Campbell Biology [11th Edition]

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Campbell Biology [11th Edition]

Table of contents :
1 Evolution, the Themes of Biology, and Scientific Inquiry

Unit 1 The Chemistry of Life
2 The Chemical Context of Life
3 Water and Life
4 Carbon and the Molecular Diversity of Life
5 The Structure and Function of Large Biological Molecules

Unit 2 The Cell
6 A Tour of the Cell
7 Membrane Structure and Function
8 An Introduction to Metabolism
9 Cellular Respiration and Fermentation
10 Photosynthesis
11 Cell Communication
12 The Cell Cycle

Unit 3 Genetics
13 Meiosis and Sexual Life Cycles
14 Mendel and the Gene Idea
15 The Chromosomal Basis of Inheritance
16 The Molecular Basis of Inheritance
17 Gene Expression: From Gene to Protein
18 Regulation of Gene Expression
19 Viruses
20 DNA Tools and Biotechnology
21 Genomes and Their Evolution

Unit 4 Mechanisms of Evolution
22 Descent with Modification: A Darwinian View of Life
23 The Evolution of Populations
24 The Origin of Species
25 The History of Life on Earth

Unit 5 The Evolutionary History of Biological Diversity
26 Phylogeny and the Tree of Life
27 Bacteria and Archaea
28 Protists
29 Plant Diversity I: How Plants Colonized Land
30 Plant Diversity Ii: the Evolution of Seed Plants
31 Fungi
32 An Overview of Animal Diversity
33 An Introduction to Invertebrates
34 The Origin and Evolution of Vertebrates

Unit 6 Plant Form and Function
35 Vascular Plant Structure, Growth, and Development
36 Resource Acquisition and Transport in Vascular Plants
37 Soil and Plant Nutrition
38 Angiosperm Reproduction and Biotechnology
39 Plant Responses to Internal and External Signals

Unit 7 Animal Form and Function
40 Basic Principles of Animal Form and Function
41 Animal Nutrition
42 Circulation and Gas Exchange
43 The Immune System
44 Osmoregulation and Excretion
45 Hormones and the Endocrine System
46 Animal Reproduction
47 Animal Development
48 Neurons, Synapses, and Signaling
49 Nervous Systems
50 Sensory and Motor Mechanisms
51 Animal Behavior

Unit 8 Ecology
52 An Introduction to Ecology and the Biosphere
53 Population Ecology
54 Community Ecology
55 Ecosystems and Restoration Ecology
56 Conservation Biology and Global Change

Appendix A Answers
Appendix B Periodic Table of the Elements
Appendix C The Metric System
Appendix D A Comparison of the Light Microscope and the Electron Microscope
Appendix E Classification of Life
Appendix F Scientific Skills Review

Citation preview

Brief Contents 1 Evolution, the Themes of Biology, and Scientific Inquiry  2

Unit 1 The Chemistry of Life 27

2 3 4 5

The Chemical Context of Life 28 Water and Life 44 Carbon and the Molecular Diversity of Life 56 The Structure and Function of Large Biological Molecules 66

Unit 2 The Cell  92 6 A Tour of the Cell 93 7 Membrane Structure and Function 126 8 An Introduction to Metabolism 143 9 Cellular Respiration and Fermentation 164 10 Photosynthesis 187 11 Cell Communication 212 12 The Cell Cycle 234

Unit 3 Genetics 253 13 Meiosis and Sexual Life Cycles 254 14 Mendel and the Gene Idea 269 15 The Chromosomal Basis of Inheritance 294 16 The Molecular Basis of Inheritance 314 17 Gene Expression: From Gene to Protein 335 18 Regulation of Gene Expression 363 19 Viruses 396 20 DNA Tools and Biotechnology 413 21 Genomes and Their Evolution 440

Unit 4 Mechanisms of Evolution 465 22 Descent with Modification: A Darwinian View of Life 466

23 The Evolution of Populations 484 24 The Origin of Species 504 25 The History of Life on Earth 523

Unit 5 The Evolutionary History of Biological Diversity 550 26 Phylogeny and the Tree of Life 551 27 Bacteria and Archaea 571

28 Protists 591 29 Plant Diversity I: How Plants Colonized Land 616 30 Plant Diversity II: The Evolution of Seed 31 32 33 34

Plants 634 Fungi 652 An Overview of Animal Diversity 671 An Introduction to Invertebrates 684 The Origin and Evolution of Vertebrates 716

Unit 6 Plant Form and Function  755 35 Vascular Plant Structure, Growth, and Development 756

36 Resource Acquisition and Transport in Vascular

Plants 782 37 Soil and Plant Nutrition 803 38 Angiosperm Reproduction and Biotechnology 820 39 Plant Responses to Internal and External Signals 840

Unit 7 Animal Form and Function 870 40 Basic Principles of Animal Form and Function 871 41 Animal Nutrition 896 42 Circulation and Gas Exchange 919 43 The Immune System 950 44 Osmoregulation and Excretion 975 45 Hormones and the Endocrine System 997 46 Animal Reproduction 1017 47 Animal Development 1041 48 Neurons, Synapses, and Signaling 1065 49 Nervous Systems 1083 50 Sensory and Motor Mechanisms 1105 51 Animal Behavior 1137

Unit 8 Ecology  1161 52 An Introduction to Ecology and the Biosphere 1162 53 Population Ecology 1188 54 Community Ecology 1212 55 Ecosystems and Restoration Ecology 1236 56 Conservation Biology and Global Change 1258

Campbell

B IOLO G Y Eleventh Edition

Lisa A. Urry

Michael L. Cain

Steven A. Wasserman

Mills College, Oakland, California

Bowdoin College, Brunswick, Maine

University of California, San Diego

Peter V. Minorsky

Jane B. Reece

Mercy College, Dobbs Ferry, New York

Berkeley, California

330 Hudson Street, New York, NY 10013

Courseware Portfolio Management Director: Beth Wilbur Courseware Portfolio Management Specialist: Josh Frost Courseware Director, Content Development: Ginnie Simione Jutson Supervising Editors: Beth N. Winickoff, Pat Burner Courseware Senior Analysts: John Burner, Mary Ann Murray, Hilair Chism Courseware Specialist: Laura Southworth Development Editor: Mary Hill Director, Content Production and Digital Studio: Erin Gregg Managing Producer, Science: Michael Early Content Producer, Science: Lori Newman Production Management and Composition: Integra Software Services, Inc. Illustrations: Lachina Design Manager: Marilyn Perry Cover and Text Design: Elise Lansdon Rights & Permissions Manager: Ben Ferrini Photo Researcher: Maureen Spuhler

Art/Text/Photo Permissions Specialist: Integra Software Services, Inc. Senior Content Developer, MasteringBiology™: Sarah Jensen Senior Rich Media Content Producer: Lee Ann Doctor Rich Media Content Producer: Tod Regan Content Producers: Jackie Jakob, Ziki Dekel Associate Content Producer: Libby Reiser Associate Instructional Designer: Cady Owens Associate Mastering™ Media Producer: Charles Hall Project Manager: Katie Cook VP Product Marketing: Christy Lesko Executive Product Marketing Manager: Lauren Harp Field Marketing Manager: Kelly Galli Manufacturing Buyer: Stacey Weinberger Text & Cover Printer: Courier/Kendallville Cover Photo: Helianthus annuus (cultivated sunflower) Cover Photo Credit: © Radius Images/Getty Images

Copyright © 2017, 2014, 2011 Pearson Education, Inc. All Rights Reserved. Printed in the United States of America. This publication is protected by copyright, and permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise. For information regarding permissions, request forms and the appropriate contacts within the Pearson Education Global Rights & Permissions department, please visit www.pearsoned.com/permissions/. Readers may view, browse, and/or download material for temporary copying purposes only, provided these uses are for noncommercial personal purposes. Except as provided by law, this material may not be further reproduced, distributed, transmitted, modified, adapted, performed, displayed, published, or sold in whole or in part, without prior written permission from the publisher. Acknowledgments of third-party content appear on page CR-1, which constitutes an extension of this copyright page. PEARSON, ALWAYS LEARNING, MasteringBiology™ and BioFlix® are exclusive trademarks owned by Pearson Education, Inc. or its affiliates in the U.S. and/or other countries. Unless otherwise indicated herein, any third-party trademarks that may appear in this work are the property of their respective owners, and any references to third-party trademarks, logos, or other trade dress are for demonstrative or descriptive purposes only. Such references are not intended to imply any sponsorship, endorsement, authorization, or promotion of Pearson’s products by the owners of such marks, or any relationship between the owner and Pearson Education, Inc. or its affiliates, authors, licensees, or distributors. Library of Congress Cataloging-in-Publication Data Names: Reece, Jane B., author. | Urry, Lisa A., author. | Campbell, Neil A.,   1946-2004, author. Title: Campbell biology / Lisa Urry, Michael Cain, Steven Wasserman, Peter    Minorsky, Jane Reece. Description: Eleventh edition. | Hoboken : Pearson Higher Education, 2016. |    Previous edition: Campbell biology / Jane B. Reece, Lisa A. Urry, Michael    L. Cain, Steven A. Wasserman, Peter V. Minorsky, Robert B. Jackson.    Tenth edition. 2014. | 1st edition: Biology by Neil A. Campbell, 1987. Identifiers: LCCN 2016025573 Subjects: LCSH: Biology. Classification: LCC QH308.2 .C34 2016 | DDC 570—dc23 LC record available at https://lccn.loc.gov/2016025573 1 16

ISBN 10: 0-134-09341-0; ISBN 13: 978-0-134-09341-3 (Student Edition) ISBN 10: 0-134-15412-6; ISBN 13: 978-0-134-15412-1 (Books a la Carte Edition)

About the Authors Lisa A. Urry is Professor of Biology and Chair of the Biology Department at Mills College. After earning a B.A. at Tufts University, she completed her Ph.D. at the Massachusetts Institute of Technology (MIT). Lisa has conducted research on gene expression during embryonic and larval development in sea urchins. Deeply committed to promoting opportunities in science for women and underrepresented minorities, she has taught courses ranging from introductory and developmental biology to a nonmajors course called Evolution for Future Presidents. Lisa is a coauthor of Campbell Biology in Focus. Michael L. Cain is an ecologist and evolutionary biologist who is now writing full-time. Michael earned an A.B. from Bowdoin College, an M.Sc. from Brown University, and a Ph.D. from Cornell University. As a faculty member at New Mexico State University, he taught introductory biology, ecology, evolution, botany, and conservation biology. Michael is the author of dozens of scientific papers on topics that include foraging behavior in insects and plants, long-distance seed dispersal, and speciation in crickets. He is a coauthor of Campbell Biology in Focus and of an ecology textbook. Steven A. Wasserman is Professor of Biology at the University of California, San Diego (UCSD). He earned an A.B. from Harvard University and a Ph.D. from MIT. Working on the fruit fly Drosophila, Steve has done research on developmental biology, reproduction, and immunity. Having taught genetics, development, and physiology to undergraduate, graduate, and medical students, he now focuses on introductory biology, for which he has been honored with UCSD’s Distinguished Teaching Award. He is a coauthor of Campbell Biology in Focus. Peter V. Minorsky is Professor of Biology at Mercy College in New York, where he teaches introductory biology, ecology, and botany. He received his A.B. from Vassar College and his Ph.D. from Cornell University. Peter taught at Kenyon College, Union College, Western Connecticut State University, and Vassar College; he is also the science writer for the journal Plant Physiology. His research interests concern how plants sense environmental change. Peter received the 2008 Award for Teaching Excellence at Mercy College and is a coauthor of Campbell Biology in Focus. Jane B. Reece, the head of the author team for Editions 8–10 of Campbell BIOLOGY, was Neil Campbell’s longtime collaborator. Jane taught biology at Middlesex County College and Queensborough Community College. She holds an A.B. from Harvard University, an M.S. from Rutgers University, and a Ph.D. from the University of California, Berkeley. Jane’s research as a doctoral student at UC Berkeley and postdoctoral fellow at Stanford University focused on genetic recombination in bacteria. Besides her work on Campbell BIOLOGY, Jane has been a coauthor on all the Campbell texts. Neil A. Campbell (1946–2004) earned his M.A. from the University of California, Los Angeles, and his Ph.D. from the University of California, Riverside. His research focused on desert and coastal plants. Neil’s 30 years of teaching included introductory biology courses at Cornell University, Pomona College, and San Bernardino Valley College, where he received the college’s first Outstanding Professor Award in 1986. For many years he was also a visiting scholar at UC Riverside. Neil was the founding author of Campbell BIOLOGY.



ABOUT THE AUTHORS iii

Preface

W

e are honored to present the Eleventh Edition of Campbell BIOLOGY. For the last three decades, Campbell BIOLOGY has been the leading college text in the biological sciences. It has been translated into 19 languages and has provided millions of students with a solid foundation in college-level biology. This success is a testament not only to Neil Campbell’s original vision but also to the dedication of hundreds of reviewers (listed on pages xxviii–xxxi), who, together with editors, artists, and contributors, have shaped and inspired this work. Our goals for the Eleventh Edition include: increasing visual literacy through new figures, questions, and exercises that build students’ skills in understanding and creating visual representations of biological structures and processes asking students to practice scientific skills by applying scientific skills to real-world problems supporting instructors by providing teaching modules with tools and materials for introducing, teaching, and assessing important and often challenging topics integrating text and media to engage, guide, and inform students in an active process of inquiry and learning Our starting point, as always, is our commitment to crafting text and visuals that are accurate, are current, and reflect our passion for teaching biology.

New to This Edition Here we provide an overview of the new features that we have developed for the Eleventh Edition; we invite you to explore pages ix–xxvi for more information and examples.

Visualizing Figures and Visual Skills Questions give students practice in interpreting and creating visual representations in biology. The Visualizing Figures have embedded questions that guide students in exploring how diagrams, photographs, and models represent and reflect biological systems and processes. Assignable questions are also available in MasteringBiology to give students practice with the visual skills addressed in the figures. Problem-Solving Exercises challenge students to apply scientific skills and interpret data in solving real-world problems. These exercises are designed to engage students through compelling case studies and provide practice with data analysis skills. Problem-Solving Exercises have assignable versions in MasteringBiology. Some also have iv

PREFACE

more extensive “Solve It” investigations to further explore a given topic. Ready-to-Go Teaching Modules on key topics provide instructors with assignments to use before and after class, as well as in-class activities that use clickers or Learning Catalytics™ for assessment. Integrated text and media: Media references in the printed book direct students to the wealth of online self-study resources available to them in the Study Area section of MasteringBiology. In eText 2.0 (which will be available for Fall 2017 classes), these resources will also be integrated directly into the eText. The new online learning tools include: Get Ready for This Chapter questions provide a quick check of student understanding of the background information needed to learn a new chapter’s content, with feedback to bolster their preparation. Figure Walkthroughs guide students through key figures with narrated explanations, figure markups, and questions that reinforce important points. Additional questions can be assigned in MasteringBiology. More than 450 animations and videos bring biology to life. These include resources from HHMI BioInteractive that engage students in topics from the discovery of the double helix to evolution. QR codes and URLs within the Chapter Review provide easy access to Vocabulary Self-Quizzes and Practice Tests for each chapter that can be used on smartphones, tablets, and computers. Interviews from the First Edition through the Eleventh Edition of Campbell BIOLOGY are referenced in the chapter where they are most relevant. The interviews show students the human side of science by featuring diverse scientists talking about how they became interested in what they study, how they began, and what inspires them. The impact of climate change at all levels of the biological hierarchy is explored throughout the text, starting with a new figure (Figure 1.12) and discussion in Chapter 1 and concluding with a new Make Connections Figure (Figure 56.30) and expanded coverage on causes and effects of climate change in Chapter 56. As in each new edition of Campbell BIOLOGY, the Eleventh Edition incorporates new content and pedagogical improvements. These are summarized on pp. vi–viii, following this Preface. Content updates reflect rapid, ongoing changes in technology and knowledge in the fields of genomics, gene editing technology (CRISPR), evolutionary

biology, microbiology, and more. In addition, significant revisions to Unit 8, Ecology, improve the conceptual framework for core ecological topics (such as population growth, species interactions, and community dynamics) and more deeply integrate evolutionary principles.

Our Hallmark Features Teachers of general biology face a daunting challenge: to help students acquire a conceptual framework for organizing an everexpanding amount of information. The hallmark features of Campbell BIOLOGY provide such a framework, while promoting a deeper understanding of biology and the process of science. As such, they are well aligned with the core competencies outlined by the 2009 Vision and Change national conference. Furthermore, the core concepts defined by Vision and Change have close parallels in the unifying themes that are introduced in Chapter 1 and integrated throughout the book. Chief among the themes of both Vision and Change and Campbell BIOLOGY is evolution. Each chapter of this text includes at least one Evolution section that explicitly focuses on evolutionary aspects of the chapter material, and each chapter ends with an Evolution Connection Question and a Write About a Theme Question. To help students distinguish the “forest from the trees,” each chapter is organized around a framework of three to seven carefully chosen Key Concepts. The text, Concept Check Questions, Summary of Key Concepts, and MasteringBiology resources all reinforce these main ideas and essential facts. Because text and illustrations are equally important for learning biology, integration of text and figures has been a hallmark of this text since the First Edition. In addition to the new Visualizing Figures, our popular Exploring Figures and Make Connections Figures epitomize this approach. Each Exploring Figure is a learning unit of core content that brings together related illustrations and text. Make Connections Figures reinforce fundamental conceptual connections throughout biology, helping students overcome tendencies to compartmentalize information. The Eleventh Edition features two new Make Connections Figures. There are also Guided Tour Figures that walk students through complex figures as an instructor would. To encourage active reading of the text, Campbell BIOLOGY includes numerous opportunities for students to stop and think about what they are reading, often by putting pencil to paper to draw a sketch, annotate a figure, or graph data. Active reading questions include Visual Skills Questions, Draw It Questions, Make Connections Questions, What If? Questions, Figure Legend Questions, Summary Questions, Synthesize Your Knowledge Questions, and Interpret the Data Questions. Answering these questions requires students to write or draw as well as think and thus helps develop the core competency of communicating science. Finally, Campbell BIOLOGY has always featured scientific inquiry, an essential component of any biology course. Complementing stories of scientific discovery in the text narrative

and the unit-opening interviews, our standard-setting Inquiry Figures deepen the ability of students to understand how we know what we know. Scientific Inquiry Questions give students opportunities to practice scientific thinking, along with the Problem-Solving Exercises, Scientific Skills Exercises, and Interpret the Data Questions. Together, these activities provide students practice in both applying the process of science and using quantitative reasoning, addressing additional core competencies outlined in Vision and Change.

MasteringBiology, the most widely used online assessment and tutorial program for biology, provides an extensive library of homework assignments that are graded automatically. In addition to the new Get Ready for This Chapter Questions, Figure Walkthroughs, Problem-Solving Exercises, and Visualizing Figures, MasteringBiology offers Dynamic Study Modules, Adaptive Follow-Up Assignments, Scientific Skills Exercises, Interpret the Data Questions, Solve It Tutorials, HHMI BioInteractive Short Films, BioFlix® Tutorials with 3-D Animations, Experimental Inquiry Tutorials, Interpreting Data Tutorials, BLAST Tutorials, Make Connections Tutorials, Video Field Trips, Video Tutor Sessions, Get Ready for Biology, Activities, Reading Quiz Questions, Student Misconception Questions, 4,500 Test Bank Questions, and MasteringBiology Virtual Labs. MasteringBiology also includes the Campbell BIOLOGY eText, Study Area, Instructor Resources, and Ready-to-Go Teaching Modules. See pages ix–xxiii and www.masteringbiology.com for more details.

Our Partnership with Instructors and Students A core value underlying our work is our belief in the importance of a partnership with instructors and students. One primary way of serving instructors and students, of course, is providing a text that teaches biology well. In addition, Pearson offers a rich variety of instructor and student resources, in both print and electronic form (see pp. ix–xxiii). In our continuing efforts to improve the book and its supplements, we benefit tremendously from instructor and student feedback, not only in formal reviews from hundreds of scientists, but also via e-mail and other avenues of informal communication. The real test of any textbook is how well it helps instructors teach and students learn. We welcome comments from both students and instructors. Please address your suggestions to: Lisa Urry (Chapter 1 and Units 1–3) [email protected] Michael Cain (Units 4, 5, and 8) [email protected] Peter Minorsky (Unit 6) [email protected] Steven Wasserman (Unit 7) [email protected]

PREFACE v

Highlights of New Content

T

his section highlights selected new content and pedagogical changes in Campbell BIOLOGY, Eleventh Edition.

Chapter 1  Evolution, the Themes of Biology, and Scientific Inquiry Chapter 1 opens with a new introduction to a case study on the evolution of coloration in mice. New text and a new photo (Figure 1.12) relate climate change to species survival.

Unit 1  The Chemistry of Life In Unit 1, new content engages students in learning this foundational material. The opening of Chapter 3 and new Figure 3.7 show organisms affected by loss of Arctic sea ice. Chapter 5 has updates on lactose intolerance, trans fats, the effects of diet on blood cholesterol, protein Figure 3.7 Effects of climate change sequences and on the Arctic. structures, and intrinsically disordered proteins. New Visualizing Figure 5.16 helps students understand various ways proteins are depicted. A new ProblemSolving Exercise engages students by having them compare DNA sequences in a case of possible fish fraud.

Unit 2  The Cell Our main goal for this unit was to make the material more accessible and inviting to students. New Visualizing Figure 6.32 shows the profusion of molecules and structures in a cell, all drawn to scale. In Chapter 7, a new figure illustrates levels of LDL receptors in people with and without familial hypercholesterolemia. Chapter 8 includes a beautiful new photo of a geyser with thermophilic bacteria in Figure 8.17, bringing to life the graphs of optimal temperatures for enzyme function. Chapter 10 discusses current research trying to genetically modify rice (a C3 crop) so that it is capable of carrying out C4 photosynthesis to increase yields. Chapter 11 includes a new Problem-Solving Exercise that guides students through assessing possible new treatments for bacterial infections by blocking quorum sensing. In Chapter 12, the mechanism of chromosome movement in bacteria has been updated and more cell cycle control checkpoints have been added, including one proposed by researchers in 2014.

Unit 3  Genetics In Chapters 13–17, we have incorporated changes that help students grasp the more abstract concepts of genetics and their chromosomal and molecular underpinnings. For example, a new Visual Skills Question with Figure 13.6 asks students to identify

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Highlights of New Content

where in the three life cycles haploid cells undergo mitosis, and what type of cells are formed. Chapter 14 includes new information from a 2014 genomic study on the number of genes and genetic variants contributing to height. Figure 14.15b now uses “inability to taste PTC” rather than “attached earlobe.” Chapters 14 and 15 are more inclusive, clarifying the meaning of the term “normal” in genetics and explaining that sex is no longer thought to be simply binary. Other updates in Chapter 15 include new research in sex determination and a technique being developed to avoid passing on mitochondrial diseases. New Visualizing Figure 16.7 shows students various ways that DNA is illustrated. Chapter 17 has a new opening photo and story about albino donkeys to pique student interest in gene expression. To help students understand the Beadle and Tatum experiment, new Figure 17.2 explains how they obtained nutritional mutants. A new ProblemSolving Exercise asks students to identify mutations in the insulin gene and predict their effect on the protein. Chapters 18–21 are extensively updated, driven by exciting new discoveries based on DNA sequencing and gene-editing technology. Chapter 18 has updates on histone modifications, nuclear location and the persistence of transcription factories, chromatin remodeling by ncRNAs, long noncoding RNAs (lncRNAs), the role of master regulatory genes in modifying chromatin structure, and the possible role of p53 in the low incidence of cancer in elephants. Make Connections Figure 18.27, “Genomics, Cell Signaling, and Cancer,” has Figure 20.14 Gene editing been expanded to include using the CRISPR-Cas9 system. more information on cell signaling. Chapter 19 features a new section that covers bacterial defenses against bacteriophages and describes the CRISPRCas9 system (Figure 19.7); updates include the Ebola, Chikungunya, and Zika viruses (Figure 19.10) and discovery of the largest virus known to date. A discussion has been added of mosquito transmission of diseases and concerns about the effects of global climate change on disease transmission. Chapter 20 has a new photo of nextgeneration DNA sequencing machines (Figure 20.2) and a new illustration of the widely used technique of RNA sequencing (Figure 20.13). A new section titled Editing Genes and Genomes has been added describing the CRISPR-Cas9 system (Figure 20.14) that has been developed to edit genes in living cells. Information has also been

added later in the chapter on use of the CRISPR-Cas9 system, including a study in which a genetic mutation for the disease tyrosinemia was corrected in mice. Finally, the discussion of ethical considerations has been updated to include a recent report of scientists using the CRISPR-Cas9 system to edit a gene in human embryos, along with a discussion of the ethical questions raised by such experiments, such as its usage in the gene drive approach to combat carrying of diseases by mosquitoes. In Chapter 21, in addition to the usual updates of sequence-related data (speed of sequencing, number of species’ genomes sequenced, etc.), there are several research updates, including some early results from the new Roadmap Epigenomics Project and results from a 2015 study focusing on 414 important yeast genes.

Unit 4  Mechanisms of Evolution A major goal for this revision was to strengthen how we help students understand and interpret visual representations of evolutionary data and concepts. Toward this end, we have added a new figure (Figure 25.8), “Visualizing the Scale of Geologic Time,” and a new figure (Figure 23.12) on gene flow. Several figures have been revised to improve the presentation of data, including Figure 24.6 (on reproductive isolation in mosquitofish), Figure 24.10 (on allopolyploid speciation), and Figure 25.25 (on the origin of the insect body plan). The Figure 23.12 Gene flow and local unit also features new adaptation in the Lake Erie water material that connects snake (Nerodia sipedon). evolutionary concepts and societal problems. Examples include text in Chapter 22 on the 2015 discovery of teix­ obactin, an antibiotic that is effective against some hard-to-treat pathogens, a new discussion in Chap­ter 24 on the impact of climate change on hybrid zones, and a new Problem-Solving Exercise in Chapter 24 on how hybridization may have led to the spread of insecticide resistance genes in mosquitoes that transmit malaria. The unit also includes new chapteropening stories in Chapter 22 (on a moth whose features illustrate the concepts of unity, diversity, and adaptation) and Chapter 25 (on the discovery of whale bones in the Sahara Desert). Additional changes include new text in ­Concept 22.3 emphasizing how populations can evolve over short periods of time, a new table (Table 23.1) highlighting the five ­conditions required for a population to be in Hardy-Weinberg equilibrium, and new material in Concept 25.1 describing how researchers recently succeeded for the first time in constructing a “protocell” in which replication of a template strand of RNA could occur.



Unit 5 The Evolutionary History of Biological Diversity In keeping with our goal of improving how students interpret and create visual representations in biology, we have added a new figure (Figure 26.5, “Visualizing Phylogenetic Relationships”) that introduces the visual conventions used in phylogenetic trees and helps students understand what such trees do and don’t convey. Students are also provided many opportunities to practice their visual skills, with more than ten new Visual Skills Questions on topics ranging from interpreting phylogenetic trees to predicting which regions of a bacterial flagellum are hydrophobic. The unit also contains new content on tree thinking, emphasizing such key points as how sister groups provide a clear way to describe evolutionary relationships and how trees do not show a “direction” in evolution. Other major content changes include new text in Concepts 26.6, 27.4, and 28.1 on the 2015 discovery of the Lokiarchaeota, a group of archaea that may represent the sister group of the eukaryotes, new text and a new figure (Figure 26.22) on horizontal gene transfer from prokaryotes to eukaryotes, new text in Concept 27.6 describing the CRISPR-Cas9 system and a new figure (Figure 27.21) that illustrates one example of how CRISPR-Cas 9 technology has opened new avenues of research on HIV, and new material in Concept 29.3 describing how early forests contributed to global climate change (in this case, global cooling). A new ProblemSolving Exercise in Chapter 34 engages students in interpreting data from a study investigating whether frogs can acquire resistance to a fungal pathogen through controlled exposure to it. Other updates include the revision of many phylogenies to reflect recent phylogenomic data, new chapter-opening stories in Chapter 31 (on how mycorrhizae link trees of different species) and Chapter 33 (on the “blue dragon,” a mollusc that preys on the highly toxic Portuguese man-of-war), new text and a new figure (Figure 34.37) on the adaptations of the kangaroo rat to its arid environment, and new material in Concept 34.7, including a new figure (Figure 34.52) describing fossil and DNA evidence indicating that humans and NeanderFigure 34.53 Fossils of hand and thals interbred, producing viable offspring. The foot bones of Homo naledi. discussion of human evolution also includes new text and a new figure (Figure 34.53) on Homo naledi, the most recently discovered member of the human evolutionary lineage.

Unit 6  Plant Form and Function A major aim in revising Chapter 35 was to help students better understand how primary and secondary growth are related. New Visualizing Figure 35.11 enables students to picture growth at the cellular level. Also, the terms protoderm, procambium, and ground meristem have been introduced to underscore the transition of meristematic to mature tissues. A new flowchart (Figure 35.24) summarizes growth in a woody shoot. New text and a figure (Figure 35.26) focus on genome analysis of Arabidopsis ecotypes, relating plant morphology to ecology and evolution. In Chapter 36, new Figure 36.8 illustrates the fine branching of leaf veins, and information on

Highlights of New Content

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phloem-xylem water transfer has been updated. New Make Connections Figure 37.10 highlights mutualism across kingdoms and domains. Figure 37.13 and the related text include new findings on how some soil nitrogen derives from weathering of rocks. New Figure 38.3 clarifies how the terms carpel and pistil are related. The text on flower structure and the angiosperm life cycle figure identify carpels as megasporophylls and stamens as microsporophylls, correlating with the plant evolution discussion in Unit 5. In Concept 38.3, the current problem of glyphosate-resistant crops is discussed in detail. A revised Figure 39.7 helps students visualize how cells elongate. Figure 39.8 now addresses apical dominance in a Guided Tour format. Information about the role of sugars in controlling apical dominance has been added. In Concept 39.4, a new Problem-Solving Exercise highlights how global climate change affects crop productivity. Figure 39.26 on defense responses against pathogens has been simplified and improved.

Unit 7  Animal Form and Function A major goal of the Unit 7 revision was to transform how students interact with and learn from representations of anatomy and physiology. For example, gastrulation is now introduced with a Visualizing Figure (Figure 47.8) that provides a clear and carefully paced introduction to three-dimensional processes that may be difficult for students to grasp. In addition, a number of the new and revised figures help students explore spatial relationships in anatomical contexts, such as the interplay of lymphatic and cardiovascular circulation (Figure 42.15) and the relationship of the limbic system to overall brain structure (Figure 49.14). A new Problem-Solving Exercise in Chapter 45 taps into student interest in medical mysteries through a case study that explores the science behind laboratory testing and diagnosis. Content updates help students appreciate the continued evolution of our understanding of even familiar phenomena, such as the sensation of thirst (Concept 44.4) and the locomotion of kangaroos and jellies (Concept 50.6). Furthermore, new text and figures introduce students to cutting-edge technology relating to such topics as RNA-based antiviral defense in invertebrates (Figure 43.4) and rapid, comprehensive characterization of viral exposure (Figure 43.24), as well as recent discoveries regarding brown fat in adult humans (Figure 40.16), the microbiome (Figure 41.17), parthenogenesis (Concept 46.1), and magnetoreception (Concept 50.1). As always, there is fine-tuning of pedagogy, as in discussions of the complementary roles of inactivation and voltage gating of ion channels during action potential formation (Concept 48.3) and of the experimental characterization of genetic determinants in bird migration (Figure 51.24). Figure 41.17 Variation in human gut microbiome at different life stages.

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Highlights of New Content

Unit 8  Ecology The Ecology Unit has been extensively revised for the Eleventh Edition. We have reorganized and improved the conceptual framework with which students are introduced to the following core ecological topics: life tables, per capita population growth, intrinsic rate of increase (“r ”), exponential population growth, logistic population growth, density dependence, species interactions (in particular, parasitism, commensalism, and mutualism), and MacArthur and Wilson’s island biogeography model. The revision also includes a deeper integration of evolutionary principles, including a new Key Concept (52.5) and two new figures (Figures 52.22 and 52.23) on the reciprocal effects of ecology and evolution, new material in Concept 52.4 on how the geographic distributions of species are shaped by a combination of evolutionary history and ecological factors, and five new Make Connections Questions that ask students to examine how ecological and evolutionary mechanisms interact. In keeping with our goal of expanding and strengthening our coverage of climate change, we have added a new discussion and a new figure (Figure 52.20) on how climate change has affected the distribution of a keystone species, a new Figure 55.8 Climate change, section of text in wildfires, and insect outbreaks. Concept 55.2 on how climate change affects NPP, a new figure (Figure 55.8) on how climate change has caused an increase in wildfires and insect outbreaks, a new Problem-Solving Exercise in Chapter 55 that explores how insect outbreaks induced by climate change can cause an ecosystem to switch from a carbon sink to a carbon source, a new figure (Figure 56.29) on the greenhouse effect, new text in Concept 56.4 on biological effects of climate change, and a new Make Connections Figure (Figure 56.30) on how climate change affects all levels of biological organization. Additional updates include a new figure (Figure 53.25) on per capita ecological footprints, a new chapter-opening story in Chapter 54 on a seemingly unlikely mutualism between a shrimp and a much larger predatory fish, new text in Concept 54.1 emphasizing that each partner in a mutualism experiences both benefits and costs, new text in Concept 54.1 describing how the outcome of an ecological interaction can change over time, two new figures (Figures 54.29 and 54.30) on the island equilibrium model, a new figure (Figure 54.31) documenting two shrew species as unexpected hosts of the Lyme disease, new text in Concept 56.1 comparing extinction rates today with those typically seen in the fossil record, and a new discussion and figure (Figure 56.22) on the restoration of a degraded urban stream.

Ready-to-Go Teaching Modules for Instructors NEW! Ready-to-Go Teaching Modules help instructors efficiently make use of the best teaching tools before, during, and after class. The Ready-to-Go Teaching Modules incorporate the best that the text, MasteringBiology, and Learning Catalytics have to offer, along with new ideas for in-class activities. The modules can be accessed through the Instructor Resources area of MasteringBiology.

Instructors can easily incorporate active learning into their courses using suggested activity ideas and questions. Videos demonstrate how the activities can be used in class.

Learning CatalyticsTM allows students to use their smartphone, tablet, or laptop to respond to questions in class. Visit learningcatalytics.com.

READY-TO-GO TEACHING MODULES FOR INSTRUCTORS

ix

See the Big Picture Each chapter is organized around a framework of 3 to 7 Key Concepts that focus on the big picture and provide a context for supporting details.

Every chapter opens with a visually dynamic photo accompanied by an intriguing question that invites students into the chapter. The List of Key Concepts introduces the big ideas covered in the chapter.

Get Ready for This Chapter

NEW! Get Ready for This Chapter questions provide a quick check of students’ knowledge of basic information needed to learn the new content of a chapter, with feedback.

After reading a Key Concept section, students can check their understanding using the Concept Check Questions. Questions throughout the chapter encourage students to read the text actively. What If? Questions ask students to apply what they’ve learned. Make Connections Questions ask students to relate content in the chapter to material presented earlier in the course.

x

SEE THE BIG PICTURE

These questions are available as MasteringBiology assignments and as self-study quizzes in the Study Area.

The Summary of Key Concepts refocuses students on the main points of the chapter. NEW! QR codes and URLs at the end of every chapter give students quick access to Vocabulary SelfQuizzes and Practice Tests on their smartphones, tablets, and computers.

Summary of Key Concepts Questions check students’ understanding of a key idea from each concept.

Summary Figures recap key information visually.

Evolution, the fundamental theme of biology, is emphasized throughout. Every chapter has a section explicitly relating the chapter content to evolution: Evolution Connection Questions are included in every Chapter Review.

Synthesize Your Knowledge Questions ask students to apply their understanding of the chapter content to explain an intriguing photo.

SEE THE BIG PICTURE

xi

Build Visual Skills NEW! Visualizing Figures teach students how to interpret diagrams and models in biology. Embedded questions give students practice applying visual skills as they read the figure.

For more practice, each Visualizing Figure is accompanied by an automatically graded assignment in MasteringBiology with feedback for students. Visualizing Figures include: Figure 5.16 Visualizing Proteins, p. 79 Figure 6.32 Visualizing the Scale of the Molecular Machinery in a Cell, pp. 122-123

Figure 16.7 Visualizing DNA, p. 319 Figure 25.8 Visualizing the Scale of Geologic Time, pp. 530-531

Figure 26.5 Visualizing Phylogenetic Relationships, shown at left and on p. 554

Figure 35.11 Visualizing Primary and Secondary Growth, p. 765 Figure 47.8 Visualizing Gastrulation, p. 1048 Figure 55.13 Visualizing Biogeochemical Cycles, p. 1247

NEW! Visual Skills Questions give students practice interpreting illustrations and photos in the text. xii

BUILD VISUAL SKILLS

NEW! Figure Walkthroughs guide students through key figures with narrated explanations, figure markups, and questions that reinforce important points.

A note in the print book lets students and instructors know when a Figure Walkthrough is available in the Study Area.

Questions embedded in each Figure Walkthrough encourage students to be active participants in their learning. The Figure Walkthroughs can also be assigned in MasteringBiology with higher-level questions.

EXPANDED! Draw It exercises give students practice creating visuals. Students are asked to put pencil to paper and draw a structure, annotate a figure, or graph experimental data.



BUILD VISUAL SKILLS

xiii

Make Connections Visually Eleven Make Connections Figures pull together content from different chapters, providing a visual representation of “big picture” relationships. Make Connections Figures include: Figure 5.26  Contributions of Genomics and Proteomics to Biology, p. 88 Figure 10.23 The Working Cell, pp. 208-209

Figure 18.27 Genomics, Cell Signaling, and Cancer, pp. 390-391 Figure 23.18 The Sickle-Cell Allele, shown at right and on pp. 500-501

Figure 33.9  Maximizing Surface Area, p. 693 NEW!  Figure 37.10 Mutualism Across Kingdoms and Domains, p. 811

Figure 39.27 Levels of Plant Defenses Against Herbivores, pp. 866-867 Figure 40.23 Life Challenges and Solutions in Plants and Animals, pp. 892-893

Figure 44.17 Ion Movement and Gradients, p. 991 Figure 55.19 The Working Ecosystem, pp. 1254-1255 NEW!  Figure 56.30  Climate Change Has Effects at All Levels of Biological Organization, pp. 1278-1279

NEW! Media references integrated into the text direct students to digital content in the MasteringBiology Study Area that will help them prepare for class and succeed on exams. Video resources include HHMI BioInteractive Short Films (documentaryquality movies from the Howard Hughes Medical Institute) and much more.

xiv

MAKE CONNECTIONS VISUALLY

Make Connections Questions in every chapter ask students to relate content in the chapter to material presented earlier in the course.



MAKE CONNECTIONS VISUALLY

xv

Practice Scientific Skills Scientific Skills Exercises use real data to build key skills needed for biology, including data analysis, graphing, experimental design, and math skills.

Each Scientific Skills Exercise is based on an experiment related to the chapter content.

Most Scientific Skills Exercises use data from published research, which is cited in the exercise. Questions build in difficulty, walking students through new skills step by step and providing opportunities for higherlevel critical thinking. All Scientific Skills Exercises are available as interactive assignments in MasteringBiology that are automatically graded.

Scientific Skills ExerciseS are available for every chapter: 1 Interpreting a Pair of Bar Graphs, p. 23

15 Using the Chi-Square (x2) Test, p. 304

2 Calibrating a Standard Radioactive Isotope Decay Curve and Interpreting Data, p. 33

16 Working with Data in a Table, p. 318

3 Interpreting a Scatter Plot with a Regression Line, p. 54

18 Analyzing DNA Deletion Experiments, p. 373

4 Working with Moles and Molar Ratios, p. 58 5 Analyzing Polypeptide Sequence Data, p. 89

17 Interpreting a Sequence Logo, p. 351 19 Analyzing a DNA Sequence-Based Phylogenetic Tree to Understand Viral Evolution, p. 409

6 Using a Scale Bar to Calculate Volume and Surface Area of a Cell, p. 99

20 Analyzing Quantitative and Spatial Gene Expression Data*

7 Interpreting a Scatter Plot with Two Sets of Data, shown above and on p. 136

22 Making and Testing Predictions, p. 481

8 Making a Line Graph and Calculating a Slope, p. 157 9 Making a Bar Graph and Evaluating a Hypothesis, p. 179 10 Making Scatter Plots with Regression Lines, p. 205 11 Using Experiments to Test a Model* 12 Interpreting Histograms, p. 250 13 Making a Line Graph and Converting Between Units of Data, p. 264 14 Making a Histogram and Analyzing a Distribution Pattern, p. 283

xvi

PRACTICE SCIENTIFIC SKILLS

21 Reading an Amino Acid Sequence Identity Table, p. 456 23 Using the Hardy-Weinberg Equation to Interpret Data and Make Predictions, p. 491 24 Identifying Independent and Dependent Variables, Making a Scatter Plot, and Interpreting Data, p. 511 25 Estimating Quantitative Data from a Graph and Developing Hypotheses, p. 536 26 Using Protein Sequence Data to Test an Evolutionary Hypothesis, p. 568 27 Calculating and Interpreting Means and Standard Errors, p. 588 Making a Bar Graph and Interpreting the Data*

Apply Scientific Skills to Solving Problems NEW! Problem-Solving Exercises guide students in applying scientific skills and interpreting real data in the context of solving a real-world problem. Problem-Solving Exercises include: Ch. 5: Are you a victim of fish fraud? Shown at left and on p. 89

Ch. 11:  Can a skin wound turn deadly? p. 214 Ch. 17: Are insulin mutations the cause of three infants’ neonatal diabetes? p. 359 Ch. 24: Is hybridization promoting insecticide resistance in mosquitoes that transmit malaria? p. 516 Ch. 34:  Can declining amphibian populations be saved by a vaccine? p. 731 Ch. 39:  How will climate change impact crop productivity? p. 861 Ch. 45: Is thyroid regulation normal in this patient? p. 1008

A version of each Problem-Solving Exercise can also be assigned in MasteringBiology.

Ch. 55:  Can an insect outbreak threaten a forest’s ability to absorb CO2 from the atmosphere? p. 1243

28 Interpreting Comparisons of Genetic Sequences, p. 593

43 Comparing Two Variables on a Common x-Axis, p. 971

29 Making Bar Graphs and Interpreting Data, p. 627

44 Describing and Interpreting Quantitative Data, p. 979

30 Using Natural Logarithms to Interpret Data, p. 637

45 Designing a Controlled Experiment, p. 1012

31 Interpreting Genomic Data and Generating Hypotheses, p. 655

46 Making Inferences and Designing an Experiment, p. 1028

Synthesizing Information from Multiple Data Sets*

47 Interpreting a Change in Slope, p. 1047

32 Calculating and Interpreting Correlation Coefficients, p. 676

48 Interpreting Data Values Expressed in Scientific Notation, p. 1080

33 Understanding Experimental Design and Interpreting Data, p. 698

49 Designing an Experiment using Genetic Mutants, p. 1093

34 Determining the Equation of a Regression Line, p. 749

51 Testing a Hypothesis with a Quantitative Model, p. 1148

35 Using Bar Graphs to Interpret Data, p. 760 36 Calculating and Interpreting Temperature Coefficients, p. 788

50 Interpreting a Graph with Log Scales, p. 1134 52 Making a Bar Graph and a Line Graph to Interpret Data, p. 1184

37 Making Observations, p. 810

53 Using the Logistic Equation to Model Population Growth, p. 1198

38 Using Positive and Negative Correlations to Interpret Data, p. 832

54 Making a Bar Graph and a Scatter Plot, p. 1215

39 Interpreting Experimental Results from a Bar Graph, p. 862

55 Interpreting Quantitative Data in a Table, p. 1245

40 Interpreting Pie Charts, p. 890

56 Graphing Cyclic Data, p. 1276

41 Interpreting Data from an Experiment with Genetic Mutants, p. 916 42 Making and Interpreting Histograms, p. 936

* Available only in MasteringBiology. All other Scientific Skills Exercises are in the print book, eText, and MasteringBiology.

APPLY SCIENTIFIC SKILLS TO SOLVING PROBLEMS

xvii

Bring Biology to Life NEW! Over 450 carefully chosen and edited videos and animations have been integrated into the print book, MasteringBiology Study Area, and eText 2.0 at point of use to help students learn biology visually. (eText 2.0 will be live for Fall 2017 classes.) Media references in the print book direct students to digital resources in the Study Area and eText 2.0: • NEW! Get Ready for This Chapter questions • NEW! Figure Walkthroughs • NEW! HHMI BioInteractive Videos and Animations • NEW! QR codes and URLs for easy access to Vocabulary Self-Quizzes and Practice Tests • BioFlix Animations • ABC News Videos • Campbell Interviews and much more

Access the complete textbook online!  The Campbell eText includes powerful interactive and customization functions, such as instructor and student note-taking, highlighting, bookmarking, search, and links to glossary terms. eText 1.0 will be available for Spring 2017 classes. NEW! eText 2.0 will be available for Fall 2017 classes with all of the above plus links to animations, videos, and quizzes, all accessible from a smartphone, tablet, and computer.

xviii

BRING BIOLOGY TO LIFE

Succeed with MasteringBiology MasteringBiology improves results by engaging students before, during, and after class. Before Class Dynamic Study Modules provide students with multiple sets of questions with extensive feedback so that they can test, learn, and retest until they achieve mastery of the textbook material. NEW! Get Ready for This Chapter quizzes help students review content they need to understand from previous chapters (see p. x). Pre-Class Reading Quizzes help students pinpoint concepts that they understand and concepts that they need to review.

During Class NEW! For ideas for in-class activities, see the Ready-to-Go Teaching Modules (see p. ix).

After Class Hundreds of self-paced tutorials and coaching activities provide students with individualized coaching with specific hints and feedback on the toughest topics in the course. Optional Adaptive Follow-up Assignments are based on each student’s performance on the original MasteringBiology assignment and provide additional questions and activities tailored to each student’s needs.

Learning CatalyticsTM allows students to use their smartphone, tablet, or laptop to respond individually or in groups to questions in class. Visit learningcatalytics. com to learn more.

SUCCEED WITH MASTERINGBIOLOGY

xix

Personalized Coaching in MasteringBiology Instructors can assign self-paced MasteringBiology tutorials that provide students with individualized coaching with specific hints and feedback on the toughest topics in the course.

1. If a student gets stuck... 2. Specific wrong-answer feedback appears in the purple feedback box.

3. Hints coach students to the correct response. 4. Optional Adaptive Follow-Up Assignments are based on the original homework assignment and provide additional coaching and practice as needed.

Question sets in the Adaptive Follow-Up Assignments continuously adapt to each student’s needs, making efficient use of study time.

xx

PERSONALIZED COACHING IN MASTERINGBIOLOGY

MasteringBiology offers thousands of tutorials, activities, and questions that can be assigned as homework. A few examples are shown below.

BioFlix Tutorials use 3-D, moviequality animations and coaching exercises to help students master tough topics outside of class. Animations are also available in the Study Area and eText 2.0, and can be shown in class.

EXPANDED! HHMI BioInteractive Short Films, documentary-quality movies from the Howard Hughes Medical Institute, engage students in topics from the discovery of the double helix to evolution, with assignable questions.

NEW! Galápagos Evolution Video Activities, filmed on the Galápagos Islands by Peter and Rosemary Grant, bring to life the dynamic evolutionary processes that impact Darwin’s finches on Daphne Major Island. Videos explore important concepts and data from the Grants’ field research, with assignable activities.

The MasteringBiology Gradebook provides instructors with quick results and easy-to-interpret insights into student performance. Every assignment is automatically graded. Shades of red highlight vulnerable students and challenging assignments.

Student scores on the optional Adaptive Follow-Up Assignments are recorded in the gradebook and offer additional diagnostic information for instructors to monitor learning outcomes and more.



PERSONALIZED COACHING IN MASTERINGBIOLOGY

xxi

Student and Lab Supplements For Students Study Guide, Eleventh Edition

Get Ready for Biology

by Martha R. Taylor, Ithaca, New York, and Michael Pollock, Mount Royal University

by Lori K. Garrett, Parkland College

978-0-134-44377-5 / 0-134-44377-2

This engaging workbook helps students brush up on important math and study skills and get up to speed on biological terminology and the basics of chemistry and cell biology. Also available in MasteringBiology.

This popular study aid provides concept maps, chapter summaries, word roots, and a variety of interactive activities including multiplechoice, short-answer essay, art labeling, and graph interpretation questions.

Inquiry in Action: Interpreting Scientific Papers, Fourth Edition* by Ruth V. Buskirk, University of Texas at Austin and Christopher M. Gillen, Kenyon College 978-0-134-47861-6 / 0-134-47861-4 This guide helps students learn how to read and understand primary research articles. Part A presents complete articles accompanied by questions that help students analyze the article. Related Inquiry Figures are included in the supplement. Part B covers every part of a research paper, explaining the aim of the sections and how the paper works as a whole.

Practicing Biology: A Student Workbook, Sixth Edition* by Jean Heitz and Cynthia Giffen, University of Wisconsin, Madison 978-0-134-48603-1 / 0-134-48603-X This workbook offers a variety of activities to suit different learning styles. Activities such as modeling and concept mapping allow students to visualize and understand biological processes. Other activities focus on basic skills, such as reading and drawing graphs.

Biological Inquiry: A Workbook of Investigative Cases, Fifth Edition* by Margaret Waterman, Southeast Missouri State University and Ethel Stanley, BioQUEST Curriculum Consortium and Beloit College 978-0-134-48646-8 / 0-134-48646-3 This workbook offers ten investigative cases. Each case study requires students to synthesize information from multiple chapters of the text and apply that knowledge to a real-world scenario as they pose hypotheses, gather new information, analyze evidence, graph data, and draw conclusions. A link to a student website is in the Study Area in MasteringBiology.

Study Card, Eleventh Edition 978-0-134-48648-2 / 0-134-48648-X This quick-reference card provides students with an overview of the entire field of biology, helping them see the connections between topics.

978-0-32150057-1 / 0-321-50057-1

A Short Guide to Writing About Biology, Ninth Edition by Jan A. Pechenik, Tufts University 978-0-13414373-6 / 0-134-14373-6 This best-selling writing guide teaches students to think as biologists and to express ideas clearly and concisely through their writing.

For Lab Investigating Biology Laboratory Manual, Ninth Edition by Judith Giles Morgan, Emory University, and  M. Eloise Brown Carter, Oxford College of Emory University 978-0-13447346-8 / 0-134-47346-9 With its distinctive investigative approach to learning, this best-selling laboratory manual is now more engaging than ever, with full-color art and photos throughout. The lab manual encourages students to participate in the process of science and develop creative and criticalreasoning skills.

Annotated Instructor’s Edition for Investigating Biology Laboratory Manual, Ninth Edition 978-0-13451922-7 / 0-134-51922-1 This Annotated Instructor Edition features teaching information including margin notes with suggestions for lab procedures and answers to questions from the Student Edition. Also included is a detailed Teaching Plan at the end of each lab with specific suggestions for organizing labs, including estimated time allotments for each part of the lab and suggestions for encouraging independent thinking and collaborative discussions.

Preparation Guide for Investigating Biology, Ninth Edition 978-013451801-5 / 0-134-51801-2 Contains materials lists, suggested vendors, instructions for preparing solutions and constructing materials, schedules for planning advance preparation, and more. Available for downloading through the “Instructor Resources” area of MasteringBiology.

Pearson Collections Custom Library gives instructors the

By Laura P. Zanello, University of California, Riverside

power to create custom lab manuals using Pearson content as well as original materials. Learn more at www.pearsonhighered.com/ collections.

978-0-32183498-0 / 0-321-83498-4

MasteringBiology® Virtual Labs is an online environment

Spanish Glossary for Biology

This resource provides definitions in Spanish for the glossary terms.

Into The Jungle: Great Adventures in the Search for Evolution

that promotes critical thinking skills using virtual experiments and explorations that can supplement or substitute for existing wet labs in microscopy, molecular biology, genetics, ecology, and systematics.

by Sean B. Carroll, University of Wisconsin, Madison 978-0-32155671-4 / 0-321-55671-2 These nine short tales vividly depict key discoveries in evolutionary biology and the excitement of the scientific process. Online resources available at www.aw-bc.com/carroll. * An Instructor Guide is available for downloading through the “Instructor Resources” area of MasteringBiology.

xxii

STUDENT AND LAB SUPPLEMENTS

Instructor Resources Instructor’s Resource CD/DVD-ROM Set for Campbell Biology, Eleventh Edition 978-0-13445458-0 / 0-134-45458-8 The Instructor’s Resource DVD Package consists of a multi-disk set of assets for each chapter. Specific features include: • Editable figures (art and photos) and tables from the text in PowerPoint® • Prepared PowerPoint Lecture Presentations for each chapter with lecture notes, editable figures (art and photos), tables, and links to animations and videos • JPEG images, including labeled and unlabeled art, photos from the text, and extra photos • Clicker Questions in PowerPoint

Customizable PowerPoint Lectures provide a jumpstart for instruction.

• 400 instructor animations and videos, including BioFlix® 3-D animations and ABC News Videos • Test Bank questions in TestGen® software and Microsoft® Word • Digital Transparencies • Quick Reference Guide: An at-a-glance reference to all instructor and student media resources available for each chapter

The Instructor Resources area of MasteringBiology includes: • NEW! Ready-to-Go Teaching Modules help instructors efficiently make use of the available teaching tools for the toughest topics. Before-class assignments, in-class activities, and afterclass assignments are provided for ease of use. Instructors can incorporate active learning into their course with the suggested activity ideas and clicker questions or Learning Catalytics questions. • Editable figures (art and photos) and tables from the text in PowerPoint • Prepared PowerPoint Lecture Presentations for each chapter with lecture notes, editable figures (art and photos), tables, and links to animations and videos • JPEG images, including labeled and unlabeled art, photos from the text, and extra photos • Clicker Questions in PowerPoint • 400 instructor animations and videos, including BioFlix 3-D animations and ABC News Videos • Test Bank (in Word and TestGen) • Digital Transparencies • Answers to Scientific Skills Exercises, Problem-Solving Exercises, Interpret the Data Questions, and Essay Questions; includes Rubric and Tips for Grading Short-Answer Essays

All of the art, graphs, and photos from the text are provided with customizable labels. More than 1,600 photos from the text and other sources are included.

• Instructor Guides for Supplements: Instructor Guide for Practicing Biology: A Student Workbook, Instructor Guide for Biological Inquiry: A Workbook of Investigative Cases, Answer Key for Inquiry in Action: Interpreting Scientific Papers, Investigating Biology Lab Prep Guide, and Investigating Biology Lab Data Tables • Quick Reference Guide

Printed Test Bank for Campbell Biology, Eleventh Edition 978-0-13447864-7 / 0-134-47864-9 This invaluable resource contains over 4,500 questions, including scenario-based questions and art, graph, and data interpretation questions. In addition to a print version, the Test Bank is available electronically through the “Instructor Resources” area of MasteringBiology, and in the Instructor’s Resource DVD Package.

Clicker Questions can be used to stimulate effective classroom discussions (for use with or without clickers). INSTRUCTOR RESOURCES

xxiii

Featured Figures Visualizing Figures

5.16 Visualizing Proteins  79 6.32 Visualizing the Scale of the Molecular Machinery in a Cell  122 16.7 Visualizing DNA  319 25.8 Visualizing the Scale of Geologic Time  530 26.5 Visualizing Phylogenetic Relationships  554 35.11 Visualizing Primary and Secondary Growth  765 47.8 Visualizing Gastrulation  1048 55.13 Visualizing Biogeochemical Cycles  1247

Make Connections Figures

5.26 Contributions of Genomics and Proteomics to Biology  88 10.23 The Working Cell  208 18.27 Genomics, Cell Signaling, and Cancer  390 23.18 The Sickle-Cell Allele  500 33.9 Maximizing Surface Area  693 37.10 Mutualism Across Kingdoms and Domains  811 39.27 Levels of Plant Defenses Against Herbivores  866 40.23 Life Challenges and Solutions in Plants and Animals 892 44.17 Ion Movement and Gradients  991 55.19 The Working Ecosystem  1254 56.30 Climate Change Has Effects at All Levels of Biological Organization  1278

Exploring Figures

1.3 5.18 6.3 6.8 6.30 7.19 11.8 12.7 13.8 16.22



24.3 25.7 27.16 28.2 29.3 29.8 29.14 30.7 30.17

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Levels of Biological Organization  4 Levels of Protein Structure  80 Microscopy 95 Eukaryotic Cells  100 Cell Junctions in Animal Tissues  120 Endocytosis in Animal Cells  140 Cell-Surface Transmembrane Receptors  218 Mitosis in an Animal Cell  238 Meiosis in an Animal Cell  260 Chromatin Packing in a Eukaryotic Chromosome 330 Reproductive Barriers  506 The Origin of Mammals  529 Selected Major Groups of Bacteria  582 Protistan Diversity  594 Derived Traits of Plants  618 Bryophyte Diversity  624 Seedless Vascular Plant Diversity  630 Gymnosperm Diversity  640 Angiosperm Diversity  648

FEATURED FIGURES



31.10 33.3 33.43 34.42 35.10 37.16 38.4 38.12 40.5 41.5 44.12 46.11 49.11 50.10 50.17 52.2 52.3 52.12 52.15 53.18

Fungal Diversity  659 Invertebrate Diversity  685 Insect Diversity  710 Mammalian Diversity  742 Examples of Differentiated Plant Cells  762 Unusual Nutritional Adaptations in Plants  817 Flower Pollination  822 Fruit and Seed Dispersal  830 Structure and Function in Animal Tissues  875 Four Main Feeding Mechanisms of Animals  901 The Mammalian Excretory System  984 Human Gametogenesis  1026 The Organization of the Human Brain  1090 The Structure of the Human Ear  1111 The Structure of the Human Eye  1116 The Scope of Ecological Research  1163 Global Climate Patterns  1164 Terrestrial Biomes  1171 Aquatic Biomes  1177 Mechanisms of Density-Dependent Regulation 1202 55.14 Water and Nutrient Cycling  1248 55.17 Restoration Ecology Worldwide  1252

Inquiry Figures





1.25 Does camouflage affect predation rates on two populations of mice?  21 4.2 Can organic molecules form under conditions estimated to simulate those on the early Earth?  57 7.4 Do membrane proteins move?  128 † 10.10 Which wavelengths of light are most effective in driving photosynthesis?  194 12.9 At which end do kinetochore microtubules shorten during anaphase?  241 12.14 Do molecular signals in the cytoplasm regulate the cell cycle?  245 14.3 When F1 hybrid pea plants self- or cross-pollinate, which traits appear in the F2 generation?  271 14.8 Do the alleles for one character segregate into gametes dependently or independently of the alleles for a different character?  276 † 15.4 In a cross between a wild-type female fruit fly and a mutant white-eyed male, what color eyes will the F1 and F2 offspring have?  297 15.9 How does linkage between two genes affect inheritance of characters?  301 16.2 Can a genetic trait be transferred between different bacterial strains?  315 16.4 Is protein or DNA the genetic material of phage T2?  316

† * 16.11 Does DNA replication follow the conservative, semiconservative, or dispersive model?  322 17.3 Do individual genes specify the enzymes that function in a biochemical pathway?  337 18.22 Could Bicoid be a morphogen that determines the anterior end of a fruit fly?  385 19.2 What causes tobacco mosaic disease?  397 20.16 Can the nucleus from a differentiated animal cell direct development of an organism?  427 20.21 Can a fully differentiated human cell be “deprogrammed” to become a stem cell?  430 21.18 What is the function of a gene (FOXP2) that is rapidly evolving in the human lineage?  459 22.13 Can a change in a population’s food source result in evolution by natural selection?  475 *23.16 Do females select mates based on traits indicative of “good genes”?  498 24.7 Can divergence of allopatric populations lead to reproductive isolation?  510 24.12 Does sexual selection in cichlids result in reproductive isolation?  513 24.18 How does hybridization lead to speciation in sunflowers? 519 25.26 What causes the loss of spines in lake stickleback fish? 544 26.6 What is the species identity of food being sold as whale meat?  555 27.10 Can prokaryotes evolve rapidly in response to environmental change?  576 28.24 What is the root of the eukaryotic tree?  609 29.9 Can bryophytes reduce the rate at which key nutrients are lost from soils?  625 31.20 Do fungal endophytes benefit a woody plant?  665 33.30 Did the arthropod body plan result from new Hox genes? 705 34.51 Did gene flow occur between Neanderthals and humans? 751 36.18 Does phloem sap contain more sugar near sources than near sinks?  799 37.11 How variable are the compositions of bacterial communities inside and outside of roots?  812 39.5 What part of a grass coleoptile senses light, and how is the signal transmitted?  845 39.6 What causes polar movement of auxin from shoot tip to base?  846 39.16 How does the order of red and far-red illumination affect seed germination?  855 40.17 How does a Burmese python generate heat while incubating eggs?  886 40.22 What happens to the circadian clock during hibernation? 891 *41.4 Can diet influence the frequency of birth defects? 900 42.25 What causes respiratory distress syndrome?  942 44.20 Can aquaporin mutations cause diabetes?  993

46.8 Why is sperm usage biased when female fruit flies mate twice?  1022 † 47.4 Does the distribution of Ca2+ in an egg correlate with formation of the fertilization envelope?  1044 47.23 How does distribution of the gray crescent affect the developmental potential of the first two daughter cells?  1059 47.24 Can the dorsal lip of the blastopore induce cells in another part of the amphibian embryoto change their developmental fate?  1060 47.26 What role does the zone of polarizing activity (ZPA) play in limb pattern formation in vertebrates?  1061 50.23 How do mammals detect different tastes?  1121 51.8 Does a digger wasp use landmarks to find her nest? 1143 51.24 Are differences in migratory orientation within a species genetically determined?  1155 53.14 How does caring for offspring affect parental survival in kestrels?  1199 † 54.3 Can a species’ niche be influenced by interspecific competition? 1214 54.18 Is Pisaster ochraceus a keystone species?  1224 55.6 Which nutrient limits phytoplankton production along the coast of Long Island?  1241 55.12 How does temperature affect litter decomposition in an ecosystem?  1246 *56.13 What caused the drastic decline of the Illinois greater prairie chicken population?  1265

Research Method Figures

5.21 6.4 10.9 13.3 14.2 14.7 15.11 20.3

20.7 20.11 26.15

35.21 37.7 48.8 53.2

54.12

X-Ray Crystallography  83 Cell Fractionation  96 Determining an Absorption Spectrum  193 Preparing a Karyotype  256 Crossing Pea Plants  270 The Testcross  275 Constructing a Linkage Map  305 Sequencing by Synthesis: Next-Generation Sequencing 415 The Polymerase Chain Reaction (PCR)  419 RT-PCR Analysis of the Expression of Single Genes 423 Applying Parsimony to a Problem in Molecular Systematics 561 Using Dendrochronology to Study Climate  771 Hydroponic Culture  808 Intracellular Recording  1070 Determining Population Size Using the MarkRecapture Method  1189 Determining Microbial Diversity Using Molecular Tools 1221

*The Inquiry Figure, original research paper, and a worksheet to guide you through the paper are provided in Inquiry in Action: Interpreting Scientific Papers, Fourth Edition. †

A related Experimental Inquiry Tutorial can be assigned in MasteringBiology.®

FEATURED FIGURES

xxv

Interviews Unit 1  The Chemistry of Life  27

Lovell Jones Prairie View A&M University and University of Texas MD Anderson Cancer Center

Unit 2  The Cell  92

University of Texas at Austin

Unit 6  Plant Form And Function  755

Philip Benfey

New Mexico State University

Duke University

Unit 7  Animal Form And Function  870

Shirley Tilghman

Harald zur Hausen

Princeton University

German Cancer Research Center

Unit 4  Mechanisms of Evolution  465

INTERVIEWS

Nancy Moran

Elba Serrano

Unit 3  Genetics  253

xxvi

Unit 5  The Evolutionary History of Biological Diversity 550

Unit 8 Ecology  1161

Jack Szostak

Tracy Langkilde

Harvard University

Penn State University

Acknowledgments

T

he authors wish to express their gratitude to the global community of instructors, researchers, students, and publishing professionals who have contributed to the Eleventh Edition of Campbell BIOLOGY. As authors of this text, we are mindful of the daunting challenge of keeping up to date in all areas of our rapidly expanding subject. We are grateful to the many scientists who helped shape this text by discussing their research fields with us, answering specific questions in their areas of expertise, and sharing their ideas about biology education. We are especially grateful to the following, listed alphabetically: Graham Alexander, John Archibald, Kristian Axelsen, Barbara Bowman, Joanne Chory, Roger Craig, Michael Hothorn, Patrick Keeling, Barrett Klein, Rachel Kramer Green, James Nieh, Kevin Peterson, T.K. Reddy, Andrew Roger, Alastair Simpson, Marty Taylor, and Elisabeth Wade. In addition, the biologists listed on pages xxviii–xxxi provided detailed reviews, helping us ensure the text’s scientific accuracy and improve its pedagogical effectiveness. We thank Michael Pollock, author of the Study Guide, for his many contributions to the accuracy, clarity, and consistency of the text; and we thank Carolyn Wetzel, Ruth Buskirk, Joan Sharp, Jennifer Yeh, and Charlene D’Avanzo for their contributions to the Scientific Skills Exercises and Problem-Solving Exercises. Thanks also to the other professors and students, from all over the world, who contacted the authors directly with useful suggestions. We alone bear the responsibility for any errors that remain, but the dedication of our consultants, reviewers, and other correspondents makes us confident in the accuracy and effectiveness of this text. Interviews with prominent scientists have been a hallmark of Campbell BIOLOGY since its inception, and conducting these interviews was again one of the great pleasures of revising the book. To open the eight units of this edition, we are proud to include interviews with Lovell Jones, Elba Serrano, Shirley Tilghman, Jack Szostak, Nancy Moran, Philip Benfey, Harald zur Hausen, and Tracy Langkilde. We are especially grateful to Rebecca Orr for her hard work on the digital resources for the eText, Study Area, and Ready-to-Go Teaching Modules. And thanks to the rest of the Ready-to-Go Teaching Modules team: Molly Jacobs, Karen Resendes, Eileen Gregory, Angela Hodgson, Maureen Leupold, Jennifer Metzler, Allison Silveus, Jered Studinski, Sara Tallarovic, Judy Schoonmaker, Michael Pollock, and Chad Brassil. We would also like to extend our sincere appreciation to Carolyn Wetzel, Jennifer Yeh, Matt Lee, and Sherry Seston for their hard work on the Figure Walkthroughs. And our gratitude goes to Katie Cook for keeping these projects so well organized. Thanks also to Kaddee Lawrence for writing the questions that accompany the Visualizing Figures in MasteringBiology and to Mikaela Schmitt-Harsh for converting the Problem-Solving Exercises to MasteringBiology tutorials. The value of Campbell BIOLOGY as a learning tool is greatly enhanced by the supplementary materials that have been created for instructors and students. We recognize that the dedicated authors of these materials are essentially writing mini (and not so mini) books. We appreciate the hard work and creativity of all the authors listed, with their creations, on page xxii. We are also grateful to Kathleen Fitzpatrick and Nicole Tunbridge (PowerPoint® Lecture Presentations); Roberta Batorsky, Douglas Darnowski, James Langeland, and David Knochel (Clicker Questions); Sonish Azam, Kevin Friesen, Murty Kambhampati, Janet Lanza, Ford Lux, Chris Romero, Ruth Sporers, and David Knochel (Test Bank); Natalie Bronstein, Linda Logdberg, Matt McArdle, Ria Murphy, Chris Romero, and Andy Stull (Dynamic Study Modules); and Eileen Gregory, Rebecca Orr, and Elena Pravosudova (Adaptive Follow-up Assignments). MasteringBiology™ and the other electronic accompaniments for this text are invaluable teaching and learning aids. We thank the hardworking, industrious instructors who worked on the

revised and new media: Roberta Batorsky, Beverly Brown, Erica Cline, Willy Cushwa, Tom Kennedy, Tom Owens, Michael Pollock, Frieda Reichsman, Rick Spinney, Dennis Venema, Carolyn Wetzel, Heather Wilson-Ashworth, and Jennifer Yeh. We are also grateful to the many other people—biology instructors, editors, and production experts—who are listed in the credits for these and other elements of the electronic media that accompany the text. Campbell BIOLOGY results from an unusually strong synergy between a team of scientists and a team of publishing professionals. Our editorial team at Pearson Education again demonstrated unmatched talents, commitment, and pedagogical insights. Our Courseware Portfolio Management Specialist, Josh Frost, brought publishing savvy, intelligence, and a much-appreciated level head to leading the whole team. The clarity and effectiveness of every page owe much to our extraordinary Supervising Editors Beth Winickoff and Pat Burner, who worked with a top-notch team of Courseware Senior Analysts in John Burner, Mary Ann Murray, Mary Hill, Laura Southworth, and Hilair Chism. Our unsurpassed Courseware Director of Content Development Ginnie Simione Jutson and Courseware Portfolio Management Director Beth Wilbur were indispensable in moving the project in the right direction. We also want to thank Robin Heyden for organizing the annual Biology Leadership Conferences and keeping us in touch with the world of AP Biology. You would not have this beautiful text if not for the work of the production team: Director of Product Management Services Erin Gregg; Managing Producer Michael Early; Content Producer Lori Newman; Photo Researcher Maureen Spuhler; Copy Editor Joanna Dinsmore; Proofreader Pete Shanks; Rights & Permissions Manager Ben Ferrini; Managing Editor Angel Chavez and the rest of the staff at Integra Software Services, Inc.; Art Production Manager Rebecca Marshall, Artist Kitty Auble, and the rest of the staff at Lachina; Design Manager Marilyn Perry; Text and Cover Designer Elise Lansdon; and Manufacturing Buyer Stacey Weinberger. We also thank those who worked on the text’s supplements: Josey Gist, Margaret Young, Kris Langan, Pete Shanks, Crystal Clifton and Jennifer Hastings at Progressive Publishing Alternatives, and Margaret McConnell at Integra. And for creating the wonderful package of electronic media that accompanies the text, we are grateful to Tania Mlawer, Sarah Jensen, Charles Hall, Katie Foley, Laura Tommasi, Lee Ann Doctor, Tod Regan, Libby Reiser, Jackie Jakob, Sarah YoungDualan, Cady Owens, Caroline Ayres, Katie Cook, and Ziki Dekel as well as VP of Production and Digital Studio Lauren Fogel and Director of Digital Content Development Portfolio Management Stacy Treco. For their important roles in marketing the text and media, we thank Christy Lesko, Lauren Harp, Kelly Galli, and Jane Campbell. For her market development support, we thank Jessica Moro. We are grateful to Adam Jaworski, VP Portfolio Management, Science, and Paul Corey, Managing Director, Higher Education Courseware, for their enthusiasm, encouragement, and support. The Pearson sales team, which represents Campbell BIOLOGY on campus, is an essential link to the users of the text. They tell us what you like and don’t like about the text, communicate the features of the text, and provide prompt service. We thank them for their hard work and professionalism. For representing our text to our international audience, we thank our sales and marketing partners throughout the world. They are all strong allies in biology education. Finally, we wish to thank our families and friends for their encouragement and patience throughout this long project. Our special thanks to Ross, Lily, and Alex (L.A.U.); Debra and Hannah (M.L.C.); Aaron, Sophie, Noah, and Gabriele (S.A.W.); Natalie (P.V.M.); and Paul, Dan, Maria, Armelle, and Sean (J.B.R.). And, as always, thanks to Rochelle, Allison, Jason, McKay, and Gus. Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, and Jane B. Reece

ACKNOWLEDGMENTS xxvii

Reviewers Eleventh Edition Reviewers Steve Abedon, Ohio State University John Alcock, Arizona State University Philip Allman, Florida Gulf Coast College Rodney Allrich, Purdue University Jim Barron, Montana State University Billings Stephen Bauer, Belmont Abbey College Aimee Bernard, University of Colorado Denver Teresa Bilinski, St. Edward’s University Sarah Bissonnette, University of California, Berkeley Jeffery Bowen, Bridgewater State University Scott Bowling, Auburn University David Broussard, Lycoming College Tessa Burch, University of Tennessee Warren Burggren, University of North Texas Patrick Cafferty, Emory University Michael Campbell, Penn State University Jeffrey Carmichael, University of North Dakota P. Bryant Chase, Florida State University Steve Christenson, Brigham Young University Curt Coffman, Vincennes University Bill Cohen, University of Kentucky Sean Coleman, University of the Ozarks Erin Connolly, University of South Carolina Ron Cooper, University of California, Los Angeles Curtis Daehler, University of Hawaii at Manoa Deborah Dardis, Southeastern Louisiana University Douglas Darnowski, Indiana University Southeast Jeremiah Davie, D’Youville College Melissa Deadmond, Truckee Meadows Community College Jennifer Derkits, J. Sergeant Reynolds Community College Jean DeSaix, University of Northern Carolina Kevin Dixon, Florida State University David Dunbar, Cabrini College Anna Edlund, Lafayette College Rob Erdman, Florida Gulf Coast College Dale Erskine, Lebanon Valley College Susan Erster, Stony Brook University Linda Fergusson-Kolmes, Portland Community College, Sylvania Campus Danilo Fernando, SUNY College of Environmental Science and Forestry, Syracuse Christina Fieber, Horry-Georgetown Technical College Melissa Fierke, SUNY College of Environmental Science and Forestry Mark Flood, Fairmont State University Robert Fowler, San Jose State University Stewart Frankel, University of Hartford Eileen Gregory, Rollins College Gokhan Hacisalihoglu, Florida A&M University Monica Hall-Woods, St. Charles Community College Jean Hardwick, Ithaca College Deborah Harris, Case Western Reserve University Chris Haynes, Shelton State Community College Albert Herrera, University of Southern California Karen Hicks, Kenyon College Elizabeth Hobson, New Mexico State University Mark Holbrook, University of Iowa Erin Irish, University of Iowa Sally Irwin, University of Hawaii, Maui College Jamie Jensen, Brigham Young University Jerry Johnson, Corban University Ann Jorgensen, University of Hawaii Ari Jumpponen, Kansas State University Doug Kane, Defiance College Kasey Karen, Georgia College & State University Paul Kenrick, Natural History Museum, London Stephen T. Kilpatrick, University of Pittsburgh at Johnstown Shannon King, North Dakota State University Karen M. Klein, Northampton Community College Jacob Krans, Western New England University Dubear Kroening, University of Wisconsin Barbara Kuemerle, Case Western Reserve University Jim Langeland, Kalamazoo College Grace Lasker, Lake Washington Institute of Technology Jani Lewis, State University of New York at Geneseo Eric W. Linton, Central Michigan University Tatyana Lobova, Old Dominion University David Longstreth, Louisiana State University Donald Lovett, College of New Jersey Lisa Lyons, Florida State University Mary Martin, Northern Michigan University

xxviii

REVIEWERS

Scott Meissner, Cornell University Jenny Metzler, Ball State University Grace Miller, Indiana Wesleyan University Jonathan Miller, Edmonds Community College Mill Miller, Wright State University Barbara Nash, Mercy College Karen Neal, J. Sargeant Reynolds Community College, Richmond Shawn Nordell, Saint Louis University Olabisi Ojo, Southern University at New Orleans Fatimata Pale, Thiel College Susan Parrish, McDaniel College Eric Peters, Chicago State University Jarmila Pittermann, University of California, Santa Cruz Jason Porter, University of the Sciences in Philadelphia Elena Pravosudova, University of Nevada, Reno Steven Price, Virginia Commonwealth University Samiksha Raut, University of Alabama at Birmingham Robert Reavis, Glendale Community College Wayne Rickoll, University of Puget Sound Luis Rodriguez, San Antonio College Kara Rosch, Blinn College Scott Russell, University of Oklahoma Jodi Rymer, College of the Holy Cross Per Salvesen, University of Bergen Davison Sangweme, University of North Georgia Karin Scarpinato, Georgia Southern University Cara Schillington, Eastern Michigan University David Schwartz, Houston Community College Carrie Schwarz, Western Washington University Joan Sharp, Simon Fraser University Alison Sherwood, University of Hawaii at Manoa Eric Shows, Jones County Junior College Brian Shmaefsky, Lone Star College John Skillman, California State University, San Bernardino Rebecca Sperry, Salt Lake Community College Clint Springer, Saint Joseph’s University Mark Sturtevant, Oakland University Diane Sweeney, Punahou School Kristen Taylor, Salt Lake Community College Rebecca Thomas, College of St. Joseph Martin Vaughan, Indiana University-Purdue University Indianapolis Meena Vijayaraghavan, Tulane University James T. Warren Jr.., Pennsylvania State University Jim Wee, Loyola University, New Orleans Charles Wellman, Sheffield University Christopher Whipps, State University of New York College of Environmental Science and Forestry Philip White, James Hutton Institute Jessica White-Phillip, Our Lady of the Lake University Robert Yost, Indiana University-Purdue University Indianapolis Tia Young, Pennsylvania State University

Reviewers of Previous Editions Kenneth Able, State University of New York, Albany; Thomas Adams, Michigan State University; Martin Adamson, University of British Columbia; Dominique Adriaens, Ghent University; Ann Aguanno, Marymount Manhattan College; Shylaja Akkaraju, Bronx Community College of CUNY; Marc Albrecht, University of Nebraska; John Alcock, Arizona State University; Eric Alcorn, Acadia University; George R. Aliaga, Tarrant County College; Rodney Allrich, Purdue University; Richard Almon, State University of New York, Buffalo; Bonnie Amos, Angelo State University; Katherine Anderson, University of California, Berkeley; Richard J. Andren, Montgomery County Community College; Estry Ang, University of Pittsburgh, Greensburg; Jeff Appling, Clemson University; J. David Archibald, San Diego State University; David Armstrong, University of Colorado, Boulder; Howard J. Arnott, University of Texas, Arlington; Mary Ashley, University of Illinois, Chicago; Angela S. Aspbury, Texas State University; Robert Atherton, University of Wyoming; Karl Aufderheide, Texas A&M University; Leigh Auleb, San Francisco State University; Terry Austin, Temple College; P. Stephen Baenziger, University of Nebraska; Brian Bagatto, University of Akron; Ellen Baker, Santa Monica College; Katherine Baker, Millersville University; Virginia Baker, Chipola College; Teri Balser, University of Wisconsin, Madison; William Barklow, Framingham State College; Susan Barman, Michigan State University; Steven Barnhart, Santa Rosa Junior College; Andrew Barton, University of Maine Farmington; Rebecca A. Bartow, Western Kentucky University; Ron Basmajian, Merced College; David Bass, University of Central Oklahoma; Bonnie Baxter, Westminster College; Tim Beagley, Salt Lake Community College; Margaret E. Beard, College of the Holy Cross; Tom Beatty, University of British Columbia; Chris Beck, Emory University; Wayne Becker, University of Wisconsin, Madison; Patricia Bedinger, Colorado State University; Jane Beiswenger, University of Wyoming; Anne Bekoff, University of Colorado, Boulder; Marc Bekoff, University of Colorado, Boulder; Tania Beliz, College of

San Mateo; Adrianne Bendich, Hoffman-La Roche, Inc.; Marilee Benore, University of Michigan, Dearborn; Barbara Bentley, State University of New York, Stony Brook; Darwin Berg, University of California, San Diego; Werner Bergen, Michigan State University; Gerald Bergstrom, University of Wisconsin, Milwaukee; Anna W. Berkovitz, Purdue University; Dorothy Berner, Temple University; Annalisa Berta, San Diego State University; Paulette Bierzychudek, Pomona College; Charles Biggers, Memphis State University; Kenneth Birnbaum, New York University; Catherine Black, Idaho State University; Michael W. Black, California Polytechnic State University, San Luis Obispo; William Blaker, Furman University; Robert Blanchard, University of New Hampshire; Andrew R. Blaustein, Oregon State University; Judy Bluemer, Morton College; Edward Blumenthal, Marquette University; Robert Blystone, Trinity University; Robert Boley, University of Texas, Arlington; Jason E. Bond, East Carolina University; Eric Bonde, University of Colorado, Boulder; Cornelius Bondzi, Hampton University; Richard Boohar, University of Nebraska, Omaha; Carey L. Booth, Reed College; Allan Bornstein, Southeast Missouri State University; David Bos, Purdue University; Oliver Bossdorf, State University of New York, Stony Book; James L. Botsford, New Mexico State University; Lisa Boucher, University of Nebraska, Omaha; J. Michael Bowes, Humboldt State University; Richard Bowker, Alma College; Robert Bowker, Glendale Community College, Arizona; Scott Bowling, Auburn University; Barbara Bowman, Mills College; Barry Bowman, University of California, Santa Cruz; Deric Bownds, University of Wisconsin, Madison; Robert Boyd, Auburn University; Sunny Boyd, University of Notre Dame; Jerry Brand, University of  Texas, Austin; Edward Braun, Iowa State University; Theodore A. Bremner, Howard University; James Brenneman, University of Evansville; Charles H. Brenner, Berkeley, California; Lawrence Brewer, University of Kentucky; Donald P. Briskin, University of Illinois, Urbana; Paul Broady, University of Canterbury; Chad Brommer, Emory University; Judith L. Bronstein, University of Arizona; Danny Brower, University of Arizona; Carole Browne, Wake Forest University; Beverly Brown, Nazareth College; Mark Browning, Purdue University; David Bruck, San Jose State University; Robb T. Brumfield, Louisiana State University; Herbert Bruneau, Oklahoma State University; Gary Brusca, Humboldt State University; Richard C. Brusca, University of Arizona, Arizona-Sonora Desert Museum; Alan H. 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Colbert, Iowa State University; Sean Coleman, University of the Ozarks; Jan Colpaert, Hasselt University; Robert Colvin, Ohio University; Jay Comeaux, McNeese State University; David Cone, Saint Mary’s University; Elizabeth Connor, University of Massachusetts; Joanne Conover, University of Connecticut; Gregory Copenhaver, University of North Carolina, Chapel Hill; John Corliss, University of Maryland; James T. Costa, Western Carolina University; Stuart J. Coward, University of Georgia; Charles Creutz, University of Toledo; Bruce Criley, Illinois Wesleyan University; Norma Criley, Illinois Wesleyan University; Joe W. Crim, University of Georgia; Greg Crowther, University of Washington; Karen Curto, University of Pittsburgh; William Cushwa, Clark College; Anne Cusic, University of Alabama, Birmingham; Richard Cyr, Pennsylvania State University; Marymegan Daly, The Ohio State University; Deborah Dardis, Southeastern Louisiana University; W. Marshall Darley, University of Georgia; Cynthia Dassler, The Ohio State University; Shannon Datwyler, California State University, Sacramento; Marianne Dauwalder, University of Texas, Austin; Larry Davenport, Samford University; Bonnie J. Davis, San Francisco State University; Jerry Davis, University of Wisconsin, La Crosse; Michael A. Davis, Central Connecticut State University; Thomas Davis, University of New Hampshire; Melissa Deadmond, Truckee Meadows Community College; John Dearn, University of Canberra; Maria E. de Bellard, California State University, Northridge; Teresa DeGolier, Bethel College; James Dekloe, University of California, Santa Cruz; Eugene Delay, University of Vermont; Patricia A. DeLeon, University of Delaware; Veronique Delesalle, Gettysburg College; T. Delevoryas, University of Texas, Austin; Roger Del Moral, University of Washington; Charles F.

