194 disorders of, 386, 386 fractures, 386 functions of, 366 growth and development of, 365, 365 ossification of, 365, 365 remodeling of, 365 spongy, 194, 363, 364 structure of, 363, 364 Bony fishes, 658, 658 Book lung, 643 Botox®, 380, 381 Bottleneck effect, 549, 549 Botulinum toxin, 381 Botulism, 381, 575 Bovine spongiform encephalopathy (BSE), 580–581 Bowman’s (glomerular) capsule, 301–302 BPH (benign prostatic hyperplasia), 432–433, 433 BPV (benign positional vertigo), 358 Brachial artery, 224 Brachiopods, 633 Brain auditory pathway to, 352–353 crayfish, 643 diseases and disorders of, 335–336 earthworm, 639 fish, 657 grasshopper, 645 human, 320–323, 321–322 odor information and, 346–347 taste information and, 345 visual pathway to, 348–351 Brain stem, 323 BRCA genes, 79, 83–84, 96 “Breaking water,” 455 Breast cancer, 79, 83, 84, 96 Breast-feeding, 457 Breasts, lactation and, 456–457, 457 Breathing, 283–286, 284–286 Brittle stars, 647 Broca’s area, 323, 325 Bronchi, 282 Bronchial tree, 282 Bronchioles, 282 Bronchitis, 291–292, 292 Brown algae, 588, 588 Bryophytes, 610 Bryozoans, 633 BSE (bovine spongiform encephalopathy), 580–581 Budding, viral, 578 Bud scales, 153 Buffers, 28, 306 Bulbourethral glands (Cowper glands), 413, 413 Bulimia nervosa, 272, 272–273 Bulk transport, 72–73, 73–74 Bull moose, 727, 727 Bundle scars, 153

Bundle sheaths, 159 Bursae, 372 Bursitis, 373, 400 Butterflies, 173, 644, 729 Butterfly weed, 618 Byssal threads, 735

C

Cadherins, 75 Caenorhabditis elegans, 446, 446, 448, 458, 640 Caffeine, 332, 333 Cairns-Smith, Alexander, 569 Calcaneus, 371, 371 Calcitonin, 393, 398 Calcitriol, 269 Calcium, 270, 271, 380, 382, 398, 398 California drought, 712 Calories, defined, 25 Calvin, Melvin, 135 Calvin cycle reactions, 130, 133, 135–137 Calyx, 619 Cambarus, 643, 643 Cambium, 154–156 Cambrian period, 541 CAM photosynthesis, 139 Canadian lynx, 697–698, 698 Canaliculi, 363 Cancer alcohol and, 515 angiogenesis and, 516 apoptosis and, 514 benign, 514, 514 bladder, 309 breast, 79, 83, 84, 96 cell characteristics in, 514, 516 cell cycle and, 82–83, 82–84 cervical, 428 colon, 83, 275 defined, 207 diet and, 515 exercise and, 515 free radicals and, 127, 140 genetic mutations and, 513, 513–514, 516 genetic testing for cancer genes, 83 hormone therapy and, 515 HPV and, 515 immune therapies for, 244 leukemias, 207, 215, 746 lung, 293, 294, 294 malignant, 514, 514 melanoma, 65, 201, 201 metastasis of, 235, 516 occupational hazards and, 515 oncogenes and, 83, 83, 513, 514 ovarian, 83, 84, 433–434 pancreatic, 275 p53 gene and, 83, 514 prevention strategies, 515 progression of, 513, 513 prostate, 433 proto-oncogenes and, 82–83, 82–83, 513, 514 from radiation, 20 radon and, 515 skin, 201 smoking and, 293, 515 sun exposure and, 515 testicular, 433 testing for, 515 tumor suppressor genes and, 82–83, 82–83 weight and, 515 X-rays and, 515 Candida albicans, 602 Candidiasis, 249 CAPD (continuous ambulatory peritoneal dialysis), 308–309 Capillaries, 191, 211–212, 211–212 Capillary beds, 211, 212 Capillary exchange, 217–218, 218 Capitulum, 370 Capsaicin, 63, 76 Capsids, 577, 577 Capsule of bacteria, 46, 47 of nonvascular plants, 611

Carapace, crayfish, 643 Carbaminohemoglobin, 289 Carbohydrates complex, 30–31, 31, 265 defined, 30 in diet, 34, 265–266, 265–266 disaccharides, 30, 30 metabolic fate of, 123 monosaccharides, 30, 30 polysaccharides, 30–31, 31 in sports drinks, 17 Carbon emissions and climate change, 719 isotopes of, 19, 20 in organic molecules, 28 Carbon cycle, 717, 717–718 Carbon dioxide fixation, 134–137, 135 Carbon dioxide reduction, 135, 135, 136–137 Carbonic acid, 28 Carbonic anhydrase, 287 Carboniferous period, 541, 606, 612, 613 Carbonyl group, 29 Carboxyl group, 29 Carcinogenesis, 82, 513 Carcinomas, 207 Cardiac cycle, 220, 221 Cardiac muscle, 195, 196 Cardiac output, 220 Cardiac stomach, 648 Cardinals, 661 Cardiovascular disease (CVD), 225–228, 267 Cardiovascular system, 210–228 aging and, 458–459 arteries of, 211, 211 blood in, 212–218, 212–218 blood vessels in, 211, 211–212 capillaries of, 191, 211–212, 211–212 diseases and disorders of, 210, 215–216, 225–228, 225–228 heart and, 218–222, 219–222 pathways of, 222–224, 223–225 structure and function of, 198, 199 veins in, 211, 212 Caribou, 726, 727 Carnivores, 707, 707 Carotenoids, 127, 128, 140 Carotid bodies, 286 Carpal bones, 366, 370, 370 Carpels, 171, 619 Carpooling, 761 Carrier proteins, 66, 66, 67, 70–71, 71–72 Carruthers, Jean, 381 Carrying capacity, 693 Carson, Rachel, 435 Cartilage articular, 363 defined, 194 elastic, 194, 363 fibrocartilage, 194, 363 hyaline, 193, 194, 363, 364 Cartilaginous fishes, 653, 655, 657–658 Cartilaginous joints, 372 Casparian strips, 151, 151 Caspases, 81 Cassini-Huygens space probe, 1 Catabolism, 102 Catalase, 54 Catalysts, 104 Cataracts, 356, 356 Cavities, body, 197, 197 CCD (colony collapse disorder), 747 CCK (cholecystokinin), 259 Cech, Thomas, 567 Cecum, 259 Celiac disease, 246 Cell body, of neurons, 314, 315 Cell cycle apoptosis and, 80, 81, 81 cancer and, 82–83, 82–84 control of, 81–84, 82–83 interphase in, 80, 80–81 mitotic stage of, 81 overview, 80, 80 Cell-mediated immunity, 240, 241 Cell plates, 88

Cell recognition proteins, 66, 66 Cells. See also Eukaryotes; Prokaryotes anatomy of, 49 anchor, 446 animal, 49 cancer, 514, 516 collar, 629 communication between, 65, 65 defined, 2, 3, 4, 44 dendritic, 215, 236 entropy and, 101, 101–102 epitheliomuscular, 632 evolution of, 58–60, 60, 568–569 extracellular matrix, 75, 75 fermentation in, 382–383 flame, 633 germ, 511 guard, 145, 163 junctions between, 75–76, 76 passage of molecules into and out of, 67, 67 plant, 50, 144–146, 144–146 Schwann, 196, 315, 315 size of, 44, 44, 46 somatic, 80, 511 stem, 217, 217, 521 totipotent, 179, 217 Cell signaling, 65, 65 Cell surface modifications, 74–76, 75–76 Cell theory, 11, 44 Cellular differentiation cytoplasmic segregation and, 445 defined, 444 factors influencing, 444–445 induction and, 445–446, 446 Cellular level of organization, 44, 44, 46, 626, 628 Cellular respiration, 113–124, 124 citric acid cycle and, 114, 115, 119, 120 defined, 55, 113, 114 electron transport chain and, 114–115, 115, 119–121, 121–122 equation for, 55 FAD and, 114 fermentation and, 118, 118–119 glycolysis and, 114, 115, 116, 117 in humans, 110, 110 mitochondria and, 109, 109–110 in mitochondria and, 119–121, 120–122, 124 muscles and, 383, 383 NAD+ and, 114, 114 overview, 114, 115 phases of, 114–115, 115 photosynthesis vs., 139, 140 preparatory reaction and, 114, 115, 119 pyruvate and, 114, 115 Cellular slime molds, 592 Cellular stages of development, 442, 442 Cellulose, 31, 31, 48 Cell walls of bacteria, 46, 47 in eukaryotes, 48, 48 in plants, 50, 76 Cenozoic era, 541 Centenarians, 440 Centipede, 642 Central canal, 319, 363 Central disk, 648 Central dogma of genetics, 568 Central nerve ring, 648 Central nervous system (CNS), 314, 319–323, 319–323 Central sulcus, 321 Central vacuoles, 50 Centrioles, 48–49, 58, 58, 85 Centrosomes, 49–50, 58, 84, 85 Cephalization, 626, 655 Cephalopods, 636, 637 Cerebellum, 321, 323 Cerebral cortex, 321 Cerebral hemispheres, 321, 321 Cerebrospinal fluid, 319 Cerebrum, 321–322, 321–323 Cervical cancer, 428 Cervical caps, 423, 423



I-4

Index

Cervical vertebrae, 368, 369 Cervix, 416–417, 417 Cervus elaphus, 684 Cesium, 21 CF. See Cystic fibrosis CFCs (chlorofluorocarbons), 752 Chagas disease, 593 Chalk sponges, 629 Chameleons, 729 Channel proteins, 66, 66, 67 Chaparral, 730, 730 Chara, 607, 607 Charcot-Marie-Tooth disease, 512 Charophytes, 586, 608 Chase, Martha, 498 Chelicera, 644 Chemical bonding, 23–25, 23–25 Chemical communication, 679, 679 Chemical cycling, 4, 4 Chemical digestion, 254 Chemical energy, 100, 100 Chemical messengers, 65 Chemical senses, 345–347, 346 Chemiosmosis, 121, 134 Chemistry, 18–29 acids and bases, 27–28, 28 atomic structure, 18–19, 19 electrons and, 18–19, 19, 22, 22 molecules and compounds in, 22–25, 23–25 neutrons and, 18–19, 19 organic molecules in, 28–29, 29 periodic table, 19, 19–20 protons and, 18–19, 19 of water, 25–28, 26–28 Chemoautotrophs, 568 Chemoreceptors, 342, 342 Chemosynthesis, 568 Chewing tobacco, 293 Chick embryos, 448, 448 Chickenpox, 580 Childbirth, 455–457, 456–457 Children birth weight of, 551–552, 552 vaccinations for, 242 Chili peppers, 63, 76 Chimpanzees, 679–680 Chipmunks, 728, 728 Chitin, 31, 48, 596 Chitons, 636 Chlamydia, 430, 431 Chlamydia trachomatis, 430 Chlamydomonas, 586–587, 587 Chloride, 270, 271 Chlorofluorocarbons (CFCs), 752 Chlorophylls, 55, 127, 128, 131, 140 Chlorophytes, 586 Chloroplasts in cell anatomy, 50 composition and function of, 48 energy and, 54, 55 evolution of, 59–60 photosynthesis and, 109 structure of, 55, 55 Choanoflagellates, 592, 592 Cholecystokinin (CCK), 259 Cholera, 381 Cholesterol cardiovascular disease and, 226, 267 functions of, 34–35 in plasma membrane, 64 structure of, 34, 35 transport of, 214 Cholinesterase, 460 Chordae tendineae, 219 Chordates, 627, 653–654, 653–654 Chorion, 448, 448, 660 Chorionic villi, 450 Chorionic villus sampling (CVS), 455, 490 Choroid, 347–348, 348 Chromalveolates, 585, 588–589, 588–590 Chromatin, 49–50, 50, 81, 84 Chromoplasts, 55 Chromosomes in bacteria, 572 of bacteria, 46 in Barr body, 490

changes in number of, 487–490, 488–489 daughter, 84–85 deletion of, 491, 491 duplication of, 491–492, 492 homologous, 89, 465, 465, 470, 470 inversion of, 491, 492, 493 karyotypes of, 488, 488–489, 489–490 meiosis and, 88–91, 89–92 mitosis and, 84, 84–88, 86–87 in monosomy, 489 mutations of (See Genetic mutations) nondisjunction and, 488, 488–489 in nucleus, 50 sex, 484, 490 structural changes in, 491–492, 491–493 translocation of, 492, 492 in trisomy, 489 Chronic bronchitis, 291–292 Chronic diseases, 207 Chronic mountain sickness, 6 Chronic wasting disease, 581 Chthamalus stellatus, 697, 697 Chyme, 257 Chytrid fungi, 595, 597, 597 Cigarettes. See Smoking Cilia, 48, 58, 59, 191, 282, 589 Ciliary body, 347–348, 348 Ciliary muscle, 348 Ciliates, 589, 590 Circadian rhythms, 403, 404 Circular muscle, of earthworms, 639 Circulatory system in amphibians, 658, 658–659 closed, 638 fetal, 452, 452–453 open, 638 Circumcision, 414 Cirrhosis, 275, 332 CITES (Convention on International Trade in Endangered Species), 753 Cities, sustainable, 761 Citric acid cycle, 114, 115, 119, 120 CJD (Creutzfeldt-Jakob disease), 37, 336, 581 Clades, 558 Cladistics, 558 Cladograms, 558, 558 Clams, 636–638, 637 Clam worm, 638, 638–639 Claspers, crayfish, 643 Classical conditioning, 678, 678 Classification defined, 557 Linnaean, 558 of organisms, 6–8, 7–8 in systematics, 8, 557 taxonomy and, 6, 557 three-domain system of, 558–559, 560 Class (taxonomy), 6 Claviceps purpurea, 600, 600 Clavicle, 366, 370, 370 Cleaning symbiosis, 701, 701 Cleavage, 442, 628 Cleavage furrows, 85 Cleft palate, 255 Climate defined, 723 El Niño- Southern Oscillation and, 738 rain shadow and, 724, 724 regulation of, 748–749 solar radiation and, 723, 723–724 terrestrial biomes and, 726, 726 wind circulation and, 723, 723–724 Climate change, 14, 718, 718, 719, 752, 752 Climax community, 702 Clitellum, 639 Clitoris, 417, 418 Cloaca, 661 Clock, molecular, 558, 664 Clogged arteries, 227 Clonal selection, 238, 238, 240 Cloning, 520–522, 520–523 Clonorchis sinensis, 634 Closed circulatory system, 638 Clostridium botulinum, 381, 575 Clostridium difficile, 576 Clostridium tetani, 572

Clotting, blood, 215–216, 216 Club drugs, 334 Club fungi, 598–599, 599 Club mosses, 613 Cnidarians anatomy of, 630, 630–631 diversity of, 631, 631 hydra, 632, 632 in phylogenetic tree, 627 Cnidocytes, 630 CNS (central nervous system), 314, 319–323, 319–323 Coal, 606, 621 Coastal ecosystems, 734–737, 735–736 Cobras, 656 Cocaine, 331, 332, 332–333 Coccidioides immitis, 602 Coccidioidomycosis, 602 Coccus (cocci), 571, 571 Coccyx, 366, 368, 369, 371 Cochlea, 351–352, 352 Cochlear canal, 352 Cochlear nerve, 352, 352 Code, genetic, 504, 504 Codominance, 476 Codons, 504, 504, 506 Coelacanth, 657 Coelom in deuterostomes, 628 development of, 442 in earthworms, 639 in humans, 197 in protostomes, 628 true, 628 Coenzyme A (CoA), 119 Coenzymes, 107, 268 Coevolution, 172, 173, 698, 699 Cofactors, enzyme, 107 Coffee, 333 Cohesion, 26, 162 Cohesion-tension model of xylem transport, 162, 162–163 Cohorts, 693 Coitus interruptus, 423 Colds, 290, 579–580 Coleochaete, 607, 607 Colitis, 576 Collagen fibers, 75, 75, 191 Collar cells, 629 Collecting ducts, 302, 304, 305, 305 Collenchyma, 145, 146 Colon, 259 Colon cancer, 83, 275 Colony collapse disorder (CCD), 747 Colony-stimulating factors, 236 Color, of skin, 477, 477–478 Color blindness, 354–355, 355, 485, 550, 550–551 Color vision, 349 Colostrum, 242, 457 Columnar epithelium, 191, 192 Comb jellies, 630, 630 Commensalism, 701, 701 Commercial products, genetic engineering and, 181 Common ancestry principle, 565 Common cold, 290, 579–580 Communication in animals, 678–680, 679–680, 682, 682 auditory, 679, 679–680 in bees, 680, 682, 682 cells and, 65, 65 chemical, 679, 679 defined, 678 tactile, 680, 682, 682 visual, 680, 680 Communities climax, 702 defined, 3, 4, 691 Compact bone, 193, 194, 363, 364 Companion cells, 146 Comparative genomics, 531 Competition in human mating, 684 intraspecific, 697 natural selection and, 538

between populations, 696–697, 697 Competitive exclusion principle, 696 Complementary base pairing, 39, 499, 499 Complement system, 236–237, 237 Complete flowers, 171 Complete fractures, 386 Complex carbohydrates, 30–31, 31, 265 Compound eye, 642 Compound fruits, 176 Compound light microscopes, 45, 45 Compounds, molecular, 23–25, 23–25 Concentration gradients, 67 Conclusions, in scientific studies, 13 Condensation, 711 Conditioning, of behavior, 678, 678 Condoms, 423, 424, 426 Conduction of action potential, 317 Cone cells, 349, 349 Congenital hypothyroidism, 405, 406 Congenital syphilis, 432 Conidia, 597 Coniferous forests, 726–728, 727 Conifers, 617, 617 Conjugation, 572, 587, 589 Conjunctiva, 347–348 Conjunctivitis, 348 Connective tissue, 191, 193–195, 193–195, 363 Conservation biology, 744 Conservation corridors, 754 Conservation of habitat, 754–756, 755 Conservation reserves, 756 Constipation, 275 Consumers, 707 Consumptive use value of species, 748 Continental drift, 542, 542 Continuous ambulatory peritoneal dialysis (CAPD), 308–309 Continuous variation, 476 Contour farming, 761, 761 Contraceptive implants, 423, 424 Contraceptive injections, 423, 424 Contraceptive patches, 423, 424 Contraceptives, 422, 423 Contraceptive vaccines, 424 Contractile rings, 85 Contractile vacuoles, 589 Contraction of muscles, 377–380, 378–380, 382–385, 382–385 Contraction period, 384 Contrast, in microscopes, 45 Control groups, 10 Controlled studies, 12, 12–13 Convention on International Trade in Endangered Species (CITES), 753 Convergent evolution, 558 Copper, 270, 271 Copperhead snakes, 656 Coracoid process, 370 Coral bleaching, 630 Coral reefs, 13, 630, 737, 739 Corals, 589, 631, 631 Cork cambium, 156 Cork cells, 145, 156 Corn, genetically engineered, 182, 525 Cornea, 347, 347–348 Corolla, 619 Corona, in rotifers, 634 Corona radiata, 441, 441 Coronary arteries, 224, 226 Coronary bypass, 227, 228 Corpus callosum, 321, 323 Corpus luteum, 419–420 Cortex, of plants, 151, 151 Cortical granules, 441, 441 Cortisol, 393, 400 Cortisone, 400 Costal cartilages, 366, 369 Cotyledons, 149, 175, 177 Coughing, 293 Coumadin, 108 Coupled reactions, 102–103, 104 Covalent bonds, 23–25, 24–25 Cover crops, 761 Cowbirds, 699, 699 Cowper glands, 413, 413