Delwiche, University of Maryland; Diane C. DeNagel, Northwestern University; William L. Dentler, University of Kansas; Daniel DerVartanian, University of Georgia; Jean DeSaix, University of North Carolina, Chapel Hill; Janet De Souza-Hart, Massachusetts College of Pharmacy & Health Sciences; Biao Ding, Ohio State University; Michael Dini, Texas Tech University; Andrew Dobson, Princeton University; Stanley Dodson, University of Wisconsin, Madison; Jason Douglas, Angelina College; Mark Drapeau, University of California, Irvine; John Drees, Temple University School of Medicine; Charles Drewes, Iowa State University; Marvin Druger, Syracuse University; Gary Dudley, University of Georgia; Susan Dunford, University of Cincinnati; Kathryn A. Durham, Lorain Community College; Betsey Dyer, Wheaton College; Robert Eaton, University of Colorado; Robert S. Edgar, University of California, Santa Cruz; Anna Edlund, Lafayette College; Douglas J. Eernisse, California State University, Fullerton; Betty J. Eidemiller, Lamar University; Brad Elder, Doane College; Curt Elderkin, College of New Jersey; William D. Eldred, Boston University; Michelle Elekonich, University of Nevada, Las Vegas; George Ellmore, Tufts University; Mary Ellard-Ivey, Pacific Lutheran University; Kurt Elliott, North West Vista College; Norman Ellstrand, University of California, Riverside; Johnny El-Rady, University of South Florida; Dennis Emery, Iowa State University; John Endler, University of California, Santa Barbara; Rob Erdman, Florida Gulf Coast College; Dale Erskine, Lebanon Valley College; Margaret T. Erskine, Lansing Community College; Gerald Esch, Wake Forest University; Frederick B. Essig, University of South Florida; Mary Eubanks, Duke University; David Evans, University of Florida; Robert C. Evans, Rutgers University, Camden; Sharon Eversman, Montana State University; Olukemi Fadayomi, Ferris State University; Lincoln Fairchild, Ohio State University; Peter Fajer, Florida State University; Bruce Fall, University of Minnesota; Sam Fan, Bradley University; Lynn Fancher, College of DuPage; Ellen H. Fanning, Vanderbilt University; Paul Farnsworth, University of New Mexico; Larry Farrell, Idaho State University; Jerry F. Feldman, University of California, Santa Cruz; Lewis Feldman, University of California, Berkeley; Myriam Alhadeff Feldman, Cascadia Community College; Eugene Fenster, Longview Community College; Russell Fernald, University of Oregon; Rebecca Ferrell, Metropolitan State College of Denver; Kim Finer, Kent State University; Milton Fingerman, Tulane University; Barbara Finney, Regis College; Teresa Fischer, Indian River Community College; Frank Fish, West Chester University; David Fisher, University of Hawaii, Manoa; Jonathan S. Fisher, St. Louis University; Steven Fisher, University of California, Santa Barbara; David Fitch, New York University; Kirk Fitzhugh, Natural History Museum of Los Angeles County; Lloyd Fitzpatrick, University of North Texas; William Fixsen, Harvard University; T. Fleming, Bradley University; Abraham Flexer, Manuscript Consultant, Boulder, Colorado; Margaret Folsom, Methodist College; Kerry Foresman, University of Montana; Norma Fowler, University of Texas, Austin; Robert G. Fowler, San Jose State University; David Fox, University of Tennessee, Knoxville; Carl Frankel, Pennsylvania State University, Hazleton; Robert Franklin, College of Charleston; James Franzen, University of Pittsburgh; Art Fredeen, University of Northern British Columbia; Kim Fredericks, Viterbo University; Bill Freedman, Dalhousie University; Matt Friedman, University of Chicago; Otto Friesen, University of Virginia; Frank Frisch, Chapman University; Virginia Fry, Monterey Peninsula College; Bernard Frye, University of Texas, Arlington; Jed Fuhrman, University of Southern California; Alice Fulton, University of Iowa; Chandler Fulton, Brandeis University; Sara Fultz, Stanford University; Berdell Funke, North Dakota State University; Anne Funkhouser, University of the Pacific; Zofia E. Gagnon, Marist College; Michael Gaines, University of Miami; Cynthia M. Galloway, Texas A&M University, Kingsville; Arthur W. Galston, Yale University; Stephen Gammie, University of Wisconsin, Madison; Carl Gans, University of Michigan; John Gapter, University of Northern Colorado; Andrea Gargas, University of Wisconsin, Madison; Lauren Garner, California Polytechnic State University, San Luis Obispo; Reginald Garrett, University of Virginia; Craig Gatto, Illinois State University; Kristen Genet, Anoka Ramsey Community College; Patricia Gensel, University of North Carolina; Chris George, California Polytechnic State University, San Luis Obispo; Robert George, University of Wyoming; J. Whitfield Gibbons, University of Georgia; J. Phil Gibson, University of Oklahoma; Frank Gilliam, Marshall University; Eric Gillock, Fort Hayes State University; Simon Gilroy, University of Wisconsin, Madison; Edwin Ginés-Candelaria, Miami Dade College; Alan D. Gishlick, Gustavus Adolphus College; Todd Gleeson, University of Colorado; Jessica Gleffe, University of California, Irvine; John Glendinning, Barnard College; David Glenn-Lewin, Wichita State University; William Glider, University of Nebraska; Tricia Glidewell, Marist School; Elizabeth A. Godrick, Boston University; Jim Goetze, Laredo Community College; Lynda Goff, University of California, Santa Cruz; Elliott Goldstein, Arizona State University; Paul Goldstein, University of Texas, El Paso; Sandra Gollnick, State University of New York, Buffalo; Roy Golsteyn, University of Lethbridge; Anne Good, University of California, Berkeley; Judith Goodenough, University of Massachusetts, Amherst; Wayne Goodey, University of British Columbia; Barbara E. Goodman, University of South Dakota; Robert Goodman, University of Wisconsin, Madison; Ester Goudsmit, Oakland University; Linda Graham, University of Wisconsin, Madison; Robert Grammer, Belmont University; Joseph Graves, Arizona State University; Eileen Gregory, Rollins College; Phyllis Griffard, University of Houston, Downtown; A. J. F. Griffiths, University of British Columbia; Bradley Griggs, Piedmont Technical College; William Grimes, University of Arizona; David Grise, Texas A&M University, Corpus Christi; Mark Gromko, Bowling Green State University; Serine Gropper, Auburn University; Katherine L. Gross, Ohio State University; Gary Gussin, University of Iowa; Edward Gruberg, Temple University; Carla Guthridge, Cameron University; Mark Guyer, National Human Genome Research Institute; Ruth Levy Guyer, Bethesda, Maryland; Carla Haas, Pennsylvania State University; R. Wayne Habermehl, Montgomery County Community College; Pryce Pete Haddix, Auburn University; Mac Hadley, University of Arizona; Joel Hagen, Radford University; Jack P. Hailman, University of Wisconsin; Leah Haimo, University of California, Riverside; Ken Halanych, Auburn University; Jody Hall, Brown University; Monica Hall-Woods, St. Charles Community College; Heather Hallen-Adams, University of Nebraska, Lincoln; Douglas Hallett, Northern Arizona University; Rebecca Halyard, Clayton State College; Devney Hamilton, Stanford University (student); E. William Hamilton, Washington and Lee University;

Reviewers

xxix

Matthew B. Hamilton, Georgetown University; Sam Hammer, Boston University; Penny Hanchey-Bauer, Colorado State University; William F. Hanna, Massasoit Community College; Dennis Haney, Furman University; Laszlo Hanzely, Northern Illinois University; Jeff Hardin, University of Wisconsin, Madison; Jean Hardwick, Ithaca College; Luke Harmon, University of Idaho; Lisa Harper, University of California, Berkeley; Jeanne M. Harris, University of Vermont; Richard Harrison, Cornell University; Stephanie Harvey, Georgia Southwestern State University; Carla Hass, Pennsylvania State University; Chris Haufler, University of Kansas; Bernard A. Hauser, University of Florida; Chris Haynes, Shelton State Community College; Evan B. Hazard, Bemidji State University (emeritus); H. D. Heath, California State University, East Bay; George Hechtel, State University of New York, Stony Brook; S. Blair Hedges, Pennsylvania State University; Brian Hedlund, University of Nevada, Las Vegas; David Heins, Tulane University; Jean Heitz, University of Wisconsin, Madison; Andreas Hejnol, Sars International Centre for Marine Molecular Biology; John D. Helmann, Cornell University; Colin Henderson, University of Montana; Susan Hengeveld, Indiana University; Michelle Henricks, University of California, Los Angeles; Caroll Henry, Chicago State University; Frank Heppner, University of Rhode Island; Albert Herrera, University of Southern California; Scott Herrick, Missouri Western State College; Ira Herskowitz, University of California, San Francisco; Paul E. Hertz, Barnard College; Chris Hess, Butler University; David Hibbett, Clark University; R. James Hickey, Miami University; Kendra Hill, San Diego State University; William Hillenius, College of Charleston; Kenneth Hillers, California Polytechnic State University, San Luis Obispo; Ralph Hinegardner, University of California, Santa Cruz; William Hines, Foothill College; Robert Hinrichsen, Indiana University of Pennsylvania; Helmut Hirsch, State University of New York, Albany; Tuan-hua David Ho, Washington University; Carl Hoagstrom, Ohio Northern University; Jason Hodin, Stanford University; James Hoffman, University of Vermont; A. Scott Holaday, Texas Tech University; N. Michele Holbrook, Harvard University; James Holland, Indiana State University, Bloomington; Charles Holliday, Lafayette College; Lubbock Karl Holte, Idaho State University; Alan R. Holyoak, Brigham Young University, Idaho; Laura Hoopes, Occidental College; Nancy Hopkins, Massachusetts Institute of Technology; Sandra Horikami, Daytona Beach Community College; Kathy Hornberger, Widener University; Pius F. Horner, San Bernardino Valley College; Becky Houck, University of Portland; Margaret Houk, Ripon College; Laura Houston, Northeast Lakeview College; Daniel J. Howard, New Mexico State University; Ronald R. Hoy, Cornell University; Sandra Hsu, Skyline College; Sara Huang, Los Angeles Valley College; Cristin Hulslander, University of Oregon; Donald Humphrey, Emory University School of Medicine; Catherine Hurlbut, Florida State College, Jacksonville; Diane Husic, Moravian College; Robert J. Huskey, University of Virginia; Steven Hutcheson, University of Maryland, College Park; Linda L. Hyde, Gordon College; Bradley Hyman, University of California, Riverside; Jeffrey Ihara, Mira Costa College; Mark Iked, San Bernardino Valley College; Cheryl Ingram-Smith, Clemson University; Harry Itagaki, Kenyon College; Alice Jacklet, State University of New York, Albany; John Jackson, North Hennepin Community College; Thomas Jacobs, University of Illinois; Kathy Jacobson, Grinnell College; Mark Jaffe, Nova Southeastern University; John C. Jahoda, Bridgewater State College; Douglas Jensen, Converse College; Dan Johnson, East Tennessee State University; Lance Johnson, Midland Lutheran College; Lee Johnson, The Ohio State University; Randall Johnson, University of California, San Diego; Roishene Johnson, Bossier Parish Community College; Stephen Johnson, William Penn University; Wayne Johnson, Ohio State University; Kenneth C. Jones, California State University, Northridge; Russell Jones, University of California, Berkeley; Cheryl Jorcyk, Boise State University; Chad Jordan, North Carolina State University; Alan Journet, Southeast Missouri State University; Walter Judd, University of Florida; Thomas W. Jurik, Iowa State University; Caroline M. Kane, University of California, Berkeley; Thomas C. Kane, University of Cincinnati; The-Hui Kao, Pennsylvania State University; Tamos Kapros, University of Missouri; E. L. Karlstrom, University of Puget Sound; Jennifer Katcher, Pima Community College; Laura A. Katz, Smith College; Judy Kaufman, Monroe Community College; Maureen Kearney, Field Museum of Natural History; Eric G. Keeling, Cary Institute of Ecosystem Studies; Patrick Keeling, University of British Columbia; Thomas Keller, Florida State University; Elizabeth A. Kellogg, University of Missouri, St. Louis; Norm Kenkel, University of Manitoba; Chris Kennedy, Simon Fraser University; George Khoury, National Cancer Institute; Rebecca T. Kimball, University of Florida; Mark Kirk, University of Missouri, Columbia; Robert Kitchin, University of Wyoming; Hillar Klandorf, West Virginia University; Attila O. Klein, Brandeis University; Daniel Klionsky, University of Michigan; Mark Knauss, Georgia Highlands College; Janice Knepper, Villanova University; Charles Knight, California Polytechnic State University; Jennifer Knight, University of Colorado; Ned Knight, Linfield College; Roger Koeppe, University of Arkansas; David Kohl, University of California, Santa Barbara; Greg Kopf, University of Pennsylvania School of Medicine; Thomas Koppenheffer, Trinity University; Peter Kourtev, Central Michigan University; Margareta Krabbe, Uppsala University; Jacob Krans, Western New England University; Anselm Kratochwil, Universität Osnabrück; Eliot Krause, Seton Hall University; Deborah M. Kristan, California State University, San Marcos; Steven Kristoff, Ivy Tech Community College; William Kroll, Loyola University, Chicago; Janis Kuby, San Francisco State University; Barb Kuemerle, Case Western Reserve University; Justin P. Kumar, Indiana University; Rukmani Kuppuswami, Laredo Community College; David Kurijaka, Ohio University; Lee Kurtz, Georgia Gwinnett College; Michael P. Labare, United States Military Academy, West Point; Marc-André Lachance, University of Western Ontario; J. A. Lackey, State University of New York, Oswego; Elaine Lai, Brandeis University; Mohamed Lakrim, Kingsborough Community College; Ellen Lamb, University of North Carolina, Greensboro; William Lamberts, College of St Benedict and St John’s University; William L’Amoreaux, College of Staten Island; Lynn Lamoreux, Texas A&M University; Carmine A. Lanciani, University of Florida; Kenneth Lang, Humboldt State University; Dominic Lannutti, El Paso Community College; Allan Larson, Washington University; John Latto, University of California, Santa Barbara; Diane K. Lavett, State University of New York, Cortland, and Emory University; Charles Leavell, Fullerton College; C. S. Lee, University of Texas; Daewoo Lee, Ohio University; Tali D. Lee, University of Wisconsin,

xxx

Reviewers

Eau Claire; Hugh Lefcort, Gonzaga University; Robert Leonard, University of California, Riverside; Michael R. Leonardo, Coe College; John Lepri, University of North Carolina, Greensboro; Donald Levin, University of Texas, Austin; Joseph Levine, Boston College; Mike Levine, University of California, Berkeley; Alcinda Lewis, University of Colorado, Boulder; Bill Lewis, Shoreline Community College; Jani Lewis, State University of New York; John Lewis, Loma Linda University; Lorraine Lica, California State University, East Bay; Harvey Liftin, Broward Community College; Harvey Lillywhite, University of Florida, Gainesville; Graeme Lindbeck, Valencia Community College; Clark Lindgren, Grinnell College; Diana Lipscomb, George Washington University; Christopher Little, The University of Texas, Pan American; Kevin D. Livingstone, Trinity University; Andrea Lloyd, Middlebury College; Sam Loker, University of New Mexico; Christopher A. Loretz, State University of New York, Buffalo; Jane Lubchenco, Oregon State University; Douglas B. Luckie, Michigan State University; Hannah Lui, University of California, Irvine; Margaret A. Lynch, Tufts University; Steven Lynch, Louisiana State University, Shreveport; Richard Machemer Jr., St. John Fisher College; Elizabeth Machunis-Masuoka, University of Virginia; James MacMahon, Utah State University; Nancy Magill, Indiana University; Christine R. Maher, University of Southern Maine; Linda Maier, University of Alabama, Huntsville; Jose Maldonado, El Paso Community College; Richard Malkin, University of California, Berkeley; Charles Mallery, University of Miami; Keith Malmos, Valencia Community College, East Campus; Cindy Malone, California State University, Northridge; Mark Maloney, University of South Mississippi; Carol Mapes, Kutztown University of Pennsylvania; William Margolin, University of Texas Medical School; Lynn Margulis, Boston University; Julia Marrs, Barnard College (student); Kathleen A. Marrs, Indiana University-Purdue University, Indianapolis; Edith Marsh, Angelo State University; Diane L. Marshall, University of New Mexico; Karl Mattox, Miami University of Ohio; Joyce Maxwell, California State University, Northridge; Jeffrey D. May, Marshall University; Mike Mayfield, Ball State University; Kamau Mbuthia, Bowling Green State University; Lee McClenaghan, San Diego State University; Richard McCracken, Purdue University; Andrew McCubbin, Washington State University; Kerry McDonald, University of Missouri, Columbia; Tanya McGhee, Craven Community College; Jacqueline McLaughlin, Pennsylvania State University, Lehigh Valley; Neal McReynolds, Texas A&M International; Darcy Medica, Pennsylvania State University; Lisa Marie Meffert, Rice University; Susan Meiers, Western Illinois University; Michael Meighan, University of California, Berkeley; Scott Meissner, Cornell University; Paul Melchior, North Hennepin Community College; Phillip Meneely, Haverford College; John Merrill, Michigan State University; Brian Metscher, University of California, Irvine; Ralph Meyer, University of Cincinnati; James Mickle, North Carolina State University; Jan Mikesell, Gettysburg College; Roger Milkman, University of Iowa; Helen Miller, Oklahoma State University; John Miller, University of California, Berkeley; Kenneth R. Miller, Brown University; Alex Mills, University of Windsor; Sarah Milton, Florida Atlantic University; Eli Minkoff, Bates College; John E. Minnich, University of Wisconsin, Milwaukee; Subhash Minocha, University of New Hampshire; Michael J. Misamore, Texas Christian University; Kenneth Mitchell, Tulane University School of Medicine; Ivona Mladenovic, Simon Fraser University; Alan Molumby, University of Illinois, Chicago; Nicholas Money, Miami University; Russell Monson, University of Colorado, Boulder; Joseph P. Montoya, Georgia Institute of Technology; Frank Moore, Oregon State University; Janice Moore, Colorado State University; Linda Moore, Georgia Military College; Randy Moore, Wright State University; William Moore, Wayne State University; Carl Moos, Veterans Administration Hospital, Albany, New York; Linda Martin Morris, University of Washington; Michael Mote, Temple University; Alex Motten, Duke University; Jeanette Mowery, Madison Area Technical College; Deborah Mowshowitz, Columbia University; Rita Moyes, Texas A&M, College Station; Darrel L. Murray, University of Illinois, Chicago; Courtney Murren, College of Charleston; John Mutchmor, Iowa State University; Elliot Myerowitz, California Institute of Technology; Gavin Naylor, Iowa State University; Karen Neal, Reynolds University; John Neess, University of Wisconsin, Madison; Ross Nehm, Ohio State University; Tom Neils, Grand Rapids Community College; Kimberlyn Nelson, Pennsylvania State University; Raymond Neubauer, University of Texas, Austin; Todd Newbury, University of California, Santa Cruz; James Newcomb, New England College; Jacalyn Newman, University of Pittsburgh; Harvey Nichols, University of Colorado, Boulder; Deborah Nickerson, University of South Florida; Bette Nicotri, University of Washington; Caroline Niederman, Tomball College; Eric Nielsen, University of Michigan; Maria Nieto, California State University, East Bay; Anders Nilsson, University of Umeå; Greg Nishiyama, College of the Canyons; Charles R. Noback, College of Physicians and Surgeons, Columbia University; Jane Noble-Harvey, Delaware University; Mary C. Nolan, Irvine Valley College; Kathleen Nolta, University of Michigan; Peter Nonacs, University of California, Los Angeles; Mohamed A. F. Noor, Duke University; Shawn Nordell, St. Louis University; Richard S. Norman, University of Michigan, Dearborn (emeritus); David O. Norris, University of Colorado, Boulder; Steven Norris, California State University, Channel Islands; Gretchen North, Occidental College; Cynthia Norton, University of Maine, Augusta; Steve Norton, East Carolina University; Steve Nowicki, Duke University; Bette H. Nybakken, Hartnell College; Brian O’Conner, University of Massachusetts, Amherst; Gerard O’Donovan, University of North Texas; Eugene Odum, University of Georgia; Mark P. Oemke, Alma College; Linda Ogren, University of California, Santa Cruz; Patricia O’Hern, Emory University; Nathan O. Okia, Auburn University, Montgomery; Jeanette Oliver, St. Louis Community College, Florissant Valley; Gary P. Olivetti, University of Vermont; Margaret Olney, St. Martin’s College; John Olsen, Rhodes College; Laura J. Olsen, University of Michigan; Sharman O’Neill, University of California, Davis; Wan Ooi, Houston Community College; Aharon Oren, The Hebrew University; John Oross, University of California, Riverside; Rebecca Orr, Collin College; Catherine Ortega, Fort Lewis College; Charissa Osborne, Butler University; Gay Ostarello, Diablo Valley College; Henry R. Owen, Eastern Illinois University; Thomas G. Owens, Cornell University; Penny Padgett, University of North Carolina, Chapel Hill; Kevin Padian, University of California, Berkeley; Dianna Padilla, State University of New York, Stony Brook; Anthony T. Paganini, Michigan State University; Barry Palevitz, University of Georgia; Michael A. Palladino,

Monmouth University; Matt Palmtag, Florida Gulf Coast University; Stephanie Pandolfi, Michigan State University; Daniel Papaj, University of Arizona; Peter Pappas, County College of Morris; Nathalie Pardigon, Institut Pasteur; Bulah Parker, North Carolina State University; Stanton Parmeter, Chemeketa Community College; Cindy Paszkowski, University of Alberta; Robert Patterson, San Francisco State University; Ronald Patterson, Michigan State University; Crellin Pauling, San Francisco State University; Kay Pauling, Foothill Community College; Daniel Pavuk, Bowling Green State University; Debra Pearce, Northern Kentucky University; Patricia Pearson, Western Kentucky University; Andrew Pease, Stevenson University; Nancy Pelaez, Purdue University; Shelley Penrod, North Harris College; Imara Y. Perera, North Carolina State University; Beverly Perry, Houston Community College; Irene Perry, University of Texas of the Permian Basin; Roger Persell, Hunter College; Eric Peters, Chicago State University; Larry Peterson, University of Guelph; David Pfennig, University of North Carolina, Chapel Hill; Mark Pilgrim, College of Coastal Georgia; David S. Pilliod, California Polytechnic State University, San Luis Obispo; Vera M. Piper, Shenandoah University; Deb Pires, University of California, Los Angeles; J. Chris Pires, University of Missouri, Columbia; Bob Pittman, Michigan State University; James Platt, University of Denver; Martin Poenie, University of Texas, Austin; Scott Poethig, University of Pennsylvania; Crima Pogge, San Francisco Community College; Michael Pollock, Mount Royal University; Roberta Pollock, Occidental College; Jeffrey Pommerville, Texas A&M University; Therese M. Poole, Georgia State University; Angela R. Porta, Kean University; Jason Porter, University of the Sciences, Philadelphia; Warren Porter, University of Wisconsin; Daniel Potter, University of California, Davis; Donald Potts, University of California, Santa Cruz; Robert Powell, Avila University; Andy Pratt, University of Canterbury; David Pratt, University of California, Davis; Elena Pravosudova, University of Nevada, Reno; Halina Presley, University of Illinois, Chicago; Eileen Preston, Tarrant Community College Northwest; Mary V. Price, University of California, Riverside; Mitch Price, Pennsylvania State University; Terrell Pritts, University of Arkansas, Little Rock; Rong Sun Pu, Kean University; Rebecca Pyles, East Tennessee State University; Scott Quackenbush, Florida International University; Ralph Quatrano, Oregon State University; Peter Quinby, University of Pittsburgh; Val Raghavan, Ohio State University; Deanna Raineri, University of Illinois, Champaign-Urbana; David Randall, City University Hong Kong; Talitha Rajah, Indiana University Southeast; Charles Ralph, Colorado State University; Pushpa Ramakrishna, Chandler-Gilbert Community College; Thomas Rand, Saint Mary’s University; Monica Ranes-Goldberg, University of California, Berkeley; Robert S. Rawding, Gannon University; Robert H. Reavis, Glendale Community College; Kurt Redborg, Coe College; Ahnya Redman, Pennsylvania State University; Brian Reeder, Morehead State University; Bruce Reid, Kean University; David Reid, Blackburn College; C. Gary Reiness, Lewis & Clark College; Charles Remington, Yale University; Erin Rempala, San Diego Mesa College; David Reznick, University of California, Riverside; Fred Rhoades, Western Washington State University; Douglas Rhoads, University of Arkansas; Eric Ribbens, Western Illinois University; Christina Richards, New York University; Sarah Richart, Azusa Pacific University; Christopher Riegle, Irvine Valley College; Loren Rieseberg, University of British Columbia; Bruce B. Riley, Texas A&M University; Todd Rimkus, Marymount University; John Rinehart, Eastern Oregon University; Donna Ritch, Pennsylvania State University; Carol Rivin, Oregon State University East; Laurel Roberts, University of Pittsburgh; Diane Robins, University of Michigan; Kenneth Robinson, Purdue University; Thomas Rodella, Merced College; Deb Roess, Colorado State University; Heather Roffey, Marianopolis College; Rodney Rogers, Drake University; Suzanne Rogers, Seton Hill University; William Roosenburg, Ohio University; Mike Rosenzweig, Virginia Polytechnic Institute and State University; Wayne Rosing, Middle Tennessee State University; Thomas Rost, University of California, Davis; Stephen I. Rothstein, University of California, Santa Barbara; John Ruben, Oregon State University; Albert Ruesink, Indiana University; Patricia Rugaber, College of Coastal Georgia; Scott Russell, University of Oklahoma; Neil Sabine, Indiana University; Tyson Sacco, Cornell University; Glenn-Peter Saetre, University of Oslo; Rowan F. Sage, University of Toronto; Tammy Lynn Sage, University of Toronto; Sanga Saha, Harold Washington College; Don Sakaguchi, Iowa State University; Walter Sakai, Santa Monica College; Mark F. Sanders, University of California, Davis; Kathleen Sandman, Ohio State University; Louis Santiago, University of California, Riverside; Ted Sargent, University of Massachusetts, Amherst; K. Sathasivan, University of Texas, Austin; Gary Saunders, University of New Brunswick; Thomas R. Sawicki, Spartanburg Community College; Inder Saxena, University of Texas, Austin; Carl Schaefer, University of Connecticut; Andrew Schaffner, Cal Poly San Luis Obispo; Maynard H. Schaus, Virginia Wesleyan College; Renate Scheibe, University of Osnabrück; David Schimpf, University of Minnesota, Duluth; William H. Schlesinger, Duke University; Mark Schlissel, University of California, Berkeley; Christopher J. Schneider, Boston University; Thomas W. Schoener, University of California, Davis; Robert Schorr, Colorado State University; Patricia M. Schulte, University of British Columbia; Karen S. Schumaker, University of Arizona; Brenda Schumpert, Valencia Community College; David J. Schwartz, Houston Community College; Christa Schwintzer, University of Maine; Erik P. Scully, Towson State University; Robert W. Seagull, Hofstra University; Edna Seaman, Northeastern University; Duane Sears, University of California, Santa Barbara; Brent Selinger, University of Lethbridge; Orono Shukdeb Sen, Bethune-Cookman College; Wendy Sera, Seton Hill University; Alison M. Shakarian, Salve Regina University; Timothy E. Shannon, Francis Marion University; Joan Sharp, Simon Fraser University; Victoria C. Sharpe, Blinn College; Elaine Shea, Loyola College, Maryland; Stephen Sheckler, Virginia Polytechnic Institute and State University; Robin L. Sherman, Nova Southeastern University; Richard Sherwin, University of Pittsburgh; Lisa Shimeld, Crafton Hills College; James Shinkle, Trinity University; Barbara Shipes, Hampton University; Richard M. Showman, University of South Carolina; Eric Shows, Jones County Junior College; Peter Shugarman, University of Southern California; Alice Shuttey, DeKalb Community College; James Sidie, Ursinus College; Daniel Simberloff, Florida State University; Rebecca Simmons, University of North Dakota; Anne Simon, University of Maryland, College Park; Robert Simons, University of California, Los Angeles; Alastair Simpson, Dalhousie University; Susan Singer, Carleton College; Sedonia

Sipes, Southern Illinois University, Carbondale; John Skillman, California State University, San Bernardino; Roger Sloboda, Dartmouth University; John Smarrelli, Le Moyne College; Andrew T. Smith, Arizona State University; Kelly Smith, University of North Florida; Nancy Smith-Huerta, Miami Ohio University; John Smol, Queen’s University; Andrew J. Snope, Essex Community College; Mitchell Sogin, Woods Hole Marine Biological Laboratory; Doug Soltis, University of Florida, Gainesville; Julio G. Soto, San Jose State University; Susan Sovonick-Dunford, University of Cincinnati; Frederick W. Spiegel, University of Arkansas; John Stachowicz, University of California, Davis; Joel Stafstrom, Northern Illinois University; Alam Stam, Capital University; Amanda Starnes, Emory University; Karen Steudel, University of Wisconsin; Barbara Stewart, Swarthmore College; Gail A. Stewart, Camden County College; Cecil Still, Rutgers University, New Brunswick; Margery Stinson, Southwestern College; James Stockand, University of Texas Health Science Center, San Antonio; John Stolz, California Institute of Technology; Judy Stone, Colby College; Richard D. Storey, Colorado College; Stephen Strand, University of California, Los Angeles; Eric Strauss, University of Massachusetts, Boston; Antony Stretton, University of Wisconsin, Madison; Russell Stullken, Augusta College; Mark Sturtevant, University of Michigan, Flint; John Sullivan, Southern Oregon State University; Gerald Summers, University of Missouri; Judith Sumner, Assumption College; Marshall D. Sundberg, Emporia State University; Cynthia Surmacz, Bloomsburg University; Lucinda Swatzell, Southeast Missouri State University; Daryl Sweeney, University of Illinois, Champaign-Urbana; Samuel S. Sweet, University of California, Santa Barbara; Janice Swenson, University of North Florida; Michael A. Sypes, Pennsylvania State University; Lincoln Taiz, University of California, Santa Cruz; David Tam, University of North Texas; Yves Tan, Cabrillo College; Samuel Tarsitano, Southwest Texas State University; David Tauck, Santa Clara University; Emily Taylor, California Polytechnic State University, San Luis Obispo; James Taylor, University of New Hampshire; John W. Taylor, University of California, Berkeley; Martha R. Taylor, Cornell University; Franklyn Tan Te, Miami Dade College; Thomas Terry, University of Connecticut; Roger Thibault, Bowling Green State University; Kent Thomas, Wichita State University; William Thomas, Colby-Sawyer College; Cyril Thong, Simon Fraser University; John Thornton, Oklahoma State University; Robert Thornton, University of California, Davis; William Thwaites, Tillamook Bay Community College; Stephen Timme, Pittsburg State University; Mike Toliver, Eureka College; Eric Toolson, University of New Mexico; Leslie Towill, Arizona State University; James Traniello, Boston University; Paul Q. Trombley, Florida State University; Nancy J. Trun, Duquesne University; Constantine Tsoukas, San Diego State University; Marsha Turell, Houston Community College; Victoria Turgeon, Furman University; Robert Tuveson, University of Illinois, Urbana; Maura G. Tyrrell, Stonehill College; Catherine Uekert, Northern Arizona University; Claudia Uhde-Stone, California State University, East Bay; Gordon Uno, University of Oklahoma; Lisa A. Urry, Mills College; Saba Valadkhan, Center for RNA Molecular Biology; James W. Valentine, University of California, Santa Barbara; Joseph Vanable, Purdue University; Theodore Van Bruggen, University of South Dakota; Kathryn VandenBosch, Texas A&M University; Gerald Van Dyke, North Carolina State University; Brandi Van Roo, Framingham State College; Moira Van Staaden, Bowling Green State University; Sarah VanVickle-Chavez, Washington University, St. Louis; William Velhagen, New York University; Steven D. Verhey, Central Washington University; Kathleen Verville, Washington College; Sara Via, University of Maryland; Frank Visco, Orange Coast College; Laurie Vitt, University of California, Los Angeles; Neal Voelz, St. Cloud State University; Thomas J. Volk, University of Wisconsin, La Crosse; Leif Asbjørn Vøllestad, University of Oslo; Amy Volmer, Swarthmore College; Janice Voltzow, University of Scranton; Margaret Voss, Penn State Erie; Susan D. Waaland, University of Washington; Charles Wade, C.S. Mott Community College; William Wade, Dartmouth Medical College; John Waggoner, Loyola Marymount University; Jyoti Wagle, Houston Community College; Edward Wagner, University of California, Irvine; D. Alexander Wait, Southwest Missouri State University; Claire Walczak, Indiana University; Jerry Waldvogel, Clemson University; Dan Walker, San Jose State University; Robert Lee Wallace, Ripon College; Jeffrey Walters, North Carolina State University; Linda Walters, University of Central Florida; James Wandersee, Louisiana State University; Nickolas M. Waser, University of California, Riverside; Fred Wasserman, Boston University; Margaret Waterman, University of Pittsburgh; Charles Webber, Loyola University of Chicago; Peter Webster, University of Massachusetts, Amherst; Terry Webster, University of Connecticut, Storrs; Beth Wee, Tulane University; James Wee, Loyola University; Andrea Weeks, George Mason University; John Weishampel, University of Central Florida; Peter Wejksnora, University of Wisconsin, Milwaukee; Kentwood Wells, University of Connecticut; David J. Westenberg, University of Missouri, Rolla; Richard Wetts, University of California, Irvine; Matt White, Ohio University; Susan Whittemore, Keene State College; Murray Wiegand, University of Winnipeg; Ernest H. Williams, Hamilton College; Kathy Williams, San Diego State University; Kimberly Williams, Kansas State University; Stephen Williams, Glendale Community College; Elizabeth Willott, University of Arizona; Christopher Wills, University of California, San Diego; Paul Wilson, California State University, Northridge; Fred Wilt, University of California, Berkeley; Peter Wimberger, University of Puget Sound; Robert Winning, Eastern Michigan University; E. William Wischusen, Louisiana State University; Clarence Wolfe, Northern Virginia Community College; Vickie L. Wolfe, Marshall University; Janet Wolkenstein, Hudson Valley Community College; Robert T. Woodland, University of Massachusetts Medical School; Joseph Woodring, Louisiana State University; Denise Woodward, Pennsylvania State University; Patrick Woolley, East Central College; Sarah E. Wyatt, Ohio University; Grace Wyngaard, James Madison University; Shuhai Xiao, Virginia Polytechnic Institute, Ramin Yadegari, University of Arizona; Paul Yancey, Whitman College; Philip Yant, University of Michigan; Linda Yasui, Northern Illinois University; Anne D. Yoder, Duke University; Hideo Yonenaka, San Francisco State University; Gina M. Zainelli, Loyola University, Chicago; Edward Zalisko, Blackburn College; Nina Zanetti, Siena College; Sam Zeveloff, Weber State University; Zai Ming Zhao, University of Texas, Austin; John Zimmerman, Kansas State University; Miriam Zolan, Indiana University; Theresa Zucchero, Methodist University; Uko Zylstra, Calvin College.

Reviewers

xxxi

Detailed Contents 1

Evolution, the Themes of Biology, and Scientific Inquiry 2

Inquiring About Life 2 Concept 1.1  The study of life reveals unifying themes 4 Theme: New Properties Emerge at Successive Levels of Biological Organization 5 Theme: Life’s Processes Involve the Expression and Transmission of Genetic Information 7 Theme: Life Requires the Transfer and Transformation of Energy and Matter 9 Theme: From Molecules to Ecosystems, Interactions Are Important in Biological Systems 10

Concept 1.2  The Core Theme: Evolution accounts for the unity and diversity of life 11 Classifying the Diversity of Life 12 Charles Darwin and the Theory of Natural Selection 14 The Tree of Life 15

Concept 1.3  In studying nature, scientists make observations and form and test hypotheses 16 Exploration and Observation 17 Forming and Testing Hypotheses 17 The Flexibility of the Scientific Process 18 A Case Study in Scientific Inquiry: Investigating Coat Coloration in Mouse Populations 20 Experimental Variables and Controls 20 Theories in Science 21

Concept 1.4  Science benefits from a cooperative approach and diverse viewpoints 22 Building on the Work of Others 22 Science, Technology, and Society 23 The Value of Diverse Viewpoints in Science 24

Unit 1  The Chemistry of Life  27

Concept 2.2  An element’s properties depend on the structure of its atoms 30 Subatomic Particles 30 Atomic Number and Atomic Mass 31 Isotopes 31 The Energy Levels of Electrons 32 Electron Distribution and Chemical Properties 34 Electron Orbitals 35

Concept 2.3  The formation and function of molecules depend on chemical bonding between atoms 36 Covalent Bonds 36 Ionic Bonds 37 Weak Chemical Interactions 38 Molecular Shape and Function 39

Concept 2.4  Chemical reactions make and break chemical

bonds 40

3 Water and Life 44 The Molecule That Supports All of Life 44 Concept 3.1  Polar covalent bonds in water molecules result in hydrogen bonding 45 Concept 3.2  Four emergent properties of water contribute to Earth’s suitability for life 45 Cohesion of Water Molecules 45 Moderation of Temperature by Water 46 Floating of Ice on Liquid Water 47 Water: The Solvent of Life 49 Possible Evolution of Life on Other Planets 50

Concept 3.3  Acidic and basic conditions

affect living organisms 51 Acids and Bases 51 The pH Scale 51 Buffers 52 Acidification: A Threat to Our Oceans 53

Interview: Lovell Jones 27

2

The Chemical Context of Life 28

A Chemical Connection to Biology 28 Concept 2.1  Matter consists of chemical elements in pure form and in combinations called compounds 29 Elements and Compounds 29 The Elements of Life 29 Case Study: Evolution of Tolerance to Toxic Elements 30

4 Carbon and the Molecular Diversity of Life 56 Carbon: The Backbone of Life 56 Concept 4.1  Organic chemistry is the study of carbon compounds 57 Organic Molecules and the Origin of Life on Earth 57

Concept 4.2  Carbon atoms can form diverse molecules

by bonding to four other atoms 58 The Formation of Bonds with Carbon 59 Molecular Diversity Arising from Variation in Carbon Skeletons 60

Concept 4.3  A few chemical groups are key to molecular

function 62 The Chemical Groups Most Important in the Processes of Life 62 ATP: An Important Source of Energy for Cellular Processes 64 The Chemical Elements of Life: A Review 64

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DETAILED CONTENTS

Concept 6.2  Eukaryotic cells have internal membranes that compartmentalize their functions 97 Comparing Prokaryotic and Eukaryotic Cells 97 A Panoramic View of the Eukaryotic Cell 99

Concept 6.3  The eukaryotic cell’s genetic instructions are housed in the nucleus and carried out by the ribosomes 102 The Nucleus: Information Central 102 Ribosomes: Protein Factories 102

Concept 6.4  The endomembrane system regulates protein traffic and performs metabolic functions 104 The Endoplasmic Reticulum: Biosynthetic Factory 104 The Golgi Apparatus: Shipping and Receiving Center 105 Lysosomes: Digestive Compartments 107 Vacuoles: Diverse Maintenance Compartments 108 The Endomembrane System: A Review 108

5 The Structure and Function of Large Biological Molecules 66 The Molecules of Life 66 Concept 5.1  Macromolecules are polymers, built from monomers 67 The Synthesis and Breakdown of Polymers 67 The Diversity of Polymers 67

Concept 5.2  Carbohydrates serve as fuel and building

material 68 Sugars 68 Polysaccharides 70

Concept 5.3  Lipids are a diverse group of hydrophobic molecules 72 Fats 72 Phospholipids 74 Steroids 75

Concept 5.4  Proteins include a diversity of structures, resulting in a wide range of functions 75 Amino Acid Monomers 75 Polypeptides (Amino Acid Polymers) 78 Protein Structure and Function 78

Concept 5.5  Nucleic acids store, transmit, and help express hereditary information 84 The Roles of Nucleic Acids 84 The Components of Nucleic Acids 84 Nucleotide Polymers 85 The Structures of DNA and RNA Molecules 86

Concept 5.6  Genomics and proteomics have transformed biological inquiry and applications 86 DNA and Proteins as Tape Measures of Evolution 87

Unit 2  The Cell 92 Interview: Elba Serrano 92

6 A Tour of the Cell 93 The Fundamental Units of Life 93 Concept 6.1  Biologists use microscopes and biochemistry to study cells 94 Microscopy 94 Cell Fractionation 96

Concept 6.5  Mitochondria and chloroplasts change energy from one form to another 109 The Evolutionary Origins of Mitochondria and Chloroplasts 109 Mitochondria: Chemical Energy Conversion 110 Chloroplasts: Capture of Light Energy 110 Peroxisomes: Oxidation 112

Concept 6.6  The cytoskeleton is a network of fibers that organizes structures and activities in the cell 112 Roles of the Cytoskeleton: Support and Motility 112 Components of the Cytoskeleton 113

Concept 6.7  Extracellular components and connections between cells help coordinate cellular activities 118 Cell Walls of Plants 118 The Extracellular Matrix (ECM) of Animal Cells 118 Cell Junctions 119

Concept 6.8  A cell is greater than the sum of

its parts 121

7 Membrane Structure and Function 126 Life at the Edge 126 Concept 7.1  Cellular membranes are fluid mosaics of lipids and proteins 127 The Fluidity of Membranes 128 Evolution of Differences in Membrane Lipid Composition 129 Membrane Proteins and Their Functions 129 The Role of Membrane Carbohydrates in Cell-Cell Recognition 130 Synthesis and Sidedness of Membranes 131

Concept 7.2  Membrane structure results in selective permeability 131 The Permeability of the Lipid Bilayer 132 Transport Proteins 132

Concept 7.3  Passive transport is diffusion of a substance across a membrane with no energy investment 132 Effects of Osmosis on Water Balance 133 Facilitated Diffusion: Passive Transport Aided by Proteins 135

Concept 7.4  Active transport uses energy to move solutes against their gradients 136 The Need for Energy in Active Transport 136 How Ion Pumps Maintain Membrane Potential 137 Cotransport: Coupled Transport by a Membrane Protein 138

Concept 7.5  Bulk transport across the plasma membrane occurs by exocytosis and endocytosis 139 Exocytosis 139 Endocytosis 139

DETAILED CONTENTS

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Concept 9.4  During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis 174 The Pathway of Electron Transport 174 Chemiosmosis: The Energy-Coupling Mechanism 175 An Accounting of ATP Production by Cellular Respiration 177

Concept 9.5  Fermentation and anaerobic respiration enable cells to produce ATP without the use of oxygen 179 Types of Fermentation 180 Comparing Fermentation with Anaerobic and Aerobic Respiration 181 The Evolutionary Significance of Glycolysis 181

Concept 9.6  Glycolysis and the citric acid cycle connect

to many other metabolic pathways 182

8 An Introduction to Metabolism 143 The Energy of Life 143 Concept 8.1  An organism’s metabolism transforms matter and energy, subject to the laws of thermodynamics 144 Organization of the Chemistry of Life into Metabolic Pathways 144 Forms of Energy 144 The Laws of Energy Transformation 145

Concept 8.2  The free-energy change of a reaction tells us whether or not the reaction occurs spontaneously 147 Free-Energy Change, ΔG 147 Free Energy, Stability, and Equilibrium 147 Free Energy and Metabolism 148

Concept 8.3  ATP powers cellular work by coupling exergonic reactions to endergonic reactions 150 The Structure and Hydrolysis of ATP 150 How the Hydrolysis of ATP Performs Work 151 The Regeneration of ATP 153

Concept 8.4  Enzymes speed up metabolic reactions by lowering energy barriers 153 The Activation Energy Barrier 153 How Enzymes Speed Up Reactions 154 Substrate Specificity of Enzymes 155 Catalysis in the Enzyme’s Active Site 156 Effects of Local Conditions on Enzyme Activity 157 The Evolution of Enzymes 159

Concept 8.5  Regulation of enzyme activity helps control metabolism 159 Allosteric Regulation of Enzymes 160 Localization of Enzymes Within the Cell 161

9 Cellular Respiration and Fermentation 164 Life Is Work 164 Concept 9.1  Catabolic pathways yield energy by oxidizing organic fuels 165 Catabolic Pathways and Production of ATP 165 Redox Reactions: Oxidation and Reduction 165 The Stages of Cellular Respiration: A Preview 168

Concept 9.2  Glycolysis harvests chemical energy by oxidizing glucose to pyruvate 170 Concept 9.3  After pyruvate is oxidized, the citric acid cycle completes the energy-yielding oxidation of organic molecules 171 Oxidation of Pyruvate to Acetyl CoA 171 The Citric Acid Cycle 172

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The Versatility of Catabolism 182 Biosynthesis (Anabolic Pathways) 183 Regulation of Cellular Respiration via Feedback Mechanisms 183

10 Photosynthesis 187 The Process That Feeds the Biosphere 187 Concept 10.1  Photosynthesis converts light energy to the chemical energy of food 189 Chloroplasts: The Sites of Photosynthesis in Plants 189 Tracking Atoms Through Photosynthesis: Scientific Inquiry 190 The Two Stages of Photosynthesis: A Preview 191

Concept 10.2  The light reactions convert solar energy to the chemical energy of ATP and NADPH 192 The Nature of Sunlight 192 Photosynthetic Pigments: The Light Receptors 192 Excitation of Chlorophyll by Light 195 A Photosystem: A Reaction-Center Complex Associated with Light-Harvesting Complexes 195 Linear Electron Flow 197 Cyclic Electron Flow 198 A Comparison of Chemiosmosis in Chloroplasts and Mitochondria 199

Concept 10.3  The Calvin cycle uses the chemical energy of ATP and NADPH to reduce CO2 to sugar 201 Concept 10.4  Alternative mechanisms of carbon fixation have evolved in hot, arid climates 203 Photorespiration: An Evolutionary Relic? 203 C4 Plants 203 CAM Plants 205

Concept 10.5  Life depends on photosynthesis 206 The Importance of Photosynthesis: A Review 206

11 Cell Communication 212 Cellular Messaging 212 Concept 11.1  External signals are converted to responses within the cell 213 Evolution of Cell Signaling 213 Local and Long-Distance Signaling 215 The Three Stages of Cell Signaling: A Preview 216

Concept 11.2  Reception: A signaling molecule binds

to a receptor protein, causing it to change shape 217 Receptors in the Plasma Membrane 217 Intracellular Receptors 220

Concept 11.3  Transduction: Cascades of molecular interactions relay signals from receptors to target molecules in the cell 221 Signal Transduction Pathways 221 Protein Phosphorylation and Dephosphorylation 222 Small Molecules and Ions as Second Messengers 223

Concept 11.4  Response: Cell signaling leads to regulation of transcription or cytoplasmic activities 226 Nuclear and Cytoplasmic Responses 226 Regulation of the Response 226

Concept 11.5  Apoptosis integrates multiple cell-signaling

pathways 229 Apoptosis in the Soil Worm Caenorhabditis elegans 230 Apoptotic Pathways and the Signals That Trigger Them 230

12 The Cell Cycle 234 The Key Roles of Cell Division 234 Concept 12.1  Most cell division results in genetically identical daughter cells 235 Cellular Organization of the Genetic Material 235 Distribution of Chromosomes During Eukaryotic Cell Division 236

Concept 12.2  The mitotic phase alternates with interphase

in the cell cycle 237 Phases of the Cell Cycle 237 The Mitotic Spindle: A Closer Look 237 Cytokinesis: A Closer Look 241 Binary Fission in Bacteria 242 The Evolution of Mitosis 243

Concept 12.3  The eukaryotic cell cycle is regulated

by a molecular control system 244 The Cell Cycle Control System 244 Loss of Cell Cycle Controls in Cancer Cells 248

Unit 3  Genetics 253 Interview: Shirley Tilghman 253

13 Meiosis and Sexual Life Cycles 254 Variations on a Theme 254 Concept 13.1  Offspring acquire genes from parents by inheriting chromosomes 255 Inheritance of Genes 255 Comparison of Asexual and Sexual Reproduction 255

Concept 13.2  Fertilization and meiosis alternate in sexual

life cycles 256 Sets of Chromosomes in Human Cells 256 Behavior of Chromosome Sets in the Human Life Cycle 257 The Variety of Sexual Life Cycles 258

Concept 13.3  Meiosis reduces the number of chromosome sets from diploid to haploid 259 The Stages of Meiosis 259 Crossing Over and Synapsis During Prophase I 262 A Comparison of Mitosis and Meiosis 262

Concept 13.4  Genetic variation produced in sexual life cycles contributes to evolution 265 Origins of Genetic Variation Among Offspring 265 The Evolutionary Significance of Genetic Variation Within Populations 266

14 Mendel and the Gene Idea 269 Drawing from the Deck of Genes 269 Concept 14.1  Mendel used the scientific approach to identify two laws of inheritance 270 Mendel’s Experimental, Quantitative Approach 270 The Law of Segregation 271 The Law of Independent Assortment 274

Concept 14.2  Probability laws govern Mendelian inheritance 276 The Multiplication and Addition Rules Applied to Monohybrid Crosses 277 Solving Complex Genetics Problems with the Rules of Probability 277

Concept 14.3  Inheritance patterns are often more complex than predicted by simple Mendelian genetics 278 Extending Mendelian Genetics for a Single Gene 278 Extending Mendelian Genetics for Two or More Genes 281 Nature and Nurture: The Environmental Impact on Phenotype 282 A Mendelian View of Heredity and Variation 282

Concept 14.4  Many human traits follow Mendelian patterns of inheritance 284 Pedigree Analysis 284 Recessively Inherited Disorders 285 Dominantly Inherited Disorders 287 Multifactorial Disorders 287 Genetic Testing and Counseling 288 DETAILED CONTENTS

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15 The Chromosomal Basis of Inheritance 294 Locating Genes Along Chromosomes 294 Concept 15.1  Morgan showed that Mendelian inheritance has its physical basis in the behavior of chromosomes: scientific inquiry 296 Morgan’s Choice of Experimental Organism 296 Correlating Behavior of a Gene’s Alleles with Behavior of a Chromosome Pair 297

Concept 15.2  Sex-linked genes exhibit

unique patterns of inheritance 298 The Chromosomal Basis of Sex 298 Inheritance of X-Linked Genes 299 X Inactivation in Female Mammals 300

Concept 15.3  Linked genes tend to be inherited together because they are located near each other on the same chromosome 301 How Linkage Affects Inheritance 301 Genetic Recombination and Linkage 302 Mapping the Distance Between Genes Using Recombination Data: Scientific Inquiry 305

Concept 15.4  Alterations of chromosome number or

structure cause some genetic disorders 306 Abnormal Chromosome Number 307 Alterations of Chromosome Structure 307 Human Disorders Due to Chromosomal Alterations 308

Concept 15.5  Some inheritance patterns are exceptions

to standard Mendelian inheritance 310 Genomic Imprinting 310 Inheritance of Organelle Genes 311

16 The Molecular Basis of Inheritance 314 Life’s Operating Instructions 314 Concept 16.1  DNA is the genetic material 315 The Search for the Genetic Material: Scientific Inquiry 315 Building a Structural Model of DNA: Scientific Inquiry 317

Concept 16.2  Many proteins work together in DNA replication and repair 320 The Basic Principle: Base Pairing to a Template Strand 320 DNA Replication: A Closer Look 322 Proofreading and Repairing DNA 327 Evolutionary Significance of Altered DNA Nucleotides 328 Replicating the Ends of DNA Molecules 328

Concept 16.3  A chromosome consists of a DNA molecule

packed together with proteins 330

17 Gene Expression: From Gene to Protein 335 The Flow of Genetic Information 335 Concept 17.1  Genes specify proteins via transcription and translation 336 Evidence from Studying Metabolic Defects 336 Basic Principles of Transcription and Translation 338 The Genetic Code 339

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Concept 17.2  Transcription is the DNA-directed synthesis of RNA: a closer look 342 Molecular Components of Transcription 342 Synthesis of an RNA Transcript 342

Concept 17.3  Eukaryotic cells modify RNA after transcription 345 Alteration of mRNA Ends 345 Split Genes and RNA Splicing 345

Concept 17.4  Translation is the RNA-directed synthesis of a polypeptide: a closer look 347 Molecular Components of Translation 348 Building a Polypeptide 350 Completing and Targeting the Functional Protein 352 Making Multiple Polypeptides in Bacteria and Eukaryotes 355

Concept 17.5  Mutations of one or a few nucleotides can affect protein structure and function 357 Types of Small-Scale Mutations 357 New Mutations and Mutagens 360 What Is a Gene? Revisiting the Question 360

18 Regulation of Gene Expression 363 Beauty in the Eye of the Beholder 363 Concept 18.1  Bacteria often respond to environmental change by regulating transcription 364 Operons: The Basic Concept 364 Repressible and Inducible Operons: Two Types of Negative Gene Regulation 366 Positive Gene Regulation 367

Concept 18.2  Eukaryotic gene expression is regulated at many stages 368 Differential Gene Expression 368 Regulation of Chromatin Structure 369 Regulation of Transcription Initiation 370 Mechanisms of Post-Transcriptional Regulation 375

Concept 18.3  Noncoding RNAs play multiple roles in controlling gene expression 377 Effects on mRNAs by MicroRNAs and Small Interfering RNAs 377 Chromatin Remodeling and Effects on Transcription by ncRNAs 378 The Evolutionary Significance of Small ncRNAs 379

Concept 18.4  A program of differential gene expression

leads to the different cell types in a multicellular organism 379 A Genetic Program for Embryonic Development 379 Cytoplasmic Determinants and Inductive Signals 380 Sequential Regulation of Gene Expression During Cellular Differentiation 381 Pattern Formation: Setting Up the Body Plan 382

Concept 18.5  Cancer results from genetic changes

that affect cell cycle control 386 Types of Genes Associated with Cancer 386 Interference with Normal Cell-Signaling Pathways 387 The Multistep Model of Cancer Development 389 Inherited Predisposition and Environmental Factors Contributing to Cancer 392 The Role of Viruses in Cancer 392

21 Genomes and Their Evolution 440 Reading the Leaves from the Tree of Life 440 Concept 21.1  The Human Genome Project fostered development of faster, less expensive sequencing techniques 441 Concept 21.2  Scientists use bioinformatics to analyze genomes and their functions 442

19 Viruses 396 A Borrowed Life 396 Concept 19.1  A virus consists of a nucleic acid surrounded by a protein coat 397

Centralized Resources for Analyzing Genome Sequences 442 Identifying Protein-Coding Genes and Understanding Their Functions 443 Understanding Genes and Gene Expression at the Systems Level 444

Concept 21.3  Genomes vary in size, number of genes, and gene density 446 Genome Size 446 Number of Genes 447 Gene Density and Noncoding DNA 447

The Discovery of Viruses: Scientific Inquiry 397 Structure of Viruses 397

Concept 19.2  Viruses replicate only in host cells 399 General Features of Viral Replicative Cycles 399 Replicative Cycles of Phages 400 Replicative Cycles of Animal Viruses 402 Evolution of Viruses 404

Concept 19.3  Viruses and prions are formidable pathogens in animals and plants 406 Viral Diseases in Animals 406 Emerging Viruses 407 Viral Diseases in Plants 410 Prions: Proteins as Infectious Agents 410

20 DNA Tools and Biotechnology 413 The DNA Toolbox 413 Concept 20.1  DNA sequencing and DNA cloning are valuable tools for genetic engineering and biological inquiry 414 DNA Sequencing 414 Making Multiple Copies of a Gene or Other DNA Segment 416 Using Restriction Enzymes to Make a Recombinant DNA Plasmid 417 Amplifying DNA: The Polymerase Chain Reaction (PCR) and Its Use in DNA Cloning 418 Expressing Cloned Eukaryotic Genes 420

Concept 20.2  Biologists use DNA technology to study gene expression and function 421 Analyzing Gene Expression 421 Determining Gene Function 424

Concept 20.3  Cloned organisms and stem cells are useful for basic research and other applications 426 Cloning Plants: Single-Cell Cultures 427 Cloning Animals: Nuclear Transplantation 427 Stem Cells of Animals 429

Concept 20.4  The practical applications of DNA-based biotechnology affect our lives in many ways 431 Medical Applications 431 Forensic Evidence and Genetic Profiles 434 Environmental Cleanup 435 Agricultural Applications 436 Safety and Ethical Questions Raised by DNA Technology 436

Concept 21.4  Multicellular eukaryotes have a lot of noncoding DNA and many multigene families 448 Transposable Elements and Related Sequences 449 Other Repetitive DNA, Including Simple Sequence DNA 450 Genes and Multigene Families 450

Concept 21.5  Duplication, rearrangement, and mutation of DNA contribute to genome evolution 452 Duplication of Entire Chromosome Sets 452 Alterations of Chromosome Structure 452 Duplication and Divergence of Gene-Sized Regions of DNA 453 Rearrangements of Parts of Genes: Exon Duplication and Exon Shuffling 454 How Transposable Elements Contribute to Genome Evolution 457

Concept 21.6  Comparing genome sequences provides clues to evolution and development 457 Comparing Genomes 457 Widespread Conservation of Developmental Genes Among Animals 461

Sexual Selection 497 Balancing Selection 498 Why Natural Selection Cannot Fashion Perfect Organisms 499

24 The Origin of Species 504 That “Mystery of Mysteries” 504 Concept 24.1  The biological species concept emphasizes

reproductive isolation 505 The Biological Species Concept 505 Other Definitions of Species 508

Unit 4  Mechanisms of Evolution 465 Interview: Jack Szostak 465

Concept 24.2  Speciation can take place with or without geographic separation 509 Allopatric (“Other Country”) Speciation 509 Sympatric (“Same Country”) Speciation 511 Allopatric and Sympatric Speciation: A Review 514

22 Descent With Modification: A Darwinian View of Life 466

Concept 24.3  Hybrid zones reveal factors that cause reproductive isolation 514

Endless Forms Most Beautiful 466 Concept 22.1  The Darwinian revolution challenged traditional views of a young Earth inhabited by unchanging species 467

Concept 24.4  Speciation can occur rapidly or slowly

Scala Naturae and Classification of Species 468 Ideas About Change over Time 468 Lamarck’s Hypothesis of Evolution 468

Concept 22.2  Descent with modification by natural selection explains the adaptations of organisms and the unity and diversity of life 469 Darwin’s Research 469 Ideas from The Origin of Species 471 Key Features of Natural Selection 474

Concept 22.3  Evolution is supported by an overwhelming amount of scientific evidence 475 Direct Observations of Evolutionary Change 475 Homology 477 The Fossil Record 479 Biogeography 480 What Is Theoretical About Darwin’s View of Life? 481

23 The Evolution of Populations 484 The Smallest Unit of Evolution 484 Concept 23.1  Genetic variation makes evolution possible 485 Genetic Variation 485 Sources of Genetic Variation 486

Concept 23.2  The Hardy-Weinberg equation can be used to test whether a population is evolving 487 Gene Pools and Allele Frequencies 488 The Hardy-Weinberg Equation 488

Concept 23.3  Natural selection, genetic drift, and gene

flow can alter allele frequencies in a population 491 Natural Selection 492 Genetic Drift 492 Case Study: Impact of Genetic Drift on the Greater Prairie Chicken 493 Effects of Genetic Drift: A Summary 494 Gene Flow 494

Concept 23.4  Natural selection is the only mechanism

that consistently causes adaptive evolution 495 Natural Selection: A Closer Look 495 The Key Role of Natural Selection in Adaptive Evolution 497

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Patterns Within Hybrid Zones 514 Hybrid Zones and Environmental Change 515 Hybrid Zones over Time 515

and can result from changes in few or many genes 518 The Time Course of Speciation 518 Studying the Genetics of Speciation 520 From Speciation to Macroevolution 521

25 The History of Life on Earth 523 A Surprise in the Desert 523 Concept 25.1  Conditions on early Earth made the origin of life possible 524 Synthesis of Organic Compounds on Early Earth 524 Abiotic Synthesis of Macromolecules 525 Protocells 525 Self-Replicating RNA 526

Concept 25.2  The fossil record documents the history

of life 526 The Fossil Record 526 How Rocks and Fossils Are Dated 528 The Origin of New Groups of Organisms 528

Concept 25.3 Key events in life’s history include the origins of unicellular and multicellular organisms and the colonization of land 530 The First Single-Celled Organisms 532 The Origin of Multicellularity 533 The Colonization of Land 534

Concept 25.4  The rise and fall of groups of organisms reflect differences in speciation and extinction rates 535 Plate Tectonics 536 Mass Extinctions 538 Adaptive Radiations 540

Concept 25.5  Major changes in body form can result from changes in the sequences and regulation of developmental genes 542 Effects of Developmental Genes 542 The Evolution of Development 543

Concept 25.6 Evolution is not goal oriented 545 Evolutionary Novelties 545 Evolutionary Trends 546

Concept 27.2  Rapid reproduction, mutation, and genetic recombination promote genetic diversity in prokaryotes 576 Rapid Reproduction and Mutation 576 Genetic Recombination 577

Concept 27.3  Diverse nutritional and metabolic adaptations have evolved in prokaryotes 579 The Role of Oxygen in Metabolism 579 Nitrogen Metabolism 579 Metabolic Cooperation 580

Concept 27.4  Prokaryotes have radiated into a diverse set of lineages 581

Unit 5 The Evolutionary History of Biological Diversity 550 Interview: Nancy Moran 550

26 Phylogeny and the Tree of Life 551 Investigating the Tree of Life 551 Concept 26.1  Phylogenies show evolutionary relationships 552 Binomial Nomenclature 552 Hierarchical Classification 552 Linking Classification and Phylogeny 553 What We Can and Cannot Learn from Phylogenetic Trees 553 Applying Phylogenies 555

Concept 26.2  Phylogenies are inferred from morphological

and molecular data 556 Morphological and Molecular Homologies 556 Sorting Homology from Analogy 556 Evaluating Molecular Homologies 556

Concept 26.3  Shared characters are used to construct phylogenetic trees 557 Cladistics 557 Phylogenetic Trees with Proportional Branch Lengths 559 Maximum Parsimony and Maximum Likelihood 560 Phylogenetic Trees as Hypotheses 562

Concept 26.4  An organism’s evolutionary history is

documented in its genome 563 Gene Duplications and Gene Families 563 Genome Evolution 564

Concept 26.5  Molecular clocks help track evolutionary

time 564 Molecular Clocks 564 Applying a Molecular Clock: Dating the Origin of HIV 565

Concept 26.6  Our understanding of the tree of life continues to change based on new data 566 From Two Kingdoms to Three Domains 566 The Important Role of Horizontal Gene Transfer 566

27 Bacteria and Archaea 571 Masters of Adaptation 571 Concept 27.1  Structural and functional adaptations contribute to prokaryotic success 572

An Overview of Prokaryotic Diversity 581 Bacteria 581 Archaea 584

Concept 27.5  Prokaryotes play crucial roles in the biosphere 585 Chemical Recycling 585 Ecological Interactions 586

Concept 27.6  Prokaryotes have both beneficial and harmful impacts on humans 586 Mutualistic Bacteria 586 Pathogenic Bacteria 586 Prokaryotes in Research and Technology 587

28 Protists 591 Living Small 591 Concept 28.1  Most eukaryotes are single-celled organisms 592 Structural and Functional Diversity in Protists 592 Four Supergroups of Eukaryotes 592 Endosymbiosis in Eukaryotic Evolution 593

Concept 28.2  Excavates include protists with modified mitochondria and protists with unique flagella 597 Diplomonads and Parabasalids 597 Euglenozoans 598

Concept 28.3  SAR is a highly diverse group of protists defined by DNA similarities 599 Stramenopiles 599 Alveolates 602 Rhizarians 605

Concept 28.4  Red algae and green algae are the closest relatives of plants 606 Red Algae 606 Green Algae 607

Concept 28.5  Unikonts include protists that are closely related to fungi and animals 608 Amoebozoans 609 Opisthokonts 611

Concept 28.6  Protists play key roles in ecological communities 612 Symbiotic Protists 612 Photosynthetic Protists 612

Cell-Surface Structures 572 Motility 574 Internal Organization and DNA 575 Reproduction 575 DETAILED CONTENTS

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29 Plant Diversity I: How Plants Colonized Land 616 The Greening of Earth 616 Concept 29.1  Plants evolved from green algae 617 Morphological and Molecular Evidence 617 Adaptations Enabling the Move to Land 617 Derived Traits of Plants 617 The Origin and Diversification of Plants 620

Concept 29.2  Mosses and other nonvascular plants have life cycles dominated by gametophytes 622 Bryophyte Gametophytes 622 Bryophyte Sporophytes 625 The Ecological and Economic Importance of Mosses 625

Concept 29.3  Ferns and other seedless vascular plants

were the first plants to grow tall 626 Origins and Traits of Vascular Plants 626 Classification of Seedless Vascular Plants 629 The Significance of Seedless Vascular Plants 631

30 Plant Diversity II: The Evolution of Seed Plants 634 Transforming the World 634 Concept 30.1  Seeds and pollen grains are key adaptations for life on land 635 Advantages of Reduced Gametophytes 635 Heterospory: The Rule Among Seed Plants 636 Ovules and Production of Eggs 636 Pollen and Production of Sperm 636 The Evolutionary Advantage of Seeds 636

Concept 30.2  Gymnosperms bear “naked” seeds, typically

on cones 637 The Life Cycle of a Pine 638 Early Seed Plants and the Rise of Gymnosperms 639 Gymnosperm Diversity 639

Concept 30.3  The reproductive

adaptations of angiosperms include flowers and fruits 642

Characteristics of Angiosperms 642 Angiosperm Evolution 645 Angiosperm Diversity 647

Concept 30.4  Human welfare depends on seed plants 649 Products from Seed Plants 649 Threats to Plant Diversity 649

31 Fungi 652 Hidden Networks 652 Concept 31.1  Fungi are heterotrophs that feed by absorption 653 Nutrition and Ecology 653 Body Structure 653 Specialized Hyphae in Mycorrhizal Fungi 654

Concept 31.2  Fungi produce spores through sexual

or asexual life cycles 655 Sexual Reproduction 656 Asexual Reproduction 656

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Concept 31.3  The ancestor of fungi was an aquatic, single-celled, flagellated protist 657 The Origin of Fungi 657 Basal Fungal Groups 658 The Move to Land 658

Concept 31.4  Fungi have radiated into a diverse set of lineages 658 Chytrids 658 Zygomycetes 660 Glomeromycetes 661 Ascomycetes 661 Basidiomycetes 663

Concept 31.5  Fungi play key roles in nutrient cycling, ecological interactions, and human welfare 665 Fungi as Decomposers 665 Fungi as Mutualists 665 Fungi as Parasites 667 Practical Uses of Fungi 668

32 An Overview of Animal Diversity 671 A Kingdom of Consumers 671 Concept 32.1  Animals are multicellular, heterotrophic eukaryotes with tissues that develop from embryonic layers 672 Nutritional Mode 672 Cell Structure and Specialization 672 Reproduction and Development 672

Concept 32.2  The history of animals spans more than half a billion years 673 Steps in the Origin of Multicellular Animals 673 Neoproterozoic Era (1 Billion–541 Million Years Ago) 674 Paleozoic Era (541–252 Million Years Ago) 675 Mesozoic Era (252–66 Million Years Ago) 677 Cenozoic Era (66 Million Years Ago to the Present) 677

Concept 32.3  Animals can be characterized by “body plans” 677 Symmetry 677 Tissues 678 Body Cavities 678 Protostome and Deuterostome Development 679

Concept 32.4  Views of animal phylogeny

continue to be shaped by new molecular and morphological data 680 The Diversification of Animals 680 Future Directions in Animal Systematics 681

Concept 34.4  Tetrapods are gnathostomes that have limbs 728 Derived Characters of Tetrapods 728 The Origin of Tetrapods 729 Amphibians 729

Concept 34.5  Amniotes are tetrapods that have a terrestrially adapted egg 732 Derived Characters of Amniotes 732 Early Amniotes 733 Reptiles 733

Concept 34.6  Mammals are amniotes that have hair and produce milk 738

33 An Introduction to Invertebrates 684 A Dragon Without a Backbone 684 Concept 33.1  Sponges are basal animals that lack tissues 688 Concept 33.2  Cnidarians are an ancient phylum of eumetazoans 689 Medusozoans 690 Anthozoans 691

Concept 33.3  Lophotrochozoans, a clade identified by molecular data, have the widest range of animal body forms 692 Flatworms 692 Rotifers and Acanthocephalans 695 Lophophorates: Ectoprocts and Brachiopods 696 Molluscs 697 Annelids 701

Concept 33.4  Ecdysozoans are the most species-rich animal group 703 Nematodes 703 Arthropods 704

Concept 33.5  Echinoderms and chordates are

deuterostomes 711 Echinoderms 711 Chordates 713

34 The Origin and Evolution of Vertebrates 716 Half a Billion Years of Backbones 716 Concept 34.1  Chordates have a notochord and a dorsal, hollow nerve cord 717 Derived Characters of Chordates 717 Lancelets 718 Tunicates 719 Early Chordate Evolution 720

Concept 34.2  Vertebrates are chordates that have a

backbone 720 Derived Characters of Vertebrates 720 Hagfishes and Lampreys 721 Early Vertebrate Evolution 722

Concept 34.3  Gnathostomes are vertebrates that have

jaws 723 Derived Characters of Gnathostomes 723 Fossil Gnathostomes 724 Chondrichthyans (Sharks, Rays, and Their Relatives) 724 Ray-Finned Fishes and Lobe-Fins 726

Derived Characters of Mammals 739 Early Evolution of Mammals 739 Monotremes 740 Marsupials 740 Eutherians (Placental Mammals) 741

Concept 34.7  Humans are mammals that have a large brain and bipedal locomotion 746 Derived Characters of Humans 746 The Earliest Hominins 747 Australopiths 747 Bipedalism 748 Tool Use 748 Neanderthals 750

Unit 6  Plant Form and Function 755 Interview: Philip Benfey 755

35 Vascular Plant Structure, Growth, and Development 756 Are Plants Computers? 756 Concept 35.1  Plants have a hierarchical organization

consisting of organs, tissues, and cells 757 Basic Vascular Plant Organs: Roots, Stems, and Leaves 757 Dermal, Vascular, and Ground Tissues 760 Common Types of Plant Cells 761

Concept 35.2  Different meristems generate new cells for primary and secondary growth 764 Concept 35.3  Primary growth lengthens roots and shoots 766 Primary Growth of Roots 766 Primary Growth of Shoots 767

Concept 35.4  Secondary growth increases the diameter of stems and roots in woody plants 770 The Vascular Cambium and Secondary Vascular Tissue 771 The Cork Cambium and the Production of Periderm 772 Evolution of Secondary Growth 772

Concept 35.5  Growth, morphogenesis, and cell differentiation produce the plant body 773 Model Organisms: Revolutionizing the Study of Plants 774 Growth: Cell Division and Cell Expansion 774 Morphogenesis and Pattern Formation 776 Gene Expression and the Control of Cell Differentiation 776 Shifts in Development: Phase Changes 777 Genetic Control of Flowering 778

36 Resource Acquisition and Transport in Vascular Plants 782 A Whole Lot of Shaking Going On 782 Concept 36.1  Adaptations for acquiring resources were key steps in the evolution of vascular plants 783 Shoot Architecture and Light Capture 783 Root Architecture and Acquisition of Water and Minerals 785

Concept 36.2  Different mechanisms transport substances

over short or long distances 785 The Apoplast and Symplast: Transport Continuums 785 Short-Distance Transport of Solutes Across Plasma Membranes 786 Short-Distance Transport of Water Across Plasma Membranes 786 Long-Distance Transport: The Role of Bulk Flow 789

Concept 36.3  Transpiration drives the transport of water

and minerals from roots to shoots via the xylem 790 Absorption of Water and Minerals by Root Cells 790 Transport of Water and Minerals into the Xylem 790 Bulk Flow Transport via the Xylem 790 Xylem Sap Ascent by Bulk Flow: A Review 794

Concept 36.4  The rate of transpiration is regulated by

stomata 794 Stomata: Major Pathways for Water Loss 795 Mechanisms of Stomatal Opening and Closing 795 Stimuli for Stomatal Opening and Closing 796 Effects of Transpiration on Wilting and Leaf Temperature 796 Adaptations That Reduce Evaporative Water Loss 796

Concept 36.5  Sugars are transported from sources to sinks

via the phloem 797 Movement from Sugar Sources to Sugar Sinks 797 Bulk Flow by Positive Pressure: The Mechanism of Translocation in Angiosperms 798

Concept 36.6  The symplast is highly dynamic 799 Changes in Plasmodesmatal Number and Pore Size 800 Phloem: An Information Superhighway 800 Electrical Signaling in the Phloem 800

37 Soil and Plant Nutrition 803 The Corkscrew Carnivore 803 Concept 37.1  Soil contains a living, complex ecosystem 804 Soil Texture 804 Topsoil Composition 804 Soil Conservation and Sustainable Agriculture 805

Concept 37.2  Plant roots absorb essential elements from the soil 807 Essential Elements 807

Symptoms of Mineral Deficiency 808 Improving Plant Nutrition by Genetic Modification 809

Concept 37.3  Plant nutrition often involves relationships with other organisms 810 Bacteria and Plant Nutrition 812 Fungi and Plant Nutrition 815 Epiphytes, Parasitic Plants, and Carnivorous Plants 816

38 Angiosperm Reproduction and Biotechnology 820 Flowers of Deceit 820 Concept 38.1  Flowers, double fertilization, and fruits are key features of the angiosperm life cycle 821 Flower Structure and Function 821 Methods of Pollination 823 The Angiosperm Life Cycle: An Overview 824 Seed Development and Structure: A Closer Look 826 Sporophyte Development from Seed to Mature Plant 827 Fruit Structure and Function 828

Concept 38.2  Flowering plants reproduce sexually, asexually, or both 831 Mechanisms of Asexual Reproduction 831 Advantages and Disadvantages of Asexual and Sexual Reproduction 831 Mechanisms That Prevent Self-Fertilization 832 Totipotency, Vegetative Reproduction, and Tissue Culture 833

Concept 38.3  People modify crops by breeding and genetic engineering 834 Plant Breeding 835 Plant Biotechnology and Genetic Engineering 835 The Debate over Plant Biotechnology 837

39 Plant Responses to Internal and External Signals 840 Stimuli and a Stationary Life 840 Concept 39.1  Signal transduction pathways link signal reception to response 841 Reception 842 Transduction 842 Response 843

Concept 39.2  Plant hormones help coordinate growth, development, and responses to stimuli 844 A Survey of Plant Hormones 845

Concept 39.3  Responses to light are critical for plant

success 853 Blue-Light Photoreceptors 853 Phytochrome Photoreceptors 854 Biological Clocks and Circadian Rhythms 855 The Effect of Light on the Biological Clock 856 Photoperiodism and Responses to Seasons 857

Concept 39.4  Plants respond to a wide variety of stimuli other than light 859 Gravity 859 Mechanical Stimuli 859 Environmental Stresses 860

Concept 39.5  Plants respond to attacks by pathogens

and herbivores 864 Defenses Against Pathogens 864 Defenses Against Herbivores 865

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Unit 7 Animal Form and Function 870 Interview: Harald zur Hausen 870

40 Basic Principles of Animal Form and Function 871 Diverse Forms, Common Challenges 871 Concept 40.1  Animal form and function are correlated at all levels of organization 872 Evolution of Animal Size and Shape 872 Exchange with the Environment 872 Hierarchical Organization of Body Plans 874 Coordination and Control 878

Concept 40.2  Feedback control maintains the internal environment in many animals 879 Regulating and Conforming 879 Homeostasis 879

Concept 40.3  Homeostatic processes for thermoregulation involve form, function, and behavior 882 Endothermy and Ectothermy 882 Variation in Body Temperature 882 Balancing Heat Loss and Gain 883 Acclimatization in Thermoregulation 886 Physiological Thermostats and Fever 886