Index I-5 Coxal bones, 366, 371, 371 C3 photosynthesis, 138, 138 C4 photosynthesis, 138, 138–139 Crabs, 642 Crack, 332–333 Cranial cavity, 197, 197 Cranial nerves, 327, 328 Cranium, 367, 367 Crayfish, 84, 643, 643 Creams, spermicidal, 423 Creatine phosphate, 299, 382, 383 Creatinine, 299 Crenation, 70 “Cretaceous crisis,” 542 Cretaceous period, 541 Creutzfeldt-Jakob disease (CJD), 37, 336, 581 Crick, Francis, 498–500 Cri du chat syndrome, 491 Crime scene investigation, 646 Cristae, 56, 56, 119–121 Crocodiles, 659, 660 Crohn’s disease, 274, 576 Cro-Magnons, 669 Crop of birds, 661 of earthworms, 639 of grasshoppers, 645 Crop rotation, 760 Crops. See Agriculture Crosses monohybrid, 468, 468 one-trait, 468–469, 468–469 testcrosses, 469, 469–472, 472 two-trait, 471, 471 Crossing-over, 90, 90, 484 Crowns (teeth), 256 Crustaceans, 643, 643 Crustose lichens, 600–601, 601 Cryptococcus neoformans, 602 Cryptosporidium parvum, 593 Ctenophores, 627, 630, 630 Cuboidal epithelium, 191, 192 Cucumaria, 647 Cultural eutrophication, 714 Culture, sociobiology and, 686 Cup coral, 631 Curie, Marie, 20 Currents, ocean, 737 Cushing syndrome, 405, 406 Cutaneous receptors, 344, 345 Cuticles earthworm, 640 plants, 144 CVD (cardiovascular disease), 225–228, 267 CVS (chorionic villus sampling), 455, 490 Cyanide gas, 107, 108 Cyanobacteria, 128, 128, 572–573, 573 Cycads, 617, 618 Cyclic AMP (cAMP), 394 Cyclic electron pathway, 134, 134 Cyclins, 82 Cyclura, 655 Cysteine, 36 Cysticercosis, 624 Cystic fibrosis (CF), 71, 292, 294, 473, 474, 528 Cystitis, 309 Cysts, 585 Cytochrome c, 544 Cytochromes, 119 Cytokines, 236, 244 Cytokinesis, 81, 85, 88, 88 Cytokinins, 184 Cytolysis, 69 Cytomegalovirus, 249 Cytoplasm in animal cells, 49 in plant cells, 50 in prokaryotes, 46 Cytoplasmic segregation, 445 Cytosine, 38, 38, 39 Cytoskeleton in cell anatomy, 49 composition and function of, 48 structure of, 56–58, 57–58 Cytotoxic T cells, 239

D

Dalton, John, 18 Damage accumulation hypothesis, 458 Dams, 760 Darwin, Charles, 536–537, 536–538, 566 Data, scientific, 10–11, 10–11 Daughter chromosomes, 84–85 Day-neutral plants, 186 DCT (distal convoluted tubule), 302, 305 DDT, 722, 740 Deafness, 356, 357 Deciduous plants, 148 Deciduous teeth, 255 Decomposers, 565, 707, 707, 748 Deductive reasoning, 9 Deer, 663 Defecation, 259 Defecation reflex, 259, 260 Defenses, antipredator, 698–700, 700 Dehydration reactions, 29, 29 Delayed allergic response, 245 Deletion of chromosomes, 491, 491 Delta cells, 401 Deltoid, 376–377 Demographic transition, 694 Denaturation, 37, 106 Dendrites, 314, 315 Dendritic cells, 215, 236 Denisovans, 6, 669 Denitrification, 715 Dense fibrous connective tissue, 193, 193, 363 Density, of water, 26–27, 27 Density-dependent factors, 696 Density-independent factors, 696 Dental caries, 256 Dentin, 256 Deoxyribonucleic acid (DNA) aging and, 458 analysis using, 523, 523–524 cloning, 520–522, 520–523 comparison with RNA, 38, 38 complementary base pairing and, 39, 499, 499 double helix and, 38, 39, 499, 499 evolution of cells and, 568–569 gene expression and, 5, 502–503, 502–504, 506–507, 508 Hershey-Chase experiments with, 498, 498 histones in packaging of, 84 hydrogen bonds in, 25 nature of, 497–498, 497–498 Okazaki fragments and, 502 polymerase chain reaction and, 522, 522–523 purines and, 499, 499 pyrimidines and, 499, 499 radiation damage to, 20 recombinant, 180, 520, 520, 522 replication of, 501, 501–502 RNA and evolution of, 568–569 sequencing of, 522 structure of, 38, 38–39, 498–500, 499 technology and, 520–523, 520–524 testing for genetic disorders, 529 in viruses, 577 wildlife conservation and, 750 Depolarization, 317 Derived traits, shared, 558 Dermatitis, 203 Dermatophytes, 601 Dermis, 200, 200, 202 Descending colon, 259 Deserts, 731, 732 Design, experimental, 10, 12 Desmosomes, 75 Detection, 342 Detrital food chains, 710 Detrital food webs, 708–709, 709 Detritus, 707, 708 Deuterostomes characteristics of, 626 embryonic development in, 628, 628 invertebrate, 647–648, 647–649 protostomes vs., 628, 628 tissue layers of, 628

Development, 440–461 aging and, 457–461 of animals, 442–443, 442–444 apoptosis and, 448 birth defects, testing for, 454, 455 cellular differentiation in, 444–446, 445–446 cellular stages of, 442, 442 as characteristic of life, 4–5, 5 childbirth, 455–457, 456–457 in deuterostomes, 628, 628 embryonic, 448, 449–451, 449–451 extraembryonic membranes and, 448, 448–449 fertilization and, 441–443, 441–444 fetal, 448, 451–452, 451–453 in flowering plants, 175–178, 176–178 morphogenesis and, 444, 446–448, 447 organ stages of, 443, 444 processes of, 444–448, 445–447 in protostomes, 628, 628 tissue stages of, 442–443, 442–443 Devonian period, 541 Diabetes insipidus (DI), 405 Diabetes mellitus biotechnology and, 519, 532 cell signaling and, 65 pathophysiology, 407–408 tubular reabsorption and, 304 type 1, 407–408, 519 type 2, 408 in young adults, 391, 408 Diabetic retinopathy, 355 Dialysate, 308 Diapheromera, 644 Diaphragm (contraceptive), 422–423, 423 Diaphragm (respiratory), 282 Diaphysis, 363 Diarrhea, 274 Diastole, 220 Diastolic pressure, 224 Diatomaceous earth, 588 Diatoms, 588, 588 Dictyostelium discoideum, 592 Didelphis virginianus, 662 Didinium nasutum, 697 Diencephalon, 323 Diesel power, from algae, 136 Diet and nutrition, 263–273 aging and, 458 birth defects and, 454 caffeine and, 333 cancer and, 515 carbohydrates and, 34, 265–266, 265–266 cardiovascular disease and, 226 defined, 263 eating, 272, 272–273 fats in, 34 of filter feeders, 629 food additives and, 271–272 food poisoning, 274, 575 garter snakes, 674, 674, 675, 675 genetically engineered foods, 182, 525 lipids and, 266–268, 268 metabolic fate of food, 123 minerals, 270, 270–271 MyPlate guide to, 34, 263, 264 obesity and, 263–265, 264 osteoporosis and, 270 plants in, 608, 608 probiotics, 576 proteins and, 266 for vegetarians, 267 vitamins, 268–269, 268–269 Dietary supplements, 270–271 Differential reproductive success, 538, 550 Diffusion, 67–68, 68 Digestive gland, crayfish, 643 Digestive system, 253–263 accessory organs of, 260–261, 260–261 aging and, 459 diseases and disorders of, 273–275, 274 enzymes of, 262–263, 262–263 esophagus, 256–257 gallbladder, 260, 260, 261 grasshoppers, 644–645, 645

hormones of, 259, 259 large intestine, 259, 259–260 liver, 260, 260–261, 261 mouth, 254–256, 255 pancreas, 260, 260, 261 pharynx, 256, 256 regulation of secretions in, 259, 259 small intestine, 258–259, 258–259 stomach, 257, 258 structure and function of, 198, 199, 254, 254–255 Digestive tract, 254–260, 257 Dihybrid cross, 471, 471 Dinoflagellates, 589, 589 Dioecious plants, 171 Diphtheria, 242, 381 Diploids, 84 Diplomonads, 590 Directional selection, 552, 552 Direct value of species, 746, 746–748 Disaccharides, 30, 30 Diseases and disorders. See also Infections acute, 207 bacterial, 573–575, 574–575 cardiovascular system, 210, 215–216, 225–228, 225–228 chronic, 207 defined, 206 digestive system, 273–275, 274 emerging, 14 endocrine system, 403, 405–407, 405–408 of eyes, 354–356, 355–356 fungal, 601–602, 602 homeostasis and, 206–207 immune system, 232, 248, 250 integumentary system, 201, 202–204 kidneys, 298, 307, 307–309 localized, 207 microbes and, 564 musculoskeletal system, 385–387, 386 nervous system, 334–337, 335–337 protozoal, 592–595, 593 reemerging, 14 reproductive system, 432–434, 433–434, 436 respiratory system, 289–294, 291–294 STDs (See Sexually transmitted diseases (STDs)) systemic, 207 of taste and smell, 354 as threat to biodiversity, 753, 754 tropical, 594, 624 urinary system, 298, 307, 307–309, 309–310 viral, 579, 579–580 Disruptive selection, 552, 554 Distal, defined, 197 Distal convoluted tubule (DCT), 302, 305 Dittmer, H. J., 147 Diuresis, 306 Diuretics, 306 DIY biology, 573 DNA. See Deoxyribonucleic acid DNA analysis, 523, 523–524 DNA fingerprinting, 523, 523–524 DNA forensics, 523, 524 DNA helicase, 501 DNA ligase, 502, 520 DNA microarrays, 529 DNA polymerase, 501 DNase, 498 DNA sequencing, 522 DNA testing, 529 DNA vaccines, 241 Dogs artificial selection of, 544–545, 545 evolution of, 652 Domain Archaea, 6–8, 7, 559, 560. See also Archaea Domain Bacteria, 6–8, 7, 559, 560. See also Bacteria Domain Eukarya, 6–8, 7, 559, 560. See also Eukarya Domains, 6–8, 7 Dominance, incomplete, 475, 475–476 Dominant alleles, 466, 468, 468



I-6

Index

Dopamine, 331, 332–333, 335 Dormancy, of seeds, 177 Dorsal, defined, 197 Dorsal abdominal artery, crayfish, 643 Dorsal artery of penis, 414 Dorsal cavity, 197, 197 Dorsal root, 320, 327 Dorsal root ganglion, 327, 328 Dorsal vein of penis, 414 Double-blind studies, 13 Double fertilization, 174 Double helix, 38, 39, 499, 499 Doubling time, 693 Down syndrome (trisomy 21), 482, 489, 489–490, 492, 493 Drag nets, 753, 753 Dragonfly, 644 Drinks, sports, 17, 39 Drought, 712 Drug abuse birth defects and, 454 cardiovascular disease and, 226 disease transmission and, 426 nervous system and, 331–332, 331–334 Drug testing, 203 Dry fruits, 176 Duchenne muscular dystrophy, 387, 485, 485 Duckbill platypus, 662 Ductus arteriosus, 452, 453 Ductus venosus, 452, 453 Dungeness crabs, 642 Duodenum, 258 Duplication of chromosomes, 491–492, 492 Dwarfism, 405, 405, 549 Dynamic equilibrium, 204 Dysmenorrhea, 434

E

Eagles, 661, 745, 745 Earlobe, 351 Ears, 351–353, 351–354 Earth, elements in crust of, 18, 18 Earthworms, 639, 639–640 Earwax, 351 Easterlies, 724 Eating disorders, 272, 272–273 Eating disorders not otherwise specified (EDNOS), 273 Ebola virus, 14, 563, 581 EBV (Epstein-Barr virus), 215, 580 Ecdysozoa, 626, 628, 640–647, 641–646 ECGs (electrocardiograms), 221–222, 222 Echinoderms, 627, 647–648, 647–649 E-cigs, 290 ECM (extracellular matrix), 75, 75 Ecological considerations bamboo, 157 climate change and carbon emissions, 719 colony collapse disorder, 747 diesel power from algae, 136 El Niño- Southern Oscillation, 738 endocrine-disrupting contaminants, 435 photochemical smog, 716 rice production, 138 Ecological levels of organization, 691, 691 Ecological niche, 696 Ecological pyramids, 710, 710 Ecological succession, 702, 702–703 Ecology, defined, 691 Ecosystems abiotic components of, 707 aquatic, 732–737, 733–737, 739, 739–740 biotic components of, 707–708, 707–708 chemical cycling and energy flow in, 4, 4, 708–710, 708–710 coastal, 734–737, 735–736 defined, 3, 4, 691 destruction of, 13–14 diversity of, 745 ecological pyramids of, 710, 710 populations within, 707, 707 terrestrial, 724–731, 725–732 trophic levels, 709–710 Ecotourism, 749 Ecstasy, 334 Ectoderm, 442, 443

Ectopic pregnancy, 417 Ectothermic organisms, 660 EDCs (endocrine-disrupting contaminants), 435 Edema, 234, 307 ED (erectile dysfunction), 414, 432 Edge effects, 755, 755 EDNOS (eating disorders not otherwise specified), 273 Edwards syndrome, 489, 489 Edward VII (king of England), 487, 487 EEGs (electroencephalograms), 323, 326 EEOC (U.S. Equal Employment Opportunity Commission), 83 Efferent arteriole, 301 Egg formation, 96 Egg-laying hormone (ELH), 675 Ejaculation, 414 Elastic cartilage, 194, 363 Elastic fibers, 75, 75, 191 Electrocardiograms (ECGs), 221–222, 222 Electroencephalograms (EEGs), 323, 326 Electrolytes, in sports drinks, 17 Electromagnetic spectrum, 131, 131 Electromyograms (EMGs), 384 Electronegativity, 24 Electrons, 18–19, 19, 22, 22 Electron shells, 19, 22 Electron transport chain, 114–115, 115, 119–121, 121–122 Elements defined, 18 in Earth’s crust, 18, 18 periodic table of, 19, 19–20 Elephantiasis, 624, 641 ELH (egg-laying hormone), 675 Elk, 684, 684 Ellis–van Creveld syndrome, 549 El Niño- Southern Oscillation (ENSO), 738 Elongation phase of translation, 506, 507 Embolus, 225 Embryonic development of animals, 626, 628, 628 defined, 448 first week of, 449, 449 fourth and fifth weeks of, 450, 450–451 second week of, 449, 449–450 sixth through eighth weeks of, 450–451 third week of, 449–450 Embryonic disk, 449 Embryonic stem cells (ESCs), 217, 521 Embryos chicks, 448, 448 crocodile, 660 development of, 448, 449–451, 449–451 in flowering plants, 174, 175–176 frog, 443, 444 human, 448, 448–449 somatic, 179 vertebrate, 443, 444 Embryo sac, 172 Emergency contraception, 424 Emerging diseases, 14 EMGs (electromyograms), 384 Emiquon floodplain restoration, 756–757, 756–757 Emotions, in animals, 681 Emphysema, 292, 294 Emulsification, 33 Enamel, tooth, 256 Enceladus (moon), 1 Endangered species, 744–745 Endergonic reactions, 102 Endocardium, 219 Endochondral ossification, 365, 365 Endocrine-disrupting contaminants (EDCs), 435 Endocrine glands, 392, 393 Endocrine system, 391–408 adrenal glands, 393–394, 399–400, 399–401 aging and, 460–461 diseases and disorders of, 403, 405–407, 405–408 gonads, 393–394, 402 hormones of, 392–395, 393–395

hypothalamus, 323, 393–394, 395, 396–397, 396–397 pancreas, 393–394, 401, 401–402 parathyroid glands, 393–394, 398, 398, 399 pineal gland, 323, 393–394, 403 pituitary gland, 323, 393–394, 395–397, 396–397 structure and function of, 199, 199–200, 392, 392 thymus, 235, 393–394, 403 thyroid gland, 393–394, 398, 398 Endocytosis, 67, 73, 74 Endoderm, 442, 443 Endodermis, 151, 151 Endolymph, 352 Endomembrane system, 51–54, 52–54 Endometriosis, 433, 434 Endometrium, 417 Endoplasmic reticulum (ER), 48–50, 52, 52–53, 59 Endoskeleton, 655 Endosperm, 174 Endospores, 572 Endosymbiotic theory, 59–60, 585 Endothelium, 211 Endothermic organisms, 660 Energy acquisition by organisms, 4, 4 of activation, 104–105, 105 adenosine diphosphate and, 102 adenosine triphosphate and, 102–103, 103 chemical, 100, 100 defined, 4, 100 in ecosystems, 4, 4, 708–710, 708–710 flow of, 100–101, 100–102 forms of, 100 free, 102 geothermal, 759 kinetic, 100, 100 mechanical, 100, 100 metabolism and, 102–103, 103 for muscle contraction, 380, 382–383, 382–383 organelle production of, 54–56, 54–56 potential, 100, 100 pregnancy and, 453 renewable sources of, 758–760, 759 solar, 4, 54, 127, 131–134, 131–134, 140, 759 transformations of, 102–103 Energy drinks, 333 Engineering, genetic. See Genetic engineering Enhancers, 510 ENSO (El Niño- Southern Oscillation), 738 Entropy, 101, 101–102 Entry, of viruses, 578 Entry inhibitors, 428 Environment adaptation to, 6 behavior and, 675–678, 677–678 genetics and, 478, 478–479 mutagens and, 511 Environmental resistance, 693 Environmental tobacco smoke, 293 Enzymatic proteins, 66, 66 Enzymes activation of, 106–107 active sites of, 105, 105 coenzymes, 107 cofactors and, 107 defined, 104 denaturation of, 106 digestive, 262–263, 262–263 factors affecting speed of, 106–107, 106–107 in fertilization, 441 functions of, 35 induced fit model of, 105, 105 inhibition of, 107, 107, 108 mechanism of action, 105, 105–106 metabolic pathways and, 103–107 pH and, 106, 107 restriction, 520 substrates and, 104, 106

temperature and, 106, 106 vitamins and, 107 Eocene epoch, 541 Eosinophils, 213, 215 Ependymal cells, 197 Ephedra, 617 Epicuticle, 642 Epidermal growth factor, 403 Epidermal tissue, in plants, 144–145, 144–145 Epidermis of plants, 144, 151, 151 of skin, 200, 200, 202 Epididymis, 413, 413, 415 Epigenetics, 676 Epiglottis, 256, 282 Epinephrine, 329, 393–394, 394, 400 Epipelagic zone, 739 Epiphyseal plate, 363 Epiphyses, 363 Epiphytes, 152, 729 Episiotomy, 456 Episodic memory, 324–325 Epithelial tissue, 191, 192 Epitheliomuscular cells, 632 Epithelium, 191, 192 EPO (erythropoietin), 214, 299 EPSA1 gene, 6 Epstein-Barr virus (EBV), 215, 580 Equilibrium disorders of, 356, 358 genetic, 546–547, 547–548 punctuated, 556, 557 sense of, 353, 353–354 Equilibrium life history patterns, 696 Equisetum, 615 Erectile dysfunction (ED), 414, 432 Erectile tissue, 414 Erection, 414 ER (endoplasmic reticulum), 48–50, 52, 52–53, 59 Ergot, 598, 600, 600 Erosion, 748 ERT (estrogen replacement therapy), 386 ERV (expiratory reserve volume), 284 Erythrocytes. See Red blood cells Erythropoietin (EPO), 214, 299 Escherichia coli (E. coli), 2, 260, 274, 307, 498, 508, 532, 573 ESCs (embryonic stem cells), 217, 521 Esophagus bird, 661 in digestive system, 256–257 earthworm, 638 sea star, 648 Essential amino acids, 266 Essential fatty acids, 266 Estrogen replacement therapy (ERT), 386 Estrogens, 35, 35, 393, 403, 419, 421 Estuaries, 734, 735 Ethanol, 332. See also Alcohol Ethical considerations. See Bioethical considerations Ethmoid bone, 367, 367 Ethnic groups, 670 Ethylene, 185, 185 Euchromatin, 509–510 Eudicots embryo development in, 175, 176 flowers of, 171, 172 monocots vs., 618–619, 619 seed germination in, 177–178, 178 structure of, 148–149, 149, 151, 151 Eudicotyledones, 618–619 Euglenids, 2, 589–590, 590 Eukarya characteristics of, 6–8, 7, 44 comparison with Archaea, 569, 569 as domain, 6–8, 7, 559, 560 kingdoms of, 585 Eukaryotes, 48–60 characteristics of, 7, 8 comparison with prokaryotes, 47, 47 gene expression in, 509–510, 510–511 origin and evolution of, 58–60, 60 structure of, 48–58, 48–58