Concept 40.4  Energy requirements are related to animal

size, activity, and environment 887 Energy Allocation and Use 887 Quantifying Energy Use 888 Minimum Metabolic Rate and Thermoregulation 888 Influences on Metabolic Rate 889 Torpor and Energy Conservation 890

41 Animal Nutrition 896 The Need to Feed 896 Concept 41.1  An animal’s diet must supply chemical energy, organic building blocks, and essential nutrients 897 Essential Nutrients 897 Dietary Deficiencies 899 Assessing Nutritional Needs 900

Concept 41.2  Food processing involves ingestion, digestion, absorption, and elimination 900 Digestive Compartments 902

Concept 41.3  Organs specialized for sequential stages of

food processing form the mammalian digestive system 903 The Oral Cavity, Pharynx, and Esophagus 903 Digestion in the Stomach 905 Digestion in the Small Intestine 906 Absorption in the Small Intestine 907 Processing in the Large Intestine 908

Concept 41.4  Evolutionary adaptations of vertebrate digestive systems correlate with diet 909 Dental Adaptations 909 Stomach and Intestinal Adaptations 910 Mutualistic Adaptations 910

Concept 41.5  Feedback circuits regulate digestion,

energy storage, and appetite 912 Regulation of Digestion 913 Regulation of Energy Storage 913 Regulation of Appetite and Consumption 915

42 Circulation and Gas Exchange 919 Trading Places 919 Concept 42.1  Circulatory systems link exchange surfaces with cells throughout the body 920 Gastrovascular Cavities 920 Open and Closed Circulatory Systems 921 Organization of Vertebrate Circulatory Systems 922

Concept 42.2  Coordinated cycles of heart contraction drive double circulation in mammals 924 Mammalian Circulation 924 The Mammalian Heart: A Closer Look 924 Maintaining the Heart’s Rhythmic Beat 926

Concept 42.3  Patterns of blood pressure and flow reflect the structure and arrangement of blood vessels 927 Blood Vessel Structure and Function 927 Blood Flow Velocity 928 Blood Pressure 928 Capillary Function 930 Fluid Return by the Lymphatic System 931

Concept 42.4  Blood components function in exchange, transport, and defense 932 Blood Composition and Function 932 Cardiovascular Disease 935

Concept 42.5  Gas exchange occurs across specialized respiratory surfaces 937 Partial Pressure Gradients in Gas Exchange 937 Respiratory Media 937 Respiratory Surfaces 938 Gills in Aquatic Animals 938 Tracheal Systems in Insects 939 Lungs 940

Concept 42.6  Breathing ventilates the lungs 942 How an Amphibian Breathes 942 How a Bird Breathes 942 How a Mammal Breathes 943 Control of Breathing in Humans 944

Concept 42.7  Adaptations for gas exchange include pigments that bind and transport gases 945 Coordination of Circulation and Gas Exchange 945 Respiratory Pigments 945 Respiratory Adaptations of Diving Mammals 947

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Concept 44.2  An animal’s nitrogenous wastes reflect its phylogeny and habitat 980 Forms of Nitrogenous Waste 980 The Influence of Evolution and Environment on Nitrogenous Wastes 981

Concept 44.3  Diverse excretory systems are variations on a tubular theme 982 Excretory Processes 982 Survey of Excretory Systems 982

Concept 44.4  The nephron is organized for stepwise processing of blood filtrate 985 From Blood Filtrate to Urine: A Closer Look 986 Solute Gradients and Water Conservation 987 Adaptations of the Vertebrate Kidney to Diverse Environments 989

Concept 44.5  Hormonal circuits link kidney function, water balance, and blood pressure 992 Homeostatic Regulation of the Kidney 992

43 The Immune System 950 Recognition and Response 950 Concept 43.1  In innate immunity, recognition and response rely on traits common to groups of pathogens 951 Innate Immunity of Invertebrates 951 Innate Immunity of Vertebrates 953 Evasion of Innate Immunity by Pathogens 956

Concept 43.2  In adaptive immunity, receptors provide pathogen-specific recognition 956 Antigen Recognition by B Cells and Antibodies 956 Antigen Recognition by T Cells 957 B Cell and T Cell Development 958

Concept 43.3  Adaptive immunity defends against infection of body fluids and body cells 961 Helper T Cells: Activating Adaptive Immunity 961 B Cells and Antibodies: A Response to Extracellular Pathogens 962 Cytotoxic T Cells: A Response to Infected Host Cells 964 Summary of the Humoral and Cell-Mediated Immune Responses 965 Immunization 966 Active and Passive Immunity 966 Antibodies as Tools 967 Immune Rejection 967

Concept 43.4  Disruptions in immune system function

can elicit or exacerbate disease 968 Exaggerated, Self-Directed, and Diminished Immune Responses 968 Evolutionary Adaptations of Pathogens That Underlie Immune System Avoidance 970 Cancer and Immunity 972

4 4 Osmoregulation and Excretion 975 A Balancing Act 975 Concept 44.1  Osmoregulation balances the uptake and loss of water and solutes 976 Osmosis and Osmolarity 976 Osmoregulatory Challenges and Mechanisms 976 Energetics of Osmoregulation 978 Transport Epithelia in Osmoregulation 979

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45 Hormones and the Endocrine System 997 The Body’s Long-Distance Regulators 997 Concept 45.1  Hormones and other signaling molecules bind to target receptors, triggering specific response pathways 998 Intercellular Communication 998 Chemical Classes of Local Regulators and Hormones 999 Cellular Hormone Response Pathways 1000 Endocrine Tissues and Organs 1002

Concept 45.2  Feedback regulation and coordination with the nervous system are common in hormone pathways 1003 Simple Endocrine Pathways 1003 Simple Neuroendocrine Pathways 1003 Feedback Regulation 1004 Coordination of Endocrine and Nervous Systems 1004 Thyroid Regulation: A Hormone Cascade Pathway 1007 Hormonal Regulation of Growth 1007

Concept 45.3  Endocrine glands respond to diverse stimuli in regulating homeostasis, development, and behavior 1009 Parathyroid Hormone and Vitamin D: Control of Blood Calcium 1009 Adrenal Hormones: Response to Stress 1010 Sex Hormones 1012 Hormones and Biological Rhythms 1013 Evolution of Hormone Function 1013

48 Neurons, Synapses, and Signaling 1065 Lines of Communication 1065 Concept 48.1  Neuron structure and organization reflect function in information transfer 1066 Neuron Structure and Function 1066 Introduction to Information Processing 1066

46 Animal Reproduction 1017 Let Me Count the Ways 1017 Concept 46.1  Both asexual and sexual reproduction occur in the animal kingdom 1018 Mechanisms of Asexual Reproduction 1018 Variation in Patterns of Sexual Reproduction 1018 Reproductive Cycles 1019 Sexual Reproduction: An Evolutionary Enigma 1019

Concept 46.2  Fertilization depends on mechanisms that bring together sperm and eggs of the same species 1020 Ensuring the Survival of Offspring 1021 Gamete Production and Delivery 1021

Concept 46.3  Reproductive organs produce and transport

gametes 1023 Human Male Reproductive Anatomy 1023 Human Female Reproductive Anatomy 1024 Gametogenesis 1025

Concept 46.4  The interplay of tropic and sex hormones

regulates reproduction in mammals 1028 Hormonal Control of the Male Reproductive System 1029 Hormonal Control of Female Reproductive Cycles 1029 Human Sexual Response 1031

Concept 46.5  In placental mammals, an embryo develops

fully within the mother’s uterus 1032 Conception, Embryonic Development, and Birth 1032 Maternal Immune Tolerance of the Embryo and Fetus 1035 Contraception and Abortion 1036 Modern Reproductive Technologies 1037

47 Animal Development 1041 A Body-Building Plan 1041 Concept 47.1  Fertilization and cleavage initiate embryonic development 1042 Fertilization 1042 Cleavage 1045

Concept 47.2  Morphogenesis in animals involves specific changes in cell shape, position, and survival 1047 Gastrulation 1047 Developmental Adaptations of Amniotes 1051 Organogenesis 1052 The Cytoskeleton in Morphogenesis 1054

Concept 47.3  Cytoplasmic determinants and inductive signals regulate cell fate 1055 Fate Mapping 1056 Axis Formation 1057 Restricting Developmental Potential 1058 Cell Fate Determination and Pattern Formation by Inductive Signals 1059 Cilia and Cell Fate 1062

Concept 48.2  Ion pumps and ion channels establish the resting potential of a neuron 1068 Formation of the Resting Potential 1068 Modeling the Resting Potential 1069

Concept 48.3  Action potentials are the signals conducted

by axons 1070 Hyperpolarization and Depolarization 1070 Graded Potentials and Action Potentials 1071 Generation of Action Potentials: A Closer Look 1072 Conduction of Action Potentials 1073

Concept 48.4  Neurons communicate with other cells

at synapses 1075 Generation of Postsynaptic Potentials 1076 Summation of Postsynaptic Potentials 1077 Termination of Neurotransmitter Signaling 1078 Modulated Signaling at Synapses 1078 Neurotransmitters 1078

49 Nervous Systems 1083 Command and Control Center 1083 Concept 49.1  Nervous systems consist of circuits of neurons and supporting cells 1084 Glia 1085 Organization of the Vertebrate Nervous System 1086 The Peripheral Nervous System 1087

Concept 49.2  The vertebrate brain is regionally specialized 1089 Arousal and Sleep 1092 Biological Clock Regulation 1092 Emotions 1093 Functional Imaging of the Brain 1094

Concept 49.3  The cerebral cortex controls voluntary movement and cognitive functions 1094 Information Processing 1095 Language and Speech 1096 Lateralization of Cortical Function 1096 Frontal Lobe Function 1096 Evolution of Cognition in Vertebrates 1097

Concept 49.4  Changes in synaptic connections underlie memory and learning 1097 Neuronal Plasticity 1098 Memory and Learning 1098 Long-Term Potentiation 1099

Concept 49.5  Many nervous system disorders can now

be explained in molecular terms 1100 Schizophrenia 1100 Depression 1100 The Brain’s Reward System and Drug Addiction 1101 Alzheimer’s Disease 1101 Parkinson’s Disease 1102 Future Directions 1102

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50 Sensory and Motor Mechanisms 1105 Sense and Sensibility 1105 Concept 50.1  Sensory receptors transduce stimulus energy and transmit signals to the central nervous system 1106 Sensory Reception and Transduction 1106 Transmission 1107 Perception 1107 Amplification and Adaptation 1107 Types of Sensory Receptors 1108

Concept 50.2  In hearing and equilibrium, mechanoreceptors detect moving fluid or settling particles 1110 Sensing of Gravity and Sound in Invertebrates 1110 Hearing and Equilibrium in Mammals 1110 Hearing and Equilibrium in Other Vertebrates 1114

Concept 50.3  The diverse visual receptors of animals depend on light-absorbing pigments 1115 Evolution of Visual Perception 1115 The Vertebrate Visual System 1117

Concept 50.4  The senses of taste and smell rely on similar

sets of sensory receptors 1121 Taste in Mammals 1121 Smell in Humans 1122

Concept 50.5  The physical interaction of protein filaments is required for muscle function 1123 Vertebrate Skeletal Muscle 1124 Other Types of Muscle 1129

Concept 50.6  Skeletal systems transform muscle contraction into locomotion 1130 Types of Skeletal Systems 1130 Types of Locomotion 1133

51 Animal Behavior 1137 The How and Why of Animal Activity 1137 Concept 51.1  Discrete sensory inputs can stimulate both simple and complex behaviors 1138 Fixed Action Patterns 1138 Migration 1138 Behavioral Rhythms 1139 Animal Signals and Communication 1139

Concept 51.2  Learning establishes specific links between

experience and behavior 1141 Experience and Behavior 1141 Learning 1142

Concept 51.3  Selection for individual survival and

reproductive success can explain diverse behaviors 1146 Evolution of Foraging Behavior 1147 Mating Behavior and Mate Choice 1147

Concept 51.4  Genetic analyses and the concept of

inclusive fitness provide a basis for studying the evolution of behavior 1152 Genetic Basis of Behavior 1153 Genetic Variation and the Evolution of Behavior 1153 Altruism 1154 Inclusive Fitness 1155 Evolution and Human Culture 1157

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Unit 8  Ecology 1161 Interview: Tracy Langkilde 1161

52 An Introduction to Ecology and the Biosphere 1162 Discovering Ecology 1162 Concept 52.1  Earth’s climate varies by latitude and season and is changing rapidly 1165 Global Climate Patterns 1165 Regional and Local Effects on Climate 1165 Microclimate 1167 Global Climate Change 1167

Concept 52.2  The distribution of terrestrial biomes is controlled by climate and disturbance 1168 Climate and Terrestrial Biomes 1168 General Features of Terrestrial Biomes 1169 Disturbance and Terrestrial Biomes 1170

Concept 52.3  Aquatic biomes are diverse and dynamic systems that cover most of Earth 1175 Zonation in Aquatic Biomes 1175

Concept 52.4  Interactions between organisms and the

environment limit the distribution of species 1176 Dispersal and Distribution 1181 Biotic Factors 1182 Abiotic Factors 1182

Concept 52.5  Ecological change and evolution affect one another over long and short periods of time 1185

53 Population Ecology 1188 Turtle Tracks 1188 Concept 53.1  Biotic and abiotic factors affect population density, dispersion, and demographics 1189 Density and Dispersion 1189 Demographics 1191

Concept 53.2  The exponential model describes population growth in an idealized, unlimited environment 1194 Changes in Population Size 1194 Exponential Growth 1194

Concept 53.3  The logistic model describes how a population

grows more slowly as it nears its carrying capacity 1195 The Logistic Growth Model 1196 The Logistic Model and Real Populations 1197

Concept 53.4  Life history traits are products of natural

selection 1198 Diversity of Life Histories 1198 “Trade-offs” and Life Histories 1199

Concept 53.5  Density-dependent factors regulate population growth 1200 Population Change and Population Density 1201 Mechanisms of Density-Dependent Population Regulation 1201 Population Dynamics 1202 Population Cycles: Scientific Inquiry 1203

Concept 53.6  The human population is no longer growing exponentially but is still increasing rapidly 1205 The Global Human Population 1205 Global Carrying Capacity 1207

Concept 55.4  Biological and geochemical processes cycle nutrients and water in ecosystems 1246 Decomposition and Nutrient Cycling Rates 1246 Biogeochemical Cycles 1247 Case Study: Nutrient Cycling in the Hubbard Brook Experimental Forest 1250

Concept 55.5  Restoration ecologists return degraded ecosystems to a more natural state 1251 Bioremediation 1251 Biological Augmentation 1253 Ecosystems: A Review 1253

54 Community Ecology 1212 Communities in Motion 1212 Concept 54.1  Community interactions are classified by whether they help, harm, or have no effect on the species involved 1213 Competition 1213 Exploitation 1215 Positive Interactions 1218

Concept 54.2  Diversity and trophic structure characterize biological communities 1220 Species Diversity 1220 Diversity and Community Stability 1221 Trophic Structure 1221 Species with a Large Impact 1223 Bottom-Up and Top-Down Controls 1225

Concept 54.3  Disturbance influences species diversity and composition 1226 Characterizing Disturbance 1226 Ecological Succession 1227 Human Disturbance 1229

Concept 54.4  Biogeographic factors affect community

diversity 1229 Latitudinal Gradients 1230 Area Effects 1230 Island Equilibrium Model 1230

Concept 54.5  Pathogens alter community structure locally and globally 1232 Pathogens and Community Structure 1232 Community Ecology and Zoonotic Diseases 1233

55 Ecosystems and Restoration Ecology 1236 Transformed to Tundra 1236 Concept 55.1  Physical laws govern energy flow and chemical cycling in ecosystems 1237 Conservation of Energy 1237 Conservation of Mass 1237 Energy, Mass, and Trophic Levels 1238

Concept 55.2  Energy and other limiting factors control

primary production in ecosystems 1239 Ecosystem Energy Budgets 1239 Primary Production in Aquatic Ecosystems 1240 Primary Production in Terrestrial Ecosystems 1241

Concept 55.3  Energy transfer between trophic levels is typically only 10% efficient 1244 Production Efficiency 1244 Trophic Efficiency and Ecological Pyramids 1244

56 Conservation Biology and Global Change 1258 Psychedelic Treasure 1258 Concept 56.1  Human activities threaten Earth’s biodiversity 1259 Three Levels of Biodiversity 1259 Biodiversity and Human Welfare 1260 Threats to Biodiversity 1261

Concept 56.2  Population conservation focuses on population size, genetic diversity, and critical habitat 1264 Small-Population Approach 1264 Declining-Population Approach 1267 Weighing Conflicting Demands 1268

Concept 56.3  Landscape and regional conservation help sustain biodiversity 1268 Landscape Structure and Biodiversity 1268 Establishing Protected Areas 1270 Urban Ecology 1272

Concept 56.4  Earth is changing rapidly as a result of human actions 1272 Nutrient Enrichment 1273 Toxins in the Environment 1274 Greenhouse Gases and Climate Change 1275 Depletion of Atmospheric Ozone 1280

Concept 56.5  Sustainable development can improve human lives while conserving biodiversity 1281 Sustainable Development 1281 The Future of the Biosphere 1282 A ppendi x A Answers A-1 A ppendi x B

Periodic Table of the Elements  B-1

A ppendi x C

The Metric System  C-1

A ppendi x D

A Comparison of the Light Microscope and the Electron Microscope D-1

A ppendi x E

Classification of Life E-1

A ppendi x F

Scientific Skills Review F-1

C redits

CR-1

G lossar y

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Evolution, the Themes of Biology, and Scientific Inquiry

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Figure 1.1  What can this beach mouse (Peromyscus polionotus) teach us about biology?

Key Concepts 1.1

The study of life reveals unifying themes

1.2

The Core Theme: Evolution accounts for the unity and diversity of life

1.3

In studying nature, scientists make observations and form and test hypotheses

1.4

Science benefits from a cooperative approach and diverse viewpoints

An inland mouse of the species Peromyscus polionotus. This mouse has a much darker back, side, and face than mice of the same species that inhabit sand dunes.

2

Inquiring About Life There are few hiding places for a mouse among the sparse clumps of beach grass that dot the brilliant white sand dunes along the Florida seashore. However, the beach mice that live there have light, dappled fur, allowing them to blend into their surroundings (Figure 1.1). Mice of the same species (Peromyscus polionotus) also inhabit nearby inland areas. These mice are much darker in color, as are the soil and vegetation where they live (see smaller photo). For both beach mice and inland mice, the close color match of coat (fur) and environment is vital for survival, since hawks, herons, and other sharp-eyed predators periodically scan the landscape for prey. How has the color of each group of mice come to be so well matched, or adapted, to the local background? An organism’s adaptations to its environment, such as the mouse’s protective camouflage, are the result of evolution, the process of change over time that has resulted in the astounding array of organisms found on Earth. Evolution is the fundamental principle of biology and the core theme of this book. Although biologists know a great deal about life on Earth, many mysteries remain. Posing questions about the living world and seeking answers through scientific inquiry are the central activities of biology, the scientific study of life. Biologists’ questions can be ambitious. They may ask how a single tiny cell

When you see this blue icon, log in to MasteringBiology and go to the Study Area for digital resources.

Get Ready for This Chapter

While limited to a handful of images, Figure 1.2 reminds us that the living world is wondrously varied. How do biologists make sense of this diversity and complexity? This opening chapter sets up a framework for answering this question. The first part of the chapter provides a panoramic view of the biological “landscape,” organized around some unifying themes. We then focus on biology’s core theme, evolution, which accounts for life’s unity and diversity. Next, we look at scientific inquiry—how scientists ask and attempt to answer questions about the natural world. Finally, we address the culture of science and its effects on society.

becomes a tree or a dog, how the human mind works, or how the different forms of life in a forest interact. When questions occur to you as you observe the natural world, you are thinking like a biologist. More than anything else, biology is a quest, an ongoing inquiry about the nature of life. At the most fundamental level, we may ask: What is life? Even a child realizes that a dog or a plant is alive, while a rock or a car is not. Yet the phenomenon we call life defies a simple, one-sentence definition. We recognize life by what living things do. Figure 1.2 highlights some of the properties and processes we associate with life.

Figure 1.2  Some properties of life. ▼ Order. This close-up of a sunflower illustrates the highly ordered structure that characterizes life.

▲ Regulation. The regulation of blood flow through the blood vessels of this jackrabbit’s ears helps maintain a constant body temperature by adjusting heat exchange with the ▲ Evolutionary adaptation. The overall surrounding air. appearance of this pygmy sea horse camouflages the animal in its environment. Such adaptations evolve over countless generations by the reproductive success of those individuals with ▼ Reproduction. heritable traits that are best suited to Organisms (living their environments. things) reproduce their own kind.

▲ Energy processing. This butterfly obtains fuel in the form of nectar from flowers. The butterfly will use chemical energy stored in its food to power flight and other work.

Animation: Signs of Life Video: Sea Horse Camouflage

▲ Growth and development. Inherited information carried by genes controls the pattern of growth and development of organisms, such as this oak seedling.

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▲ Response to the environment. The Venus flytrap on the left closed its trap rapidly in response to the environmental stimulus of a grasshopper landing on the open trap.

Evolution, the Themes of Biology, and Scientific Inquiry

3

Concept  1.1

Here are five unifying themes—ways of thinking about life that will still hold true decades from now.

The study of life reveals unifying themes



Biology is a subject of enormous scope, and exciting new biological discoveries are being made every day. How can you organize into a comprehensible framework all the information you’ll encounter as you study the broad range of topics included in biology? Focusing on a few big ideas will help.

Figure 1.3 



Organization Information Energy and Matter Interactions Evolution

In this section and the next, we’ll briefly define and explore each theme.

Exploring Levels of Biological Organization ◀1

The Biosphere

Even from space, we can see signs of Earth’s life—in the green mosaic of the forests, for example. We can also see the entire biosphere, which consists of all life on Earth and all the places where life exists: most regions of land, most bodies of water, the atmosphere to an altitude of several kilometers, and even sediments far below the ocean floor.

◀2

Ecosystems

Our first scale change brings us to a North American mountain meadow, which is an example of an ecosystem, as are tropical forests, grasslands, deserts, and coral reefs. An ecosystem consists of all the living things in a particular area, along with all the nonliving components of the environment with which life interacts, such as soil, water, atmospheric gases, and light.

▶3

Communities

The array of organisms inhabiting a particular ecosystem is called a biological community. The community in our meadow ecosystem includes many kinds of plants, various animals, mushrooms and other fungi, and enormous numbers of diverse microorganisms, such as bacteria, that are too small to see without a microscope. Each of these forms of life belongs to a species—a group whose members can only reproduce with other members of the group.

▶4

Populations

A population consists of all the individuals of a species living within the bounds of a specified area. For example, our meadow includes a population of lupine (some of which are shown here) and a population of mule deer. A community is therefore the set of populations that inhabit a particular area.

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Evolution, the Themes of Biology, and Scientific Inquiry

▲5

Organisms

Individual living things are called organisms. Each plant in the meadow is an organism, and so is each animal, fungus, and bacterium.

Theme: New Properties Emerge at Successive Levels of Biological Organization Organization The study of life on Earth extends from the microscopic scale of the molecules and cells that make up organisms to the global scale of the entire living planet. As biologists, we can divide this enormous range into different levels of biological organization. In Figure 1.3, we zoom in from space to take a closer and closer look at life in a mountain meadow. This journey, depicted as a series of numbered steps, highlights the hierarchy of biological organization.

▼6

Organs

▼7

The structural hierarchy of life continues to unfold as we explore the architecture of a complex organism. A leaf is an example of an organ, a body part that is made up of multiple tissues and has specific functions in the body. Leaves, stems, and roots are the major organs of plants. Within an organ, each tissue has a distinct arrangement and contributes particular properties to organ function.

Cell

▶8

Cells

The cell is life’s fundamental unit of structure and function. Some organisms consist of a single cell, which performs all the functions of life. Other organisms are multicellular and feature a division of labor among specialized cells. Here we see a magnified view of a cell in a leaf tissue. This cell is about 40 micrometers (μm) across—about 500 of them would reach across a small coin. Within these tiny cells are even smaller green structures called chloroplasts, which are responsible for photosynthesis.

Zooming in through the levels of the biological hierarchy at ever-finer resolution illustrates an approach called reductionism. This method is so named because it reduces complex systems to simpler components that are more manageable to study. Reductionism is a powerful strategy in biology. For example, by studying the molecular structure of DNA that had been extracted from cells, James Watson and Francis Crick inferred the chemical basis of biological inheritance. Reductionism has propelled many major discoveries, but it provides a necessarily incomplete view of life on Earth, as we’ll discuss next.

Tissues

Viewing the tissues of a leaf requires a microscope. Each tissue is a group of cells that work together, performing a specialized function. The leaf shown here has been cut on an angle. The honeycombed tissue in the interior of the leaf (left side of photo) is the main location of photosynthesis, the process that converts light energy to the chemical energy of sugar. The jigsaw puzzle–like “skin” on the surface of the leaf is a tissue called epidermis (right side of photo). The pores through the epidermis allow entry of the gas CO2, a raw material for sugar production.

50 μm

10 μm

▼9

Organelles

▼ 10

Chloroplasts are examples of organelles, the various functional components present in cells. The image below, taken by a powerful microscope, shows a single chloroplast.

Chloroplast

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1 μm

Molecules

Our last scale change drops us into a chloroplast for a view of life at the molecular level. A molecule is a chemical structure consisting of two or more units called atoms, represented as balls in this computer graphic of a chlorophyll molecule. Chlorophyll is the pigment that makes a leaf green, and it absorbs sunlight during photosynthesis. Within each chloroplast, millions of chlorophyll Atoms molecules are organized into Chlorophyll systems that convert molecule light energy to the chemical energy of food.

Evolution, the Themes of Biology, and Scientific Inquiry

5

Emergent Properties Let’s reexamine Figure 1.3, beginning this time at the molecular level and then zooming out. This approach allows us to see novel properties emerge at each level that are absent from the preceding one. These emergent properties are due to the arrangement and interactions of parts as complexity increases. For example, although photosynthesis occurs in an intact chloroplast, it will not take place in a disorganized test-tube mixture of chlorophyll and other chloroplast molecules. The coordinated processes of photosynthesis require a specific organization of these molecules in the chloroplast. Isolated components of living systems—the objects of study in a reductionist approach—lack a number of significant properties that emerge at higher levels of organization. Emergent properties are not unique to life. A box of bicycle parts won’t transport you anywhere, but if they are arranged in a certain way, you can pedal to your chosen destination. Compared with such nonliving examples, however, biological systems are far more complex, making the emergent properties of life especially challenging to study. To fully explore emergent properties, biologists today complement reductionism with systems biology, the exploration of a biological system by analyzing the interactions among its parts. In this context, a single leaf cell can be considered a system, as can a frog, an ant colony, or a desert ecosystem. By examining and modeling the dynamic behavior of an integrated network of components, systems biology enables us to pose new kinds of questions. For example, how do networks of molecular interactions in our bodies generate our 24-hour cycle of wakefulness and sleep? At a larger scale, how does a gradual increase in atmospheric carbon dioxide alter ecosystems and the entire biosphere? Systems biology can be used to study life at all levels.

Structure and Function At each level of the biological hierarchy, we find a correlation of structure and function. Consider a leaf in Figure 1.3: Its thin, flat shape maximizes the capture of sunlight by chloroplasts. Because such correlations of structure and function are common in all forms of life, analyzing a biological structure gives us clues about what it does and how it works. Conversely, knowing the function of something provides insight into its structure and organization. Many examples from the animal kingdom show a correlation between structure and function. For example, the hummingbird’s anatomy allows the wings to rotate at the shoulder, so hummingbirds have the ability, unique among birds, to fly backward or hover in place. While hovering, the birds can extend their long, 6

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slender beaks into flowers and feed on nectar. The elegant match of form and function in the structures of life is explained by natural selection, which we’ll explore shortly.

The Cell: An Organism’s Basic Unit of Structure and Function In life’s structural hierarchy, the cell is the smallest unit of organization that can perform all activities required for life. The so-called Cell Theory was first developed in the 1800s, based on the observations of many scientists. The theory states that all living organisms are made of cells, which are the basic unit of life. In fact, the actions of organisms are all based on the functioning of cells. For instance, the movement of your eyes as you read this sentence results from the activities of muscle and nerve cells. Even a process that occurs on a global scale, such as the recycling of carbon atoms, is the product of cellular functions, including the photosynthetic activity of chloroplasts in leaf cells. All cells share certain characteristics. For instance, every cell is enclosed by a membrane that regulates the passage of materials between the cell and its surroundings. Nevertheless, we distinguish two main forms of cells: prokaryotic and eukaryotic. The cells of two groups of single-celled microorganisms— bacteria (singular, bacterium) and archaea (singular, archaean)— are prokaryotic. All other forms of life, including plants and animals, are composed of eukaryotic cells. A eukaryotic cell contains membrane-enclosed organelles (Figure 1.4). Some organelles, such as the DNA-containing nucleus, are found in the cells of all eukaryotes; other organelles Figure 1.4  Contrasting eukaryotic and prokaryotic cells in size and complexity. The cells are shown to scale here; to see a larger magnification of a prokaryotic cell, see Figure 6.5. Eukaryotic cell Membrane

Prokaryotic cell

DNA (no nucleus) Membrane

Cytoplasm

Nucleus (membraneenclosed) Membraneenclosed organelles

DNA (throughout nucleus)

1 μm

VISUAL SKILLS Measure the scale bar and use its length to estimate the length of the prokaryotic cell and the longest dimension of the eukaryotic cell.

Evolution, the Themes of Biology, and Scientific Inquiry

are specific to particular cell types. For example, the chloroplast in Figure 1.3 is an organelle found only in eukaryotic cells that carry out photosynthesis. In contrast to eukaryotic cells, a prokaryotic cell lacks a nucleus or other membrane-enclosed organelles. Furthermore, prokaryotic cells are generally smaller than eukaryotic cells, as shown in Figure 1.4.

Figure 1.6  Inherited DNA directs development of an organism.

Nuclei containing DNA Sperm cell

Egg cell

Theme: Life’s Processes Involve the Expression and Transmission of Genetic Information

Fertilized egg with DNA from both parents

Within cells, structures called chromosomes contain genetic material in the form of DNA (deoxyribonucleic acid). In cells that are preparing to divide, the chromosomes may be made visible using a dye that appears blue when bound to the DNA (Figure 1.5). Information

Embryo’s cells with copies of inherited DNA Offspring with traits inherited from both parents

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Figure 1.5  A lung cell from a newt divides into two smaller cells that will grow and divide again.

Nucleus

Figure 1.7  DNA: The genetic material.

DNA

Cell A C Nucleotide

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DNA, the Genetic Material

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Before a cell divides, the DNA is first replicated, or copied, and each of the two cellular offspring inherits a complete set of chromosomes, identical to that of the parent cell. Each chromosome contains one very long DNA molecule with hundreds or thousands of genes, each a section of the DNA of the chromosome. Transmitted from parents to offspring, genes are the units of inheritance. They encode the information necessary to build all of the molecules synthesized within a cell, which in turn establish that cell’s identity and function. You began as a single cell stocked with DNA inherited from your parents. The replication of that DNA prior to each cell division transmitted copies of the DNA to what eventually became the trillions of cells of your body. As the cells grew and divided, the genetic information encoded by the DNA directed your development (Figure 1.6). The molecular structure of DNA accounts for its ability to store information. A DNA molecule is made up of two long chains, called strands, arranged in a double helix. Each chain is made up of four kinds of chemical building blocks called nucleotides, abbreviated A, T, C, and G (Figure 1.7). Specific sequences of these four nucleotides encode the information

A

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C C G T A G T A (b) Single strand of DNA. These (a) DNA double helix. This geometric shapes and letters are model shows the atoms simple symbols for the nucleoin a segment of DNA. Made tides in a small section of one up of two long chains (strands) strand of a DNA molecule. Genetic of building blocks called information is encoded in specific nucleotides, a DNA molecule sequences of the four types of takes the three-dimensional nucleotides. Their names are form of a double helix. abbreviated A, T, C, and G. Animation: Heritable Information: DNA

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in genes. The way DNA encodes information is analogous to how we arrange the letters of the alphabet into words and phrases with specific meanings. The word rat, for example, evokes a rodent; the words tar and art, which contain the same letters, mean very different things. We can think of nucleotides as a four-letter alphabet. For many genes, the sequence provides the blueprint for making a protein. For instance, a given bacterial gene may specify a particular protein (an enzyme) required to break down a certain sugar molecule, while a human gene may denote a different protein (an antibody) that helps fight off infection. Overall, proteins are major players in building and maintaining the cell and carrying out its activities. Protein-encoding genes control protein production indirectly, using a related molecule called RNA as an intermediary (Figure 1.8). The sequence of nucleotides along a gene is transcribed into mRNA, which is then translated into a linked series of protein building blocks called amino acids. Once completed, the amino acid chain forms a specific protein with a unique shape and function. The entire process by which the information in a gene directs the manufacture of a cellular product is called gene expression. In carrying out gene expression, all forms of life employ essentially the same genetic code: A particular sequence of nucleotides says the same thing in one organism as it does in another. Differences between organisms reflect differences between their nucleotide sequences rather than between their genetic codes. This universality of the genetic code is a strong piece of evidence that all life is related. Comparing the sequences in several species for a gene that codes for a particular protein can provide valuable information both about the protein and about the relationship of the species to each other. The mRNA molecule in Figure 1.8 is translated into a protein, but other cellular RNAs function differently. For example, we have known for decades that some types of RNA are actually components of the cellular machinery that manufactures proteins. Recently, scientists have discovered whole new classes of RNA that play other roles in the cell, such as regulating the functioning of protein-coding genes. Genes specify all of these RNAs as well, and their production is also referred to as gene expression. By carrying the instructions for making proteins and RNAs and by replicating with each cell division, DNA ensures faithful inheritance of genetic information from generation to generation.

Figure 1.8  Gene expression: Cells use information encoded in a gene to synthesize a functional protein.

(a) The lens of the eye (behind the pupil) is able to focus light because lens cells are tightly packed with transparent proteins called crystallin. How do lens cells make crystallin proteins?

Lens cell

(b) A lens cell uses information in DNA to make crystallin proteins. Crystallin gene The crystallin gene is a section of DNA in a chromosome.

DNA (part of the crystallin gene)

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The cell translates the information in the sequence of mRNA nucleotides to make a protein, a series of linked amino acids.

TRANSLATION

Chain of amino acids

PROTEIN FOLDING

Genomics: Large-Scale Analysis of DNA Sequences The entire “library” of genetic instructions that an organism inherits is called its genome. A typical human cell has two similar sets of chromosomes, and each set has approximately 3 billion nucleotide pairs of DNA. If the one-letter abbreviations for the nucleotides of a set were written in letters the

T

Using the information in the sequence of DNA nucleotides, the cell makes (transcribes) a specific RNA molecule called mRNA.

TRANSCRIPTION

mRNA

T

Protein Crystallin protein

The chain of amino acids folds into the specific shape of a crystallin protein. Crystallin proteins can then pack together and focus light, allowing the eye to see.

Figure Walkthrough

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Evolution, the Themes of Biology, and Scientific Inquiry

size of those you are now reading, the genomic text would fill about 700 biology textbooks. Since the early 1990s, the pace at which researchers can determine the sequence of a genome has accelerated at an astounding rate, enabled by a revolution in technology. The genome sequence—the entire sequence of nucleotides for a representative member of a species—is now known for humans and many other animals, as well as numerous plants, fungi, bacteria, and archaea. To make sense of the deluge of data from genome-sequencing projects and the growing catalog of known gene functions, scientists are applying a systems biology approach at the cellular and molecular levels. Rather than investigating a single gene at a time, researchers study whole sets of genes (or other DNA) in one or more species—an approach called genomics. Likewise, the term proteomics refers to the study of sets of proteins and their properties. (The entire set of proteins expressed by a given cell, tissue, or organism is called a proteome). Three important research developments have made the genomic and proteomic approaches possible. One is “highthroughput” technology, tools that can analyze many biological samples very rapidly. The second major development is bioinformatics, the use of computational tools to store, organize, and analyze the huge volume of data that results from high-throughput methods. The third development is the formation of interdisciplinary research teams—groups of diverse specialists that may include computer scientists, mathematicians, engineers, chemists, physicists, and, of course, biologists from a variety of fields. Researchers in such teams aim to learn how the activities of all the proteins and RNAs encoded by the DNA are coordinated in cells and in whole organisms.

Figure 1.9  Energy flow and chemical cycling. There is a one-way flow of energy in an ecosystem: During photosynthesis, plants convert energy from sunlight to chemical energy (stored in food molecules such as sugars), which is used by plants and other organisms to do work and is eventually lost from the ecosystem as heat. In contrast, chemicals cycle between organisms and the physical environment.

Theme: Life Requires the Transfer and Transformation of Energy and Matter Energy and Matter A fundamental characteristic of living organisms is their use of energy to carry out life’s activities. Moving, growing, reproducing, and the various cellular activities of life are work, and work requires energy. The input of energy, primarily from the sun, and the transformation of energy from one form to another make life possible (Figure 1.9). When a plant’s leaves absorb sunlight, molecules within the leaves convert the energy of sunlight to the chemical energy of food, such as sugars, in the process of photosynthesis. The chemical energy in the food molecules is then passed along by plants and other photosynthetic organisms (producers) to consumers. Consumers are organisms, such as animals, that feed on other organisms or their remains. When an organism uses chemical energy to perform work, such as muscle contraction or cell division, some of that energy is lost to the surroundings as heat. As a result, energy flows through an ecosystem in one direction, usually entering as light and exiting as heat. In contrast, chemicals cycle within an ecosystem, where they are used and then recycled (see Figure 1.9). Chemicals that a plant absorbs from the air or soil may be incorporated into the plant’s body and then passed to an animal that eats the plant. Eventually, these chemicals will be returned to the environment by decomposers such as bacteria and fungi that break down waste products, leaf litter, and the bodies of dead organisms. The chemicals are then available to be taken up by plants again, thereby completing the cycle.

ENERGY FLOW

LC EMICA YCLING CH

Plants convert sunlight to chemical energy.

Light energy comes from the sun.

Organisms use chemical energy to do work.

Plants take up chemicals from the soil and air.

Chemicals

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Chemicals in plants are passed to organisms that eat the plants.

Heat is lost from the ecosystem.

Decomposers such as fungi and bacteria break down leaf litter and dead organisms, returning chemicals to the soil.

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Theme: From Molecules to Ecosystems, Interactions Are Important in Biological Systems

Figure 1.10  Feedback regulation. The human body regulates use and storage of glucose, a major cellular fuel. This figure shows negative feedback: The response to insulin reduces the initial stimulus. Insulin-producing Glucose cell in pancreas in blood Insulin 1 High blood glucose levels stimulate cells in the pancreas – to secrete insulin into the blood.

At any level of the biological hierarchy, interactions between the components of the system ensure smooth integration of all the parts, such that they function as a whole. This holds true equally well for molecules in a cell and the components of an ecosystem; we’ll discuss both as examples.

Molecules: Interactions Within Organisms

Negative feedback

Interactions

2 Insulin circulates in the blood throughout the body.

3 Insulin binds to body cells, At lower levels of organization, the interactions between causing them to take up components that make up living organisms—organs, tissues, glucose and liver cells to store glucose. This lowers glucose cells, and molecules—are crucial to their smooth operation. levels in the blood. Consider the regulation of blood sugar levels, for instance. Cells in the body must match the supply of fuel (sugar) to 4 Lowered blood glucose demand, regulating the opposing processes of sugar breaklevels do not stimulate secretion of insulin. down and storage. The key is the ability of many biological VISUAL SKILLS In this example, what is the response to insulin? What is processes to self-regulate by a mechanism called feedback. the initial stimulus that is reduced by the response? In feedback regulation, the output or product of a process regulates that very process. The most common form of regulation in living systems is negative feedback, a loop in which with its roots, insects that live on it, and animals that eat its the response reduces the initial stimulus. As seen in the examleaves and fruit (Figure 1.11). Interactions between organple of insulin signaling (Figure 1.10), after a meal the level of isms include those that are mutually beneficial (as when the sugar glucose in your blood rises, which stimulates cells of “cleaner fish” eat small parasites on a turtle), and those the pancreas to secrete insulin. Insulin, in turn, causes body in which one species benefits and the other is harmed (as cells to take up glucose and liver cells to store it, thus decreaswhen a lion kills and eats a zebra). In some interactions ing blood glucose levels. This eliminates the stimulus for insubetween species, both are harmed—for example, when two lin secretion, shutting off the pathway. Thus, the output of the plants compete for a soil resource that is in short supply. process negatively regulates that process. Though less common than processes Figure 1.11  Interactions of an African acacia tree with other organisms regulated by negative feedback, there are and the physical environment. also many biological processes regulated Sunlight by positive feedback, in which an end product speeds up its own production. Leaves absorb light Leaves take in The clotting of your blood in response to energy from the sun. CO2 carbon dioxide injury is an example. When a blood vesfrom the air and sel is damaged, structures in the blood release oxygen. called platelets begin to aggregate at the O2 site. Positive feedback occurs as chemicals released by the platelets attract more Leaves fall to the ground and are platelets. The platelet pileup then initidecomposed by ates a complex process that seals the organisms that wound with a clot. return minerals

to the soil.

Ecosystems: An Organism’s Interactions with Other Organisms and the Physical Environment At the ecosystem level, every organism interacts with other organisms. For instance, an acacia tree interacts with soil microorganisms associated 10

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Water and minerals in the soil are taken up by the tree through its roots.

Evolution, the Themes of Biology, and Scientific Inquiry

Animals eat leaves and fruit from the tree, returning nutrients and minerals to the soil in their waste products.

Interactions among organisms help regulate the functioning of the ecosystem as a whole. Each organism also interacts continuously with physical factors in its environment. The leaves of a tree, for example, absorb light from the sun, take in carbon dioxide from the air, and release oxygen to the air (see Figure 1.11). The environment is also affected by organisms. For instance, in addition to taking up water and minerals from the soil, the roots of a plant break up rocks as they grow, contributing to the formation of soil. On a global scale, plants and other photosynthetic organisms have generated all the oxygen in the atmosphere. Like other organisms, we humans interact with our environment. Our interactions sometimes have dire consequences: For example, over the past 150 years, humans have greatly increased the burning of fossil fuels (coal, oil, and gas). This practice releases large amounts of carbon dioxide (CO2) and other gases into the atmosphere, causing heat to be trapped close to the Earth’s surface (see Figure 56.29). Scientists calculate that the CO2 that human activities have added to the atmosphere has increased the average temperature of the planet by about 1ºC since 1900. At the current rates that CO2 and other gases are being added to the atmosphere, global models predict an additional rise of at least 3ºC before the end of this century. This ongoing global warming is a major aspect of climate change, a directional change to the global climate that lasts for three decades or more (as opposed to short-term changes in the weather). But global warming is not the only way the climate is changing: Wind and precipitation patterns are also shifting, and extreme weather events such as storms and droughts are occurring more often. Climate change has already affected organisms and their habitats all over the planet. For example, polar bears have lost much of the ice platform from which they hunt, leading to food shortages and increased mortality rates. As habitats deteriorate, hundreds of plant and animal species are shifting their ranges to more suitable locations—but for some, there is insufficient suitable habitat, or they may not be able to migrate quickly enough. As a result, the populations of many species are shrinking in size or even disappearing (Figure 1.12). This Figure 1.12  Threatened by global warming. A warmer environment causes lizards in the genus Sceloporus to spend more time in refuges from the heat, reducing time for foraging. Their food intake drops, decreasing reproductive success. Surveys show that 12% of the 200 populations in Mexico have disappeared since 1975. For more examples of climate change affecting life on Earth, see Make Connections Figure 56.30.

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trend can result in extinction, the permanent loss of a species. As we’ll discuss in greater detail in Concept 56.4, the consequences of these changes for humans and other organisms may be profound. Having considered four of the unifying themes (organization, information, energy and matter, and interactions), let’s now turn to evolution. There is consensus among biologists that evolution is the core theme of biology, and it is discussed in detail in the next section.

Concept Check 1.1 1. Starting with the molecular level in Figure 1.3, write a sentence that includes components from the previous (lower) level of biological organization, for example: “A molecule consists of atoms bonded together.” Continue with organelles, moving up the biological hierarchy. 2. Identify the theme or themes exemplified by (a) the sharp quills of a porcupine, (b) the development of a multicellular organism from a single fertilized egg, and (c) a hummingbird using sugar to power its flight. 3. WHAT IF? For each theme discussed in this section, give an example not mentioned in the text. For suggested answers, see Appendix A.

Concept  1.2 The Core Theme: Evolution accounts for the unity and diversity of life Evolution Evolution is the one idea that makes logical sense of everything we know about living organisms. As the fossil record clearly shows, life has been evolving on Earth for billions of years, resulting in a vast diversity of past and present organisms. But along with the diversity there is also unity, in the form of shared features. For example, while sea horses, jackrabbits, hummingbirds, and giraffes all look very different, their skeletons are organized in the same basic way. The scientific explanation for the unity and diversity of organisms—as well as for the adaptation of organisms to their particular environments—is evolution: the concept that the organisms living on Earth today are the modified descendants of common ancestors. As a result of descent with modification, two species share certain traits (unity) simply because they have descended from a common ancestor. Furthermore, we can account for differences between two species (diversity) with the idea that certain heritable changes occurred after the two species diverged from their common ancestor. An abundance of evidence of different types supports the occurrence of evolution and the theory that describes how it takes place, which we’ll discuss in detail in Chapters 22–25. To quote one of the founders of modern evolutionary theory, Theodosius Dobzhansky, “Nothing in biology makes sense except in the light of evolution.” To understand Dobzhansky’s statement, we need to discuss how biologists think about the vast diversity of life on the planet.

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Classifying the Diversity of Life

The Three Domains of Life

Diversity is a hallmark of life. Biologists have so far identified and named about 1.8 million species of organisms. Each species is given a two-part name: The first part is the name of the genus (plural, genera) to which the species belongs, and the second part is unique to the species within the genus. (For example, Homo sapiens is the name of our species.) To date, known species include at least 100,000 species of fungi, 290,000 plant species, 57,000 vertebrate species (animals with backbones), and 1 million insect species (more than half of all known forms of life)—not to mention the myriad types of single-celled organisms. Researchers identify thousands of additional species each year. Estimates of the total number of species range from about 10 million to over 100 million. Whatever the actual number, the enormous variety of life gives biology a very broad scope. Biologists face a major challenge in attempting to make sense of this variety.

Historically, scientists have classified the diversity of life-forms into species and broader groupings by careful comparisons of structure, function, and other obvious features. In the last few decades, new methods of assessing species relationships, such as comparisons of DNA sequences, have led to a reevaluation of the classification of life. Although this reevaluation is ongoing, biologists currently divide all organisms into three groups called domains: Bacteria, Archaea, and Eukarya (Figure 1.13). The organisms making up two of the three domains— Bacteria and Archaea—are prokaryotic. All the eukaryotes (organisms with eukaryotic cells) are in domain Eukarya. This domain includes four subgroups: kingdom Plantae, kingdom Fungi, kingdom Animalia, and the protists. The three kingdoms are distinguished partly by their modes of nutrition: Plants produce their own sugars and other food molecules by photosynthesis, fungi absorb nutrients in

Figure 1.13  The three domains of life.

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(b) Domain Archaea

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(a) Domain Bacteria

Bacteria are the most diverse and widespread prokaryotes and are now classified into multiple kingdoms. Each rod-shaped structure in this photo is a bacterial cell.

Domain Archaea includes multiple kingdoms. Some of the prokaryotes known as archaea live in Earth’s extreme environments, such as salty lakes and boiling hot springs. Each round structure in this photo is an archaeal cell.

(c) Domain Eukarya ◀ Kingdom Animalia consists of multicellular eukaryotes that ingest other organisms.

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▲ Kingdom Plantae (land plants) consists of terrestrial multicellular eukaryotes that carry out photosynthesis, the conversion of light energy to the chemical energy in food. ▶ Kingdom Fungi is defined in part by the nutritional mode of its members (such as this mushroom), which absorb nutrients from outside their bodies.

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▶ Protists are mostly unicellular eukaryotes and some relatively simple multicellular relatives. Pictured here is an assortment of protists inhabiting pond water. Scientists are currently debating how to classify protists in a way that accurately reflects their evolutionary relationships.

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dissolved form from their surroundings, and animals obtain food by eating and digesting other organisms. Animalia is, of course, the kingdom to which we belong. The most numerous and diverse eukaryotes are the protists, which are mostly single-celled organisms. Although protists were once placed in a single kingdom, they are now classified into several groups. One major reason for this change is the recent DNA evidence showing that some protists are less closely related to other protists than they are to plants, animals, or fungi.

fossils and other evidence, is the saga of a changing Earth billions of years old, inhabited by an evolving cast of living forms (Figure 1.15). Figure 1.15  Digging into the past. Paleontologists carefully excavate the hind leg of a long-necked dinosaur (Rapetosaurus krausei ) from rocks in Madagascar.

Unity in the Diversity of Life As diverse as life is, it also displays remarkable unity. Consider, for example, the similar skeletons of different animals and the universal genetic language of DNA (the genetic code), both mentioned earlier. In fact, similarities between organisms are evident at all levels of the biological hierarchy. For example, unity is obvious in many features of cell structure, even among distantly related organisms (Figure 1.14). How can we account for life’s dual nature of unity and diversity? The process of evolution, explained next, illuminates both the similarities and differences in the world of life. It also introduces another important dimension of biology: the passage of time. The history of life, as documented by Figure 1.14  An example of unity underlying the diversity of life: the architecture of cilia in eukaryotes. Cilia (singular, cilium) are extensions of cells that function in locomotion. They occur in eukaryotes as diverse as Paramecium (found in pond water) and humans. Even organisms so different share a common architecture for their cilia, which have an elaborate system of tubules that is striking in cross-sectional views.

5 μm Cross section of a cilium, as viewed with an electron microscope

15 μm Cilia of Paramecium. The cilia of the single-celled Paramecium propel the organism through pond water.

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Cilia of windpipe cells. The cells that line the human windpipe are equipped with cilia that help keep the lungs clean by sweeping a film of debris-trapping mucus upward.

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Figure 1.16  Charles Darwin as a young man. His revolutionary book On the Origin of Species was first published in 1859.

Figure 1.17  Unity and diversity among birds. These four birds are variations on a common body plan. For example, each has feathers, a beak, and wings. However, these common features are highly specialized for the birds’ diverse lifestyles. ▼ Red-shouldered hawk

▼ American flamingo ABC News Video: Exploring Evolution in the Solomon Islands

▲ European robin

Charles Darwin and the Theory of Natural Selection An evolutionary view of life came into sharp focus in November 1859, when Charles Darwin published one of the most important and influential books ever written, On the Origin of Species by Means of Natural Selection (Figure 1.16). The Origin of Species articulated two main points. The first point was that contemporary species arose from a succession of ancestors that differed from them. Darwin called this process “descent with modification.” This insightful phrase captured the duality of life’s unity and diversity—unity in the kinship among species that descended from common ancestors and diversity in the modifications that evolved as species branched from their common ancestors (Figure 1.17). Darwin’s second main point was his proposal that “natural selection” is a primary cause of descent with modification. Darwin developed his theory of natural selection from observations that by themselves were neither new nor profound. However, although others had described the pieces of the puzzle, it was Darwin who saw how they fit together. He started with the following three observations from nature: First, individuals in a population vary in their traits, many of which seem to be heritable (passed on from parents to offspring). Second, a population can produce far more offspring than can survive to produce offspring of their own. With more individuals than the environment is able to support, competition is inevitable. Third, species generally are suited to their environments—in other words, they are adapted to their circumstances. For instance, a common adaptation among birds that eat mostly hard seeds is an especially strong beak. By making inferences from these three observations, Darwin arrived at his theory of evolution. He reasoned that individuals 14

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▲ Gentoo penguin

with inherited traits that are better suited to the local environment are more likely to survive and reproduce than less wellsuited individuals. Over many generations, a higher and higher proportion of individuals in a population will have the advantageous traits. Evolution occurs as the unequal reproductive success of individuals ultimately leads to adaptation to their environment, as long as the environment remains the same. Darwin called this mechanism of evolutionary adaptation natural selection because the natural environment consistently “selects” for the propagation of certain traits among naturally occurring variant traits in the population. The example in Figure 1.18 illustrates the ability of natural selection to “edit” a population’s heritable variations in color. We see the products of natural selection in the exquisite adaptations of various organisms to the special circumstances of their way of life and their environment. The wings of the bat shown in Figure 1.19 are an excellent example of adaptation.

Evolution, the Themes of Biology, and Scientific Inquiry

Figure 1.18  Natural selection. This imaginary beetle population has colonized a locale where the soil has been blackened by a recent brush fire. Initially, the population varies extensively in the inherited coloration of the individuals, from very light gray to charcoal. For hungry birds that prey on the beetles, it is easiest to spot the beetles that are lightest in color.

1 Population with varied inherited traits

2 Elimination of individuals with certain traits

3 Reproduction of survivors

DRAW IT Over time, the soil will gradually become lighter in color. Draw another step to show how the soil, when lightened to medium color, would affect natural selection. Write a caption for this new step. Then explain how the population would change over time as the soil becomes lighter.

Figure 1.19  Evolutionary adaptation. Bats, the only mammals capable of active flight, have wings with webbing between extended “fingers.” Darwin proposed that such adaptations are refined over time by natural selection.

The Tree of Life Take another look at the skeletal architecture of the bat’s wings in Figure 1.19. These wings are not like those of feathered birds; the bat is a mammal. The bat’s forelimbs, though adapted for flight, actually have all the same bones, joints, nerves, and blood vessels found in other limbs as diverse as the human arm, the foreleg of a horse, and the flipper of a whale. Indeed, all mammalian forelimbs are anatomical variations of a common architecture. According to the Darwinian concept of descent with modification, the shared anatomy of mammalian limbs reflects inheritance of the limb structure from a common ancestor—the “prototype” mammal from which all other mammals descended. The diversity of mammalian forelimbs results from modification by natural selection operating over millions of years in different environmental contexts. Fossils and other evidence corroborate anatomical unity in supporting this view of mammalian descent from a common ancestor.

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4 Increased frequency of traits that enhance survival and reproductive success

HHMI Video: The Making of The Fittest: Natural Selection and Adaptation (Rock Pocket Mouse)

Darwin proposed that natural selection, by its cumulative effects over long periods of time, could cause an ancestral species to give rise to two or more descendant species. This could occur, for example, if one population fragmented into several subpopulations isolated in different environments. In these separate arenas of natural selection, one species could gradually radiate into multiple species as the geographically isolated populations adapted over many generations to different sets of environmental factors. The Galápagos finches are a famous example of the process of radiation of new species from a common ancestor. Darwin collected specimens of these birds during his 1835 visit to the remote Galápagos Islands, 900 kilometers (km) off the Pacific coast of South America. These relatively young volcanic islands are home to many species of plants and animals found nowhere else in the world, though many Galápagos organisms are clearly related to species on the South American mainland. The Galápagos finches are believed to have descended from an ancestral finch species that reached the archipelago from South America or the Caribbean. Over time, the Galápagos finches diversified from their ancestor as populations became adapted to different food sources on their particular islands. Years after Darwin collected the finches, researchers began to sort out their evolutionary relationships, first from anatomical and geographic data and more recently with the help of DNA sequence comparisons. ABC News Video: Protecting the Galápagos Islands Video: Galápagos Biodiversity by Peter and Rosemary Grant

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Figure 1.20  Descent with modification: adaptive radiation of finches on the Galápagos Islands. This “tree” illustrates a current model for the evolution of finches on the Galápagos. Note the different beaks, which are adapted to different food sources on the different islands. For example, heavier, thicker beaks are better at cracking seeds, while the more slender beaks are better at grasping insects.

Green warbler finch Certhidea olivacea (insect-eater) Vegetarian finch Platyspiza crassirostris (fruit-eater)

ANCESTRAL FINCH

Woodpecker finch Camarhynchus pallidus (insect-eater)

HHMI Video: The Origin of Species: The Beak of the Finch

Small tree finch Camarhynchus parvulus (insect-eater)

Branch point: where lineages diverge from a common ancestor.

Common cactus finch Geospiza scandens (cactus-eater)

This branch point represents the common ancestor of finches in genera Camarhynchus and Geospiza.

as in Figure 1.20. Tree diagrams make sense: Just as an individual has a genealogy that can be diagrammed as a family tree, each species is one twig of a branching tree of life extending back in time through ancestral species more and more remote. Species that are very similar, such as the Galápagos finches, share a common ancestor. Through an ancestor that lived much farther back in time, finches are related to sparrows, hawks, penguins, and all other birds. Furthermore, finches and other birds are related to us through a common ancestor even more ancient. Trace life back far enough, and we reach the early prokaryotes that inhabited Earth over 3.5 billion years ago. We can recognize their vestiges in our own cells—in the universal genetic code, for example. Indeed, all of life is connected through its long evolutionary history.

Concept Check 1.2 1. Explain why “editing” is a metaphor for how natural selection acts on a population’s heritable variation. 2. Referring to Figure 1.20, provide a possible explanation for how, over a very long time, the green warbler finch came to have a slender beak. 3. DRAW IT The three domains you learned about in Concept 1.2 can be represented in the tree of life as the three main branches, with three subbranches on the eukaryotic branch being the kingdoms Plantae, Fungi, and Animalia. What if fungi and animals are more closely related to each other than either of these kingdoms is to plants—as recent evidence strongly suggests? Draw a simple branching pattern that symbolizes the proposed relationship between these three eukaryotic kingdoms. For suggested answers, see Appendix A.

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Large ground finch Geospiza magnirostris (seed-eater)

Concept  1.3 In studying nature, scientists make observations and form and test hypotheses Science is a way of knowing—an approach to understanding the natural world. It developed out of our curiosity about ourselves, other life-forms, our planet, and the universe. The word science is derived from a Latin verb meaning “to know.” Striving to understand seems to be one of our basic urges. At the heart of science is inquiry, a search for information and explanations of natural phenomena. There is no formula for successful scientific inquiry, no single scientific method that researchers must rigidly follow. As in all quests, science includes elements of challenge, adventure, and luck, along with careful planning, reasoning, creativity, patience, and the persistence to overcome setbacks. Such diverse elements of inquiry make science far less structured than most people realize. That said, it is possible to highlight certain characteristics that help to distinguish science from other ways of describing and explaining nature. Scientists use a process of inquiry that includes making observations, forming logical, testable explanations (hypotheses), and testing them. The process is necessarily repetitive: In testing a hypothesis, more observations may inspire revision of the original hypothesis or formation of a new one, thus leading to further testing. In this way,

Evolution, the Themes of Biology, and Scientific Inquiry

scientists circle closer and closer to their best estimation of the laws governing nature.

Figure 1.21  Jane Goodall collecting qualitative data on chimpanzee behavior. Goodall recorded her observations in field notebooks, often with sketches of the animals’ behavior.

Exploration and Observation Our innate curiosity often stimulates us to pose questions about the natural basis for the phenomena we observe in the world. For example, what causes the roots of a plant seedling to grow downward? In fine-tuning their questions, biologists rely heavily on the scientific literature, the published contributions of fellow scientists. By reading about and understanding past studies, scientists can build on the foundation of existing knowledge, focusing their investigations on observations that are original and on hypotheses that are consistent with previous findings. Identifying publications relevant to a new line of research is now easier than at any point in the past, thanks to indexed and searchable electronic databases. In the course of their work, biologists make careful observations. In gathering information, they often use tools such as microscopes, precision thermometers, or high-speed cameras that extend their senses or facilitate careful measurement. Observations can reveal valuable information about the natural world. For example, a series of detailed observations have shaped our understanding of cell structure, and another set of observations is currently expanding our databases of genome sequences from diverse species and databases of genes whose expression is altered in various diseases. Recorded observations are called data. Put another way, data are items of information on which scientific inquiry is based. The term data implies numbers to many people. But some data are qualitative, often in the form of recorded descriptions rather than numerical measurements. For example, Jane Goodall spent decades recording her observations of chimpanzee behavior during field research in a Tanzanian jungle (Figure 1.21). In her studies, Goodall also enriched the field of animal behavior with volumes of quantitative data, such as the frequency and duration of specific behaviors for different members of a group of chimpanzees in a variety of situations. Quantitative data are generally expressed as numerical measurements and often organized into tables and graphs. Scientists analyze their data using a type of mathematics called statistics to test whether their results are significant or merely due to random fluctuations. All results presented in this text have been shown to be statistically significant. Collecting and analyzing observations can lead to important conclusions based on a type of logic called inductive reasoning. Through induction, we derive generalizations from a large number of specific observations. “The sun always rises in the east” is an example. And so is “All organisms are made of cells.” Careful observations and data analyses, along

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Interview with Jane Goodall: Living with chimpanzees

with generalizations reached by induction, are fundamental to our understanding of nature.

Forming and Testing Hypotheses After carrying out preliminary observations and collecting and analyzing data, scientists begin to form tentative answers to their original questions and to test their hypothetical explanations—that is, their hypotheses. In science, a hypothesis is an explanation, based on observations and assumptions, that leads to a testable prediction. Said another way, a hypothesis is an explanation on trial. The hypothesis is usually a rational accounting for a set of observations, based on the available data and guided by inductive reasoning. A scientific hypothesis must lead to predictions that can be tested by making additional observations or by performing experiments. An experiment is a scientific test, carried out under controlled conditions. We all make observations and develop questions and hypotheses in solving everyday problems. Let’s say, for example, that your desk lamp is plugged in and turned on but the bulb isn’t lit. That’s an observation. The question is obvious: Why doesn’t the lamp work? Two reasonable hypotheses based on your experience are that (1) the bulb is not screwed in properly or (2) the bulb is burnt out. Each of these alternative hypotheses leads to predictions you can test with experiments. For example, the improperly screwed-in

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Figure 1.22  A simplified view of the scientific process. The idealized process sometimes called the “scientific method” is shown in this flow chart, which illustrates hypothesis testing for a desk lamp that doesn’t work. Observation: Desk lamp doesn’t work.

Question: Why doesn’t the desk lamp work?

Hypothesis #1: Bulb is improperly screwed in.

Hypothesis #2: Bulb is burnt out.

Prediction: Reinstalling bulb will fix problem.

Prediction: Replacing bulb will fix problem.

Test of prediction: Reinstall bulb.

Test of prediction: Replace bulb.

Result: Desk lamp doesn't work. Hypothesis is not supported.

Result: Lamp works. Hypothesis is supported.

bulb hypothesis predicts that carefully re-installing the bulb will fix the problem. Figure 1.22 diagrams this informal inquiry. Figuring things out in this way by trial and error is a hypothesis-based approach.

Deductive Reasoning A type of logic called deduction is also built into the use of hypotheses in science. While induction entails reasoning from a set of specific observations to reach a general conclusion, deductive reasoning involves logic that flows in the opposite direction, from the general to the specific. From general premises, we extrapolate to the specific results we should expect if the premises are true. In the scientific process, deductions usually take the form of predictions of results that will be found if a particular hypothesis (premise) is correct. We then test the hypothesis by carrying out experiments or observations to see whether or not the results are as predicted. This deductive testing takes the form of “If . . . then” logic. In the case of the desk lamp example: If the burnt-out bulb hypothesis is correct, then the lamp should work if you replace the bulb with a new one. We can use the desk lamp example to illustrate two other key points about the use of hypotheses in science. First, one can always devise additional hypotheses to explain a set of observations. For instance, another hypothesis to explain our nonworking desk lamp is that the electrical socket is

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broken. Although you could design an experiment to test this hypothesis, you can never test all possible hypotheses. Second, we can never prove that a hypothesis is true. Based on the experiments shown in Figure 1.22, the burnt-out bulb hypothesis is the most likely explanation, but testing supports that hypothesis not by proving that it is correct, but rather by failing to prove it incorrect. For example, even if replacing the bulb fixed the desk lamp, it might have been because there was a temporary power outage that just happened to end while the bulb was being changed. Although a hypothesis can never be proved beyond the shadow of a doubt, testing it in various ways can significantly increase our confidence in its validity. Often, rounds of hypothesis formulation and testing lead to a scientific consensus—the shared conclusion of many scientists that a particular hypothesis explains the known data well and stands up to experimental testing.

Questions That Can and Cannot Be Addressed by Science Scientific inquiry is a powerful way to learn about nature, but there are limitations to the kinds of questions it can answer. A scientific hypothesis must be testable; there must be some observation or experiment that could reveal if such an idea is likely to be true or false. The hypothesis that a burnt-out bulb is the sole reason the lamp doesn’t work would not be supported if replacing the bulb with a new one didn’t fix the lamp. Not all hypotheses meet the criteria of science: You wouldn’t be able to test the hypothesis that invisible ghosts are fooling with your desk lamp! Because science only deals with natural, testable explanations for natural phenomena, it can neither support nor contradict the invisible ghost hypothesis, nor whether spirits or elves cause storms, rainbows, or illnesses. Such supernatural explanations are simply outside the bounds of science, as are religious matters, which are issues of personal faith. Science and religion are not mutually exclusive or contradictory; they are simply concerned with different issues.

The Flexibility of the Scientific Process The desk lamp example of Figure 1.22 traces an idealized process of inquiry sometimes called the scientific method. However, very few scientific inquiries adhere rigidly to the sequence of steps that are typically used to describe this approach. For example, a scientist may start to design an experiment, but then backtrack after realizing that more preliminary observations are necessary. In other cases, observations remain too puzzling to prompt well-defined questions until further study provides a new context in which to view those observations. For example, scientists

Evolution, the Themes of Biology, and Scientific Inquiry

could not unravel the details of how genes encode proteins until after the discovery of the structure of DNA (an event that took place in 1953). A more realistic model of the scientific process is shown in Figure 1.23. The focus of this model, shown in the central circle in the figure, is the forming and testing of hypotheses. This core set of activities is the reason that science does so well in explaining phenomena in the natural world. These activities, however, are shaped by exploration and discovery (the upper circle in Figure 1.23) and influenced by interactions with other scientists and with society more generally

EXP LO

• Predicting results • Doing experiments and/or making observations • Gathering data • Analyzing results Interpreting Results Data may… • Support a hypothesis • Contradict a hypothesis • Inspire a revised or new hypothesis

COMM UN

F

SOC IET A

TESTING HY PO AND G TH IN ES Testing Ideas M E R O • Forming hypotheses

LYSIS AN ANA D Y F T I • Feedback and peer review • Replication of findings • Publication

ACK DB EE

• Developing technology • Informing policy • Solving problems • Building knowledge

• Observing nature • Asking questions • Reading the scientific literature

S

ES OM TC

EFITS AND O EN B U L

N AND DIS TIO CO RA

RY VE

Figure 1.23  The process of science: A realistic model. In reality, the process of science is not linear, but is more circular, involving backtracking, repetitions, and interactions of different parts of the process. This illustration is based on a model (How Science Works) from the website Understanding Science (www.understandingscience.org).

(lower circles). For example, the community of scientists influences which hypotheses are tested, how test results are interpreted, and what value is placed on the findings. Similarly, societal needs—such as the push to cure cancer or understand the process of climate change—may help shape what research projects are funded and how extensively the results are discussed. Now that we have highlighted the key features of scientific inquiry—making observations and forming and testing hypotheses—you should be able to recognize these features in a case study of actual scientific research.

• Consensus building

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A Case Study in Scientific Inquiry: Investigating Coat Coloration in Mouse Populations Our case study begins with a set of observations and inductive generalizations. Color patterns of animals vary widely in nature, sometimes even among members of the same species. What accounts for such variation? As you may recall, the two mice depicted at the beginning of this chapter are members of the same species (Peromyscus polionotus), but they have different color patterns and reside in different environments. The beach mouse lives along the Florida seashore, a habitat of brilliant white sand dunes with sparse clumps of beach grass. The inland mouse lives on darker, more fertile soil farther inland (Figure 1.24). Even a brief glance at the photographs in Figure 1.24 reveals a striking match of mouse coloration to its habitat. The natural predators of these mice, including hawks, owls, foxes, and coyotes, are all visual hunters (they use their eyes to look for prey). It was logical, therefore, for Francis Bertody Sumner, a naturalist studying populations of these mice in the 1920s, to form the hypothesis that their coloration patterns had evolved as adaptations that camouflage the mice in their native environments, protecting them from predation. As obvious as the camouflage hypothesis may seem, it still required testing. In 2010, biologist Hopi Hoekstra of Harvard University and a group of her students headed to Florida to test the prediction that mice with coloration that did not match their habitat would be preyed on more heavily than the native, well-matched mice. Figure 1.25 summarizes this field experiment. The researchers built hundreds of models of mice and spraypainted them to resemble either beach or inland mice, so that

the models differed only in their color patterns. The researchers placed equal numbers of these model mice randomly in both habitats and left them overnight. The mouse models resembling the native mice in the habitat were the control group (for instance, light-colored mouse models in the beach habitat), while the mouse models with the non-native coloration were the experimental group (for example, darker models in the beach habitat). The following morning, the team counted and recorded signs of predation events, which ranged from bites and gouge marks on some models to the outright disappearance of others. Judging by the shape of the predators’ bites and the tracks surrounding the experimental sites, the predators appeared to be split fairly evenly between mammals (such as foxes and coyotes) and birds (such as owls, herons, and hawks). For each environment, the researchers then calculated the percentage of predation events that targeted camouflaged models. The results were clear-cut: Camouflaged models showed much lower predation rates than those lacking camouflage in both the beach habitat (where light mice were less vulnerable) and the inland habitat (where dark mice were less vulnerable). The data thus fit the key prediction of the camouflage hypothesis.

Experimental Variables and Controls In carrying out an experiment, a researcher often manipulates one factor in a system and observes the effects of this change. The mouse camouflage experiment described in Figure 1.25 is an example of a controlled experiment, one that is designed to compare an experimental group (the noncamouflaged mice models, in this case) with a control group (the camouflaged models). Both the factor that is manipulated and the factor that is subsequently measured are types of experimental variables—a feature or quantity that varies in

Figure 1.24  Different coloration in beach and inland populations of Peromyscus polionotus. Beach population

Florida

GULF OF MEXICO

Inland population

Beach population

Beach mice living on sparsely vegetated sand dunes along the coast have light tan, dappled fur on their backs that allows them to blend into their surroundings, providing camouflage.

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Inland population

Members of the same species living about 30 km inland have dark fur on their backs, camouflaging them against the dark ground of their habitat.

Evolution, the Themes of Biology, and Scientific Inquiry

Figure 1.25

Inquiry  Does camouflage affect predation rates on two populations of mice? Experiment  Hopi Hoekstra and colleagues tested the hypothesis that coat coloration provides camouflage that protects beach and inland populations of Peromyscus polionotus mice from predation in their habitats. The researchers spray-painted mouse models with light or dark color patterns that matched those of the beach and inland mice and placed models with each of the patterns in both habitats. The next morning, they counted damaged or missing models. Results  For each habitat, the researchers calculated the percentage of attacked models that were camouflaged or non-camouflaged. In both habitats, the models whose pattern did not match their surroundings suffered much higher “predation” than did the camouflaged models.

Percentage of attacked models

Beach habitat

Inland habitat

A common misconception is that the term controlled experiment means that scientists control all features of the experimental environment. But that’s impossible in field research and can be very difficult even in highly regulated laboratory environments. Researchers usually “control” unwanted variables not by eliminating them through environmental regulation, but by canceling out their effects by using control groups. Animation: Introduction to Experimental Design

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Theories in Science

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“It’s just a theory!” Our everyday use of the term theory often implies an untested Light models Dark models Light models Dark models speculation. But the term theory has a different meaning in science. What is a scientific theory, and how is it different from a hypothesis or from mere speculation? Camouflaged Non-camouflaged Non-camouflaged Camouflaged First, a scientific theory is much (control) (experimental) (experimental) (control) broader in scope than a hypothesis. This is a hypothesis: “Coat coloration wellConclusion  The results are consistent with the researchers’ prediction: that mouse models matched to their habitat is an adaptation with camouflage coloration would be preyed on less often than non-camouflaged mouse models. Thus, the experiment supports the camouflage hypothesis. that protects mice from predators.” But this is a theory: “Evolutionary adaptaData from  S. N. Vignieri, J. G. Larson, and H. E. Hoekstra, The selective advantage of crypsis in mice, Evolution 64:2153–2158 (2010). tions arise by natural selection.” This INTERPRET THE DATA The bars indicate the percentage of the attacked models that were either light theory proposes that natural selection or dark. Assume 100 mouse models were attacked in each habitat. For the beach habitat, how many were is the evolutionary mechanism that light models? Dark models? Answer the same questions for the inland habitat. accounts for an enormous variety of Interview with Hopi Hoekstra: Investigating the genetics adaptations, of which coat color in mice and natural selection of mouse coat color is but one example. Second, a theory is general enough to spin off many new, testable hypotheses. For example, an experiment. In our example, the color of the mouse model the theory of natural selection motivated two researchers was the independent variable—the factor being manipuat Princeton University, Peter and Rosemary Grant, to test lated by the researchers. The dependent variable is the factor the specific hypothesis that the beaks of Galápagos finches being measured that is predicted to be affected by the indepenevolve in response to changes in the types of available food. dent variable; in this case, the researchers measured the preda(Their results supported their hypothesis; see the introduction rate in response to variation in color of the mouse model. tion to Chapter 23.) Ideally, the experimental and control groups differ in only one And third, compared to any one hypothesis, a theory is independent variable—in the mouse experiment, color. generally supported by a much greater body of evidence. The Without the control group, the researchers would not theory of natural selection has been supported by a vast quanhave been able to rule out other factors as causes of the more tity of evidence, with more being found every day, and has frequent attacks on the non-camouflaged mice—such as not been contradicted by any scientific data. Those theories different numbers of predators or different temperatures in that become widely adopted in science (such as the theory the different test areas. The clever experimental design left of natural selection and the theory of gravity) explain a great coloration as the only factor that could account for the low diversity of observations and are supported by a vast accumupredation rate on models camouflaged with respect to the lation of evidence. surrounding environment. 0

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In spite of the body of evidence supporting a widely accepted theory, scientists will sometimes modify or even reject theories when new research produces results that don’t fit. For example, biologists once lumped bacteria and archaea together as a kingdom of prokaryotes. When new methods for comparing cells and molecules could be used to test such relationships, the evidence led scientists to reject the theory that bacteria and archaea are members of the same kingdom. If there is “truth” in science, it is at best conditional, based on the weight of available evidence.

Concept Check 1.3 1. What qualitative observation led to the quantitative study in Figure 1.25? 2. Contrast inductive reasoning with deductive reasoning. 3. Why is natural selection called a theory? 4. WHAT IF? In the deserts of New Mexico, the soils are mostly sandy, with occasional regions of black rock derived from lava flows that occurred about 1,000 years ago. Mice are found in both sandy and rocky areas, and owls are known predators. What might you expect about coat color in these two mouse populations? Explain. How would you use this ecosystem to further test the camouflage hypothesis? For suggested answers, see Appendix A.

Concept  1.4 Science benefits from a cooperative approach and diverse viewpoints Movies and cartoons sometimes portray scientists as loners in white lab coats, working in isolated labs. In reality, science is an intensely social activity. Most scientists work in teams, which often include both graduate and undergraduate students. And to succeed in science, it helps to be a good communicator. Research results have no impact until shared with a community of peers through seminars, publications, and websites. And, in fact, research papers aren’t published until they are vetted by colleagues in what is called the “peer review” process. The examples of scientific inquiry described in this book, for instance, have all been published in peer-reviewed journals.

Building on the Work of Others The great scientist Isaac Newton once said: “To explain all nature is too difficult a task for any one man or even for any one age. ’Tis much better to do a little with certainty, and leave the rest for others that come after you. . . .” Anyone who becomes a scientist, driven by curiosity about how nature works, is sure to benefit greatly from the rich storehouse of discoveries by others who have come before. In fact, Hopi Hoekstra’s experiment benefited from the work of another

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researcher, D. W. Kaufman, 40 years earlier. You can study the design of Kaufman’s experiment and interpret the results in the Scientific Skills Exercise. Scientific results are continually scrutinized through the repetition of observations and experiments. Scientists working in the same research field often check one another’s claims by attempting to confirm observations or repeat experiments. If scientific colleagues cannot repeat experimental findings, this failure may reflect some underlying weakness in the original claim, which will then have to be revised. In this sense, science polices itself. Integrity and adherence to high professional standards in reporting results are central to the scientific endeavor, since the validity of experimental data is key to designing further lines of inquiry. It is not unusual for several scientists to converge on the same research question. Some scientists enjoy the challenge of being first with an important discovery or key experiment, while others derive more satisfaction from cooperating with fellow scientists working on the same problem. Cooperation is facilitated when scientists use the same organism. Often it is a widely used model organism—a species that is easy to grow in the lab and lends itself particularly well to the questions being investigated. Because all species are evolutionarily related, such an organism may be viewed as a model for understanding the biology of other species and their diseases. For example, genetic studies of the fruit fly Drosophila melanogaster have taught us a lot about how genes work in other species, even humans. Some other popular model organisms are the mustard plant Arabidopsis thaliana, the soil worm Caenorhabditis elegans, the zebrafish Danio rerio, the mouse Mus musculus, and the bacterium Escherichia coli. As you read through this book, note the many contributions that these and other model organisms have made to the study of life. Biologists may approach interesting questions from different angles. Some biologists focus on ecosystems, while others study natural phenomena at the level of organisms or cells. This text is divided into units that look at biology at different levels and investigate problems through different approaches. Yet any given problem can be addressed from many perspectives, which in fact complement each other. For example, Hoekstra’s work uncovered at least one genetic mutation that underlies the differences between beach and inland mouse coloration. Her lab includes biologists specializing in different biological levels, allowing links to be made between the evolutionary adaptations she focuses on and their molecular basis in DNA sequences. As a biology student, you can benefit from making connections between the different levels of biology. You can develop this skill by noticing when certain topics crop up again and again in different units. One such topic is sickle-cell disease, a well-understood genetic condition that is prevalent among native inhabitants of Africa and other warm regions and their

Evolution, the Themes of Biology, and Scientific Inquiry

Scientific Skills Exercise

Interpreting a Pair of Bar Graphs

Interpret The Data

How Much Does Camouflage Affect Predation on Mice by Owls With and Without Moonlight?  D. W. Kaufman hypothesized that the extent to which the coat color of a mouse contrasted with the color of its surroundings would affect the rate of nighttime predation by owls. He also hypothesized that the contrast would be affected by the amount of moonlight. In this exercise, you will analyze data from his studies of owl predation on mice that tested these hypotheses.

How the Experiment Was Done  Pairs of mice (Peromyscus polionotus) with different coat colors, one light brown and one dark brown, were released simultaneously into an enclosure that contained a hungry owl. The researcher recorded the color of the mouse that was first caught by the owl. If the owl did not catch either mouse within 15 minutes, the test was recorded as a zero. The release trials were repeated multiple times in enclosures with either a dark-colored soil surface or a light-colored soil surface. The presence or absence of moonlight during each assay was recorded.

40

Light coat

40

Light coat

35

Dark coat

35

Dark coat

30 25 20 15 10 5

Number of mice caught

Number of mice caught

Data from the Experiment

30 25 20 15 10 5

0 Full moon No moon A: Light-colored soil

0 Full moon No moon B: Dark-colored soil

Data from  D. W. Kaufman, Adaptive coloration in Peromyscus polionotus: Experimental selection by owls, Journal of Mammalogy 55:271–283 (1974).

descendants. Sickle-cell disease will appear in several units of the text, each time addressed at a new level. In addition, we have designed a number of figures that make connections between the content in different chapters, as well as questions that ask you to make the connections yourselves. We hope these features will help you integrate the material you’re learning and enhance your enjoyment of biology by encouraging you to keep the big picture in mind.

Science, Technology, and Society The research community is part of society at large, and the relationship of science to society becomes clearer when we add technology to the picture (see Figure 1.23). Though

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1. First, make sure you understand how the graphs are set up. Graph A shows data from the light-colored soil enclosure and graph B from the darkcolored enclosure, but in all other respects the graphs are the same. (a) There is more than one independent variable in these graphs. What are the independent variables, the variables that were tested by the researcher? Which axis of the graphs has the independent variables? (b) What is the dependent variable, the response to the variables being tested? Which axis of the graphs has the dependent variable? 2. (a) How many dark brown mice were caught in the light-colored soil enclosure on a moonlit night? (b) How many dark brown mice were caught in the dark-colored soil enclosure on a moonlit night? (c) On a moonlit night, would a dark brown mouse be more likely to escape predation by owls on dark- or light-colored soil? Explain your answer. 3. (a) Is a dark brown mouse on dark-colored soil more likely to escape predation under a full moon or with no moon? (b) What about a light brown mouse on light-colored soil? Explain. 4. (a) Under which conditions would a dark brown mouse be most likely to escape predation at night? (b) A light brown mouse? 5. (a) What combination of independent variables led to the highest predation level in enclosures with light-colored soil? (b) What combination of independent variables led to the highest predation level in enclosures with dark-colored soil? 6. Thinking about your answers to question 5, provide a simple statement describing conditions that are especially deadly for either color of mouse. 7. Combining the data from both graphs, estimate the number of mice caught in moonlight versus no-moonlight conditions. Which condition is optimal for predation by the owl? Explain. Instructors: A version of this Scientific Skills Exercise can be assigned in MasteringBiology.

science and technology sometimes employ similar inquiry patterns, their basic goals differ. The goal of science is to understand natural phenomena, while that of technology is to apply scientific knowledge for some specific purpose. Biologists and other scientists usually speak of “discoveries,” while engineers and other technologists more often speak of “inventions.” Because scientists put new technology to work in their research, science and technology are interdependent. The potent combination of science and technology can have dramatic effects on society. Sometimes, the applications of basic research that turn out to be the most beneficial come out of the blue, from completely unanticipated observations in the course of scientific exploration. For example, discovery of the structure of DNA by Watson and

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Figure 1.26  DNA technology and forensics. In 2011, forensic analysis of DNA samples from a crime scene led to the release of Michael Morton from prison after he had served nearly 25 years for a crime he didn’t commit, the brutal murder of his wife. The DNA analysis linked another man, also charged in a second murder, to the crime. The photo shows Mr. Morton hugging his parents after his conviction was overturned. The details of forensic analysis of DNA will be described in Chapter 20.

Crick 60 years ago and subsequent achievements in DNA science led to the technologies of DNA manipulation that are transforming applied fields such as medicine, agriculture, and forensics (Figure 1.26). Perhaps Watson and Crick envisioned that their discovery would someday lead to important applications, but it is unlikely that they could have predicted exactly what all those applications would be. The directions that technology takes depend less on the curiosity that drives basic science than on the current needs and wants of people and on the social environment of the times. Debates about technology center more on “should we do it” than “can we do it.” With advances in technology come difficult choices. For example, under what circumstances is it acceptable to use DNA technology to find out if particular people have genes for hereditary diseases? Should such tests always be voluntary, or are there circumstances when genetic testing should be mandatory? Should insurance companies or employers have access to the information, as they do for many other types of personal health data? These questions are becoming much more urgent as the sequencing of individual genomes becomes quicker and cheaper. Ethical issues raised by such questions have as much to do with politics, economics, and cultural values as with science and technology. All citizens—not only professional scientists—have a responsibility to be informed about how science works and about the potential benefits and risks of technology. The relationship between science, technology, and society increases the significance and value of any biology course. 24

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The Value of Diverse Viewpoints in Science Many of the technological innovations with the most profound impact on human society originated in settlements along trade routes, where a rich mix of different cultures ignited new ideas. For example, the printing press, which helped spread knowledge to all social classes, was invented by the German Johannes Gutenberg around 1440. This invention relied on several innovations from China, including paper and ink. Paper traveled along trade routes from China to Baghdad, where technology was developed for its mass production. This technology then migrated to Europe, as did water-based ink from China, which was modified by Gutenberg to become oil-based ink. We have the cross-fertilization of diverse cultures to thank for the printing press, and the same can be said for other important inventions. Along similar lines, science stands to gain much from embracing a diversity of backgrounds and viewpoints among its practitioners. But just how diverse a population are scientists in relation to gender, race, ethnicity, and other attributes? The scientific community reflects the cultural standards and behaviors of the society around it. It is therefore not surprising that until recently, women and certain minorities have faced huge obstacles in their pursuit to become professional scientists in many countries around the world. Over the past 50 years, changing attitudes about career choices have increased the proportion of women in biology and some other sciences, so that now women constitute roughly half of undergraduate biology majors and biology Ph.D. students. The pace has been slow at higher levels in the profession, however, and women and many racial and ethnic groups are still significantly underrepresented in many branches of science and technology. This lack of diversity hampers the progress of science. The more voices that are heard at the table, the more robust, valuable, and productive the scientific interchange will be. The authors of this text welcome all students to the community of biologists, wishing you the joys and satisfactions of this exciting field of science.

Concept Check 1.4 1. How does science differ from technology? 2. MAKE CONNECTIONS The gene that causes sickle-cell disease is present in a higher percentage of residents of sub-Saharan Africa than among those of African descent living in the United States. Even though this gene causes sickle-cell disease, it also provides some protection from malaria, a serious disease that is widespread in sub-Saharan Africa but absent in the United States. Discuss an evolutionary process that could account for the different percentages of the sickle-cell gene among residents of the two regions. (See Concept 1.2.)

Evolution, the Themes of Biology, and Scientific Inquiry

For suggested answers, see Appendix A.

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Chapter Review

Go to MasteringBiology™ for Videos, Animations, Vocab Self-Quiz, Practice Tests, and more in the Study Area.

Summary of Key Concepts Concept  1.1

The study of life reveals unifying themes (pp. 4–11)

Organisms interact continuously with physical factors. Plants take up nutrients from the soil and chemicals from the air and use energy from the sun.

? VOCAB

Organization Theme: New Properties Emerge SELF-QUIZ goo.gl/6u55ks at Successive Levels of Biological Organization The hierarchy of life unfolds as follows: biosphere 7 ecosystem 7 community 7 population 7 organism 7 organ system 7 organ 7 tissue 7 cell 7 organelle 7 molecule 7 atom. With each step upward from atoms, new emergent properties result from interactions among components at the lower levels. In an approach called reductionism, complex systems are broken down to simpler components that are more manageable to study. In systems biology, scientists attempt to model the dynamic behavior of whole biological systems by studying the interactions among the system’s parts. Structure and function are correlated at all levels of biological organization. The cell, an organism’s basic unit of structure and function, is the lowest level that can perform all activities required for life. Cells are either prokaryotic or eukaryotic. Eukaryotic cells contain membrane-enclosed organelles, including a DNAcontaining nucleus. Prokaryotic cells lack such organelles.

Thinking about the muscles and nerves in your hand, how does the activity of text messaging reflect the four themes of biology described in this section?

Concept  1.2

The Core Theme: Evolution accounts for the unity and diversity of life (pp. 11–16)





Information Theme: Life’s Processes Involve the Expression and Transmission of Genetic Information Genetic information is encoded in the nucleotide sequences of DNA. It is DNA that transmits heritable information from parents to offspring. DNA sequences (called genes) program a cell’s protein production by being transcribed into mRNA and then translated into specific proteins, a process called gene expression. Gene expression also produces RNAs that are not translated into protein but serve other important functions. Genomics is the large-scale analysis of the DNA sequences of a species (its genome) as well as the comparison of genomes between species. Bioinformatics uses computational tools to deal with huge volumes of sequence data.

Evolution, the process of change that has transformed life on Earth, accounts for the unity and diversity of life. It also explains evolutionary adaptation—the match of organisms to their environments. Biologists classify species according to a system of broader and broader groups. Domain Bacteria and domain Archaea consist of prokaryotes. Domain Eukarya, the eukaryotes, includes various groups of protists and the kingdoms Plantae, Fungi, and Animalia. As diverse as life is, there is also evidence of remarkable unity, revealed in the similarities between different species. Darwin proposed natural selection as the mechanism for evolutionary adaptation of populations to their environments. Natural selection is the evolutionary process that occurs when a population is exposed to environmental factors that consistently cause individuals with certain heritable traits to have greater reproductive success than do individuals with other heritable traits.

Population of organisms

Hereditary variations

Overproduction of offspring and competition Environmental factors

Differences in reproductive success of individuals

Energy and Matter Theme: Life Requires the Transfer and Transformation of Energy and Matter Energy flows through an ecoENERGY FLOW system. All organisms must AL CYC perform work, which requires EMIC LIN CH energy. Producers convert energy from sunlight to chemical energy, some of which is then passed on to consumers. (The rest is lost from the ecosystem as heat.) Chemicals cycle between organisms and the environment.

Evolution of adaptations in the population

G

Each species is one twig of a branching tree of life extending back in time through more and more remote ancestral species. All of life is connected through its long evolutionary history.

?

How could natural selection have led to the evolution of adaptations such as camouflaging coat color in beach mice?

Concept  1.3

Interactions Theme: From Molecules to Ecosystems, Interactions Are Important in Biological Systems In feedback regulation, a process is regulated by its output or end product. In negative feedback, accumulation of the end product slows its production. In positive feedback, an end product speeds up its own production. chapter 1

In studying nature, scientists make observations and form and test hypotheses (pp. 16–22)

In scientific inquiry, scientists make and record observations (collect data) and use inductive reasoning to draw a general conclusion, which can be developed into a testable hypothesis. Deductive reasoning makes predictions that can be used to

Evolution, the Themes of Biology, and Scientific Inquiry

25





test hypotheses. Hypotheses must be testable; science can address neither the possibility of supernatural phenomena nor the validity of religious beliefs. Hypotheses can be tested by conducting experiments or, when that is not possible, by making observations. In the process of science, the core activity is testing ideas. This endeavor is influenced by exploration and discovery, community analysis and feedback, and societal outcomes. Controlled experiments, such as the investigation of coat color in mouse populations, are designed to demonstrate the effect of one variable by testing control groups and experimental groups that differ in only that one variable. A scientific theory is broad in scope, generates new hypotheses, and is supported by a large body of evidence.

?

What are the roles of gathering and interpreting data in the process of scientific inquiry?

Concept  1.4

Science benefits from a cooperative approach and diverse viewpoints (pp. 22–24)





Science is a social activity. The work of each scientist builds on the work of others who have come before. Scientists must be able to repeat each other’s results, and integrity is key. Biologists approach questions at different levels; their approaches complement each other. Technology consists of any method or device that applies scientific knowledge for some specific purpose that affects society. The impact of basic research is not always immediately obvious. Diversity among scientists promotes progress in science.

?

Explain why different approaches and diverse backgrounds among scientists are important.

6. Which of the following is an example of qualitative data? (A) The fish swam in a zigzag motion. (B) The contents of the stomach are mixed every 20 seconds. (C) The temperature decreased from 20°C to 15°C. (D) The six pairs of robins hatched an average of three chicks each. 7. Which sentence best describes the logic of scientific inquiry? (A) If I generate a testable hypothesis, tests and observations will support it. (B) If my prediction is correct, it will lead to a testable hypothesis. (C) If my observations are accurate, they will support my hypothesis. (D) If my hypothesis is correct, I can expect certain test results. 8. DRAW IT  With rough sketches, draw a biological hierarchy similar to the one in Figure 1.3 but using a coral reef as the ecosystem, a fish as the organism, its stomach as the organ, and DNA as the molecule. Include all levels in the hierarchy.

Level 3: Synthesis/Evaluation 9. EVOLUTION CONNECTION  A typical prokaryotic cell has about 3,000 genes in its DNA, while a human cell has almost 21,000 genes. About 1,000 of these genes are present in both types of cells. Based on your understanding of evolution, explain how such different organisms could have this same subset of 1,000 genes. What sorts of functions might these shared genes have? 10. SCIENTIFIC INQUIRY  Based on the results of the mouse coloration case study, suggest another hypothesis researchers might use to further study the role of predators in the natural selection process. 11. SCIENTIFIC INQUIRY  Scientists search the scientific literature by means of electronic databases such as PubMed, a free online database maintained by the National Center for Biotechnology Information. Use PubMed to find the abstract of a scientific article that Hopi Hoekstra published in 2015 or later.

Test Your Understanding Level 1: Knowledge/Comprehension 1. All the organisms on your campus make up (A) an ecosystem. (C) a population. (B) a community. (D) a taxonomic domain.

Level 2: Application/Analysis

PRACTICE TEST

goo.gl/CUYGKD

2. Systems biology is mainly an attempt to (A) analyze genomes from different species. (B) simplify complex problems by reducing the system into smaller, less complex units. (C) understand the behavior of entire biological systems by studying interactions among its component parts. (D) build high-throughput machines for the rapid acquisition of biological data.

12. WRITE ABOUT A THEME: EVOLUTION  In a short essay (100–150 words), discuss Darwin’s view of how natural selection resulted in both unity and diversity of life on Earth. Include in your discussion some of his evidence. (See a suggested grading rubric and tips for writing good essays in the Study Area of MasteringBiology under “Write About a Theme.”) 13. SYNTHESIZE YOUR KNOWLEDGE Can you pick out the mossy leaf-tailed gecko lying against the tree trunk in this photo? How is the appearance of the gecko a benefit in terms of survival? Given what you learned about evolution, natural selection, and genetic information in this chapter, describe how the gecko’s coloration might have evolved.

3. Which of the following best demonstrates the unity among all organisms? (A) emergent properties (C) the structure and function (B) descent with of DNA modification (D) natural selection 4. A controlled experiment is one that (A) proceeds slowly enough that a scientist can make careful records of the results. (B) tests experimental and control groups in parallel. (C) is repeated many times to make sure the results are accurate. (D) keeps all variables constant. 5. Which of the following statements best distinguishes hypotheses from theories in science? (A) Theories are hypotheses that have been proved. (B) Hypotheses are guesses; theories are correct answers. (C) Hypotheses usually are relatively narrow in scope; theories have broad explanatory power. (D) Theories are proved true; hypotheses are often contradicted by experimental results.

26

chapter 1

For selected answers, see Appendix A.

Evolution, the Themes of Biology, and Scientific Inquiry

For additional practice questions, check out the Dynamic Study Modules in MasteringBiology. You can use them to study on your smartphone, tablet, or computer anytime, anywhere!

UNIT 1 Born in Baton Rouge, Louisiana, Lovell Jones was one of the first students to integrate the schools in that city. He went on to major in biology at California State University at Hayward (now CSU East Bay). He then earned a Ph.D. in Zoology with an emphasis in endocrinology and tumor biology from the University of California, Berkeley, in 1977. As a postdoctoral fellow at the University of California, San Francisco, he carried out research on the effects of estrogen on breast cancer. In 1980, after his post-doc, he moved to the MD Anderson Cancer Center as an assistant professor in the Department of Gynecology and Biochemistry, where he remained until his retirement in 2013. In addition to making major contributions to our understanding of the role of steroid hormones in reproductive cancers, Dr. Jones is considered a pioneer in addressing health inequities and social justice. He has continued the latter work during his retirement. During his long, distinguished career, Dr. Jones has received numerous awards and has been commended in the U.S. House of Representatives for his lifelong work.

“Studying biochemistry in a holistic way is going to be the key to the answers we need.” Dr. Lovell Jones in his lab, where he has studied the relationship between hormones and cancers.

THE CHEMISTRY OF LIFE An Interview with Lovell Jones What did you study in graduate school?

I got my Ph.D. at UC Berkeley, working with Howard Bern on the effects of hormones on cervical and vaginal development. I developed an interest in the effects of an artificial estrogen called diethylstilbestrol (DES), a medication used to stop spontaneous abortion in pregnant women. Women that used DES during their first trimester gave birth to girls who had an increased incidence of a rare form of cervical cancer at a very early age, as young as 7. At that time, it was thought that DES was acting as a chemical to initiate cancer, but I thought it was a hormonal action because it was mimicking estrogen, and I wanted to test that using mice. By exposing mice to natural estrogen when their cervical and vaginal tracts were developing, I showed that this exposure produced related types of tumors.

How did you continue to work on the relationship between hormones and cancer? After I got my Ph.D., I worked with Finn Siiteri at UCSF, who was an expert on endocrinerelated hormones but had no cancer knowledge. He brought me on because I had shown that hormones could initiate cancer. I started to work on finding a way to measure estrogen levels as a risk factor for developing breast cancer. I found that the level of free estrogen—estrogen that isn’t bound to other molecules—was the risk factor because only free estrogen was able to function. At this point, I was recruited to MD Anderson Cancer Center to develop hormonal therapies using agents that blocked estrogens. We developed treatments for lung cancer, head and neck cancers, and salivary cancers. At the same time, I was looking at long-term effects of estrogens on neonatal development and developing a method to use hormone levels to predict breast cancer risk.

Why study biochemistry, and what are the most exciting avenues of biochemical research? Studying biochemistry in a holistic way is going to be the key to the answers we need. We have to get out of our silos and do what some people call trans-disciplinary research. As far as future research goes, the most exciting area is precision medicine, an approach to treatment that considers a person’s genes, environment, and lifestyle. We can now get away from looking at individuals in terms of their race, which is not a biological construct, and instead look at them as members of populations: subgroups that have a particular history together. For example, one of the highest incidences of pre-menopausal breast cancer is among blacks and whites in the Chesapeake Bay area. Why? Well, most white females who came into Maryland during colonial times were Scotch–Irish Catholics who arrived as indentured servants, a higher form of slaves. Roughly a third of them had interactions with African slaves. So a lot of people in that area have genes that came from Africa, and some people who look white have black ancestry. A disease called triple-negative breast cancer, where tumor cells are missing three types of receptors, is spoken of as a “black cancer.” But it’s really just that a group of genes that arose in one place in Africa leads to early onset of breast cancer. Also, triple-negative breast cancer is a disease of the young, not of black females in particular. So that’s why I say precision medicine will need to analyze all of an individual’s genes and consider their evolutionary history rather than just assigning them to one race or another.

What is your advice to an undergraduate considering a career in biology? Find a good mentor. Some of the greatest mentors put their students ahead of themselves. Howard Bern, my Ph.D. advisor, said that the greatest contribution to science is not your papers or grants, but the people you leave behind. 27

Superset

The Chemical Context of Life

2

Figure 2.1 What weapon are these wood ants shooting into the air?

KEY CONCEPTS

A Chemical Connection to Biology

2.1

Matter consists of chemical elements in pure form and in combinations called compounds

2.2

An element’s properties depend on the structure of its atoms

2.3

The formation and function of molecules depend on chemical bonding between atoms

2.4

Chemical reactions make and break chemical bonds

Like other animals, ants have structures and mechanisms that defend them from attack. Wood ants live in colonies of hundreds or thousands, and the colony as a whole has a particularly effective way of dealing with enemies. When threatened from above, the ants shoot volleys of formic acid into the air from their abdomens, and the acid bombards the potential predator, such as a hungry bird (Figure 2.1). Formic acid is produced by many species of ants and in fact got its name from the Latin word for ant, formica. For quite a few ant species, the formic acid isn’t shot out, but probably serves as a disinfectant that protects the ants against microbial parasites. Scientists have long known that chemicals play a major role in insect communication, the attraction of mates, and defense against predators. Research on ants and other insects is a good example of how relevant chemistry is to the study of life. Unlike college courses, nature is not neatly packaged into individual sciences—biology, chemistry, physics, and so forth. Biologists specialize in the study of life, but organisms and their environments are natural systems to which the concepts of chemistry and physics apply. Biology is multidisciplinary. This unit of chapters introduces some basic concepts of chemistry that apply to the study of life. Somewhere in the transition from molecules to cells, we will cross the blurry boundary between nonlife and life. This chapter focuses on the chemical components that make up all matter. When you see this blue icon, log in to MasteringBiology and go to the Study Area for digital resources. Interview with Deborah Gordon: Investigating ant behavior

28

Get Ready for This Chapter

CONCEPT

2.1

The Elements of Life

Matter consists of chemical elements in pure form and in combinations called compounds Organisms are composed of matter, which is anything that takes up space and has mass.* Matter exists in many forms. Rocks, metals, oils, gases, and living organisms are a few examples of what seems to be an endless assortment of matter.

Elements and Compounds Matter is made up of elements. An element is a substance that cannot be broken down to other substances by chemical reactions. Today, chemists recognize 92 elements occurring in nature; gold, copper, carbon, and oxygen are examples. Each element has a symbol, usually the first letter or two of its name. Some symbols are derived from Latin or German; for instance, the symbol for sodium is Na, from the Latin word natrium. A compound is a substance consisting of two or more different elements combined in a fixed ratio. Table salt, for example, is sodium chloride (NaCl), a compound composed of the elements sodium (Na) and chlorine (Cl) in a 1:1 ratio. Pure sodium is a metal, and pure chlorine is a poisonous gas. When chemically combined, however, sodium and chlorine form an edible compound. Water (H2O), another compound, consists of the elements hydrogen (H) and oxygen (O) in a 2:1 ratio. These are simple examples of organized matter having emergent properties: A compound has characteristics different from those of its elements (Figure 2.2).

Figure 2.2 The emergent properties of a compound. The metal sodium combines with the poisonous gas chlorine, forming the edible compound sodium chloride, or table salt.

+

Na Sodium

+

Cl Chlorine

NaCl Sodium chloride

Of the 92 natural elements, about 20–25% are essential elements that an organism needs to live a healthy life and reproduce. The essential elements are similar among organisms, but there is some variation—for example, humans need 25 elements, but plants need only 17. Just four elements—oxygen (O), carbon (C), hydrogen (H), and nitrogen (N)—make up approximately 96% of living matter. Calcium (Ca), phosphorus (P), potassium (K), sulfur (S), and a few other elements account for most of the remaining 4% or so of an organism’s mass. Trace elements are required by an organism in only minute quantities. Some trace elements, such as iron (Fe), are needed by all forms of life; others are required only by certain species. For example, in vertebrates (animals with backbones), the element iodine (I) is an essential ingredient of a hormone produced by the thyroid gland. A daily intake of only 0.15 milligram (mg) of iodine is adequate for normal activity of the human thyroid. An iodine deficiency in the diet causes the thyroid gland to grow to abnormal size, a condition called goiter. Consuming seafood or iodized salt reduces the incidence of goiter. Relative amounts of all the elements in the human body are listed in Table 2.1. Some naturally occurring elements are toxic to organisms. In humans, for instance, the element arsenic has been linked to numerous diseases and can be lethal. In some areas of the world, arsenic occurs naturally and can make its way into the groundwater. As a result of using water from drilled wells in southern Asia, millions of people have been inadvertently exposed to arsenic-laden water. Efforts are under way to reduce arsenic levels in their water supply.

Table 2.1

Elements in the Human Body Symbol

Oxygen

O

65.0%

Carbon

C

18.5%

Hydrogen

H

9.5%

Nitrogen

N

3.3%

Calcium

Ca

1.5%

Phosphorus

P

1.0%

Potassium

K

0.4%

Sulfur

S

0.3%

Sodium

Na

0.2%

Chlorine

Cl

0.2%

Mg

0.1%

Magnesium *In everyday language we tend to substitute the term weight for mass, although the two are not identical. Mass is the amount of matter in an object, whereas the weight of an object is how strongly that mass is pulled by gravity. The weight of an astronaut walking on the moon is approximately 1 ⁄ 6 the astronaut’s weight on Earth, but his or her mass is the same. However, as long as we are earthbound, the weight of an object is a measure of its mass; in everyday language, therefore, we tend to use the terms interchangeably.

Percentage of Body Mass (including water)

Element

u 96.3%

u 3.7%

Trace elements (less than 0.01% of mass): Boron (B), chromium (Cr), cobalt (Co), copper (Cu), fluorine (F), iodine (I), iron (Fe), manganese (Mn), molybdenum (Mo), selenium (Se), silicon (Si), tin (Sn), vanadium (V), zinc (Zn)

INTERPRET THE DATA Given the makeup of the human body, what compound do you think accounts for the high percentage of oxygen?

CHAPTER 2

The Chemical Context of Life

29

Figure 2.3 Serpentine plant community. These plants are growing on serpentine soil, which contains elements that are usually toxic to plants. The insets show a close-up of serpentine rock and one of the plants, a Tiburon Mariposa lily (Calochortus tiburonensis). This specially-adapted species is found only on this one hill in Tiburon, a peninsula that juts into San Francisco Bay.

CONCEPT

2.2

An element’s properties depend on the structure of its atoms Each element consists of a certain type of atom that is different from the atoms of any other element. An atom is the smallest unit of matter that still retains the properties of an element. Atoms are so small that it would take about a million of them to stretch across the period printed at the end of this sentence. We symbolize atoms with the same abbreviation used for the element that is made up of those atoms. For example, the symbol C stands for both the element carbon and a single carbon atom.

Subatomic Particles

Case Study: Evolution of Tolerance to Toxic Elements EVOLUTION Some species have become adapted to environments containing elements that are usually toxic; an example is serpentine plant communities. Serpentine is a jade-like mineral that contains elevated concentrations of elements such as chromium, nickel, and cobalt. Although most plants cannot survive in soil that forms from serpentine rock, a small number of plant species have adaptations that allow them to do so (Figure 2.3). Presumably, variants of ancestral, nonserpentine species arose that could survive in serpentine soils, and subsequent natural selection resulted in the distinctive array of species we see in these areas today. Researchers are studying whether serpentine-adapted plants could be used to take up toxic heavy metals in contaminated areas, concentrating them for safer disposal.

Although the atom is the smallest unit having the properties of an element, these tiny bits of matter are composed of even smaller parts, called subatomic particles. Using high-energy collisions, physicists have produced more than 100 types of particles from the atom, but only three kinds of particles are relevant here: neutrons, protons, and electrons. Protons and electrons are electrically charged. Each proton has one unit of positive charge, and each electron has one unit of negative charge. A neutron, as its name implies, is electrically neutral. Protons and neutrons are packed together tightly in a dense core, or atomic nucleus, at the center of an atom; protons give the nucleus a positive charge. The rapidly moving electrons form a “cloud” of negative charge around the nucleus, and it is the attraction between opposite charges that keeps the electrons in the vicinity of the nucleus. Figure 2.4 shows

Figure 2.4 Simplified models of a helium (He) atom. The helium nucleus consists of 2 neutrons (brown) and 2 protons (pink). Two electrons (yellow) exist outside the nucleus. These models are not to scale; they greatly overestimate the size of the nucleus in relation to the electron cloud. Cloud of negative charge (2 electrons)

Electrons Nucleus

CONCEPT CHECK 2.1





1. MAKE CONNECTIONS Explain how table salt has emergent properties. (See Concept 1.1.)

+

+

2. Is a trace element an essential element? Explain.

+

+

(a) This model represents the two electrons as a cloud of negative charge, a result of their motion around the nucleus.

(b) In this more simplified model, the electrons are shown as two small yellow spheres on a circle around the nucleus.

3. WHAT IF? In humans, iron is a trace element required for the proper functioning of hemoglobin, the molecule that carries oxygen in red blood cells. What might be the effects of an iron deficiency? 4. MAKE CONNECTIONS Explain how natural selection might have played a role in the evolution of species that are tolerant of serpentine soils. (Review Concept 1.2.) For suggested answers, see Appendix A.

30

UNIT ONE

The Chemistry of Life

two commonly used models of the structure of the helium atom as an example. The neutron and proton are almost identical in mass, each about 1.7 * 10-24 gram (g). Grams and other conventional units are not very useful for describing the mass of objects that are so minuscule. Thus, for atoms and subatomic particles (and for molecules, too), we use a unit of measurement called the dalton, in honor of John Dalton, the British scientist who helped develop atomic theory around 1800. (The dalton is the same as the atomic mass unit, or amu, a unit you may have encountered elsewhere.) Neutrons and protons have masses close to 1 dalton. Because the mass of an electron is only about 1/2,000 that of a neutron or proton, we can ignore electrons when computing the total mass of an atom.

Atomic Number and Atomic Mass Atoms of the various elements differ in their number of subatomic particles. All atoms of a particular element have the same number of protons in their nuclei. This number of protons, which is unique to that element, is called the atomic number and is written as a subscript to the left of the symbol for the element. The abbreviation 2He, for example, tells us that an atom of the element helium has 2 protons in its nucleus. Unless otherwise indicated, an atom is neutral in electrical charge, which means that its protons must be balanced by an equal number of electrons. Therefore, the atomic number tells us the number of protons and also the number of electrons in an electrically neutral atom. We can deduce the number of neutrons from a second quantity, the mass number, which is the total number of protons and neutrons in the nucleus of an atom. The mass number is written as a superscript to the left of an element’s symbol. For example, we can use this shorthand to write an atom of helium as 42He. Because the atomic number indicates how many protons there are, we can determine the number of neutrons by subtracting the atomic number from the mass number. In our example, the helium atom 42He has 2 neutrons. For sodium (Na):

Animation: Atomic Number and Atomic Mass

Isotopes All atoms of a given element have the same number of protons, but some atoms have more neutrons than other atoms of the same element and therefore have greater mass. These different atomic forms of the same element are called isotopes of the element. In nature, an element may occur as a mixture of its isotopes. As an example, the element carbon, which has the atomic number 6, has three naturally occurring isotopes. The most common isotope is carbon-12, 126C, which accounts for about 99% of the carbon in nature. The isotope 126C has 6 neutrons. Most of the remaining 1% of carbon consists of atoms of the isotope 136C, with 7 neutrons. A third, even rarer isotope, 146C, has 8 neutrons. Notice that all three isotopes of carbon have 6 protons; otherwise, they would not be carbon. Although the isotopes of an element have slightly different masses, they behave identically in chemical reactions. (For an element with more than one naturally occurring isotope, the atomic mass is an average of those isotopes, weighted by their abundance. Thus carbon has an atomic mass of 12.01 daltons.) Both 12C and 13C are stable isotopes, meaning that their nuclei do not have a tendency to lose subatomic particles, a process called decay. The isotope 14C, however, is unstable, or radioactive. A radioactive isotope is one in which the nucleus decays spontaneously, giving off particles and energy. When the radioactive decay leads to a change in the number of protons, it transforms the atom to an atom of a different element. For example, when an atom of carbon-14 (14C) decays, it loses a proton, becoming an atom of nitrogen (14N). Radioactive isotopes have many useful applications in biology.

æ

æ

Mass number = number of protons + neutrons = 23 for sodium 23 Na 11

Neutrons and protons each have a mass very close to 1 dalton, so the mass number is close to, but slightly different from, the total mass of an atom, called its atomic mass. For example, the mass number of sodium (23 11Na) is 23, but its atomic mass is 22.9898 daltons.

Atomic number = number of protons = number of electrons in a neutral atom = 11 for sodium

Number of neutrons = mass number - atomic number = 23 - 11 = 12 for sodium The simplest atom is hydrogen 11H, which has no neutrons; it consists of a single proton with a single electron. Because the contribution of electrons to mass is negligible, almost all of an atom’s mass is concentrated in its nucleus.

Radioactive Tracers Radioactive isotopes are often used as diagnostic tools in medicine. Cells can use radioactive atoms just as they would use nonradioactive isotopes of the same element. The radioactive isotopes are incorporated into biologically active molecules, which are then used as tracers to track atoms during metabolism, the chemical processes of an organism. For example, certain kidney disorders are diagnosed by injecting small doses of radioactively labeled substances into the blood and then analyzing the tracer molecules excreted in the urine. Radioactive tracers are also used in combination with sophisticated imaging instruments, such as PET scanners

CHAPTER 2

The Chemical Context of Life

31

Figure 2.5 A PET scan, a medical use for radioactive isotopes. PET, an acronym for positron-emission tomography, detects locations of intense chemical activity in the body. The bright yellow spot marks an area with an elevated level of radioactively labeled glucose, which in turn indicates high metabolic activity, a hallmark of cancerous tissue.

The Energy Levels of Electrons

Cancerous throat tissue

that can monitor growth and metabolism of cancers in the body (Figure 2.5). Although radioactive isotopes are very useful in biological research and medicine, radiation from decaying isotopes also poses a hazard to life by damaging cellular molecules. The severity of this damage depends on the type and amount of radiation an organism absorbs. One of the most serious environmental threats is radioactive fallout from nuclear accidents. The doses of most isotopes used in medical diagnosis, however, are relatively safe.

Radiometric Dating EVOLUTION Researchers measure radioactive decay in fossils to date these relics of past life. Fossils provide a large body of evidence for evolution, documenting differences between organisms from the past and those living at present and giving us insight into species that have disappeared over time. While the layering of fossil beds establishes that deeper fossils are older than more shallow ones, the actual age (in years) of the fossils in each layer cannot be determined by position alone. This is where radioactive isotopes come in. A “parent” isotope decays into its “daughter” isotope at a fixed rate, expressed as the half-life of the isotope—the time it takes for 50% of the parent isotope to decay. Each radioactive isotope has a characteristic half-life that is not affected by temperature, pressure, or any other environmental variable. Using a process called radiometric dating, scientists measure the ratio of different isotopes and calculate how many half-lives (in years) have passed since an organism was fossilized or a rock was formed. Half-life values range from very short for some isotopes, measured in seconds or days, to extremely long—uranium-238 has a half-life of 4.5 billion years! Each isotope can best “measure” a particular range of years: Uranium-238 was used to determine that moon rocks are approximately 4.5 billion years old, similar to the estimated age of Earth. In the Scientific Skills Exercise, you can work with data from an experiment that used carbon-14 to determine the age of an important fossil. (Figure 25.6 explains more about radiometric dating of fossils.)

32

UNIT ONE

The Chemistry of Life

The simplified models of the atom in Figure 2.4 greatly exaggerate the size of the nucleus relative to that of the whole atom. If an atom of helium were the size of a typical football stadium, the nucleus would be the size of a pencil eraser in the center of the field. Moreover, the electrons would be like two tiny gnats buzzing around the stadium. Atoms are mostly empty space. When two atoms approach each other during a chemical reaction, their nuclei do not come close enough to interact. Of the three subatomic particles we have discussed, only electrons are directly involved in chemical reactions. An atom’s electrons vary in the amount of energy they possess. Energy is defined as the capacity to cause change— for instance, by doing work. Potential energy is the energy that matter possesses because of its location or structure. For example, water in a reservoir on a hill has potential energy because of its altitude. When the gates of the reservoir’s dam are opened and the water runs downhill, the energy can be used to do work, such as moving the blades of turbines to generate electricity. Because energy has been expended, the water has less energy at the bottom of the hill than it did in the reservoir. Matter has a natural tendency to move toward the lowest possible state of potential energy; in our example, the water runs downhill. To restore the potential energy of a reservoir, work must be done to elevate the water against gravity. The electrons of an atom have potential energy due to their distance from the nucleus (Figure 2.6). The negatively charged Figure 2.6 Energy levels of an atom’s electrons. Electrons exist only at fixed levels of potential energy called electron shells. (a) A ball bouncing down a flight of stairs can come to rest only on each step, not between steps. Similarly, an electron can exist only at certain energy levels, not between levels. Third shell (highest energy level in this model) Second shell (higher energy level)

First shell (lowest energy level)

Energy absorbed

Energy lost

Atomic nucleus (b) An electron can move from one shell to another only if the energy it gains or loses is exactly equal to the difference in energy between the energy levels of the two shells. Arrows in this model indicate some of the stepwise changes in potential energy that are possible. Figure Walkthrough

Calibrating a Standard Radioactive Isotope Decay Curve and Interpreting Data How Long Might Neanderthals Have Co-Existed with Modern Humans (Homo (Homo sapiens)? sapiens)? Neanderthals (Homo (Homo neanderthalensis) were living in Europe by 350,000 years ago and may have coexisted with early Homo sapiens in parts of Eurasia for hundreds or thousands of years before Neanderthals became extinct. Researchers sought to more accurately determine the extent of their overlap by pinning down the latest date Neanderthals still lived in the area. They used carbon-14 dating to determine the age of a Neanderthal fossil from the most recent (uppermost) archeological layer containing Neanderthal bones. In this exercise you will calibrate a standard carbon-14 decay curve and use it to determine the age of this Neanderthal fossil. The age will help you approximate the last time the two species may have coexisted at the site where this fossil was collected.

How the Experiment Was Done Carbon-14 (14C) is a radioactive 14

14

isotope of carbon that decays to N at a constant rate. C is present in the atmosphere in small amounts at a constant ratio with both 13 C and 12C, two other isotopes of carbon. When carbon is taken up from the atmosphere by a plant during photosynthesis, 12C, 13C, and 14 C isotopes are incorporated into the plant in the same proportions in which they were present in the atmosphere. These proportions remain the same in the tissues of an animal that eats the plant. While an organism is alive, the 14C in its body constantly decays to 14N but is constantly replaced by new carbon from the environment. Once an organism dies, it stops taking in new 14C but the 14C in its tissues continues to decay, while the 12C in its tissues remains the same because it is not radioactive and does not decay. Thus, scientists can calculate how long the pool of original 14C has been decaying in a fossil by measuring the ratio of 14C to 12C and comparing it to the ratio of 14 C to 12C present originally in the atmosphere. The fraction of 14C in a fossil compared to the original fraction of 14C can be converted to years because we know that the half-life of 14C is 5,730 years—in other words, half of the 14C in a fossil decays every 5,730 years.

Data from the Experiment The researchers found that the Neanderthal fossil had approximately 0.0078 (or, in scientific notation, 7.8 * 10-3) as much 14C as the atmosphere. The following questions will guide you through translating this fraction into the age of the fossil. INTERPRET THE DATA 1. A standard curve of radioactive isotope decay is shown at the top of the right column. The graph line shows the fraction of the radioactive isotope over time (before present) in units of halflives. Recall that a half-life is the amount of time it takes for half of the radioactive isotope to decay. Labeling each data point with the corresponding fractions will help orient you to this graph. Draw an arrow to the data point for half-life = 1 and write the

electrons are attracted to the positively charged nucleus. It takes work to move a given electron farther away from the nucleus, so the more distant an electron is from the nucleus, the greater its potential energy. Unlike the continuous flow of water downhill, changes in the potential energy of electrons can occur only in steps of fixed amounts. An electron having a

Fraction of isotope remaining in fossil

SCIENTIFIC SKILLS EXERCISE 1.0

Neanderthal fossils

0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

0

1

2

3 4 5 6 7 8 Time before present (half-lives)

9

10

Data from R. Pinhasi et al., Revised age of late Neanderthal occupation and the end of the Middle Paleolithic in the northern Caucasus, Proceedings of the National Academy of Sciences USA 147:8611–8616 (2011). doi 10.1073/pnas.1018938108

fraction of 14C that will remain after one half-life. Calculate the fraction of 14C remaining at each half-life and write the fractions on the graph near arrows pointing to the data points. Convert each fraction to a decimal number and round off to a maximum of three significant digits (zeros at the beginning of the number do not count as significant digits). Also write each decimal number in scientific notation. 2. Recall that 14C has a half-life of 5,730 years. To calibrate the x-axis for 14C decay, write the time before present in years below each half-life. 3. The researchers found that the Neanderthal fossil had approximately 0.0078 as much 14C as found originally in the atmosphere. (a) Using the numbers on your graph, determine how many half-lives have passed since the Neanderthal died. (b) Using your 14C calibration on the x-axis, what is the approximate age of the Neanderthal fossil in years (round off to the nearest thousand)? (c) Approximately when did Neanderthals become extinct according to this study? (d) The researchers cite evidence that modern humans (H. sapiens) became established in the same region as the last Neanderthals approximately 39,000–42,000 years ago. What does this suggest about the overlap of Neanderthals and modern humans? 4. Carbon-14 dating works for fossils up to about 75,000 years old; fossils older than that contain too little 14C to be detected. Most dinosaurs went extinct 65.5 million years ago. (a) Can 14C be used to date dinosaur bones? Explain. (b) Radioactive uranium-235 has a half-life of 704 million years. If it was incorporated into dinosaur bones, could it be used to date the dinosaur fossils? Explain. Instructors: A version of this Scientific Skills Exercise can be assigned in MasteringBiology.

certain amount of energy is something like a ball on a staircase (Figure 2.6a). The ball can have different amounts of potential energy, depending on which step it is on, but it cannot spend much time between the steps. Similarly, an electron’s potential energy is determined by its energy level. An electron can exist only at certain energy levels, not between them. CHAPTER 2

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An electron’s energy level is correlated with its average distance from the nucleus. Electrons are found in different electron shells, each with a characteristic average distance and energy level. In diagrams, shells can be represented by concentric circles, as they are in Figure 2.6b. The first shell is closest to the nucleus, and electrons in this shell have the lowest potential energy. Electrons in the second shell have more energy, and electrons in the third shell even more energy. An electron can move from one shell to another, but only by absorbing or losing an amount of energy equal to the difference in potential energy between its position in the old shell and that in the new shell. When an electron absorbs energy, it moves to a shell farther out from the nucleus. For example, light energy can excite an electron to a higher energy level. (Indeed, this is the first step taken when plants harness the energy of sunlight for photosynthesis, the process that produces food from carbon dioxide and water. You’ll learn more about photosynthesis in Chapter 10.) When an electron loses energy, it “falls back” to a shell closer to the nucleus, and the lost energy is usually released to the environment as heat. For example, sunlight

excites electrons in the surface of a car to higher energy levels. When the electrons fall back to their original levels, the car’s surface heats up. This thermal energy can be transferred to the air or to your hand if you touch the car.

Electron Distribution and Chemical Properties The chemical behavior of an atom is determined by the distribution of electrons in the atom’s electron shells. Beginning with hydrogen, the simplest atom, we can imagine building the atoms of the other elements by adding 1 proton and 1 electron at a time (along with an appropriate number of neutrons). Figure 2.7, a modified version of what is called the periodic table of the elements, shows this distribution of electrons for the first 18 elements, from hydrogen (1H) to argon (18Ar). The elements are arranged in three rows, or periods, corresponding to the number of electron shells in their atoms. The left-to-right sequence of elements in each row corresponds to the sequential addition of electrons and protons. (See Appendix B for the complete periodic table.)

Figure 2.7 Electron distribution diagrams for the first 18 elements in the periodic table. In a standard periodic table (see Appendix B), information for each element is presented as shown for helium in the inset. In the diagrams in this table, electrons are represented as yellow dots and electron shells as concentric circles. These diagrams are a convenient way to picture the distribution of an atom’s electrons among its electron shells, but these simplified models do not accurately represent the shape of the atom or the location of its electrons. The elements are arranged in rows, each representing the filling of an electron shell. As electrons are added, they occupy the lowest available shell. Hydrogen 1H

Atomic number

2

He Atomic mass

First shell

4.003

Helium 2He

Element symbol Electron distribution diagram

Lithium 3Li

Beryllium 4Be

Boron 5B

Carbon 6C

Nitrogen 7N

Oxygen 8O

Fluorine 9F

Neon 10Ne

Sodium 11Na

Magnesium 12Mg

Aluminum 13Al

Silicon 14Si

Phosphorus 15P

Sulfur 16S

Chlorine 17Cl

Argon 18Ar

Second shell

Third shell

VISUAL SKILLS Looking at the depictions of atoms in this chart, what is the atomic number of magnesium? How many protons and electrons does it have? How many electron shells? How many valence electrons?

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Animation: Electron Distribution Diagrams

Hydrogen’s 1 electron and helium’s 2 electrons are located in the first shell. Electrons, like all matter, tend to exist in the lowest available state of potential energy. In an atom, this state is in the first shell. However, the first shell can hold no more than 2 electrons; thus, hydrogen and helium are the only elements in the first row of the table. In an atom with more than 2 electrons, the additional electrons must occupy higher shells because the first shell is full. The next element, lithium, has 3 electrons. Two of these electrons fill the first shell, while the third electron occupies the second shell. The second shell holds a maximum of 8 electrons. Neon, at the end of the second row, has 8 electrons in the second shell, giving it a total of 10 electrons. The chemical behavior of an atom depends mostly on the number of electrons in its outermost shell. We call those outer electrons valence electrons and the outermost electron shell the valence shell. In the case of lithium, there is only 1 valence electron, and the second shell is the valence shell. Atoms with the same number of electrons in their valence shells exhibit similar chemical behavior. For example, fluorine (F) and chlorine (Cl) both have 7 valence electrons, and both form compounds when combined with the element sodium (Na): Sodium fluoride (NaF) is commonly added to toothpaste to prevent tooth decay, and, as described earlier, NaCl is table salt (see Figure 2.2). An atom with a completed valence shell is unreactive; that is, it will not interact readily with other atoms. At the far right of the periodic table are helium, neon, and argon, the only three elements shown in Figure 2.7 that have full valence shells. These elements are said to be inert, meaning chemically unreactive. All the other atoms in Figure 2.7 are chemically reactive because they have incomplete valence shells.

Figure 2.8 Electron orbitals.

First shell Neon, with two filled shells (10 electrons) Second shell (a) Electron distribution diagram. An electron distribution diagram is shown here for a neon atom, which has a total of 10 electrons. Each concentric circle represents an electron shell, which can be subdivided into electron orbitals.

First shell

Second shell

y

x

1s orbital

2s orbital

z Three 2p orbitals

(b) Separate electron orbitals. The three-dimensional shapes represent electron orbitals—the volumes of space where the electrons of an atom are most likely to be found. Each orbital holds a maximum of 2 electrons. The first electron shell, on the left, has one spherical (s) orbital, designated 1s. The second shell, on the right, has one larger s orbital (designated 2s for the second shell) plus three dumbbell-shaped orbitals called p orbitals (2p for the second shell). The three 2p orbitals lie at right angles to one another along imaginary x-, y-, and z-axes of the atom. Each 2p orbital is outlined here in a different color.

Electron Orbitals In the early 1900s, the electron shells of an atom were visualized as concentric paths of electrons orbiting the nucleus, somewhat like planets orbiting the sun. It is still convenient to use two-dimensional concentric-circle diagrams, as in Figure 2.7, to symbolize three-dimensional electron shells. However, you need to remember that each concentric circle represents only the average distance between an electron in that shell and the nucleus. Accordingly, the concentric-circle diagrams do not give a real picture of an atom. In reality, we can never know the exact location of an electron. What we can do instead is describe the space in which an electron spends most of its time. The three-dimensional space where an electron is found 90% of the time is called an orbital. Each electron shell contains electrons at a particular energy level, distributed among a specific number of orbitals of distinctive shapes and orientations. Figure 2.8 shows the orbitals of neon as an example, with its electron distribution diagram for reference. You can think of an orbital as a component of an electron shell. The first electron shell has only one spherical s orbital (called 1s), but the second shell has

1s, 2s, and 2p orbitals

(c) Superimposed electron orbitals. To reveal the complete picture of the electron orbitals of neon, we superimpose the 1s orbital of the first shell and the 2s and three 2p orbitals of the second shell.

four orbitals: one large spherical s orbital (called 2s) and three dumbbell-shaped p orbitals (called 2p orbitals). (The third shell and other higher electron shells also have s and p orbitals, as well as orbitals of more complex shapes.) No more than 2 electrons can occupy a single orbital. The first electron shell can therefore accommodate up to 2 electrons in its s orbital. The lone electron of a hydrogen atom occupies the 1s orbital, as do the 2 electrons of a helium atom. The four orbitals of the second electron shell can hold up to 8 electrons, CHAPTER 2

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2 in each orbital. Electrons in each of the four orbitals in the second shell have nearly the same energy, but they move in different volumes of space. The reactivity of an atom arises from the presence of unpaired electrons in one or more orbitals of the atom’s valence shell. As you will see in the next section, atoms interact in a way that completes their valence shells. When they do so, it is the unpaired electrons that are involved.

CONCEPT CHECK 2.2 1. A lithium atom has 3 protons and 4 neutrons. What is its mass number? 2. A nitrogen atom has 7 protons, and the most common isotope of nitrogen has 7 neutrons. A radioactive isotope of nitrogen has 8 neutrons. Write the atomic number and mass number of this radioactive nitrogen as a chemical symbol with a subscript and superscript. 3. How many electrons does fluorine have? How many electron shells? Name the orbitals that are occupied. How many electrons are needed to fill the valence shell? 4. VISUAL SKILLS In Figure 2.7, if two or more elements are in the same row, what do they have in common? If two or more elements are in the same column, what do they have in common? For suggested answers, see Appendix A.

CONCEPT

2.3

The formation and function of molecules depend on chemical bonding between atoms Now that we have looked at the structure of atoms, we can move up the hierarchy of organization and see how atoms combine to form molecules and ionic compounds. Atoms with incomplete valence shells can interact with certain other atoms in such a way that each partner atom completes its valence shell: The atoms either share or transfer valence electrons. These interactions usually result in atoms staying close together, held by attractions called chemical bonds. The strongest kinds of chemical bonds are covalent bonds and ionic bonds in dry ionic compounds. (Ionic bonds in aqueous, or water-based, solutions are weak interactions, as we will see later.) Animation: Introduction to Chemical Bonds

Covalent Bonds A covalent bond is the sharing of a pair of valence electrons by two atoms. For example, let’s consider what happens when two hydrogen atoms approach each other. Recall that hydrogen has 1 valence electron in the first shell, but the shell’s capacity is 2 electrons. When the two hydrogen atoms come close enough for their 1s orbitals to overlap, they can share their electrons (Figure 2.9). Each hydrogen atom is now associated with 2 electrons in what amounts to a completed valence shell. Two 36

UNIT ONE

The Chemistry of Life

Figure 2.9 Formation of a covalent bond. Hydrogen atoms (2 H) 1 In each hydrogen

atom, the single electron is held in its orbital by its attraction to the proton in the nucleus.

+

+

2 When two hydrogen

atoms approach each other, the electron of each atom is also attracted to the proton in the other nucleus.

+

+

3 The two electrons

become shared in a covalent bond, forming an H2 molecule.

+

+

Hydrogen molecule (H2)

or more atoms held together by covalent bonds constitute a molecule, in this case a hydrogen molecule. Figure 2.10a shows several ways of representing a hydrogen molecule. Its molecular formula, H2, simply indicates that the molecule consists of two atoms of hydrogen. Electron sharing can be depicted by an electron distribution diagram or by a Lewis dot structure, in which element symbols are surrounded by dots that represent the valence electrons (H : H). We can also use a structural formula, H ¬ H, where the line represents a single bond, a pair of shared electrons. A space-filling model comes closest to representing the actual shape of the molecule. (You may also be familiar with ball-and-stick models, which are shown in Figure 2.15.) Oxygen has 6 electrons in its second electron shell and therefore needs 2 more electrons to complete its valence shell. Two oxygen atoms form a molecule by sharing two pairs of valence electrons (Figure 2.10b). The atoms are thus joined by what is called a double bond (O“O). Each atom that can share valence electrons has a bonding capacity corresponding to the number of covalent bonds the atom can form. When the bonds form, they give the atom a full complement of electrons in the valence shell. The bonding capacity of oxygen, for example, is 2. This bonding capacity is called the atom’s valence and usually equals the number of unpaired electrons required to complete the atom’s outermost (valence) shell. See if you can determine the valences of hydrogen, oxygen, nitrogen, and carbon by studying the electron distribution diagrams in Figure 2.7. You can see that the valence of hydrogen is 1; oxygen, 2; nitrogen, 3; and carbon, 4. The situation is more complicated

Figure 2.10 Covalent bonding in four molecules. The number of electrons required to complete an atom’s valence shell generally determines how many covalent bonds that atom will form. This figure shows several ways of indicating covalent bonds. Name and Molecular Formula

Electron Distribution Diagram

(a) Hydrogen (H2). Two hydrogen atoms share one pair of electrons, forming a single bond.

Lewis Dot Structure and Structural Formula

δ– δ– O

δ+

H H

Because oxygen (O) is more electronegative than hydrogen (H), shared electrons are pulled more toward oxygen.

SpaceFilling Model

H •• H H

Figure 2.11 Polar covalent bonds in a water molecule.

H

H H2O

H

δ+

This results in two regions of partial negative charge on oxygen and a partial positive charge on each hydrogen.

(d) Methane (CH4 ). Four hydrogen atoms can satisfy the valence of H one carbon atom, forming methane.

• •

• • • •

O

O

O

• •

O

O •• H H

• •

• •

H

O

O

H

H

H

H H •• C •• H H

H

• •

(c) Water (H2O). Two hydrogen atoms and one oxygen atom are joined by single bonds, forming a molecule of water.

O •• •• O

• •

(b) Oxygen (O2). Two oxygen atoms share two pairs of electrons, forming a double bond.

• •

Animation: Nonpolar and Polar Molecules

C H

H

H H

C

H

H Animation: Covalent Bonds

for phosphorus, in the third row of the periodic table, which can have a valence of 3 or 5 depending on the combination of single and double bonds it makes. The molecules H2 and O2 are pure elements rather than compounds because a compound is a combination of two or more different elements. Water, with the molecular formula H2O, is a compound. Two atoms of hydrogen are needed to satisfy the valence of one oxygen atom. Figure 2.10c shows the structure of a water molecule. (Water is so important to life that Chapter 3 is devoted entirely to its structure and behavior.) Methane, the main component of natural gas, is a compound with the molecular formula CH4. It takes four hydrogen atoms, each with a valence of 1, to complement one atom of carbon, with its valence of 4 (Figure 2.10d). (We will look at many other compounds of carbon in Chapter 4.) Atoms in a molecule attract shared bonding electrons to varying degrees, depending on the element. The attraction of a particular atom for the electrons of a covalent bond is called its

electronegativity. The more electronegative an atom is, the more strongly it pulls shared electrons toward itself. In a covalent bond between two atoms of the same element, the electrons are shared equally because the two atoms have the same electronegativity—the tug-of-war is at a standoff. Such a bond is called a nonpolar covalent bond. For example, the single bond of H2 is nonpolar, as is the double bond of O2. However, when an atom is bonded to a more electronegative atom, the electrons of the bond are not shared equally. This type of bond is called a polar covalent bond. Such bonds vary in their polarity, depending on the relative electronegativity of the two atoms. For example, the bonds between the oxygen and hydrogen atoms of a water molecule are quite polar (Figure 2.11). Oxygen is one of the most electronegative elements, attracting shared electrons much more strongly than hydrogen does. In a covalent bond between oxygen and hydrogen, the electrons spend more time near the oxygen nucleus than near the hydrogen nucleus. Because electrons have a negative charge and are pulled toward oxygen in a water molecule, the oxygen atom has two regions of partial negative charge (each indicated by the Greek letter δ with a minus sign, δ-, “delta minus”), and each hydrogen atom has a partial positive charge (δ+, “delta plus”). In contrast, the individual bonds of methane (CH4) are much less polar because the electronegativities of carbon and hydrogen are similar.

Ionic Bonds In some cases, two atoms are so unequal in their attraction for valence electrons that the more electronegative atom strips an electron completely away from its partner. The two resulting oppositely charged atoms (or molecules) are called ions. A positively charged ion is called a cation, while a negatively charged ion is called an anion. Because of their opposite charges, cations and anions attract each other; this attraction is called an ionic bond. Note that the transfer of an electron is not, by itself, the formation of a bond; rather, it allows a bond to form because it results in two ions of opposite charge. Any two ions of opposite charge can form an ionic bond. The ions do not need to have acquired their charge by an electron transfer with each other. CHAPTER 2

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37

which consists of molecules having a definite size and number of atoms, an ionic compound does not consist of 1 The lone valence electron of a sodium 2 Each resulting ion has a completed molecules. The formula for an ionic atom is transferred to join the 7 valence valence shell. An ionic bond can form compound, such as NaCl, indicates only electrons of a chlorine atom. between the oppositely charged ions. the ratio of elements in a crystal of the + – salt. “NaCl” by itself is not a molecule. Not all salts have equal numbers of cations and anions. For example, the ionic compound magnesium chloride Cl Cl Na Na (MgCl2) has two chloride ions for each magnesium ion. Magnesium (12Mg) must lose 2 outer electrons if the atom is to Na Cl Na+ Cl– have a complete valence shell, so it has a Sodium atom Chlorine atom Sodium ion Chloride ion tendency to become a cation with a net (a cation) (an anion) charge of 2+ (Mg2+). One magnesium Sodium chloride (NaCl) Animation: Formation of Ions and Ionic Bonds cation can therefore form ionic bonds with two chloride anions (Cl-). The term ion also applies to entire molecules that are elecThis is what happens when an atom of sodium (11Na) trically charged. In the salt ammonium chloride (NH4Cl), for encounters an atom of chlorine (17Cl) (Figure 2.12). A sodium instance, the anion is a single chloride ion (Cl-), but the catatom has a total of 11 electrons, with its single valence elecion is ammonium (NH4+), a nitrogen atom covalently bonded tron in the third electron shell. A chlorine atom has a total to four hydrogen atoms. The whole ammonium ion has an of 17 electrons, with 7 electrons in its valence shell. When electrical charge of 1+ because it has given up 1 electron and these two atoms meet, the lone valence electron of sodium thus is 1 electron short. is transferred to the chlorine atom, and both atoms end up Environment affects the strength of ionic bonds. In a dry with their valence shells complete. (Because sodium no longer salt crystal, the bonds are so strong that it takes a hammer has an electron in the third shell, the second shell is now the and chisel to break enough of them to crack the crystal in valence shell.) The electron transfer between the two atoms two. If the same salt crystal is dissolved in water, however, moves one unit of negative charge from sodium to chlorine. the ionic bonds are much weaker because each ion is partially Sodium, now with 11 protons but only 10 electrons, has a net shielded by its interactions with water molecules. Most electrical charge of 1+; the sodium atom has become a cation. drugs are manufactured as salts because they are quite stable Conversely, the chlorine atom, having gained an extra electron, when dry but can dissociate (come apart) easily in water. now has 17 protons and 18 electrons, giving it a net electrical (In Concept 3.2, you will learn how water dissolves salts.) charge of 1- ; it has become a chloride ion—an anion. Compounds formed by ionic bonds are called ionic Weak Chemical Interactions compounds, or salts. We know the ionic compound sodium chloride (NaCl) as table salt (Figure 2.13). Salts are In organisms, most of the strongest chemical bonds are covaoften found in nature as crystals of various sizes and shapes. lent bonds, which link atoms to form a cell’s molecules. But Each salt crystal is an aggregate of vast numbers of cations weaker interactions within and between molecules are also and anions bonded by their electrical attraction and arranged indispensable, contributing greatly to the emergent properties in a three-dimensional lattice. Unlike a covalent compound, of life. Many large biological molecules are held in their funcFigure 2.12 Electron transfer and ionic bonding. The attraction between oppositely charged atoms, or ions, is an ionic bond. An ionic bond can form between any two oppositely charged ions, even if they have not been formed by transfer of an electron from one to the other.

Figure 2.13 A sodium chloride (NaCl) crystal. The sodium ions (Na+) and chloride ions (Cl-) are held together by ionic bonds. The formula NaCl tells us that the ratio of Na+ to Cl- is 1:1.

Na+ Cl–

tional form by weak interactions. In addition, when two molecules in the cell make contact, they may adhere temporarily by weak interactions. The reversibility of weak interactions can be an advantage: Two molecules can come together, affect one another in some way, and then separate. Several types of weak chemical interactions are important in organisms. One is the ionic bond as it exists between ions dissociated in water, which we just discussed. Hydrogen bonds and van der Waals interactions are also crucial to life.

Hydrogen Bonds Among weak chemical interactions, hydrogen bonds are so central to the chemistry of life that they deserve special 38

UNIT ONE

The Chemistry of Life

Figure 2.14 A hydrogen bond. δ+

δ– δ– O

Water (H2O)

H

H This hydrogen bond (dotted line) results from the attraction between the partial positive charge on a hydrogen atom of water and the partial negative charge on the nitrogen atom of ammonia.

δ+ δ–

Ammonia (NH3)

N H δ+

H

H δ+

δ+ DRAW IT

Draw one water molecule surrounded by four other water molecules, arranged so that they can make hydrogen bonds with each other. Use simple outlines of space-filling models. Draw the partial charges on the water molecules and use dots for the hydrogen bonds.

Animation: Hydrogen Bonds

attention. When a hydrogen atom is covalently bonded to an electronegative atom, the hydrogen atom has a partial positive charge that allows it to be attracted to a different electronegative atom nearby. This attraction between a hydrogen and an electronegative atom is called a hydrogen bond. In living cells, the electronegative partners are usually oxygen or nitrogen atoms. Refer to Figure 2.14 to examine the simple case of hydrogen bonding between water (H2O) and ammonia (NH3).

Van der Waals Interactions Even a molecule with nonpolar covalent bonds may have positively and negatively charged regions. Electrons are not always evenly distributed; at any instant, they may accumulate by chance in one part of a molecule or another. The results are ever-changing regions of positive and negative charge that enable all atoms and molecules to stick to one another. These van der Waals interactions are individually weak and occur only when atoms and molecules are very close together. When many such interactions occur simultaneously, however, they can be powerful: Van der Waals interactions allow the gecko lizard shown here to walk straight up a wall! The anatomy of the gecko’s foot— including many minuscule hairlike projections from the toes and strong tendons underlying the skin—strikes a balance between maximum surface contact with the wall and necessary stiffness of the foot. The van der Waals interactions between the foot molecules and the molecules of

the wall’s surface are so numerous that despite their individual weakness, together they can support the gecko’s body weight. This discovery has inspired development of an artificial adhesive called Geckskin: A patch the size of an index card can hold a 700-pound weight to a wall! Van der Waals interactions, hydrogen bonds, ionic bonds in water, and other weak interactions may form not only between molecules but also between parts of a large molecule, such as a protein. The cumulative effect of weak interactions is to reinforce the three-dimensional shape of the molecule. (You will learn more about the very important biological roles of weak interactions in Chapter 5.)

Molecular Shape and Function A molecule has a characteristic size and shape, which are key to its function in the living cell. A molecule consisting of two atoms, such as H2 or O2, is always linear, but most molecules with more than two atoms have more complicated shapes. These shapes are determined by the positions of the atoms’ orbitals (Figure 2.15). When an atom forms covalent bonds,

Figure 2.15 Molecular shapes due to hybrid orbitals. s orbital

z

Three p orbitals

Four hybrid orbitals

x

y Tetrahedron (a) Hybridization of orbitals. The single s and three p orbitals of a valence shell involved in covalent bonding combine to form four teardrop-shaped hybrid orbitals. These orbitals extend to the four corners of an imaginary tetrahedron (outlined in pink). Space-Filling Model

Ball-and-Stick Model

O H

104.5°

H

Hybrid-Orbital Model (with ball-and-stick model superimposed)

Unbonded electron pairs H

O H

Water (H2O) H

H

C H

C H

H

H

H

H

Methane (CH4) (b) Molecular-shape models. Three models representing molecular shape are shown for water and methane. The positions of the hybrid orbitals determine the shapes of the molecules. CHAPTER 2 CHAP

The Chemical Context of Life

39

the orbitals in its valence shell undergo rearrangement. For atoms with valence electrons in both s and p orbitals (review Figure 2.8), the single s and three p orbitals form four new hybrid orbitals shaped like identical teardrops extending from the region of the atomic nucleus, as shown in Figure 2.15a. If we connect the larger ends of the teardrops with lines, we have the outline of a geometric shape called a tetrahedron, a pyramid with a triangular base. For water molecules (H2O), two of the hybrid orbitals in the oxygen’s valence shell are shared with hydrogens (see Figure 2.15b). The result is a molecule shaped roughly like a V, with its two covalent bonds at an angle of 104.5°. The methane molecule (CH4) has the shape of a completed tetrahedron because all four hybrid orbitals of the carbon atom are shared with hydrogen atoms (see Figure 2.15b). The carbon nucleus is at the center, with its four covalent bonds radiating to hydrogen nuclei at the corners of the tetrahedron. Larger molecules containing multiple carbon atoms, including many of the molecules that make up living matter, have more complex overall shapes. However, the tetrahedral shape of a carbon atom bonded to four other atoms is often a repeating motif within such molecules. Molecular shape is crucial: It determines how biological molecules recognize and respond to one another with specificity. Biological molecules often bind temporarily to each other by forming weak interactions, but only if their shapes are complementary. Consider the effects of opiates, drugs such as morphine and heroin derived from opium. Opiates relieve pain and alter mood by weakly binding to specific receptor molecules on the surfaces of brain cells. Why would brain cells carry receptors for opiates, compounds that are not made by the body? In 1975, the discovery of endorphins answered this question. Endorphins are signaling molecules made by the pituitary gland that bind to the receptors, relieving pain and producing euphoria during times of stress, such as intense exercise. Opiates have shapes similar to endorphins and mimic them by binding to endorphin receptors in the brain. That is why opiates and endorphins have similar effects (Figure 2.16). The role of molecular shape in brain chemistry illustrates how biological organization leads to a match between structure and function, one of biology’s unifying themes.

Figure 2.16 A molecular mimic. Morphine affects pain perception and emotional state by mimicking the brain’s natural endorphins. Key

Carbon

Nitrogen

Hydrogen

Sulfur Oxygen

Natural endorphin

Morphine

(a) Structures of endorphin and morphine. The boxed portion of the endorphin molecule (left) binds to receptor molecules on target cells in the brain. The boxed portion of the morphine molecule (right) is a close match.

Natural endorphin

Morphine

Endorphin receptors

Brain cell

(b) Binding to endorphin receptors. Both endorphin and morphine can bind to endorphin receptors on the surface of a brain cell. Interview with Candace Pert: Discovering opiate receptors in the brain

CONCEPT

2.4

Chemical reactions make and break chemical bonds The making and breaking of chemical bonds, leading to changes in the composition of matter, are called chemical reactions. An example is the reaction between hydrogen and oxygen molecules that forms water:

CONCEPT CHECK 2.3 1. Why does the structure H ¬ C “ C ¬ H fail to make sense chemically?

+

2. What holds the atoms together in a crystal of magnesium chloride (MgCl2)? 3. WHAT IF? If you were a pharmaceutical researcher, why would you want to learn the three-dimensional shapes of naturally occurring signaling molecules? For suggested answers, see Appendix A.

40

UNIT ONE

The Chemistry of Life

2 H2

+ Reactants

O2

2 H2O Chemical reaction

Products

This reaction breaks the covalent bonds of H2 and O2 and forms the new bonds of H2O. When we write the equation for a chemical reaction, we use an arrow to indicate the conversion of the starting materials, called the reactants, to the resulting materials, or products. The coefficients indicate the number of molecules involved; for example, the coefficient 2 before the H2 means that the reaction starts with two molecules of hydrogen. Notice that all atoms of the reactants must be accounted for in the products. Matter is conserved in a chemical reaction: Reactions cannot create or destroy atoms but can only rearrange (redistribute) the electrons among them. Photosynthesis, which takes place within the cells of green plant tissues, is an important biological example of how chemical reactions rearrange matter. Humans and other animals ultimately depend on photosynthesis for food and oxygen, and this process is at the foundation of almost all ecosystems. The following summarizes the process of photosynthesis:

Reactants 6 CO2 + Carbon dioxide

Products 6 H2O Water

Sunlight

C6H12O6 Glucose

+

6 O2 Oxygen

The raw materials of photosynthesis are carbon dioxide (CO2) and water (H2O), which land plants absorb from the air and soil, respectively. Within the plant cells, sunlight powers the conversion of these ingredients to a sugar called glucose (C6H12O6) and oxygen molecules (O2), a by-product that can be seen when released by a water plant (Figure 2.17). Although photosynthesis is actually a sequence of many chemical reactions, we still end up with the same number and types of atoms that we had when we started. Matter has simply been rearranged, with an input of energy provided by sunlight. All chemical reactions are theoretically reversible, with the products of the forward reaction becoming the reactants for the reverse reaction. For example, hydrogen and nitrogen molecules can combine to form ammonia, but ammonia can also decompose to regenerate hydrogen and nitrogen:

Figure 2.17 Photosynthesis: a solarpowered rearrangement of matter. Elodea, a freshwater plant, produces sugar by rearranging the atoms of carbon dioxide and water in the chemical process known as photosynthesis, which is powered by sunlight. Much of the sugar is then converted to other food molecules. Oxygen gas (O2) is a by-product of photosynthesis; notice the bubbles of O2 gas escaping from the leaves submerged in water.

Leaf

Bubbles of O2

DRAW IT Add labels and arrows on the photo showing the reactants and products of photosynthesis as it takes place in a leaf.

The same holds true for products. As products accumulate, collisions resulting in the reverse reaction become more frequent. Eventually, the forward and reverse reactions occur at the same rate, and the relative concentrations of products and reactants stop changing. The point at which the reactions offset one another exactly is called chemical equilibrium. This is a dynamic equilibrium; reactions are still going on in both directions, but with no net effect on the concentrations of reactants and products. Equilibrium does not mean that the reactants and products are equal in concentration, but only that their concentrations have stabilized at a particular ratio. The reaction involving ammonia reaches equilibrium when ammonia decomposes as rapidly as it forms. In some chemical reactions, the equilibrium point may lie so far to the right that these reactions go essentially to completion; that is, virtually all the reactants are converted to products. We will return to the subject of chemical reactions after more detailed study of the various types of molecules that are important to life. In the next chapter, we focus on water, the substance in which all the chemical processes of organisms occur.

CONCEPT CHECK 2.4 1. MAKE CONNECTIONS Consider the reaction between hydrogen and oxygen that forms water, shown with balland-stick models at the beginning of Concept 2.4. After studying Figure 2.10, draw and label the Lewis dot structures representing this reaction.

3 H2 + N2 L 2 NH3

2. Which type of chemical reaction, if any, occurs faster at equilibrium: the formation of products from reactants or that of reactants from products?

The two opposite-headed arrows indicate that the reaction is reversible. One of the factors affecting the rate of a reaction is the concentration of reactants. The greater the concentration of reactant molecules, the more frequently they collide with one another and have an opportunity to react and form products.

3. WHAT IF? Write an equation that uses the products of photosynthesis as reactants and the reactants of photosynthesis as products. Add energy as another product. This new equation describes a process that occurs in your cells. Describe this equation in words. How does this equation relate to breathing? For suggested answers, see Appendix A.

CHAPTER 2

The Chemical Context of Life

41

2

Chapter Review

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SUMMARY OF KEY CONCEPTS CONCEPT 2.1

Matter consists of chemical elements in pure form and in combinations called compounds (pp. 29–30)

VOCAB SELF-QUIZ

goo.gl/6u55ks

Elements cannot be broken down chemically to other substances. A compound contains two or more different elements in a fixed ratio. Oxygen, carbon, hydrogen, and nitrogen make up approximately 96% of living matter.

?

Molecules consist of two or more covalently bonded atoms. The attraction of an atom for the electrons of a covalent bond is its electronegativity. If both atoms are the same, they have the same electronegativity and share a nonpolar covalent bond. Electrons of a polar covalent bond are pulled closer to the more electronegative atom, such as the oxygen in H2O. An ion forms when an atom or molecule gains or loses an electron and becomes charged. An ionic bond is the attraction between two oppositely charged ions: Ionic bond

Compare an element and a compound.

Electron transfer forms ions

+



Na

Cl

Na

Cl

Na Sodium atom

Cl Chlorine atom

Na+ Sodium ion (a cation)

Cl– Chloride ion (an anion)

CONCEPT 2.2

An element’s properties depend on the structure of its atoms (pp. 30–36) An atom, the smallest unit of an element, has the following components: Nucleus Protons (+ charge) determine element



+ +

Neutrons (no charge) determine isotope

Electrons (– charge) form negative cloud and determine chemical behavior



Atom

An electrically neutral atom has equal numbers of electrons and protons; the number of protons determines the atomic number. The atomic mass is measured in daltons and is roughly equal to the mass number, the sum of protons plus neutrons. Isotopes of an element differ from each other in neutron number and therefore mass. Unstable isotopes give off particles and energy as radioactivity. In an atom, electrons occupy specific electron shells; the electrons in a shell have a characteristic energy level. Electron distribution in shells determines the chemical behavior of an atom. An atom that has an incomplete outer shell, the valence shell, is reactive. Electrons exist in orbitals, three-dimensional spaces with specific shapes that are components of electron shells. DRAW IT

Draw the electron distribution diagrams for neon (10Ne) and argon (18 Ar). Use these diagrams to explain why these elements are chemically unreactive.

The formation and function of molecules depend on chemical bonding between atoms (pp. 36–40)

Single covalent bond

UNIT ONE

The Chemistry of Life

• •

• • • •

• •

• •

O• • + • O• ••

• •

H •• H

• •

Chemical bonds form when atoms interact and complete their valence shells. Covalent bonds form when pairs of electrons are shared:

42

?

In terms of electron sharing between atoms, compare nonpolar covalent bonds, polar covalent bonds, and the formation of ions.

CONCEPT 2.4

Chemical reactions make and break chemical bonds (pp. 40–41) Chemical reactions change reactants into products while conserving matter. All chemical reactions are theoretically reversible. Chemical equilibrium is reached when the forward and reverse reaction rates are equal.

?

What would happen to the concentration of products if more reactants were added to a reaction that was in chemical equilibrium? How would this addition affect the equilibrium?

TEST YOUR UNDERSTANDING

CONCEPT 2.3

H• + H•

Weak interactions reinforce the shapes of large molecules and help molecules adhere to each other. A hydrogen bond is an attraction between a hydrogen atom carrying a partial positive charge (δ+) and an electronegative atom carrying a partial negative charge (δ-). Van der Waals interactions occur between transiently positive and negative regions of molecules. A molecule’s shape is determined by the positions of its atoms’ valence orbitals. Covalent bonds result in hybrid orbitals, which are responsible for the shapes of H2O, CH4, and many more complex biological molecules. Molecular shape is usually the basis for the recognition of one biological molecule by another.

O •• •• O

Double covalent bond

Level 1: Knowledge/Comprehension 1. In the term trace element, the adjective trace means that (A) the element is required in very small amounts. PRACTICE TEST (B) the element can be used as a label to trace goo.gl/CUYGKD atoms through an organism’s metabolism. (C) the element is very rare on Earth. (D) the element enhances health but is not essential for the organism’s long-term survival.

2. Compared with 31P, the radioactive isotope 32P has (A) a different atomic number. (B) one more proton. (C) one more electron. (D) one more neutron. 3. The reactivity of an atom arises from (A) the average distance of the outermost electron shell from the nucleus. (B) the existence of unpaired electrons in the valence shell. (C) the sum of the potential energies of all the electron shells. (D) the potential energy of the valence shell. 4. Which statement is true of all atoms that are anions? (A) The atom has more electrons than protons. (B) The atom has more protons than electrons. (C) The atom has fewer protons than does a neutral atom of the same element. (D) The atom has more neutrons than protons. 5. Which of the following statements correctly describes any chemical reaction that has reached equilibrium? (A) The concentrations of products and reactants are equal. (B) The reaction is now irreversible. (C) Both forward and reverse reactions have halted. (D) The rates of the forward and reverse reactions are equal.

Level 2: Application/Analysis 6. We can represent atoms by listing the number of protons, neutrons, and electrons—for example, 2p+, 2n0, 2e- for helium. Which of the following represents the 18O isotope of oxygen? (A) 7p+, 2n0, 9e(B) 8p+, 10n0, 8e(C) 9p+, 9n0, 9e(D) 10p+, 8n0, 9e-

11. SCIENTIFIC INQUIRY Female luna moths (Actias luna) attract males by emitting chemical signals that spread through the air. A male hundreds of meters away can detect these molecules and fly toward their source. The sensory organs responsible for this behavior are the comblike antennae visible in the photograph shown here. Each filament of an antenna is equipped with thousands of receptor cells that detect the sex attractant. Based on what you learned in this chapter, propose a hypothesis to account for the ability of the male moth to detect a specific molecule in the presence of many other molecules in the air. What predictions does your hypothesis make? Design an experiment to test one of these predictions. 12. WRITE ABOUT A THEME: ORGANIZATION While waiting at an airport, Neil Campbell once overheard this claim: “It’s paranoid and ignorant to worry about industry or agriculture contaminating the environment with their chemical wastes. After all, this stuff is just made of the same atoms that were already present in our environment.” Drawing on your knowledge of electron distribution, bonding, and emergent properties (see Concept 1.1), write a short essay (100–150 words) countering this argument. 13. SYNTHESIZE YOUR KNOWLEDGE

7. The atomic number of sulfur is 16. Sulfur combines with hydrogen by covalent bonding to form a compound, hydrogen sulfide. Based on the number of valence electrons in a sulfur atom, predict the molecular formula of the compound. (A) HS (C) H2S (B) HS2 (D) H4S 8. What coefficients must be placed in the following blanks so that all atoms are accounted for in the products? C6H12O6 S

_________

C2H6O +

(A) 2; 1 (B) 3; 1

_________

CO2

(C) 1; 3 (D) 2; 2

9. DRAW IT Draw Lewis dot structures for each hypothetical molecule shown below, using the correct number of valence electrons for each atom. Determine which molecule makes sense because each atom has a complete valence shell and each bond has the correct number of electrons. Explain what makes the other molecule nonsensical, considering the number of bonds each type of atom can make.

H

(a)

O

H

H

C

C

H

H O

H

(b)

C

H H

C

O

H

Level 3: Synthesis/Evaluation 10. EVOLUTION CONNECTION The percentages of naturally occurring elements making up the human body (see Table 2.1) are similar to the percentages of these elements found in other organisms. How could you account for this similarity among organisms?

This bombardier beetle is spraying a boiling hot liquid that contains irritating chemicals, used as a defense mechanism against its enemies. The beetle stores two sets of chemicals separately in its glands. Using what you learned about chemistry in this chapter, propose a possible explanation for why the beetle is not harmed by the chemicals it stores and what causes the explosive discharge. For selected answers, see Appendix A.

For additional practice questions, check out the Dynamic Study Modules in MasteringBiology. You can use them to study on your smartphone, tablet, or computer anytime, anywhere!

CHAPTER 2

The Chemical Context of Life

43

Superset

3

Water and Life

Figure 3.1 How does life on Earth depend on the chemistry of water?

KEY CONCEPTS

The Molecule That Supports All of Life

3.1

Polar covalent bonds in water molecules result in hydrogen bonding

3.2

Four emergent properties of water contribute to Earth’s suitability for life

3.3

Acidic and basic conditions affect living organisms

Life on Earth began in water and evolved there for 3 billion years before spreading onto land. Water is the substance that makes life possible as we know it here on Earth, and possibly on other planets as well. All organisms familiar to us are made mostly of water and live in an environment dominated by water. Three-quarters of Earth’s surface is covered by water. Although most of this water is in liquid form, water is also present on Earth as a solid (ice) and a gas (water vapor). Water is the only common substance on Earth to exist in the natural environment in all three physical states of matter. Furthermore, the solid form of water floats on the liquid form, a rare property emerging from the chemistry of the water molecule. As the Earth is warming from climate change (see Concept 1.1), the ratio of ice to liquid water is changing. Arctic sea ice and glaciers are melting, affecting life on, under, and around them. In the Arctic, warmer waters and the smaller ice pack are resulting in blooms of phytoplankton (microscopic aquatic photosynthetic organisms), seen from space as the “cloudy” seawater in Figure 3.1. Organisms that depend on Arctic ice, however, are suffering. For instance, a population of black guillemots in Alaska is declining due to the warming climate and reduction of Arctic sea ice. In this chapter, you will learn how the structure of a water molecule allows it to interact with other molecules, including other water molecules. This ability leads to water’s unique emergent properties that help make Earth suitable for life.