Index I-7 Europa (moon), 1 Eustachian tubes, 291 Eutrophication, 714, 732, 752 Eutrophic lakes, 732, 733 Evaporation, 711 Evergreens, 148 Evolution, 535–554. See also Natural selection agents of change in, 548–549, 551–552 analogous structures and, 543 anatomical evidence for, 543, 543–544 of animals, 625–628, 625–628 of antibiotic resistance, 535, 553, 560 of ATP, 568 biochemical evidence for, 544, 544 biogeographical evidence for, 540, 542, 542 bottleneck effect and, 549, 549 of cells, 58–60, 60, 568–569 of chloroplasts, 59–60 of chordates, 653, 653–654 coevolution, 172, 173, 698, 699 continental drift and, 542, 542 convergent, 558 Darwin and, 536–537, 536–538 defined, 5, 536 of DNA, 568–569 of eukaryotes, 58–60, 60 evidence for, 539–545, 539–545 fossil evidence for, 539–540, 539–540 founder effect and, 549, 549 of fungi, 595, 595 gene flow and, 547, 549 genetic drift and, 548–549, 549 genetic equilibrium and, 546–547, 547–548 geological timescale and, 540, 541 of hominins, 664–668, 665–668 homologous structures and, 543, 543 human, 666–669, 667–669 of jaws, 655, 657 Lamarck and, 537, 537 macroevolution, 555–556, 555–557 mass extinctions and, 541, 542–543 of membranes, 567–568, 568 of microbial life, 566–569, 567–568 microevolution, 545–547, 546–548 of mitochondria, 59–60 of monomers, 566–567, 567 mosaic, 666 mutations and, 548 nonrandom mating and, 549 observations of, 544–545, 545 of plants, 607–609, 607–609 of plasma membrane, 567–568, 568 of polymers, 567 of primates, 665 processes of, 548–549, 551–554 of protobionts, 567–568, 568 speciation and, 555–556, 555–557 systematics and, 557–559, 558–560 theory of, 11–12, 536–538 of vertebrates, 655 vestigial structures and, 543 Excavates, 585, 589–590, 590–591 Exchange pools, 710 Excretion aging and, 459 in earthworms, 638 urinary system and, 299 Excurrent siphon of clam, 637 of sea squirt, 654 Executioner caspases, 81 Exercise cancer and, 515 cardiovascular disease and, 226 muscle size and, 384–385 sports drinks and, 17, 39 water loss in, 17 Exergonic reactions, 102 Exocrine glands, 392 Exocuticle, 642 Exocytosis, 67, 72–73, 73 Exons, 503, 503 Exophthalmia, 406, 406

Exoskeleton, 641, 643 Exotic species, 690, 751, 751 Experimental design, 10, 12 Experimental variables, 10 Experiments, scientific, 9–10 Expiration, 284, 285, 285–286 Expiratory reserve volume (ERV), 284 Exponential growth, 692, 692 Expression, gene. See Gene expression Extension, of muscles, 375 Extensor carpi muscles, 376–377 Extensor digitorum longus, 376–377 External auditory canal, 367 External oblique, 376–377 External respiration, 286–287, 287, 289 Exteroceptors, 342, 342 Extinctions defined, 13 mass, 541, 542–543 Extracellular matrix (ECM), 75, 75 Extracorporeal membrane oxygenation, 283 Extraembryonic membranes, 448, 448–449 Ex vivo gene therapy, 528, 528 Eyes aging and, 459 anatomy and physiology of, 347–350, 347–351 arthropod, 641 disorders of, 354–356, 355–356 LASIK surgery for, 341, 355, 358 Eyespot apparatus, 590 Eyespots, 648 Eyestrain, 348

F

Facial bones, 367–368, 368 Facilitated transport, 67, 71, 71 Facilitation model of ecological succession, 702 FAD (flavin adenine dinucleotide), 107, 114 Fall, colors of, 127, 140 Fallopian tubes, 416–417, 417 Fall overturn, 733 False ribs, 369, 370 Familial hypercholesteremia, 73, 476, 476, 528 Family (taxonomy), 6 FAPs (fixed action patterns), 675, 677 Farsightedness, 355, 355 Fascia, 375 Fasciculations, 387 Fast-twitch muscle fibers, 385, 385 Fatal familial insomnia, 336 Fate maps, 446, 446, 448 Fats, 32, 33, 34. See also Lipids Fat-soluble vitamins, 268, 269 Fatty acids, 32, 33, 123, 266 Fear, in animals, 681 Feathers, 660, 661 Feather stars, 647 Feedback negative, 204–205, 204–205, 393 positive, 205–206, 396 Female condoms, 423, 424 Female reproductive system, 416–422, 416–422, 433–434, 434, 436 Femur, 366, 371, 371 Fermentation alcohol, 118–119 in cells, 382–383 defined, 118 energy yield of, 118–119 lactic acid, 118 yeasts and, 598 Ferns, 607, 608, 613–615, 613–615 Fertilization defined, 441 double, 174 early development and, 441–443, 441–444 enzymes in, 441 in flowering plants, 172, 174, 175 implantation and, 421, 422 meiosis and, 89 polyspermy and, 441

Fertilization membrane, 441, 441 Fetal alcohol syndrome (FAS), 454 Fetal circulation, 452, 452–453 Fetal development circulation in, 452, 452–453 extraembryonic membranes and, 448, 448–449 fifth through seventh months of, 452 placenta and, 453 third and fourth months of, 451, 451 Fiber, dietary, 260, 265 Fibers muscle, 378, 378–379 plants, 146 Fibrils, 76 Fibrin, 99, 110, 215 Fibrinogen, 214, 215 Fibrinolysin, 420 Fibroblasts, 193 Fibrocartilage, 194, 363 Fibromyalgia, 387 Fibronectin, 75, 75 Fibrous joints, 372 Fibrous root systems, 151–152, 152 Fibula, 366, 371, 371 Filaments, 171, 619 Filter feeders, 629 Fimbriae bacterial, 46, 47, 572 in reproductive system, 417, 417 Finches, 538, 538, 545, 545 Fingerprintin, DNA, 523, 523–524 Fireflies, 680, 680 First law of thermodynamics, 100–101 First messenger, 394 Fish evolution of, 657–658, 657–658 overexploitation of, 753, 753 in phylogenetic tree of chordates, 653 Fission, binary, 572, 572 Fitness. See also Exercise behavior and, 682–687, 683–684, 686–687 defined, 550 inclusive, 686, 687 natural selection and, 550–551 reproductive strategies and, 683–684, 684 societies and, 685–686 territoriality and, 682–683, 683 Fixed action patterns (FAPs), 675, 677 Flagella of bacteria, 46, 47, 572 in dinoflagellates, 589 in euglenids, 590 of eukaryotes, 48 origin of, 60 in protists, 587, 587 of sponges, 629 structure of, 58, 59 Flagellates, 589 Flagship species, 754 Flame cells, 633 Flamingos, 661 Flat-backed millipede, 642 Flatworms, 627, 633–634 Flavin adenine dinucleotide (FAD), 107, 114 Fleming, Alexander, 9, 553, 576 “Flesh-eating” bacteria, 574, 574 Fleshy fruits, 176 Flexion, of muscles, 375 Flexor carpi muscles, 376–377 Flexor digitorum, 376–377 Flexor digitorum longus, 376–377 Floating ribs, 369, 370 Flood prevention, 748 Flowering plants. See also Angiosperms anatomy of, 170–171, 171 double fertilization in, 174 embryos in, 174, 175–176 evolutionary history of, 607 fertilization in, 172, 174, 175 growth and development of, 175–178, 176–178 insects and, 173 life cycle of, 171–172, 174, 174–175, 619, 620–621 photosynthesis in, 128–130, 129

pollination in, 172–174, 175 sexual reproduction in, 170–172, 170–172, 174, 174–175 stimuli, responses to, 186–187, 186–187 Flowers, 170–171, 171–172, 619–621. See also Flowering plants Fluid-mosaic model, 64, 64 Flukes, 634 Flu virus, 579–580 Foams, spermicidal, 423 Folic acid, 269 Foliose lichens, 601, 601 Follicles, 398, 418 Follicle-stimulating hormone (FSH), 393, 396–397, 416, 419 Follicular phase, 419 Fontanels, 367, 451 Food. See Diet and nutrition Food additives, 271–272 Food allergies, 246, 576 Food chains, 709 Food Plate guide (USDA), 34 Food poisoning, 274, 575 Food webs, 708–709, 709 Foot, in molluscs, 635, 638 Foot ganglion, 637 Foraging, 683, 683 Foramen magnum, 367, 367 Foramen ovale, 453 Foraminiferans, 592, 592 Forensics DNA and, 523, 524 maggots in, 646 Foreskin, 413, 414 Forests coniferous, 726–728, 727 temperate deciduous, 728, 728 tropical, 13, 728–729, 729 Forewing, 645 Formed elements, 213, 214 FosB gene, 674 Fossil fuels, 606, 718 Fossils evidence, for evolution, 539–540, 539–540 Founder effect, 549, 549 Fovea centralis, 347–348, 348 Fox, Michael J., 335 Fox, Sidney, 567 FOX03A gene, 440, 461 Foxes, 732 Fractures, 386 Fragile X syndrome, 485–486, 486, 492 Franklin, Rosalind, 500, 500 Free energy, 102 Free-living flatworms, 633 Free radicals, 127, 140, 268, 458 Frequency, allele, 546–547, 547, 549, 549 Frogs, 443, 444, 445–446 Fronds, 613 Frontal bone, 367, 367, 368 Frontalis, 376–377 Frontal lobe, 321, 322 Fructose, 30 Fruit development of, 170 in dispersal of seeds, 619–620 ripening of, 127 types of, 621 Fruit flies, morphogenesis in, 446–447, 447 Fruits accessory, 176 aggregate, 176 compound, 176 dry, 176 fleshy, 176 multiple, 176 simple, 176 structure and function of, 176, 177 types of, 176 Fruticose lichens, 601, 601 FSH (follicle-stimulating hormone), 393, 396–397, 416, 419 Fukushima nuclear disaster, 21, 21 Functional genomics, 531 Functional groups, 28, 29, 29



I-8

Index

Fungi, 595–602 AM, 601 biology of, 595–596, 596 characteristics of, 7, 8, 595 chytrids, 595, 597, 597 club, 598–599, 599 diseases caused by, 601–602, 602 diversity of, 596–599, 597–599 drugs for, 602 evolution of, 595, 595 Microsporidia, 595, 596–597 nonseptate, 595 poisonous, 600 reproduction of, 596, 596, 597, 598 sac, 597–598, 598 symbiotic relationships of, 599, 601, 601 zygospore, 597, 597 Fusion inhibitors, 426

G

GABA (gamma aminobutyrate), 332 Galactose, 30 Galápagos finches, 538, 538, 545, 545 Gallbladder in digestive system, 260, 260, 261 fish, 657 Gallstones, 275 Gamete formation, 468 Gamete intrafallopian transfer (GIFT), 436 Gametes, 89, 413 Gametophytes, 170, 171–172, 607, 609, 609–610, 611 Gamma aminobutyrate (GABA), 332 Ganglia, 327 Ganglioside GM2, 43, 60 Gap junctions, 76, 76, 191 Garden peas, Mendel’s study of, 466, 466–467 Garter snakes, food choice in, 674, 674, 675, 675 Gart gene, 490 Gases diffusion of, 68 exchanges in body, 68, 68, 286–287, 287,  289 noble, 20, 22 Gastric bypass surgery, 253, 275 Gastric glands, 257 Gastric inhibitory peptide (GIP), 259 Gastrin, 259 Gastrocnemius, 376–377 Gastrodermis, 632 Gastroesophageal reflux disease (GERD), 256 Gastrointestinal tract, 254 Gastropods, 636, 637 Gastrovascular cavity, 631 Gastrulation, 442 GBS (Guillain-Barré syndrome), 337 Geiger counters, 20 Gel electrophoresis, 523 Gene expression, 502–511 control of, 508–510, 508–511 DNA and, 5, 502–503, 502–504, 506–507, 508 elongation and, 506, 507 in eukaryotes, 509–510, 510–511 initiation and, 506, 506 messenger RNA and, 502–503, 502–504 overview, 507, 508 posttranscriptional control of, 510, 511 posttranslational control of, 510, 511 pretranscriptional control of, 509–510, 510–511 in prokaryotes, 508–509, 509 ribosomal RNA and, 505, 505 ribosomes and, 505, 505 RNA and, 502–507, 502–508 termination and, 506, 507 transcriptional control of, 510, 511 transcription and, 502–503, 502–504 transcription factors and, 6 transfer RNA and, 504, 504–505 translational control of, 510, 511 translation and, 502, 502, 504–506, 504–507

Gene flow, 547, 549 Gene linkage, 483, 483–484 Gene pools, 546 Generations, alternation of, 170, 607, 609, 609 Genes cloning, 520–522, 520–523 homeotic, 446–447, 447 morphogen, 446 redefining, 530–531 reproduction and, 5 in transcription, 502 tumor suppressor, 82–83, 82–83, 96, 513 Gene segments, 237 Gene therapy, 527–528, 527–528 Genetically modified organisms (GMOs), 169, 180–182, 187, 524–525 Genetic code, 504, 504 Genetic disorders pedigree analysis and, 472–475, 473–475 testing for, 529 Genetic diversity, 745 Genetic drift bottleneck effect and, 549, 549 defined, 548 equilibrium and, 547 evolution and, 548–549, 549 founder effect and, 549, 549 Genetic engineering defined, 520 of plants, 180–181, 180–181 Genetic equilibrium, 546–547, 547–548 Genetic markers, 529 Genetic mutations cancer and, 513, 513–514, 516 causes of, 511–512, 512 defined, 491, 511 deletions, 491, 491 duplications, 491–492, 492 equilibrium and, 546–547 evolution and, 548 induced, 511 inversions, 491, 492, 493 mutagens and, 511 point, 512, 512, 548 protein activity and, 512–513, 512–513 replication errors and, 511 spontaneous, 511 testing for, 529 translocations, 492, 492 transposons and, 511–512, 512 Genetic profiles, 523, 531 Genetics, 464–479. See also Inheritance behavior and, 674–675, 674–675 of breast cancer, 79, 96 central dogma of, 568 environmental influences on, 478, 478–479 epigenetics, 676 evidence for common ancestry, 670 incomplete dominance and, 475–476 Mendel’s laws and, 465–472, 465–472 pedigree analysis and, 472–475, 473–475 population, 546 taste and, 477 temperature and, 478, 478 Genetic testing for birth defects, 454, 455 for cancer genes, 83 for genetic disorders, 529 Genetic variation, 89 Gene transfer, in bacteria, 572 Genital herpes, 428, 430, 580 Genital tract female, 417 male, 413–414 Genital warts, 428 Genomes, 520, 530, 531 Genomics, 530, 531 Genotype, 466, 468 Genus (taxonomy), 6 Geological timescale, 540, 541 Geothermal energy, 759 GERD (gastroesophageal reflux disease), 256 Germ cells, 511 Germination, 177–178, 178 Germ layers, 443, 443

Gerontology, 457 Gerstein, Mark, 531 GFP (green fluorescent protein), 573 GH (growth hormone), 393, 397, 405 Giardia lamblia, 590, 591 Gibberellins, 184, 184 Gibbons, 683, 683–684 GIFT (gamete intrafallopian transfer), 436 Gigantism, 405, 405 Gills crayfish, 643 fish, 657 Gills Onions, 743, 762 Gingiva, 256 Gingivitis, 256 Ginkgos, 617, 618 GIP (gastric inhibitory peptide), 259 Giraffes, 2, 731 Gizzard, 661 Glands, 191 Glans clitoris, 417, 418 Glans penis, 414, 414 Glass sponges, 629–630 Glaucoma, 348, 355–356 Glenoid cavity, 370, 370 Glia-derived growth factor, 196 Global warming, 14, 630, 718, 752, 752 Glomerular (Bowman’s) capsule, 301–302 Glomerular filtrate, 303 Glomerular filtration, 302–303, 303 Glomerulus, 301 Glossodoris macfarlandi, 636 Glottis, 256, 282 Glucagon, 393, 401 Glucocorticoids, 393, 399, 400 Glucose cellular respiration and, 114 regulation of, 401, 401 structure of, 30, 30 Glucose phosphate, 137 Glucose tolerance test, 407, 407 Glutamate, 337 Glycemic index, 265, 265 Glyceraldehyde 3-phosphate (G3P), 116 Glycogen, 30, 31 Glycolipids, 65 Glycolysis, 114, 115, 116, 117 Glycoproteins, 65, 66 GMOs. See Genetically modified organisms Gnetophytes, 617 Gnetum, 617 GnRH (gonadotropin-releasing hormone), 416, 419 Goiter, 406, 406 Golgi, Camillo, 52 Golgi apparatus, 48–50, 52, 53, 59 Golgi tendon organs, 343–344, 344 Gonadotropic hormones, 393, 397 Gonadotropin-releasing hormone (GnRH), 416, 419 Gonads crayfish, 643 in endocrine system, 393–394, 402, 414 fish, 657 sea star, 648 Gonopore, 648 Gonorrhea, 430–431, 575 Gonorrhea proctitis, 431 Gonyaulax, 589, 589 Gould, Steven Jay, 571 Gout, 299 Gradualistic model of speciation, 556, 557 Gram, Hans Christian, 571 Gram-negative bacteria, 571 Gram-positive bacteria, 571 Gram stains, 571 Grana, 55, 55, 129, 129 Grant, Peter, 545 Grant, Rosemary, 545 Granular leukocytes, 213, 215 Granulocyte-macrophage colony-stimulating factor, 403 Granzymes, 239 Grasshoppers, 644–645, 645 Grasslands, 4, 730–731, 730–731 Graves disease, 406

Gravitational equilibrium, 353, 353 Gravitational equilibrium pathway, 354 Gravitropism, 183, 185, 186, 186 Gray crescent, 444–445, 445 Gray matter, 315, 319–320 Grazing food chains, 709 Grazing food webs, 708, 709 Greater trochanter, 371 Green algae, 585–587, 607 Green fluorescent protein (GFP), 573 “Green” fuels, 136 Green glands, 643 Greenhouse effect, 14, 718 Griffith, Frederick, 497 Ground pines, 613 Ground tissue, in plants, 144, 144–145, 145–146 Groundwater mining, 713 Growth factors, 65, 392, 403 Growth hormone (GH), 393, 397, 405 Growth plate, 363, 364 Guanine, 38, 38, 39 Guard cells, 145, 163 Guillain-Barré syndrome (GBS), 337 Gulls, 677, 677 Gum disease, 256 Gustation (taste), 345, 346, 354, 459, 477 Gymnosperms, 607, 608, 617, 617 Gyrus, 321

H

HAART (highly active antiretroviral therapy), 428 Habitat conservation and restoration of, 754–757, 755–757 ecological niche and, 696 loss of, 13, 13–14, 749, 749–751 Haemophilus influenzae, 336 Hagfishes, 657 Hair cells, in ear, 351, 352 Hairs drug testing with, 203 follicles, 202 root, 144, 147 Haldane, J. B. S., 566 Half-life, 540 Haliaetus leucocephalus, 661 Halocynthia, 654 Halophiles, 569, 569–570 Halorhodopsin, 570 Hamstring group, 376–377 Haploids, 84 Haplorhini, 663 Hard palate, 255, 281 Hardy-Weinberg equilibrium, 546–547, 547–548 Hare, snowshoe, 697–698, 698 Hashimoto thyroiditis, 406 Hawking, Stephen, 337, 337 Hay fever, 245 H bands, 378 HBV (hepatitis B virus), 241, 243, 275, 430 HCG (guman chorionic gonadotropin), 421, 449 HDLs (high-density lipoproteins), 226, 267 Health and medicine artificial lungs, 288 birth defects, testing for, 454, 455 Botox®, 380, 381 cancer prevention strategies, 515 cell signaling in, 65, 65 enzyme inhibitors, 108 immediate allergic response, 246 immunization schedules, 243 medicinal leeches, 216 melatonin, 404 neglected tropical diseases, 594 opportunistic infections and HIV, 249 organ transplants, 308 radiation applied to, 20, 20–22 sleep, 326 smoking, 293 STDs, preventing transmission of, 426 sun, impact on skin, 201 trinucleotide repeat expansion disorders, 486 vertebrates and, 656