Black guillemots, threatened by climate change

44

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Get Ready for This Chapter

CONCEPT

3.1

Polar covalent bonds in water molecules result in hydrogen bonding Water is so familiar to us that it is easy to overlook its many extraordinary qualities. Following the theme of emergent properties, we can trace water’s unique behavior to the structure and interactions of its molecules. Studied on its own, the water molecule is deceptively simple. It is shaped like a wide V, with its two hydrogen atoms joined to the oxygen atom by single covalent bonds. Oxygen is more electronegative than hydrogen, so the electrons of the covalent bonds spend more time closer to oxygen than to hydrogen; these are polar covalent bonds (see Figure 2.11). This unequal sharing of electrons and water’s V-like shape make it a polar molecule, meaning that its overall charge is unevenly distributed. In water, the oxygen of the molecule has two regions of partial negative charge (δ-), and each hydrogen has a partial positive charge (δ+). The properties of water arise from attractions between oppositely charged atoms of different water molecules: The partially positive hydrogen of one molecule is attracted to the partially negative oxygen of a nearby molecule. The two molecules are thus held together by a hydrogen bond (Figure 3.2). When water is in its liquid form, its hydrogen bonds are very fragile, each only about 1/20 as strong as a covalent bond. The hydrogen bonds form, break, and re-form with great frequency. Each lasts only a few trillionths of a second, but the molecules are constantly forming new hydrogen bonds with a succession of partners. Therefore, at any instant, most of the water molecules Figure 3.2 Hydrogen bonds between water molecules. δ+ H

δ+ O

δ– Because of its electron arrangement, oxygen has two regions with partial negative charge.

The charged regions in a water molecule are due to its polar covalent bonds.

H

δ–

Regions of neighboring water molecules with opposite partial charges are attracted to each other, forming hydrogen bonds.

δ+ H

δ+

O

δ–

H δ– δ+

DRAW IT

δ+

Each water molecule can hydrogen-bond to several others; these associations are constantly changing. δ–

Draw partial charges on the water molecule at the far left, and draw three more water molecules hydrogen-bonded to it.

Animation: Polarity of Water

are hydrogen-bonded to their neighbors. The extraordinary properties of water emerge from this hydrogen bonding, which organizes water molecules into a higher level of structural order.

CONCEPT CHECK 3.1 1. MAKE CONNECTIONS What is electronegativity, and how does it affect interactions between water molecules? (Review Figure 2.11.) 2. VISUAL SKILLS Look at Figure 3.2 and explain why the central water molecule can hydrogen bond to four (rather than three or five) other water molecules. 3. Why is it unlikely that two neighboring water molecules would be arranged like this?

O

HH

O

HH 4. WHAT IF? What would be the effect on the properties of the water molecule if oxygen and hydrogen had equal electronegativity? For suggested answers, see Appendix A.

CONCEPT

3.2

Four emergent properties of water contribute to Earth’s suitability for life We will examine four emergent properties of water that contribute to Earth’s suitability as an environment for life: cohesive behavior, ability to moderate temperature, expansion upon freezing, and versatility as a solvent.

Cohesion of Water Molecules Water molecules stay close to each other as a result of hydrogen bonding. Although the arrangement of molecules in a sample of liquid water is constantly changing, at any given moment many of the molecules are linked by multiple hydrogen bonds. These linkages make water more structured than most other liquids. Collectively, the hydrogen bonds hold the substance together, a phenomenon called cohesion. Cohesion due to hydrogen bonding contributes to the transport of water and dissolved nutrients against gravity in plants. Water from the roots reaches the leaves through a network of water-conducting cells (Figure 3.3). As water evaporates from a leaf, hydrogen bonds cause water molecules leaving the veins to tug on molecules farther down, and the upward pull is transmitted through the water-conducting cells all the way to the roots. Adhesion, the clinging of one substance to another, also plays a role. Adhesion of water by hydrogen bonds to the molecules of cell walls helps counter the downward pull of gravity (see Figure 3.3). Related to cohesion is surface tension, a measure of how difficult it is to stretch or break the surface of a liquid. At the interface between water and air is an ordered arrangement of water molecules, hydrogen-bonded to one another and to the water below, but not to the air above. This asymmetry gives CHAPTER 3

Water and Life

45

Figure 3.3 Water transport in plants. Because of the properties of cohesion and adhesion, the tallest trees can transport water more than 100 m upward—approximately one-quarter the height of the Empire State Building in New York City. Evaporation from leaves pulls water upward from the roots through water-conducting cells. H 2O

Adhesion of the water to cell walls by hydrogen bonds helps resist the downward pull of gravity. Two types of water-conducting cells

Direction of water movement

300 μm

H2O

H2O

Cohesion due to hydrogen bonds between water molecules helps hold together the column of water within the cells.

BioFlix® Animation: Adhesion and Cohesion in Plants Animation: Cohesion of Water

water an unusually high surface tension, making it behave as though it were coated with an invisible film. You can observe the surface tension of water by slightly overfilling a drinking glass; the water will stand above the rim. The spider in Figure 3.4 takes advantage of the surface tension of water to walk across a pond without breaking the surface.

Moderation of Temperature by Water Water moderates air temperature by absorbing heat from air that is warmer and releasing the stored heat to air that is cooler. Water is effective as a heat bank because it can absorb or release a relatively large amount of heat with only a slight change in its own temperature. To understand this capability of water, let’s first look at temperature and heat. Figure 3.4 Walking on water. The high surface tension of water, resulting from the collective strength of its hydrogen bonds, allows this raft spider to walk on the surface of a pond.

46

UNIT ONE

The Chemistry of Life

Temperature and Heat Anything that moves has kinetic energy, the energy of motion. Atoms and molecules have kinetic energy because they are always moving, although not necessarily in any particular direction. The faster a molecule moves, the greater its kinetic energy. The kinetic energy associated with the random movement of atoms or molecules is called thermal energy. Thermal energy is related to temperature, but they are not the same thing. Temperature represents the average kinetic energy of the molecules in a body of matter, regardless of volume, whereas the thermal energy of a body of matter reflects the total kinetic energy, and thus depends on the matter’s volume. When water is heated in a coffeemaker, the average speed of the molecules increases, and the thermometer records this as a rise in temperature of the liquid. The total amount of thermal energy also increases in this case. Note, however, that although the pot of coffee has a much higher temperature than, say, the water in a swimming pool, the swimming pool contains more thermal energy because of its much greater volume. Whenever two objects of different temperature are brought together, thermal energy passes from the warmer to the cooler object until the two are the same temperature. Molecules in the cooler object speed up at the expense of the thermal energy of the warmer object. An ice cube cools a drink not by adding coldness to the liquid, but by absorbing thermal energy from the liquid as the ice itself melts. Thermal energy in transfer from one body of matter to another is defined as heat. One convenient unit of heat used in this book is the calorie (cal). A calorie is the amount of heat it takes to raise the temperature of 1 g of water by 1°C. Conversely, a calorie is also the amount of heat that 1 g of water releases when it cools by 1°C. A kilocalorie (kcal), 1,000 cal, is the quantity of heat required to raise the temperature of 1 kilogram (kg) of water by 1°C. (The “Calories” on food packages are actually kilocalories.) Another energy unit used in this book is the joule ( J). One joule equals 0.239 cal; one calorie equals 4.184 J.

Water’s High Specific Heat The ability of water to stabilize temperature stems from its relatively high specific heat. The specific heat of a substance is defined as the amount of heat that must be absorbed or lost for 1 g of that substance to change its temperature by 1°C. We already know water’s specific heat because we have defined a calorie as the amount of heat that causes 1 g of water to change its temperature by 1°C. Therefore, the specific heat of water is 1 calorie per gram and per degree Celsius, abbreviated as 1 cal/(g # °C). Compared with most other substances, water has an unusually high specific heat. For example, ethyl alcohol, the type of alcohol in alcoholic beverages, has a specific heat of 0.6 cal/(g # °C); that is, only 0.6 cal is required to raise the temperature of 1 g of ethyl alcohol by 1°C. Because of the high specific heat of water relative to other materials, water will change its temperature less than other

liquids when it absorbs or loses a given amount of heat. The reason you can burn your fingers by touching the side of an iron pot on the stove when the water in the pot is still lukewarm is that the specific heat of water is ten times greater than that of iron. In other words, the same amount of heat will raise the temperature of 1 g of the iron much faster than it will raise the temperature of 1 g of the water. Specific heat can be thought of as a measure of how well a substance resists changing its temperature when it absorbs or releases heat. Water resists changing its temperature; when it does change its temperature, it absorbs or loses a relatively large quantity of heat for each degree of change. We can trace water’s high specific heat, like many of its other properties, to hydrogen bonding. Heat must be absorbed in order to break hydrogen bonds; by the same token, heat is released when hydrogen bonds form. A calorie of heat causes a relatively small change in the temperature of water because much of the heat is used to disrupt hydrogen bonds before the water molecules can begin moving faster. And when the temperature of water drops slightly, many additional hydrogen bonds form, releasing a considerable amount of energy in the form of heat. What is the relevance of water’s high specific heat to life on Earth? A large body of water can absorb and store a huge amount of heat from the sun in the daytime and during summer while warming up only a few degrees. At night and during winter, the gradually cooling water can warm the air. This capability of water serves to moderate air temperatures in coastal areas (Figure 3.5). The high specific heat of water also tends to stabilize ocean temperatures, creating a favorable environment for marine life. Thus, because of its high specific heat, the water that covers most of Earth keeps temperature fluctuations on land and in water within limits that permit life. Also, because organisms are made primarily of water, they are better able to resist changes in their own temperature than if they were made of a liquid with a lower specific heat.

Evaporative Cooling Molecules of any liquid stay close together because they are attracted to one another. Molecules moving fast enough to Figure 3.5 Temperatures for the Pacific Ocean and Southern California on an August day.

Los Angeles (Airport) 75° 70s (°F) 80s 90s

Pacific Ocean 68°

100s INTERPRET THE DATA in this diagram.

San Bernardino 100° Riverside 96° Santa Ana Palm Springs 84° 106°

Burbank 90°

Santa Barbara 73°

San Diego 72°

40 miles

Explain the pattern of temperatures shown

overcome these attractions can depart the liquid and enter the air as a gas (vapor). This transformation from a liquid to a gas is called vaporization, or evaporation. Recall that the speed of molecular movement varies and that temperature is the average kinetic energy of molecules. Even at low temperatures, the speediest molecules can escape into the air. Some evaporation occurs at any temperature; a glass of water at room temperature, for example, will eventually evaporate completely. If a liquid is heated, the average kinetic energy of molecules increases and the liquid evaporates more rapidly. Heat of vaporization is the quantity of heat a liquid must absorb for 1 g of it to be converted from the liquid to the gaseous state. For the same reason that water has a high specific heat, it also has a high heat of vaporization relative to most other liquids. To evaporate 1 g of water at 25°C, about 580 cal of heat is needed—nearly double the amount needed to vaporize a gram of alcohol or ammonia. Water’s high heat of vaporization is another emergent property resulting from the strength of its hydrogen bonds, which must be broken before the molecules can exit from the liquid in the form of water vapor. The high amount of energy required to vaporize water has a wide range of effects. On a global scale, for example, it helps moderate Earth’s climate. A considerable amount of solar heat absorbed by tropical seas is consumed during the evaporation of surface water. Then, as moist tropical air circulates poleward, it releases heat as it condenses and forms rain. On an organismal level, water’s high heat of vaporization accounts for the severity of steam burns. These burns are caused by the heat energy released when steam condenses into liquid on the skin. As a liquid evaporates, the surface of the liquid that remains behind cools down (its temperature decreases). This evaporative cooling occurs because the “hottest” molecules, those with the greatest kinetic energy, are the most likely to leave as gas. It is as if the 100 fastest runners at a college transferred to another school; the average speed of the remaining students would decline. Evaporative cooling of water contributes to the stability of temperature in lakes and ponds and also provides a mechanism that prevents terrestrial organisms from overheating. For example, evaporation of water from the leaves of a plant helps keep the tissues in the leaves from becoming too warm in the sunlight. Evaporation of sweat from human skin dissipates body heat and helps prevent overheating on a hot day or when excess heat is generated by strenuous activity. High humidity on a hot day increases discomfort because the high concentration of water vapor in the air inhibits the evaporation of sweat from the body.

Floating of Ice on Liquid Water Water is one of the few substances that are less dense as a solid than as a liquid. In other words, ice floats on liquid water. While other materials contract and become denser when they solidify, water expands. The cause of this exotic behavior is, once again, CHAPTER 3

Water and Life

47

Figure 3.6 Ice: crystalline structure and floating barrier. In ice, each molecule is hydrogen-bonded to four neighbors in a three-dimensional crystal. Because the crystal is spacious, ice has fewer molecules than an equal volume of liquid water. In other words, ice is less dense than liquid water. Floating ice becomes a barrier that insulates the liquid water below from the colder air. The marine organism shown here is a type of shrimp called krill; it was photographed beneath floating ice in the Southern Ocean near Antarctica. WHAT IF?

If water did not form hydrogen bonds, what would happen to the shrimp’s habitat, shown here?

Hydrogen bond

Liquid water: Hydrogen bonds break and re-form

Ice: Hydrogen bonds are stable

hydrogen bonding. At temperatures above 4°C, water behaves Many scientists are worried that these bodies of ice are at risk like other liquids, expanding as it warms and contracting as it of disappearing. Global warming, which is caused by carbon cools. As the temperature falls from 4°C to 0°C, water begins to dioxide and other “greenhouse” gases in the atmosphere (see freeze because more and more of its molecules are moving too Figure 56.28), is having a profound effect on icy environments slowly to break hydrogen bonds. At 0°C, the molecules become around the globe. In the Arctic, the average air temperature locked into a crystalline lattice, each water molecule hydrogenhas risen 2.2°C just since 1961. This temperature increase has bonded to four partners (Figure 3.6). The hydrogen bonds affected the seasonal balance between Arctic sea ice and liquid keep the molecules at “arm’s length,” far enough apart to make water, causing ice to form later in the year, to melt earlier, and ice about 10% less dense (10% fewer molecules in the same volto cover a smaller area. The rate at which glaciers and Arctic sea ume) than liquid water at 4°C. When ice absorbs enough heat ice are disappearing is posing an extreme challenge to animals for its temperature to rise above 0°C, hydrogen bonds between that depend on ice for their survival (Figure 3.7). molecules are disrupted. As the crystal collapses, the ice melts and molecules have Figure 3.7 Effects of climate change on the Arctic. Warmer temperatures in the Arctic fewer hydrogen bonds, allowing them cause more sea ice to melt in the summer, benefiting some organisms and harming others. to slip closer together. Water reaches its greatest density at 4°C and then begins to Species that are benefiting from loss of ice: expand as the molecules move faster. Even More light and warmer Bowhead Some fish species, in liquid water, many of the molecules are waters result in more whales, which feed such as capelin, connected by hydrogen bonds, though phytoplankton, which on plankton they benefit from are eaten by filter, are thriving. having more only transiently: The hydrogen bonds are other organplankton to constantly breaking and re-forming. isms. eat. The ability of ice to float due to its Species being harmed by loss of ice: lower density is an important factor Russia in the suitability of the environment Loss of ice has Arctic reduced feeding for life. If ice sank, then eventually all ocean opportunities for ponds, lakes, and even oceans would Extent of sea ice in Sept. 2014 polar bears, who freeze solid, making life as we know it hunt from the ice. Extent of sea ice in Sept. 1979 impossible on Earth. During summer, Bering Strait only the upper few inches of the ocean The Pacific walrus depends North Pole on the ice to rest; its would thaw. Instead, when a deep body fate is uncertain. Greenland of water cools, the floating ice insulates the liquid water below, preventing it Black guillemots in Alaska from freezing and allowing life to exist cannot fly from their nests Alaska under the frozen surface, as shown in the on land to their fishing grounds at the edge photo in Figure 3.6. Besides insulating of the ice, which is now too far from land; the water below, ice also provides a solid Canada young birds are starving. habitat for some animals, such as polar Interview with Susan Solomon: bears and seals. Sea ice in Sept. 2014 Understanding climate change

48

UNIT ONE

The Chemistry of Life

Ice lost from Sept. 1979 to Sept. 2014

Water: The Solvent of Life A sugar cube placed in a glass of water will dissolve with a little stirring. The glass will then contain a uniform mixture of sugar and water; the concentration of dissolved sugar will be the same everywhere in the mixture. A liquid that is a completely homogeneous mixture of two or more substances is called a solution. The dissolving agent of a solution is the solvent, and the substance that is dissolved is the solute. In this case, water is the solvent and sugar is the solute. An aqueous solution is one in which the solute is dissolved in water; water is the solvent. Water is a very versatile solvent, a quality we can trace to the polarity of the water molecule. Suppose, for example, that a spoonful of table salt, the ionic compound sodium chloride (NaCl), is placed in water (Figure 3.8). At the surface of each crystal of salt, the sodium and chloride ions are exposed to the solvent. These ions and regions of the water molecules are attracted to each other due to their opposite charges. The oxygens of the water molecules have regions of partial negative charge that are attracted to sodium cations. The hydrogen regions are partially positively charged and are attracted to chloride anions. As a result, water molecules surround the individual sodium and chloride ions, separating and shielding them from one another. The sphere of water molecules around each dissolved ion is called a hydration shell. Working inward from the surface of each salt crystal, water eventually dissolves all the ions. The result is a solution of two solutes, sodium cations and chloride anions, homogeneously mixed with water, the solvent. Other ionic compounds also dissolve in water. Seawater, for instance, contains a great variety of dissolved ions, as do living cells. A compound does not need to be ionic to dissolve in water; many compounds made up of nonionic polar molecules, Negative oxygen regions of polar water molecules are attracted to sodium cations (Na+).

_

Na+

+

_

_ Positive hydrogen regions of water molecules are attracted to chloride anions (Cl–).

Na+

+ Cl–

_ + _

Cl–

+

+_ + _ +

_ + _



_

Figure 3.8 Table salt dissolving in water. A sphere of water molecules, called a hydration shell, surrounds each solute ion. WHAT IF?

What would happen if you heated this solution for a long time?

Figure 3.9 A water-soluble protein. Human lysozyme is a protein found in tears and saliva that has antibacterial action (see Figure 5.16). This model shows the lysozyme molecule (purple) in an aqueous environment. Ionic and polar regions on the protein’s surface attract the partially charged regions on water molecules. This oxygen is attracted to a slight positive charge on the lysozyme molecule. δ+ δ–

δ– δ+

This hydrogen is attracted to a slight negative charge on the lysozyme molecule.

such as the sugar in the sugar cube mentioned earlier, are also water-soluble. Such compounds dissolve when water molecules surround each of the solute molecules, forming hydrogen bonds with them. Even molecules as large as proteins can dissolve in water if they have ionic and polar regions on their surface (Figure 3.9). Many different kinds of polar compounds are dissolved (along with ions) in the water of such biological fluids as blood, the sap of plants, and the liquid within all cells. Water is the solvent of life.

Hydrophilic and Hydrophobic Substances Any substance that has an affinity for water is said to be hydrophilic (from the Greek hydro, water, and philos, loving). In some cases, substances can be hydrophilic without actually dissolving. For example, some molecules in cells are so large that they do not dissolve. Another example of a hydrophilic substance that does not dissolve is cotton, a plant product. Cotton consists of giant molecules of cellulose, a compound with numerous regions of partial positive and partial negative charges that can form hydrogen bonds with water. Water adheres to the cellulose fibers. Thus, a cotton towel does a great job of drying the body, yet it does not dissolve in the washing machine. Cellulose is also present in the walls of water-conducting cells in a plant; you read earlier how the adhesion of water to these hydrophilic walls helps water move up the plant against gravity. There are, of course, substances that do not have an affinity for water. Substances that are nonionic and nonpolar (or otherwise cannot form hydrogen bonds) actually seem to repel water; these substances are said to be hydrophobic (from the Greek phobos, fearing). An example from the kitchen is vegetable oil, which, as you know, does not mix stably with water-based substances such as vinegar. The hydrophobic CHAPTER 3

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49

behavior of the oil molecules results from a prevalence of relatively nonpolar covalent bonds, in this case bonds between carbon and hydrogen, which share electrons almost equally. Hydrophobic molecules related to oils are major ingredients of cell membranes. (Imagine what would happen to a cell if its membrane dissolved!)

Solute Concentration in Aqueous Solutions Most of the chemical reactions in organisms involve solutes dissolved in water. To understand such reactions, we must know how many atoms and molecules are involved and calculate the concentration of solutes in an aqueous solution (the number of solute molecules in a volume of solution). When carrying out experiments, we use mass to calculate the number of molecules. We must first calculate the molecular mass, which is the sum of the masses of all the atoms in a molecule. As an example, let’s calculate the molecular mass of table sugar (sucrose), C12H22O11, by multiplying the number of atoms by the atomic mass of each element (see Appendix B). In round numbers of daltons, the mass of a carbon atom is 12, the mass of a hydrogen atom is 1, and the mass of an oxygen atom is 16. Thus, sucrose has a molecular mass of (12 * 12) + (22 * 1) + (11 * 16) = 342 daltons. Because we can’t weigh out small numbers of molecules, we usually measure substances in units called moles. Just as a dozen always means 12 objects, a mole (mol) represents an exact number of objects: 6.02 * 1023, which is called Avogadro’s number. Because of the way in which Avogadro’s number and the unit dalton were originally defined, there are 6.02 * 1023 daltons in 1 g. Once we determine the molecular mass of a molecule such as sucrose, we can use the same number (342), but with the unit gram, to represent the mass of 6.02 * 1023 molecules of sucrose, or 1 mol of sucrose (this is sometimes called the molar mass). To obtain 1 mol of sucrose in the lab, therefore, we weigh out 342 g. The practical advantage of measuring a quantity of chemicals in moles is that a mole of one substance has exactly the same number of molecules as a mole of any other substance. If the molecular mass of substance A is 342 daltons and that of substance B is 10 daltons, then 342 g of A will have the same number of molecules as 10 g of B. A mole of ethyl alcohol (C2H6O) also contains 6.02 * 1023 molecules, but its mass is only 46 g because the mass of a molecule of ethyl alcohol is less than that of a molecule of sucrose. Measuring in moles makes it convenient for scientists working in the laboratory to combine substances in fixed ratios of molecules. How would we make a liter (L) of solution consisting of 1 mol of sucrose dissolved in water? We would measure out 342 g of sucrose and then gradually add water, while stirring, until the sugar was completely dissolved. We would then add enough water to bring the total volume of the solution up to 1 L. At that point, we would have a 1-molar (1 M) solution of sucrose. Molarity—the number of moles of solute per liter of solution—is the unit of concentration most often used by biologists for aqueous solutions. 50

UNIT ONE

The Chemistry of Life

Water’s capacity as a versatile solvent complements the other properties discussed in this chapter. Since these remarkable properties allow water to support life on Earth so well, scientists who seek life elsewhere in the universe look for water as a sign that a planet might sustain life. MP3 Tutor: The Properties of Water

Possible Evolution of Life on Other Planets EVOLUTION Biologists who look for life elsewhere in the universe (known as astrobiologists) have concentrated their search on planets that might have water. More than 800 planets have been found outside our solar system, and there is evidence for the presence of water vapor on a few of them. In our own solar system, Mars has been a focus of study. Like Earth, Mars has an ice cap at both poles. Images from spacecraft sent to Mars showed that ice is present just under the surface of Mars and enough water vapor exists in its atmosphere for frost to form. In 2015, scientists found evidence of water flowing on Mars (Figure 3.10), and other studies suggested conditions existed that could have supported microorganismal life. Drilling below the surface may be the next step in the search for signs of life on Mars. If any life-forms or fossils are found, their study will shed light on the process of evolution from an entirely new perspective.

Figure 3.10 Evidence for liquid water on Mars. Water appears to have helped form these dark streaks that run downhill on Mars during the summer. NASA scientists also found evidence of hydrated salts, indicating water is present. (This digitally treated photograph was taken by the Mars Reconnaissance Orbiter.)

Dark streaks

CONCEPT CHECK 3.2 1. Describe how properties of water contribute to the upward movement of water in a tree. 2. Explain the saying “It’s not the heat; it’s the humidity.” 3. How can the freezing of water crack boulders? 4. WHAT IF? A water strider (an insect that can walk on water) has legs that are coated with a hydrophobic substance. What might be the benefit? What would happen if the substance were hydrophilic? 5. INTERPRET THE DATA The concentration of the appetiteregulating hormone ghrelin is about 1.3 * 10-10 M in the blood of a fasting person. How many molecules of ghrelin are in 1 L of blood? For selected answers, see Appendix A.

3.3

CONCEPT

Acids and Bases

Acidic and basic conditions affect living organisms Occasionally, a hydrogen atom participating in a hydrogen bond between two water molecules shifts from one molecule to the other. When this happens, the hydrogen atom leaves its electron behind, and what is actually transferred is a hydrogen ion (H+), a single proton with a charge of 1+. The water molecule that lost a proton is now a hydroxide ion (OH-), which has a charge of 1-. The proton binds to the other water molecule, making that molecule a hydronium ion (H3O+). We can picture the chemical reaction as follows: + H O

H

H

O H

2 H2O

H

O H H

Hydronium ion (H3O+)



+

O

What would cause an aqueous solution to have an imbalance in H+ and OH- concentrations? When acids dissolve in water, they donate additional H+ to the solution. An acid is a substance that increases the hydrogen ion concentration of a solution. For example, when hydrochloric acid (HCl) is added to water, hydrogen ions dissociate from chloride ions: HCl S H+ + ClThis source of H+ (dissociation of water is the other source) results in an acidic solution—one having more H+ than OH-. A substance that reduces the hydrogen ion concentration of a solution is called a base. Some bases reduce the H+ concentration directly by accepting hydrogen ions. Ammonia (NH3), for instance, acts as a base when the unshared electron pair in nitrogen’s valence shell attracts a hydrogen ion from the solution, resulting in an ammonium ion (NH4+): NH3 + H+ L NH4+

H

Hydroxide ion (OH–)

Animation: Dissociation of Water Molecules

By convention, H+ (the hydrogen ion) is used to represent H3O+ (the hydronium ion), and we follow that practice in this book. Keep in mind, though, that H+ does not exist on its own in an aqueous solution. It is always associated with a water molecule in the form of H3O+. As indicated by the double arrows, this is a reversible reaction that reaches a state of dynamic equilibrium when water molecules dissociate at the same rate that they are being reformed from H+ and OH-. At this equilibrium point, the concentration of water molecules greatly exceeds the concentrations of H+ and OH-. In pure water, only one water molecule in every 554 million is dissociated; the concentration of H+ and of OH- in pure water is therefore 10-7 M (at 25°C). This means there is only one ten-millionth of a mole of hydrogen ions per liter of pure water and an equal number of hydroxide ions. (Even so, this is a huge number—over 60,000 trillion—of each ion in a liter of pure water.) Although the dissociation of water is reversible and statistically rare, it is exceedingly important in the chemistry of life. H+ and OH- are very reactive. Changes in their concentrations can drastically affect a cell’s proteins and other complex molecules. As we have seen, the concentrations of H+ and OH- are equal in pure water, but adding certain kinds of solutes, called acids and bases, disrupts this balance. Biologists use something called the pH scale to describe how acidic or basic (the opposite of acidic) a solution is. In the remainder of this chapter, you will learn about acids, bases, and pH and why changes in pH can adversely affect organisms.

Other bases reduce the H+ concentration indirectly by dissociating to form hydroxide ions, which combine with hydrogen ions and form water. One such base is sodium hydroxide (NaOH), which in water dissociates into its ions: NaOH S Na+ + OHIn either case, the base reduces the H+ concentration. Solutions with a higher concentration of OH- than H+ are known as basic solutions. A solution in which the H+ and OH- concentrations are equal is said to be neutral. Notice that single arrows were used in the reactions for HCl and NaOH. These compounds dissociate completely when mixed with water, so hydrochloric acid is called a strong acid and sodium hydroxide a strong base. In contrast, ammonia is a weak base. The double arrows in the reaction for ammonia indicate that the binding and release of hydrogen ions are reversible reactions, although at equilibrium there will be a fixed ratio of NH4+ to NH3. Weak acids are acids that reversibly release and accept back hydrogen ions. An example is carbonic acid: H2CO3 Carbonic acid

L

HCO3Bicarbonate ion

+

H+ Hydrogen ion

Here the equilibrium so favors the reaction in the left direction that when carbonic acid is added to pure water, only 1% of the molecules are dissociated at any particular time. Still, that is enough to shift the balance of H+ and OH- from neutrality.

The pH Scale In any aqueous solution at 25°C, the product of the H+ and OH- concentrations is constant at 10-14. This can be written [H+][OH-] = 10-14 (The brackets indicate molar concentration.) As previously mentioned, in a neutral solution at 25°C, [H+] = 10-7 and [OH-] = 10-7. Therefore, the product of [H+] and [OH-] in a CHAPTER 3

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51

neutral solution at 25°C is 10-14. If enough acid is added to a solution to increase [H+] to 10-5 M, then [OH-] will decline by an equivalent factor to 10-9 M (note that 10-5 * 10-9 = 10-14). This constant relationship expresses the behavior of acids and bases in an aqueous solution. An acid not only adds hydrogen ions to a solution, but also removes hydroxide ions because of the tendency for H+ to combine with OH-, forming water. A base has the opposite effect, increasing OH- concentration but also reducing H+ concentration by the formation of water. If enough of a base is added to raise the OH- concentration to 10-4 M, it will cause the H+ concentration to drop to 10-10 M. Whenever we know the concentration of either H+ or OH- in an aqueous solution, we can deduce the concentration of the other ion. Because the H+ and OH- concentrations of solutions can vary by a factor of 100 trillion or more, scientists have developed a way to express this variation more conveniently than in moles per liter. The pH scale (Figure 3.11) compresses the range of H+ and OH- concentrations by employing logarithms. Figure 3.11 The pH scale and pH values of some aqueous solutions. pH Scale 0 1

Increasingly Acidic [H+] > [OH–]

H+

H+

+ OH– H

+

H + OH– H H+ H+ H+

Acidic solution

Battery acid

2 Gastric juice (in stomach), lemon juice 3 Vinegar, wine, cola 4 Tomato juice Beer Black coffee 5 Rainwater 6 Urine

OH–

OH– –

H+ H+ OH – OH– OH + H H+ H+

Neutral [H+] = [OH–]

Saliva 7 Pure water Human blood, tears 8 Seawater Inside small intestine

Neutral solution

OH– –

OH–

H+ OH– OH– – OH OH H+

OH



Basic solution

Increasingly Basic [H+] < [OH–]

9 10 Milk of magnesia 11 Household ammonia 12 Household 13 bleach 14

Oven cleaner

Animation: Acids, Bases, and pH

52

UNIT ONE

The Chemistry of Life

The pH of a solution is defined as the negative logarithm (base 10) of the hydrogen ion concentration: pH = -log [H+] For a neutral aqueous solution, [H+] is 10-7 M, giving us -log 10-7 = -(-7) = 7 Notice that pH decreases as H+ concentration increases (see Figure 3.11). Notice, too, that although the pH scale is based on H+ concentration, it also implies OH- concentration. A solution of pH 10 has a hydrogen ion concentration of 10-10 M and a hydroxide ion concentration of 10-4 M. The pH of a neutral aqueous solution at 25°C is 7, the midpoint of the pH scale. A pH value less than 7 denotes an acidic solution; the lower the number, the more acidic the solution. The pH for basic solutions is above 7. Most biological fluids, such as blood and saliva, are within the range of pH 6–8. There are a few exceptions, however, including the strongly acidic digestive juice of the human stomach (gastric juice), which has a pH of about 2. Remember that each pH unit represents a tenfold difference in H+ and OH- concentrations. It is this mathematical feature that makes the pH scale so compact. A solution of pH 3 is not twice as acidic as a solution of pH 6, but 1,000 times (10 * 10 * 10) more acidic. When the pH of a solution changes slightly, the actual concentrations of H+ and OH- in the solution change substantially.

Buffers The internal pH of most living cells is close to 7. Even a slight change in pH can be harmful because the chemical processes of the cell are very sensitive to the concentrations of hydrogen and hydroxide ions. The pH of human blood is very close to 7.4, which is slightly basic. A person cannot survive for more than a few minutes if the blood pH drops to 7 or rises to 7.8, and a chemical system exists in the blood that maintains a stable pH. If 0.01 mol of a strong acid is added to a liter of pure water, the pH drops from 7.0 to 2.0. If the same amount of acid is added to a liter of blood, however, the pH decrease is only from 7.4 to 7.3. Why does the addition of acid have so much less of an effect on the pH of blood than it does on the pH of water? The presence of substances called buffers allows biological fluids to maintain a relatively constant pH despite the addition of acids or bases. A buffer is a substance that minimizes changes in the concentrations of H+ and OH- in a solution. It does so by accepting hydrogen ions from the solution when they are in excess and donating hydrogen ions to the solution when they have been depleted. Most buffer solutions contain a weak acid and its corresponding base, which combine reversibly with hydrogen ions. Several buffers contribute to pH stability in human blood and many other biological solutions. One of these is carbonic

acid (H2CO3), which is formed when CO2 reacts with water in blood plasma. As mentioned earlier, carbonic acid dissociates to yield a bicarbonate ion (HCO3-) and a hydrogen ion (H+): Response to a rise in pH

H2CO3



H + donor (acid)

Response to a drop in pH

Figure 3.12 Atmospheric CO2 from human activities and its fate in the ocean.

Some carbon dioxide (CO2) in the atmosphere dissolves in the ocean, where it reacts with water to form carbonic acid (H2CO3).



HCO

3

H + acceptor (base)

H

+

+

CO2

Hydrogen ion

The chemical equilibrium between carbonic acid and bicarbonate acts as a pH regulator, the reaction shifting left or right as other processes in the solution add or remove hydrogen ions. If the H+ concentration in blood begins to fall (that is, if pH rises), the reaction proceeds to the right and more carbonic acid dissociates, replenishing hydrogen ions. But when the H+ concentration in blood begins to rise (when pH drops), the reaction proceeds to the left, with HCO3- (the base) removing the hydrogen ions from the solution and forming H2CO3. Thus, the carbonic acid–bicarbonate buffering system consists of an acid and a base in equilibrium with each other. Most other buffers are also acid-base pairs.

CO2 + H2O

H2CO3 H+ + HCO3–

H2CO3 H+

+ CO3

2–

CO32– + Ca2+

HCO3



CaCO3

The added H+ combines with carbonate ions (CO32–), forming more HCO3–. Less CO32– is available for calcification —the formation of calcium carbonate (CaCO3)—by marine organisms such as corals.

Acidification: A Threat to Our Oceans Among the many threats to water quality posed by human activities is the burning of fossil fuels, which releases CO2 into the atmosphere. The resulting increase in atmospheric CO2 levels has caused global warming and other aspects of climate change (see Concept 56.4). In addition, about 25% of human-generated CO2 is absorbed by the oceans. In spite of the huge volume of water in the oceans, scientists worry that the absorption of so much CO2 will harm marine ecosystems. Recent data have shown that such fears are well founded. When CO2 dissolves in seawater, it reacts with water to form carbonic acid, which lowers ocean pH. This process, known as ocean acidification, alters the delicate balance of conditions for life in the oceans (Figure 3.12). Based on measurements of CO2 levels in air bubbles trapped in ice over thousands of years, scientists calculate that the pH of the oceans is 0.1 pH unit lower now than at any time in the past 420,000 years. Recent studies predict that it will drop another 0.3–0.5 pH unit by the end of this century. As seawater acidifies, the extra hydrogen ions combine with carbonate ions (CO32-) to form bicarbonate ions (HCO3-), thereby reducing the carbonate ion concentration (see Figure 3.12). Scientists predict that ocean acidification will cause the carbonate ion concentration to decrease by 40% by the year 2100. This is of great concern because carbonate ions are required for calcification, the production of calcium carbonate (CaCO3) by many marine organisms, including reef-building corals and animals that build shells. The Scientific Skills Exercise allows you to work with data from an experiment examining the effect of carbonate ion concentration on coral reefs. Coral reefs are sensitive ecosystems that act as havens for a great diversity of marine life.

Carbonic acid dissociates into hydrogen ions (H+) and bicarbonate ions (HCO3–).

VISUAL SKILLS Looking at all the chemical equations above, summarize the effect of adding excess CO2 to the oceans on the calcification process in the final equation.

ABC News Video: Ocean Acidification

The disappearance of coral reef ecosystems would be a tragic loss of biological diversity. If there is any reason for optimism about the future quality of water resources on our planet, it is that we have made progress in learning about the delicate chemical balances in oceans, lakes, and rivers. Continued progress can come only from the actions of informed individuals, like yourselves, who are concerned about environmental quality. This requires understanding the crucial role that water plays in the suitability of the environment for continued life on Earth.

CONCEPT CHECK 3.3 1. Compared with a basic solution at pH 9, the same volume of an acidic solution at pH 4 has _________ times as many hydrogen ions (H+). 2. HCl is a strong acid that dissociates in water: HCl S H+ + Cl-. What is the pH of 0.01 M HCl? 3. Acetic acid (CH3COOH) can be a buffer, similar to carbonic acid. Write the dissociation reaction, identifying the acid, base, H+ acceptor, and H+ donor. 4. WHAT IF? Given a liter of pure water and a liter solution of acetic acid, what would happen to the pH, in general, if you added 0.01 mol of a strong acid to each? Use the reaction from question 3 to explain the result. For suggested answers, see Appendix A. CHAPTER 3

Water and Life

53

SCIENTIFIC SKILLS EXERCISE

Interpreting a Scatter Plot with a Regression Line that acidification of the ocean due to higher levels of atmospheric CO2 will lower the concentration of dissolved carbonate ions, which living corals use to build calcium carbonate reef structures. In this exercise, you will analyze data from a controlled experiment that examined the effect of carbonate ion concentration ([CO32-]) on calcium carbonate deposition, a process called calcification.

How the Experiment Was Done For several years, scientists conducted research on ocean acidification using a large coral reef aquarium at Biosphere 2 in Arizona. They measured the rate of calcification by the reef organisms and examined how the calcification rate changed with differing amounts of dissolved carbonate ions in the seawater.

Calcification rate [mmol CaCO3 /(m2 • day)]

How Does the Carbonate Ion Concentration of Seawater Affect the Calcification Rate of a Coral Reef? Scientists predict

20

10

0 220

Data from the Experiment The black data points in the graph

1. When presented with a graph of experimental data, the first step in analysis is to determine what each axis represents. (a) In words, explain what is being shown on the x-axis. Be sure to include the units. (b) What is being shown on the yy-axis (including units)? (c) Which variable is the independent variable—the variable that was manipulated by the researchers? (d) Which variable is the dependent variable—the variable that responded to or depended on the treatment, which was measured by the researchers? (For additional information about graphs, see the Scientific Skills Review in Appendix F.) 2. Based on the data shown in the graph, describe in words the relationship between carbonate ion concentration and calcification rate. 3. (a) If the seawater carbonate ion concentration is 270 µmol/kg, what is the approximate rate of calcification, and approximately how many days would it take 1 square meter of reef to accumulate 30 mmol of calcium carbonate (CaCO3)? (b) If the seawater

3

Chapter Review

260

280

2–

[CO3 ] (μmol/kg of seawater)

form a scatter plot. The red line, known as a linear regression line, is the best-fitting straight line for these points. INTERPRET THE DAT ATA AT TA

240

Data from C. Langdon et al., Effect of calcium carbonate saturation state on the calcification rate of an experimental coral reef, Global Biogeochemical Cycles 14:639–654 (2000).

carbonate ion concentration is 250 µmol/kg, what is the approximate rate of calcification, and approximately how many days would it take 1 square meter of reef to accumulate 30 mmol of calcium carbonate? (c) If carbonate ion concentration decreases, how does the calcification rate change, and how does that affect the time it takes coral to grow? 4. (a) Referring to the equations in Figure 3.12, determine which step of the process is measured in this experiment. (b) Are the results of this experiment consistent with the hypothesis that increased atmospheric [CO2] will slow the growth of coral reefs? Why or why not? Instructors: A version of this Scientific Skills Exercise can be assigned in MasteringBiology.

Go to MasteringBiology™ for Videos, Animations, Vocab Self-Quiz, Practice Tests, and more in the Study Area. DRAW IT

Label a hydrogen bond and a polar covalent bond in the diagram of five water molecules. Is a hydrogen bond a covalent bond? Explain.

SUMMARY OF KEY CONCEPTS CONCEPT 3.1

CONCEPT 3.2

Polar covalent bonds in water molecules result in hydrogen bonding (p. 45)

Four emergent properties of water contribute to Earth’s suitability for life (pp. 45–50)

δ– δ+ H

VOCAB SELF-QUIZ

goo.gl/6u55ks

Water is a polar molecule. A hydrogen bond forms when a partially negatively charged region on the oxygen of one O δ– δ+ H water molecule is attracted to the δ+ δ– partially positively charged hydroδ– δ+ gen of a nearby water molecule. Hydrogen bonding between water molecules is the basis for water’s properties.

54

UNIT ONE

The Chemistry of Life

Hydrogen bonding keeps water molecules close to each other, giving water cohesion. Hydrogen bonding is also responsible for water’s surface tension. Water has a high specific heat: Heat is absorbed when hydrogen bonds break and is released when hydrogen bonds form. This helps keep temperatures relatively steady, within limits that permit life. Evaporative cooling is based on water’s high heat of vaporization. The evaporative loss of the most energetic water molecules cools a surface.

Ice floats because it is less dense than liquid water. This property allows life to exist under the frozen surfaces of lakes and polar seas. Water is an unusually versatile solvent because its polar molecules are attracted to ions and polar substances that can form hydrogen bonds. Hydrophilic substances have an affinity for water; hydrophobic substances do not. Molarity, the number of moles of solute per liter of solution, is used as a measure of solute concentration in solutions. A mole is a certain number of molecules of a substance. The mass of a mole of a substance in grams is the same as the molecular mass in daltons. The emergent properties of water support life on Earth and may contribute to the potential for life to have evolved on other planets.

?

Describe how different types of solutes dissolve in water. Explain what a solution is.

CONCEPT 3.3

Acidic and basic conditions affect living organisms (pp. 51–54) A water molecule can transfer an H+ to another water molecule to form H3O+ (represented simply by H+) and OH-. The concentration of H+ is 0 expressed as pH; pH = -log [H+]. Acidic A buffer consists of an acid-base [H+] > [OH–] Acids donate H+ in pair that combines reversibly aqueous solutions. with hydrogen ions, allowing it to resist pH changes. Neutral The burning of fossil fuels 7 [H+] = [OH–] increases the amount of CO2 in the atmosphere. Some CO2 Bases donate OH– dissolves in the oceans, causing or accept H+ in ocean acidification, which Basic aqueous solutions. has potentially grave conse[H+] < [OH–] quences for marine organisms 14 that rely on calcification.

?

Explain what happens to the concentration of hydrogen ions in an aqueous solution when you add a base and cause the concentration of OH - to rise to 10-3. What is the pH of this solution?

TEST YOUR UNDERSTANDING

Level 3: Synthesis/Evaluation 7. In agricultural areas, farmers pay close attention to the weather forecast. Right before a predicted overnight freeze, farmers spray water on crops to protect the plants. Use the properties of water to explain how this method works. Be sure to mention why hydrogen bonds are responsible for this phenomenon. 8. MAKE CONNECTIONS What do climate change (see Concept 1.1 and Concept 3.2) and ocean acidification have in common? 9. EVOLUTION CONNECTION This chapter explains how the emergent properties of water contribute to the suitability of the environment for life. Until fairly recently, scientists assumed that other physical requirements for life included a moderate range of temperature, pH, atmospheric pressure, and salinity, as well as low levels of toxic chemicals. That view has changed with the discovery of organisms known as extremophiles, which have been found flourishing in hot, acidic sulfur springs, around hydrothermal vents deep in the ocean, and in soils with high levels of toxic metals. Why would astrobiologists be interested in studying extremophiles? What does the existence of life in such extreme environments say about the possibility of life on other planets? 10. SCIENTIFIC INQUIRY Design a controlled experiment to test the hypothesis that water acidification caused by acidic rain would inhibit the growth of Elodea, a freshwater plant (see Figure 2.17). 11. WRITE ABOUT A THEME: ORGANIZATION Several emergent properties of water contribute to the suitability of the environment for life. In a short essay (100–150 words), describe how the ability of water to function as a versatile solvent arises from the structure of water molecules. 12. SYNTHESIZE YOUR KNOWLEDGE

Level 1: Knowledge/Comprehension 1. Which of the following is a hydrophobic material? (A) paper (B) table salt (C) wax (D) sugar

6. DRAW IT Draw the hydration shells that form around a potassium ion and a chloride ion when potassium chloride (KCl) dissolves in water. Label the positive, negative, and partial charges on the atoms.

PRACTICE

TEST 2. We can be sure that a mole of table sugar and a goo.gl/CUYGKD mole of vitamin C are equal in their (A) mass. (B) volume. (C) number of atoms. (D) number of molecules.

3. Measurements show that the pH of a particular lake is 4.0. What is the hydrogen ion concentration of the lake? (A) 4.0 M (B) 10-10 M -4 (C) 10 M (D) 104 M 4. What is the hydroxide ion concentration of the lake described in question 3? (A) 10-10 M (B) 10-4 M (C) 10-7 M (D) 10.0 M

Level 2: Application/Analysis 5. A slice of pizza has 500 kcal. If we could burn the pizza and use all the heat to warm a 50-L container of cold water, what would be the approximate increase in the temperature of the water? (Note: A liter of cold water weighs about 1 kg.) (A) 50°C (B) 5°C (C) 100°C (D) 10°C

How do cats drink? Scientists using highspeed video have shown that cats use an interesting technique to drink aqueous substances like water and milk. Four times a second, the cat touches the tip of its tongue to the water and draws a column of water up into its mouth (as you can see in the photo), which then shuts before gravity can pull the water back down. Describe how the properties of water allow cats to drink in this fashion, including how water’s molecular structure contributes to the process. For selected answers, see Appendix A. For additional practice questions, check out the Dynamic Study Modules in MasteringBiology. You can use them to study on your smartphone, tablet, or computer anytime, anywhere!

CHAPTER 3

Water and Life

55

Superset

Carbon and the Molecular Diversity of Life

4

Figure 4.1 What properties make carbon the basis of all life?

KEY CONCEPTS

Carbon: The Backbone of Life

4.1

Organic chemistry is the study of carbon compounds

4.2

Carbon atoms can form diverse molecules by bonding to four other atoms

4.3

A few chemical groups are key to molecular function

Living organisms, such as the plants and the Qinling golden snub-nosed monkeys shown in Figure 4.1, are made up of chemicals based mostly on the element carbon. Carbon enters the biosphere through the action of producers—plants and other photosynthetic organisms that use solar energy to transform atmospheric CO2 into the carbon-based molecules of life. These molecules are then taken up by consumers, which feed on other organisms. Of all the chemical elements, carbon is unparalleled in its ability to form molecules that are large, complex, and varied, making possible the diversity of organisms that have evolved on Earth. Proteins, DNA, carbohydrates, and other molecules that distinguish living matter from inanimate material are all composed of carbon atoms bonded to one another and to atoms of other elements. Hydrogen (H), oxygen (O), nitrogen (N), sulfur (S), and phosphorus (P) are other common ingredients of these compounds, but it is the element carbon (C) that accounts for the enormous variety of biological molecules. Large biological molecules, such as proteins, are the main focus of Chapter 5. In this chapter, we investigate the properties of smaller molecules. We will use these small molecules to illustrate concepts of molecular architecture that help explain why carbon is so important to life, while highlighting the theme that emergent properties arise from the organization of matter in living organisms.

Carbon can bond to four other atoms or groups of atoms, making a large variety of molecules possible.

56

When you see this blue icon, log in to MasteringBiology and go to the Study Area for digital resources.

Get Ready for This Chapter

CONCEPT

4.1

Organic chemistry is the study of carbon compounds For historical reasons, compounds containing carbon are said to be organic, and their study is called organic chemistry. By the early 1800s, chemists had learned to make simple compounds in the laboratory by combining elements under the right conditions. Artificial synthesis of the complex molecules extracted from living matter seemed impossible, however. Organic compounds were thought to arise only in living organisms, which were believed to contain a life force beyond the jurisdiction of physical and chemical laws. Chemists began to chip away at this notion when they learned to synthesize organic compounds in the laboratory. In 1828, Friedrich Wöhler, a German chemist, tried to make an “inorganic” salt, ammonium cyanate, by mixing solutions of ammonium ions (NH4+) and cyanate ions (CNO-). Wöhler was astonished to find that instead he had made urea, an organic compound present in the urine of animals. The next few decades saw laboratory synthesis of increasingly complex organic compounds, supporting the view that physical and chemical laws govern the processes of life. Organic chemistry was redefined as the study of carbon compounds, regardless of origin. Organic compounds range from simple molecules, such as methane (CH4), to colossal ones, such as proteins, with thousands of atoms.

Figure 4.2

Inquiry Can organic molecules form under conditions estimated to simulate those on the early Earth? Experiment In 1953, Stanley Miller set up a closed system to mimic conditions thought at that time to have existed on the early Earth. A flask of water simulated the primeval sea. The water was heated so that some vaporized and moved into a second, higher flask containing the “atmosphere”—a mixture of gases. Sparks were discharged in the synthetic atmosphere to mimic lightning. 2 The “atmosphere” contained a mixture of hydrogen gas (H2), methane (CH4), ammonia (NH3), and water vapor.

“Atmosphere” CH4

Water vapor 1 The water

mixture in the ”sea” flask was heated; vapor entered the ”atmosphere” flask.

Electrode NH

3

H2

Condenser Cooled “rain” containing organic molecules

Cold water

H2O “sea”

Organic Molecules and the Origin of Life on Earth EVOLUTION In 1953, Stanley Miller, a graduate student of Harold Urey at the University of Chicago, helped bring the abiotic (nonliving) synthesis of organic compounds into the context of evolution. Study Figure 4.2 to learn about his classic experiment. From his results, Miller concluded that complex organic molecules could arise spontaneously under conditions thought at that time to have existed on the early Earth. You can work with the data from a related experiment in the Scientific Skills Exercise. These experiments support the idea that abiotic synthesis of organic compounds, perhaps near volcanoes, could have been an early stage in the origin of life (see Figure 25.2). The overall percentages of the major elements of life—C, H, O, N, S, and P—are quite uniform from one organism to another, reflecting the common evolutionary origin of all life. Because of carbon’s ability to form four bonds, however, this limited assortment of atomic building blocks can be used to build an inexhaustible variety of organic molecules. Different species of organisms, and different individuals within a species, are distinguished by variations in the types of organic molecules they make. In a sense, the great diversity

3 Sparks were discharged to mimic lightning.

Sample for chemical analysis 5 As material cycled through the apparatus, Miller periodically collected samples for analysis.

4 A condenser cooled the atmosphere, raining water and dissolved molecules into the sea flask.

Results Miller identified a variety of organic molecules that are common in organisms. These included simple compounds, such as formaldehyde (CH2O) and hydrogen cyanide (HCN), and more complex molecules, such as amino acids and long chains of carbon and hydrogen known as hydrocarbons. Conclusion Organic molecules, a first step in the origin of life, may have been synthesized abiotically on the early Earth. Although new evidence indicates that the early Earth’s atmosphere was different from the “atmosphere” used by Miller in this experiment, recent experiments using the revised list of chemicals also produced organic molecules. (We will explore this hypothesis in more detail in Concept 25.1.) Data from S. L. Miller, A production of amino acids under possible primitive Earth conditions, Science 117:528–529 (1953).

WHAT IF If Miller had increased the concentration of NH3 in his experiment, how might the relative amounts of the products HCN and CH2O have differed? Interview with Stanley Miller: Investigating the origin of life

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Carbon and the Molecular Diversity of Life

57

SCIENTIFIC SKILLS EXERCISE

Working with Moles and Molar Ratios Could the First Biological Molecules Have Formed Near Volcanoes on Early Earth? In 2007, Jeffrey Bada, a former gradgrad uate student of Stanley Miller, discovered some vials of samples that had never been analyzed from an experiment performed by Miller in 1958. In that experiment, Miller used hydrogen sulfide gas (H2S) as one of the gases in the reactant mixture. Since H2S is released by volcanoes, the H2S experiment was designed to mimic conditions near volcanoes on early Earth. In 2011, Bada and colleagues published the results of their analysis of these “lost” samples. In this exercise, you will make calculations using the molar ratios of reactants and products from the H2S experiment.

How the Experiment Was Done According to his laboratory notebook, Miller used the same apparatus as in his original experiment (see Figure 4.2), but the mixture of gaseous reactants included methane (CH4), carbon dioxide (CO2), hydrogen sulfide (H2S), and ammonia (NH3). After three days of simulated volcanic activity, he collected samples of the liquid, partially purified the chemicals, and sealed the samples in sterile vials. In 2011, Bada’s research team used modern analytical methods to analyze the products in the vials for the presence of amino acids, the building blocks of proteins. Data from the Experiment The table below shows 4 of the 23 amino acids detected in the 2011 analysis of the samples from Miller’s 1958 H2S experiment. Product Compound

Molecular Formula

Molar Ratio (Relative to Glycine)

Glycine

C2H5NO2

1.0

Serine

C3H7NO3

3.0 * 10-2

C5H11NO2S

1.8 * 10-3

C3H7NO2

1.1

Methionine Alanine

Data from E. T. Parker et al., Primordial synthesis of amines and amino acids in a 1958 Miller H2S-rich spark discharge experiment, Proceedings of the National Academy of Sciences USA 108:5526-5531 (2011). www.pnas.org/cgi/doi/10.1073/ pnas.1019191108.

INTERPRET THE DATA 1. A mole is the number of particles of a substance with a mass equivalent to its molecular (or atomic) mass in daltons. There are 6.02 * 1023 molecules (or atoms) in 1.0 mole (Avogadro’s number; see Concept 3.2). The data table shows the “molar ratios” of some of the products from the Miller H2S experiment. In a molar ratio, each unitless value is expressed relative to a standard for

of living organisms we see on the planet (and in fossil remains) is made possible by the unique chemical versatility of the element carbon.

CONCEPT CHECK 4.1 1. Why was Wöhler astonished to find he had made urea? 2. VISUAL SKILLS See Figure 4.2. Miller carried out a control experiment without discharging sparks and found no organic compounds. What might explain this result? For suggested answers, see Appendix A.

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Some of Stanley Miller’s notes from his 1958 hydrogen sulfide (H2S) experiment along with his original vials. that experiment. Here, the standard is the number of moles of the amino acid glycine, which is set to a value of 1.0. For instance, serine has a molar ratio of 3.0 * 10-2, meaning that for every mole of glycine, there is 3.0 * 10-2 mole of serine. (a) Give the molar ratio of methionine to glycine and explain what it means. (b) How many molecules of glycine are present in 1.0 mole? (c) For every 1.0 mole of glycine in the sample, how many molecules of methionine are present? (Recall that to multiply two numbers with exponents, you add their exponents; to divide them, you subtract the exponent in the denominator from that in the numerator.) 2. (a) Which amino acid is present in higher amounts than glycine? (b) How many more molecules of that amino acid are present than the number of molecules in 1.0 mole of glycine? 3. The synthesis of products is limited by the amount of reactants. (a) If one mole each of CH4, NH3, H2S, and CO2 is added to 1 liter of water (= 55.5 moles of H2O) in a flask, how many moles of hydrogen, carbon, oxygen, nitrogen, and sulfur are in the flask? (b) Looking at the molecular formula in the table, how many moles of each element would be needed to make 1.0 mole of glycine? (c) What is the maximum number of moles of glycine that could be made in that flask, with the specified ingredients, if no other molecules were made? Explain. (d) If serine or methionine were made individually, which element(s) would be used up first for each? How much of each product could be made? 4. The earlier published experiment carried out by Miller did not include H2S in the reactants (see Figure 4.2). Which of the compounds shown in the data table can be made in the H2S experiment but could not be made in the earlier experiment? Instructors: A version of this Scientific Skills Exercise can be assigned in MasteringBiology.

CONCEPT

4.2

Carbon atoms can form diverse molecules by bonding to four other atoms The key to an atom’s chemical characteristics is its electron configuration. This configuration determines the kinds and number of bonds an atom will form with other atoms. Recall that it is the valence electrons, those in

Figure 4.3 The shapes of three simple organic molecules. Molecule and Molecular Shape (a) Methane. When a carbon atom has four single bonds to other atoms, the molecule is tetrahedral.

Molecular Formula

Structural Formula

Ball-and-Stick Model (molecular shape in pink)

Space-Filling Model

H CH4

C

H

H

H

C

C

H

H

C

C

H

The Formation of Bonds with Carbon Carbon has 6 electrons, with 2 in the first electron shell and 4 in the second shell; thus, it has 4 valence electrons in a shell that can hold up to 8 electrons. A carbon atom usually completes its valence shell by sharing its 4 electrons with other atoms so that 8 electrons are present. Each pair of shared electrons constitutes a covalent bond (see Figure 2.10d). In organic molecules, carbon usually forms single or double covalent bonds. Each carbon atom acts as an intersection point from which a molecule can branch off in as many as four directions. This enables carbon to form large, complex molecules. When a carbon atom forms four single covalent bonds, the arrangement of its four hybrid orbitals causes the bonds to angle toward the corners of an imaginary tetrahedron. The bond angles in methane (CH4) are 109.5° (Figure 4.3a), and they are roughly the same in any group of atoms where carbon has four single bonds. For example, ethane (C2H6) is shaped like two overlapping tetrahedrons (Figure 4.3b). In molecules with more carbons, every grouping of a carbon bonded to four other atoms has a tetrahedral shape. But when two carbon atoms are joined by a double bond, as in ethene (C2H4), the bonds from both carbons are all in the same plane, so the atoms joined to those carbons are in the same plane as well (Figure 4.3c). We find it convenient to write molecules as structural formulas, as if the molecules being represented are two-dimensional,

Figure 4.4 Valences of the major elements of organic molecules. Valence, the number of covalent bonds an atom can form, is generally equal to the number of electrons required to complete the valence shell. All electrons are shown in the electron distribution diagrams (top), but only valence shell electrons are shown in the Lewis dot structures (bottom). Note that carbon can form four bonds. Hydrogen (valence = 1)

Oxygen (valence = 2)

Nitrogen (valence = 3)

Carbon (valence = 4)

H

O

N

C

H•



O ••



N•



the outermost shell, that are available to form bonds with other atoms.

H



C• •

H

• •

H C2H4

H



H

H

• •

(c) Ethene (ethylene). When two carbon atoms are joined by a double bond, all atoms attached to those carbons are in the same plane, and the molecule is flat.

C2H6

H



(b) Ethane. A molecule may have more than one tetrahedral group of single-bonded atoms. (Ethane consists of two such groups.)

MAKE CONNECTIONS Draw the Lewis dot structures for sodium, phosphorus, sulfur, and chlorine. (Refer to Figure 2.7.)

but keep in mind that molecules are three-dimensional and that the shape of a molecule is central to its function. The number of unpaired electrons in the valence shell of an atom is generally equal to the atom’s valence, the number of covalent bonds it can form. Figure 4.4 shows the valences of carbon and its most frequent bonding partners— hydrogen, oxygen, and nitrogen. These are the four main atoms in organic molecules. The electron configuration of carbon gives it covalent compatibility with many different elements. Let’s consider how valence and the rules of covalent bonding apply to carbon atoms with partners other than hydrogen. We’ll look at two examples, the simple molecules carbon dioxide and urea. CHAPTER 4

Carbon and the Molecular Diversity of Life

59

In the carbon dioxide molecule (CO2), a single carbon atom is joined to two atoms of oxygen by double covalent bonds. The structural formula for CO2 is shown here:

Figure 4.5 Four ways that carbon skeletons can vary. (a) Length

O“ C“O Each line in a structural formula represents a pair of shared electrons. Thus, the two double bonds in CO2 have the same number of shared electrons as four single bonds. The arrangement completes the valence shells of all atoms in the molecule:

H

H

H

C

C

H

H

H

H

Ethane

H

H

H

C

C

C

H

H

H

H

Propane

Carbon skeletons vary in length. (b) Branching

O

C

H

O H

Because CO2 is a very simple molecule and lacks hydrogen, it is often considered inorganic, even though it contains carbon. Whether we call CO2 organic or inorganic, however, it is clearly important to the living world as the source of carbon, via photosynthetic organisms, for all organic molecules in organisms (see Concept 2.4). Urea, CO(NH2)2, is the organic comO pound found in urine that Wöhler syntheH H C sized in the early 1800s. Again, each atom N N has the required number of covalent bonds. H H In this case, one carbon atom participates in Urea both single and double bonds. Urea and carbon dioxide are molecules with only one carbon atom. But as Figure 4.3 shows, a carbon atom can also use one or more valence electrons to form covalent bonds to other carbon atoms, each of which can also form four bonds. Thus, the atoms can be linked into chains of seemingly infinite variety.

Molecular Diversity Arising from Variation in Carbon Skeletons Carbon chains form the skeletons of most organic molecules. The skeletons vary in length and may be straight, branched, or arranged in closed rings (Figure 4.5). Some carbon skeletons have double bonds, which vary in number and location. Such variation in carbon skeletons is one important source of the molecular complexity and diversity that characterize living matter. In addition, atoms of other elements can be bonded to the skeletons at available sites.

Hydrocarbons All of the molecules that are shown in Figures 4.3 and 4.5 are hydrocarbons, organic molecules consisting of only carbon and hydrogen. Atoms of hydrogen are attached to the carbon skeleton wherever electrons are available for covalent bonding. Hydrocarbons are the major components of petroleum, which is called a fossil fuel because it consists of the partially decomposed remains of organisms that lived millions of years ago. 60

UNIT ONE

The Chemistry of Life

H

H

H

H

H

C

C

C

C

H

H

H

H

H

C

H H

H

H

C

C

C

H

H

H

H

2-Methylpropane (commonly called isobutane)

Butane

Skeletons may be unbranched or branched. (c) Double bond position

H

H

H

H

H

C

C

C

C

H

H

H

H

H

H

H

H

C

C

C

C

H

1-Butene

H

H

2-Butene

The skeleton may have double bonds, which can vary in location. (d) Presence of rings H H H H H

C

C

C H C

H

H C C H

H H H H

Cyclohexane

H

H

C C

C

C H

C C

H

H

Benzene

Some carbon skeletons are arranged in rings. In the abbreviated structural formula for each compound (to its right), each corner represents a carbon and its attached hydrogens. Animation: Diversity of Carbon-Based Molecules

Although hydrocarbons are not prevalent in most living organisms, many of a cell’s organic molecules have regions consisting of only carbon and hydrogen. For example, the molecules known as fats have long hydrocarbon tails attached to a nonhydrocarbon component (Figure 4.6). Neither petroleum nor fat dissolves in water; both are hydrophobic compounds because the great majority of their bonds are relatively nonpolar carbon-to-hydrogen linkages. Another characteristic of hydrocarbons is that they can undergo reactions that release a relatively large amount of energy. The gasoline that fuels a car consists of hydrocarbons, and the hydrocarbon tails of fats serve as stored fuel for plant embryos (seeds) and animals.

Nucleus Fat droplets

Figure 4.7 Three types of isomers. Isomers are compounds that have the same molecular formula but different structures. (a) Structural isomers H

H

H

H

H

H

C

C

C

C

C

H

H

H H

C

H H

H

C

C

C

C

H

H H H H H H H H Pentane 2-Methylbutane Structural isomers differ in covalent partners, as shown in this example of two isomers of C5H12. H

10 μm (a) Part of a human adipose cell

H

(b) A fat molecule

Figure 4.6 The role of hydrocarbons in fats. (a) Mammalian adipose cells stockpile fat molecules as a fuel reserve. This colorized micrograph shows part of a human adipose cell with many fat droplets, each containing a large number of fat molecules. (b) A fat molecule consists of a small, nonhydrocarbon component joined to three hydrocarbon tails that account for the hydrophobic behavior of fats. The tails can be broken down to provide energy. (Black = carbon; gray = hydrogen; red = oxygen.) MAKE CONNECTIONS

How do the tails account for the hydrophobic nature of fats? (See Concept 3.2.)

Isomers Variation in the architecture of organic molecules can be seen in isomers, compounds that have the same numbers of atoms of the same elements but different structures and hence different properties. We will examine three types of isomers: structural isomers, cis-trans isomers, and enantiomers. Structural isomers differ in the covalent arrangements of their atoms. Compare, for example, the two five-carbon compounds in Figure 4.7a. Both have the molecular formula C5H12, but they differ in the covalent arrangement of their carbon skeletons. The skeleton is straight in one compound but branched in the other. The number of possible isomers increases tremendously as carbon skeletons increase in size. There are only three forms of C5H12 (two of which are shown in Figure 4.7a), but there are 18 variants of C8H18 and 366,319 possible structural isomers of C20H42. Structural isomers may also differ in the location of double bonds. In cis-trans isomers, carbons have covalent bonds to the same atoms, but these atoms differ in their spatial arrangements due to the inflexibility of double bonds. Single bonds allow the atoms they join to rotate freely about the bond axis without changing the compound. In contrast, double bonds do not permit such rotation. If a double bond joins two carbon atoms, and each C also has two different atoms (or groups of atoms) attached to it, then two distinct cis-trans isomers are possible. Consider a simple molecule with two double-bonded carbons, each of which has an H and an X attached to it (Figure 4.7b). The arrangement with both Xs on the same side of the double bond is called a cis isomer, and that with the Xs on opposite sides is called a trans isomer. The subtle difference in shape between such isomers can have a

(b) Cis-trans isomers X

X C

C

H

X

H C

C

X

H

cis isomer: The two Xs are on the same side.

H

trans isomer: The two Xs are on opposite sides.

Cis-trans isomers differ in arrangement about a double bond. In these diagrams, X represents an atom or group of atoms attached to a double-bonded carbon. (c) Enantiomers CO2H

CO2H

C H

C NH2

CH3 L isomer

NH2

H CH3 D isomer

Enantiomers differ in spatial arrangement around an asymmetric carbon, resulting in molecules that are mirror images, like left and right hands. The two isomers here are designated the L and D isomers from the Latin for “left” and ”right” (levo and dextro). Enantiomers cannot be superimposed on each other. DRAW IT shown in (a).

There are three structural isomers of C5H12 ; draw the one not

Animation: Isomers

dramatic effect on the biological activities of organic molecules. For example, the biochemistry of vision involves a light-induced change of retinal, a chemical compound in the eye, from the cis isomer to the trans isomer (see Figure 50.17). Another example involves trans fats, harmful fats formed during food processing that are discussed in Concept 5.3. Enantiomers are isomers that are mirror images of each other and that differ in shape due to the presence of an asymmetric carbon, one that is attached to four different atoms or groups of atoms. (See the middle carbon in the balland-stick models shown in Figure 4.7c.) The four groups can

CHAPTER 4

Carbon and the Molecular Diversity of Life

61

Figure 4.8 The pharmacological importance of enantiomers. Ibuprofen and albuterol are drugs whose enantiomers have different effects. (S and R are used here to distinguish between enantiomers.) Ibuprofen is commonly sold as a mixture of the two enantiomers; the S enantiomer is 100 times more effective than the R form. Albuterol is synthesized and sold only as the R form of that particular drug; the S form counteracts the active R form. Drug

Effects

Ibuprofen

Reduces inflammation and pain

Albuterol

Relaxes bronchial (airway) muscles, improving airflow in asthma patients

Effective Enantiomer

Ineffective Enantiomer

S-Ibuprofen

R-Ibuprofen

R-Albuterol

S-Albuterol

be arranged in space around the asymmetric carbon in two different ways that are mirror images. Enantiomers are, in a way, left-handed and right-handed versions of the molecule. Just as your right hand won’t fit into a left-handed glove, a “right-handed” molecule won’t fit into the same space as the “left-handed” version. Usually, only one isomer is biologically active because only that form can bind to specific molecules in an organism. The concept of enantiomers is important in the pharmaceutical industry because the two enantiomers of a drug may not be equally effective, as is the case for both ibuprofen and the asthma medication albuterol (Figure 4.8). Methamphetamine also occurs in two enantiomers that have very different effects. One enantiomer is the highly addictive stimulant drug known as “crank,” sold illegally in the street drug trade. The other has a much weaker effect and is the active ingredient in an over-the-counter vapor inhaler for treatment of nasal congestion. The differing effects of enantiomers in the body demonstrate that organisms are sensitive to even the subtlest variations in molecular architecture. Once again, we see that molecules have emergent properties that depend on the specific arrangement of their atoms.

CONCEPT CHECK 4.2 1. DRAW IT (a) Draw a structural formula for C2H4. (b) Draw the trans isomer of C2H2Cl2. 2. VISUAL SKILLS Which two pairs of molecules in Figure 4.5 are isomers? For each pair, identify the type of isomer. 3. How are gasoline and fat chemically similar? 4. VISUAL SKILLS See Figures 4.5a and 4.7. Can propane (C3H8) form isomers? Explain. For suggested answers, see Appendix A.

62

UNIT ONE

The Chemistry of Life

CONCEPT

4.3

A few chemical groups are key to molecular function The properties of an organic molecule depend not only on the arrangement of its carbon skeleton but also on the various chemical groups attached to that skeleton. We can think of hydrocarbons, the simplest organic molecules, as the underlying framework for more complex organic molecules. A number of chemical groups can replace one or more hydrogens of the hydrocarbon. These groups may participate in chemical reactions or may contribute to function indirectly by their effects on molecular shape; they help give each molecule its unique properties.

The Chemical Groups Most Important in the Processes of Life Consider the differences between estradiol (a type of estrogen) and testosterone. These compounds are female and male sex hormones, respectively, in humans and other vertebrates. Both are steroids, organic molecules with a common carbon skeleton in the form of four fused rings. They differ only in the chemical groups attached to the rings (shown here in abbreviated form); the distinctions in molecular architecture are shaded in blue: Estradiol

CH3

OH

Testosterone

CH3

OH

CH3

HO

O

The different actions of these two molecules on many targets throughout the body are the basis of gender, producing the contrasting features of male and female vertebrates. In this case, the chemical groups are important because they affect molecular shape, contributing to function. In other cases, chemical groups are directly involved in chemical reactions; such groups are known as functional groups. Each has certain properties, such as shape and charge, that cause it to participate in chemical reactions in a characteristic way. The seven chemical groups most important in biological processes are the hydroxyl, carbonyl, carboxyl, amino, sulfhydryl, phosphate, and methyl groups. The first six groups can be chemically reactive; of these six, all except the sulfhydryl group are also hydrophilic and thus increase the solubility of organic compounds in water. The methyl group is not reactive, but instead often serves as a recognizable tag on biological molecules. Study Figure 4.9 to become familiar with these biologically important chemical groups.

Figure 4.9 Some biologically important chemical groups. Group Properties and Compound Name

Chemical Group Hydroxyl group (

Is polar due to electronegative oxygen. Forms hydrogen bonds with water, helping dissolve compounds such as sugars.

OH ) OH

(may be written HO

Carbonyl group (

C

)

Sugars with ketone groups are called ketoses; those with aldehydes are called aldoses.

O)

Compound name: Ketone (carbonyl group is within a carbon skeleton) or aldehyde (carbonyl group is at the end of a carbon skeleton)

C

Acts as an acid (can donate H+) because the covalent bond between oxygen and hydrogen is so polar.

COOH)

O

H

Compound name: Amine

N

SH (may be written HS

)

OPO32–) O O–

O–

H

O

H

C

C

C

H

H

H

H H

C

H H

H

C

H

H

H

H

H

Propanal, an aldehyde

+ H+

H H+

+

N H

H

O

C

Ionized form of COOH (carboxylate ion), found in cells

H

C

O

O–

OH

C

+N

H

H Ionized form of found in cells

NH2,

OH

C C

CH2

SH

Cysteine, a sulfurcontaining amino acid

N H

OH OH H H

Compound name: Organic phosphate

Compound name: Methylated compound

C

C

H

Contributes negative charge (1– when positioned inside a chain of phosphates; 2– when at the end). When attached, confers on a molecule the ability to react with water, releasing energy.

H

O

C

O

Two — SH groups can react, forming a “cross-link” that helps stabilize protein structure. Hair protein cross-links maintain the straightness or curliness of hair; in hair salons, permanent treatments break cross-links, then re-form them while the hair is in the desired shape.

H

O

H

Affects the expression of genes when on DNA or on proteins bound to DNA. Affects the shape and function of male and female sex hormones.

CH3)

C

H

Ethanol, the alcohol present in alcoholic beverages

OH

Acetone, the simplest ketone

Compound name: Thiol

Methyl group (

C

Glycine, an amino acid (note its carboxyl group)

SH)

P

C

H

HO

H

O

H

H

Acts as a base; can pick up an H+ from the surrounding solution (water, in living organisms).

NH2)

Phosphate group (

H

Acetic acid, which gives vinegar its sour taste

OH

Sulfhydryl group (

H

Compound name: Carboxylic acid, or organic acid

C

Amino group (

H

Compound name: Alcohol (specific name usually ends in -ol )

O

Carboxyl group (

Examples

C

C

C

H

H

H

O O

P O–

O–

Glycerol phosphate, which takes part in many important chemical reactions in cells

NH2 N O

C

C

N

C C

CH3

H

5-Methylcytosine, a component of DNA that has been modified by addition of a methyl group

H

Animation: Functional Groups CHAPTER 4

Carbon and the Molecular Diversity of Life

63

ATP: An Important Source of Energy for Cellular Processes

The Chemical Elements of Life: A Review Living matter, as you have learned, consists mainly of carbon, oxygen, hydrogen, and nitrogen, with smaller amounts of sulfur and phosphorus. These elements all form strong covalent bonds, an essential characteristic in the architecture of complex organic molecules. Of all these elements, carbon is the virtuoso of the covalent bond. The versatility of carbon makes possible the great diversity of organic molecules, each with particular properties that emerge from the unique arrangement of its carbon skeleton and the chemical groups appended to that skeleton. This variation at the molecular level provides the foundation for the rich biological diversity found on our planet.

The “Phosphate group” row in Figure 4.9 shows a simple example of an organic phosphate molecule. A more complicated organic phosphate, adenosine triphosphate, or ATP, is worth mentioning here because its function in the cell is so important. ATP consists of an organic molecule called adenosine attached to a string of three phosphate groups: O –O

O

P

O

O–

P

O O

O–

P

O

Adenosine

O–

Where three phosphates are present in series, as in ATP, one phosphate may be split off as a result of a reaction with water. This inorganic phosphate ion, HOPO32-, is often abbreviated P i in this book, and a phosphate group in an organic molecule is often written as P . Having lost one phosphate, ATP becomes adenosine diphosphate, or ADP. Although ATP is sometimes said to store energy, it is more accurate to think of it as storing the potential to react with water. This reaction releases energy that can be used by the cell. You will learn about this in more detail in Concept 8.3. Reacts with H2O P P

P

Adenosine

ATP

4

P

P

Adenosine ADP

+

Pi

+

CONCEPT CHECK 4.3 1. VISUAL SKILLS What does the term amino acid signify about the structure of such a molecule? See Figure 4.9. 2. What chemical change occurs to ATP when it reacts with water and releases energy?

Energy

Inorganic phosphate

Chapter Review

CONCEPT 4.1

Organic chemistry is the study of carbon compounds (pp. 57–58) VOCAB

Organic compounds, once thought to arise only SELF-QUIZ goo.gl/6u55ks within living organisms, were finally synthesized in the laboratory. Living matter is made mostly of carbon, oxygen, hydrogen, and nitrogen. Biological diversity results from carbon’s ability to form a huge number of molecules with particular shapes and properties. How did Stanley Miller’s experiments support the idea that, even at life’s origins, physical and chemical laws govern the processes of life?

CONCEPT 4.2

Carbon atoms can form diverse molecules by bonding to four other atoms (pp. 58–62) Carbon, with a valence of 4, can bond to various other atoms, including O, H, and N. Carbon can also bond to other carbon

64

UNIT ONE

The Chemistry of Life

For suggested answers, see Appendix A.

Go to MasteringBiology™ for Videos, Animations, Vocab Self-Quiz, Practice Tests, and more in the Study Area.

SUMMARY OF KEY CONCEPTS

?

3. DRAW IT Suppose you had an organic molecule such as cysteine (see Figure 4.9, sulfhydryl group example), and you chemically removed the —NH2 group and replaced it with —COOH. Draw this structure. How would this change the chemical properties of the molecule? Is the central carbon asymmetric before the change? After?

atoms, forming the carbon skeletons of organic compounds. These skeletons vary in length and shape and have bonding sites for atoms of other elements. Hydrocarbons consist of carbon and hydrogen. Isomers are compounds that have the same molecular formula but different structures and therefore different properties. Three types of isomers are structural isomers, cis-trans isomers, and enantiomers. VISUAL SKILLS

Refer back to Figure 4.9. What type of isomers are acetone and propanal? How many asymmetric carbons are present in acetic acid, glycine, and glycerol phosphate? Can these three molecules exist as forms that are enantiomers?

CONCEPT 4.3

A few chemical groups are key to molecular function (pp. 62–64) Chemical groups attached to the carbon skeletons of organic molecules participate in chemical reactions (functional groups) or contribute to function by affecting molecular shape (see Figure 4.9). ATP (adenosine triphosphate) consists of adenosine attached to three phosphate groups. ATP can react with water, forming

Level 3: Synthesis/Evaluation

ADP (adenosine diphosphate) and inorganic phosphate. This reaction releases energy that can be used by the cell.

P

P

P

Adenosine

Reacts with H2O

P

P

ATP

?

+

Adenosine ADP

Pi +

Energy

Inorganic phosphate

In what ways does a methyl group differ chemically from the other six important chemical groups shown in Figure 4.9?

TEST YOUR UNDERSTANDING Level 1: Knowledge/Comprehension 1. Organic chemistry is currently defined as (A) the study of compounds made only by living cells. (B) the study of carbon compounds. (C) the study of natural (as opposed to synthetic) compounds. (D) the study of hydrocarbons.

PRACTICE TEST

goo.gl/CUYGKD

2. VISUAL SKILLS Which functional group is not present in this molecule? HO O (A) carboxyl C H (B) sulfhydryl H C C OH (C) hydroxyl (D) amino N H

H

H

3. MAKE CONNECTIONS Which chemical group is most likely to be responsible for an organic molecule behaving as a base (see Concept 3.3)? (A) hydroxyl (C) amino (B) carbonyl (D) phosphate

Level 2: Application/Analysis 4. VISUAL SKILLS Visualize the structural formula of each of the following hydrocarbons. Which hydrocarbon has a double bond in its carbon skeleton? (A) C3H8 (C) C2H4 (B) C2H6 (D) C2H2

9. EVOLUTION CONNECTION • DRAW IT Some scientists think that life elsewhere in the universe might be based on the element silicon, rather than on carbon, as on Earth. Look at the electron distribution diagram for silicon in Figure 2.7 and draw the Lewis dot structure for silicon. What properties does silicon share with carbon that would make silicon-based life more likely than, say, neon-based life or aluminum-based life? 10. SCIENTIFIC INQUIRY 50 years ago, pregnant women who were prescribed thalidomide for morning sickness gave birth to children with birth defects. Thalidomide is a mixture of two enantiomers; one reduces morning sickness, but the other causes severe birth defects. Today, the FDA has approved this drug for non-pregnant individuals with Hansen’s disease (leprosy) or newly diagnosed multiple myeloma, a blood and bone marrow cancer. The beneficial enantiomer can be synthesized and given to patients, but over time, both the beneficial and the harmful enantiomer can be detected in the body. Propose a possible explanation for the presence of the harmful enantiomer. 11. WRITE ABOUT A THEME: ORGANIZATION In 1918, an epidemic of sleeping sickness caused an unusual rigid paralysis in some survivors, similar to symptoms of advanced Parkinson’s disease. Years later, l-dopa (below, left), a chemical used to treat Parkinson’s disease, was given to some of these patients. l-dopa was remarkably effective at eliminating the paralysis, at least temporarily. However, its enantiomer, d-dopa (right), was subsequently shown to have no effect at all, as is the case for Parkinson’s disease. In a short essay (100–150 words), discuss how the effectiveness of one enantiomer and not the L-dopa D-dopa other illustrates the theme of structure and function. 12. SYNTHESIZE YOUR KNOWLEDGE Explain how the chemical structure of the carbon atom accounts for the differences between the male and female lions seen in the photo.