Index I-9 Health span, 457 Hearing, 351–353, 351–354, 459 Hearing loss, 356, 357 Heart aging and, 458–459 artificial, 228, 228 birds, 661 crayfish, 643 earthworm, 639 fish, 657 grasshopper, 645 heartbeat, 220–221, 221–222 “lub-dub” sound of, 220–221 path of blood through, 219–220 smoking and, 293 structure and function of, 218–219, 219 transplants, 228 Heart attack, 227 Heartbeat, 220–221, 221–222 Heartburn, 256 Heart disease, 127 Heart transplants, 228 Heart valve disease, 227 Heartwood, 158 Heat capacity, of water, 25 Heat of vaporization, 25–26, 26 Helicobacter pylori, 274 Helilx, double, 38, 39, 499, 499 Helium atoms, 19, 19 Helper T cells, 239 Heme, 286 Hemocoel, 643 Hemodialysis, 307, 307–309 Hemoglobin, 6, 194, 214, 214, 286, 289 Hemolysis, 69, 247 Hemolytic disease of the newborn, 247, 248 Hemophilia, 215–216, 486–487, 487 Hemorrhagic fever, 563, 581 Hemorrhoids, 212, 275 Hepatic portal system, 224, 261, 261 Hepatic portal vein, 224 Hepatitis A, 275 Hepatitis B virus (HBV), 241, 243, 275, 430 Hepatitis C, 275 Herbaceous stems, 154, 155 Herbicide-resistant plants, 180 Herbivores, 707, 707 Herd immunity, 242 Hermaphroditic organisms, 629, 633, 636 Heroin, 334 Herpes, genital, 428, 430, 580 Herpes simplex virus (HSV), 428, 580 Herpesviruses, 578, 580 Hershey, 498 Hershey-Chase experiments, 498, 498 Heterochromatin, 509 Heterosporous plants, 617 Heterotrophs, 128, 568, 707 Heterozygote advantage, 553–554 Heterozygous organisms, 467 HEXA gene, 43 Hexoses, 30, 30 HGH (human growth hormone), 397, 461 HGP (Human Genome Project), 530 High blood pressure. See Hypertension High-density lipoproteins (HDLs), 226, 267 High elevations, adaptation to living in, 6 Highly active antiretroviral therapy (HAART), 428 Hindwings, 645 Hinge joints, 373, 374 Hippocampus, 324, 324 Hip replacements, 362 Hirudo medicinalis, 640 Histamine, 236 Histones, 84 Histoplasma capsulatum, 602 Histoplasmosis, 602 HIV/AIDS opportunistic infections and, 241, 249, 250, 425 overview, 424–425 reproduction of, 427, 428 stages of infection, 425, 425, 427 T cells and, 215, 241, 425, 425

treatments for, 426–427 vaccine for, 429 HMS Beagle, 536, 536, 537 Hodgkin disease, 746 Holocene epoch, 541 Homeobox, 446 Homeodomain, 446–447 Homeostasis body systems and, 206 body temperature regulation and, 204–205, 205 capillaries and, 211 as characteristic of life, 5 control systems and, 206 defined, 5, 204 disease and, 206–207 hormones and, 392–393 maintenance systems and, 206 mechanical, 204, 205 negative feedback and, 204–205, 204–205 plasma membrane and, 64 positive feedback and, 205–206 support systems and, 206 theory of, 11 transport systems and, 206, 206 Homeotic genes, 446–447, 447 Hominins, evolution of, 664–668, 665–668 Homo erectus, 667 Homo ergaster, 667, 668 Homo floresiensis, 667–668 Homo habilis, 666 Homo heidelbergensis, 668 Homologous chromosomes, 89, 465, 465, 470, 470 Homologous structures, 543, 543 Homo neandertalensis, 668–669 Homo sapiens, 668–669 Homozygous organisms, 467 Honeybees, 173, 619, 680, 682, 682, 747 Hooke, Robert, 44 Horizontal gene transfer, 572 Hormone replacement therapy (HRT), 461, 515 Hormones. See also specific hormones action of, 394–395, 394–395 aging and, 458 antagonistic, 393 cancer and, 515 defined, 259, 392 in digestive system, 259, 259 endocrine, 392–395, 393–395 endocrine disrupters and, 435 homeostasis and, 392–393 local, 392 ovaries and, 419–420, 420 peptide, 394, 394 in plants, 183–185, 183–185 secretion of, 299 steroids, 34–35, 35, 64, 394, 395, 395 testes and, 416, 416 urinary system and, 299 Hornworts, 608 Horseshoe crabs, 746–747 Horsetails, 615, 615 Hosts, in parasitism, 700 Hot peppers, 63, 76 Housefly, 644 HPV. See Human papillomavirus HRT (hormone replacement therapy), 461, 515 HSV (herpes simplex virus), 428, 580 Human chorionic gonadotropin (HCG), 421, 449 Human Genome Project (HGP), 530 Human growth hormone (HGH), 397, 461 Human nutrition. See Diet and nutrition Human papillomavirus (HPV), 204, 428, 430, 430, 515 Humans body cavities and body membranes in, 197, 197–198 cellular respiration and, 110, 110 chromosomes in, 84 classification of, 8, 8 embryos of, 448, 448–449 evolution of, 666–669, 667–669

life cycle of, 94–96, 95 mating in, 684–685 organ systems in, 3, 4, 198–200, 199 plants and, 608–609, 608–609 population growth of, 693–695, 694–695 tissues in, 191–197, 192–196 Humerus, 366, 370, 370 Hummingbirds, 173 Humoral immunity, 238 Huntington disease, 474, 474–475, 492, 529 Hyaline cartilage, 193, 194, 363, 364 Hybridization, of plants, 180 Hybridomas, 245 Hybrid rice production, 138 Hybrid vehicles, 759–760 Hydras, 632, 632 Hydrocarbons, 1, 28 Hydrofluorocarbons, 718 Hydrogen, in organic molecules, 28 Hydrogenation, 33, 266–267 Hydrogen bonds, 25, 25 Hydrogen cars, 759, 760 Hydrologic (water) cycle, 711–713, 713 Hydrolysis reactions, 29, 29 Hydrophilic molecules, 26 Hydrophobic molecules, 26 Hydropower, 758–759, 759 Hydrostatic skeleton, 638 Hydrothermal vents, 740 Hydroxyl group, 29, 29, 30 Hydrozoans, 631 Hymen, 417 Hyoid bone, 368, 368 Hypersensitivities, 245, 246 Hypertension, 6, 210, 225, 225, 227, 228, 270 Hyperthyroidism, 406 Hypertonic solutions, 70, 70 Hypertrophy, 385 Hyphae, 595–596, 596 Hypothalamic-inhibiting hormones, 396 Hypothalamic-releasing hormones, 396 Hypothalamus, 323, 393–394, 395, 396–397, 396–397 Hypothesis, 9 Hypothyroidism, 405–406, 406 Hypotonic solutions, 69–70, 70

I

IAA (indoleacetic acid), 183 I bands, 378 ICSI (intracytoplasmic sperm injection), 436, 436 Iguanas, 655 Ileum, 258 Iliopsoas, 376–377 Ilium, 371, 371 Immediate allergic response, 245–246, 246 Immune complexes, 238 Immune system, 232–250. See also Immunity; Lymphatic system adverse effects of immune responses, 245–248, 245–248 aging and, 459 chemical barriers in, 235 diseases and disorders of, 232, 248, 250 inflammatory response in, 235–236, 236 physical barriers in, 235 protective proteins in, 236–237, 237 structure and function of, 198, 199 Immune therapies, 244, 244–245 Immunity active, 241, 241–242 adaptive, 235, 237–241, 238–240 antibody-mediated, 238 cell-mediated, 240, 241 cytokines and, 244 defined, 235, 241 herd, 242 innate, 235–237, 236–237 passive, 241, 242, 243, 244 Immunizations. See also Vaccines active immunity due to, 241, 241 defined, 241 schedule for, 243 Immunodeficiency disease, 250 Immunoglobulins, 214, 238, 239, 239, 242

Immunosuppressive drugs, 248 Impact fractures, 386 Imperfect flowers, 171 Impetigo, 574, 574 Implantation, 421, 422 Imprinting, 677 Impulses, nerve, 315–319, 316–318 Inbreeding, 549, 550–551, 551 Inclusive fitness, 686, 687 Incomplete dominance, 475, 475–476 Incomplete flowers, 171 Incomplete fractures, 386 Incompletely dominant disorders, 476, 476 Incontinence, 300 Incurrent siphon of clam, 637, 638 of sea squirt, 654 Incus, 351, 351 Independent assortment, 90, 91, 470–471 Indirect value of species, 748–749 Indoleacetic acid (IAA), 183 Induced fit model, 105, 105 Induced mutations, 511 Induced pluripotent stem cells, 217 Induction, cellular differentiation and, 445–446, 446 Inductive reasoning, 9 Infant respiratory distress syndrome, 283 Infections. See also Diseases and disorders birth defects and, 454 opportunistic, 241, 249, 250, 425 streptococcal, 574 Infectious mononucleosis, 215, 580 Inferior, defined, 197 Inferior vena cava, 212, 219 Infertility assisted reproductive technology for, 412, 436 causes of, 434 defined, 434 Inflammatory response, 235–236, 236 Influenza, 579–580 Inguinal nodes, 235 Inheritance. See also Genetics of acquired characteristics, 537 blending concept of, 466 of blood type, 476, 477 dominance and codominance, 475, 475–476 gene linkage and, 483, 483–484 multiple allele, 476 natural selection and, 550 particulate theory of, 466 patterns of, 472–475 polygenic, 476–477, 477–478, 478 sex-linked, 484–487, 484–487 of single trait, 467–470, 468–469 temperature and, 478, 478 of two traits, 470–472, 470–472 Inhibin, 416 Inhibition model of ecological succession, 702 Initiation phase of translation, 506, 506 Initiator caspases, 81 Ink sac, 637 Innate immunity, 235–237, 236–237 Inner ear, 351, 352 Insect-resistant plants, 180 Insects characteristics of, 643–645, 644–645 plants and, 619 as pollinators, 173 Insertion, of muscles, 375 Insomnia, 326 Inspiration, 284–285, 285 Inspiratory reserve volume (IRV), 284 Insulin actions of, 393, 401 biotechnology and, 519, 532 cell signaling and release of, 65 regulated secretion of, 73 Insulin pumps, 408 Integral proteins, 64, 65–66, 66 Integrase inhibitors, 428 Integrated pest management, 760–761 Integration, 343



I-10

Index

Integration, synaptic, 318 Integrin, 75, 75 Integumentary system accessory organs in, 202, 202 aging and, 458 diseases and disorders of, 201, 202–204 regions of skin in, 200, 200, 202 structure and function of, 198, 199 sun and, 201, 201 Intercalated disks, 196 Interferons, 237 Interkinesis, 90, 91 Interleukins, 236, 244 Intermediate filaments, 49, 57, 58 Intermembrane space, 120, 121 Internal respiration, 287, 289 International Code of Phylogenetic Nomenclature, 558 Interneurons, 314, 314–315 Internodes, 148 Interoceptors, 342 Interphase, 80, 80–81 Interstitial cells, 415, 416 Interstitial fluid, 194, 206, 218 Intervertebral disks, 319, 369, 369 Intestines disorders of, 274–275 fish, 657 grasshopper, 645 large, 259, 259–260 small, 258, 258–259 Intracytoplasmic sperm injection (ICSI), 436, 436 Intraspecific competition, 697 Intrauterine devices (IUDs), 422, 423 Intrauterine insemination (IUI), 436 Introns, 503, 503 Inversion of chromosomes, 491, 492, 493 Invertebrates. See also specific animals annelids, 627, 638–640, 638–640 arthropods, 627, 641–646, 641–647 characteristics of, 625, 626 cnidarians, 627, 630–632, 630–632 comb jellies, 630, 630 defined, 625 deuterostomes, 647–648, 647–649 Ecdysozoa, 626, 628, 640–647, 641–646 echinoderms, 627, 647–648, 647–649 flatworms, 627, 633–634 hydras, 632, 632 Lophotrochozoa, 628, 632–640, 633–640 molluscs, 627, 635–638, 636–637 Porifera, 629, 629–630 rotifers, 627, 634, 635 roundworms, 446, 446, 627, 640–641, 641 trochozoans, 628, 632, 633–640, 634–640 In vitro fertilization (IVF), 412, 436 In vivo gene therapy, 528 Iodine, 21, 270, 271 Ionic bonds, 23, 23 Ions, 23 Iris, 347–348, 348 Irish potato famine, 181 Iron, 270, 271 “Iron-sulfur world” hypothesis, 566–567 Irritable bowel syndrome, 576 IRV (inspiratory reserve volume), 284 Ischium, 371, 371 Isolating mechanisms, 555, 555 Isomers, 28 Isotonic solutions, 69, 70 Isotopes, 19, 20–22 IUDs (intrauterine devices), 422, 423 IUI (intrauterine insemination), 436 IVF (in vitro fertilization), 412, 436 Ixodes, 646

J

Jacobs syndrome, 489, 490 Jaguars, 729, 750, 750 Jaundice, 275 Jawless fishes, 653, 657 Jaws, 655, 657 Jejunum, 258 Jellyfishes, 631, 631 Jet fuels, 136

Jointed appendages, 641, 658 Joints ball-and-socket, 373, 374 bones and, 363 cartilaginous, 372 disorders of, 386, 386 fibrous, 372 hinge, 373, 374 pivot, 373, 374 replacement of, 362, 387 structure, 372 synovial, 372–373, 374 Jolie, Angelina, 79, 96 Junctions between cells, 75–76, 76, 191 Jupiter, moons of, 1 Jurassic period, 541 Juxtaglomerular apparatus, 305, 305

K

Kangaroo rats, 732 Kaposi sarcoma, 249 Karyotypes, 488, 488–489, 489–490 Katydids, 729 Kelp, 588 Kerner, Justinus, 381 Ketamine, 334 Keystone species, 754 Kidney disease, polycystic, 298 Kidney failure, 307–309 Kidneys. See also Urinary system acid-base balance and, 299, 306–307, 307 anatomy of, 301–302, 301–302 artificial, 307, 307 bird, 661 diseases and disorders of, 298, 307, 307–309 fish, 657 glomerular filtration and, 302–303, 303 nephrons and, 301–302, 301–302 regulatory functions of, 304–307, 305, 307 salt reabsorption and, 304–305 solute gradient and, 305 structure and function of, 298, 299 transplantation of, 308, 309 urine production and, 299, 300, 300, 302–303, 303 water reabsorption and, 305, 305–306 Kidney stones, 307 Killer whales, 663 Kinetic energy, 100, 100 Kinetochore spindle fibers, 85 Kingdoms, 6, 7, 8 Kinocillium, 354 Kin selection, 686 Klinefelter syndrome, 489, 490 Knee replacements, 362, 387 Koala, 662 Koch, Robert, 574 Krause end bulbs, 344, 345 Krebs cycle. See Citric acid cycle Krill, 739 Kudzu, 751, 751 Kuru, 336, 581 Kyoto Protocol, 719 Kyphosis, 368

L

Labial palps of clams, 637, 638 of grasshoppers, 645 Labium majora, 417, 418 Labium minora, 417, 418 Labor (childbirth), 455–457, 456–457 Lac operon, 508 Lac repressors, 508 Lacrimal bone, 368 Lacrimal glands, 281 Lactate, 113 Lactation, 456–457, 457 Lacteal, 258 Lactic acid fermentation, 118 Lactobacillus, 576, 602 Lactose, 30, 508–509, 509 Lactose intolerance, 263 Lacunae, 194, 363 Ladybugs, 746

Lagging strand, of DNA, 502 Lag phase, of population growth, 693 Lakes, 732–734, 733–734 Lamarck, Jean-Baptiste de, 537, 537 Lampreys, 657 Lancelets, 442, 442, 653–654, 654 Landscape conservation, 754–756, 755 Landscape diversity, 746 Language, 325–326, 327 Lanugo, 452 Large intestine, 259, 259–260 Laryngitis, 291 Laryngopharynx, 281 Larynx, 281, 281–282 LASIK surgery, 341, 355, 358 Last universal common ancestor (LUCA), 565 Latency, viral, 578 Latent period, 384 Lateral, defined, 197 Lateral line, 657 Latimeria calumniae, 657 Latissimus dorsi, 376–377 Laughing gulls, 677, 677 Laws of independent assortment, 470 scientific, 12 of segregation, 465 of thermodynamics, 100–102 LCA (Leber’s congenital amaurosis), 356 LDCs (less-developed countries), 694, 694–695, 758 LDLs (low-density lipoproteins), 73, 226, 267 Leading strand, of DNA, 502 Leaf veins, 146, 159 Learning associative, 678 in birds, 677, 677 defined, 324, 675 limbic system and, 324–326 social interactions and, 677 Leaves aging of, 184 classification of, 160, 161 color changing of, 127, 140 diversity of, 160, 161 organization of, 159–160, 160–161 photosynthesis and, 128–130, 129 in shoot system, 148 structure of, 159, 160 Leber’s congenital amaurosis (LCA), 356 Leeches, 216, 640, 640 Left optic tract, 350 Left ventricular assist device (LVAD), 228 Legionnaires’ disease, 14 Legumes, 176 Lens, of eye, 347–348, 348 Lenticels, 145 Leprosy, 574, 746 Leptin, 403 Less-developed countries (LDCs), 694, 694–695, 758 Lesser trochanter, 371 Leucophaeus atricilla, 677, 677 Leukemias, 207, 215, 746 Leukocytes. See White blood cells LH (luteinizing hormone), 393, 397, 416, 419 Lichens, 599, 601, 601 Life adaptation as characteristic of, 5 characteristics of, 2–5 classification of organisms, 6–8, 7–8 flow of energy and, 100–101, 100–102 materials and energy for, 4, 4 organization as characteristic of, 2–4, 3 origin of, 565, 565 reproduction and development as characteristic of, 4–5, 5 response to stimuli as characteristic of, 5 Life cycle of ferns, 614, 614 of flowering plants, 171–172, 174, 174–175, 619, 620–621 of humans, 94–96, 95 of moss, 610, 612 of pines, 616 of tapeworms, 634, 635

Life expectancy, 440, 458 Life history patterns, 695–696, 696 Life span, 457 Life zones, 733–734, 734 Ligaments, 193, 363 Light, visible, 131, 131 Light reactions, 130, 132–134, 132–134 Lignin, 146 Limb buds, 450, 451 Limbic system, 324–325, 324–326, 327 Limnetic zone, 733–734, 734 Limpets, 736–737 Limulus amoebocyte lysate, 746 Lineage, 664 Linkage, gene, 483, 483–484 Linkage groups, 483, 483 Linnaean classification, 558 Linoleic acid, 266 Linolenic acid, 266 Lions, 663, 673, 687 Lipase, 262 Lipids defined, 33 in diet, 266–268, 268 fats and oils, 32, 33 phospholipids, 33, 33–34 in plasma membrane, 64, 64, 65 steroids, 34–35, 35 Lipopolysaccharide molecules, 571 Lipoproteins, 214 Liposomes, 568 Lithotripsy, 307 Littoral zone, 733, 734, 735 Liver alcohol and, 332 birds, 661 cirrhosis of, 275, 332 in digestive system, 260, 260–261, 261 fish, 657 Liverworts, 608, 610, 611 Lobe-finned fishes, 653, 657, 658 Lobules, 415 Local hormones, 392 Localized diseases, 207 Locus, 465 Loggerhead turtles, 750, 750 Logistic growth, 692, 693 Long-day plants, 186, 186 Longitudinal fissure, 321 Longitudinal muscle, in earthworms, 639 Long-term memory, 324, 325, 325 Long-term potentiation (LTP), 325 Loop of Henle, 302 Loop of the nephron (loop of Henle), 302 Loose fibrous connective tissue, 193, 193 Lophophorans characteristics of, 626 classification of, 633 in phylogenetic tree, 627 in protostomes, 628, 632 Lophotrochozoa, 628, 632–640, 633–640 Lordosis, 368 Lou Gehrig’s disease, 337, 337 Low-density lipoproteins (LDLs), 73, 226, 267 Lower limbs, 371, 371 Lower respiratory tract, 281, 291–294 LTP (long-term potentiation), 325 “Lub-dub” sound, 220–221 LUCA (last universal common ancestor), 565 “Lucy” (hominin), 666 Lumbar vertebrae, 368, 369 Lumbricus, 639, 639 Lumen, 257 Lung cancer, 293, 294, 294 Lungs of birds, 661 diseases and disorders of, 292–294, 292–294 of fish, 658 gas exchange in, 68, 68 structure and function of, 282–283, 283 Lupus, 250 Luteal phase, 419–420 Luteinizing hormone (LH), 393, 397, 416, 419

Index I-11 LVAD (left ventricular assist device), 228 Lycophytes, 607, 608, 613, 613 Lycopodium, 613, 613 Lymph, 218, 233 Lymphatic capillaries, 206, 218, 233 Lymphatic system. See also Immune system organs of, 234, 234–235 structure and function of, 198, 199, 206, 233, 233 vessels of, 233, 233–234 Lymphatic vessels, 233, 233–234 Lymph nodes, 206, 235 Lymphocytes, 194, 213, 215, 234 Lymphoid organs, 234, 234–235 Lymphomas, 207 Lynx, 697–698, 698 Lysosomes in cell anatomy, 49, 52, 53 composition and function of, 48 Tay-Sachs disease and, 43, 53, 60

M

Macaws, 729 MacLeod, Colin, 498 MAC (Mycobacterium avium complex), 249 Macroevolution, 555–556, 555–557 Macromolecules, 29 Macronucleus, 589 Macrophages, 215, 215, 236 Macular degeneration, 355 Mad cow disease, 336, 580–581 Madreporite, 648 Maggots, 646 Magnesium, 270, 271 Magnification, of microscopes, 45 Major histocompatibility complex (MHC), 236, 239, 685 Malaria, 592–593, 593 Male condoms, 423, 424 Male hormonal contraception, 422 Male reproductive system, 413–416, 413–416, 432–433, 433 Malignant cancer, 514, 514 Malleus, 351, 351 Malpighi, Marcello, 164 Malpighian tubules, 645, 645 Maltase, 262 Malthus, Thomas, 538 Maltose, 30, 30, 262 Mammals, 653, 661–663, 662–663 Mandible fish, 657 human, 366, 367, 368 Manganese, 270 Mantle, in molluscs, 635, 638 Manubrium, 370 Marfan syndrome, 474 Marijuana, 332 Marine diesel, 136 Marine snails, egg-laying behavior of, 675 Marsupials, 662, 662 Mass, atomic, 19 Masseter, 376–377 Mass extinctions, 541, 542–543 Mass number, 19 Mast cells, 236 Mastoid sinuses, 367 Materials, acquisition by organisms, 4, 4 Maternal determinants, 445 Mating, 549, 684–685 Matrix of connective tissue, 191 of mitochondria, 56, 56, 119 Matter, defined, 18 Maxillae, 367, 368 McCarty, Maclyn, 498 McClintock, Barbara, 512, 530 MDCs (more-developed countries), 694, 694–695, 758 MDMA, 334 MD (muscular dystrophy), 387, 485, 485 Measles, 242, 580 Measles-mumps-rubella (MMR) vaccine, 242, 580 Mechanical digestion, 254 Mechanical energy, 100, 100

Mechanoreceptors, 342, 342, 352 Medial, defined, 197 Medial condyle, 371 Medial malleolus, 371 Medicinal leeches, 640, 640 Medicinal value of species, 746–747 Medicine. See Health and medicine Medulla oblongata, 323 Medullary cavity, 364 Medusa stage, 631 Megakaryocytes, 215 Megaphylls, 607, 613 Megaspores, 170, 171 Meiosis anaphase in, 90, 90–91, 92, 93–94 in animal cells, 90, 92 chromosomes and, 88–91, 89–92 comparison with mitosis, 92–94, 93–94 crossing-over and, 90, 90, 484 fertilization and, 89 genetic variation and, 89 importance of, 91 independent assortment and, 90, 91 interkinesis in, 90, 91 meiosis I, 89, 89–90, 93–94 meiosis II, 89, 89, 91, 92, 94 metaphase in, 90, 90–91, 92, 93–94 occurrence of, 93–94 overview, 88–89, 89 prophase in, 90, 90, 91, 92, 93–94 telophase in, 90, 91, 92, 93–94 Meissner corpuscles, 344, 345 Melanin, 202 Melanocytes, 202 Melanocyte-stimulating hormone (MSH), 393, 397 Melanoma, 65, 201, 201 Melatonin, 323, 393, 403, 404 Membrane-assisted transport, 72 Membrane attack complex, 237 Membrane-first hypothesis, 568 Membranes. See also Plasma membrane basement, 191 basilar, 352 evolution of, 567–568, 568 extraembryonic, 448, 448–449 fertilization, 441, 441 in human body, 197, 197–198 mucous, 197 otolithic, 354 serous, 197–198 synovial, 198, 372 tectorial, 352 thylakoid, 133, 133–134 tympanic, 351, 351 Memory, 324–325, 325 Memory B cells, 238 Memory T cells, 241 Mendel, Gregor, 465, 465–466 Mendel’s laws, 465–472, 465–472 Meniere’s disease, 358 Meninges, 198, 319 Meningitis, 336 Menisci, 372 Menopause, 421–422, 461 Menstruation, 420–421, 421 Menstruation disorders, 434 Meristematic tissue, 85, 144, 144 Meristems, 144, 154, 154–155 Merkel disks, 344, 345 MERS (Middle East respiratory syndrome), 14 Mesoderm, 442, 443, 628 Mesoglea, 630, 632 Mesopelagic zone, 739, 739 Mesophyll, 159 Mesozoic era, 541 Messenger RNA (mRNA), 39, 502–503, 502–504 Metabolic pathways, 103–107 Metabolic wastes, excretion of, 299 Metabolism aerobic, 113, 114 anaerobic, 113, 114 athletes and, 113 bacterial, 572–573, 573

defined, 4, 102, 566 energy and, 102–103, 103 oxidation-reduction reactions and, 108–110, 109–110 Metacarpal bones, 366, 370, 371 Metamorphosis, 643, 645 Metaphase, 85, 87, 93–94 Metaphase I, 90, 90–91, 93–94 Metaphase II, 91, 92, 94 Metaphase plates, 85 Metaphysis, 363 Metastasis, 235, 516 Metatarsals, 366, 371, 371 Methamphetamine, 334 Methane, 566, 718 Methanogens, 570, 570 Methicillin-resistant Staphylococcus aureus (MRSA), 535, 553, 574 MG (myasthenia gravis), 248, 337 MHC (major histocompatibility complex), 236, 239, 685 Mice, nurturing behavior in, 674, 674 Micelles, 567 Microarrays, DNA, 529 Microbes. See also Archaea; Bacteria; Eukarya decomposition and, 565 defined, 564 disease and, 564 evolution of, 566–569, 567–568 number of, 564 origin of, 565–568, 565–569 Pasteur’s experiments with, 564, 564 Microbiology, defined, 564 Microbiota, 264, 564 Microevolution, 545–547, 546–548 Microfibrils, 76 Microglia, 196 Micronucleus, 589 Microphylls, 613 Microscopes, 44, 45, 45 Microspores, 170, 171 Microsporidia, 595, 596–597 Microtubules, 49–50, 57, 58, 85 Microvilli, 46, 191, 258 Micturition, 300 Midbrain, 323 Middle ear, 351, 351–352 Middle East respiratory syndrome (MERS), 14 Middle lamella, 76 Miller, Stanely, 566 Miller-Urey experiment, 566, 566 Millipede, 642 Mimicry, 698–700, 700 Mineralocorticoids, 393, 399, 400 Minerals, 270, 270–271 Miocene epoch, 541 Mites, 645 Mitochondria aging and, 458 in cell anatomy, 49 in cellular respiration, 119–121, 120–122, 124 cellular respiration and, 109, 109–110 composition and function of, 48 energy and, 54, 55–56 evolution of, 59–60 in plant cells, 50 structure of, 55–56, 56 Mitosis anaphase, 85, 87, 93–94 in animal cells, 85, 86–87 chromosomes and, 84, 84–88, 86–87 comparison with meiosis, 92–94, 93–94 defined, 81, 85 metaphase, 85, 87, 93–94 occurrence of, 93–94 overview, 84, 84–85 in plant cells, 85, 86–87 prometaphase, 85, 86 prophase, 85, 86, 93–94 telophase, 85, 87, 93–94 Mitotic stage of cell cycle, 81 MMR (measles-mumps-rubella) vaccine, 242, 580

Models, in science, 10 Molds slime, 591, 591–592 water, 588–589, 589 Molecular clock, 558, 664 Molecules in compounds, 23–25, 23–25 defined, 3, 23 formation of, 2 hydrophilic, 26 hydrophobic, 26 motor, 57 organic, 28–29, 29 passage into and out of cells, 67, 67 shape of, 24 signaling, 65 Molluscs, 627, 635–638, 636–637 Molting, 641 Mongooses, 751, 751 Monkeys, 663, 679, 679 Monoclonal antibodies, 244, 244–245 Monocots embryo development in, 175 eudicots vs., 618–619, 619 flowering of, 172, 172 seed germination in, 177–178, 178 structure of, 149, 149, 151, 151 Monocotyledones, 618 Monocytes, 213, 215, 236 Monoecious plants, 171 Monohybrid crosses, 468, 468 Monomers defined, 29, 566 evolution of, 566–567, 567 organic, 566 primordial soup hypothesis and, 566 in synthesis and degradation of polymers, 29, 29 Mononuclear cells, 215 Mononucleosis, 215, 580 Monosaccharides, 30, 30 Monosomy, 489 Monotremes, 662, 662 Monounsaturated fatty acids, 266 Mons pubis, 417, 418 Montane coniferous forest, 726 Moose, 727, 727 More-developed countries (MDCs), 694, 694–695, 758 Morels, 598, 598 Morning-after pills, 424 Morning sickness, 453 Morphine, 334 Morphogenesis apoptosis and, 448 defined, 444 homeotic genes and, 446–447, 447 morphogen genes and, 446 Morphogen genes, 446 Morphogen gradients, 446, 447 Morula, 442, 449 Mosaic evolution, 666 Mosses, 607, 608, 610–611, 612 Moths, 173, 546–547, 546–548 Motor molecules, 57 Motor neurons, 314, 315 Motor unit, 384 Mouth crayfish, 643 in digestive system, 254–256, 255 earthworm, 639 roundworm, 641 MPTP, 108 mRNA (messenger RNA), 39, 502–503, 502–504 MRSA (methicillin-resistant Staphylococcus aureus), 535, 553, 574 MS. See Multiple sclerosis MSH (melanocyte-stimulating hormone), 393, 397 Mucosa, 257 Mucous membranes, 197, 281 Mucus plugs, 455 Müllerian mimicry, 699 Multidrug combination products, 428 Multiple allele inheritance, 476



I-12

Index

Multiple fruits, 176 Multiple sclerosis (MS), 248, 250, 313, 335, 336, 337 Mumps, 255 Musca, 644 Muscle fiber contraction, 377–380, 378–380, 382–383, 382–383 Muscle fibers, 378, 378–379 Muscles actions of, 377 antagonistic pairs of, 375, 375 cardiac, 195, 196 contraction of, 377–380, 378–380, 382–385, 382–385 disorders of, 387 insertion of, 375 nomenclature of, 375, 377 origin of, 375 shape of, 375 size of, 375 skeletal, 195, 195, 375–377, 375–377 smooth, 195, 195 tone of, 384 twitching, 384 Muscle spindles, 343–344, 344 Muscular dystrophy (MD), 387, 485, 485 Muscularis, 257 Muscular system. See Musculoskeletal system Muscular tissue, 195, 195–196 Musculoskeletal system, 362–387. See also Bones; Muscles aging and, 460 diseases and disorders of, 385–387, 386 structure and function of, 198–199, 199 tissues in, 363, 364 Mushrooms, 2 Mustard plants, 163 Mutagens, 511 Mutations. See Genetic mutations Mutualism, 701, 701 Myasthenia gravis (MG), 248, 337 Mycelium, 595–596, 596 Mycobacterium avium complex (MAC), 249 Mycobacterium leprae, 574 Mycobacterium tuberculosis, 292, 553, 574–575, 575 Mycorrhizae, 152, 153, 595, 601 Mycoses, 601–602, 602 Myelin sheaths, 196, 315, 315 Myocardial infarction, 227 Myocardium, 219 Myofibrils, 378 Myofilaments, 378 Myoglobin, 383 Myogram, 384 Myosin, 57, 378 MyPlate, 34, 263, 264 Myxedema, 406

N

NAD+ (nicotinamide adenine dinucleotide), 107, 114, 114 NADP+ (nicotinamide adenine dinucleotide phosphate), 130 Naegleria fowleri, 584, 595, 603 Nails, 202, 202 Names, scientific, 8 Nasal bones, 368, 368 Nasal cavities, 255, 281, 281 Nasolacrimal canal, 368 Nasopharynx, 256, 281 Natriuresis, 401 Natural family planning, 423 Natural killer (NK) cells, 236, 459 Natural selection. See also Evolution adaptation and, 538 defined, 5 directional, 552, 552 disruptive, 552, 554 fitness and, 550–551 inheritance and, 550 principles of, 538, 549–550 reproductive success and, 538, 550 stabilizing, 551–552, 552 variation and, 538, 550

Neandertals, 668–669 Nearsightedness, 355, 355 Nectar guides, 173 Neem trees, 143, 166 Negative feedback, 204–205, 204–205, 393 Neglected tropical diseases (NTDs), 594, 624 Neisseria gonorrhoeae, 430–431, 575 Neisseria meningitidis, 336 Nematocysts, 630, 632 Nematodes, 446, 446, 640 NE (norepinephrine), 318, 329, 393, 400 Nephridia, 639 Nephridium, 638 Nephrons anatomy of, 301–302, 301–302 reabsorption from, 303–304, 304 Nereis, 638, 638–639 Neritic province, 737, 739 Nerve cord in annelids, 638 in chordates, 653 Nerve fibers, 327 Nerve ganglion, grasshopper, 645 Nerve growth factor, 403 Nerve impulse transmission, 315–319, 316–318 Nerves cranial, 327, 328 defined, 327 formation of, 196 spinal, 319, 327–328, 328 Nervous system, 313–337 aging and, 459 in arthropods, 641 brain, 320–323, 321–322 central, 314, 319–323, 319–323 diseases and disorders of, 334–337, 335–337 drug abuse and, 331–332, 331–334 impulse transmission in, 315–319, 316–318 limbic system and, 324–325, 324–326, 327 neurons of, 196, 196, 314, 314–315 organization of, 314, 314, 319 peripheral, 314, 327–331, 328–331 spinal cord, 319–320, 320 structure and function of, 198, 199 tissue in, 4, 196, 196–197, 314–315 Nervous tissue, 4, 196, 196–197, 314–315 Neural crest, 443 Neural plate, 443 Neural tube, 443, 444 Neurofibrillary tangles, 460 Neuroglia, 196, 196–197, 314 Neuromuscular junction, 378, 380 Neurons, 196, 196, 314, 314–315 Neurotransmitters, 318–319. See also specific neurotransmitters Neurula, 443 Neutrons, 18–19, 19 Neutrophils, 213, 215, 236 Niacin, 107, 269 Niche, ecological, 696 Nicotinamide adenine dinucleotide (NAD+), 107, 114, 114 Nicotinamide adenine dinucleotide phosphate (NADP+), 130 Nicotine, 226, 293, 332. See also Smoking Nile crocodiles, 660 Nitrates, 163 Nitrification, 715 Nitrogen cycle, 714–715, 715, 717 Nitrogen fixation, 714–715 Nitrous oxide, 718 NK (natural killer) cells, 236, 459 Noble gases, 20, 22 Nociceptors, 63, 344 Nodes, of plants, 148 Nodes of Ranvier, 315, 315 Noise levels, hearing loss and, 356, 357 Nomenclature, of muscles, 375, 377 Noncyclic electron pathway, 132–133, 133 Nondisjunction, 488, 488–489 Nonfunctioning proteins, 513 Nonpolar covalent bonds, 24 Nonrandom mating, 549

Nonrenewable resources, 757 Nonseptate fungi, 595 Nonvascular plants, 610–611, 611 Nonvertebrate chordates, 654, 654 Norepinephrine (NE), 318, 329, 393, 400 Nose, 281, 281 Nostrils bird, 661 fish, 657 Notochord in chordates, 653 in human embryos, 443 NTDs (neglected tropical diseases), 594, 624 Nuclear envelope, 49–51, 50, 58 Nuclear pores, 50, 50–51 Nucleic acids, 38–39, 38–39. See also Deoxyribonucleic acid (DNA); Ribonucleic acid (RNA) Nucleoid, 46, 47, 572 Nucleoli, 48–50, 50 Nucleoplasm, 50 Nucleotides, 38, 38 Nucleus, 48–51, 49–51 Nudibranchs, 636, 636 Nurturing behavior, 674, 674 Nutrients, defined, 263. See also Diet and nutrition Nutrient uptake, in plants, 161–166, 162, 164–165

O

Obesity cardiovascular disease and, 226 nutrition and, 263–265, 264 surgery for, 253, 265, 275 Observation, in science, 9 Occipital bone, 367, 367 Occipitalis, 376–377 Occipital lobe, 321, 322 Occupational hazards, cancer and, 515 Oceans, 737, 737, 739, 739–740 Octet rule, 22 Octopuses, 636, 636 Odocoileus virginarius, 663 Oil glands, 202 Oils, 32, 33, 268, 268 Okazaki fragments, 502 Olecranon, 370 Olfaction (smell), 345–346, 346, 354, 459 Olfactory bulb, 324, 346, 346–347 Olfactory cells, 346 Olfactory epithelium, 346 Olfactory tract, 324, 347 Oligocene epoch, 541 Oligochaetes, 639, 639–640 Oligodendrocytes, 196, 315 Oligotrophic lakes, 732, 733 Omega-3 fatty acids, 266 Omnivores, 707 Oncogenes, 83, 83, 513, 514 One-trait genetic crosses, 468–469, 468–469 One-trait testcrosses, 469, 469–470 Oocytes, 95, 416–418, 441 Oogenesis, 94, 95, 95–96, 416 Oparin, Alexander, 566 Oparin-Haldane hypothesis, 566 Open circulatory system, 638 Operant conditioning, 678 Operons, 508 Ophiothrix, 647 Opisthokonts, 585, 592 Opium, 334 Opossum, 662 Opportunistic infections, 241, 249, 250, 425 Opportunistic life history patterns, 696 Optic chiasma, 350, 350 Optic nerve, 348, 348 Optic vesicle, 446, 451 Oral cavity, 254 Oral contraception, 422, 423 Orbicularis oculi, 376–377 Orbicularis oris, 376–377 Orbits, 367 Orchids, 729 Orcinus orca, 663 Order (taxonomy), 6

Ordovician period, 541 Organ development, 443, 444 Organelles energy-producing, 54–56, 54–56 in eukaryotes, 48, 48–49 origin of, 59 Organic chemicals, 752 Organic farms, 760–761 Organic molecules, 28–29, 29 Organic monomers, 566–567, 567 Organic nutrient transport, 164–166, 165 Organic polymers, 566, 567 Organisms classification of, 6–8, 7–8 defined, 3 Organization biological, 2, 3 cellular level of, 44, 44, 46 as characteristic of life, 2–4, 3 Organ of Corti, 352 Organs, defined, 3, 4 Organ systems, 3, 4, 198–200, 199 Organ transplants from animals, 656 heart, 228 kidney, 308, 309 rejection of, 248 Orgasm, 414, 418 Origin of eukaryotes, 58–60, 60 of life, 565, 565 of microbes, 565–568, 565–569 of muscle, 375 Ornithorhynchus anatinus, 662 Oropharynx, 281 Osculum, 629 Osmoregulation, 299, 304–306, 305 Osmosis, 68–70, 69–70 Osmotic pressure, 69 Ossicles, 351 Ossification, 365, 365 Osteoarthritis, 386 Osteoblasts, 365 Osteoclasts, 365 Osteocytes, 363, 364 Osteons, 194, 363, 364 Osteoporosis, 270, 386, 386 Otitis media, 291 Otolithic membrane, 354 Otoliths, 353, 354 Outer ear, 351, 351 Out-of-Africa hypothesis, 668, 668 Oval window, 351, 351 Ovarian cancer, 83, 84, 433–434 Ovarian cycle, 418–420, 419–421 Ovarian cysts, 434 Ovaries in endocrine system, 393–394, 402 in flowers, 171, 619 grasshopper, 645 hormonal control of, 419–420, 420 human, 416, 416–417 Overexploitation, 752–753, 753 Oviduct, grasshopper, 645 Oviducts, 416–417, 417 Ovipositors, grasshopper, 645 Ovulation, 416, 419 Ovules, 171, 617 Owls, 732 Oxidation-reduction reactions, 108–110, 109–110 Oxygen debt, 118, 383 Oxyhemoglobin, 289 Oxytocin, 393, 395–396 Ozone depletion, 752