5. VISUAL SKILLS Choose the term that correctly describes the relationship between these two sugar molecules: (A) structural isomers H (B) cis-trans isomers O H H C OH C (C) enantiomers (D) isotopes C O H C OH

H

C

H

OH

H

C

OH

H

6. VISUAL SKILLS Identify the asymmetric carbon in this molecule:

OH H

O C H

a

C H

b

C

H c

H

C

d

H

H C

e

H

H

7. Which action could produce a carbonyl group? (A) the replacement of the ¬ OH of a carboxyl group with hydrogen (B) the addition of a thiol to a hydroxyl (C) the addition of a hydroxyl to a phosphate (D) the replacement of the nitrogen of an amine with oxygen 8. VISUAL SKILLS Which of the molecules shown in question 5 has an asymmetric carbon? Which carbon is asymmetric?

For selected answers, see Appendix A. For additional practice questions, check out the Dynamic Study Modules in MasteringBiology. You can use them to study on your smartphone, tablet, or computer anytime, anywhere!

CHAPTER 4

Carbon and the Molecular Diversity of Life

65

5

The Structure and Function of Large Biological Molecules

Figure 5.1  Why is the structure of a protein important for its function?

Key Concepts 5.1

Macromolecules are polymers, built from monomers

5.2

Carbohydrates serve as fuel and building material

5.3

Lipids are a diverse group of hydrophobic molecules

5.4

Proteins include a diversity of structures, resulting in a wide range of functions

5.5

Nucleic acids store, transmit, and help express hereditary information

5.6

Genomics and proteomics have transformed biological inquiry and applications

The Molecules of Life Given the rich complexity of life on Earth, it might surprise you that the most important large molecules found in all living things—from bacteria to elephants— can be sorted into just four main classes: carbohydrates, lipids, proteins, and nucleic acids. On the molecular scale, members of three of these classes—carbohydrates, proteins, and nucleic acids—are huge and are therefore called macromolecules. For example, a protein may consist of thousands of atoms that form a molecular colossus with a mass well over 100,000 daltons. Considering the size and complexity of macromolecules, it is noteworthy that biochemists have determined the detailed structure of so many of them. The image in Figure 5.1 is a molecular model of a protein called alcohol dehydrogenase, which breaks down alcohol in the body. The architecture of a large biological molecule plays an essential role in its function. Like water and simple organic molecules, large biological molecules exhibit unique emergent properties arising from the orderly arrangement of their atoms. In this chapter, we’ll first consider how macromolecules are built. Then we’ll examine the structure and function of all four classes of large biological molecules: carbohydrates, lipids, proteins, and nucleic acids.

The scientist in the foreground is using 3-D glasses to help her visualize the structure of the protein displayed on her screen. When you see this blue icon, log in to MasteringBiology and go to the Study Area for digital resources.

66

Get Ready for This Chapter

Concept  5.1 Macromolecules are polymers, built from monomers Large carbohydrates, proteins, and nucleic acids are chainlike molecules called polymers (from the Greek polys, many, and meros, part). A polymer is a long molecule consisting of many similar or identical building blocks linked by covalent bonds, much as a train consists of a chain of cars. The repeating units that serve as the building blocks of a polymer are smaller molecules called monomers (from the Greek monos, single). In addition to forming polymers, some monomers have functions of their own.

Figure 5.2  The synthesis and breakdown of polymers. (a) Dehydration reaction: synthesizing a polymer HO

The Diversity of Polymers A cell has thousands of different macromolecules; the collection varies from one type of cell to another. The inherited chapter 5 

2

3

H

Short polymer

HO

HO

1

H

Unlinked monomer

Dehydration removes a water molecule, forming a new bond.

2

3

H 2O

4

H

4

H

Longer polymer (b) Hydrolysis: breaking down a polymer

The Synthesis and Breakdown of Polymers Although each class of polymer is made up of a different type of monomer, the chemical mechanisms by which cells make and break down polymers are basically the same in all cases. In cells, these processes are facilitated by enzymes, specialized macromolecules that speed up chemical reactions. The reaction connecting monomers is a good example of a dehydration reaction, a reaction in which two molecules are covalently bonded to each other with the loss of a water molecule (Figure 5.2a). When a bond forms between two monomers, each monomer contributes part of the water molecule that is released during the reaction: One monomer provides a hydroxyl group ( ¬ OH), while the other provides a hydrogen ( ¬ H). This reaction is repeated as monomers are added to the chain one by one, making a polymer (also called polymerization). Polymers are disassembled to monomers by hydrolysis, a process that is essentially the reverse of the dehydration reaction (Figure 5.2b). Hydrolysis means water breakage (from the Greek hydro, water, and lysis, break). The bond between monomers is broken by the addition of a water molecule, with a hydrogen from water attaching to one monomer and the hydroxyl group attaching to the other. An example of hydrolysis within our bodies is the process of digestion. The bulk of the organic material in our food is in the form of polymers that are much too large to enter our cells. Within the digestive tract, various enzymes attack the polymers, speeding up hydrolysis. Released monomers are then absorbed into the bloodstream for distribution to all body cells. Those cells can then use dehydration reactions to assemble the monomers into new, different polymers that can perform specific functions required by the cell. (Dehydration reactions and hydrolysis can also be involved in the formation and breakdown of molecules that are not polymers, such as some lipids.)

1

HO

1

2

3

H2O

Hydrolysis adds a water molecule, breaking a bond.

HO

1

2

3

H

HO

H

Animation: Making and Breaking Polymers

differences between close relatives, such as human siblings, reflect small variations in polymers, particularly DNA and proteins. Molecular differences between unrelated individuals are more extensive, and those between species greater still. The diversity of macromolecules in the living world is vast, and the possible variety is effectively limitless. What is the basis for such diversity in life’s polymers? These molecules are constructed from only 40 to 50 common monomers and some others that occur rarely. Building a huge variety of polymers from such a limited number of monomers is analogous to constructing hundreds of thousands of words from only 26 letters of the alphabet. The key is arrangement—the particular linear sequence that the units follow. However, this analogy falls far short of describing the great diversity of macromolecules because most biological polymers have many more monomers than the number of letters in even the longest word. Proteins, for example, are built from 20 kinds of amino acids arranged in chains that are typically hundreds of amino acids long. The molecular logic of life is simple but elegant: Small molecules common to all organisms act as building blocks that are ordered into unique macromolecules. Despite this immense diversity, molecular structure and function can still be grouped roughly by class. Let’s examine each of the four major classes of large biological molecules. For each class, the large molecules have emergent properties not found in their individual components. The Structure and Function of Large Biological Molecules

67

Concept Check 5.1 1. What are the four main classes of large biological molecules? Which class does not consist of polymers? 2. How many molecules of water are needed to completely hydrolyze a polymer that is ten monomers long? 3. WHAT IF? If you eat a piece of fish, what reactions must occur for the amino acid monomers in the protein of the fish to be converted to new proteins in your body?

Figure 5.3  The structure and classification of some monosaccharides. Sugars vary in the location of their carbonyl groups (orange), the length of their carbon skeletons, and the way their parts are arranged spatially around asymmetric carbons (compare, for example, the purple portions of glucose and galactose). Aldoses (Aldehyde Sugars) Carbonyl group at end of carbon skeleton

For suggested answers, see Appendix A.

Trioses: three-carbon sugars (C3H6O3)

Concept  5.2

H

Carbohydrates include sugars and polymers of sugars. The simplest carbohydrates are the monosaccharides, or simple sugars; these are the monomers from which more complex carbohydrates are built. Disaccharides are double sugars, consisting of two monosaccharides joined by a covalent bond. Carbohydrate macromolecules are polymers called polysaccharides, composed of many sugar building blocks.

C

OH

H

C

OH

The Chemistry of Life

H

C

OH

C

O

C

OH

H

H

Glyceraldehyde An initial breakdown product of glucose

Dihydroxyacetone An initial breakdown product of glucose

Pentoses: five-carbon sugars (C5H10O5) H

H

O H

C

Sugars

unit one 

H

H

Animation: Carbohydrates

68

H

O C

Carbohydrates serve as fuel and building material

Monosaccharides (from the Greek monos, single, and sacchar, sugar) generally have molecular formulas that are some multiple of the unit CH2O. Glucose (C6H12O6), the most common monosaccharide, is of central importance in the chemistry of life. In the structure of glucose, we can see the trademarks of a sugar: The molecule has a carbonyl group, l C “ O, and √ multiple hydroxyl groups, ¬ OH (Figure 5.3). Depending on the location of the carbonyl group, a sugar is either an aldose (aldehyde sugar) or a ketose (ketone sugar). Glucose, for example, is an aldose; fructose, an isomer of glucose, is a ketose. (Most names for sugars end in -ose.) Another criterion for classifying sugars is the size of the carbon skeleton, which ranges from three to seven carbons long. Glucose, fructose, and other sugars that have six carbons are called hexoses. Trioses (three-carbon sugars) and pentoses (five-carbon sugars) are also common. Still another source of diversity for simple sugars is in the way their parts are arranged spatially around asymmetric carbons. (Recall that an asymmetric carbon is a carbon attached to four different atoms or groups of atoms.) Glucose and galactose, for example, differ only in the placement of parts around one asymmetric carbon (see the purple boxes in Figure 5.3). What seems like a small difference is significant enough to give the two sugars distinctive shapes and binding activities, thus different behaviors. Although it is convenient to draw glucose with a linear carbon skeleton, this representation is not completely accurate. In aqueous solutions, glucose molecules, as well as most other

Ketoses (Ketone Sugars) Carbonyl group within carbon skeleton

C

OH

C

O

H

C

OH

H

C

OH

H

C

OH

H

C

OH

H

C

OH

H

C

OH

H

C

OH

H

H

Ribose A component of RNA

Ribulose An intermediate in photosynthesis

Hexoses: six-carbon sugars (C6H12O6) H

O

H C

C

H

O H

C

OH

C

O

H

C

OH

H

HO

C

H

HO

C

H

C

OH

HO

C

H

H

C

OH

H

C

OH

H

C

OH

H

C

OH

H

C

OH

H

C

OH

H

C

OH

H

C

OH

HO

C

H

H

H

Glucose Galactose Energy sources for organisms MAKE CONNECTIONS

H Fructose An energy source for organisms

In the 1970s, a process was developed that converts the glucose in corn syrup to its sweeter-tasting isomer, fructose. High-fructose corn syrup, a common ingredient in soft drinks and processed food, is a mixture of glucose and fructose. What type of isomers are glucose and fructose? (See Figure 4.7.)

Animation: Monosaccharides

Figure 5.4  Linear and ring forms of glucose. H

O 1C

H HO

3

H

H

4

OH

H

5

OH

H

6

OH

C C C C C

6 CH2OH

6 CH2OH

2

OH

H 4C

OH

O

5C

H OH

H

H

H

1C

H OH

4C

O

2

3C

H

5C

OH

C

3C

OH

H

H

CH2OH 6

O

H

H 1C

H

HO

3

OH

2

C

O

5

H OH

4

H

OH

2

H

OH

H 1

OH

(b) Abbreviated ring structure. Each unlabeled corner represents a carbon. The ring’s thicker edge indicates that you are looking at the ring edge-on; the components attached to the ring lie above or below the plane of the ring.

H (a) Linear and ring forms. Chemical equilibrium between the linear and ring structures greatly favors the formation of rings. The carbons of the sugar are numbered 1 to 6, as shown. To form the glucose ring, carbon 1 (magenta) bonds to the oxygen (blue) attached to carbon 5. DRAW IT Start with the linear form of fructose (see Figure 5.3) and draw the formation of the fructose ring in two steps, as shown in (a). First, number the carbons starting at the top of the linear structure. Then draw the molecule in a ringlike orientation, attaching carbon 5 via its oxygen to carbon 2. Compare the number of carbons in the fructose and glucose rings.

are generally incorporated as monomers into disaccharides or polysaccharides, discussed next. A disaccharide consists of two monosaccharides joined by a glycosidic linkage, a covalent bond formed between two monosaccharides by a dehydration reaction (glyco refers to carbohydrate). For example, maltose is a disaccharide formed by the linking of two molecules of glucose (Figure 5.5a). Also known as malt sugar, maltose is an ingredient used in brewing beer. The most prevalent disaccharide is sucrose, or table sugar. Its two monomers are glucose and fructose (Figure 5.5b). Plants generally transport carbohydrates from leaves to roots

five- and six-carbon sugars, form rings, because they are the most stable form of these sugars under physiological conditions (Figure 5.4). Monosaccharides, particularly glucose, are major nutrients for cells. In the process known as cellular respiration, cells extract energy from glucose molecules by breaking them down in a series of reactions. Not only are simple-sugar molecules a major fuel for cellular work, but their carbon skeletons also serve as raw material for the synthesis of other types of small organic molecules, such as amino acids and fatty acids. Sugar molecules that are not immediately used in these ways

Figure 5.5  Examples of disaccharide synthesis. (a) Dehydration reaction in CH2OH the synthesis of maltose. O H The bonding of two glucose H H units forms maltose. The 1–4 H glycosidic linkage joins the OH H OH HO number 1 carbon of one HO glucose to the number 4 H OH carbon of the second glucose. Joining the glucose monomers H2O in a different way would reGlucose sult in a different disaccharide. (b) Dehydration reaction in the synthesis of sucrose. Sucrose is a disaccharide formed from glucose and fructose. Notice that fructose forms a five-sided ring, though it is a hexose like glucose.

CH2OH H HO

O H OH H

H

O H OH

H

H

H OH

HO

H OH

H HO

O H OH

H

H

H

HO CH2OH

DRAW IT

Referring to Figures 5.3 and 5.4, number the carbons in each sugar in this figure. How does the name of each linkage relate to the numbers?

chapter 5 

H 4

O H OH

H

H OH

H

OH

Maltose

H

Fructose

CH2OH

OH

CH2OH

H

1– 4 H glycosidic 1 linkage O

Glucose

OH H2O

CH2OH

OH

CH2OH O

OH

Glucose

CH2OH

H HO

O H OH

H

1– 2 H glycosidic 1 linkage

CH2OH O 2

H

H

HO CH2OH

O H

OH

OH

H

Sucrose Animation: Synthesis of Sucrose

The Structure and Function of Large Biological Molecules

69

and other nonphotosynthetic organs in the form of sucrose. Lactose, the sugar present in milk, is another disaccharide, in this case a glucose molecule joined to a galactose molecule. Disaccharides must be broken down into monosaccharides to be used for energy by organisms. Lactose intolerance is a common condition in humans who lack lactase, the enzyme that breaks down lactose. The sugar is instead broken down by intestinal bacteria, causing formation of gas and subsequent cramping. The problem may be avoided by taking the enzyme lactase when eating or drinking dairy products or consuming dairy products that have already been treated with lactase to break down the lactose.

Polysaccharides Polysaccharides are macromolecules, polymers with a few hundred to a few thousand monosaccharides joined by

glycosidic linkages. Some polysaccharides serve as storage material, hydrolyzed as needed to provide sugar for cells. Other polysaccharides serve as building material for structures that protect the cell or the whole organism. The architecture and function of a polysaccharide are determined by its sugar monomers and by the positions of its glycosidic linkages.

Storage Polysaccharides Both plants and animals store sugars for later use in the form of storage polysaccharides (Figure 5.6). Plants store starch, a polymer of glucose monomers, as granules within cellular structures known as plastids. (Plastids include chloroplasts.) Synthesizing starch enables the plant to stockpile surplus glucose. Because glucose is a major cellular fuel, starch represents stored energy. The sugar can later be withdrawn by the plant from this carbohydrate “bank” by hydrolysis, which breaks the

Figure 5.6  Polysaccharides of plants and animals. (a) Starch stored in plant cells, (b) glycogen stored in muscle cells, and (c) structural cellulose fibers in plant cell walls are all polysaccharides composed entirely of glucose monomers (green hexagons). In starch and glycogen, the polymer chains tend to form helices in unbranched regions because of the angle of the linkages between glucose molecules. There are two kinds of starch: amylose and amylopectin. Cellulose, with a different kind of glucose linkage, is always unbranched. Storage structures (plastids) containing starch granules in a potato tuber cell

Amylose (unbranched)

O

O

O

O O

O

O

O

(a) Starch O

Muscle tissue

O

O

O

O

O

O

O

O

O

O O

Glucose monomer

Amylopectin (somewhat branched) O

O

O

O

50 μm

O

O

O

O O

O

O

O

Glycogen (extensively branched)

Glycogen granules stored in muscle tissue

O O

O

O

O O O

1 μm (b) Glycogen

O

10 μm

Microfibril (bundle of about 80 cellulose molecules)

O

0.5 μm

unit one 

O

The Chemistry of Life

O

O

O

O

O

O

O

O

OH

O

OH

O

O

O O

70

O

Cellulose molecule (unbranched)

O O

(c) Cellulose

O

O

Cellulose microfibrils in a plant cell wall Plant cell, surrounded by cell wall

O

O

O

O

O

Cell wall

O

O

O

O

O

Hydrogen bonds between parallel cellulose molecules hold them together. O

O

O

O O

O

O O

O

The difference is based on the fact that there are actually two slightly different ring structures for glucose (Figure 5.7a). When glucose forms a ring, the hydroxyl group attached to the number 1 carbon is positioned either below or above the plane of the ring. These two ring forms for glucose are called alpha (α) and beta (β), respectively. (Greek letters are often used as a “numbering” system for different versions of biological structures, much as we use the letters a, b, c, and so on for the parts of a question or a figure.) In starch, all the glucose monomers are in the α configuration (Figure 5.7b), the arrangement we saw in Figures 5.4 and 5.5. In contrast, the glucose monomers of cellulose are all in the β configuration, making every glucose monomer “upside down” with respect to its neighbors (Figure 5.7c; see also Figure 5.6c). The differing glycosidic linkages in starch and cellulose give the two molecules distinct three-dimensional shapes. Certain starch molecules are largely helical, fitting their function of efficiently storing glucose units. Conversely, a cellulose molecule is straight. Cellulose is never branched, and some hydroxyl groups on its glucose monomers are free to hydrogen-bond with the hydroxyls of other cellulose molecules lying parallel to it. In plant cell walls, parallel cellulose molecules held together in this way are grouped into units called microfibrils (see Figure 5.6c). These cable-like microfibrils are a strong building material for plants and an important substance for humans because cellulose is the major constituent of paper and the only component of cotton. The unbranched structure of cellulose thus fits its function: imparting strength to parts of the plant. Enzymes that digest starch by hydrolyzing its α linkages are unable to hydrolyze the β linkages of cellulose due to the different shapes of these two molecules. In fact,

bonds between the glucose monomers. Most animals, including humans, also have enzymes that can hydrolyze plant starch, making glucose available as a nutrient for cells. Potato tubers and grains—the fruits of wheat, maize (corn), rice, and other grasses—are the major sources of starch in the human diet. Most of the glucose monomers in starch are joined by 1–4 linkages (number 1 carbon to number 4 carbon), like the glucose units in maltose (see Figure 5.5a). The simplest form of starch, amylose, is unbranched. Amylopectin, a more complex starch, is a branched polymer with 1–6 linkages at the branch points. Both of these starches are shown in Figure 5.6a. Animals store a polysaccharide called glycogen, a polymer of glucose that is like amylopectin but more extensively branched (Figure 5.6b). Vertebrates store glycogen mainly in liver and muscle cells. Hydrolysis of glycogen in these cells releases glucose when the demand for sugar increases. (The extensively branched structure of glycogen fits its function: More free ends are available for hydrolysis.) This stored fuel cannot sustain an animal for long, however. In humans, for example, glycogen stores are depleted in about a day unless they are replenished by eating. This is an issue of concern in lowcarbohydrate diets, which can result in weakness and fatigue.

Structural Polysaccharides Organisms build strong materials from structural polysaccharides. For example, the polysaccharide called cellulose is a major component of the tough walls that enclose plant cells (Figure 5.6c). Globally, plants produce almost 1014 kg (100 billion tons) of cellulose per year; it is the most abundant organic compound on Earth. Like starch, cellulose is a polymer of glucose with 1–4 glycosidic linkages, but the linkages in these two polymers differ. Figure 5.7  Starch and cellulose structures. (a) α and β glucose ring structures. These two interconvertible forms of glucose differ in the placement of the hydroxyl group (highlighted in blue) attached to the number 1 carbon.

H

CH2OH H 4

HO

O H OH

O C

H

H

HO

1

H

OH

C C

H

H

4

H

C

OH

OH

H

C

OH

α Glucose

H

C

OH

H

CH2OH

OH

HO

H OH

O

OH

H

1

H H

OH

β Glucose

H CH2OH O HO

CH2OH O 1 4

OH

O

OH

OH

CH2OH O O

OH

OH

CH2OH O O

OH

OH

CH2OH O OH

OH

(b) Starch: 1–4 linkage of α glucose monomers. All monomers are in the same orientation. Compare the positions of the OH groups highlighted in yellow with those in cellulose (c).

HO

O

OH

1 4

OH O

OH

CH2OH O

OH

CH2OH

O

OH O

OH

OH O

OH

OH

CH2OH

(c) Cellulose: 1–4 linkage of β glucose monomers. In cellulose, every β glucose monomer is upside down with respect to its neighbors. (See the highlighted OH groups.)

Animation: Starch, Cellulose, and Glycogen Structures chapter 5 

The Structure and Function of Large Biological Molecules

71

Figure 5.8  Chitin, a structural polysaccharide. CH2OH H OH

O H OH

H

H

NH

OH

C

◀ The structure of the chitin monomer

H

O

Concept Check 5.2 1. Write the formula for a monosaccharide that has three carbons. 2. A dehydration reaction joins two glucose molecules to form maltose. The formula for glucose is C6H12O6. What is the formula for maltose? 3. WHAT IF? After a cow is given antibiotics to treat an infection, a vet gives the animal a drink of “gut culture” containing various prokaryotes. Why is this necessary? For suggested answers, see Appendix A.

CH3

Concept  5.3 ◀ Chitin, embedded in proteins, forms the exoskeleton of arthropods. This emperor dragonfly (Anax imperator) is molting—shedding its old exoskeleton (brown) and emerging upside down in adult form.

few organisms possess enzymes that can digest cellulose. Almost all animals, including humans, do not; the cellulose in our food passes through the digestive tract and is eliminated with the feces. Along the way, the cellulose abrades the wall of the digestive tract and stimulates the lining to secrete mucus, which aids in the smooth passage of food through the tract. Thus, although cellulose is not a nutrient for humans, it is an important part of a healthful diet. Most fruits, vegetables, and whole grains are rich in cellulose. On food packages, “insoluble fiber” refers mainly to cellulose. Some microorganisms can digest cellulose, breaking it down into glucose monomers. A cow harbors cellulosedigesting prokaryotes and protists in its gut. These microbes hydrolyze the cellulose of hay and grass and convert the glucose to other compounds that nourish the cow. Similarly, a termite, which is unable to digest cellulose by itself, has prokaryotes or protists living in its gut that can make a meal of wood. Some fungi can also digest cellulose in soil and elsewhere, thereby helping recycle chemical elements within Earth’s ecosystems. Another important structural polysaccharide is chitin, the carbohydrate used by arthropods (insects, spiders, crustaceans, and related animals) to build their exoskeletons (Figure 5.8). An exoskeleton is a hard case that surrounds the soft parts of an animal. Made up of chitin embedded in a layer of proteins, the case is leathery and flexible at first, but becomes hardened when the proteins are chemically linked to each other (as in insects) or encrusted with calcium carbonate (as in crabs). Chitin is also found in fungi, which use this polysaccharide rather than cellulose as the building material for their cell walls. Chitin is similar to cellulose, with β linkages, except that the glucose monomer of chitin has a nitrogen-containing attachment (see Figure 5.8). 72

unit one 

The Chemistry of Life

Lipids are a diverse group of hydrophobic molecules Lipids are the one class of large biological molecules that does not include true polymers, and they are generally not big enough to be considered macromolecules. The compounds called lipids are grouped with each other because they share one important trait: They mix poorly, if at all, with water. The hydrophobic behavior of lipids is based on their molecular structure. Although they may have some polar bonds associated with oxygen, lipids consist mostly of hydrocarbon regions. Lipids are varied in form and function. They include waxes and certain pigments, but we will focus on the types of lipids that are most important biologically: fats, phospholipids, and steroids. Animation: Lipids

Fats Although fats are not polymers, they are large molecules assembled from smaller molecules by dehydration reactions, like the dehydration reaction described for the polymerization of monomers in Figure 5.2a. A fat is constructed from two kinds of smaller molecules: glycerol and fatty acids (Figure 5.9a). Glycerol is an alcohol; each of its three carbons bears a hydroxyl group. A fatty acid has a long carbon skeleton, usually 16 or 18 carbon atoms in length. The carbon at one end of the skeleton is part of a carboxyl group, the functional group that gives these molecules the name fatty acid. The rest of the skeleton consists of a hydrocarbon chain. The relatively nonpolar C ¬ H bonds in the hydrocarbon chains of fatty acids are the reason fats are hydrophobic. Fats separate from water because the water molecules hydrogen-bond to one another and exclude the fats. This is why vegetable oil (a liquid fat) separates from the aqueous vinegar solution in a bottle of salad dressing. In making a fat, three fatty acid molecules are each joined to glycerol by an ester linkage, a bond formed by a dehydration reaction between a hydroxyl group and a carboxyl group. The resulting fat, also called a triacylglycerol, thus consists of three fatty acids linked to one glycerol molecule. (Still another name for a fat is triglyceride, a word often found

Figure 5.9  The synthesis and structure of a fat, or triacylglycerol. The molecular building blocks of a fat are one molecule of glycerol and three molecules of fatty acids. (a) One water molecule is removed for each fatty acid joined to the glycerol. (b) A fat molecule with three fatty acid units, two of them identical. The carbons of the fatty acids are arranged zigzag to suggest the actual orientations of the four single bonds extending from each carbon (see Figures 4.3a and 4.6b). H H H

C C

O OH

H

H

C

C

HO

OH

C

H

H

Fatty acid (in this case, palmitic acid)

H2O H

At room temperature, the molecules of a saturated fat, such as the fat in butter, are packed closely together, forming a solid.

C

H

H

(a) Saturated fat

H

C

C

H

H

H

H

C

C

H

H

H

H

C

C

H

H

H

H

C

C

H

H

H

H

C

C

H

H

H

H

C

C

H

H

H

H

C

Figure 5.10  Saturated and unsaturated fats and fatty acids.

OH

H

Structural formula of a saturated fat molecule (Each hydrocarbon chain is represented as a zigzag line, where each bend represents a carbon atom; hydrogens are not shown.)

H

O

H

C

O

C

H

C

O

C

H

C

O

C

O O

H

Glycerol (a) One of three dehydration reactions in the synthesis of a fat Ester linkage H H

C

O O

C

H C H

O H

C

O

C

H C H

O H

C H

O

C

H C H

H C H H C H H C H

H C H H C H H C H

H C H H C H H C H

H C H H C H H C H

H C H H C H H C H

H C H H C H H C H

H C H H C H H C H

H C H H C H H C H

H C H H C H H C H

H C H H C H H C H

H C H H C H H C H

H C H

H C H

H C

H

(b) Unsaturated fat

H

At room temperature, the molecules of an unsaturated fat such as olive oil cannot pack together closely enough to solidify because of the kinks in some of their fatty acid hydrocarbon chains.

H C

H

H H C H

H C H

Space-filling model of stearic acid, a saturated fatty acid (red = oxygen, black = carbon, gray = hydrogen)

H C

H

H

(b) Fat molecule (triacylglycerol)

in the list of ingredients on packaged foods.) The fatty acids in a fat can all be the same, or they can be of two or three different kinds, as in Figure 5.9b. The terms saturated fats and unsaturated fats are commonly used in the context of nutrition (Figure 5.10). These terms refer to the structure of the hydrocarbon chains of the fatty acids. If there are no double bonds between carbon atoms composing a chain, then as many hydrogen atoms as possible are bonded to the carbon skeleton. Such a structure is said to be saturated with hydrogen, and the resulting fatty acid is therefore called a saturated fatty acid (Figure 5.10a). An unsaturated fatty acid has one or more double bonds, with one fewer hydrogen atom on each double-bonded carbon. Nearly every double bond in naturally occurring fatty acids is a cis double bond, which creates a kink in the hydrocarbon chain wherever it occurs (Figure 5.10b). (See Figure 4.7b to remind yourself about cis and trans double bonds.) A fat made from saturated fatty acids is called a saturated fat. Most animal fats are saturated: The hydrocarbon chains of their fatty acids—the “tails” of the fat molecules—lack double bonds, and their flexibility allows the fat molecules to pack together tightly. Saturated animal fats—such as lard chapter 5 

H

Structural formula of an unsaturated fat molecule

O

H

C

O

C

H

C

O

C

H

C

O

C

O O

H

Space-filling model of oleic acid, an unsaturated fatty acid Cis double bond causes bending.

and butter—are solid at room temperature. In contrast, the fats of plants and fishes are generally unsaturated, meaning that they are built of one or more types of unsaturated fatty acids. Usually liquid at room temperature, plant and fish fats are referred to as oils—olive oil and cod liver oil are examples. The kinks where the cis double bonds are located prevent the molecules from packing together closely enough to solidify at room temperature. The phrase “hydrogenated vegetable oils” The Structure and Function of Large Biological Molecules

73

on food labels means that unsaturated fats have been synthetically converted to saturated fats by adding hydrogen, allowing them to solidify. Peanut butter, margarine, and many other products are hydrogenated to prevent lipids from separating out in liquid (oil) form. A diet rich in saturated fats is one of several factors that may contribute to the cardiovascular disease known as atherosclerosis. In this condition, deposits called plaques develop within the walls of blood vessels, causing inward bulges that impede blood flow and reduce the resilience of the vessels. The process of hydrogenating vegetable oils produces not only saturated fats but also unsaturated fats with trans double bonds. It appears that trans fats can contribute to coronary heart disease (see Concept 42.4). Because trans fats are especially common in baked goods and processed foods, the U.S. Food and Drug Administration (FDA) requires nutritional labels to include information on trans fat content. In addition, the FDA has ordered trans fats to be removed from the U.S. food supply by 2018. Some countries, such as Denmark and Switzerland, have already banned trans fats in foods. The major function of fats is energy storage. The hydrocarbon chains of fats are similar to gasoline molecules and just as rich in energy. A gram of fat stores more than twice as much energy as a gram of a polysaccharide, such as starch. Because plants are relatively immobile, they can function with bulky energy storage in the form of starch. (Vegetable oils are generally obtained from seeds, where more compact storage is an asset to the plant.) Animals, however, must carry their energy stores with them, so there is an advantage to having a more compact reservoir of fuel—fat. Humans

CH2

Choline

O O

O–

P

Phosphate

O CH2

CH

O

O

C

O C

CH2

Glycerol

Cells as we know them could not exist without another type of lipid—phospholipids. Phospholipids are essential for cells because they are major constituents of cell membranes. Their structure provides a classic example of how form fits function at the molecular level. As shown in Figure 5.11, a phospholipid is similar to a fat molecule but has only two fatty acids attached to glycerol rather than three. The third hydroxyl group of glycerol is joined to a phosphate group, which has a negative electrical charge in the cell. Typically, an additional small charged or polar molecule is also linked to the phosphate group. Choline is one such molecule (see Figure 5.11), but there are many others as well, allowing formation of a variety of phospholipids that differ from each other. The two ends of phospholipids show different behaviors with respect to water. The hydrocarbon tails are hydrophobic and are excluded from water. However, the phosphate group and its attachments form a hydrophilic head that has an affinity for water. When phospholipids are added to water, they self-assemble into a double-layered sheet called a “bilayer”

DRAW IT O

Draw an oval around the hydrophilic head of the space-filling model.

Figure Walkthrough Animation: Space-Filling Model of a Phospholipid

Fatty acids

Hydrophobic tails

Hydrophilic head

N(CH3)3

Phospholipids

Figure 5.11  The structure of a phospholipid. A phospholipid has a hydrophilic (polar) head and two hydrophobic (nonpolar) tails. This particular phospholipid, called a phosphatidylcholine, has a choline attached to a phosphate group. Shown here are (a) the structural formula, (b) the space-filling model (yellow = phosphorus, blue = nitrogen), (c) the symbol for a phospholipid that will appear throughout this book, and (d) the bilayer structure formed by self-assembly of phospholipids in an aqueous environment.

+

CH2

and other mammals stock their long-term food reserves in adipose cells (see Figure 4.6a), which swell and shrink as fat is deposited and withdrawn from storage. In addition to storing energy, adipose tissue also cushions such vital organs as the kidneys, and a layer of fat beneath the skin insulates the body. This subcutaneous layer is especially thick in whales, seals, and most other marine mammals, insulating their bodies in cold ocean water.

Kink due to cis double bond

Hydrophilic head Hydrophobic tails

(a) Structural formula

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The Chemistry of Life

(b) Space-filling model

(c) Phospholipid symbol

(d) Phospholipid bilayer

that shields their hydrophobic fatty acid tails from water (Figure 5.11d). At the surface of a cell, phospholipids are arranged in a similar bilayer. The hydrophilic heads of the molecules are on the outside of the bilayer, in contact with the aqueous solutions inside and outside of the cell. The hydrophobic tails point toward the interior of the bilayer, away from the water. The phospholipid bilayer forms a boundary between the cell and its external environment and establishes separate compartments within eukaryotic cells; in fact, the existence of cells depends on the properties of phospholipids.

Steroids Steroids are lipids characterized by a carbon skeleton consisting of four fused rings. Different steroids are distinguished by the particular chemical groups attached to this ensemble of rings. Cholesterol, a type of steroid, is a crucial molecule in animals (Figure 5.12). It is a common component of animal cell membranes and is also the precursor from which other steroids, such as the vertebrate sex hormones, are synthesized. In vertebrates, cholesterol is synthesized in the liver and is also obtained from the diet. A high level of cholesterol in the blood may contribute to atherosclerosis, although some researchers are questioning the roles of cholesterol and saturated fats in the development of this condition. Figure 5.12  Cholesterol, a steroid. Cholesterol is the molecule from which other steroids, including the sex hormones, are synthesized. Steroids vary in the chemical groups attached to their four interconnected rings (shown in gold). CH3

H3C CH3

CH3

CH3

HO MAKE CONNECTIONS

Compare cholesterol with the sex hormones shown in the figure at the beginning of Concept 4.3. Circle the chemical groups that cholesterol has in common with estradiol; put a square around the chemical groups that cholesterol has in common with testosterone.

Interview with Lovell Jones: Investigating the effects of sex hormones on cancer (see the interview before Chapter 2)

Concept Check 5.3 1. Compare the structure of a fat (triglyceride) with that of a phospholipid. 2. Why are human sex hormones considered lipids? 3. WHAT IF? Suppose a membrane surrounded an oil droplet, as it does in the cells of plant seeds and in some animal cells. Describe and explain the form it might take. For suggested answers, see Appendix A.

chapter 5 

Concept  5.4 Proteins include a diversity of structures, resulting in a wide range of functions Nearly every dynamic function of a living being depends on proteins. In fact, the importance of proteins is underscored by their name, which comes from the Greek word proteios, meaning “first,” or “primary.” Proteins account for more than 50% of the dry mass of most cells, and they are instrumental in almost everything organisms do. Some proteins speed up chemical reactions, while others play a role in defense, storage, transport, cellular communication, movement, or structural support. Figure 5.13 shows examples of proteins with these functions, which you’ll learn more about in later chapters. Life would not be possible without enzymes, most of which are proteins. Enzymatic proteins regulate metabolism by acting as catalysts, chemical agents that selectively speed up chemical reactions without being consumed in the reaction. Because an enzyme can perform its function over and over again, these molecules can be thought of as workhorses that keep cells running by carrying out the processes of life. A human has tens of thousands of different proteins, each with a specific structure and function; proteins, in fact, are the most structurally sophisticated molecules known. Consistent with their diverse functions, they vary extensively in structure, each type of protein having a unique threedimensional shape. Proteins are all constructed from the same set of 20 amino acids, linked in unbranched polymers. The bond between amino acids is called a peptide bond, so a polymer of amino acids is called a polypeptide. A protein is a biologically functional molecule made up of one or more polypeptides, each folded and coiled into a specific three-dimensional structure.

Amino Acid Monomers All amino acids share a common strucSide chain (R group) ture. An amino acid is an organic R molecule with both an amino group α carbon and a carboxyl group (see Figure 4.9); O H the small figure shows the general forN C C mula for an amino acid. At the center H OH H of the amino acid is an asymmetric Amino Carboxyl carbon atom called the alpha (α) group group carbon. Its four different partners are an amino group, a carboxyl group, a hydrogen atom, and a variable group symbolized by R. The R group, also called the side chain, differs with each amino acid. The Structure and Function of Large Biological Molecules

75

Figure 5.13  An overview of protein functions.

Animation: Protein Functions

Enzymatic proteins

Defensive proteins

Function: Selective acceleration of chemical reactions Example: Digestive enzymes catalyze the hydrolysis of bonds in food molecules.

Function: Protection against disease Example: Antibodies inactivate and help destroy viruses and bacteria.

Enzyme

Antibodies Bacterium

Virus

Storage proteins

Transport proteins

Function: Storage of amino acids Examples: Casein, the protein of milk, is the major source of amino acids for baby mammals. Plants have storage proteins in their seeds. Ovalbumin is the protein of egg white, used as an amino acid source for the developing embryo.

Function: Transport of substances Examples: Hemoglobin, the iron-containing protein of vertebrate blood, transports oxygen from the lungs to other parts of the body. Other proteins transport molecules across membranes, as shown here.

Ovalbumin

Transport protein

Amino acids for embryo

Cell membrane

Hormonal proteins

Receptor proteins

Function: Coordination of an organism‘s activities Example: Insulin, a hormone secreted by the pancreas, causes other tissues to take up glucose, thus regulating blood sugar concentration.

Function: Response of cell to chemical stimuli Example: Receptors built into the membrane of a nerve cell detect signaling molecules released by other nerve cells. Receptor protein

Insulin secreted

High blood sugar

Normal blood sugar

Signaling molecules

Contractile and motor proteins

Structural proteins

Function: Movement Examples: Motor proteins are responsible for the undulations of cilia and flagella. Actin and myosin proteins are responsible for the contraction of muscles.

Function: Support Examples: Keratin is the protein of hair, horns, feathers, and other skin appendages. Insects and spiders use silk fibers to make their cocoons and webs, respectively. Collagen and elastin proteins provide a fibrous framework in animal connective tissues.

Actin

Myosin Collagen

Muscle tissue

30 μm

Figure 5.14 shows the 20 amino acids that cells use to

build their thousands of proteins. Here the amino groups and carboxyl groups are all depicted in ionized form, the way they usually exist at the pH found in a cell. The side chain (R group) may be as simple as a hydrogen atom, as in the amino acid glycine, or it may be a carbon skeleton with various functional groups attached, as in glutamine. The physical and chemical properties of the side chain determine the unique characteristics of a particular amino acid, thus affecting its functional role in a polypeptide. In Figure 5.14, the amino acids are grouped according to the properties of their 76

unit one 

The Chemistry of Life

Connective tissue

60 μm

side chains. One group consists of amino acids with nonpolar side chains, which are hydrophobic. Another group consists of amino acids with polar side chains, which are hydrophilic. Acidic amino acids have side chains that are generally negative in charge due to the presence of a carboxyl group, which is usually dissociated (ionized) at cellular pH. Basic amino acids have amino groups in their side chains that are generally positive in charge. (Notice that all amino acids have carboxyl groups and amino groups; the terms acidic and basic in this context refer only to groups in the side chains.) Because they are charged, acidic and basic side chains are also hydrophilic.

Figure 5.14  The 20 amino acids of proteins. The amino acids are grouped here according to the properties of their side chains (R groups) and shown in their prevailing ionic forms at pH 7.2, the pH within a cell. The three-letter and one-letter abbreviations for the amino acids are in parentheses. All of the amino acids used in proteins are L enantiomers (see Figure 4.7c). Nonpolar side chains; hydrophobic Side chain (R group)

CH3 CH3

CH3

CH

CH2

CH3 CH3 CH3

H H3N+

C

C

H

O

O–

H3N+

Glycine (Gly or G)

CH2

CH

C

C

H

O

O–

H3N+

Alanine (Ala or A)

C

C

H

O

O–

N+

H3

Valine (Val or V)

CH

H3C

C

C

H

O

O–

N+

H3

C

C

H

O

O–

Isoleucine (Ile or I)

Leucine (Leu or L)

CH3 S NH

CH2 CH2 H3N+

CH2

C

C

H

O

O–

H3N+

Methionine (Met or M)

CH2

CH2

C

C

H

O

O–

H3N+

C

C

H

O

Phenylalanine (Phe or F)

Tryptophan (Trp or W)

Since cysteine is only weakly polar, it is sometimes classified as a nonpolar amino acid.

OH

O–

H2C

CH2

H2N+

C

C

H

O

O–

Proline (Pro or P)

Polar side chains; hydrophilic

OH CH2 H3N+

O–

C

C

H

O

H3N+

Serine (Ser or S)

C

C

H

O

NH2 O C

SH

OH CH3 CH

CH2 O–

H3N+

Threonine (Thr or T)

C

H

O

O–

H3N+

Cysteine (Cys or C)

CH2

CH2

CH2

C

NH2 O C

C

C

H

O

O–

H3N+

Tyrosine (Tyr or Y)

CH2

C

C

H

O

O–

H3N+

Asparagine (Asn or N)

C

C

H

O

O–

Glutamine (Gln or Q)

Electrically charged side chains; hydrophilic Basic (positively charged) NH2

Acidic (negatively charged)

NH3

C

CH2

NH

C

CH2

CH2

C

CH2

CH2

CH2

CH2

CH2

CH2

CH2

O– O–

H3N+

+

O

C

C

H

O

Aspartic acid (Asp or D)

O–

H3N+

O

C

C

H

O

Glutamic acid (Glu or E)

O–

H3N+

C

C

H

O

Lysine (Lys or K)

chapter 5 

O–

H3N+

NH2+ NH+ NH CH2

C

C

H

O

Arginine (Arg or R)

O–

H3N+

C

C

H

O

O–

Histidine (His or H)

The Structure and Function of Large Biological Molecules

77

Polypeptides (Amino Acid Polymers) Now that we have examined amino acids, let’s see how they are linked to form polymers (Figure 5.15). When two amino acids are positioned so that the carboxyl group of one is adjacent to the amino group of the other, they can become joined by a dehydration reaction, with the removal of a water molecule. The resulting covalent bond is called a peptide bond. Repeated over and over, this process yields a polypeptide, a polymer of many amino acids linked by peptide bonds. You’ll learn more about how cells synthesize polypeptides in Concept 17.4. The repeating sequence of atoms highlighted in purple in Figure 5.15 is called the polypeptide backbone. Extending from this backbone are the different side chains (R groups) of the amino acids. Polypeptides range in length from a few amino acids to 1,000 or more. Each specific polypeptide has a unique linear sequence of amino acids. Note that one end of the polypeptide chain has a free amino group (the N-terminus of the polypeptide), while the opposite end has a free carboxyl group (the C-terminus). The chemical nature of the molecule Figure 5.15  Making a polypeptide chain. Peptide bonds are formed by dehydration reactions, which link the carboxyl group of one amino acid to the amino group of the next. The peptide bonds are formed one at a time, starting with the amino acid at the amino end (N-terminus). The polypeptide has a repetitive backbone (purple) to which the amino acid side chains (yellow and green) are attached. CH3

OH

S CH2

SH CH2

CH2 H H

N

CH2

H C

C

H

O

N

H C

C

H

O

OH

N

H

C

C

H

O

OH

Peptide bond H2O

CH3 Side chains (R groups)

OH

S CH2

SH CH2

CH2 Backbone

New peptide bond forming

H H

N

CH2

H C

C

H

O

Amino end (N-terminus)

N

H C

C

H

O

N

Peptide bond

C

C

H

O

OH

Carboxyl end (C-terminus)

DRAW IT Label the three amino acids in the upper part of the figure using three-letter and one-letter codes. Circle and label the carboxyl and amino groups that will form the new peptide bond.

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The Chemistry of Life

as a whole is determined by the kind and sequence of the side chains, which determine how a polypeptide folds and thus its final shape and chemical characteristics. The immense variety of polypeptides in nature illustrates an important concept introduced earlier—that cells can make many different polymers by linking a limited set of monomers into diverse sequences.

Protein Structure and Function The specific activities of proteins result from their intricate three-dimensional architecture, the simplest level of which is the sequence of their amino acids. What can the amino acid sequence of a polypeptide tell us about the three-dimensional structure (commonly referred to simply as the “structure”) of the protein and its function? The term polypeptide is not synonymous with the term protein. Even for a protein consisting of a single polypeptide, the relationship is somewhat analogous to that between a long strand of yarn and a sweater of particular size and shape that can be knitted from the yarn. A functional protein is not just a polypeptide chain, but one or more polypeptides precisely twisted, folded, and coiled into a molecule of unique shape, which can be shown in several different types of models (Figure 5.16). And it is the amino acid sequence of each polypeptide that determines what three-dimensional structure the protein will have under normal cellular conditions. When a cell synthesizes a polypeptide, the chain may fold spontaneously, assuming the functional structure for that protein. This folding is driven and reinforced by the formation of various bonds between parts of the chain, which in turn depends on the sequence of amino acids. Many proteins are roughly spherical (globular proteins), while others are shaped like long fibers (fibrous proteins). Even within these broad categories, countless variations exist. A protein’s specific structure determines how it works. In almost every case, the function of a protein depends on its ability to recognize and bind to some other molecule. In an especially striking example of the marriage of form and function, Figure 5.17 shows the exact match of shape between an antibody (a protein in the body) and the particular foreign substance on a flu virus that the antibody binds to and marks for destruction. Also, you may recall another example of molecules with matching shapes from Concept 2.3: endorphin molecules (produced by the body) and morphine molecules (a manufactured drug), both of which fit into receptor proteins on the surface of brain cells in humans, producing euphoria and relieving pain. Morphine, heroin, and other opiate drugs are able to mimic endorphins because they all have a shape similar to that of endorphins and can thus fit into and bind to endorphin receptors in the brain. This fit is very specific, something like a lock and key (see Figure 2.16). The endorphin receptor, like other receptor molecules, is a protein. The function of a protein—for instance, the ability of a receptor protein to bind to a particular pain-relieving signaling molecule—is an emergent property resulting from exquisite molecular order.

Figure 5.16 

Visualizing Proteins

Proteins can be represented in different ways, depending on the goal of the illustration.

Target molecule (on bacterial cell surface) bound to lysozyme

Structural Models Using data from structural studies of proteins, computers can generate various types of models. Each model emphasizes a different aspect of the protein’s structure, but no model can show what a protein actually looks like. These three models depict lysozyme, a protein in tears and saliva that helps prevent infection by binding to target molecules on bacteria. 1 In which model is it easiest to follow the polypeptide backbone?

Instructors: The tutorial “Molecular Model: Lysozyme,” in which students rotate 3-D models of lysozyme, can be assigned in MasteringBiology.

Space-filling model: Shows all the atoms of the protein (except hydrogen), emphasizing the overall globular shape. The atoms are color-coded: gray = carbon, red = oxygen, blue = nitrogen, and yellow = sulfur.

Ribbon model: Shows only the backbone of the polypeptide, emphasizing how it folds and coils to form a 3-D shape, in this case stabilized by disulfide bridges (yellow lines).

Wireframe model (blue): Shows the backbone of the polypeptide chain with side chains (R groups) extending from it (see Figure 5.15). A ribbon model (purple) is superimposed on the wireframe model. Insulin-producing cell in pancreas

Simplified Diagrams It isn‘t always necessary to use a detailed computer model; simplified diagrams are useful when the focus of the figure is on the function of the protein, not the structure.

Enzyme In this diagram of the protein rhodopsin, a simple transparent shape is drawn around the contours of a ribbon model, showing the overall shape of the molecule as well as some internal details.

When structural details are not needed, a solid shape can be used to represent a protein.

A simple shape is used here to represent a generic enzyme because the diagram focuses on enzyme action in general.

2 Draw a simple version of lysozyme that shows its overall shape, based on the molecular models in the top section of the figure.

Figure 5.17  Complementarity of shape between two protein surfaces. A technique called X-ray crystallography was used to generate a computer model of an antibody protein (blue and orange, left) bound to a flu virus protein (yellow and green, right). This is a wireframe model modified by adding an “electron density map” in the region where the two proteins meet. Computer software was then used to back the images away from each other slightly.

Insulin

Antibody protein

Sometimes a protein is represented simply as a dot, as shown here for insulin. 3 Why is it unnecessary

to show the actual shape of insulin here?

Protein from flu virus

Four Levels of Protein Structure In spite of their great diversity, proteins share three superimposed levels of structure, known as primary, secondary, and tertiary structure. A fourth level, quaternary structure, arises when a protein consists of two or more polypeptide chains. Figure 5.18 describes these four levels of protein structure. Be sure to study this figure thoroughly before going on to the next section. Animation: Protein Structure chapter 5 

VISUAL SKILLS

What do these computer models allow you to see about

the two proteins?

The Structure and Function of Large Biological Molecules

79

Exploring Levels of Protein Structure

Figure 5.18 

Primary Structure

Secondary Structure

Linear chain of amino acids

Regions stabilized by hydrogen bonds between atoms of the polypeptide backbone

H

Amino acids

O

H +

R

O

H

H C

N H

C

C

N

R

H

H

N

H

C

C

O

R

C

A region of α helix in transthyretin Hydrogen bond

+H N Gly 3 Amino end 1

Pro Thr Gly Thr Gly Glu Ser Lys Cys

10

5

Pro

A region of β pleated sheet in transthyretin

Leu

β strand, often shown as a folded or flat arrow pointing toward the carboxyl end

Met

30

25

15

20

Val

His Val Ala Val Asn Ile Ala Pro Ser Gly Arg Val Ala Asp Leu Val Lys

Val Phe

Hydrogen bond

Arg

35

Lys

40

45

50

Ala Ala Asp Asp Thr Trp Glu Pro Phe Ala Ser Gly Lys Thr Ser Glu Ser

Glu

Primary structure of transthyretin 70

65

55 Leu

60

Ile Glu Val Lys Tyr Ile Gly Glu Val Phe Glu Glu Glu Thr Thr Leu Gly

Asp

Gly

His

Thr 75 Lys

80

Ser

85

90

Tyr Trp Lys Ala Leu Gly Ile Ser Pro Phe His Glu His Ala Glu Val Val Phe

95

115 Tyr

110

105

100

Ser Tyr Pro Ser Leu Leu Ala Ala Ile Thr Tyr Arg Arg Pro Gly Ser Asp

Thr Ala Asn

Ser Thr Thr

120

O

125

Ala Val Val Thr Asn Pro Lys Glu

C

O–

Carboxyl end

The primary structure of a protein is its sequence of amino acids. As an example, let’s consider transthyretin, a globular blood protein that transports vitamin A and one of the thyroid hormones throughout the body. Transthyretin is made up of four identical polypeptide chains, each composed of 127 amino acids. Shown here is one of these chains unraveled for a closer look at its primary structure. Each of the 127 positions along the chain is occupied by one of the 20 amino acids, indicated here by its threeletter abbreviation. The primary structure is like the order of letters in a very long word. If left to chance, there would be 20127 different ways of making a polypeptide chain 127 amino acids long. However, the precise primary structure of a protein is determined not by the random linking of amino acids, but by inherited genetic information. The primary structure in turn dictates secondary and tertiary structure, due to the chemical nature of the backbone and the side chains (R groups) of the amino acids along the polypeptide. 80

Unit one 

The Chemistry of Life

Most proteins have segments of their polypeptide chains repeatedly coiled or folded in patterns that contribute to the protein’s overall shape. These coils and folds, collectively referred to as secondary structure, are the result of hydrogen bonds between the repeating constituents of the polypeptide backbone (not the amino acid side chains). Within the backbone, the oxygen atoms have a partial negative charge, and the hydrogen atoms attached to the nitrogens have a partial positive charge (see Figure 2.14); therefore, hydrogen bonds can form between these atoms. Individually, these hydrogen bonds are weak, but because they are repeated many times over a relatively long region of the polypeptide chain, they can support a particular shape for that part of the protein. One such secondary structure is the α helix, a delicate coil held together by hydrogen bonding between every fourth amino acid, as shown above. Although each transthyretin polypeptide has only one α helix region (see the Tertiary Structure section), other globular proteins have multiple stretches of α helix separated by nonhelical regions (see hemoglobin in the Quaternary Structure section). Some fibrous proteins, such as α-keratin, the structural protein of hair, have the α helix formation over most of their length. The other main type of secondary structure is the β pleated sheet. As shown above, in this structure two or more segments of the polypeptide chain lying side by side (called β strands) are connected by hydrogen bonds between parts of the two parallel segments of polypeptide backbone. β pleated sheets make up the core of many globular proteins, as is the case for transthyretin (see Tertiary Structure), and dominate some fibrous proteins, including the silk protein of a spider’s web. The teamwork of so many hydrogen bonds makes each spider silk fiber stronger than a steel strand of the same weight. ▶ Spiders secrete silk fibers made of a structural protein containing β pleated sheets, which allow the spider web to stretch and recoil.

Tertiary Structure

Quaternary Structure

Three-dimensional shape stabilized by interactions between side chains

Association of two or more polypeptides (some proteins only)

α helix Transthyretin polypeptide β pleated sheet

Superimposed on the patterns of secondary structure is a protein’s tertiary structure, shown here in a ribbon model of the transthyretin polypeptide. While secondary structure involves interactions between backbone constituents, tertiary structure is the overall shape of a polypeptide resulting from interactions between the side chains (R groups) of the various amino acids. One type of interaction that contributes to tertiary structure is called—somewhat misleadingly—a hydrophobic interaction. As a polypeptide folds into its functional shape, amino acids with hydrophobic (nonpolar) side chains usually end up in clusters at the core of the protein, out of contact with water. Thus, a “hydrophobic interaction” is actually caused by the exclusion of nonpolar substances by water molecules. Once nonpolar amino acid side chains are close together, van der Waals interactions help hold them together. Meanwhile, hydrogen bonds between polar side chains and ionic bonds between positively and negatively charged side chains also help stabilize tertiary structure. These are all weak interactions in the aqueous cellular environment, but their cumulative effect helps give the protein a unique shape. Covalent bonds called disulfide bridges may further reinforce the shape of a protein. Disulfide bridges form where two cysteine monomers, which have sulfhydryl groups (—SH) on their side chains (see Figure 4.9), are brought close together by the folding of the protein. The sulfur of one cysteine bonds to the sulfur of the second, and the disulfide bridge (—S—S—) rivets parts of the protein together (see yellow lines in Figure 5.16 ribbon model). All of these different kinds of interactions can contribute to the tertiary structure of a protein, as shown here in a small part of a hypothetical protein:

Single polypeptide subunit

Some proteins consist of two or more polypeptide chains aggregated into one functional macromolecule. Quaternary structure is the overall protein structure that results from the aggregation of these polypeptide subunits. For example, shown above is the complete globular transthyretin protein, made up of its four polypeptides. Another example is collagen, which is a fibrous protein that has three identical helical polypeptides intertwined into a larger triple helix, giving the long fibers great strength. This suits collagen fibers to their function as the girders of connective tissue in skin, bone, tendons, ligaments, and other body parts. (Collagen accounts for 40% of the protein in a human body.) Collagen

Hemoglobin, the oxygen-binding protein of red blood cells, is another example of a globular protein with quaternary structure. It consists of four polypeptide subunits, two of one kind (α) and two of another kind (β). Both α and β subunits consist primarily of α-helical secondary structure. Each subunit has a nonpolypeptide component, called heme, with an iron atom that binds oxygen. Heme Iron

CH2 OH NH2 O C

Hydrogen bond

CH2

Disulfide bridge Polypeptide backbone of small part of a protein

β subunit

CH CH3 CH3 CH3 CH3 CH CH2

CH2 S

Transthyretin protein (four identical polypeptides)

α subunit

C

S

O– NH3+

CH2

CH2

O

Hydrophobic interactions and van der Waals interactions Ionic bond

α subunit

CH2 CH2 CH2

β subunit Hemoglobin

Animation: Proteins

MP3 Tutor: Protein Structure and Function chapter 5 

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Figure 5.19  A single amino acid substitution in a protein causes sickle-cell disease. Primary Structure

Sickle-cell hemoglobin

Normal hemoglobin

1

Val

2

His

3

Leu

4

Thr

5

Pro

6

Glu

7

Glu

1

Val

2

His

3

Leu

4

Thr

5

Pro

6

Val

7

Glu

Secondary and Tertiary Structures

Quaternary Structure

Normal β subunit

Normal hemoglobin

Function

Red Blood Cell Shape

β α

β Sickle-cell β subunit

5 μm

α Sickle-cell hemoglobin β

β

Hydrophobic interactions between sickle-cell hemoglobin proteins lead to their aggregation into a fiber; α capacity to carry oxygen is greatly reduced.

5 μm

Considering the chemical characteristics of the amino acids valine and glutamic acid (see Figure 5.14), propose a possible explanation for the dramatic effect on protein function that occurs when valine is substituted for glutamic acid.

Sickle-Cell Disease: A Change in Primary Structure Even a slight change in primary structure can affect a protein’s shape and ability to function. For instance, sickle-cell disease, an inherited blood disorder, is caused by the substitution of one amino acid (valine) for the normal one (glutamic acid) at the position of the sixth amino acid in the primary structure of hemoglobin, the protein that carries oxygen in red blood cells. Normal red blood cells are disk-shaped, but in sickle-cell disease, the abnormal hemoglobin molecules tend to aggregate into chains, deforming some of the cells into a sickle shape (Figure 5.19). A person with the disease has periodic “sicklecell crises” when the angular cells clog tiny blood vessels, impeding blood flow. The toll taken on such patients is a dramatic example of how a simple change in protein struc­ ture can have devastating effects on protein function. Interview with Linus Pauling: Winner of the Nobel Prize in Chemistry and the Nobel Peace Prize

HHMI Animation: Sickle-Cell Disease

synthesized in the crowded environment within a cell, aided by other proteins. However, protein structure also depends on the physical and chemical conditions of the protein’s environment. If the pH, salt concentration, temperature, or other aspects of its environment are altered, the weak chemical bonds and interactions within a protein may be destroyed, causing the protein to unravel and lose its native shape, a change called denaturation (Figure 5.20). Because it is misshapen, the denatured protein is biologically inactive. Most proteins become denatured if they are transferred from an aqueous environment to a nonpolar solvent, such as ether or chloroform; the polypeptide chain refolds so that Figure 5.20  Denaturation and renaturation of a protein. High temperatures or various chemical treatments will denature a protein, causing it to lose its shape and hence its ability to function. If the denatured protein remains dissolved, it may renature when the chemical and physical aspects of its environment are restored to normal.

What Determines Protein Structure? You’ve learned that a unique shape endows each protein with a specific function. But what are the key factors determining protein structure? You already know most of the answer: A polypeptide chain of a given amino acid sequence can be arranged into a three-dimensional shape determined by the interactions responsible for secondary and tertiary struc­ ture. This folding normally occurs as the protein is being unit one 

The Chemistry of Life

Fibers of abnormal hemoglobin deform red blood cell into sickle shape.

α

MAKE CONNECTIONS

82

Normal red blood cells are full of individual hemoglobin proteins.

Normal hemoglobin proteins do not associate with one another; each carries oxygen.

naturation De

Normal protein

Re n

aturation

Denatured protein

its hydrophobic regions face outward toward the solvent. Other denaturation agents include chemicals that disrupt the hydrogen bonds, ionic bonds, and disulfide bridges that maintain a protein’s shape. Denaturation can also result from excessive heat, which agitates the polypeptide chain enough to overpower the weak interactions that stabilize the structure. The white of an egg becomes opaque during cooking because the denatured proteins are insoluble and solidify. This also explains why excessively high fevers can be fatal: Proteins in the blood tend to denature at very high body temperatures. When a protein in a test-tube solution has been denatured by heat or chemicals, it can sometimes return to its functional shape when the denaturing agent is removed. (Sometimes this is not possible: For example, a fried egg will not become liquefied when placed back into the refrigerator!) We can conclude that the information for building specific shape is intrinsic to the protein’s primary structure; this is often the case for small proteins. The sequence of amino acids determines the protein’s shape—where an α helix can form, where β pleated sheets can exist, where disulfide bridges are located, where ionic bonds can form, and so on. But how does protein folding occur in the cell?

Protein Folding in the Cell Biochemists now know the amino acid sequence for about 65 million proteins, with roughly 1.5 million added each month, and the three-dimensional shape for almost 35,000. Researchers have tried to correlate the primary structure of many proteins with their three-dimensional structure to discover the rules of protein folding. Unfortunately, however, the protein-folding process is not that simple. Most proteins probably go through several intermediate structures on their way to a stable shape, and looking at the mature structure does not reveal the stages of folding required to achieve that form. However, biochemists have developed methods for tracking a protein through such stages and learning more about this important process. Misfolding of polypeptides in cells is a serious problem that has come under increasing scrutiny by medical researchers. Many diseases—such as cystic fibrosis, Alzheimer’s, Parkinson’s, and mad cow disease—are associated with an accumulation of misfolded proteins. In fact, misfolded versions of the transthyretin protein featured in Figure 5.18 have been implicated in several diseases, including one form of senile dementia. Even when scientists have a correctly folded protein in hand, determining its exact three-dimensional structure is not simple, for a single protein has thousands of atoms. The method most commonly used to determine the 3-D structure of a protein is X-ray crystallography, which depends on the diffraction of an X-ray beam by the atoms of a crystallized molecule. Using this technique, scientists can build a 3-D model that shows the exact position of every atom in a protein molecule (Figure 5.21). Nuclear magnetic resonance (NMR) chapter 5 

Figure 5.21

Research Method  X-Ray Crystallography Application  Scientists use X-ray crystallography to determine the three-dimensional (3-D) structure of macromolecules such as nucleic acids and proteins.

Technique  Researchers aim an X-ray beam through a crystallized

protein or nucleic acid. The atoms of the crystal diffract (bend) the X-rays into an orderly array that a digital detector records as a pattern of spots called an X-ray diffraction pattern, an example of which is shown here. Diffracted X-rays X-ray source

X-ray beam Crystal

Digital detector

X-ray diffraction pattern

Results  Using data from X-ray diffraction patterns and the sequence of monomers determined by chemical methods, researchers can build a 3-D computer model of the macromolecule being studied, such as the foursubunit protein transthyretin (see Figure 5.18) shown here.

spectroscopy and bioinformatics (see Concept 5.6) are complementary approaches to understanding protein structure and function. The structure of some proteins is difficult to determine for a simple reason: A growing body of biochemical research has revealed that a significant number of proteins, or regions of proteins, do not have a distinct 3-D structure until they interact with a target protein or other molecule. Their flexibility and indefinite structure are important for their function, which may require binding with different targets at different times. These proteins, which may account for 20–30% of mammalian proteins, are called intrinsically disordered proteins and are the focus of current research.

Concept Check 5.4 1. What parts of a polypeptide participate in the bonds that hold together secondary structure? Tertiary structure? 2. Thus far in the chapter, the Greek letters α and β have been used to specify at least three different pairs of structures. Name and briefly describe them. 3. WHAT IF? Where would you expect a polypeptide region rich in the amino acids valine, leucine, and isoleucine to be located in a folded polypeptide? Explain. For suggested answers, see Appendix A.

The Structure and Function of Large Biological Molecules

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Concept  5.5 Nucleic acids store, transmit, and help express hereditary information If the primary structure of polypeptides determines a protein’s shape, what determines primary structure? The amino acid sequence of a polypeptide is programmed by a discrete unit of inheritance known as a gene. Genes consist of DNA, which belongs to the class of compounds called nucleic acids. Nucleic acids are polymers made of monomers called nucleotides.

The Roles of Nucleic Acids The two types of nucleic acids, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), enable living organisms to reproduce their complex components from one generation to the next. Unique among molecules, DNA provides directions for its own replication. DNA also directs RNA synthesis and, through RNA, controls protein synthesis; this entire process is called gene expression (Figure 5.22). DNA is the genetic material that organisms inherit from their parents. Each chromosome contains one long DNA molecule, usually carrying several hundred or more genes. When a cell reproduces itself by dividing, its DNA molecules are copied and passed along from one generation of cells to the next. The information that programs all the cell’s activities is encoded in the structure of the DNA. The DNA, however, is not directly involved in running the operations of the cell, any more than computer software by itself can read the bar code on a box of cereal. Just as a scanner is needed to read a bar code, proteins are required to implement genetic programs. The molecular hardware of the cell—the tools that carry out biological functions—consists mostly of proteins. For example, the oxygen carrier in red blood cells is the protein hemoglobin that you saw earlier (see Figure 5.18), not the DNA that specifies its structure. How does RNA, the other type of nucleic acid, fit into gene expression, the flow of genetic information from DNA to proteins? A given gene along a DNA molecule can direct synthesis of a type of RNA called messenger RNA (mRNA). The mRNA molecule interacts with the cell’s protein-synthesizing machinery to direct production of a polypeptide, which folds into all or part of a protein. We can summarize the flow of genetic information as DNA S RNA S protein (see Figure 5.22). The sites of protein synthesis are cellular structures called ribosomes. In a eukaryotic cell, ribosomes are in the cytoplasm— the region between the nucleus and the plasma membrane, the cell’s outer boundary—but DNA resides in the nucleus. Messenger RNA conveys genetic instructions for building proteins from the nucleus to the cytoplasm. Prokaryotic cells lack nuclei but still use mRNA to convey a message from the DNA to ribosomes and other cellular equipment that translate the coded information into amino acid sequences. Later in the 84

unit one 

The Chemistry of Life

Figure 5.22  Gene expression: DNA S RNA S protein. In a eukaryotic cell, DNA in the nucleus programs protein production in the cytoplasm by dictating synthesis of messenger RNA (mRNA). DNA

1 Synthesis of mRNA in the nucleus

mRNA

NUCLEUS CYTOPLASM mRNA 2 Movement of mRNA into cytoplasm via nuclear pore 3 Synthesis of protein using information carried on mRNA Polypeptide

Ribosome

Amino acids

BioFlix® Animation: Gene Expression

book, you’ll read about other functions of some recently discovered RNA molecules; the stretches of DNA that direct synthesis of these RNAs are also considered genes (see Concept 18.3).

The Components of Nucleic Acids Nucleic acids are macromolecules that exist as polymers called polynucleotides (Figure 5.23a). As indicated by the name, each polynucleotide consists of monomers called nucleotides. A nucleotide, in general, is composed of three parts: a five-carbon sugar (a pentose), a nitrogen-containing (nitrogenous) base, and one to three phosphate groups (Figure 5.23b). The beginning monomer used to build a polynucleotide has three phosphate groups, but two are lost during the polymerization process. The portion of a nucleotide without any phosphate groups is called a nucleoside. To understand the structure of a single nucleotide, let’s first consider the nitrogenous bases (Figure 5.23c). Each nitrogenous base has one or two rings that include nitrogen atoms. (They are called nitrogenous bases because the nitrogen atoms tend to take up H+ from solution, thus acting as bases.) There are two families of nitrogenous bases: pyrimidines and purines. A pyrimidine has one six-membered ring of carbon and nitrogen atoms. The members of the pyrimidine family are cytosine (C), thymine (T), and uracil (U). Purines are larger, with a six-membered ring fused to a

group that links the sugars of two nucleotides. This bonding results in a repeating pattern of sugar-phosphate units called the sugar-phosphate backbone (see Figure 5.23a). (Note that the nitrogenous bases are not part of the backbone.) The two free ends of the polymer are distinctly different from each other. One end has a phosphate attached to a 5′ carbon, and the other end has a hydroxyl group on a 3′ carbon; we refer to these as the 5′ end and the 3′ end, respectively. We can say that a polynucleotide has a built-in directionality along its sugar-phosphate backbone, from 5′ to 3′, somewhat like a one-way street. The bases are attached all along the sugar-phosphate backbone. The sequence of bases along a DNA (or mRNA) polymer is unique for each gene and provides very specific information to the cell. Because genes are hundreds to thousands of nucleotides long, the number of possible base sequences is effectively limitless. The information carried by the gene is encoded in its specific sequence of the four DNA bases. For example, the sequence 5′-AGGTAACTT-3′ means one thing, whereas the sequence 5′-CGCTTTAAC-3′ has a different meaning. (Entire genes, of course, are much longer.) The linear order of bases in a gene specifies the amino acid sequence—the primary ­structure—of a protein, which in turn specifies that protein’s 3-D structure, thus enabling its function in the cell.

five-membered ring. The purines are adenine (A) and guanine (G). The specific pyrimidines and purines differ in the chemical groups attached to the rings. Adenine, guanine, and cytosine are found in both DNA and RNA; thymine is found only in DNA and uracil only in RNA. Now let’s add the sugar to which the nitrogenous base is attached. In DNA the sugar is deoxyribose; in RNA it is ribose (see Figure 5.23c). The only difference between these two sugars is that deoxyribose lacks an oxygen atom on the second carbon in the ring, hence the name deoxyribose. So far, we have built a nucleoside (base plus sugar). To complete the construction of a nucleotide, we attach one to three phosphate groups to the 5′ carbon of the sugar (the carbon numbers in the sugar include ′, the prime symbol; see Figure 5.23b). With one phosphate, this is a nucleoside monophosphate, more often called a nucleotide.

Nucleotide Polymers The linkage of nucleotides into a polynucleotide involves a dehydration reaction. (You will learn the details in Concept 16.2.) In the polynucleotide, adjacent nucleotides are joined by a phosphodiester linkage, which consists of a phosphate

Figure 5.23  Components of nucleic acids. (a) A polynucleotide has a sugar-phosphate backbone with variable appendages, the nitrogenous bases. (b) In a polynucleotide, each nucleotide monomer includes a nitrogenous base, a sugar, and a phosphate group. Note that carbon numbers in the sugar include primes (′). (c) A nucleoside includes a nitrogenous base (purine or pyrimidine) and a five-carbon sugar (deoxyribose or ribose).

Pyrimidines O

NH2 C

N O

Sugar-phosphate backbone (on blue background)

5′ end

NITROGENOUS BASES

C

CH CH

N H

O

C

N H

CH3

C CH

HN O

C

C N H

CH CH

Thymine (T, in DNA) Uracil (U, in RNA)

Cytosine (C) 5′C

C

HN

O

O

Purines

3′C

O

NH2 Nucleoside O

O

Nitrogenous base O –O

P

O

O

Phosphate group

OH

N

O 1′C

N

N CH

HC N H

HOCH2 Sugar (pentose)

4′

(b) Nucleotide monomer in a polynucleotide

3′ end (a) Polynucleotide, or nucleic acid

OH

O

H 3′

H 2′

C C

C N

NH C

NH2

Guanine (G) SUGARS

5′

H

3′C

C

N H

C

Adenine (A)

CH2 3′C

C

HC

5′C

O– 5′C

N

1′

H

OH H Deoxyribose (in DNA)

5′

HOCH2 4′

H

H

OH

O H

1′

H 3′ 2′ OH OH

Ribose (in RNA)

(c) Nucleoside components

HHMI Animation: The Chemical Structure of DNA chapter 5 

The Structure and Function of Large Biological Molecules

85

Figure 5.24  The structures of DNA and tRNA molecules. (a) The DNA molecule is usually a double helix, with the sugar-phosphate backbones of the antiparallel polynucleotide strands (symbolized here by blue ribbons) on the outside of the helix. Hydrogen bonds between pairs of nitrogenous bases hold the two strands together. As illustrated here with symbolic shapes for the bases, adenine (A) can pair only with thymine (T), and guanine (G) can pair only with cytosine (C). Each DNA strand in this figure is the structural equivalent of the polynucleotide diagrammed in Figure 5.23a. (b) A tRNA molecule has a roughly L-shaped structure due to complementary base pairing of antiparallel stretches of RNA. In RNA, A pairs with U. HHMI Animation: Paired DNA Strands

5′

3′

Sugar-phosphate backbones Hydrogen bonds

T

G C A C

T

3′

5′

T

G

G C C A

A

Base pair joined by hydrogen bonding

DNA molecules have two polynucleotides, or “strands,” that wind around an imaginary axis, forming a double helix (Figure 5.24a). The two sugar-phosphate backbones run in opposite 5′ S 3′ directions from each other; this arrangement is referred to as antiparallel, somewhat like a divided highway. The sugar-phosphate backbones are on the outside of the helix, and the nitrogenous bases are paired in the interior of the helix. The two strands are held together by hydrogen bonds between the paired bases (see Figure 5.24a). Most DNA molecules are very long, with thousands or even millions of base pairs. The one long DNA double helix in a eukaryotic chromosome includes many genes, each one a particular segment of the molecule. In base pairing, only certain bases in the double helix are compatible with each other. Adenine (A) in one strand always pairs with thymine (T) in the other, and guanine (G) always pairs with cytosine (C). Reading the sequence of bases along one strand of the double helix would tell us the sequence of bases along the other strand. If a stretch of one strand has the base sequence 5′-AGGTCCG-3′, then the base-pairing rules tell us that the same stretch of the other strand must have the sequence 3′-TCCAGGC-5′ The two strands of the double helix are complementary, each the predictable counterpart of the other. It is this feature of DNA that makes it possible to generate two identical copies of each DNA molecule in a cell that is preparing to divide. When the cell divides, the copies are distributed to the daughter cells, making them genetically identical to the parent cell. Thus, the structure of DNA accounts for its function of transmitting genetic information whenever a cell reproduces. RNA molecules, by contrast, exist as single strands. Complementary base pairing can occur, however, between regions of two RNA molecules or even between two stretches of nucleotides in the same RNA molecule. In fact, base pairing within an RNA molecule allows it to take on the particular The Chemistry of Life

C

G

U

The Structures of DNA and RNA Molecules

unit one 

Base pair joined by hydrogen bonding

G

(a) DNA

86

A

(b) Transfer RNA

three-dimensional shape necessary for its function. Consider, for example, the type of RNA called transfer RNA (tRNA), which brings amino acids to the ribosome during the synthesis of a polypeptide. A tRNA molecule is about 80 nucleotides in length. Its functional shape results from base pairing between nucleotides where complementary stretches of the molecule can run antiparallel to each other (Figure 5.24b). Note that in RNA, adenine (A) pairs with uracil (U); thymine (T) is not present in RNA. Another difference between RNA and DNA is that DNA almost always exists as a double helix, whereas RNA molecules are more variable in shape. RNAs are versatile molecules, and many biologists believe RNA may have preceded DNA as the carrier of genetic information in early forms of life (see Concept 25.1). Animation: Nucleic Acid Structure

Concept Check 5.5 1. DRAW IT Go to Figure 5.23a and, for the top three nucleotides, number all the carbons in the sugars, circle the nitrogenous bases, and star the phosphates. 2. DRAW IT In a DNA double helix, a region along one DNA strand has this sequence of nitrogenous bases: 5′-TAGGCCT-3′. Copy this sequence, and write down its complementary strand, clearly indicating the 5′ and 3′ ends of the complementary strand. For suggested answers, see Appendix A.

Concept  5.6 Genomics and proteomics have transformed biological inquiry and applications Experimental work in the first half of the 20th century established the role of DNA as the bearer of genetic information, passed from generation to generation, that

specified the functioning of living cells and organisms. Once the structure of the DNA molecule was described in 1953, and the linear sequence of nucleotide bases was understood to specify the amino acid sequence of proteins, biologists sought to “decode” genes by learning their nucleotide sequences (often called “base sequences”). The first chemical techniques for DNA sequencing, or determining the sequence of nucleotides along a DNA strand, one by one, were developed in the 1970s. Researchers began to study gene sequences, gene by gene, and the more they learned, the more questions they had: How was expression of genes regulated? Genes and their protein products clearly interacted with each other, but how? What was the function, if any, of the DNA that is not part of genes? To fully understand the genetic functioning of a living organism, the entire sequence of the full complement of DNA, the organism’s genome, would be most enlightening. In spite of the apparent impracticality of this idea, in the late 1980s several prominent biologists put forth an audacious proposal to launch a project that would sequence the entire human genome—all 3 billion bases of it! This endeavor began in 1990 and was effectively completed in the early 2000s. An unplanned but profound side benefit of this project— the Human Genome Project—was the rapid development of faster and less expensive methods of sequencing. This trend has continued: The cost for sequencing 1 million bases in 2001, well over $5,000, has decreased to less than $0.02 in 2016. And a human genome, the first of which took over 10 years to sequence, could be completed at today’s pace in just a few days (Figure 5.25). The number of genomes that have been fully sequenced has burgeoned, generating reams of data and prompting development of bioinformatics, the use of computer software and other computational tools that can handle and analyze these large data sets. The reverberations of these developments have transformed the study of biology and related fields. Biologists often look at problems by analyzing large sets of genes or even comparing whole genomes of different species, an approach called genomics. A similar analysis of large sets of proteins, including their sequences, is called proteomics. (Protein sequences can be determined either by using biochemical techniques or by translating the DNA sequences that code for them.) These approaches permeate all fields of biology, some examples of which are shown in Figure 5.26.

Figure 5.25  Automatic DNA sequencing machines and abundant computing power enable rapid sequencing of genes and genomes.

Perhaps the most significant impact of genomics and proteomics on the field of biology as a whole has been their contributions to our understanding of evolution. In addition to confirming evidence for evolution from the study of fossils and characteristics of currently existing species, genomics has helped us tease out relationships among different groups of organisms that had not been resolved by previous types of evidence, and thus infer evolutionary history.

DNA and Proteins as Tape Measures of Evolution Evolution We are accustomed to thinking of shared traits, such as hair and milk production in mammals, as evidence of shared ancestry. Because DNA carries heritable information in the form of genes, sequences of genes and their protein products document the hereditary background of an organism. The linear sequences of nucleotides in DNA molecules are passed from parents to offspring; these sequences determine the amino acid sequences of proteins. As a result, siblings have greater similarity in their DNA and proteins than do unrelated individuals of the same species. Given our evolutionary view of life, we can extend this concept of “molecular genealogy” to relationships between species: We would expect two species that appear to be closely related based on anatomical evidence (and possibly fossil evidence) to also share a greater proportion of their DNA and protein sequences than do less closely related species. In fact, that is the case. An example is the comparison of the β polypeptide chain of human hemoglobin with the corresponding hemoglobin polypeptide in other vertebrates. In this chain of 146 amino acids, humans and gorillas differ in just 1 amino acid, while humans and frogs, more distantly related, differ in 67 amino acids. In the Scientific Skills Exercise, you can apply this sort of reasoning to additional species. And this conclusion holds true as well when comparing whole genomes: The human genome is 95–98% identical to that of the chimpanzee, but only roughly 85% identical to that of the mouse, a more distant evolutionary relative. Molecular biology has added a new tape measure to the toolkit biologists use to assess evolutionary kinship. Comparing genomic sequences has practical applications as well. In the Problem-Solving Exercise, you can see how this type of genomic analysis can help you detect consumer fraud.