P

Pacemakers, 221 Pacinian corpuscles, 344, 345 Pain receptors, 63, 344–345 Palate, 255, 281 Palatine bones, 367 Paleocene epoch, 541 Paleozoic era, 541 Palpitations, 222 Pancreas

Index I-13 bird, 661 in digestive system, 260, 260, 261 in endocrine system, 393–394, 401, 401–402 Pancreatic amylase, 262 Pancreatic cancer, 275 Pancreatic islets, 401 Pancreatic juice, 262 Pancreatitis, 275 Pandinus, 646 Panspermia, 567 Panthera leo, 663 Pantothenic acid, 269 Paper wasps, 642 Papillae, 345 Papio hamadryas, 684 Pap test, 428 Parabasalids, 590 Paramecia, 58, 589, 590, 696, 697 Paraplegia, 336 Parapodia, 638 Parasites, 700 Parasitic flatworms, 633–634 Parasitism, 699, 700, 700–701 Parasympathetic division, of autonomic system, 329, 330–331 Parathyroid gland disorders, 406 Parathyroid glands, 393–394, 398, 398, 399 Parathyroid hormone (PTH), 393, 399 Parenchyma, 145, 145–146, 159 Parietal bones, 367, 367 Parietal lobe, 321, 322 Parkinson disease (PD), 108, 196, 335, 531 Parsimony, 558 Particulate theory of inheritance, 466 Parturition, 455, 455–456 Passive immunity, 241, 242, 243, 244 Passive smoking, 294 Pasteur, Louis, 564, 564 Patau syndrome, 489, 489 Patella, 366, 371, 371 Paternity analysis, 523 Pathogens, 235 Pattern formation, 444, 446, 447 Patterns of inheritance, 472–475 Pavlov, Ivan, 678 PCR (polymerase chain reaction), 522, 522–523 PCT (proximal convoluted tubule), 302 PD (Parkinson disease), 108, 196, 335, 531 Peas, garden, 466, 466–467 Pecten, 636 Pectins, 76 Pectoral girdle, 366, 370, 370–371 Pectoralis major, 376–377 Pedicellaria, 648 Pedicels, 171 Pedigrees analysis of, 472–475, 473–475 for autosomal disorders, 472–475, 473–475 defined, 472 genetic disorders and, 472–475, 473–475 of hemophilia in European royal families, 487, 487 for X-linked disorders, 485, 485 Pedipalps, 644 Pelagic division, 737, 737 Pellagra, 107, 268 Pellicles, 590 Pelvic cavity, 197, 197 Pelvic girdle, 366, 371, 371 Pelvic inflammatory disease (PID), 430 Pen, of squid, 637 Penicillin, 107, 553 Penicillium, 9, 576, 598 Penis, 413, 413–414 Pentoses, 30, 30 Peppered moths, 546–547, 546–548 Peppers, 63, 76 Pepsin, 262 Peptidases, 262 Peptide bonds, 36, 36 Peptide hormones, 394, 394 Peptides, 35–36, 36, 262 Peptidoglycan, 46 Perception, 342

Perfect flowers, 171 Perforin, 239 Performing, biology of, 190, 207 Pericardial cavity, clam, 638 Pericardium, 198, 219 Pericycle, 151, 151 Perilymph, 352 Periodic table, 19, 19–20 Periodontitis, 256 Periosteum, 363, 364 Peripheral nerve disorders, 337 Peripheral nervous system (PNS), 314, 327–331, 328–331 Peripheral proteins, 64, 65 Peristalsis, 256 Peritoneum, 198 Peritonitis, 259 Peritubular capillary network, 301, 304 Periwinkles, 737, 746 Permafrost, 726 Permeability, of plasma membrane, 66–73, 67–74 Permian period, 541 Peroneus longus, 376–377 Peroxisomes, 48–50, 54, 54 Pertussis. See Whooping cough Pesticides, 435, 760 Petals, 171, 619 Petioles, 148 PET (positron emission tomography), 20, 20 Peyer’s patches, 258 PF (precision farming), 761 p53 gene, 83, 514, 528 pH, enzymes and, 106, 107 Phagocytes, 236 Phagocytosis, 73, 74 Phalanges (foot), 366, 371, 371 Phalanges (hand), 366, 370, 371 Pharming, 656 Pharyngeal pouches, 451, 653 Pharyngitis, 290, 291, 574 Pharynx in digestive system, 256, 256 earthworm, 639 in respiratory system, 281, 281–282 roundworm, 641 tunicate, 654 Phascolarctos cinereus, 662 Phenomena, 9 Phenotypes, 466–468, 468, 537 Phenylalanine, 36, 464 Phenylketonuria (PKU), 464, 474, 479, 513 Phenylketonurics, 464 Phenylthiocarbamide (PTC), 477 Pheromones, 395, 679 Phloem, 146, 146, 151, 151, 164, 611 Phloem ray, 155–156 Phloem transport, 164–166, 166 Phoenicopterus ruber, 661 Phoronids, 633 Phosphate group, 29, 33 Phospholipid bilayer, 46, 64, 64 Phospholipids, 33, 33–34 Phosphorus, 270, 271 Phosphorus cycle, 713, 714, 714 Phosphorylation, 103 Photochemical smog, 716 Photoperiods, 186, 186 Photoreceptors, 342, 342, 349, 349 Photosynthesis, 127–140 alternative pathways for, 137–138, 137–139 in bacteria, 572 C3, 138, 138 C4, 138, 138–139 CAM pathway in, 139 cellular respiration vs., 139, 140 cyclic electron pathway in, 134, 134 defined, 4, 128 in flowering plants, 128–130, 129 leaves and, 128–130, 129 light reactions in, 130, 132–134, 132–134 noncyclic electron pathway in, 132, 132–133 overview, 128

oxidation-reduction reactions and, 109, 109 in protists, 585–588 reaction in, 54, 130, 130 thylakoid membrane and, 133, 133–134 visible light and, 131, 131 Photosystems, 132, 133 Phototropism, 183, 185, 186, 186 Photovoltaic cells, 759 pH scale, 27–28, 28 Phycobilins, 128 Phyletic gradualism model, 556 PhyloCode, 558 Phylogenetics, 557, 558 Phylogeny, 557–558, 558–559 Phylum (taxonomy), 6 Physalia, 631 Phytochrome, 186–187, 187 Phytoplankton, 589 Phytoremediation, 163 Pica, 273 PID (pelvic inflammatory disease), 430 Pigment cells, 642 Pimples, 203 Pineal gland, 323, 393–394, 403 Pines, 613, 616 Pingelapese, inbreeding among, 550–551, 551 Pink eye, 348 Pinna, 351, 351 Pinocytosis, 73, 74 Pinworms, 641 Pistils, 619 Pisum sativum (garden pea), 466, 466–467 Pith, 151, 151, 154, 155 Pithovirus, 577 Pituitary dwarfism, 405, 405 Pituitary gland, 323, 393–394, 395–397, 396–397 Pituitary gland disorders, 405, 405–406 Pivot joints, 373, 374 PKD (polycystic kidney disease), 298 PKU (phenylketonuria), 464, 474, 479, 513 Placenta as afterbirth, 453, 456 fetal circulation and, 453 structure and function of, 421, 453 Placental mammals, 662, 663 Planarians, 633, 634 Plankton, 585, 733 Plants, 606–621 abscisic acid in, 184–185, 185 alternation of generations in, 170, 607, 609, 609 anatomy of, 50 antipredator defenses in, 698 asexual reproduction in, 178–181, 179–181 auxin in, 183, 183–184 body organization of, 147, 147 C3, 138, 138 C4, 138, 138–139 as carbon dioxide fixers, 134–137, 135 cells of, 50, 144–146, 144–146 cell walls of, 50, 76 characteristics of, 7, 8 commercial uses of, 608–609, 608–609 cytokinesis in, 88, 88 cytokinins in, 184 day-neutral, 186 deciduous, 148 dioecious, 171 epidermal tissue in, 144–145, 144–145 ethylene in, 185, 185 evergreens, 148 evolution of, 607–609, 607–609 flowering (See Flowering plants) as food, 608, 608 genetic engineering of, 180–181, 180–181 gibberellins in, 184, 184 ground tissue in, 144, 144–145, 145–146 hormones in, 183–185, 183–185 humans and, 608–609, 608–609 hybridization of, 180 importance of, 607 insects and, 619 long-day, 186, 186 meristematic tissue in, 85, 144, 144

mitosis in, 85, 86–87 monoecious, 171 nonvascular, 610–611, 611 organs and systems of, 147–148, 147–148 pollination in, 172–174, 175, 617 propagation of, 179, 179–180 root organization in, 150–153, 150–153 root systems in, 147–148, 147–148 seed, 607, 615–621, 616–621 seedless, 169, 187, 611–615, 613–615 shoot systems, 147, 147–148, 148 short-day, 186, 186 as solar energy converters, 131–134, 131–134 stems of, 148 stimuli, responses to, 185–186 stomata in, 129, 129, 130, 145, 163–164, 164 tissues of, 144–146, 144–146, 179–180 transgenic, 526 transport of nutrients in, 161–166, 162, 164–165 tropisms in, 185–186, 186 turgor pressure and, 69 uptake of nutrients in, 161–166, 162, 164–165 vascular tissue in, 144, 144, 146, 146, 151, 607, 610 viroid disease in, 580 water uptake in, 161–163, 162 woody, 158 Plaques, 34, 225, 226, 267 Plasma, 194, 194–195, 212, 214 Plasma cells, 238 Plasma membrane, 64–74 active transport across, 67, 71–72, 72 bulk transport across, 72–73, 73–74 in cell anatomy, 49 diffusion across, 67–68, 68 endocytosis and, 67, 73, 74 in eukaryotes, 48–49 evolution of, 567–568, 568 exocytosis and, 67, 72–73, 73 facilitated transport across, 67, 71, 71 fluid-mosaic model of, 64, 64 homeostasis and, 64 lipids in, 64, 64, 65 osmosis and, 68–70, 69–70 permeability of, 66–73, 67–74 phagocytosis and, 73, 74 pinocytosis and, 73, 74 in plant cells, 50 in prokaryotes, 46, 47 proteins in, 64, 64, 65–66, 66 sodium-potassium pump and, 71, 72 structure and function of, 64, 64–66, 66 Plasmapheresis, 337 Plasmids, 47, 520, 572 Plasmin, 99, 110 Plasminogen, 99, 110 Plasmodesmata, 76, 146 Plasmodial slime molds, 591, 591 Plasmodium vivax, 592–593, 593 Plasmolysis, 70 Plastids, 55, 585 Platelet-derived growth factor, 403 Platelets, 195, 213, 215–216, 216 Play, in animals, 681 Pleasure, in animals, 681 Pleistocene epoch, 541 Pleura, 198, 282 Pleurobrachia pileus, 630 Pliocene epoch, 541 Plumules, 178 PMS (premenstrual syndrome), 434 Pneumatophores, 152 Pneumocystis carinii, 292 Pneumocystis pneumonia, 249 Pneumonectomy, 294 Pneumonia, 292, 292 PNS (peripheral nervous system), 314, 327–331, 328–331 Poachers, 753 Podocytes, 301 Point mutations, 512, 512, 548 Poison-dart frogs, 700, 729



I-14

Index

Poisons as enzyme inhibitors, 107 fungi and, 600 Polar bodies, 95, 418 Polar covalent bonds, 24 Polar easterlies, 724 Polio, 242 Pollen cones, 617 Pollen grains, 170, 171, 174, 617, 619 Pollen sacs, 616 Pollen tubes, 171, 174, 617 Pollination, 172–174, 175, 617 Pollinators, 173, 617 Pollution, 751–752 Polyandrous primates, 683 Polychaetes, 638, 638–639 Polyculture, 760, 761 Polycystic kidney disease (PKD), 298 Polygenic inheritance, 476–477, 477–478, 478 Polymerase chain reaction (PCR), 522, 522–523 Polymers defined, 29 evolution of, 567 organic, 566 synthesis and degradation of, 29, 29 Polyp cnidarians, 631 Polypeptides, 35–36 Polyps, colon, 275 Polyribosomes, 49, 51, 505, 505 Polysaccharides, 30–31, 31 Polyspermy, 441 Polytrichum, 612 Polyunsaturated fatty acids, 266 Poly-X syndrome, 489, 490 Pons, 323 Poplars, 163 Population genetics, 546 Population growth age distributions and, 695, 695 biotic potential of, 692, 692 carrying capacity and, 693 exponential, 692, 692 human, 693–695, 694–695 in less-developed countries, 694, 694–695 in more-developed countries, 694, 694–695 patterns of, 692, 692–695 survivorship and, 693, 693 zero, 694 Populations competition between, 696–697, 697 defined, 3, 4, 546, 691 in ecosystems, 707, 707 interactions between, 695–701 predator-prey dynamics, 697–698, 698 Porifera, 629, 629–630 Pork tapeworm, 634, 635 Portal system, 224, 396 Portuguese man-of-war, 631, 631 Positive feedback, 205–206, 396 Positron emission tomography (PET), 20, 20 Positrons, 20 Postanal tails, 653 Posterior, defined, 197 Posterior pituitary gland, 395–396, 396 Postsynaptic membrane, 318 Posttranscriptional control of gene expression, 510, 511 Posttranslational control of gene expression, 510, 511 Postzygotic isolating mechanisms, 555, 555 Potassium, 270, 271 Potassium gates, 316, 317 Potato blight, 181 Potential energy, 100, 100 Precapillary sphincters, 212 Precipitation, 713 Precision farming (PF), 761 Predation, 697–700, 698, 700 Predators, 697 Predictions, 10 Prefrontal area, 323 Pregnancy. See also Infertility birth defects, testing for, 454, 455 childbirth, 455–457, 456–457

ectopic, 417 energy level and, 453 fertilization and, 421, 422 morning sickness and, 453 smoking and, 293 smooth muscle and, 454 Prehypertension, 225 Preimplantation genetic diagnosis, 455 Premenstrual syndrome (PMS), 434 Preparatory reaction, 114, 115, 119 Presbyopia, 348, 459 Pressure-flow model of phloem transport, 164–166, 166 Presynaptic membrane, 318 Pretranscriptional control of gene expression, 509–510, 510–511 Prey, 697 Prezygotic isolating mechanisms, 555, 555 Pride, of lions, 673, 687 Primary follicles, 418 Primary growth, 150 Primary meristems, 154, 154 Primary motor area, 321, 322 Primary oocytes, 95, 418 Primary organizers, 445 Primary producers, 565 Primary response, to vaccines, 241 Primary somatosensory area, 321, 322 Primary spermatocytes, 94 Primary structure of proteins, 36, 37 Primary succession, 702 Primates, 663, 665 Primitive streak, 443, 443 Primordial soup hypothesis, 566 Principles, scientific, 12 Prions, 37, 336, 336, 580–581 PRL (prolactin), 393, 397, 456–457 Probability one-trait crosses and, 469 two-trait crosses and, 471 Probiotics, 576 Processes, bone, 366 Processing centers, 323 Producers, 707, 707 Product rule of probability, 469 Products, in chemical reactions, 102 Profiling, genetic, 523, 531 Profundal zone, 734, 734 Progesterone, 393, 403, 419, 421 Proglottids, 633–634 Prokaryotes characteristics of, 7, 8, 44 comparison with eukaryotes, 47, 47 defined, 569 gene expression in, 508–509, 509 structure of, 46–47, 46–47 Prolactin (PRL), 393, 397, 456–457 Prometaphase, 85, 86 Promoters, 502, 508, 510 Propagation, of plants, 179, 179–180 Prophase, 85, 86, 93–94 Prophase I, 90, 90, 91, 93–94 Prophase II, 91, 92, 94 Proprioceptors, 343–344 Prop roots, 152 Prostaglandins, 403, 414 Prostate cancer, 433 Prostate disorders, 432–433, 433 Prostate gland, 413, 413 Prostate-specific antigen (PSA), 433 Protease, 428 Protease inhibitors, 428 Protective proteins, 236–237, 237 Protein-first hypothesis, 567, 569 Proteinoids, 567 Proteins carrier, 66, 66, 67, 70–71, 71–72 cell recognition, 66, 66 channel, 66, 66, 67 defined, 35 denaturation of, 37 in diet, 266 enzymatic, 66, 66 functions of, 35 genetic mutations and activity of, 512–513, 512–513

integral, 64, 65–66, 66 metabolic fate of, 123 nonfunctioning, 513 organizational structure of, 36–37, 37 peptides and, 35–36, 36 peripheral, 64, 65 in plasma membrane, 64, 64, 65–66, 66 protective, 236–237, 237 receptor, 66, 66 types of, 35 for vegetarians, 267 Proteoglycans, 75, 75 Proteomics, 531–532 Prothrombin, 215 Prothrombin activator, 110, 215 Protiotics, 576 Protists, 585–595 amoebozoans, 585, 590–592, 591 archaeplastids, 585–588, 587–588 characteristics of, 7, 8, 585 chromalveolates, 585, 588–589, 588–590 defined, 585 diseases caused by, 592–595, 593 diversity of, 585–592, 585–592 excavates, 585, 589–590, 590–591 opisthokonts, 585, 592 photosynthetic, 585–588 reproduction in, 585 rhizarians, 585, 592 Protobionts, 566, 567–568, 568 Protocells, 567 Protonema, 610 Protons, 18–19, 19 Proto-oncogenes, 82–83, 82–83, 513, 514 Protoplasts, 179, 180, 526 Protostomes characteristics of, 626 deuterostomes vs., 628, 628 embryonic development in, 628, 628 tissue layers of, 628 Protozoal diseases, 592–595, 593 Protozoans, 585, 589 Proviruses, 579 Proximal, defined, 197 Proximal convoluted tubule (PCT), 302 PSA (prostate-specific antigen), 433 Pseudocoelom, 640, 641 Pseudoephedrine, 334 Pseudomonas aeruginosa, 553, 553 Pseudoplasmodium, 592 Pseudopods, 590 Pseudostratified epithelium, 191, 192 Psilocybin, 598, 600 Psilotum, 615 PTC (phenylthiocarbamide), 477 Pteridophytes, 613 PTH (parathyroid hormone), 393, 399 Puberty, 457 Pubis, 371, 371 Publications, scientific, 10–11, 11 Puffballs, 598 Pulmonary arteries, 219, 452 Pulmonary circuit, 222–223, 223 Pulmonary fibrosis, 292, 294 Pulmonary semilunar valve, 219 Pulmonary surfactant, 283 Pulmonary tuberculosis, 292, 292 Pulmonary veins, 219, 452 Pulp (teeth), 256 Pulse, 220 Punctuated equilibrium, 556, 557 Punnett squares, 468, 468–469 Pupil, 347–348, 348 Purines, 499, 499 Purkinje fibers, 221 Pyelonephritis, 307 Pyloric stomach, 648 Pyramids, ecological, 710, 710 Pyrenoids, 587 Pyrimidines, 499, 499 Pyrogens, 403 Pyruvate, 114, 115