Concept Check 5.6 1. How would sequencing the entire genome of an organism help scientists to understand how that organism functioned? 2. Given the function of DNA, why would you expect two species with very similar traits to also have very similar genomes? For suggested answers, see Appendix A.

chapter 5 

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87

Figure 5.26

Make Connections

Paleontology New DNA sequencing techniques have allowed decoding of minute quantities of DNA found in ancient tissues from our extinct relatives, the Neanderthals (Homo neanderthalensis). Sequencing the Neanderthal genome has informed our understanding of their physical appearance (as in this reconstruction), as well as their relationship with modern humans. (See Figures 34.51 and 34.52.)

Contributions of Genomics and Proteomics to Biology Nucleotide sequencing and the analysis of large sets of genes and proteins can be done rapidly and inexpensively due to advances in technology and information processing. Taken together, genomics and proteomics have advanced our understanding of biology across many different fields.

Evolution A major aim of evolutionary biology is to understand the relationships among species, both living and extinct. For example, genome sequence comparisons have identified the hippopotamus as the land mammal sharing the most recent common ancestor with whales. (See Figure 22.20.)

Medical Science Identifying the genetic basis for human diseases like cancer helps researchers focus their search for potential future treatments. Currently, sequencing the sets of genes expressed in an individual’s tumor can allow a more targeted approach to treating the cancer, a type of “personalized medicine.” (See Figures 12.20 and 18.27.)

Species Interactions Hippopotamus Short-finned pilot whale

Conservation Biology The tools of molecular genetics and genomics are increasingly used by forensic ecologists to identify which species of animals and plants are killed illegally. In one case, genomic sequences of DNA from illegal shipments of elephant tusks were used to track down poachers and pinpoint the territory where they were operating. (See Figure 56.9.) 88

Unit one 

The Chemistry of Life

Most plant species exist in a mutually beneficial partnership with fungi (right) and bacteria associated with the plants’ roots; these interactions improve plant growth. Genome sequencing and analysis of gene expression have allowed characterization of plant-associated communities. Such studies will help advance our understanding of such interactions and may improve agricultural practices. (See the Chapter 31 Scientific Skills Exercise and Figure 37.11.) MAKE CONNECTIONS Considering the examples provided here, describe how the approaches of genomics and proteomics help us to address a variety of biological questions.

HHMI Video: The Making of the Fittest: The Birth and Death of Genes (Icefish)

Scientific Skills Exercise Human

Analyzing Polypeptide Sequence Data

Gibbon

monkeys, and gibbons. Because a complete sequence would not fit on one line here, the sequences are broken into three segments. The sequences for the three different species are aligned so that you can compare them easily. For example, you can see that for all three species, the first amino acid is V (valine) and the 146th amino acid is H (histidine).

Are Rhesus Monkeys or Gibbons More Closely Related to Humans?  In this exercise, you will look at amino acid sequence data for the β polypeptide chain of hemoglobin, often called β-globin. You will then interpret the data to hypothesize whether the monkey or the gibbon is more closely related to humans.

How Such Experiments Are Done  Researchers can isolate the polypeptide of interest from an organism and then determine the amino acid sequence. More frequently, the DNA of the relevant gene is sequenced, and the amino acid sequence of the polypeptide is deduced from the DNA sequence of its gene.

Data from the Experiments  In the data below, the letters give the sequence of the 146 amino acids in β-globin from humans, rhesus Species

Rhesus monkey

Interpret The Data 1. Scan the monkey and gibbon sequences, letter by letter, circling any amino acids that do not match the human sequence. (a) How many amino acids differ between the monkey and the human sequences? (b) Between the gibbon and human? 2. For each nonhuman species, what percent of its amino acids are identical to the human sequence of β-globin?

Alignment of Amino Acid Sequences of 𝛃-globin

Human Monkey Gibbon

   1  VHLTPEEKSA   VTALWGKVNV   DEVGGEALGR   LLVVYPWTQR   FFESFGDLST    1  VHLTPEEKNA   VTTLWGKVNV   DEVGGEALGR   LLLVYPWTQR   FFESFGDLSS    1  VHLTPEEKSA   VTALWGKVNV   DEVGGEALGR   LLVVYPWTQR   FFESFGDLST

Human Monkey Gibbon

  51  PDAVMGNPKV   KAHGKKVLGA   FSDGLAHLDN   LKGTFATLSE   LHCDKLHVDP   51  PDAVMGNPKV   KAHGKKVLGA   FSDGLNHLDN   LKGTFAQLSE   LHCDKLHVDP   51  PDAVMGNPKV   KAHGKKVLGA   FSDGLAHLDN   LKGTFAQLSE   LHCDKLHVDP

Human Monkey Gibbon

101  ENFRLLGNVL   VCVLAHHFGK   EFTPPVQAAY   QKVVAGVANA   LAHKYH 101  ENFKLLGNVL   VCVLAHHFGK   EFTPQVQAAY   QKVVAGVANA   LAHKYH 101  ENFRLLGNVL   VCVLAHHFGK   EFTPQVQAAY   QKVVAGVANA   LAHKYH

3. Based on these data alone, state a hypothesis for which of these two species is more closely related to humans. What is your reasoning? 4. What other evidence could you use to support your hypothesis? Data from  Human: http://www.ncbi.nlm.nih.gov/ protein/AAA21113.1; rhesus monkey: http://www.ncbi .nlm.nih.gov/protein/122634; gibbon: http://www.ncbi .nlm.nih.gov/protein/122616

Instructors: A version of this Scientific Skills Exercise can be assigned in MasteringBiology.

Problem-Solving Exercise

Are you a victim of fish fraud? When buying salmon, perhaps you prefer the more expensive wild-caught Pacific salmon (Oncorhynchus species) over farmed Atlantic salmon (Salmo salar). But studies reveal that about 40% of the time, you aren’t getting the fish you paid for! Watch the video in the MasteringBiology Study Area for more information.

In this exercise, you will investigate whether a piece of salmon has been fraudulently labeled. Your Approach The principle guiding your investigation is that DNA sequences from within a species or from closely related species are more similar to each other than are sequences from more distantly related species. Your Data

You’ve been sold a piece of salmon labeled as coho salmon (Oncorhynchus kisutch). To see whether your fish was labeled correctly, you will compare a short DNA sequence from your sample to standard sequences from the same gene for three salmon species. The sequences are: Sample labeled as O. kisutch   (coho salmon)

5 ’  CGGCACCGCCCTAAGTCTCT-3’  

Sequence for O. kisutch (coho salmon)  5’      AGGCACCGCCCTAAGTCTAC-3’   Standard µ Sequence for O. keta (chum salmon)  5’      AGGCACCGCCCTGAGCCTAC -3’   sequences Sequence for Salmo salar (Atlantic salmon) 5 ’  CGGCACCGCCCTAAGTCTCT-3’  

Your Analysis

ABC News Video: Fake Fish in Stores and Restaurants Instructors: A version of this ProblemSolving Exercise can be assigned in Chapter 5 of MasteringBiology. A more extensive investigation is in Chapter 26 of MasteringBiology.

1. Scan along the standard sequences (O. kisutch, O. keta, and S. salar), base by base, circling any bases that do not match the sequence from your fish sample. 2. How many bases differ between (a) O. kisutch and your fish sample? (b) O. keta and the sample? (c) S. salar and the sample? 3. For each standard, what percentage of its bases are identical to your sample? 4. Based on these data alone, state a hypothesis for the species identity of your sample. What is your reasoning?

chapter 5 

The Structure and Function of Large Biological Molecules

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5

Chapter Review

Go to MasteringBiology™ for Videos, Animations, Vocab Self-Quiz, Practice Tests, and more in the Study Area.

Summary of Key Concepts of monomers. The components of lipids vary. Monomers form larger molecules by dehydration reactions, in which water molecules are released. Polymers can disassemble by the reverse process, hydrolysis. An immense variety of polymers can be built from a small set of monomers.

Concept  5.1

Macromolecules are polymers, built from monomers (pp. 67–68)

VOCAB SELF-QUIZ

Large carbohydrates (polysaccharides), proteins, and nucleic acids are polymers, which are chains

Large Biological Molecules

goo.gl/6u55ks

CH2OH

(pp. 68–72)

?

Compare the composition, structure, and function of starch and cellulose. What role do starch and cellulose play in the human body?

Concept 5.3

What is the fundamental basis for the differences between large carbohydrates, proteins, and nucleic acids?

Components

Concept 5.2

Carbohydrates serve as fuel and building material

?

O

H

H OH

HO

H

H OH

H

OH

Monosaccharide monomer

Examples Monosaccharides: glucose, fructose Disaccharides: lactose, sucrose Polysaccharides:  Cellulose (plants)   Starch (plants)   Glycogen (animals)  Chitin (animals and fungi) Triacylglycerols (fats or oils): glycerol + three fatty acids

Glycerol

Lipids are a diverse group of hydrophobic molecules

Functions Fuel; carbon sources that can be converted to other molecules or combined into polymers

  Strengthens plant cell walls   Stores glucose for energy   Stores glucose for energy   Strengthens exoskeletons and fungal cell walls Important energy source

3 fatty acids

(pp. 72–75)

?

    

Why are lipids not considered to be polymers or macromolecules?

Phospholipids: glycerol + phosphate group + two fatty acids

Head with P 2 fatty acids

Lipid bilayers of membranes Hydrophobic tails Hydrophilic heads

Steroids: four fused rings with attached chemical groups

 Component of cell membranes (cholesterol)   Signaling molecules that travel through the body (hormones)

Steroid backbone

Concept 5.4

R

Proteins include a diversity of structures, resulting in a wide range of functions (pp. 75–83)

?

Explain the basis for the great diversity of proteins.

Concept 5.5

90

What role does complementary base pairing play in the functions of nucleic acids?

Unit one 

N H

C

C OH

H

Amino acid monomer (20 types)

Nitrogenous base

Nucleic acids store, transmit, and help express hereditary information (pp. 84–86)

?

O

H

The Chemistry of Life

Phosphate group CH2 P

O

Sugar Nucleotide (monomer of a polynucleotide)

 Enzymes   Defensive proteins   Storage proteins   Transport proteins  Hormones  Receptor proteins   Motor proteins   Structural proteins

DNA:

 Catalyze chemical reactions  Protect against disease   Store amino acids   Transport substances  Coordinate organismal responses  Receive signals from outside cell   Function in cell movement  Provide structural support

Stores hereditary information

 Sugar = deoxyribose  Nitrogenous bases = C, G, A, T  Usually double-stranded RNA:    Sugar = ribose  Nitrogenous bases = C, G, A, U  Usually single-stranded

Various functions in gene expression, including carrying instructions from DNA to ribosomes

Concept  5.6

Genomics and proteomics have transformed biological inquiry and applications (pp. 86–89)



Recent technological advances in DNA sequencing have given rise to genomics, an approach that analyzes large sets of genes or whole genomes, and proteomics, a similar approach for large sets of proteins. Bioinformatics is the use of computational tools and computer software to analyze these large data sets. The more closely two species are related evolutionarily, the more similar their DNA sequences are. DNA sequence data confirm models of evolution based on fossils and anatomical evidence.

?

Given the sequences of a particular gene in fruit flies, fish, mice, and humans, predict the relative similarity of the human sequence to that of each of the other species.

Test Your Understanding Level 1: Knowledge/Comprehension 1. Which of the following categories includes all others in the list? (A) disaccharide (B) polysaccharide (C) starch (D) carbohydrate

PRACTICE TEST

goo.gl/CUYGKD

2. The enzyme amylase can break glycosidic linkages between glucose monomers only if the monomers are in the α form. Which of the following could amylase break down? (A) glycogen, starch, and amylopectin (B) glycogen and cellulose (C) cellulose and chitin (D) starch, chitin, and cellulose 3. Which of the following is true of unsaturated fats? (A) They are more common in animals than in plants. (B) They have double bonds in their fatty acid chains. (C) They generally solidify at room temperature. (D) They contain more hydrogen than do saturated fats having the same number of carbon atoms. 4. The structural level of a protein least affected by a disruption in hydrogen bonding is the (A) primary level. (B) secondary level. (C) tertiary level. (D) quaternary level.

7. Which of the following pairs of base sequences could form a short stretch of a normal double helix of DNA? (A) 5′-AGCT-3′ with 5′-TCGA-3′ (B) 5′-GCGC-3′ with 5′-TATA-3′ (C) 5′-ATGC-3′ with 5′-GCAT-3′ (D) All of these pairs are correct. 8. Construct a table that organizes the following terms, and label the columns and rows. Monosaccharides Polypeptides Phosphodiester linkages Fatty acids Triacylglycerols Peptide bonds Amino acids Polynucleotides Glycosidic linkages Nucleotides Polysaccharides Ester linkages 9. DRAW IT  Copy the polynucleotide strand in Figure 5.23a and label the bases G, T, C, and T, starting from the 5′ end. Assuming this is a DNA polynucleotide, now draw the complementary strand, using the same symbols for phosphates (circles), sugars (pentagons), and bases. Label the bases. Draw arrows showing the 5′ S 3′ direction of each strand. Use the arrows to make sure the second strand is antiparallel to the first. Hint: After you draw the first strand vertically, turn the paper upside down; it is easier to draw the second strand from the 5′ toward the 3′ direction as you go from top to bottom.

Level 3: Synthesis/Evaluation 10. Evolution Connection  Comparisons of amino acid sequences can shed light on the evolutionary divergence of related species. If you were comparing two living species, would you expect all proteins to show the same degree of divergence? Why or why not? Justify your answer. 11. Scientific Inquiry  Suppose you are a research assistant in a lab studying DNA-binding proteins. You have been given the amino acid sequences of all the proteins encoded by the genome of a certain species and have been asked to find candidate proteins that could bind DNA. What type of amino acids would you expect to see in the DNA-binding regions of such proteins? Explain your thinking. 12. Write About A Theme: Organization  Proteins, which have diverse functions in a cell, are all polymers of the same kinds of monomers—amino acids. Write a short essay (100–150 words) that discusses how the structure of amino acids allows this one type of polymer to perform so many functions. 13. Synthesize Your Knowledge

5. Enzymes that break down DNA catalyze the hydrolysis of the covalent bonds that join nucleotides together. What would happen to DNA molecules treated with these enzymes? (A) The two strands of the double helix would separate. (B) The phosphodiester linkages of the polynucleotide backbone would be broken. (C) The pyrimidines would be separated from the deoxyribose sugars. (D) All bases would be separated from the deoxyribose sugars.

Level 2: Application/Analysis 6. The molecular formula for glucose is C6H12O6. What would be the molecular formula for a polymer made by linking ten glucose molecules together by dehydration reactions? (A) C60H120O60 (B) C60H102O51 (C) C60H100O50 (D) C60H111O51

chapter 5 

Given that the function of egg yolk is to nourish and support the developing chick, explain why egg yolks are so high in fat, protein, and cholesterol. For selected answers, see Appendix A. For additional practice questions, check out the Dynamic Study Modules in MasteringBiology. You can use them to study on your smartphone, tablet, or computer anytime, anywhere!

The Structure and Function of Large Biological Molecules

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Unit 2 Elba Serrano was born in Old San Juan, Puerto Rico. Because her father was a career sergeant in the U.S. military, she had a nomadic childhood, tempered by her family’s strong Hispanic culture. She majored in physics at the University of Rochester, one of two women among 80 majors. She then earned a Ph.D. from Stanford University, studying the permeability of nerve cells to ions, and was a postdoctoral fellow at UCLA. There, she started working on the auditory system, as a way of understanding how cells allow an organism to respond to changes in the environment. Since 1992, Dr. Serrano has been a faculty member at New Mexico State University, where she has made many discoveries about the development of the auditory system. Dr. Serrano is also an outspoken advocate and mentor for women and underrepresented groups in science.

“Membranes are like borders—very dynamic places where a lot of things happen.” Inner ear “hair” cells with bundles of rod-like protrusions that detect sound waves. Damage to these bundles by noise or aging can lead to deafness or loss of balance. (SEM)

The Cell An Interview with Elba Serrano How did you get interested in science, and in biology in particular?

When you’re a military kid, you’re asked to pick out what you want for Christmas from catalogs, and I remember always gravitating to the telescope and the chemistry set. To this day, I love math—I solve math problems for fun. In college I studied in a medical school library, and I’d take breaks from studying physics by reading about biology. I became very interested in biochemistry, and I said, “It looks like there is a field called biophysics—I think I’ll go to grad school for that.” I ended up at Stanford, and I really struggled—I was pretty much ready to drop out at the end of the first year. Then I was assigned a book called Nerve, Muscle, and Synapse, by Bernard Katz. I started reading this book at 8 AM and didn’t stop—I didn’t eat lunch. I finished it by the end of the day and knew “this is what I want to do.” It was basically membrane channel biophysics, and there were equations! I thought, “This is the science I recognize.”

What was your dissertation research? I worked on neurons, and I focused on the flow of ions like potassium through the cell membrane. Membranes are interesting because they both define a cell—they’re barriers—and at the same time they are a conduit. Membranes are like borders—very dynamic places where a lot of things happen. Within the membrane there are special proteins called ion channels, a kind of gatekeeper that permits the passage of ions. Neurons are the basic cells of the brain. The way the brain does its job is through the coordinated work of many neurons, and these cells are similar in some ways but also extremely diverse. So my dissertation question explored the diversity of neuron cell membranes: How are the proteins on different neurons different?

How did you become interested in the auditory system, and what questions are you asking? I always want to use the concepts of physics to understand biological systems. Biophysicists love the inner ear: it has sensory surfaces with beautiful cells called mechanosensory hair cells (see photo). These are similar but diverse in their function; the ear’s capacity to pick up all sound frequencies is because all the little hair cells are slightly different from each other. The mechanosensory hair cells we work on in my lab are marvelous, beautiful, very special cells. We study the structure and positioning of these cells, and at the same time we ask genetics questions. We are interested in identifying the genes for the ion channel proteins that are important in the electrical response of these cells—how are they organized in the genome, how are they turned on and off? The ear is a powerful system that has both redundancy—many similar cells—and diversity—each cell is unique.

What is your advice to an undergraduate who is considering a career in biology?

5 μm

92

First, don’t limit yourself in your possibilities for your future. Next, get a variety of experiences—part of life’s journey is about finding out what gives us satisfaction. If you’re choosing a career in biology, there are many ways you can be a biologist. You can work in labs or in field settings, among others. Finally, understanding biology today requires understanding math, physics, and chemistry, so it is important to invest in those areas. Even if you feel you will not be able to excel in them, do not let that deter you, because everything we do in science can be learned—it just requires dedication.

A Tour of the Cell

6

Figure 6.1  How do your cells help you learn about biology?

Key Concepts

The Fundamental Units of Life Cells are as fundamental to the living systems of biology as the atom is to chemistry. Many different types of cells are working for you right now. The contraction of muscle cells moves your eyes as you read this sentence. Figure 6.1 shows extensions from a nerve cell (orange) making contact with muscle cells (red). The words on the page are translated into signals that nerve cells carry to your brain, where they are passed on to other nerve cells. As you study, your cells make connections

6.1

Biologists use microscopes and biochemistry to study cells

6.2

Eukaryotic cells have internal membranes that compartmentalize their functions

6.3

The eukaryotic cell’s genetic instructions are housed in the nucleus and carried out by the ribosomes

6.4

The endomembrane system regulates protein traffic and performs metabolic functions

6.5

Mitochondria and chloroplasts change energy from one form to another

6.6

The cytoskeleton is a network of fibers that organizes structures and activities in the cell

6.7

Extracellular components and connections between cells help coordinate cellular activities

between nerve cells that solidify memories and permit learning to occur. All organisms are made of cells. In the hierarchy of biological organization, the cell is the simplest collection of matter that can be considered a living entity. Indeed, many forms of life exist as single-celled organisms, like the Paramecium shown here, a eukaryote that lives in pond water. Larger, more complex organisms, including plants and animals, are multicellular; their bodies are associations of many kinds of specialized cells that could not survive for long on their own. Even when cells are arranged into higher levels of organization, such as tissues and organs, 40 μm the cell remains the organism’s basic unit of structure and function. All cells are related by their descent from earlier cells. During the long evolutionary history of life on Earth, cells have been modified in many different ways. But although cells can differ substantially from one another, they share common features. In this chapter, we’ll first examine the tools and techniques that allow us to understand cells, then tour the cell and become acquainted with its components.

6.8

A cell is greater than the sum of its parts

When you see this blue icon, log in to MasteringBiology and go to the Study Area for digital resources.

Get Ready for This Chapter

93

Concept  6.1 Biologists use microscopes and biochemistry to study cells How can cell biologists investigate the inner workings of a cell, usually too small to be seen by the unaided eye? Before we tour the cell, it will be helpful to learn how cells are studied.

Figure 6.2  The size range of cells. Most cells are between 1 and 100 µm in diameter (yellow region of chart) and their components are even smaller (see Figure 6.32), as are viruses. Notice that the scale along the left side is logarithmic, to accommodate the range of sizes shown. Starting at the top of the scale with 10 m and going down, each reference measurement marks a tenfold decrease in diameter or length. For a complete table of the metric system, see Appendix C. 10 m

94

Unit two 

The Cell

0.1 m Chicken egg 1 cm

1 mm

100 μm

Frog egg

Human egg Most plant and animal cells

10 μm

1 μm

100 nm

10 nm

Nucleus Most bacteria Mitochondrion

Smallest bacteria Viruses

Superresolution microscopy

Ribosomes

Electron microscopy

The development of instruments that extend the human senses allowed the discovery and early study of cells. Microscopes were invented in 1590 and further refined during the 1600s. Cell walls were first seen by Robert Hooke in 1665 as he looked through a microscope at dead cells from the bark of an oak tree. But it took the wonderfully crafted lenses of Antoni van Leeuwenhoek to visualize living cells. Imagine Hooke’s excitement when he visited van Leeuwenhoek in 1674 and the world of microorganisms—what his host called “very little animalcules”—was revealed to him. The microscopes first used by Renaissance scientists, as well as the microscopes you are likely to use in the laboratory, are all light microscopes. In a light microscope (LM), visible light is passed through the specimen and then through glass lenses. The lenses refract (bend) the light in such a way that the image of the specimen is magnified as it is projected into the eye or into a camera (see Appendix D). Three important parameters in microscopy are magnification, resolution, and contrast. Magnification is the ratio of an object’s image size to its real size. Light microscopes can magnify effectively to about 1,000 times the actual size of the specimen; at greater magnifications, additional details cannot be seen clearly. Resolution is a measure of the clarity of the image; it is the minimum distance two points can be separated and still be distinguished as separate points. For example, what appears to the unaided eye as one star in the sky may be resolved as twin stars with a telescope, which has a higher resolving ability than the eye. Similarly, using standard techniques, the light microscope cannot resolve detail finer than about 0.2 micrometer (µm), or 200 nanometers (nm), regardless of the magnification (Figure 6.2). The third parameter, contrast, is the difference in brightness between the light and dark areas of an image. Methods for enhancing contrast include staining or labeling cell components to stand out visually. Figure 6.3 shows some different types of microscopy; study this figure as you read this section. Until recently, the resolution barrier prevented cell biologists from using standard light microscopy when studying organelles, the membrane-enclosed structures within eukaryotic cells. To see these structures in any detail required the development of a new instrument. In the 1950s, the electron microscope was introduced to biology. Rather than focusing light, the electron microscope (EM) focuses a

Length of some nerve and muscle cells

Light microscopy

Microscopy

Unaided eye

Human height 1m

Proteins Lipids

1 nm

0.1 nm

Small molecules Atoms 1 centimeter (cm) = 10 –2 meter (m) = 0.4 inch 1 millimeter (mm) = 10 –3 m 1 micrometer (μm) = 10 –3 mm = 10 –6 m 1 nanometer (nm) = 10 –3 μm = 10 –9 m

Animation: Metric System Review

beam of electrons through the specimen or onto its surface (see Appendix D). Resolution is inversely related to the wavelength of the light (or electrons) a microscope uses for imaging, and electron beams have much shorter wavelengths than visible light. Modern electron microscopes can theoretically achieve a resolution of about 0.002 nm, though in practice they usually cannot resolve structures smaller than about 2 nm across. Still, this is a 100-fold improvement over the standard light microscope. The scanning electron microscope (SEM) is especially useful for detailed study of the topography

Figure 6.3

  Exploring Microscopy

Light Microscopy (LM)

Brightfield (stained specimen). Staining with various dyes enhances contrast. Most staining procedures require that cells be fixed (preserved), thereby killing them.

Fluorescence. The locations of specific molecules in the cell can be revealed by labeling the molecules with fluorescent dyes or antibodies; some cells have molecules that fluoresce on their own. Fluorescent substances absorb ultraviolet radiation and emit visible light. In this fluorescently labeled uterine cell, nuclear material is blue, organelles called mitochondria are orange, and the cell’s “skeleton” is green. 10 μm

Electron Microscopy (EM)

Super-resolution. On the top is a confocal image of part of a nerve cell, using a fluorescent label that binds to a molecule clustered in small sacs in the cell (vesicles) that are 40 nm in diameter. The greenish-yellow spots are blurry because 40 nm is below the 200-nm limit of resolution for standard light microscopy. Below is an image of the same part of the cell, seen using a new super-resolution technique. Sophisticated equipment is used to light up individual fluorescent molecules and record their position. Combining information from many molecules in different places “breaks” the limit of resolution, resulting in the sharp greenish-yellow dots seen here. (Each dot is a 40-nm vesicle.) Longitudinal section of cilium

Cilia

Abbreviations used in figure legends in this text: LM = Light Micrograph SEM = Scanning Electron Micrograph TEM = Transmission Electron Micrograph VISUAL SKILLS

When the tissue was sliced for the TEM, what was the orientation of the cilia in the upper left? On the right? Explain how the orientation determined the type of section we see.



Cross section of cilium

TEM SEM

2 μm

2 μm

1 μm

Differential interference contrast (Nomarski). As in phase-contrast microscopy, optical modifications are used to exaggerate differences in density; the image appears almost 3-D.

10 μm

Deconvolution. The top of this split image is a compilation of standard fluorescence micrographs through the depth of a white blood cell. Below is an image of the same cell reconstructed from many blurry images at different planes, each of which was processed using deconvolution software. This  process digitally removes out-of-focus light and reassigns it to its source, creating a much sharper 3-D image.

Phase-contrast. Variations in density within the specimen are amplified to enhance contrast in unstained cells; this is especially useful for examining living, unpigmented cells.

Scanning electron microscopy (SEM). Micrographs taken with a scanning electron microscope show a 3-D image of the surface of a specimen. This SEM shows the surface of a cell from a trachea (windpipe) covered with cell projections called cilia. Electron micrographs are black and white but are often artificially colorized to highlight particular structures, as has been done with both electron micrographs shown here.

50 μm

Confocal. The top image is a standard fluorescence micrograph of fluorescently labeled nervous tissue (nerve cells are green, support cells are orange, and regions of overlap are yellow); below it is a confocal image of the same tissue. Using a laser, this “optical sectioning” technique eliminates out-of-focus light from a thick sample, creating a single plane of fluorescence in the image. By capturing sharp images at many different planes, a 3-D reconstruction can be created. The standard image is blurry because out-of-focus light is not excluded.

50 μm

Brightfield (unstained specimen). Light passes directly through the specimen. Unless the cell is naturally pigmented or artificially stained, the image has little contrast. (The first four light micrographs show human cheek epithelial cells; the scale bar pertains to all four micrographs.)

Transmission electron microscopy (TEM). A transmission electron microscope profiles a thin section of a specimen. This TEM shows a section through a tracheal cell, revealing its internal structure. In preparing the specimen, some cilia were cut along their lengths, creating longitudinal sections, while other cilia were cut straight across, creating cross sections.



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of a specimen (see Figure 6.3). The electron beam scans the surface of the sample, usually coated with a thin film of gold. The beam excites electrons on the surface, and these secondary electrons are detected by a device that translates the pattern of electrons into an electronic signal sent to a video screen. The result is an image of the specimen’s surface that appears three-dimensional. The transmission electron microscope (TEM) is used to study the internal structure of cells (see Figure 6.3). The TEM aims an electron beam through a very thin section of the specimen, much as a light microscope aims light through a sample on a slide. For the TEM, the specimen has been stained with atoms of heavy metals, which attach to certain cellular structures, thus enhancing the electron density of some parts of the cell more than others. The electrons passing through the specimen are scattered more in the denser regions, so fewer are transmitted. The image displays the pattern of transmitted electrons. Instead of using glass lenses, both the SEM and TEM use electromagnets as lenses to bend the paths of the electrons, ultimately focusing the image onto a monitor for viewing. Electron microscopes have revealed many subcellular structures that were impossible to resolve with the light microscope. But the light microscope offers advantages, especially in studying living cells. A disadvantage of electron microscopy is that the methods used to prepare the specimen kill the cells. Specimen preparation for any type of microscopy can introduce artifacts, structural features seen in micrographs that do not exist in the living cell. In the past several decades, light microscopy has been revitalized by major technical advances (see Figure 6.3). Labeling individual cellular molecules or structures with fluorescent markers has made it possible to see such structures with increasing detail. In addition, both confocal and deconvolution microscopy have produced sharper images of three-dimensional tissues and cells. Finally, a group of new techniques and labeling molecules developed in recent years has allowed researchers to “break” the ­resolution barrier and distinguish subcellular structures even as small as 10–20 nm across. As this super-resolution microscopy becomes more widespread, the images we see of living cells are proving as exciting for us as van Leeuwenhoek’s were for Robert Hooke 350 years ago. Microscopes are the most important tools of cytology, the study of cell structure. Understanding the function of each structure, however, required the integration of cytology and biochemistry, the study of the chemical processes (metabolism) of cells.

Cell Fractionation A useful technique for studying cell structure and function is cell fractionation (Figure 6.4), which takes cells apart and separates major organelles and other subcellular 96

Unit two 

The Cell

Figure 6.4

Research Method  Cell Fractionation Application  Cell fractionation is used to separate (fractionate) cell components based on size and density.

Technique  Cells are homogenized in a blender to break them up. The resulting mixture (homogenate) is centrifuged. The supernatant (the liquid above the pellet) is poured into another tube and centrifuged at a higher speed for a longer period. This process is repeated several times. This “differential centrifugation” results in a series of pellets, each containing different cell components.

Homogenization Tissue cells Homogenate Centrifuged at 1,000 g (1,000 times the Centrifugation force of gravity) for 10 min Supernatant poured into next tube Differential centrifugation 20,000 g 20 min

80,000 g 60 min

Pellet rich in nuclei and cellular debris

150,000 g 3 hr Pellet rich in mitochondria (and chloroplasts if cells are from a plant) Pellet rich in “microsomes” (pieces of plasma membranes and cells’ internal membranes) Pellet rich in ribosomes

Results  In early experiments, researchers used microscopy to identify the organelles in each pellet and biochemical methods to determine their metabolic functions. These identifications established a baseline for this method, enabling today’s researchers to know which cell fraction they should collect in order to isolate and study particular organelles. MAKE CONNECTIONS If you wanted to study the process of translation of proteins from mRNA, which part of which fraction would you use? (See Figure 5.22.)

structures from one another. The piece of equipment that is used for this task is the centrifuge, which spins test tubes holding mixtures of disrupted cells at a series of increasing speeds. At each speed, the resulting force causes a subset of the cell components to settle to the bottom of the tube, forming a pellet. At lower speeds, the pellet consists of larger components, and higher speeds result in a pellet with smaller components. Cell fractionation enables researchers to prepare specific cell components in bulk and identify their functions, a task not usually possible with intact cells. For example, on one of the cell fractions, biochemical tests showed the presence of enzymes involved in cellular respiration, while electron microscopy revealed large numbers of the organelles called mitochondria. Together, these data helped biologists determine that mitochondria are the sites of cellular respiration. Biochemistry and cytology thus complement each other in correlating cell function with structure.

Concept Check 6.1 1. How do stains used for light microscopy compare with those used for electron microscopy? 2. WHAT IF? Which type of microscope would you use to study (a) the changes in shape of a living white blood cell and (b) the details of surface texture of a hair? For suggested answers, see Appendix A.

Concept  6.2 Eukaryotic cells have internal membranes that compartmentalize their functions Cells—the basic structural and functional units of every organism—are of two distinct types: prokaryotic and eukaryotic. Organisms of the domains Bacteria and Archaea consist of prokaryotic cells. Protists, fungi, animals, and plants all consist of eukaryotic cells. (“Protist” is an informal term referring to a diverse group of mostly unicellular eukaryotes.)

Comparing Prokaryotic and Eukaryotic Cells All cells share certain basic features: They are all bounded by a selective barrier, called the plasma membrane (also referred to as the cell membrane). Inside all cells is a semifluid, jellylike substance called cytosol, in which subcellular components are suspended. All cells contain chromosomes, which carry genes in the form of DNA. And all cells have ribosomes, tiny complexes that make proteins according to instructions from the genes. A major difference between prokaryotic and eukaryotic cells is the location of their DNA. In a eukaryotic cell, most of the DNA is in an organelle called the nucleus, which is bounded by a double membrane (see Figure 6.8). In a prokaryotic cell, the DNA is concentrated in a region that is not membrane-enclosed, called the nucleoid (Figure 6.5).

Figure 6.5  A prokaryotic cell. Lacking a true nucleus and the other membrane-enclosed organelles of the eukaryotic cell, the prokaryotic cell appears much simpler in internal structure. Prokaryotes include bacteria and archaea; the general cell structure of these two domains is quite similar. Fimbriae: attachment structures on the surface of some prokaryotes (not visible on TEM) Nucleoid: region where the cell’s DNA is located (not enclosed by a membrane) Ribosomes: complexes that synthesize proteins Plasma membrane: membrane enclosing the cytoplasm Bacterial chromosome

Cell wall: rigid structure outside the plasma membrane Glycocalyx: outer coating of many prokaryotes, consisting of a capsule or a slime layer

(a) A typical rod-shaped bacterium

Flagella: locomotion organelles of some prokaryotes

0.5 μm (b) A thin section through the bacterium Corynebacterium diphtheriae (colorized TEM)

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Eukaryotic means “true nucleus” (from the Greek eu, true, and karyon, kernel, referring to the nucleus), and prokaryotic means “before nucleus” (from the Greek pro, before), reflecting the earlier evolution of prokaryotic cells. The interior of either type of cell is called the cytoplasm; in eukaryotic cells, this term refers only to the region between the nucleus and the plasma membrane. Within the cytoplasm of a eukaryotic cell, suspended in cytosol, are a variety of organelles of specialized form and function. These membranebounded structures are absent in almost all prokaryotic cells, another distinction between prokaryotic and eukaryotic cells. In spite of the absence of organelles, though, the ­prokaryotic cytoplasm is not a formless soup. For example, some ­prokaryotes contain regions surrounded by proteins (not membranes), within which specific reactions take place. Eukaryotic cells are generally much larger than prokaryotic cells (see Figure 6.2). Size is a general feature of cell structure that relates to function. The logistics of carrying out cellular metabolism sets limits on cell size. At the lower limit, the smallest cells known are bacteria called mycoplasmas, which have diameters between 0.1 and 1.0 µm. These are perhaps the smallest packages with enough DNA to program metabolism and enough enzymes and other cellular equipment to carry out the activities necessary for a cell to sustain itself and reproduce. Typical bacteria are 1–5 µm in diameter, about ten times the size of mycoplasmas. Eukaryotic cells are typically 10–100 µm in diameter. Metabolic requirements also impose theoretical upper limits on the size that is practical for a single cell. At the boundary of every cell, the plasma membrane functions as a selective barrier that allows passage of enough oxygen, nutrients, and wastes to service the entire cell (Figure 6.6). For each square micrometer of membrane, only a limited amount of a particular substance can cross per second, so the ratio of surface area to volume is critical. As a cell (or any other object) increases in size, its surface area grows proportionately less than its volume. (Area is proportional to a linear dimension squared, whereas volume is proportional to the linear dimension cubed.) Thus, a smaller object has a greater ratio of surface area to volume (Figure 6.7). The Scientific Skills Exercise gives you a chance to calculate the volumes and surface areas of two actual cells— a mature yeast cell and a cell budding from it. To see different ways organisms maximize the surface area of cells, see Make Connections Figure 33.9. The need for a surface area large enough to accommodate the volume helps explain the microscopic size of most cells and the narrow, elongated shapes of others, such as nerve cells. Larger organisms do not generally have larger cells than smaller organisms—they simply have more cells (see Figure 6.7). A sufficiently high ratio of surface area to volume is especially important in cells that exchange a lot of material with their surroundings, such as intestinal cells. Such cells may have many long, thin projections from their surface called microvilli, which increase surface area without an appreciable increase in volume. 98

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Figure 6.6  The plasma membrane. The plasma membrane and the membranes of organelles consist of a double layer (bilayer) of phospholipids with various proteins attached to or embedded in it. The hydrophobic parts of phospholipids and membrane proteins are found in the interior of the membrane, while the hydrophilic parts are in contact with aqueous solutions on either side. Carbohydrate side chains may be attached to proteins or lipids on the outer surface of the plasma membrane. (a) TEM of a plasma membrane. The plasma membrane, here in a red blood cell, appears as a pair of dark bands separated by a light band.

Outside of cell

Inside of cell (cytoplasm)

0.1 μm

Carbohydrate side chains Phospholipid

Hydrophilic region

Hydrophobic region Hydrophilic region

Proteins (b) Structure of the plasma membrane

VISUAL SKILLS

What parts of the membrane diagram in (b) correspond to the dark bands, and which to the light band, in the TEM in (a)? (Review Figure 5.11.)

BioFlix® Animation: Membranes

Figure 6.7  Geometric relationships between surface area and volume. In this diagram, cells are represented as boxes. Using arbitrary units of length, we can calculate the cell’s surface area (in square units, or units2), Surface area increases while volume (in cubic units, or total volume remains constant units3), and ratio of surface area to volume. A high surface-to-volume ratio facilitates the exchange of materials between a cell and 5 its environment. 1 1 Total surface area [sum of the surface areas (height × width) of all box sides × number of boxes]

Total volume [height × width × length × number of boxes] Surface-to-volume (S-to-V) ratio [surface area ÷ volume]

6

150

750

1

125

125

6

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Scientific Skills Exercise

Using a Scale Bar to Calculate Volume and Surface Area of a Cell How Much New Cytoplasm and Plasma Membrane Are Made by a Growing Yeast Cell?  The unicellular yeast Saccharomyces cerevisiae divides by budding off a small new cell that then grows to full size (see the yeast cells at the bottom of Figure 6.8). During its growth, the new cell synthesizes new cytoplasm, which increases its volume, and new plasma membrane, which increases its surface area. In this exercise, you will use a scale bar to determine the sizes of a mature parent yeast cell and a cell budding from it. You will then calculate the volume and surface area of each cell. You will use your calculations to determine how much cytoplasm and plasma membrane the new cell needs to synthesize to grow to full size.

How the Experiment Was Done  Yeast cells were grown under conditions that promoted division by budding. The cells were then viewed with a differential interference contrast light microscope and photographed.

Data from the Experiment  This light micrograph shows a budding yeast cell about to be released from the mature parent cell:

Mature parent cell Budding cell

1 μm Micrograph from  Kelly Tatchell, using yeast cells grown for experiments described in L. Kozubowski et al., Role of the septin ring in the asymmetric localization of proteins at the mother-bud neck in Saccharomyces cerevisiae, Molecular Biology of the Cell 16:3455–3466 (2005).

The evolutionary relationships between prokaryotic and eukaryotic cells will be discussed later in this chapter, and prokaryotic cells will be described in detail elsewhere (see Chapter 27). Most of the discussion of cell structure that follows in this chapter applies to eukaryotic cells.

A Panoramic View of the Eukaryotic Cell In addition to the plasma membrane at its outer surface, a eukaryotic cell has extensive, elaborately arranged internal membranes that divide the cell into compartments—the organelles mentioned earlier. The cell’s compartments provide different local environments that support specific metabolic functions, so incompatible processes can occur simultaneously in a single cell. The plasma membrane and organelle membranes also participate directly in the cell’s metabolism because many enzymes are built right into the membranes.

Interpret The Data 1. Examine the micrograph of the yeast cells. The scale bar under the photo is labeled 1 µm. The scale bar works in the same way as a scale on a map, where, for example, 1 inch equals 1 mile. In this case the bar represents one thousandth of a millimeter. Using the scale bar as a basic unit, determine the diameter of the mature parent cell and the new cell. Start by measuring the scale bar and the diameter of each cell. The units you use are irrelevant, but working in millimeters is convenient. Divide each diameter by the length of the scale bar and then multiply by the scale bar’s length value to give you the diameter in micrometers. 2. The shape of a yeast cell can be approximated by a sphere. (a) Calculate the volume of each cell using the formula for the volume of a sphere: V=

4 3 πr 3

r

d

Note that π (the Greek letter pi) is a constant with an approximate value of 3.14, d stands for diameter, and r stands for radius, which is half the diameter. (b) What volume of new cytoplasm will the new cell have to synthesize as it matures? To determine this, calculate the difference between the volume of the full-sized cell and the volume of the new cell. 3. As the new cell grows, its plasma membrane needs to expand to contain the increased volume of the cell. (a) Calculate the surface area of each cell using the formula for the surface area of a sphere: A = 4πr 2. (b) How much area of new plasma membrane will the new cell have to synthesize as it matures? 4. When the new cell matures, it will be approximately how many times greater in volume and how many times greater in surface area than its current size? Instructors: A version of this Scientific Skills Exercise can be assigned in MasteringBiology.

The basic fabric of most biological membranes is a double layer of phospholipids and other lipids. Embedded in this lipid bilayer or attached to its surfaces are diverse proteins (see Figure 6.6). However, each type of membrane has a unique composition of lipids and proteins suited to that membrane’s specific functions. For example, enzymes embedded in the membranes of the organelles called mitochondria function in cellular respiration. Because membranes are so fundamental to the organization of the cell, Chapter 7 will discuss them in detail. Before continuing with this chapter, examine the eukaryotic cells in Figure 6.8. The generalized diagrams of an animal cell and a plant cell introduce the various organelles and show the key differences between animal and plant cells. The micrographs at the bottom of the figure give you a glimpse of cells from different types of eukaryotic organisms.

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Figure 6.8

  Exploring Eukaryotic Cells Animal Cell (cutaway view of generalized cell)

Flagellum: motility structure present in some animal cells, composed of a cluster of microtubules within an extension of the plasma membrane

Nuclear envelope: double membrane enclosing the nucleus; perforated by pores; continuous with ER

ENDOPLASMIC RETICULUM (ER): network of membranous sacs and tubes; active in membrane synthesis and other synthetic and metabolic processes; has rough (ribosome-studded) and smooth regions Rough ER

Nucleolus: nonmembranous structure involved in production of ribosomes; a nucleus has one or more nucleoli

Smooth ER

NUCLEUS

Chromatin: material consisting of DNA and proteins; visible in a dividing cell as individual condensed chromosomes

Centrosome: region where the cell’s microtubules are initiated; contains a pair of centrioles

Plasma membrane: membrane enclosing the cell

CYTOSKELETON: reinforces cell’s shape; functions in cell movement; components are made of protein. Includes: Microfilaments Intermediate filaments

Ribosomes (small brown dots): complexes that make proteins; free in cytosol or bound to rough ER or nuclear envelope

Microtubules Microvilli: projections that increase the cell’s surface area

Golgi apparatus: organelle active in synthesis, modification, sorting, and secretion of cell products

Peroxisome: organelle with various specialized metabolic functions; produces hydrogen peroxide as a by-product and then converts it to water

Lysosome: digestive organelle where macromolecules are hydrolyzed

Mitochondrion: organelle where cellular respiration occurs and most ATP is generated

BioFlix® Animation: Tour of an Animal Cell

Nucleus Nucleolus Human cells from lining of uterus (colorized TEM)

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1 μm Cell wall

Buds

Vacuole

5 μm

Cell

Fungal Cells

Animal Cells

10 μm

Parent cell

Yeast cells: reproducing by budding (above, colorized SEM) and a single cell (right, colorized TEM)

Nucleus Mitochondrion

Plant Cell (cutaway view of generalized cell) Nuclear envelope NUCLEUS

Rough endoplasmic reticulum

Nucleolus Chromatin

Smooth endoplasmic reticulum

Ribosomes (small brown dots) Central vacuole: prominent organelle in older plant cells; functions include storage, breakdown of waste products, and hydrolysis of macromolecules; enlargement of the vacuole is a major mechanism of plant growth

Golgi apparatus

Microfilaments Microtubules

CYTOSKELETON

Mitochondrion Peroxisome Chloroplast: photosynthetic organelle; converts energy of sunlight to chemical energy stored in sugar molecules

Cell wall: outer layer that maintains cell’s shape and protects cell from mechanical damage; made of cellulose, other polysaccharides, and protein

Plant Cells

Cell Cell wall Chloroplast Mitochondrion Nucleus Nucleolus

Cells from duckweed (Spirodela oligorrhiza), a floating plant (colorized TEM)

BioFlix® Animation: Tour of a Plant Cell Video: Turgid Elodea Video: Chlamydomonas

8 μm

5 μm

Wall of adjacent cell

Unicellular Eukaryotes

Plasmodesmata: cytoplasmic channels through cell walls that connect the cytoplasms of adjacent cells

1 μm

Plasma membrane

Flagella Nucleus Nucleolus Vacuole

Unicellular green alga Chlamydomonas (above, colorized SEM; right, colorized TEM)

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Concept Check 6.2 1. Briefly describe the structure and function of the nucleus, the mitochondrion, the chloroplast, and the endoplasmic reticulum. 2. DRAW IT Draw a simplified elongated cell that measures 125 * 1 * 1 arbitrary units. A nerve cell would be roughly this shape. Predict how its surface-to-volume ratio would compare with those in Figure 6.7. Then calculate the ratio and check your prediction. For suggested answers, see Appendix A.

Concept  6.3 The eukaryotic cell’s genetic instructions are housed in the nucleus and carried out by the ribosomes On the first stop of our detailed tour of the eukaryotic cell, let’s look at two cellular components involved in the genetic control of the cell: the nucleus, which houses most of the cell’s DNA, and the ribosomes, which use information from the DNA to make proteins.

The Nucleus: Information Central The nucleus contains most of the genes in the eukaryotic cell. (Some genes are located in mitochondria and chloroplasts.) It is usually the most conspicuous organelle (see the purple structure in the fluorescence micrograph), averaging about 5 µm in diameter. The nuclear envelope encloses the nucleus (Figure 6.9), separating its contents from the cytoplasm. The nuclear envelope is a double membrane. The two membranes, each a lipid bilayer with associated proteins, are separated by a space of 20–40 nm. The envelope is perforated by pore structures that are about 100 nm in diameter. At the lip of each pore, the inner and outer membranes of the nuclear envelope are continuous. An intricate protein structure called a pore complex lines each pore and plays an important role in the cell by regulating the entry and exit of proteins and RNAs, as well as large complexes of macromolecules. Except at the pores, the nuclear side of the envelope is lined by the nuclear lamina, a netlike array of protein filaments (in animal cells, called intermediate filaments) that maintains the shape of the nucleus by mechanically supporting the nuclear envelope. There is also much evidence for a nuclear matrix, a framework of protein fibers extending throughout the nuclear interior. The nuclear lamina and matrix may help organize the genetic material so it functions efficiently. Within the nucleus, the DNA is organized into discrete units called chromosomes, structures that carry the genetic information. Each chromosome contains one long DNA molecule

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associated with many proteins. Some of the proteins help coil the DNA molecule of each chromosome, reducing its length and allowing it to fit into the nucleus. The complex of DNA and proteins making up chromosomes is called chromatin. When a cell is not dividing, stained chromatin appears as a diffuse mass in micrographs, and the chromosomes cannot be distinguished from one another, even though discrete chromosomes are present. As a cell prepares to divide, however, the chromosomes coil (condense) further, becoming thick enough to be distinguished under a microscope as separate structures. Each eukaryotic species has a characteristic number of chromosomes. For example, a typical human cell has 46 chromosomes in its nucleus; the exceptions are the sex cells (eggs and sperm), which have only 23 chromosomes in humans. A fruit fly cell has 8 chromosomes in most cells and 4 in the sex cells. A prominent structure within the nondividing nucleus is the nucleolus (plural, nucleoli), which appears through the electron microscope as a mass of densely stained granules and fibers adjoining part of the chromatin. Here a type of RNA called ribosomal RNA (rRNA) is synthesized from instructions in the DNA. Also in the nucleolus, proteins imported from the cytoplasm are assembled with rRNA into large and small subunits of ribosomes. These subunits then exit the nucleus through the nuclear pores to the cytoplasm, where a large and a small subunit Nucleus can assemble into a ribosome. Sometimes there are two or more nucleoli; the number depends on the species and the stage in the cell’s reproductive cycle. As we saw in Figure 5.22, the nucleus directs protein synthesis by synthesizing messenger RNA (mRNA) according to instructions provided by the DNA. The mRNA is then transported to the cytoplasm via the nuclear pores. Once an mRNA molecule reaches the cytoplasm, ribosomes 5 μm translate the mRNA’s genetic message into the primary structure of a specific polypeptide. (This process of transcribing and translating genetic information is described in detail in Chapter 17.) BioFlix® Animation: Nucleus and Ribosomes

Ribosomes: Protein Factories Ribosomes, which are complexes made of ribosomal RNAs and proteins, are the cellular components that carry out protein synthesis (Figure 6.10). (Note that ribosomes are not membrane bounded and thus are not considered organelles.) Cells that have high rates of protein synthesis have particularly large numbers of ribosomes as well as prominent nucleoli, which makes sense, given the role of nucleoli in ribosome assembly. For example, a human pancreas cell, which makes many digestive enzymes, has a few million ribosomes.

Figure 6.9  The nucleus and its envelope. Within the nucleus are the chromosomes, which appear as a mass of chromatin (DNA and associated proteins) and one or more nucleoli (singular, nucleolus), which function in ribosome synthesis. The nuclear envelope, which consists of two membranes separated by a narrow space, is perforated with pores and lined by the nuclear lamina. Nucleus 1 μm Nucleus Nucleolus Chromatin

Nuclear envelope: Outer membrane Inner membrane Nuclear pore Rough ER

▲ Surface of nuclear envelope (TEM). This specimen was prepared by a technique known as freeze-fracture, which cuts from the outer membrane to the inner membrane, revealing both.

Pore complex Ribosome

▲ Chromatin. This segment of a chromosome from a nondividing cell shows two states of coiling of the DNA (blue) and protein (purple) complex. The thicker form is sometimes also organized into long loops.

0.5 μm

0.25 μm

◀ Close-up of nuclear envelope

▲ Pore complexes (TEM). Each pore is ringed by protein particles. MAKE CONNECTIONS

Chromosomes contain the genetic material and reside in the nucleus; how does the rest of the cell get access to the information they carry? (See Figure 5.22.)

◀ Nuclear lamina (TEM). The netlike lamina lines the inner surface of the nuclear envelope. (The light circular spots are nuclear pores.) Figure 6.10  Ribosomes. This electron micrograph of a pancreas cell shows both free and bound ribosomes. The simplified diagram and computer model show the two subunits of a ribosome. 0.25 μm Ribosomes

Free ribosomes in cytosol

ER

Endoplasmic reticulum (ER) Ribosomes bound to ER Large subunit Small subunit TEM showing ER and ribosomes DRAW IT After you have read the section on ribosomes, circle a ribosome in the micrograph that might be making a protein that will be secreted.

Diagram of a ribosome

Computer model of a ribosome Interview with Venki Ramakrishnan: Studying ribosome structure

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Ribosomes build proteins in two cytoplasmic locales. At any given time, free ribosomes are suspended in the cytosol, while bound ribosomes are attached to the outside of the endoplasmic reticulum or nuclear envelope (see Figure 6.10). Bound and free ribosomes are structurally identical, and ribosomes can play either role at different times. Most of the proteins made on free ribosomes function within the cytosol; examples are enzymes that catalyze the first steps of sugar breakdown. Bound ribosomes generally make proteins that are destined for insertion into membranes, for packaging within certain organelles such as lysosomes (see Figure 6.8), or for export from the cell (secretion). Cells that specialize in protein secretion—for instance, the cells of the pancreas that secrete digestive enzymes—frequently have a high proportion of bound ribosomes. (You will learn more about ribosome structure and function in Concept 17.4.)

Concept Check 6.3 1. What role do ribosomes play in carrying out genetic instructions? 2. Describe the molecular composition of nucleoli and explain their function. 3. WHAT IF? As a cell begins the process of dividing, its chromosomes become shorter, thicker, and individually visible in an LM (light micrograph). Explain what is happening at the molecular level. For suggested answers, see Appendix A.

Concept  6.4 The endomembrane system regulates protein traffic and performs metabolic functions Many of the different membrane-bounded organelles of the eukaryotic cell are part of the endomembrane system, which includes the nuclear envelope, the endoplasmic reticulum, the Golgi apparatus, lysosomes, various kinds of vesicles and vacuoles, and the plasma membrane. This system carries out a variety of tasks in the cell, including synthesis of proteins, transport of proteins into membranes and organelles or out of the cell, metabolism and movement of lipids, and detoxification of poisons. The membranes of this system are related either through direct physical continuity or by the transfer of membrane segments as tiny vesicles (sacs made of membrane). Despite these relationships, the various membranes are not identical in structure and function. Moreover, the thickness, molecular composition, and types of chemical reactions carried out in a given membrane are not fixed, but may be modified several times during the membrane’s life. Having already discussed the nuclear envelope, we will now focus on the endoplasmic reticulum and the other endomembranes to which the endoplasmic reticulum gives rise.

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The Endoplasmic Reticulum: Biosynthetic Factory The endoplasmic reticulum (ER) is such an extensive network of membranes that it accounts for more than half the total membrane in many eukaryotic cells. (The word endoplasmic means “within the cytoplasm,” and reticulum is Latin for “little net.”) The ER consists of a network of membranous tubules and sacs called cisternae (from the Latin cisterna, a reservoir for a liquid). The ER membrane separates the internal compartment of the ER, called the ER lumen (cavity) or cisternal space, from the cytosol. And because the ER membrane is continuous with the nuclear envelope, the space between the two membranes of the envelope is continuous with the lumen of the ER (Figure 6.11). There are two distinct, though connected, regions of the ER that differ in structure and function: smooth ER and rough ER. Smooth ER is so named because its outer surface lacks ribosomes. Rough ER is studded with ribosomes on the outer surface of the membrane and thus appears rough through the electron microscope. As already mentioned, ribosomes are also attached to the cytoplasmic side of the nuclear envelope’s outer membrane, which is continuous with rough ER.

Functions of Smooth ER The smooth ER functions in diverse metabolic processes, which vary with cell type. These processes include synthesis of lipids, metabolism of carbohydrates, detoxification of drugs and poisons, and storage of calcium ions. Enzymes of the smooth ER are important in the synthesis of lipids, including oils, steroids, and new membrane phospholipids. Among the steroids produced by the smooth ER in animal cells are the sex hormones of vertebrates and the various steroid hormones secreted by the adrenal glands. The cells that synthesize and secrete these hormones—in the testes and ovaries, for example—are rich in smooth ER, a structural feature that fits the function of these cells. Other enzymes of the smooth ER help detoxify drugs and poisons, especially in liver cells. Detoxification usually involves adding hydroxyl groups to drug molecules, making them more soluble and easier to flush from the body. The sedative phenobarbital and other barbiturates are examples of drugs metabolized in this manner by smooth ER in liver cells. In fact, barbiturates, alcohol, and many other drugs induce the proliferation of smooth ER and its associated detoxification enzymes, thus increasing the rate of detoxification. This, in turn, increases tolerance to the drugs, meaning that higher doses are required to achieve a particular effect, such as sedation. Also, because some of the detoxification enzymes have relatively broad action,

the proliferation of smooth ER in response to one drug can increase the need for higher dosages of other drugs as well. Barbiturate abuse, for example, can decrease the effectiveness of certain antibiotics and other useful drugs. Figure 6.11  Endoplasmic reticulum (ER). A membranous system of interconnected tubules and flattened sacs called cisternae, the ER is also continuous with the nuclear envelope, as shown in the cutaway diagram at the top. The membrane of the ER encloses a continuous compartment called the ER lumen (or cisternal space). Rough ER, which is studded on its outer surface with ribosomes, can be distinguished from smooth ER in the electron micrograph (TEM). Transport vesicles bud off from a region of the rough ER called transitional ER and travel to the Golgi apparatus and other destinations.

Smooth ER

Rough ER

Nuclear envelope

ER lumen Cisternae Ribosomes

Transitional ER

Transport vesicle

Smooth ER

Rough ER

0.2 μm

The smooth ER also stores calcium ions. In muscle cells, for example, the smooth ER membrane pumps calcium ions from the cytosol into the ER lumen. When a muscle cell is stimulated by a nerve impulse, calcium ions rush back across the ER membrane into the cytosol and trigger contraction of the muscle cell. In other cell types, release of calcium ions from the smooth ER triggers different responses, such as secretion of vesicles carrying newly synthesized proteins.

Functions of Rough ER Many cells secrete proteins that are produced by ribosomes attached to rough ER. For example, certain pancreatic cells synthesize the protein insulin in the ER and secrete this hormone into the bloodstream. As a polypeptide chain grows from a bound ribosome, the chain is threaded into the ER lumen through a pore formed by a protein complex in the ER membrane. The new polypeptide folds into its functional shape as it enters the ER lumen. Most secretory proteins are glycoproteins, proteins with carbohydrates covalently bonded to them. The carbohydrates are attached to the proteins in the ER lumen by enzymes built into the ER membrane. After secretory proteins are formed, the ER membrane keeps them separate from proteins in the cytosol, which are produced by free ribosomes. Secretory proteins depart from the ER wrapped in the membranes of vesicles that bud like bubbles from a specialized region called transitional ER (see Figure 6.11). Vesicles in transit from one part of the cell to another are called transport vesicles; we will discuss their fate shortly. In addition to making secretory proteins, rough ER is a membrane factory for the cell; it grows in place by adding membrane proteins and phospholipids to its own membrane. As polypeptides destined to be membrane proteins grow from the ribosomes, they are inserted into the ER membrane itself and anchored there by their hydrophobic portions. Like the smooth ER, the rough ER also makes membrane phospholipids; enzymes built into the ER membrane assemble phospholipids from precursors in the cytosol. The ER membrane expands, and portions of it are transferred in the form of transport vesicles to other components of the endomembrane system.

The Golgi Apparatus: Shipping and Receiving Center After leaving the ER, many transport vesicles travel to the Golgi apparatus. We can think of the Golgi as a warehouse for receiving, sorting, shipping, and even some manufacturing. Here, products of the ER, such as proteins, are modified and stored and then sent to other destinations. Not surprisingly, the Golgi apparatus is especially extensive in cells specialized for secretion.

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The Golgi apparatus consists of a group of associated, flattened membranous sacs—cisternae—looking like a stack of pita bread (Figure 6.12). A cell may have many, even hundreds, of these stacks. The membrane of each cisterna in a stack separates its internal space from the cytosol. Vesicles concentrated in the vicinity of the Golgi apparatus are engaged in the transfer of material between parts of the Golgi and other structures. A Golgi stack has a distinct structural directionality, with the membranes of cisternae on opposite sides of the stack differing in thickness and molecular composition. The two sides of a Golgi stack are referred to as the cis face and the trans face; these act, respectively, as the receiving and shipping departments of the Golgi apparatus. The term cis means “on the same side,” and the cis face is usually located near the ER. Transport vesicles move material from the ER to the Golgi apparatus. A vesicle that buds from the ER can add its membrane and the contents of its lumen to the cis face by fusing with a Golgi membrane on that side. The trans face (“on the opposite side”) gives rise to vesicles that pinch off and travel to other sites. Products of the endoplasmic reticulum are usually modified during their transit from the cis region to the trans region of the Golgi apparatus. For example, glycoproteins formed in the ER have their carbohydrates modified, first in the ER itself, and then as they pass through the Golgi. The Golgi

Golgi apparatus

cis face (“receiving” side of Golgi apparatus)

1 Vesicles move from ER to Golgi.

6 Vesicles also

transport certain proteins back to ER, their site of function.

removes some sugar monomers and substitutes others, producing a large variety of carbohydrates. Membrane phospholipids may also be altered in the Golgi. In addition to its finishing work, the Golgi apparatus also manufactures some macromolecules. Many polysaccharides secreted by cells are Golgi products. For example, pectins and certain other noncellulose polysaccharides are made in the Golgi of plant cells and then incorporated along with cellulose into their cell walls. Like secretory proteins, nonprotein Golgi products that will be secreted depart from the trans face of the Golgi inside transport vesicles that eventually fuse with the plasma membrane. The Golgi manufactures and refines its products in stages, with different cisternae containing unique teams of enzymes. Until recently, biologists viewed the Golgi as a static structure, with products in various stages of processing transferred from one cisterna to the next by vesicles. While this may occur, research from several labs has given rise to a new model of the Golgi as a more dynamic structure. According to the cisternal maturation model, the cisternae of the Golgi actually progress forward from the cis to the trans face, carrying and modifying their cargo as they move. Figure 6.12 shows the details of this model. The reality probably lies somewhere between the two models; recent research suggests the central regions of the cisternae may remain in place, while the outer ends are more dynamic.

Figure 6.12  The Golgi apparatus. The Golgi apparatus consists of stacks of associated, flattened sacs, or cisternae. Unlike the ER cisternae, these sacs are not physically connected, as you can see in the cutaway diagram. A Golgi stack receives and dispatches transport vesicles and the products they contain. A Golgi stack has a structural and functional directionality, with a cis face that receives vesicles containing ER products and a trans face that dispatches vesicles. The cisternal maturation model proposes that the Golgi cisternae themselves “mature,” moving from the cis to the trans face while carrying some proteins along. In addition, some vesicles recycle enzymes that had been carried forward in moving cisternae, transporting them “backward” to a less mature region where their functions are needed. 2 Vesicles coalesce to form new cis Golgi cisternae.

0.1 μm

Cisternae 3 Cisternal maturation: Golgi cisternae move in a cisto-trans direction. 4 Vesicles form and leave Golgi, carrying specific products to other locations or to the plasma membrane for secretion.

5 Vesicles transport some proteins backward to less mature Golgi cisternae, where they function.

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trans face (“shipping” side of Golgi apparatus)

TEM of Golgi apparatus

Before a Golgi stack dispatches its products by budding vesicles from the trans face, it sorts these products and targets them for various parts of the cell. Molecular identification tags, such as phosphate groups added to the Golgi products, aid in sorting by acting like zip codes on mailing labels. Finally, transport vesicles budded from the Golgi may have external molecules on their membranes that recognize “docking sites” on the surface of specific organelles or on the plasma membrane, thus targeting the vesicles appropriately.

Lysosomes: Digestive Compartments A lysosome is a membranous sac of hydrolytic enzymes that many eukaryotic cells use to digest (hydrolyze) macromolecules. Lysosomal enzymes work best in the acidic environment found in lysosomes. If a lysosome breaks open or leaks its contents, the released enzymes are not very active because the cytosol has a near-neutral pH. However, excessive leakage from a large number of lysosomes can destroy a cell by self-digestion. Hydrolytic enzymes and lysosomal membrane are made by rough ER and then transferred to the Golgi apparatus for

further processing. At least some lysosomes probably arise by budding from the trans face of the Golgi apparatus (see Figure 6.12). How are the proteins of the inner surface of the lysosomal membrane and the digestive enzymes themselves spared from destruction? Apparently, the three-dimensional shapes of these proteins protect vulnerable bonds from enzymatic attack. Lysosomes carry out intracellular digestion in a variety of circumstances. Amoebas and many other unicellular eukaryotes eat by engulfing smaller organisms or food particles, a process called phagocytosis (from the Greek phagein, to eat, and kytos, vessel, referring here to the cell). The food vacuole formed in this way then fuses with a lysosome, whose enzymes digest the food (Figure 6.13a, bottom). Digestion products, including simple sugars, amino acids, and other monomers, pass into the cytosol and become nutrients for the cell. Some human cells also carry out phagocytosis. Among them are macrophages, a type of white blood cell that helps defend the body by engulfing and ­destroying bacteria and other invaders (see Figure 6.13a, top, and Figure 6.31).

Figure 6.13  Lysosomes. Nucleus

1 μm

Vesicle containing two damaged organelles

1 μm

Mitochondrion fragment Peroxisome fragment

Lysosome 1 Lysosome contains active hydrolytic enzymes.

2 Lysosome fuses with food vacuole.

3 Hydrolytic enzymes digest food particles.

1 Lysosome fuses with vesicle containing damaged organelles.

Digestive enzymes

2 Hydrolytic enzymes digest organelle components.

Lysosome

Lysosome Plasma membrane

Peroxisome Digestion Food vacuole Vesicle

(a) Phagocytosis. In phagocytosis, lysosomes digest (hydrolyze) materials taken into the cell. Top: In this macrophage (a type of white blood cell) from a rat, the lysosomes are very dark because of a stain that reacts with one of the products of digestion inside the lysosome (TEM). Macrophages ingest bacteria and viruses and destroy them using lysosomes. Bottom: This diagram shows a lysosome fusing with a food vacuole during the process of phagocytosis by a unicellular eukaryote.

Mitochondrion

Digestion

(b) Autophagy. In autophagy, lysosomes recycle intracellular materials. Top: In the cytoplasm of this rat liver cell is a vesicle containing two disabled organelles (TEM). The vesicle will fuse with a lysosome in the process of autophagy, which recycles intracellular materials. Bottom: This diagram shows fusion of such a vesicle with a lysosome. This type of vesicle has a double membrane of unknown origin. The outer membrane fuses with the lysosome, and the inner membrane is degraded along with the damaged organelles.

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Lysosomes also use their hydrolytic enzymes to recycle the cell’s own organic material, a process called autophagy. During autophagy, a damaged organelle or small amount of cytosol becomes surrounded by a double membrane (of unknown origin), and a lysosome fuses with the outer membrane of this vesicle (Figure 6.13b). The lysosomal enzymes dismantle the inner membrane with the enclosed material, and the resulting small organic compounds are released to the cytosol for reuse. With the help of lysosomes, the cell continually renews itself. A human liver cell, for example, recycles half of its macromolecules each week. The cells of people with inherited lysosomal storage diseases lack a functioning hydrolytic enzyme normally present in lysosomes. The lysosomes become engorged with indigestible material, which begins to interfere with other cellular activities. In Tay-Sachs disease, for example, a lipid-digesting enzyme is missing or inactive, and the brain becomes impaired by an accumulation of lipids in the cells. Fortunately, lysosomal storage diseases are rare in the general population.

Vacuoles: Diverse Maintenance Compartments Vacuoles are large vesicles derived from the endoplasmic reticulum and Golgi apparatus. Thus, vacuoles are an integral part of a cell’s endomembrane system. Like all cellular membranes, the vacuolar membrane is selective in transporting solutes; as a result, the solution inside a vacuole differs in composition from the cytosol. Vacuoles perform a variety of functions in different kinds of cells. Food vacuoles, formed by phagocytosis, have already been mentioned (see Figure 6.13a). Many unicellular eukaryotes living in fresh water have contractile vacuoles that pump excess water out of the cell, thereby maintaining a suitable concentration of ions and molecules inside the cell (see Figure 7.13). In plants and fungi, certain vacuoles carry out enzymatic hydrolysis, a function shared by lysosomes in animal cells. (In fact, some biologists consider these hydrolytic vacuoles to be a type of lysosome.) In plants, small vacuoles can hold reserves of important organic compounds, such as the proteins stockpiled in the storage cells in seeds. Vacuoles may also help protect the plant against herbivores by storing compounds that are poisonous or unpalatable to animals. Some plant vacuoles contain pigments, such as the red and blue pigments of petals that help attract pollinating insects to flowers. Mature plant cells generally contain a large central vacuole (Figure 6.14), which develops by the coalescence of smaller vacuoles. The solution inside the central vacuole, called cell sap, is the plant cell’s main repository of inorganic ions, including potassium and chloride. The central vacuole plays a major role in the growth of plant cells, which enlarge

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Figure 6.14  The plant cell vacuole. The central vacuole is usually the largest compartment in a plant cell; the rest of the cytoplasm is often confined to a narrow zone between the vacuolar membrane and the plasma membrane (TEM).

Central vacuole

Cytosol

Central vacuole

Nucleus Cell wall Chloroplast

5 μm BioFlix® Animation: Central Vacuole

as the vacuole absorbs water, enabling the cell to become larger with a minimal investment in new cytoplasm. The cytosol often occupies only a thin layer between the central vacuole and the plasma membrane, so the ratio of plasma membrane surface to cytosolic volume is sufficient, even for a large plant cell.

The Endomembrane System: A Review Figure 6.15 reviews the endomembrane system, showing

the flow of membrane lipids and proteins through the various organelles. As the membrane moves from the ER to the Golgi and then elsewhere, its molecular composition and metabolic functions are modified, along with those of its contents. The endomembrane system is a complex and dynamic player in the cell’s compartmental organization. We’ll continue our tour of the cell with some organelles that are not closely related to the endomembrane system but play crucial roles in the energy transformations carried out by cells.

Concept Check 6.4 1. Describe the structural and functional distinctions between rough and smooth ER. 2. Describe how transport vesicles integrate the endomembrane system. 3. WHAT IF? Imagine a protein that functions in the ER but requires modification in the Golgi apparatus before it can achieve that function. Describe the protein’s path through the cell, starting with the mRNA molecule that specifies the protein. For suggested answers, see Appendix A.

Nucleus 1 The nuclear envelope is

Nuclear envelope

connected to the rough ER, which is also continuous with the smooth ER.

Rough ER Smooth ER 2 Membranes and proteins produced by the ER move via transport vesicles to the Golgi.

cis Golgi

3 The Golgi pinches off transport vesicles and other vesicles that give rise to lysosomes, other types of specialized vesicles, and vacuoles.

Plasma membrane

trans Golgi

4 The lysosome is available for fusion with another vesicle for digestion.

5 A transport vesicle carries proteins to the plasma membrane for secretion.

6 The plasma membrane expands by fusion of vesicles; proteins are secreted from the cell.

Figure 6.15  Review: relationships among organelles of the endomembrane system. The red arrows show some of the migration pathways for membranes and the materials they enclose.

Concept  6.5 Mitochondria and chloroplasts change energy from one form to another Organisms transform the energy they acquire from their surroundings. In eukaryotic cells, mitochondria and chloroplasts are the organelles that convert energy to forms that cells can use for work. Mitochondria (singular, mitochondrion) are the sites of cellular respiration, the metabolic process that uses oxygen to drive the generation of ATP by extracting energy from sugars, fats, and other fuels. Chloroplasts, found in plants and algae, are the sites of photosynthesis. This process in chloroplasts converts solar energy to chemical energy by absorbing sunlight and using it to drive the synthesis of organic compounds such as sugars from carbon dioxide and water. In addition to having related functions, mitochondria and chloroplasts share similar evolutionary origins, which we’ll discuss briefly before describing their structures. In this section, we will also consider the peroxisome, an oxidative organelle. The evolutionary origin of the peroxisome, as well as its relation to other organelles, is still a matter of some debate.

BioFlix® Animation: Endomembrane System

The Evolutionary Origins of Mitochondria and Chloroplasts Evolution Mitochondria and chloroplasts display similarities with bacteria that led to the endosymbiont theory, illustrated in Figure 6.16. This theory states that an early ancestor of eukaryotic cells engulfed an oxygen-using nonphotosynthetic prokaryotic cell. Eventually, the engulfed cell formed a relationship with the host cell in which it was enclosed, becoming an endosymbiont (a cell living within another cell). Indeed, over the course of evolution, the host cell and its endosymbiont merged into a single organism, a eukaryotic cell with a mitochondrion. At least one of these cells may have then taken up a photosynthetic prokaryote, becoming the ancestor of eukaryotic cells that contain chloroplasts. This is a widely accepted theory, which we will discuss in more detail in Concept 25.3. This theory is consistent with many structural features of mitochondria and chloroplasts. First, rather than being bounded by a single membrane like organelles of the endomembrane system, mitochondria and typical chloroplasts have two membranes surrounding them. (Chloroplasts also have an internal system of membranous sacs.) There is evidence that the ancestral engulfed

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Figure 6.16  The endosymbiont theory of the origins of mitochondria and chloroplasts in eukaryotic cells. According to this theory, the proposed ancestors of mitochondria were oxygenusing nonphotosynthetic prokaryotes, while the proposed ancestors of chloroplasts were photosynthetic prokaryotes. The large arrows represent change over evolutionary time; the small arrows inside the cells show the process of the endosymbiont becoming an organelle, also over long periods of time. Nucleus

Endoplasmic reticulum Nuclear envelope

Engulfing of oxygenusing nonphotosynthetic prokaryote, which, over many generations of cells, becomes a mitochondrion

Ancestor of eukaryotic cells (host cell)

Engulfing of photosynthetic prokaryote

Mitochondrion

Chloroplast At least one cell

Mitochondrion

Nonphotosynthetic eukaryote

Photosynthetic eukaryote

prokaryotes had two outer membranes, which became the double membranes of mitochondria and chloroplasts. Second, like prokaryotes, mitochondria and chloroplasts contain ribosomes, as well as circular DNA molecules—like bacterial chromosomes—associated with their inner membranes. The DNA in these organelles programs the synthesis of some organelle proteins on ribosomes that have been synthesized and assembled there as well. Third, also consistent with their probable evolutionary origins as cells, mitochondria and chloroplasts are autonomous (somewhat independent) organelles that grow and reproduce within the cell. Next, we focus on the structures of mitochondria and chloroplasts, while providing an overview of their structures and functions. (In Chapters 9 and 10, we will examine their roles as energy transformers.)

Mitochondria: Chemical Energy Conversion Mitochondria are found in nearly all eukaryotic cells, including those of plants, animals, fungi, and most unicellular eukaryotes. Some cells have a single large mitochondrion,

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but more often a cell has hundreds or even thousands of mitochondria; the number correlates with the cell’s level of metabolic activity. For example, cells that move or contract have proportionally more mitochondria per volume than less active cells. Each of the two membranes enclosing the mitochondrion is a phospholipid bilayer with a unique collection of embedded proteins (Figure 6.17). The outer membrane is smooth, but the inner membrane is convoluted, with infoldings called cristae. The inner membrane divides the mitochondrion into two internal compartments. The first is the intermembrane space, the narrow region between the inner and outer membranes. The second compartment, the mitochondrial matrix, is enclosed by the inner membrane. The matrix contains many different enzymes as well as the mitochondrial DNA and ribosomes. Enzymes in the matrix catalyze some of the steps of cellular respiration. Other proteins that function in respiration, including the enzyme that makes ATP, are built into the inner membrane. As highly folded surfaces, the cristae give the inner mitochondrial membrane a large surface area, thus enhancing the productivity of cellular respiration. This is another example of structure fitting function. Mitochondria are generally in the range of 1–10 µm long. Time-lapse films of living cells reveal mitochondria moving around, changing their shapes, and fusing or dividing in two, unlike the static structures seen in electron micrographs of dead cells. These studies helped cell biologists understand that mitochondria in a living cell form a branched tubular network, seen in a whole cell in Figure 6.17b, that is in a dynamic state of flux.

Chloroplasts: Capture of Light Energy Chloroplasts contain the green pigment chlorophyll, along with enzymes and other molecules that function in the photosynthetic production of sugar. These lens-shaped organelles, about 3–6 µm in length, are found in leaves and other green organs of plants and in algae (Figure 6.18; see also Figure 6.26c). The contents of a chloroplast are partitioned from the cytosol by an envelope consisting of two membranes separated by a very narrow intermembrane space. Inside the chloroplast is another membranous system in the form of flattened, interconnected sacs called thylakoids. In some regions, thylakoids are stacked like poker chips; each stack is called a granum (plural, grana). The fluid outside the thylakoids is the stroma, which contains the chloroplast DNA and ribosomes as well as many enzymes. The membranes of the chloroplast divide the chloroplast space into three compartments: the intermembrane space, the stroma, and the thylakoid space. This compartmental organization enables the chloroplast to convert light energy to chemical energy

by the membranes: the intermembrane space and the mitochondrial matrix. Many respiratory enzymes are found in the inner membrane and the matrix. Free ribosomes are also present in the matrix. The circular DNA molecules are associated with the inner mitochondrial membrane. (b) The light micrograph shows an

Figure 6.17  The mitochondrion, site of cellular respiration. (a) The inner and outer membranes of the mitochondrion are evident in the drawing and electron micrograph (TEM). The cristae are infoldings of the inner membrane, which increase its surface area. The cutaway drawing shows the two compartments bounded

entire unicellular eukaryote (Euglena gracilis) at a much lower magnification than the TEM. The mitochondrial matrix has been stained green. The mitochondria form a branched tubular network. The nuclear DNA is stained red; molecules of mitochondrial DNA appear as bright yellow spots.

Mitochondrion

10 μm Intermembrane space Mitochondria

Outer membrane

DNA Inner membrane

Free ribosomes in the mitochondrial matrix

Mitochondrial DNA

Cristae Matrix

Nuclear DNA 0.1 μm (b) Network of mitochondria in Euglena (LM)

(a) Diagram and TEM of mitochondrion BioFlix® Animation: Mitochondria

during photosynthesis. (You will learn more about photosynthesis in Chapter 10.) As with mitochondria, the static and rigid appearance of chloroplasts in micrographs or schematic diagrams cannot accurately depict their dynamic behavior in the living cell. Their shape is changeable, and they grow and occasionally pinch in two, reproducing themselves. They are mobile and, with mitochondria and other organelles, move around the

Chloroplast

cell along tracks of the cytoskeleton, a structural network we will consider in Concept 6.6. The chloroplast is a specialized member of a family of closely related plant organelles called plastids. One type of plastid, the amyloplast, is a colorless organelle that stores starch (amylose), particularly in roots and tubers. Another is the chromoplast, which has pigments that give fruits and flowers their orange and yellow hues.

Figure 6.18  The chloroplast, site of photosynthesis. (a) Many plants have lens-shaped chloroplasts, as shown here in a diagram and a TEM. A chloroplast has three compartments: the intermembrane space, the stroma, and the thylakoid space. Free ribosomes are present in the stroma, as are copies of chloroplast DNA molecules. (b) This fluorescence micrograph, at a much lower magnification than the TEM, shows a whole cell of the green alga Spirogyra crassa, which is named for its spiral chloroplasts. Under natural light the chloroplasts appear green, but under ultraviolet light they naturally fluoresce red, as shown here. Stroma 50 μm

Ribosomes Inner and outer membranes Granum

Chloroplasts (red)

DNA Thylakoid

Intermembrane space

(a) Diagram and TEM of chloroplast

1 μm (b) Chloroplasts in an algal cell

®

BioFlix Animation: Chloroplasts and Mitochondria

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Figure 6.19  A peroxisome. Peroxisomes are roughly spherical and often have a granular or crystalline core that is thought to be a dense collection of enzyme molecules. Chloroplasts and mitochondria cooperate with peroxisomes in certain metabolic functions (TEM).