Q

Quadriceps femoris, 376–377 Quadriplegia, 336

Quaternary period, 541 Quaternary structure of proteins, 36, 37

R

Radial canal, 648, 648 Radial symmetry, 626 Radiata, 626 Radiation medical uses of, 20, 20–22, 454 solar, 723, 723–724 Radioactive dating, 540 Radioactive decay, 21 Radioactive isotopes, 20–22 Radius, 366, 370, 370 Radon, 515 Radula, 636 Rain forests temperate, 727–728 tropical, 13, 728–729, 729 Rain shadow, 724, 724 Random mating, 547 Rapid eye movement (REM) sleep, 326 RA (rheumatoid arthritis), 250, 386 Ras proteins, 83 RAS (reticular activating system), 323 Rate of natural increase, 692 Ray-finned fishes, 653, 657, 658 RB (retinoblastoma) protein, 514 rDNA (recombinant DNA), 180, 520, 520, 522 Reactants, 102 Reaction center, 132 Reactions coupled, 102–103, 104 endergonic, 102 exergonic, 102 light, 130, 132–134, 132–134 oxidation-reduction, 108–110 photosynthetic, 54, 130, 130 redox, 108 Reasoning, 9 Receptacles, 619 Receptor-mediated endocytosis, 73, 74 Receptor proteins, 66, 66 Recessive alleles, 466, 468 Reciprocal altruism, 687 Recombinant DNA (rDNA), 180, 520, 520, 522 Recombination, linkage groups and, 483, 484 Rectum bird, 661 grasshopper, 645 human, 259 sea star, 648 Rectus abdominis, 376–377 Red algae, 587–588, 588 Red blood cells, 194–195, 213–214, 214 Red bone marrow, 217, 217, 234, 363 Redox reactions, 108 Red pulp, 235 Red tide, 589 Reduced hemoglobin, 289 Reemerging diseases, 14 Reeve, Christopher, 337 Referred pain, 345 Reflex actions, 256, 328 Reflex arc, 328, 329 Refractory period, 317, 414 Regulated secretion, 73 Rejection of tissue, 248 Relaxation period, 384 Remodeling, of bones, 365, 365 REM (rapid eye movement) sleep, 326 Renal arteries, 299, 301 Renal capsules, 299 Renal cortex, 301, 302 Renal medulla, 301 Renal pelvis, 301 Renal veins, 299, 301 Renewable resources, 713, 757, 758–760 Renin, 305, 400 Replacement model, 668, 669 Replacement reproduction, 695 Replication of DNA, 501, 501–502 errors in, 511 viral, 578

Index I-15 Repolarization, 317 Repressors, 508 Reproduction. See also Pregnancy; Reproductive system in bacteria, 572, 572 as characteristic of life, 4–5, 5 control of, 422–424, 423 differential reproductive success, 538, 550 in earthworms, 639 fitness and strategies of, 683–684, 684 in flowering plants, 170–172, 170–172, 174, 174–175 of fungi, 596, 596, 597, 598 in grasshoppers, 645 in protists, 585 replacement, 695 in viruses, 578, 578–579 Reproductive cloning, 521, 521 Reproductive success, natural selection and, 538, 550 Reproductive system, 412–436. See also Reproduction aging and, 461 diseases and disorders of, 432–434, 433–434, 436 female, 416–422, 416–422, 433–434, 434, 436 male, 413–416, 413–416, 432–433, 433 sexually transmitted diseases and, 424–432 structure and function of, 199, 200 Reproductive technology, 412, 436 Reptiles, 653, 659–660, 660 RER (rough endoplasmic reticulum), 48–50, 52, 52–53 Reserve design, 756 Reservoirs, 710, 718 Residual volume, 284 Resistance, antibiotic, 535, 553, 560, 575–576 Resolution, of microscopes, 45 Resource partitioning, 696–697 Respiration. See also Cellular respiration; Respiratory system external, 286–287, 287, 289 internal, 287, 289 Respiratory acidosis, 287 Respiratory alkalosis, 287 Respiratory center, 286, 306 Respiratory system, 279–294 aging and, 459 in arthropods, 643 breathing and, 283–286, 284–286 bronchial tree in, 282 diseases and disorders of, 289–294, 291–294 gas exchange in, 68, 68, 286–287, 287, 289 in grasshopper, 645 larynx, 281, 281–282 lungs, 282–283, 283 nose, 281, 281 pathway of air in, 280–281, 281 pharynx, 281, 281–282 structure and function of, 198, 199, 280, 280 trachea, 281–282, 282 Respiratory tract, 280–283, 289–294 Respiratory volumes, 284, 284 Responding variables, 10 Response to stimuli, as characteristic of life, 5 Resting potential, 315–316, 316 Restoration ecology, 756 Restoration of habitat, 756–757, 756–757 Restriction enzymes, 520 Results, of scientific studies, 13 Resveratrol, 226 Reticular activating system (RAS), 323 Reticular connective tissue, 194 Reticular fibers, 191 Reticular formation, 323 Retina, 347–350, 348–350 Retinal, 349 Retinal detachment, 355 Retinoblastoma, 83 Retinoblastoma (RB) protein, 514 Retinopathy, 355 Retroviruses, 578, 579

Reverse transcriptase, 427, 569 Reverse transcriptase inhibitors, 428 R groups, 35, 36 Rhabdom, 642 Rheumatoid arthritis (RA), 250, 386 Rhinoviruses, 290, 579 Rhizarians, 585, 592 Rhizoids, 610 Rhizome, 615 Rhizomes, 158 Rhizopus stolonifer, 597, 597 Rhodopsin, 349 Rh system, 247–248 Rib cage, 369, 369–370 Rib facets, 369 Riboflavin, 107, 269 Ribonucleic acid (RNA) comparison with DNA, 38, 38 in evolution of DNA, 568–569 gene expression and, 502–507, 502–508 messenger RNA, 39, 502–503, 502–504 ribosomal RNA, 50, 51, 505, 505 structure of, 38, 38–39 transfer RNA, 504, 504–505 in viroids, 580 in viruses, 577, 578 Ribosomal RNA (rRNA), 50, 51, 505, 505 Ribosomes in bacteria, 47, 47 binding sites of, 505, 505 in cell anatomy, 49, 51 in eukaryotes, 48 in plant cells, 50 structure of, 505, 505 translation and, 505, 505 Ribozymes, 104 Ribs, 366, 369, 369–370 Rice production, 138, 181 Rickets, 268, 269 Right optic tract, 350 Ring canal, 648, 648 Ringworm, 601–602, 602 Ripening, of fruit, 127 RNA. See Ribonucleic acid RNA-first hypothesis, 567, 568 RNA polymerase, 502 Roadrunners, 730, 730 Rod cells, 349, 349 Rohypnol, 334 Root caps, 151, 151 Root diversity, 151–152 Root hair plexus, 344, 345 Root hairs, 144, 147 Root nodules, 152, 153 Root organization, 150–153, 150–153 Root specializations, 152, 152 Root systems, 147–148, 147–148 Rosetta space probe, 1 Rotation, 375 Rotational equilibrium, 353, 353 Rotational equilibrium pathway, 354 Rotator cuffs, 370 Rotavirus, 242 Rotifers, 627, 634, 635 Rough endoplasmic reticulum (RER), 48–50, 52, 52–53 Round window, 351, 351 Roundworms, 446, 446, 627, 640–641, 641 anatomy of, 640–641, 641 induction and, 446, 446 in phylogenetic tree, 627 Roux-en-Y procedure, 253 RuBP carboxylase, 135, 135, 136, 137 Ruffini endings, 344, 345 Rugae, 257, 299 Rusts, 598–599, 599

S

Saccule, 353, 354 Sac fungi, 597–598, 598 Sacral vertebrae, 368, 369 Sacrum, 366, 368, 369, 371 SAD (seasonal affective disorder), 323, 404 Safe sex, 426, 515 Salamanders, 655 Salivary amylase, 255, 262

Salivary glands, 255, 645 Salmonella, 575 Salt reabsorption of, 304–305 table, 23, 23 Saltatory conduction, 317 Saprotrophs, 588, 595 Sapwood, 158 Sarcolemma, 378 Sarcomas, 207 Sarcomeres, 378, 380 Sarcoplasmic reticulum, 378 Sargasso Sea, 588 Sargassum, 588 Sarin gas, 107, 108, 108 SARS (severe acute respiratory syndrome), 14 Sartorius, 376–377 Saturated fatty acids, 32, 33, 266 Saturn, moons of, 1 Savannas, 731, 731 Scales, fish, 657 Scallops, 636 Scanning electron microscopes (SEMs), 45, 45 Scapula, 366, 370, 370 Schistosoma, 634 Schistosomiasis, 624, 634 Schleiden, Matthias, 44 Schwann, Theodor, 44 Schwann cells, 196, 315, 315 SCID (severe combined immunodeficiency), 250, 528 Science challenges facing, 13–14 controlled studies in, 12, 12–13 data presentation and analysis in, 10–11, 10–11 hypothesis in, 9 models organisms used in, 10 observation in, 9 predictions and experiments in, 9–10 process of, 8–13, 9–12 results and conclusions in, 13 Scientific method, 9, 9, 12, 12–13 Scientific names, 8 Scientific publications, 10–11, 11 Scientific theories, 11–12 Sclera, 347, 347–348 Sclereids, 146 Sclerenchyma, 145, 146, 159 SCN (suprachiasmatic nucleus), 404 Scolex, 633 Scoliosis, 368 Scorpions, 644, 645 Scrapie, 580 Scrotum, 413, 414–415 Scurvy, 268 Sea anemone, 631, 631 Sea cucumbers, 647 Seashores, 735–737, 736 Sea slugs, 636 Seasonal affective disorder (SAD), 323, 404 Sea squirts, 654, 654 Sea stars, 648, 648–649, 736 Sea turtles, 750, 750 Sea urchins, 647 Seaweeds, 587 Sebaceous glands, 202 Sebum, 202 Secondary follicles, 418 Secondary growth, 154–155, 156 Secondary oocytes, 95, 418 Secondary response, to vaccines, 241 Secondary spermatocytes, 94–95 Secondary structure of proteins, 36, 37 Secondary succession, 702 Secondhand smoke, 293 Second law of thermodynamics, 101, 101–102 Second messenger, 394 Secretin, 259 Secretory phase, 420 Sedimentation, 539 Seed cones, 617 Seed development, 170

Seed dispersal, 177 Seedless plants, 169, 187, 611–615, 613–615 Seed plants, 607, 615–621, 616–621 Segmentation in annelids, 638 in arthropods, 641 in earthworms, 639 Segregation, law of, 465 Selection artificial, 544–545, 545 kin, 686 natural (See Natural selection) sexual, 684 Selective permeability, 67 Selenium, 270, 271 Self-interest, 686–687 Semantic memory, 324 Semen, 413 Semicircular canals, 351, 352, 354 Semilunar valve, 219 Seminal fluid, 413 Seminal receptacles, of grasshoppers, 645 Seminal vesicles earthworm, 639 human, 413, 413 Seminiferous tubules, 415, 415–416 SEMs (scanning electron microscopes), 45, 45 Senescence, 184. See also Aging Sensation, 342–343, 342–343 Sensitive period, 677 Sensory adaptation, 343 Sensory disorders, 354–358 Sensory neurons, 314, 314 Sensory receptors, 342, 342–343 Sensory transduction, 343 Sepals, 171, 619 Septate, 595, 596 Septum, 219 Sequoia trees, 2, 2, 128 Serosa, 257 Serotonin, 332 Serous membranes, 197–198 Sertoli cells, 415 Serum, 215 Serum sickness, 244 Sessile, 626 Setae, 638, 639 Set points, 204 Severe acute respiratory syndrome (SARS), 14 Severe combined immunodeficiency (SCID), 250, 528 Sex cancer and, 515 determination of, 451 disease transmission through, 426 Sex chromosomes, 484, 490 Sex determination, 451 Sex hormones, 393 Sex-linked alleles, 484, 484–485 Sex-linked inheritance, 484–487, 484–487 Sex pilus, 572 Sexually transmitted diseases (STDs), 424–432. See also HIV/AIDS bacterial, 430–432 chlamydia, 430, 431 genital herpes, 428, 430, 580 gonorrhea, 430–431, 575 hepatitis B virus, 241, 243, 275, 430 human papillomavirus, 204, 428, 430, 430, 515 preventing transmission of, 426 syphilis, 431, 431–432 viral, 424–430 Sexual selection, 684 Shared derived traits, 558 Sharks, 655 Shigella sonnei, 535, 560 Shingles, 249, 580 Shock, anaphylactic, 245 Shoot systems, 147, 147–148, 148 Shoot tips, 154 Short-day plants, 186, 186 Short tandem repeats (STRs), 523, 524 Short-term memory, 324



I-16

Index

Shrublands, 730, 730 Sick building syndrome, 602 Sickle cell disease, 473, 474, 512, 513, 554, 554 Sieve-tube members, 146 Sieve tubes, 166 Sight. See Eyes Sigmoid colon, 259 Sigmoria, 642 Signaling, cell, 65, 65 Signaling molecules, 65 Silent Spring (Carson), 435 Silurian period, 541 Simple epithelium, 191, 192 Simple fractures, 386 Simple fruits, 176 Simple goiter, 406, 406 Simple sugars, 30, 30 Single nucleotide polymorphisms (SNPs), 530 Single trait inheritance, 467–470, 468–469 Sinks, for sugar, 166 Sinoatrial node, 221 Sinuses, 281, 367 Sinusitis, 291 Sister chromatids, 81, 84 Skeletal muscle, 195, 195, 375–377, 375–377 Skeletal system. See Musculoskeletal system Skeleton. See also Bones appendicular, 366, 366, 370–371, 370–371 in arthropods, 641 axial, 366, 366–369, 367–370 hydrostatic, 638 in vertebrates, 655 Skeleton, of hydrocarbon chains, 28 Skill memory, 325 Skin aging and, 458 color and genetics, 477–478, 478 cutaneous receptors in, 344, 345 disorders of, 202–204 function of, 200 regions of, 200, 200, 202 sun and, 201, 201 Skin cancer, 201 Skull, 366, 367–368, 367–368 Sleep, 326, 404 SLE (systemic lupus erythematosus), 250 Sliding filament model, 378 Slime molds, 591, 591–592 Slow-twitch muscle fibers, 385, 385 Small intestine, 258–259, 258–259 Smallpox, 242 Smell, 345–346, 346, 354, 459 Smog, 716 Smoking birth defects and, 454 cancer and, 515 cardiovascular disease and, 226 e-cigs, 290 health, impact on, 293 nervous system and, 332 strategies for quitting, 293 Smooth endoplasmic reticulum, 48–50, 52, 52–53 Smooth muscle, 195, 195, 454 Smuts, 598–599, 599 Snails, 636, 637, 675 Snakes characteristics of, 659 food choice in, 674, 674, 675, 675 medicine and, 656 Snowshoe hare, 697–698, 698 Snow trillium, 618 SNPs (single nucleotide polymorphisms), 530 Snuff, 293 Social interactions, learning and, 677 Society benefits of, 673, 687 fitness and, 685–686 sustainability of, 757–761, 758–759, 761 Sociobiology, 686 Sodium, 270, 271 Sodium bicarbonate, 262 Sodium chloride, 23, 23 Sodium gates, 316, 317

Sodium-potassium pumps, 71, 72, 316 Soft palate, 255, 281 Soil erosion, 748 Solar energy, 4, 54, 127, 131–134, 131–134, 140, 759 Solar radiation, 723, 723–724 Solute gradient, 305 Solutes, 26, 68 Solutions acidic, 27–28, 28 basic, 27–28, 28 defined, 26 diffusion in, 68 hypertonic, 70, 70 hypotonic, 69–70, 70 isotonic, 69, 70 Solvents, 68 Somatic cells, 80, 511 Somatic embryos, 179 Somatic senses, 343–345, 344–345 Somatic system, 328, 329, 331 Somatosensory association area, 321, 322 Somatostain, 402 Somites, 443 Sound communication, 679, 679–680 Space-filling models, 24, 24 Speciation adaptive radiation and, 556 allopatric, 555–556, 556 defined, 555 pace of, 556, 557 postzygotic isolating mechanisms and, 555, 555 prezygotic isolating mechanisms and, 555, 555 process of, 555–556 sympatric, 556 Species defined, 4, 555 endangered, 744–745 exotic, 690, 751, 751 flagship, 754 keystone, 754 in taxonomy, 6 threatened, 745 Spectrum, electromagnetic, 131, 131 Speech, 325–326, 327 Sperm, 95, 415, 415–416, 441, 441–442 Spermatids, 95, 415 Spermatocytes, 415 Spermatogenesis, 94–95, 95, 415, 415 Spermatogonia, 415 Spermatozoa, 415 Sperm duct crayfish, 643 roundworm, 641 Spermicidal jellies, 423, 423 Sphenoid bone, 367, 367 Sphincters, 256, 300 Sphygmomanometers, 224 Spicules, 629, 641 Spiders, 645–647, 646 Spinal cord disorders of, 336–337 functions of, 320 structure of, 319–320, 320 Spinal nerves, 319, 327–328, 328 Spinal reflexes, 328, 329 Spinal taps, 319 Spindle fibers, 58, 85 Spindles, muscle, 343–344, 344 Spinous processes, 368 Spiracles, grasshopper, 645 Spiral fractures, 386 Spirillum (spirilla), 571, 571 Spirobranchus, 638 Spirogyra, 587, 587 Spleen, 235 Sponges anatomy of, 629, 629 chalk, 629 glass, 629–630 hermaphroditic, 629 in phylogenetic tree, 627 spicules in, 629 Spongy bone, 194, 363, 364

Spontaneous mutations, 511 Sporangia, 591, 597, 611 Spores, 170, 585, 596 Sporophytes, 170, 171, 607, 611, 617 Sporozoans, 589 Sporozoites, 592 Sports, metabolic demands on athletes, 113 Sports drinks, 17, 39 Spring overturn, 733 Spring wood, 156 Spruce trees, 727, 727 Squamous cell carcinoma, 201, 201 Squamous epithelium, 191, 192 Squid, 636, 637 Stabilizing selection, 551–552, 552 Stable age structures, 695, 695 Stachybotrys chartarum, 602 Stalk, 611 Stamens, 171, 619 Stapes, 351, 351 Staphylococcus aureus, methicillin-resistant, 535, 553, 574 Starch, 30, 31, 137 Starfish, 648, 648–649, 736 StarLinkTM corn, 182, 525 Start codons, 506, 506 Statistical data, 10 Statistical significance, 10 STDs. See Sexually transmitted diseases Stem cells, 217, 217, 521 Stems diversity of, 158, 159 herbaceous, 154, 155 organization of, 153–156, 153–156, 158, 158–159 plant, 148 woody, 154–156, 156, 158, 158–159 Stents, 228, 228 Stereocilia, 352, 352, 354 Sternocleidomastoid, 376–377 Sternum, 366, 370 Steroids, 34–35, 35, 64, 394, 395, 395. See also Anabolic steroids Stethoscopes, 220 Stigmas, 171, 619 Stimulants, 331, 333 Stimuli, 342 Stinkhorns, 598 Stolons, 158 Stomach crayfish, 643 in digestive system, 257, 258 fish, 657 Stomach ulcers, 273–274, 274 Stomata (stoma), 129, 129, 130, 145, 163–164, 164 Stone canal, 648, 648 Stones bladder, 309, 309 kidney, 307 Stoneworts, 607 Stop codons, 504 Stratification of lakes, 732–733, 733 Stratified epithelium, 191, 192 Strepsirhini, 663 Strep throat, 290, 574 Streptococcal infections, 574 Streptococci, 571 Streptococcus mutans, 574 Streptococcus pneumoniae, 574 Streptococcus pyogenes, 290, 574, 574 Stress, cardiovascular disease and, 226 Stress fractures, 386 Stress response, 399, 399 Stretch marks, 454 Stretch receptors, 286 Striae gravidarum, 454 Striations, of muscles, 195, 378 Stroke, 227, 336 Stroma, 55, 55, 129, 129, 133 Strong acids, 27 Strong bases, 27 Strongulcentrotus, 647 STRs (short tandem repeats), 523, 524 Style, of flowers, 171, 619 Styloid process, 367

Subatomic particles, 18–19, 19 Subcutaneous layer, 200, 200, 202 Sublittoral zone, 739, 740 Submucosa, 257 Subneural blood vessel, 639 Subsidence, 760 Substance abuse, 331–332, 331–334 Substrate concentration, 106 Substrate-level ATP synthesis, 116, 119 Substrates, 104, 106 Subunit vaccines, 241 Sucrose, 30 Sudoriferous glands, 202 Sulci, 321 Sulfhydryl group, 29 Summer wood, 156, 158 Sum rule of probability, 469 Sun, impact on skin, 201 Sunburn, 201 Sunglasses, 356 Sunscreen, 515 Superficial mycoses, 601–602 Supergroups, 8, 585, 585 Superior, defined, 197 Superior vena cava, 212, 219 Supplements, dietary, 270–271 Suprachiasmatic nucleus (SCN), 404 Surface-area-to-volume relationships, 46, 46 Surface tension, of water, 26 Surfactants, 283 Surrogate mothers, 436 Survivorship, 693, 693 Suspensory ligament, of eye, 348 Sustainable societies, 757–761, 758–759, 761 Sutures, 372 Swallowing, 256, 256 Sweat glands, 202 Swift, Taylor, 190 Swim bladder, 658 Swimmerets, 643 Swine flu, 14, 242, 580 Symbiosis cleaning, 701, 701 coevolution and, 699 commensalism and, 701, 701 defined, 700 fungi and, 599, 601, 601 mutualism and, 701, 701 parasitism and, 699, 700, 700–701 in roots, 152–153, 153 Symmetry, in animals, 625–626 Sympathetic division, of autonomic system, 329, 330–331 Sympatric speciation, 556 Synapse, 317, 317, 331 Synapsis, 89, 90 Synaptic clefts, 318, 331 Synaptic integration, 318 Synaptic transmission, 317–318, 317–319 Synovial fluid, 372 Synovial joints, 372–373, 374 Synovial membrane, 372 Synovial membranes, 198 Syphilis, 431, 431–432 Systematics classification and, 8, 557 defined, 6, 557 phylogeny and, 557–558, 558–559 three-domain classification system and, 558–559, 560 Systemic circuit, 222, 223–224, 223–225 Systemic diseases, 206 Systemic lupus erythematosus (SLE), 250 Systemic mycoses, 602 Systole, 220 Systolic pressure, 224

T

Table salt, 23, 23 Tactile communication, 680, 682, 682 Taenia solium, 634, 635 Taiga, 726, 728 Tail, postanal, 653 Talus, 371, 371 Tanning, 201 “Tapeworm diet,” 624, 649

Index I-17 Tapeworms, 624, 633–634, 635 Taproots, 151–152, 152 Tarantula, 642 Tarsal bones, 366, 371, 371 Taste, 345, 346, 354, 459, 477 Taste buds, 254, 345, 346 Taxon, 557 Taxonomists, 557 Taxonomy, 6, 557 Tay-Sachs disease, 43, 53, 60, 473, 473 TB. See Tuberculosis T-cell receptors (TCRs), 239 T cells, 215, 235, 237, 239–241, 240 Tear glands, 281 Technology defined, 13 DNA technology, 520–523, 520–524 Tectorial membrane, 352 Teeth, 255, 255–256 Tegumental gland, 642 Telomeres, 458, 516 Telophase, 85, 87, 93–94 Telophase I, 90, 91, 93–94 Telophase II, 91, 92, 94 Telson, 643 Temperate deciduous forests, 728, 728 Temperate grasslands, 730, 730–731 Temperate rain forests, 727–728 Temperature body temperature regulation, 204–205, 205 enzymes and, 106, 106 genetics and, 478, 478 Temporal bone, 367, 367, 368 Temporal lobe, 321, 322 TEMs (transmission electron microscopes), 45, 45 Tendons, 193, 363, 375 Tennis elbow, 373 Terminal buds, 153–154 Termination phase of translation, 506, 507 Terminology, anatomical, 197 Terrace farming, 761 Terrestrial ecosystems, 724–731 biomes and, 725–726, 725–726 coniferous forests, 726–728, 727 deserts, 731, 732 grasslands, 4, 730–731, 730–731 shrublands, 730, 730 temperate deciduous forests, 728, 728 tundra, 726, 727 Territoriality, 682–683, 683 Tertiary structure of proteins, 36, 37 Testcrosses one-trait, 469, 469–470 two-trait, 471–472, 472 Testes bird, 661 crayfish, 643 in endocrine system, 393–394, 402 hormone regulation and, 416, 416 human, 413, 413, 414–415, 415 Testicular cancer, 433 Testosterone, 35, 35, 393, 402, 416 Tetanus, 242, 381, 384 Tetrahedrons, 24, 24 Tetrahydrocannabinol (THC), 332 Thalamus, 32, 323 Thallus, 610 Thamnophis elegans, 674, 674, 675, 676 THC (tetrahydrocannabinol), 332 Theories cell, 11, 44 defined, 11 endosymbiotic, 59–60, 585 evolution, 11–12, 536–538 homeostasis, 11 natural selection, 538 scientific, 11–12 Therapeutic cloning, 521, 521 Therapy, gene, 527–528, 527–528 Thermal inversion, 716, 716 Thermoacidophiles, 570, 570 Thermodynamics, laws of, 100–102 Thermoreceptors, 342, 342 Thermus aquaticus, 523 Thiamine, 269

Thick filaments, 378 Thigmotropism, 185, 186 Thin filaments, 378 Thoracic cage, 366, 369, 369–370 Thoracic cavity, 197, 197 Thoracic vertebrae, 368, 369 Thorax crayfish, 643 grasshopper, 645 Threatened species, 745 Three-domain classification system, 558–559, 560 Three-parent in vitro fertilization, 412, 436 Threshold, 317 Thrombin, 110, 215 Thrombocytes. See Platelets Thromboembolism, 225, 228 Thrombus, 225 Thrush, 602, 602 Thylakoid membrane, 133, 133–134 Thylakoids, 47, 55, 55, 129, 129 Thymine, 38, 38, 39 Thymosins, 393, 403 Thymus, 235, 393–394, 403 Thyroid gland, 393–394, 398, 398 Thyroid gland disorders, 405–406, 406 Thyroid-stimulating hormone (TSH), 393, 396, 406 Thyroxine, 393, 398 Tibia, 366, 371, 371 Tibialis anterior, 376–377 Tibial tuberosity, 371 Ticks, 645, 646 Tidal volume, 284 Tiger centipede, 642 Tiger sharks, 655 Tight junctions, 76, 76, 191 Timescale, geological, 540, 541 Tineas, 601 Tissue cultures, 179, 180 Tissue fluid, 214 Tissue level of organization, 628 Tissue plasminogen activator (tPA), 228, 336 Tissues adipose, 193, 193–194 connective, 191, 193–195, 193–195, 363 defined, 3, 4, 191 developmental stages of, 442–443, 442–443 epidermal, 144–145, 144–145 epithelial, 191, 192 ground, 144, 144–145, 145–146 layers of, 628 meristematic, 85, 144, 144 muscular, 195, 195–196 in musculoskeletal system, 363, 364 nervous, 4, 196, 196–197, 314–315 in plants, 144–146, 144–146, 179–180 rejection of, 248 true, 628 vascular, 144, 144, 146, 146, 151, 607, 610 Titan (moon), 1 Tobacco, chewing, 293. See also Smoking Tobacco mosaic virus, 181 Tolerance model of ecological succession, 702–703 Tongue, 255 Tonicella, 636 Tonicity, 69 Tonsillectomy, 255, 291 Tonsillitis, 255, 290–291 Tonsils, 255, 281, 290 Total artificial hearts, 228, 228 Totipotent cells, 179, 217 Toxoplasma gondii, 593 Toxoplasmic encephalitis, 249 tPA (tissue plasminogen activator), 228, 336 Trabeculae, 363 Trace minerals, 270, 270–271 Tracers, 20 Trachea bird, 661 disorders of, 291 grasshopper, 645 in respiratory system, 281–282, 282 Tracheids, 146

Tracheotomy, 291 Tracts, 315 Trade winds, 724 Traits shared derived, 558 single trait inheritance, 467–470, 468–469 two trait inheritance, 470–472, 470–472 Transcription defined, 502 gene expression and, 502–503, 502–504 messenger RNA and, 502–503, 502–504 Transcription activators, 510 Transcriptional control of gene expression, 510, 511 Transcription factors, 6, 510 Transcription initiation complex, 510 Transduction, of bacteria, 572 Transduction pathway, 65, 65 Trans–fats, 32, 33, 266–267 Transfer RNA (tRNA), 504, 504–505 Transformation, of bacteria, 572 Transgenic animals, 526, 527 Transgenic bacteria, 524–526, 526 Transgenic organisms, 180, 180–181, 520 Transgenic plants, 526 Transitional fossils, 539–540, 539–540 Transitional links, 539 Translation defined, 502 elongation phase of, 506, 507 gene expression and, 502, 502, 504–506, 504–507 genetic code and, 504, 504 initiation phase of, 506, 506 ribosomal RNA and, 505, 505 ribosomes and, 505, 505 steps in, 506, 506–507 termination phase of, 506, 507 transfer RNA and, 504, 504–505 Translational control of gene expression, 510, 511 Translocation of chromosomes, 492, 492 Transmissible spongiform encephalopathies (TSEs), 336 Transmission electron microscopes (TEMs), 45, 45 Transpiration, 162 Transplants. See Organ transplants Transport systems, 206, 206 Transposons, 511–512, 512 Transurethral microwave thermotherapy, 433 Transurethral resection of prostate, 433 Transverse colon, 259 Transverse processes, 368 Transverse tubules, 378 Trapezius, 376–377 Trawling, 753, 753 TREDs (trinucleotide repeat expansion disorders), 486, 492 Tree trunks, 158, 158 Treponema pallidum, 431, 432 Triassic period, 541 Triceps brachii, 376–377 Trichinella spiralis, 640 Trichinosis, 640–641 Trichomes, 159 Trichomonas vaginalis, 590 Tricuspid valve, 219 Triglycerides, 32, 33 Triiodothyronine, 393, 398 Trinucleotide repeat, 486, 492 Trinucleotide repeat expansion disorders (TREDs), 486, 492 Trisomy, 489 Trisomy 21. See Down syndrome tRNA (transfer RNA), 504, 504–505 Trochlea, 370 Trochophores, 628 Trochozoans, 628, 632, 633–640, 634–640 Troides, 644 Trophic levels of ecosystems, 709–710 Trophoblasts, 449, 449–450 Tropical deciduous forests, 729 Tropical diseases, 594, 624 Tropical rain forests, 13, 728–729, 729 Tropisms, 185–186, 186

Tropomyosin, 380 Troponin, 380 True-breeding lines, 466 True coelom, 628 True ribs, 369, 369 True tissues, 628 Truffles, 597 Trypanosoma brucei, 593, 594 Trypanosoma cruzi, 593 Trypsin, 106, 262 TSEs (transmissible spongiform encephalopathies), 336 Tsetse flies, 594 TSH (thyroid-stimulating hormone), 393, 396, 406 T (transverse) tubules, 378 Tubal ligation, 422, 423 Tubastrea, 631 Tube feet, 648 Tubercles, 574 Tuberculosis (TB), 245, 245, 292, 292, 553, 574–575, 575 Tubular reabsorption, 303–304, 304 Tubular secretion, 304 Tumor necrosis factor alpha, 236 Tumors, 83 Tumor suppressor genes, 82–83, 82–83, 96, 513 Tundra, 726, 727 Tunic, 654 Tunicates, 653–654, 654 Turgor pressure, 69 Turner syndrome, 489, 490 Turtles, 750, 750 Twin studies, 676 Two-trait genetic crosses, 471, 471 Two trait inheritance, 467–470, 468–469 Two-trait testcrosses, 471–472, 472 Tympanic canal, 352 Tympanic membrane, 351, 351 Tympanostomy tubes, 291, 291 Tympanum, grasshopper, 645 Type 1 diabetes, 407–408, 519 Type 2 diabetes, 408 Typhlosole, 639

U

Ulcerative colitis, 576 Ulcers, stomach, 273–274, 274 Ulna, 366, 370, 370 Ultraviolet rays, 201 Umami receptors, 345 Umbilical arteries, 452, 453 Umbilical cord, 450, 453 Umbilical vein, 452, 453 Umbo, 637 Uncoating, of viruses, 578 United Nations Climate Change Conference, 719 Unsaturated fatty acids, 32, 33, 266 Unstable age structures, 695, 695 Upper limbs, 370, 370–371 Upper respiratory tract, 281, 281, 289–291 Upwelling, 737 Uracil, 39 Uranium, 163 Urban growth, 761 Urea, 299 Uremia, 307 Ureters bird, 661 function of, 299 male, 413 Urethra disorders of, 309, 309–310 female, 418 male, 413, 413–414 structure and function of, 300 Urethritis, 309, 430 Urey, Harold, 566 Uric acid, 299 Urinary bladder disorders of, 309, 309–310 male, 414 stones in, 309, 309 structure and function of, 299–300, 300



I-18

Index

Urinary system, 298–310. See also Kidneys diseases and disorders of, 298, 307, 307–309, 309–310 excretion and, 299 male vs. female, 309, 309 metabolic waste and, 299 organs of, 299–300 osmoregulation and, 299 structure and function of, 198, 199, 299, 300 ureters, 299 urethra, 300 urinary bladder, 299–300, 300 urination and, 300, 300 Urination, 300, 300 Urine production, 299, 300, 300, 302–303, 303 Uropods, 643 U.S. Department of Agriculture (USDA), 34, 263 U.S. Equal Employment Opportunity Commission (EEOC), 83 Uterine cycle, 420, 420–421 Uterine tubes, 416–417, 417 Uterus, 416–417, 417 Utricle, 353, 354 Uvula, 255, 281

V

Vaccines, 241, 242, 424. See also Immunizations birth defections and, 454 for children, 242 contraceptive, 424 defined, 241 HIV/AIDS, 429 human immunodeficiency virus, 428, 430 measles-mumps-rubella, 242, 580 Vacuoles, 48, 50, 54, 589 Vagina grasshopper, 645 human, 416–418, 417 Vaginal contraceptive rings, 423, 424 Vaginal film, 423 Vaginitis, 590 Valence shell, 22 Valine, 36 Valley fever, 602 Vampire bats, 99, 110 van Ermengem, Emile Pierre, 381 van Leeuwenhoek, Anton, 44, 564, 634 Vaping, 290 Vaporization, heat of, 25–26, 26 Variables, experimental, 10 Variant Creutzfeldt-Jakob disease (vCJD), 581 Variation heterozygote advantage and, 553–554 maintenance of, 553–554 natural selection and, 538, 550 Varicella-zoster virus, 580 Varicose veins, 212 Vascular bundles, 146 Vascular cambium, 154, 155 Vascular cylinders, 146, 151 Vascular pathways, 222–224, 223–225 Vascular plants, seedless, 611–615, 613–615

Vascular tissue, in plants, 144, 144, 146, 146, 151, 607, 610 Vas deferens, 413, 413, 415, 661 Vasectomy, 422, 423 vCJD (variant Creutzfeldt-Jakob disease), 581 Vectors, 520 Vegetal pole, 442 Vegetarians, 267 Vegetative organs, 147 Veins in cardiovascular system, 211, 212 leaf, 146, 159 pulmonary, 219 Venae cavae, 212, 219, 224 Ventilation, 280 Ventral, defined, 197 Ventral cavity, 197, 197 Ventral nerve cord, crayfish, 643 Ventricles brain, 319, 320 heart, 219 Ventricular fibrillation (VF), 222 Venules, 212 Vermiform appendix, 259 Vernix caseosa, 452 Vertebrae, types of, 368, 369 Vertebral cavity, 197, 197 Vertebral column, 366, 368–369, 369 Vertebrates. See also specific animals amniotic egg in, 655 amphibians, 653, 658–659, 658–659 birds, 173, 660, 661, 675, 677 characteristics of, 625, 626 chordates, 627, 653–654, 653–654 defined, 625 embryos, 443, 444 endoskeleton in, 655 evolution of, 655 fish, 653, 657–658, 657–658 jaws in, 655 mammals, 653, 661–663, 662–663 medicine and, 656 reptiles, 653, 659–660, 660 Vertigo, 356, 358 Vervet monkeys, 679, 679 Vesicles, 48–49, 51, 53, 567 Vesicular follicles, 418 Vessel elements, 146 Vestibular canal, 352 Vestibular nerve, 351, 353, 354 Vestibule, 352 Vestigial structures, 543 VF (ventricular fibrillation), 222 Victoria (queen of England), 487, 487 Villi, 258 Virchow, Rudolf, 44 Viroids, 577, 580 Viruses antigenic drift and, 579, 579 antigenic shift and, 579, 579 assembly in, 578 attachment of, 578 bacteriophages, 578 biosynthesis in, 578 budding of, 578 capsid in, 577, 577 disease caused by, 579, 579–580

drugs for, 580 entry of, 578 latency in, 578 replication of, 578 reproduction of, 578, 578–579 retroviruses, 578, 579 size and structure of, 577, 577–578 STDs caused by, 424–430 Visceral mass, 635, 638 Visible light, 131, 131 Vision. See Eyes Visual accommodation, 348 Visual association area, 321 Visual communication, 680, 680 Visual cortex, 350–351 Visual focus disorders, 355, 355 Visual pathway, 348–351 Vital capacity, 284 Vitamin A, 268–269, 269, 349, 515 Vitamin B1, 269 Vitamin B2, 107, 269 Vitamin B6, 269 Vitamin B12, 269 Vitamin C, 268–269, 269, 515 Vitamin D, 269, 269, 270, 299 Vitamin E, 268–269, 269 Vitamin K, 215, 269 Vitamins. See also specific vitamins cancer and, 515 enzymes and, 107 as nutrients, 268–269, 268–269 Vitreous humor, 348 VNO (vomeronasal organ), 679 Vocal cords, 282, 282 Volume, relationship with surface area, 46, 46 Volvox, 587, 587 Vomer bone, 367 Vomeronasal organ (VNO), 679 Vomiting, 257 Vulva, 417, 417

W

Wächtershäuser, Günter, 566–567 Walking stick, 644 Warfarin, 108 Warts genital, 428 on skin, 204 Wasps, 173, 642 Waste disposal, 748 Water chemistry of, 25–28, 26–28 cohesiveness and adhesiveness of, 26 conservation of, 760 DDT in, 722 density of, 26–27, 27 exercise and loss of, 17 heat capacity of, 25 heat of vaporization of, 25–26, 26 hydrogen bonds in, 25, 25 properties of, 25–27, 26–27 provision of fresh, 748 reabsorption of, 305, 305–306 as solvent, 26 in sports drinks, 17 surface tension of, 26 uptake in plants, 161–163, 162

Water cycle, 711–713, 713 Water lilies, 618 Watermelons, seedless, 169, 187 Water molds, 588–589, 589 Water-soluble vitamins, 268, 269 Watson, James, 498–500 Weight, cancer and, 515 Welwitschia, 617, 618 Wernicke’s area, 323, 325 Westerlies, 724 Whales, 540, 540, 663 Whisk ferns, 615, 615 White blood cells, 195, 213, 214–215 White matter, 315, 319–320, 323 White pulp, 235 Whooping cough, 242 Wildebeests, 731, 731 Wildlife conservation, DNA and, 750 Wilkins, Maurice, 500 Williams syndrome, 491, 491 Wilms tumor, 83 Wind circulation, 723, 723–724 Windpipe. See Trachea Wind power, 759, 759 Wisdom teeth, 255 Wobble effect, 505 Wolves, 706, 720, 745 Wood, 156, 156, 158, 158 Woody plants, 158 Woody stems, 154–156, 156, 158, 158–159

X

Xenon, 21 Xenotransplantation, 248, 656 Xeroderma pigmentosum (XP), 496, 516 X-inactivation, 509, 510 Xiphoid process, 370 X-linked agammaglobulinemia (XLA), 232 X-linked alleles, 484, 484–485 X-linked disorders, 485–487, 485–487 XP (xeroderma pigmentosum), 496, 516 X rays, 454, 515 Xylem, 146, 146, 148, 151, 151, 611 Xylem rays, 155–156 Xylem transport, 162, 162–163

Y

Yeasts, 118, 592, 598, 598 Yellow bone marrow, 363, 364 Yellowstone National Park, wolves of, 706, 720, 745 Yolk plug, 440, 443 Yolk sac, 448, 448, 449

Z

Zero population growth, 694 Zinc, 270, 271 Z lines, 378 Zona pellucida, 441, 441, 442 Zone of cell division, 150, 151 Zone of elongation, 150 Zone of maturation, 150 Zoospores, 597 Zygomatic bones, 367–368, 368 Zygomaticus, 376–377 Zygospore fungi, 597, 597 Zygotes, 94, 96, 417, 441