Concept  6.6

Peroxisome Mitochondrion

Chloroplasts

1 μm

Peroxisomes: Oxidation The peroxisome is a specialized metabolic compartment bounded by a single membrane (Figure 6.19). Peroxisomes contain enzymes that remove hydrogen atoms from various substrates and transfer them to oxygen (O2), producing hydrogen peroxide (H2O2) as a by-product (from which the organelle derives its name). These reactions have many different functions. Some peroxisomes use oxygen to break fatty acids down into smaller molecules that are transported to mitochondria and used as fuel for cellular respiration. Peroxisomes in the liver detoxify alcohol and other harmful compounds by transferring hydrogen from the poisonous compounds to oxygen. The H2O2 formed by peroxisomes is itself toxic, but the organelle also contains an enzyme that converts H2O2 to water. This is an excellent example of how the cell’s compartmental structure is crucial to its functions: The enzymes that produce H2O2 and those that dispose of this toxic compound are sequestered away from other cellular components that could be damaged. Specialized peroxisomes called glyoxysomes are found in the fat-storing tissues of plant seeds. These organelles contain enzymes that initiate the conversion of fatty acids to sugar, which the emerging seedling uses as a source of energy and carbon until it can produce its own sugar by photosynthesis. How peroxisomes are related to other organelles is still an open question. They grow larger by incorporating proteins made in the cytosol and ER, as well as lipids made in the ER and within the peroxisome itself. Peroxisomes may increase in number by splitting in two when they reach a certain size, sparking the suggestion of an endosymbiotic evolutionary origin, but others argue against this scenario. Discussion of this issue is ongoing.

The cytoskeleton is a network of fibers that organizes structures and activities in the cell In the early days of electron microscopy, biologists thought that the organelles of a eukaryotic cell floated freely in the cytosol. But improvements in both light microscopy and electron microscopy have revealed the cytoskeleton, a network of fibers extending throughout the cytoplasm (Figure 6.20). Bacterial cells also have fibers that form a type of cytoskeleton, constructed of proteins similar to eukaryotic ones, but here we will concentrate on eukaryotes. The eukaryotic cytoskeleton, which plays a major role in organizing the structures and activities of the cell, is composed of three types of molecular structures: microtubules, microfilaments, and intermediate filaments.

Roles of the Cytoskeleton: Support and Motility The most obvious function of the cytoskeleton is to give mechanical support to the cell and maintain its shape. This is especially important for animal cells, which lack walls. The remarkable strength and resilience of the cytoskeleton as a whole are based on its architecture. Like a dome tent, the cytoskeleton is stabilized by a balance between opposing forces exerted by its elements. And just as the skeleton of an animal helps fix the positions of other body parts, the cytoskeleton provides anchorage for many organelles and even cytosolic enzyme molecules. The cytoskeleton is more dynamic than an animal skeleton, however. It can be quickly dismantled in one part of the cell and reassembled in a new location, changing the shape of the cell. Figure 6.20  The cytoskeleton. As shown in this fluorescence micrograph, the cytoskeleton extends throughout the cell. The cytoskeletal elements have been tagged with different fluorescent molecules: green for microtubules and reddish-orange for microfilaments. A third component of the cytoskeleton, intermediate filaments, is not evident here. (The blue color tags the DNA in the nucleus.)

Concept Check 6.5 1. Describe two characteristics shared by chloroplasts and mitochondria. Consider both function and membrane structure. 3. WHAT IF? A classmate proposes that mitochondria and chloroplasts should be classified in the endomembrane system. Argue against the proposal. For suggested answers, see Appendix A.

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10 μm

2. Do plant cells have mitochondria? Explain.

BioFlix® Animation: Cytoskeleton

Some types of cell motility (movement) also involve the cytoskeleton. The term cell motility includes both changes in cell location and movements of cell parts. Cell motility generally requires interaction of the cytoskeleton with motor proteins. There are many such examples: Cytoskeletal elements and motor proteins work together with plasma membrane molecules to allow whole cells to move along fibers outside the cell. Inside the cell, vesicles and other organelles often use motor protein “feet” to “walk” to their destinations along a track provided by the cytoskeleton. For example, this is how vesicles containing neurotransmitter molecules migrate to the tips of axons, the long extensions of nerve cells that release these molecules as chemical signals to adjacent nerve cells (Figure 6.21). The cytoskeleton also manipulates the plasma membrane, bending it inward to form food vacuoles or other phagocytic vesicles.

Figure 6.21  Motor proteins and the cytoskeleton.

Vesicle

ATP

Receptor for motor protein

Microtubule of cytoskeleton

Motor protein (ATP powered)

(a) Motor proteins that attach to receptors on vesicles can “walk” the vesicles along microtubules or, in some cases, along microfilaments. Microtubule

Vesicles

0.25 μm

Interview with George Langford: Investigating how motor proteins move cargo

Components of the Cytoskeleton Now let’s look more closely at the three main types of fibers that make up the cytoskeleton: Microtubules are the thickest of the three types; microfilaments (also called actin filaments) are the thinnest; and intermediate filaments are fibers with diameters in a middle range (Table 6.1).

(b) Two vesicles containing neurotransmitters move along a microtubule toward the tip of a nerve cell extension called an axon (SEM).

Table 6.1  The Structure and Function of the Cytoskeleton  Property

Microtubules (Tubulin Polymers)

Microfilaments (Actin Filaments)

Intermediate Filaments

Structure

Hollow tubes

Two intertwined strands of actin

Fibrous proteins coiled into cables

Diameter

25 nm with 15-nm lumen

7 nm

8–12 nm

Protein subunits

Tubulin, a dimer consisting of α-tubulin and β-tubulin

Actin

One of several different proteins (such as keratins)

Main functions

Maintenance of cell shape (compression-resisting “girders”); cell motility (as in cilia or flagella); chromosome movements in cell division; organelle movements

Maintenance of cell shape (tensionbearing elements); changes in cell shape; muscle contraction; cytoplasmic streaming in plant cells; cell motility (as in amoeboid movement); division of animal cells

Maintenance of cell shape (tensionbearing elements); anchorage of nucleus and certain other organelles; formation of nuclear lamina

Fluorescence micrographs of fibroblasts. Fibroblasts are a favorite cell type for cell biology studies because they spread out flat and their internal structures are easy to see. In each, the structure of interest has been tagged with fluorescent molecules. The DNA in the nucleus has also been tagged in the first micrograph (blue) and third micrograph (orange).

10 μm

5 μm

10 μm

Column of tubulin dimers

Keratin proteins Actin subunit

Fibrous subunit (keratins coiled together)

25 nm

8–12 nm

7 nm

α

β

Tubulin dimer

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Microtubules All eukaryotic cells have microtubules, hollow rods constructed from globular proteins called tubulins. Each tubulin protein is a dimer, a molecule made up of two subunits. A tubulin dimer consists of two slightly different polypeptides, α-tubulin and β-tubulin. Microtubules grow in length by adding tubulin dimers; they can also be disassembled and their tubulins used to build microtubules elsewhere in the cell. Because of the orientation of tubulin dimers, the two ends of a microtubule are slightly different. One end can accumulate or release tubulin dimers at a much higher rate than the other, thus growing and shrinking significantly during cellular activities. (This is called the “plus end,” not because it can only add tubulin proteins but because it’s the end where both “on” and “off” rates are much higher.) Microtubules shape and support the cell and also serve as tracks along which organelles equipped with motor proteins can move. In addition to the example in Figure 6.21, microtubules guide vesicles from the ER to the Golgi apparatus and from the Golgi to the plasma membrane. Microtubules are also involved in the separation of chromosomes during cell division, as shown in Figure 12.7.

Centrosomes and Centrioles  In animal cells, microtubules grow out from a centrosome, a region that is often located near the nucleus. These microtubules function as compression-resisting girders of the cytoskeleton. Within the centrosome is a pair of centrioles, each composed of nine sets of triplet microtubules arranged in a ring (Figure 6.22). Although centrosomes with centrioles may help organize microtubule assembly in animal cells, many other eukaryotic cells lack centrosomes with centrioles and instead organize microtubules by other means. Cilia and Flagella  In eukaryotes, a specialized arrangement of microtubules is responsible for the beating of flagella (singular, flagellum) and cilia (singular, cilium), microtubule-containing extensions that project from some cells. (The bacterial flagellum, shown in Figure 6.5, has a completely different structure.) Many unicellular eukaryotes are propelled through water by cilia or flagella that act as locomotor appendages, and the sperm of animals, algae, and some plants have flagella. When cilia or flagella extend from cells that are held in place as part of a tissue layer, they can move fluid over the surface of the tissue. For example, the ciliated lining of the trachea (windpipe) sweeps mucus containing trapped debris out of the lungs (see the EMs in Figure 6.3). In a woman’s reproductive tract, the cilia lining the oviducts help move an egg toward the uterus. Motile cilia usually occur in large numbers on the cell surface. Flagella are usually limited to just one or a few per cell, and they are longer than cilia. Flagella and cilia differ in their

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Figure 6.22  Centrosome containing a pair of centrioles. Most animal cells have a centrosome, a region near the nucleus where the cell’s microtubules are initiated. Within the centrosome is a pair of centrioles, each about 250 nm (0.25 µm) in diameter. The two centrioles are at right angles to each other, and each is made up of nine sets of three microtubules. The blue portions of the drawing represent nontubulin proteins that connect the microtubule triplets.

Centrosome

Microtubule

Centrioles

0.25 μm

Longitudinal section of one centriole

Microtubules

Cross section of the other centriole

VISUAL SKILLS How many microtubules are in a centrosome? In the drawing, circle and label one microtubule and describe its structure. Circle and label a triplet.

beating patterns. A flagellum has an undulating motion like the tail of a fish. In contrast, cilia have alternating power and recovery strokes, much like the oars of a racing crew boat (Figure 6.23). A cilium may also act as a signal-receiving “antenna” for the cell. Cilia that have this function are generally nonmotile, and there is only one per cell. (In fact, in vertebrate animals, it appears that almost all cells have such a cilium, which is called a primary cilium.) Membrane proteins on this kind of cilium transmit molecular signals from the cell’s environment to its interior, triggering signaling pathways that may lead to changes in the cell’s activities. Cilium-based signaling appears to be crucial to brain function and to embryonic development.

Figure 6.23  A comparison of the beating of flagella and motile cilia.

(a) Motion of flagella. A flagellum usually undulates, its snakelike motion driving a cell in the same direction as the axis of the flagellum. Propulsion of a human sperm cell is an example of flagellate locomotion (LM).

Direction of swimming

5 μm

(b) Motion of cilia. Cilia have a back-and-forth motion. The rapid power stroke moves the cell in a direction perpendicular to the axis of the cilium. Then, during the slower recovery stroke,the cilium bends and sweeps sideways, closer to the cell surface. A dense nap of cilia, beating at a rate of about 40 to 60 strokes a second, covers this Colpidium, a freshwater protist (colorized SEM).

Direction of organism’s movement

Power stroke

Recovery stroke

Video: Flagellum Movement in Swimming Sperm Video: Flagellum Beating with ATP Video: Paramecium Cilia

Though different in length, number per cell, and beating pattern, motile cilia and flagella share a common structure. Each motile cilium or flagellum has a group of microtubules sheathed in an extension of the plasma membrane (Figure 6.24a). Nine doublets of microtubules are arranged in a ring with two single microtubules in its center (Figure 6.24b). This arrangement, referred to as the “9 + 2” pattern, is found in nearly all eukaryotic flagella and motile cilia. (Nonmotile primary cilia have a “9 + 0” pattern, lacking the central pair of microtubules.) The microtubule assembly of a cilium or flagellum is anchored in the cell by a basal body, which is structurally very similar to a centriole, with microtubule triplets in a “9 + 0” pattern (Figure 6.24c). In fact, in many animals (including humans), the basal body of the fertilizing sperm’s flagellum enters the egg and becomes a centriole. How does the microtubule assembly produce the bending movements of flagella and motile cilia? Bending involves large motor proteins called dyneins (red in the diagram in Figure 6.24) that are attached along each outer microtubule doublet. A typical dynein protein has two “feet” that “walk” along the microtubule of the adjacent doublet, using ATP for energy. One foot maintains contact, while the other releases and reattaches one step farther

15 μm

along the microtubule (see Figure 6.21). The outer doublets and two central microtubules are held together by flexible cross-linking proteins (blue in the diagram in Figure 6.24), and the walking movement is coordinated so that it happens on one side of the circle at a time. If the doublets were not held in place, the walking action would make them slide past each other. Instead, the movements of the dynein feet cause the microtubules—and the organelle as a whole—to bend.

Microfilaments (Actin Filaments) Microfilaments are thin solid rods. They are also called actin filaments because they are built from molecules of actin, a globular protein. A microfilament is a twisted double chain of actin subunits (see Table 6.1). Besides occurring as linear filaments, microfilaments can form structural networks when certain proteins bind along the side of such a filament and allow a new filament to extend as a branch. Like microtubules, microfilaments seem to be present in all eukaryotic cells. In contrast to the compression-resisting role of microtubules, the structural role of microfilaments in the cytoskeleton is to bear tension (pulling forces). A three-dimensional

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Figure 6.24  Structure of a flagellum or motile cilium.

0.1 μm

Plasma membrane

Outer microtubule doublet Motor proteins (dyneins) Central microtubule Radial spoke

Microtubules

Plasma membrane Basal body

Cross-linking protein between outer doublets (b) A cross section through a motile cilium shows the ”9 + 2“ arrangement of microtubules (TEM). The outer microtubule doublets are held together with the two central microtubules by flexible cross-linking proteins (blue in art), including the radial spokes. The doublets also have attached motor proteins called dyneins (red in art).

0.5 μm (a) A longitudinal section of a motile cilium shows microtubules running the length of the membrane-sheathed structure (TEM).

0.1 μm Triplet

(c) Basal body: The nine outer doublets of a cilium or flagellum extend into the basal body, where each doublet joins another microtubule to form a ring of nine triplets. Each triplet is connected to the next by nontubulin proteins (thinner blue lines in diagram). This is a ”9 + 0“ arrangement: The two central microtubules are not present because they terminate above the basal body (TEM). Cross section of basal body DRAW IT In (a) and (b), circle and label the central pair of microtubules. In (a), show where they terminate, and explain why they aren’t seen in the cross section of the basal body in (c).

Microvillus

Plasma membrane

Microfilaments (actin filaments)

Intermediate filaments

Figure 6.25  A structural role of microfilaments. The surface area of this nutrient-absorbing intestinal cell is increased by its many microvilli (singular, microvillus), cellular extensions reinforced by bundles of microfilaments. These actin filaments are anchored to a network of intermediate filaments (TEM).

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0.25 μm

network formed by microfilaments just inside the plasma membrane (cortical microfilaments) helps support the cell’s shape (see Figure 6.8). This network gives the outer cytoplasmic layer of a cell, called the cortex, the semisolid consistency of a gel, in contrast with the more fluid state of the interior cytoplasm. In some kinds of animal cells, such as nutrient-absorbing intestinal cells, bundles of microfilaments make up the core of microvilli, delicate projections that increase the cell’s surface area (Figure 6.25). Microfilaments are well known for their role in cell ­motility. Thousands of actin filaments and thicker filaments

Animation: Cilia and Flagella

made of a protein called myosin interact to cause contraction of muscle cells (Figure 6.26a); muscle contraction is described in detail in Concept 50.5. In the unicellular eukaryote Amoeba and some of our white blood cells, localized contractions brought about by actin and myosin are involved in the amoeboid (crawling) movement of the cells. The cell crawls along a surface by extending cellular extensions called pseudopodia (from the Greek pseudes, false, and pod, foot) and moving toward them (Figure 6.26b). In plant cells, actin-protein interactions contribute to cytoplasmic streaming, a circular flow of cytoplasm within cells (Figure 6.26c). This movement, which is especially common in large plant cells, speeds the movement of organelles and the distribution of materials within the cell.

Figure 6.26  Microfilaments and motility. In these three examples, interactions between actin filaments and motor proteins bring about cell movement.

Intermediate Filaments

(a) Myosin motors in muscle cell contraction. The ”walking” of myosin projections (the so-called heads) drives the parallel myosin and actin filaments past each other so that the actin filaments approach each other in the middle (red arrows). This shortens the muscle cell. Muscle contraction involves the shortening of many muscle cells at the same time (TEM).

Intermediate filaments are named for their diameter, which is larger than the diameter of microfilaments but smaller than that of microtubules (see Table 6.1). While microtubules and microfilaments are found in all eukaryotic cells, intermediate filaments are only found in the cells of some animals, including vertebrates. Specialized for bearing tension (like microfilaments), intermediate filaments are a diverse class of cytoskeletal elements. Each type is constructed from a particular molecular subunit belonging to a family of proteins whose members include the keratins. Microtubules and microfilaments, in contrast, are consistent in diameter and composition in all eukaryotic cells. Intermediate filaments are more permanent fixtures of cells than are microfilaments and microtubules, which are often disassembled and reassembled in various parts of a cell. Even after cells die, intermediate filament networks often persist; for example, the outer layer of our skin consists of dead skin cells full of keratin filaments. Chemical treatments that remove microfilaments and microtubules from the cytoplasm of living cells leave a web of intermediate filaments that retains its original shape. Such experiments suggest that intermediate filaments are especially sturdy and that they play an important role in reinforcing the shape of a cell and fixing the position of certain organelles. For instance, the nucleus typically sits within a cage made of intermediate filaments, fixed in location by branches of the filaments that extend into the cytoplasm. Other intermediate filaments make up the nuclear lamina, which lines the interior of the nuclear envelope (see Figure 6.9). By supporting a cell’s shape, intermediate filaments help the cell carry out its specific function. For example, the network of intermediate filaments shown in Figure 6.25 anchors the microfilaments supporting the intestinal microvilli. Thus, the various kinds of intermediate filaments may function together as the permanent framework of the entire cell.

Muscle cell 0.5 μm

Actin filament Myosin filament Myosin head

Cortex (outer cytoplasm): gel with actin network 100 μm Inner cytoplasm (more fluid)

Extending pseudopodium (b) Amoeboid movement. Interaction of actin filaments with myosin causes contraction of the cell, pulling the cell’s trailing end (at left) forward (to the right) (LM).

Organelles

30 μm

(c) Cytoplasmic streaming in plant cells. A layer of cytoplasm cycles around the cell, moving over tracks of actin filaments. Myosin motors attached to some organelles drive the streaming by interacting with the actin (LM). BioFlix® Animation: Actin and Myosin in Muscle Contraction Video: Amoeba Pseudopodia Video: Cytoplasmic Streaming

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Concept Check 6.6 1. Describe how cilia and flagella bend. 2. WHAT IF? Males afflicted with Kartagener’s syndrome are sterile because of immotile sperm, and they tend to suffer from lung infections. This disorder has a genetic basis. Suggest what the underlying defect might be.

Figure 6.27  Plant cell walls. The drawing shows several cells, each with a cell wall, large vacuole, a nucleus, and several chloroplasts and mitochondria. The TEM shows the cell walls where two cells come together. The multilayered partition between plant cells consists of adjoining walls individually secreted by the cells. Central vacuole Cytosol

For suggested answers, see Appendix A.

Plasma membrane

Concept  6.7

Plant cell walls

Extracellular components and connections between cells help coordinate cellular activities Having crisscrossed the cell to explore its interior components, we complete our tour of the cell by returning to the surface of this microscopic world, where there are additional structures with important functions. The plasma membrane is usually regarded as the boundary of the living cell, but most cells synthesize and secrete materials extracellularly (to the outside of the cell). Although these materials and the structures they form are outside the cell, their study is important to cell biology because they are involved in a great many essential cellular functions.

Plasmodesmata

Secondary cell wall Primary cell wall Middle lamella

Cell Walls of Plants The cell wall is an extracellular structure of plant cells (Figure 6.27). This is one of the features that distinguishes plant cells from animal cells. The wall protects the plant cell, maintains its shape, and prevents excessive uptake of water. On the level of the whole plant, the strong walls of specialized cells hold the plant up against the force of gravity. Prokaryotes, fungi, and some unicellular eukaryotes also have cell walls, as you saw in Figures 6.5 and 6.8, but we will postpone discussion of them until Unit Five. Plant cell walls are much thicker than the plasma membrane, ranging from 0.1 µm to several micrometers. The exact chemical composition of the wall varies from species to species and even from one cell type to another in the same plant, but the basic design of the wall is consistent. Microfibrils made of the polysaccharide cellulose (see Figure 5.6) are synthesized by an enzyme called cellulose synthase and secreted to the extracellular space, where they become embedded in a matrix of other polysaccharides and proteins. This combination of materials, strong fibers in a “ground substance” (matrix), is the same basic architectural design found in steel-reinforced concrete and in fiberglass. A young plant cell first secretes a relatively thin and flexible wall called the primary cell wall (see the micrograph in Figure 6.27). Between primary walls of adjacent cells is the middle lamella, a thin layer rich in sticky polysaccharides called pectins. The middle lamella glues adjacent cells together. (Pectin is used in cooking as a thickening agent in 118

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jams and jellies.) When the cell matures and stops growing, it strengthens its wall. Some plant cells do this simply by secreting hardening substances into the primary wall. Other cells add a secondary cell wall between the plasma membrane and the primary wall. The secondary wall, often deposited in several laminated layers, has a strong and durable matrix that affords the cell protection and support. Wood, for example, consists mainly of secondary walls. Plant cell walls are usually perforated by channels between adjacent cells called plasmodesmata, which will be discussed shortly.

The Extracellular Matrix (ECM) of Animal Cells Although animal cells lack walls akin to those of plant cells, they do have an elaborate extracellular matrix (ECM). The main ingredients of the ECM are glycoproteins and other carbohydrate-containing molecules secreted by the cells. (Recall that glycoproteins are proteins with covalently bonded carbohydrates, usually short chains of sugars.) The most abundant glycoprotein in the ECM of most animal cells is collagen, which forms strong fibers outside the cells (see Figure 5.18). In fact, collagen accounts for about 40% of the total protein in the human body. The collagen fibers are embedded in a network woven out of proteoglycans

Collagen fibers are embedded in a web of proteoglycan complexes.

A proteoglycan complex consists of hundreds of proteoglycan molecules attached noncovalently to a single long polysaccharide molecule.

EXTRACELLULAR FLUID

Fibronectin attaches the ECM to integrins embedded in the plasma membrane.

Plasma membrane

Microfilaments

CYTOPLASM

Integrins, membrane proteins with two subunits, bind to the ECM on the outside and to associated proteins attached to microfilaments on the inside. This linkage can transmit signals between the cell’s external environment and its interior and can result in changes in cell behavior.

Polysaccharide molecule Carbohydrates Core protein

Proteoglycan molecule Proteoglycan complex

Figure 6.28  Extracellular matrix (ECM) of an animal cell. The molecular composition and structure of the ECM vary from one cell type to another. In this example, three different types of ECM molecules are present: collagen, fibronectin, and proteoglycans.

secreted by cells (Figure 6.28). A proteoglycan molecule consists of a small core protein with many carbohydrate chains covalently attached, so that it may be up to 95% carbohydrate. Large proteoglycan complexes can form when hundreds of proteoglycan molecules become noncovalently attached to a single long polysaccharide molecule, as shown in Figure 6.28. Some cells are attached to the ECM by ECM glycoproteins such as fibronectin. Fibronectin and other ECM proteins bind to cell-surface receptor proteins called integrins that are built into the plasma membrane. Integrins span the membrane and bind on their cytoplasmic side to associated proteins attached to microfilaments of the cytoskeleton. The name integrin is based on the word integrate: Integrins are in a position to transmit signals between the ECM and the cytoskeleton and thus to integrate changes occurring outside and inside the cell. Current research on fibronectin, other ECM molecules, and integrins reveals the influential role of the ECM in the lives of cells. By communicating with a cell through integrins, the ECM can regulate a cell’s behavior. For example, some cells in a developing embryo migrate along specific pathways by matching the orientation of their microfilaments to the “grain” of fibers in the extracellular matrix. Researchers have also learned that the extracellular matrix around a cell can influence the activity of genes in the nucleus. Information about the ECM probably reaches the nucleus by a combination of mechanical and chemical signaling pathways. Mechanical signaling involves fibronectin, integrins, and microfilaments of the cytoskeleton. Changes in the cytoskeleton may in turn trigger signaling pathways inside the cell, leading to changes in the set

of proteins being made by the cell and therefore changes in the cell’s function. In this way, the extracellular matrix of a particular tissue may help coordinate the behavior of all the cells of that tissue. Direct connections between cells also function in this coordination, as we discuss next.

Cell Junctions Cells in an animal or plant are organized into tissues, organs, and organ systems. Neighboring cells often adhere, interact, and communicate via sites of direct physical contact.

Plasmodesmata in Plant Cells It might seem that the nonliving cell walls of plants would isolate plant cells from one another. But in fact, as shown in Figure 6.29, cell walls are perforated with ­plasmodesmata (singular, plasmodesma; from the Greek desma, bond), channels that connect cells. Cytosol passing through the Figure 6.29  Plasmodesmata between plant cells. The cytoplasm of one plant cell is continuous with the cytoplasm of its neighbors via plasmodesmata, cytoplasmic channels through the cell walls (TEM). Cell walls Interior of cell Interior of adjacent cell 0.5 μm

Plasmodesmata

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plasmodesmata joins the internal chemical environments of adjacent cells. These connections unify most of the plant into one living continuum. The plasma membranes of ­adjacent cells line the channel of each plasmodesma and thus are continuous. Water and small solutes can pass freely from cell to cell, and several experiments have shown that in some circumstances, certain proteins and RNA molecules can do this as well (see Concept 36.6). The ­macromolecules transported to neighboring cells appear to reach the ­plasmodesmata by moving along fibers of the cytoskeleton.

Figure 6.30

Tight Junctions, Desmosomes, and Gap Junctions in Animal Cells In animals, there are three main types of cell junctions: tight junctions, desmosomes, and gap junctions. (Gap junctions are most like the plasmodesmata of plants, although gap ­junction pores are not lined with membrane.) All three types of cell junctions are especially common in epithelial tissue, which lines the external and internal surfaces of the body. Figure 6.30 uses epithelial cells of the intestinal lining to illustrate these junctions.

  Exploring Cell Junctions in Animal Tissues Tight Junctions

Tight junctions prevent fluid from moving across a layer of cells.

Tight junction

TEM

0.5 μm

At tight junctions, the plasma membranes of neighboring cells are very tightly pressed against each other, bound together by specific proteins. Forming continuous seals around the cells, tight junctions establish a barrier that prevents leakage of extracellular fluid across a layer of epithelial cells (see red dashed arrow). For example, tight junctions between skin cells make us watertight.

Desmosomes Tight junction

Desmosomes (one type of anchoring junction) function like rivets, fastening cells together into strong sheets. Intermediate filaments made of sturdy keratin proteins anchor desmosomes in the cytoplasm. Desmosomes attach muscle cells to each other in a muscle. Some “muscle tears” involve the rupture of desmosomes.

Intermediate filaments Desmosome

TEM

1 μm

Gap junction

Gap Junctions

Plasma membranes of adjacent cells

Space between cells

Animation: Cell Junctions

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Extracellular matrix

TEM

Ions or small molecules

0.1 μm

Gap junctions (also called communicating junctions) provide cytoplasmic channels from one cell to an adjacent cell and in this way are similar in their function to the plasmodesmata in plants. Gap junctions consist of membrane proteins that surround a pore through which ions, sugars, amino acids, and other small molecules may pass. Gap junctions are necessary for communication between cells in many types of tissues, such as heart muscle, and in animal embryos.

Concept Check 6.7 1. In what way are the cells of plants and animals structurally different from single-celled eukaryotes? 2. WHAT IF? If the plant cell wall or the animal extracellular matrix were impermeable, what effect would this have on cell function? 3. MAKE CONNECTIONS The polypeptide chain that makes up a tight junction weaves back and forth through the membrane four times, with two extracellular loops, and one loop plus short C-terminal and N-terminal tails in the cytoplasm. Looking at Figure 5.14, what would you predict about the amino acid sequence of the tight junction protein? For suggested answers, see Appendix A.

Concept  6.8 A cell is greater than the sum of its parts From our panoramic view of the cell’s compartmental organization to our close-up inspection of each organelle’s architecture, this tour of the cell has provided many opportunities to correlate structure with function. (This would be a good time to review cell structure by returning to Figure 6.8.) BioFlix® Animation: Tour of an Animal Cell BioFlix® Animation: Tour of a Plant Cell

Even as we dissect the cell, remember that none of its components works alone. As an example of cellular integration, consider the microscopic scene in Figure 6.31. The large cell is a macrophage (see Figure 6.13a). It helps defend the mammalian body against infections by ingesting bacteria (the smaller

cells) into phagocytic vesicles. The macrophage crawls along a surface and reaches out to the bacteria with thin pseudopodia (specifically, filopodia). Actin filaments interact with other elements of the cytoskeleton in these movements. After the macrophage engulfs the bacteria, they are destroyed by lysosomes produced by the elaborate endomembrane system. The digestive enzymes of the lysosomes and the proteins of the cytoskeleton are all made by ribosomes. And the synthesis of these proteins is programmed by genetic messages dispatched from the DNA in the nucleus. All these processes require energy, which mitochondria ­supply in the form of ATP. Cellular functions arise from cellular order: The cell is a living unit greater than the sum of its parts. The cell in Figure 6.31 is a good example of integration of cellular processes, seen from the outside. But what about the internal organization of a cell? As you proceed in your study of biology to consider different cellular processes, it will be helpful to try to visualize the architecture and furnishings inside a cell. Figure 6.32 is designed to help you get a sense of the relative sizes and organization of important biological molecules and macromolecules, along with cellular structures and organelles. As you study this figure, see if you can shrink yourself down to the size of a protein and contemplate your surroundings.

Concept Check 6.8 1. Colpidium colpoda is a unicellular eukaryote that lives in freshwater, eats bacteria, and moves by cilia (see Figure 6.23b). Describe how the parts of this cell work together in the functioning of C. colpoda, including as many organelles and other cell structures as you can. For suggested answers, see Appendix A.

Figure 6.31  The emergence of cellular functions. The ability of this macrophage (brown) to recognize, apprehend, and destroy Staphylococcus bacteria (orange) is a coordinated activity of the whole cell. Its cytoskeleton, lysosomes, and plasma membrane are among the components that function in phagocytosis (colorized SEM).

Macrophage

Bacteria

Filopodium (extension of the macrophage) engulfing bacteria 10 μm Animation: Review of Animal Cell Structure and Function

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Figure 6.32 

Visualizing the Scale of the Molecular Machinery in a Cell

A slice of a plant cell’s interior is illustrated in the center panel, with all structures and molecules drawn to scale. Selected molecules and structures are shown above and below, all enlarged by the same factor so you can compare their sizes. All protein and nucleic acid structures are based on data from the Protein Data Bank; regions whose structure has not yet been determined are shown in gray. Proton pump

Calcium channel

Aquaporin

Receptor

Enzyme in signaling pathway

Transport

Signaling

Membrane proteins: Proteins embedded in the plasma membrane or other cellular membranes help transport substances across membranes, conduct signals from one side of the membrane to the other, and participate in other crucial cellular functions. Many proteins are able to move within the membrane.

Plasma membrane Cell wall

Chloroplast

Mitochondrion

Phosphofructokinase

Complex III Cyt c

Rubisco

Complex IV ATP synthase

Q Photosystem II

ATP synthase

Cytochrome complex

Fd

NADP+ reductase

Photosystem I

Pq

Complex II Hexokinase

Complex I

Isocitrate dehydrogenase

Cellular respiration: Cellular respiration includes many steps, some carried out by individual proteins or protein complexes in the cytoplasm and the mitochondrial matrix. Other proteins and protein complexes, involved in generating ATP from food molecules, form a “chain” in the inner mitochondrial membrane.

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Pc Photosynthesis: Large complexes of proteins with associated nonprotein molecules are embedded in chloroplast membranes. Together, they can trap light energy in molecules that are later used by other proteins inside the chloroplast to make sugars. This is the basis for all life on the planet.

Translation: In the cytoplasm, the information in mRNA is used to assemble a polypeptide with a specific sequence of amino acids. Both transfer RNA (tRNA) molecules and a ribosome play a role. The eukaryotic ribosome, which includes a large subunit and a small subunit, is a colossal complex composed of four large ribosomal RNA (rRNA) molecules and more than 80 proteins. Through transcription and translation, the nucleotide sequence of DNA determines the amino acid sequence of a polypeptide, via the intermediary mRNA.

Transcription: In the nucleus, the information contained in a DNA sequence is transferred to messenger RNA (mRNA) by an enzyme called RNA polymerase. After their synthesis, mRNA molecules leave the nucleus via nuclear pores. mRNA

RNA polymerase

Polypeptide made of amino acids

DNA

Nucleosome: DNA wrapped around 8 histone proteins DNA

Nuclear pore: The nuclear pore complex regulates molecular traffic in and out of the nucleus, which is bounded by a double membrane. Among the largest structures that pass through the pore are the ribosomal subunits, which are built in the nucleus.

Ribosome

Large subunit

tRNA

Small subunit mRNA

Nucleus Nuclear pore

Endoplasmic reticulum

Nuclear envelope

Scale within cell 25 nm

Microfilament Vesicle

25 nm Scale of enlarged structures 1 List the following structures from largest to

smallest: proton pump, nuclear pore, Cyt c, ribosome.

2 Considering the structures of a nucleosome and

Subunit of two tubulin proteins

Motor protein (myosin) Microtubule

Microfilament

Cytoskeleton: Cytoskeletal structures are polymers of protein subunits. Microtubules are hollow structural rods made of tubulin protein subunits, while microfilaments are cables that have two chains of actin proteins wound around each other.

Motor proteins: Responsible for transport of vesicles and movement of organelles within the cell. This requires energy, often provided by ATP hydrolysis.

of RNA polymerase, speculate about what must happen before RNA polymerase can transcribe the DNA wrapped around the histone proteins of a nucleosome.

3 Find another myosin motor protein walking on

a microfilament in this figure. What organelle is being moved by that myosin protein?

Instructors: Additional questions related to this Visualizing Figure can be assigned in MasteringBiology.

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6

Chapter Review

Go to MasteringBiology™ for Videos, Animations, Vocab Self-Quiz, Practice Tests, and more in the Study Area.

Concept  6.2

Summary of Key Concepts

Eukaryotic cells have internal membranes that compartmentalize their functions

Concept  6.1

(pp. 97–102)

Biologists use microscopes and biochemistry to study cells (pp. 94–97)





VOCAB SELF-QUIZ

Improvements in microscopy that affect the paramgoo.gl/6u55ks eters of magnification, resolution, and contrast have catalyzed progress in the study of cell structure. Light microscopy (LM) and electron microscopy (EM), as well as other types, remain important tools. Cell biologists can obtain pellets enriched in particular cellular components by centrifuging disrupted cells at sequential speeds, a process known as cell fractionation.

?

How do microscopy and biochemistry complement each other to reveal cell structure and function?

Cell Component Concept 6.3

Describe the relationship between the nucleus and ribosomes.

Concept 6.4

Unit two 

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Explain how the compartmental organization of a eukaryotic cell contributes to its biochemical functioning.

Structure

Function

Two subunits made of ribosomal RNAs and proteins; can be free in cytosol or bound to ER

Protein synthesis

Extensive network of membranebounded tubules and sacs; membrane separates lumen from cytosol; continuous with nuclear envelope

Smooth ER: synthesis of lipids, metabolism of carbohydrates, Ca2+ storage, detoxification of drugs and poisons

Golgi apparatus

Stacks of flattened membranous sacs; has polarity (cis and trans faces)

Modification of proteins, carbohydrates on proteins, and phospholipids; synthesis of many polysaccharides; sorting of Golgi products, which are then released in vesicles

Lysosome

Membranous sac of hydrolytic enzymes (in animal cells)

Breakdown of ingested substances, cell macromolecules, and damaged organelles for recycling

Large membrane-bounded vesicle

Digestion, storage, waste disposal, water balance, cell growth, and protection

Ribosome

(Nuclear envelope)

Vacuole

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?

(ER)

Endoplasmic reticulum (ER)

Describe the key role played by transport vesicles in the endomembrane system.



Houses chromosomes, which are made of chromatin (DNA and proteins); contains nucleoli, where ribosomal subunits are made; pores regulate entry and exit of materials

The endomembrane system regulates protein traffic and performs metabolic functions (pp. 104–109)

?



All cells are bounded by a plasma membrane. Prokaryotic cells lack nuclei and other membrane-enclosed organelles, while eukaryotic cells have internal membranes that compartmentalize cellular functions. The surface-to-volume ratio is an important parameter affecting cell size and shape. Plant and animal cells have most of the same organelles: a nucleus, endoplasmic reticulum, Golgi apparatus, and mitochondria. Chloroplasts are present only in cells of photosynthetic eukaryotes.

Surrounded by nuclear envelope (double membrane) perforated by nuclear pores; nuclear envelope continuous with endoplasmic reticulum (ER)

Nucleus

The eukaryotic cell’s genetic instructions are housed in the nucleus and carried out by the ribosomes (pp. 102–104)

?



Rough ER: aids in synthesis of secretory and other proteins on bound ribosomes; adds carbohydrates to proteins to make glycoproteins; produces new membrane

Cell Component Concept 6.5

Mitochondria and chloroplasts change energy from one form to another (pp. 109–112)

?

What does the endosymbiont theory propose as the origin for mitochondria and chloroplasts? Explain.

Bounded by double membrane; inner membrane has infoldings

Cellular respiration

Chloroplast

Typically two membranes around fluid stroma, which contains thylakoids stacked into grana

Photosynthesis (chloroplasts are in cells of photosynthetic eukaryotes, including plants)

Specialized metabolic compartment bounded by a single membrane

Contains enzymes that transfer H atoms from substrates to oxygen, producing H2O2 (hydrogen peroxide), which is converted to H2O.

Peroxisome

Level 2: Application/Analysis

The cytoskeleton is a network of fibers that organizes structures and activities in the cell (pp. 112–118)



The cytoskeleton functions in structural support for the cell and in motility and signal transmission. Microtubules shape the cell, guide organelle movement, and separate chromosomes in dividing cells. Cilia and flagella are motile appendages containing microtubules. Primary cilia also play sensory and signaling roles. Microfilaments are thin rods that function in muscle contraction, amoeboid movement, cytoplasmic streaming, and support of microvilli. Intermediate filaments support cell shape and fix organelles in place.

?

Describe the role of motor proteins inside the eukaryotic cell and in whole-cell movement.

Concept  6.7

Extracellular components and connections between cells help coordinate cellular activities (pp. 118–121)





Plant cell walls are made of cellulose fibers embedded in other polysaccharides and proteins. Animal cells secrete glycoproteins and proteoglycans that form the extracellular matrix (ECM), which functions in support, adhesion, movement, and regulation. Cell junctions connect neighboring cells. Plants have plasmodesmata that pass through adjoining cell walls. Animal cells have tight junctions, desmosomes, and gap junctions.

?

Compare the structure and functions of a plant cell wall and the extracellular matrix of an animal cell.

Concept  6.8

A cell is greater than the sum of its parts (pp. 121–123)

4. Cyanide binds to at least one molecule involved in producing ATP. If a cell is exposed to cyanide, most of the cyanide will be found within the (A) mitochondria. (C) peroxisomes. (B) ribosomes. (D) lysosomes. 5. Which cell would be best for studying lysosomes? (A) muscle cell (C) bacterial cell (B) nerve cell (D) phagocytic white blood cell 6. DRAW IT  From memory, draw two eukaryotic cells. Label the structures listed here and show any physical connections between the internal structures of each cell: nucleus, rough ER, smooth ER, mitochondrion, centrosome, chloroplast, vacuole, lysosome, microtubule, cell wall, ECM, microfilament, Golgi apparatus, intermediate filament, plasma membrane, peroxisome, ribosome, nucleolus, nuclear pore, vesicle, flagellum, microvilli, plasmodesma.

Level 3: Synthesis/Evaluation 7. EVOLUTION CONNECTION  (a) What cell structures best reveal evolutionary unity? (b) Provide an example of diversity related to specialized cellular modifications. 8. SCIENTIFIC INQUIRY  Imagine protein X, destined to span the plasma membrane. Assume that the mRNA carrying the genetic message for protein X has already been translated by ribosomes in a cell culture. If you fractionate the cells (see Figure 6.4), in which fraction would you find protein X? Explain by describing its transit through the cell. 9. WRITE ABOUT A THEME: Organization Considering some of the characteristics that define life and drawing on your knowledge of cellular structures and functions, write a short essay (100–150 words) that discusses this statement: Life is an emergent property that appears at the level of the cell. (See Concept 1.1.) 10. SYNTHESIZE YOUR KNOWLEDGE

Many components work together in a functioning cell.

?

Function

Mitochondrion

Concept  6.6



Structure

When a cell ingests a bacterium, what role does the nucleus play?

Test Your Understanding

Epithelial cell

Level 1: Knowledge/Comprehension 1. Which structure is not part of the endomembrane system? (A) nuclear envelope (C) Golgi apparatus (B) chloroplast (D) plasma membrane

PRACTICE TEST

goo.gl/CUYGKD

2. Which structure is common to plant and animal cells? (A) chloroplast (C) mitochondrion (B) central vacuole (D) centriole 3. Which of the following is present in a prokaryotic cell? (A) mitochondrion (C) nuclear envelope (B) ribosome (D) chloroplast

The cells in this SEM are epithelial cells from the small intestine. Discuss how aspects of their structure contribute to their specialized functions of nutrient absorption and as a barrier between the intestinal contents and the blood supply on the other side of the sheet of epithelial cells. For selected answers, see Appendix A.

For additional practice questions, check out the Dynamic Study Modules in MasteringBiology. You can use them to study on your smartphone, tablet, or computer anytime, anywhere!

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7

Membrane Structure and Function

Figure 7.1  How do cell membrane proteins like this aquaporin (blue ribbons) help regulate chemical traffic?

Key Concepts 7.1

Cellular membranes are fluid mosaics of lipids and proteins

7.2

Membrane structure results in selective permeability

7.3

Passive transport is diffusion of a substance across a membrane with no energy investment

7.4

Active transport uses energy to move solutes against their gradients

7.5

Bulk transport across the plasma membrane occurs by exocytosis and endocytosis

Potassium ion channel protein

Potassium ion

Life at the Edge The plasma membrane that surrounds the cell can be considered the edge of life, the boundary that separates a living cell from its surroundings and controls all inbound and outbound traffic. Like all biological membranes, the plasma membrane exhibits selective permeability; that is, it allows some substances to cross it more easily than others. The ability of the cell to discriminate in its chemical exchanges is fundamental to life, and it is the plasma membrane and its component molecules that make this selectivity possible. In this chapter, you will learn how cellular membranes control the passage of substances, often with transport proteins. For example, the image in Figure 7.1 shows a computer model of a short section of the phospholipid bilayer of a membrane (hydrophilic heads are yellow, and hydrophobic tails are green). The blue ribbons within the lipid bilayer represent helical regions of a membrane transport channel protein called an aquaporin. One molecule of this protein enables billions of water molecules (red and gray) to pass through the membrane every second, many more than could cross on their own. Another type of transport protein is the ion channel shown here; it allows potassium ions to pass through the membrane. To understand how the plasma membrane and its proteins enable cells to survive and function, we begin by examining membrane structure, then explore how plasma membranes control transport into and out of cells. When you see this blue icon, log in to MasteringBiology and go to the Study Area for digital resources. Animation: Membrane in Motion Interview with Peter Agre: Discovering aquaporins

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Concept  7.1

Figure 7.2  Phospholipid bilayer (cross section).

Cellular membranes are fluid mosaics of lipids and proteins

Two phospholipids

Lipids and proteins are the staple ingredients of membranes, although carbohydrates are also important. The most abundant lipids in most membranes are phospholipids. The ability of phospholipids to form membranes is inherent in their molecular structure. A phospholipid is an amphipathic molecule, meaning it has both a hydrophilic (“water-loving”) region and a hydrophobic (“water-fearing”) region (see Figure 5.11). Other types of membrane lipids are also amphipathic. A phospholipid bilayer can exist as a stable boundary between two aqueous compartments because the molecular arrangement shelters the hydrophobic tails of the phospholipids from water while exposing the hydrophilic heads to water (Figure 7.2). Like membrane lipids, most membrane proteins are amphipathic. Such proteins can reside in the phospholipid bilayer with their hydrophilic regions protruding. This molecular orientation maximizes contact of hydrophilic regions of proteins with water in the cytosol and extracellular fluid, while

Hydrophilic head

WATER

Hydrophobic tail

WATER

VISUAL SKILLS

Consulting Figure 5.11, circle and label the hydrophilic and hydrophobic portions of the enlarged phospholipids on the right. Explain what each portion contacts when the phospholipids are in the plasma membrane.

providing their hydrophobic parts with a nonaqueous environment. Figure 7.3 shows the currently accepted model of the arrangement of molecules in the plasma membrane. In this fluid mosaic model, the membrane is a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids.

Fibers of extracellular matrix (ECM)

Glycoprotein

Carbohydrates Glycolipid EXTRACELLULAR SIDE OF MEMBRANE

Phospholipid Cholesterol Microfilaments of cytoskeleton Figure 7.3  Updated model of an animal cell’s plasma membrane (cutaway view). Animation: Structure of the Plasma Membrane

Peripheral proteins Integral protein CYTOPLASMIC SIDE OF MEMBRANE 127

The proteins are not randomly distributed in the membrane, however. Groups of proteins are often associated in long-lasting, specialized patches, where they carry out common functions. Researchers have found specific lipids in these patches as well and have proposed naming them lipid rafts, but there is ongoing controversy about whether such structures exist in living cells or are an artifact of biochemical techniques. Like all models, the fluid mosaic model is continually being refined as new research reveals more about membrane structure.

The Fluidity of Membranes Membranes are not static sheets of molecules locked rigidly in place. A membrane is held together mainly by hydrophobic interactions, which are much weaker than covalent bonds (see Figure 5.18). Most of the lipids and some proteins can shift about sideways—that is, in the plane of the membrane, like partygoers elbowing their way through a crowded room. Very rarely, also, a lipid may flip-flop across the membrane, switching from one phospholipid layer to the other. The sideways movement of phospholipids within the membrane is rapid. Adjacent phospholipids switch positions about 107 times per second, which means that a phospholipid can travel about 2 µm—the length of many bacterial cells—in 1 second. Proteins are much larger than lipids and move more slowly, but some membrane proteins do drift, as shown in a classic experiment described in Figure 7.4. Some membrane proteins seem to move in a highly directed manner, perhaps driven along cytoskeletal fibers in the cell by motor proteins

Figure 7.4

connected to the membrane proteins’ cytoplasmic regions. However, many other membrane proteins seem to be held immobile by their attachment to the cytoskeleton or to the extracellular matrix (see Figure 7.3). A membrane remains fluid as temperature decreases until the phospholipids settle into a closely packed arrangement and the membrane solidifies, much as bacon grease forms lard when it cools. The temperature at which a membrane solidifies depends on the types of lipids it is made of. As the temperature decreases, the membrane remains fluid to a lower temperature if it is rich in phospholipids with unsaturated hydrocarbon tails (see Figures 5.10 and 5.11). Because of kinks in the tails where double bonds are located, unsaturated hydrocarbon tails cannot pack together as closely as saturated hydrocarbon tails, making the membrane more fluid (Figure 7.5a). The steroid cholesterol, which is wedged between phospholipid molecules in the plasma membranes of animal cells, has different effects on membrane fluidity at different temperatures (Figure 7.5b). At relatively high temperatures— at 37°C, the body temperature of humans, for example— cholesterol makes the membrane less fluid by restraining phospholipid movement. However, because cholesterol also hinders the close packing of phospholipids, it lowers the temperature required for the membrane to solidify. Thus, cholesterol can be thought of as a “fluidity buffer” for the membrane, resisting changes in membrane fluidity that can be caused by changes in temperature. Compared to animals, plants have very low levels of cholesterol; rather, related steroid lipids buffer membrane fluidity in plant cells.

Figure 7.5  Factors that affect membrane fluidity.

Inquiry  Do membrane proteins move? Experiment  Larry Frye and Michael Edidin, at Johns Hopkins University, labeled the plasma membrane proteins of a mouse cell and a human cell with two different markers and fused the cells. Using a microscope, they observed the markers on the hybrid cell.

(a) Unsaturated versus saturated hydrocarbon tails. Fluid

Viscous

Results Membrane proteins + Mouse cell

Mixed proteins after 1 hour Human cell

Hybrid cell

Unsaturated hydrocarbon tails (kinked) prevent packing, enhancing membrane fluidity.

(b) Cholesterol within the animal cell membrane.

Conclusion  The mixing of the mouse and human membrane proteins indicates that at least some membrane proteins move sideways within the plane of the plasma membrane.

Cholesterol reduces membrane fluidity at moderate temperatures by reducing phospholipid movement, but at low temperatures it hinders solidification by disrupting the regular packing of phospholipids.

Data from  L. D. Frye and M. Edidin, The rapid intermixing of cell surface antigens after formation of mouse-human heterokaryons, Journal of Cell Science 7:319 (1970).

WHAT IF? Suppose the proteins did not mix in the hybrid cell, even many hours after fusion. Would you be able to conclude that proteins don’t move within the membrane? What other explanation could there be?

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Saturated hydrocarbon tails pack together, increasing membrane viscosity.

Cholesterol

Membranes must be fluid to work properly; the fluidity of a membrane affects both its permeability and the ability of membrane proteins to move to where their function is needed. Usually, membranes are about as fluid as salad oil. When a membrane solidifies, its permeability changes, and enzymatic proteins in the membrane may become inactive if their activity requires movement within the membrane. However, membranes that are too fluid cannot support protein function either. Therefore, extreme environments pose a challenge for life, resulting in evolutionary adaptations that include differences in membrane lipid composition.

Evolution of Differences in Membrane Lipid Composition Evolution Variations in the cell membrane lipid compositions of many species appear to be evolutionary adaptations that maintain the appropriate membrane fluidity under specific environmental conditions. For instance, fishes that live in extreme cold have membranes with a high proportion of unsaturated hydrocarbon tails, enabling their membranes to remain fluid (see Figure 7.5a). At the other extreme, some bacteria and archaea thrive at temperatures greater than 90°C (194°F) in thermal hot springs and geysers. Their membranes include unusual lipids that may prevent excessive fluidity at such high temperatures. The ability to change the lipid composition of cell membranes in response to changing temperatures has evolved in organisms that live where temperatures vary. In many plants that tolerate extreme cold, such as winter wheat, the percentage of unsaturated phospholipids increases in autumn, an adjustment that keeps the membranes from solidifying during winter. Certain bacteria and archaea can also change the proportion of unsaturated phospholipids in their cell membranes, depending on the temperature at which they are growing. Overall, natural selection has apparently favored organisms whose mix of membrane lipids ensures an appropriate level of membrane fluidity for their environment.

Membrane Proteins and Their Functions Now we come to the mosaic aspect of the fluid mosaic model. Somewhat like a tile mosaic (shown here), a membrane is a collage of different proteins, often clustered together in groups, embedded in the fluid matrix of the lipid bilayer (see Figure 7.3). In the plasma membrane of red blood cells alone, for example, more than 50 kinds of proteins have been found so far. Phospholipids form the main fabric of the membrane, but proteins determine most of the membrane’s functions. Different types of cells contain different sets of membrane proteins, and the

Figure 7.6  The structure of a transmembrane protein. Bacteriorhodopsin (a bacterial transport protein) has a distinct orientation in the membrane, with its N-terminus outside the cell and its C-terminus inside. This ribbon model highlights the secondary structure of the hydrophobic parts, including seven transmembrane α helices, which lie mostly within the hydrophobic interior of the membrane. The nonhelical hydrophilic segments are in contact with the aqueous solutions on the extracellular and cytoplasmic sides of the membrane.

N-terminus

EXTRACELLULAR SIDE

α helix C-terminus CYTOPLASMIC SIDE

various membranes within a cell each have a unique collection of proteins. Notice in Figure 7.3 that there are two major populations of membrane proteins: integral proteins and peripheral proteins. Integral proteins penetrate the hydrophobic interior of the lipid bilayer. The majority are transmembrane proteins, which span the membrane; other integral proteins extend only partway into the hydrophobic interior. The hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acids (see Figure 5.14), typically 20–30 amino acids in length, usually coiled into α helices (Figure 7.6). The hydrophilic parts of the molecule are exposed to the aqueous solutions on either side of the membrane. Some proteins also have one or more hydrophilic channels that allow passage through the membrane of hydrophilic substances (even of water itself; see Figure 7.1). Peripheral proteins are not embedded in the lipid bilayer at all; they are loosely bound to the surface of the membrane, often to exposed parts of integral proteins (see Figure 7.3). On the cytoplasmic side of the plasma membrane, some membrane proteins are held in place by attachment to the cytoskeleton. And on the extracellular side, certain membrane proteins may attach to materials outside the cell. For example, in animal cells, membrane proteins may be attached to fibers of the extracellular matrix (see Figure 6.28; integrins are one type of integral, transmembrane protein). These attachments combine to give animal cells a stronger framework than the plasma membrane alone could provide. A single cell may have cell-surface membrane proteins that carry out several different functions, such as transport through the cell membrane, enzymatic activity, or attaching a cell to either a neighboring cell or the extracellular matrix. Furthermore, a single membrane protein may itself carry out multiple functions. Thus, the membrane is not only a structural mosaic, with many proteins embedded in the membrane, but also a functional mosaic, carrying out a range

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Figure 7.7  Some functions of membrane proteins. In many cases, a single protein performs multiple tasks.

Animation: Functions of the Plasma Membrane

(a) Transport. Left: A protein that spans the membrane may provide a hydrophilic channel across the membrane that is selective for a particular solute. Right: Other transport proteins shuttle a substance from one side to the other by changing shape (see Figure 7.14b). Some of these proteins hydrolyze ATP as an energy source to actively pump substances across the membrane. (b) Enzymatic activity. A protein built into the membrane may be an enzyme with its active site (where the reactant binds) exposed to substances in the adjacent solution. In some cases, several enzymes in a membrane are organized as a team that carries out sequential steps of a metabolic pathway. (c) Signal transduction. A membrane protein (receptor) may have a binding site with a specific shape that fits the shape of a chemical messenger, such as a hormone. The external messenger (signaling molecule) may cause the protein to change shape, allowing it to relay the message to the inside of the cell, usually by binding to a cytoplasmic protein (see Figure 11.6).

(d) Cell-cell recognition. Some glycoproteins serve as identification tags that are specifically recognized by membrane proteins of other cells. This type of cell-cell binding is usually short-lived compared to that shown in (e).

ATP Enzymes

Signaling molecule Receptor

Signal transduction

Glycoprotein

(f) Attachment to the cytoskeleton and extracellular matrix (ECM). Microfilaments or other elements of the cytoskeleton may be noncovalently bound to membrane proteins, a function that helps maintain cell shape and stabilizes the location of certain membrane proteins. Proteins that can bind to ECM molecules can coordinate extracellular and intracellular changes (see Figure 6.28). Some transmembrane proteins can bind to a particular ECM molecule and, when bound, transmit a signal into the cell. Use the proteins shown in (c) and (f) to explain how this might occur.

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Proteins on a cell’s surface are important in the medical field. For example, a protein called CD4 on the surface of immune cells helps the human immunodeficiency virus (HIV) infect these cells, leading to acquired immune deficiency syndrome (AIDS). Despite multiple exposures to HIV, however, a small number of people do not develop AIDS and show no evidence of HIV-infected cells. Comparing their genes with the genes of infected individuals, researchers learned that resistant people have an unusual form of a gene that codes for an immune cell-surface protein called CCR5. Further work showed that although CD4 is the main HIV receptor, HIV must also bind to CCR5 as a “co-receptor” to infect most cells (Figure 7.8a). An absence of CCR5 on the cells of resistant individuals, due to the gene alteration, prevents the virus from entering the cells (Figure 7.8b). This information has been key to developing a treatment for HIV infection. Interfering with CD4 causes dangerous side effects because of its many important functions in cells. Discovery of the CCR5 co-receptor provided a safer target for development of drugs that mask this protein and block HIV entry. One such drug, maraviroc (brand name Selzentry), was approved for treatment of HIV in 2007 and is now being tested to determine whether this drug might also work to prevent HIV infection in uninfected, at-risk patients.

The Role of Membrane Carbohydrates in Cell-Cell Recognition

(e) Intercellular joining. Membrane proteins of adjacent cells may hook together in various kinds of junctions, such as gap junctions or tight junctions (see Figure 6.30). This type of binding is more long-lasting than that shown in (d).

VISUAL SKILLS

of functions. Figure 7.7 illustrates six major functions performed by proteins of the plasma membrane.

Cell-cell recognition, a cell’s ability to distinguish one type of neighboring cell from another, is crucial to the functioning of an organism. It is important, for example, in the sorting of cells into tissues and organs in an animal embryo. It is also Figure 7.8  The genetic basis for HIV resistance. HIV

Receptor (CD4)

Co-receptor (CCR5)

(a) HIV can infect a cell with CCR5 on its surface, as in most people. MAKE CONNECTIONS

Receptor (CD4) but no CCR5

Plasma membrane

(b) HIV cannot infect a cell lacking CCR5 on its surface, as in resistant individuals.

Study Figures 2.16 and 5.17; each shows pairs of molecules binding to each other. What would you predict about CCR5 that would allow HIV to bind to it? How could a drug molecule interfere with this binding?

associated carbohydrates in the plasma membrane is determined as the membrane is being built by the endoplasmic reticulum (ER) and Golgi apparatus, components of the endomembrane system (see Figure 6.15).

the basis for the rejection of foreign cells by the immune system, an important line of defense in vertebrate animals (see Concept 43.1). Cells recognize other cells by binding to molecules, often containing carbohydrates, on the extracellular surface of the plasma membrane (see Figure 7.7d). Membrane carbohydrates are usually short, branched chains of fewer than 15 sugar units. Some are covalently bonded to lipids, forming molecules called glycolipids. (Recall that glyco refers to carbohydrate.) However, most are covalently bonded to proteins, which are thereby glycoproteins (see Figure 7.3). The carbohydrates on the extracellular side of the plasma membrane vary from species to species, among individuals of the same species, and even from one cell type to another in a single individual. The diversity of the molecules and their location on the cell’s surface enable membrane carbohydrates to function as markers that distinguish one cell from another. For example, the four human blood types designated A, B, AB, and O reflect variation in the carbohydrate part of glycoproteins on the surface of red blood cells.

Concept Check 7.1 1. VISUAL SKILLS Carbohydrates are attached to plasma membrane proteins in the ER (see Figure 7.9). On which side of the vesicle membrane are the carbohydrates during transport to the cell surface? 2. WHAT IF? How would the membrane lipid composition of a native grass found in very warm soil around hot springs compare with that of a native grass found in cooler soil? Explain. For suggested answers, see Appendix A.

Concept  7.2 Membrane structure results in selective permeability The biological membrane is an exquisite example of a supramolecular structure—many molecules ordered into a higher level of organization—with emergent properties beyond those of the individual molecules. The remainder of this chapter focuses on one of those properties: the ability to regulate transport across cellular boundaries, a function essential to the cell’s existence. We will see once again that form fits

Synthesis and Sidedness of Membranes Membranes have distinct inside and outside faces. The two lipid layers may differ in lipid composition, and each protein has directional orientation in the membrane (see Figure 7.6). Figure 7.9 shows how membrane sidedness arises: The asymmetrical arrangement of proteins, lipids, and their

Figure 7.9  Synthesis of membrane components and their orientation in the membrane. The cytoplasmic (orange) face of the plasma membrane differs from the extracellular (aqua) face. The latter arises from the inside face of ER, Golgi, and vesicle membranes.

Lipid bilayer

Transmembrane glycoproteins

11 Membrane proteins and lipids are synthesized in association with the endoplasmic reticulum (ER). In the ER, carbohydrates (green) are added to the transmembrane proteins (purple dumbbells), making them glycoproteins. The carbohydrate portions may then be modified.

Secretory protein Golgi apparatus

21 Inside the Golgi apparatus, the glycoproteins undergo further carbohydrate modification, and lipids acquire carbohydrates, becoming glycolipids.

Vesicle

31 The glycoproteins, glycolipids, and secretory proteins (purple spheres) are transported in vesicles to the plasma membrane.

Attached carbohydrate

41 As vesicles fuse with the plasma membrane, the outside face of the vesicle becomes continuous with the inside (cytoplasmic) face of the plasma membrane. This releases the secretory proteins from the cell, a process called exocytosis, and positions the carbohydrates of membrane glycoproteins and glycolipids on the outside (extracellular) face of the plasma membrane.

ER Glycolipid ER lumen Plasma membrane: Cytoplasmic face Extracellular face

Transmembrane glycoprotein

Secreted protein Membrane glycolipid

DRAW IT Draw an integral membrane protein extending from partway through the ER membrane into the ER lumen. Next, draw the protein where it would be located in a series of numbered steps ending at the plasma membrane. Would the protein contact the cytoplasm or the extracellular fluid? Explain.

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function: The fluid mosaic model helps explain how membranes regulate the cell’s molecular traffic. A steady traffic of small molecules and ions moves across the plasma membrane in both directions. Consider the chemical exchanges between a muscle cell and the extracellular fluid that bathes it. Sugars, amino acids, and other nutrients enter the cell, and metabolic waste products leave it. The cell takes in O2 for use in cellular respiration and expels CO2. Also, the cell regulates its concentrations of inorganic ions, such as Na+, K+, Ca2+, and Cl-, by shuttling them one way or the other across the plasma membrane. Although the heavy traffic through them may seem to suggest otherwise, cell membranes are selectively permeable, and substances do not cross the barrier indiscriminately. The cell is able to take up some small molecules and ions and exclude others.

The Permeability of the Lipid Bilayer Nonpolar molecules, such as hydrocarbons, CO2, and O2, are hydrophobic, as are lipids. They can all therefore dissolve in the lipid bilayer of the membrane and cross it easily, without the aid of membrane proteins. However, the hydrophobic interior of the membrane impedes direct passage through the membrane of ions and polar molecules, which are hydrophilic. Polar molecules such as glucose and other sugars pass only slowly through a lipid bilayer, and even water, a very small polar molecule, does not cross rapidly relative to nonpolar molecules. A charged atom or molecule and its surrounding shell of water (see Figure 3.8) are even less likely to penetrate the hydrophobic interior of the membrane. Furthermore, the lipid bilayer is only one aspect of the gatekeeper system responsible for a cell’s selective permeability. Proteins built into the membrane play key roles in regulating transport.

Transport Proteins Specific ions and a variety of polar molecules can’t move through cell membranes on their own. However, these hydrophilic substances can avoid contact with the lipid bilayer by passing through transport proteins that span the membrane. Some transport proteins, called channel proteins, function by having a hydrophilic channel that certain molecules or atomic ions use as a tunnel through the membrane (see Figure 7.7a, left). For example, the passage of water molecules through the membrane in certain cells is greatly facilitated by channel proteins known as aquaporins (see Figure 7.1). Each aquaporin allows entry of up to 3 billion (3 * 109) water molecules per second, passing single file through its central channel, which fits ten at a time. Without aquaporins, only a tiny fraction of these water molecules would pass through the same area of the cell membrane in a second, so the channel protein brings about a tremendous increase in rate. Other transport proteins, called carrier proteins, hold onto their passengers and change shape in a way that shuttles them across the membrane (see Figure 7.7a, right). 132

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A transport protein is specific for the substance it translocates (moves), allowing only a certain substance (or a small group of related substances) to cross the membrane. For example, a specific carrier protein in the plasma membrane of red blood cells transports glucose across the membrane 50,000 times faster than glucose can pass through on its own. This “glucose transporter” is so selective that it even rejects fructose, a structural isomer of glucose. Thus, the selective permeability of a membrane depends on both the discriminating barrier of the lipid bilayer and the specific transport proteins built into the membrane. Animation: Selective Permeability of Membranes

What establishes the direction of traffic across a membrane? And what mechanisms drive molecules across membranes? We will address these questions next as we explore two modes of membrane traffic: passive transport and active transport.

Concept Check 7.2 1. What property allows O2 and CO2 to cross a lipid bilayer without the aid of membrane proteins? 2. VISUAL SKILLS Examine Figure 7.2. Why is a transport protein needed to move many water molecules rapidly across a membrane? 3. MAKE CONNECTIONS Aquaporins exclude passage of hydronium ions (H3O+), but some aquaporins allow passage of glycerol, a three-carbon alcohol (see Figure 5.9), as well as H2O. Since H3O+ is closer in size to water than glycerol is, yet cannot pass through, what might be the basis of this selectivity? For suggested answers, see Appendix A.

Concept  7.3 Passive transport is diffusion of a substance across a membrane with no energy investment Molecules have a type of energy called thermal energy, due to their constant motion (see Concept 3.2). One result of this motion is diffusion, the movement of particles of any substance so that they spread out into the available space. Each molecule moves randomly, yet diffusion of a population of molecules may be directional. To understand this process, let’s imagine a synthetic membrane separating pure water from a solution of a dye in water. Study Figure 7.10a carefully to appreciate how diffusion would result in both solutions having equal concentrations of the dye molecules. Once that point is reached, there will be a dynamic equilibrium, with roughly as many dye molecules crossing the membrane each second in one direction as in the other. We can now state a simple rule of diffusion: In the absence of other forces, a substance will diffuse from where it is more concentrated to where it is less concentrated. Put another way, any substance will diffuse down its concentration gradient,

the region along which the density of a chemical substance increases or decreases (in this case, decreases). No work must be done to make this happen; diffusion is a spontaneous process, needing no input of energy. Note that each substance diffuses down its own concentration gradient, unaffected by the concentration gradients of other substances (Figure 7.10b). Much of the traffic across cell membranes occurs by diffusion. When a substance is more concentrated on one side of a membrane than on the other, there is a tendency for the substance to diffuse across the membrane down its concentration gradient (assuming that the membrane is permeable to that substance). One important example is the uptake of oxygen by a cell performing cellular respiration. Dissolved oxygen diffuses into the cell across the plasma membrane. As long as cellular respiration consumes the O2 as it enters, diffusion into the cell will continue because the concentration gradient favors movement in that direction. The diffusion of a substance across a biological membrane is called passive transport because the cell does not have

Figure 7.10  The diffusion of solutes across a synthetic membrane. Each of the large arrows under the diagrams shows the net diffusion of the dye molecules of that color. Molecules of dye

Membrane (cross section)

Net diffusion

Equilibrium

(a) Diffusion of one solute. The membrane has pores large enough for molecules of dye to pass through. Random movement of dye molecules will cause some to pass through the pores; this will happen more often on the side with more dye molecules. The dye diffuses from where it is more concentrated to where it is less concentrated (called diffusing down a concentration gradient). This leads to a dynamic equilibrium: The solute molecules continue to cross the membrane, but at roughly equal rates in both directions.

Net diffusion

Net diffusion

Equilibrium

Net diffusion

Net diffusion

Equilibrium

(b) Diffusion of two solutes. Solutions of two different dyes are separated by a membrane that is permeable to both. Each dye diffuses down its own concentration gradient. There will be a net diffusion of the purple dye toward the left, even though the total solute concentration was initially greater on the left side. Animation: Diffusion

Effects of Osmosis on Water Balance To see how two solutions with different solute concentrations interact, picture a U-shaped glass tube with a selectively permeable artificial membrane separating two sugar solutions (Figure 7.11). Pores in this synthetic membrane are too small Figure 7.11  Osmosis. Two sugar solutions of different concentrations are separated by a membrane that the solvent (water) can pass through but the solute (sugar) cannot. Water molecules move randomly and may cross in either direction, but overall, water diffuses from the solution with less concentrated solute to that with more concentrated solute. This passive transport of water, or osmosis, makes the sugar concentrations on both sides more nearly equal. (The concentrations are prevented from being exactly equal due to the effect of water pressure on the higher side, which is not discussed here, for simplicity.) Lower concentration of solute (sugar)

WATER

Net diffusion

to expend energy to make it happen. The concentration gradient itself represents potential energy (see Concept 2.2 and Figure 8.5b) and drives diffusion. Remember, however, that membranes are selectively permeable and therefore have different effects on the rates of diffusion of various molecules. In the case of water, the presence of aquaporin proteins allows water to diffuse very rapidly across the membranes of certain cells compared with diffusion in the absence of aquaporins. As we’ll see next, the movement of water across the plasma membrane has important consequences for cells.

Higher concentration of solute

More similar concentrations of solute

Sugar molecule H2O Selectively permeable membrane Water molecules can pass through pores, but sugar molecules cannot.

Water molecules cluster around sugar molecules.

This side has fewer solute molecules and more free water molecules.

This side has more solute molecules and fewer free water molecules. Osmosis

Water moves from an area of higher to lower free water concentration (lower to higher solute concentration). VISUAL SKILLS If an orange dye capable of passing through the membrane was added to the left side of the tube above, how would it be distributed at the end of the experiment? (See Figure 7.10.) Would the final solution levels in the tube be affected? Figure Walkthrough

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for sugar molecules to pass through but large enough for water molecules. However, tight clustering of water molecules around the hydrophilic solute molecules makes some of the water unavailable to cross the membrane. As a result, the solution with a higher solute concentration has a lower free water concentration. Water diffuses across the membrane from the region of higher free water concentration (lower solute concentration) to that of lower free water concentration (higher solute concentration) until the solute concentrations on both sides of the membrane are more nearly equal. The diffusion of free water across a selectively permeable membrane, whether artificial or cellular, is called osmosis. The movement of water across cell membranes and the balance of water between the cell and its environment are crucial to organisms. Let’s now apply what we’ve learned about osmosis in this system to ­living cells.

Water Balance of Cells Without Cell Walls

A cell without rigid cell walls can tolerate neither excessive uptake nor excessive loss of water. This problem of water balance is automatically solved if such a cell lives in isotonic surroundings. Seawater is isotonic to many marine invertebrates. The cells of most terrestrial (land-dwelling) animals are bathed in an extracellular fluid that is isotonic to the cells. In hypertonic or hypotonic environments, however, organisms that lack rigid cell walls must have other adaptations for ­osmoregulation, the control of solute concentrations and water balance. For example, the unicellular eukaryote Paramecium lives in pond water, which is hypotonic to the cell. Paramecium has a plasma membrane that is much less permeable to water than the membranes of most other cells, but this only slows the uptake of water, which continually enters the cell. The Paramecium cell doesn’t burst because it is also equipped with a contractile vacuole, an organelle that functions as a bilge pump to force water out of the cell as fast as it enters by osmosis (Figure 7.13). In contrast, the bacteria and archaea that live in hypersaline (excessively salty) environments (see Figure 27.1) have cellular mechanisms that balance the internal and external solute concentrations to ensure that water does not move out of the cell. We will examine other evolutionary adaptations for osmoregulation by animals in Concept 44.1.

To explain the behavior of a cell in a solution, we must consider both solute concentration and membrane permeability. Both factors are taken into account in the concept of tonicity, the ability of a surrounding solution to cause a cell to gain or lose water. The tonicity of a solution depends in part on its concentration of solutes that cannot cross the memWater Balance of Cells with Cell Walls brane (nonpenetrating solutes) relative to that inside the cell. The cells of plants, prokaryotes, fungi, and some unicellular If there is a higher concentration of nonpenetrating solutes eukaryotes are surrounded by cell walls (see Figure 6.27). When in the surrounding solution, water will tend to leave the cell, such a cell is immersed in a hypotonic solution—bathed in and vice versa. rainwater, for example—the cell wall helps maintain the cell’s If a cell without a cell wall, such as an animal cell, is immersed in an environment that is isotonic to the cell (iso means “same”), Figure 7.12  The water balance of living cells. How living cells react to changes in there will be no net movement of water the solute concentration of their environment depends on whether or not they have cell walls. across the plasma membrane. Water (a) Animal cells, such as this red blood cell, do not have cell walls. (b) Plant cells do have cell walls. (Arrows indicate net water movement after the cells were first placed in these solutions.) diffuses across the membrane, but at the same rate in both directions. In an Hypotonic solution Isotonic solution Hypertonic solution isotonic environment, the volume of an (a) Animal cell. An H2O H2O H2O H2O animal cell fares best animal cell is stable (Figure 7.12a). in an isotonic environLet’s transfer the cell to a solution that ment unless it has is hypertonic to the cell (hyper means special adaptations that offset the osmo“more,” in this case referring to nonpentic uptake or loss of etrating solutes). The cell will lose water, water. Lysed Normal Shriveled shrivel, and probably die. This is why an increase in the salinity (saltiness) of a lake Plasma Cell wall H2O Plasma H2O membrane can kill the animals there; if the lake water membrane H2O H2O becomes hypertonic to the animals’ cells, (b) Plant cell. Plant cells they might shrivel and die. However, are turgid (firm) and generally healthiest taking up too much water can be just as in a hypotonic envihazardous as losing water. If we place the ronment, where the cell in a solution that is ­hypotonic to uptake of water is eventually balanced the cell (hypo means “less”), water will by the wall pushing Turgid (normal) Flaccid Plasmolyzed enter the cell faster than it leaves, and back on the cell. the cell will swell and lyse (burst) like an Animation: Osmosis and Water Balance in Cells Video: Turgid Elodea overfilled water balloon. Video: Plasmolysis in Elodea

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Figure 7.13  The contractile vacuole of Paramecium. The vacuole collects fluid from canals in the cytoplasm. When full, the vacuole and canals contract, expelling fluid from the cell (LM). Contractile vacuole

50 μm

Figure 7.14  Two types of transport proteins that carry out facilitated diffusion. In both cases, the protein can transport the solute in either direction, but the net movement is down the concentration gradient of the solute. (a) A channel protein has a channel through which water molecules or a specific solute can pass.

EXTRACELLULAR FLUID

Channel protein Video: Paramecium Vacuole

water balance. Consider a plant cell. Like an animal cell, the plant cell swells as water enters by osmosis (Figure 7.12b). However, the relatively inelastic cell wall will expand only so much before it exerts a back pressure on the cell, called turgor pressure, that opposes further water uptake. At this point, the cell is turgid (very firm), the healthy state for most plant cells. Plants that are not woody, such as most houseplants, depend for mechanical support on cells kept turgid by a surrounding hypotonic solution. If a plant’s cells and surroundings are isotonic, there is no net tendency for water to enter and the cells become flaccid (limp); the plant wilts. However, a cell wall is of no advantage if the cell is immersed in a hypertonic environment. In this case, a plant cell, like an animal cell, will lose water to its surroundings and shrink. As the plant cell shrivels, its plasma membrane pulls away from the cell wall at multiple places. This phenomenon, called plasmolysis, causes the plant to wilt and can lead to plant death. The walled cells of bacteria and fungi also plasmolyze in hypertonic environments.

Facilitated Diffusion: Passive Transport Aided by Proteins Let’s look more closely at how water and certain hydrophilic solutes cross a membrane. As mentioned earlier, many polar molecules and ions impeded by the lipid bilayer of the membrane diffuse passively with the help of transport proteins that span the membrane. This phenomenon is called facilitated diffusion. Cell biologists are still trying to learn exactly how various transport proteins facilitate diffusion. Most transport proteins are very specific: They transport some substances but not others. As mentioned earlier, the two types of transport proteins are channel proteins and carrier proteins. Channel proteins simply provide corridors that allow specific molecules or ions to cross the membrane (Figure 7.14a). The hydrophilic passageways provided by these proteins can allow water molecules or small ions to diffuse very quickly from one side of the membrane to the other. Aquaporins, the water channel proteins, facilitate the massive levels of diffusion of water (osmosis) that occur in plant cells and in animal cells such as

Solute

CYTOPLASM

Carrier protein

Solute

(b) A carrier protein alternates between two shapes, moving a solute across the membrane during the shape change. Animation: Facilitated Diffusion

red blood cells (see Figure 7.12). Certain kidney cells also have a high number of aquaporins, allowing them to reclaim water from urine before it is excreted. If the kidneys did not perform this function, you would excrete about 180 L of urine per day—and have to drink an equal volume of water! Channel proteins that transport ions are called ion channels. Many ion channels function as gated channels, which open or close in response to a stimulus. For some gated channels, the stimulus is electrical. In a nerve cell, for example, an ion channel opens in response to an electrical stimulus, allowing a stream of potassium ions to leave the cell. (See the potassium ion channel at the beginning of this chapter.) This restores the cell’s ability to fire again. Other gated channels open or close when a specific substance other than the one to be transported binds to the channel. These gated channels are also important in the functioning of the nervous system, as you’ll learn in Concepts 48.2 and 48.3. Interview with Elba Serrano: Investigating how ion channels enable you to hear (see the interview before Chapter 6)

Carrier proteins, such as the glucose transporter mentioned earlier, seem to undergo a subtle change in shape that somehow translocates the solute-binding site across the membrane (Figure 7.14b). Such a change in shape may be triggered by the binding and release of the transported molecule. Like ion channels, carrier proteins involved in facilitated diffusion result in the net movement of a substance down its concentration gradient.

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Scientific Skills Exercise

Interpreting a Scatter Plot with Two Sets of Data

Glucose Uptake over Time in Guinea Pig Red Blood Cells 100

important energy source for animals, is transported into cells by facilitated diffusion using protein carriers. In this exercise, you will interpret a graph with two sets of data from an experiment that examined glucose uptake over time in red blood cells from guinea pigs of different ages. You will determine if the cells’ rate of glucose uptake depended on the age of the guinea pigs.

How the Experiment Was Done  Researchers incubated guinea pig red blood cells in a 300 mM (millimolar) radioactive glucose solution at pH 7.4 at 25°C. Every 10 or 15 minutes, they removed a sample of cells and measured the concentration of radioactive glucose inside those cells. The cells came from either a 15-day-old or a 1-month-old guinea pig. Data from the Experiment  When you have multiple sets of data, it can be useful to plot them on the same graph for comparison. In the graph here, each set of dots (of the same color) forms a scatter plot, in which every data point represents two numerical values, one for each variable. For each data set, a curve that best fits the points has been drawn to make it easier to see the trends. (For additional information about graphs, see the Scientific Skills Review in Appendix F.) Interpret The Data 1. First make sure you understand the parts of the graph. (a) Which variable is the independent variable—the variable controlled by the researchers? (b) Which variable is the dependent variable—the variable that depended on the treatment and was measured by the researchers? (c) What do the red dots represent? (d) The blue dots? 2. From the data points on the graph, construct a table of the data. Put “Incubation Time (min)” in the left column of the table.

No energy input is thus required: This is passive transport. The Scientific Skills Exercise gives you an opportunity to work with data from an experiment related to glucose transport. BioFlix® Animation: Passive Transport

Concept Check 7.3 1. How do you think a cell performing cellular respiration rids itself of the resulting CO2? 2. WHAT IF? If a Paramecium swims from a hypotonic to an isotonic environment, will its contractile vacuole become more active or less? Why? For suggested answers, see Appendix A.

Concept  7.4 Active transport uses energy to move solutes against their gradients Despite the help of transport proteins, facilitated diffusion is considered passive transport because the solute is moving down its concentration gradient, a process that requires no 136

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Concentration of radioactive glucose (mM)

Is Glucose Uptake into Cells Affected by Age?  Glucose, an

80 60 40 15-day-old guinea pig

20

0

1-month-old guinea pig 0

10

20 30 40 50 Incubation time (min)

60

Data from  T. Kondo and E. Beutler, Developmental changes in glucose transport of guinea pig erythrocytes, Journal of Clinical Investigation 65:1–4 (1980).

3. What does the graph show? Compare and contrast glucose uptake in red blood cells from 15-day-old and 1-month-old guinea pigs. 4. Develop a hypothesis to explain the difference between glucose uptake in red blood cells from 15-day-old and 1-month-old guinea pigs. (Think about how glucose gets into cells.) 5. Design an experiment to test your hypothesis. Instructors: A version of this Scientific Skills Exercise can be assigned in MasteringBiology.

energy. Facilitated diffusion speeds transport of a solute by providing efficient passage through the membrane, but it does not alter the direction of transport. Some other transport proteins, however, can move solutes against their concentration gradients, across the plasma membrane from the side where they are less concentrated (whether inside or outside) to the side where they are more concentrated.

The Need for Energy in Active Transport To pump a solute across a membrane against its gradient requires work; the cell must expend energy. Therefore, this type of membrane traffic is called active transport. The transport proteins that move solutes against their concentration gradients are all carrier proteins rather than channel proteins. This makes sense because when channel proteins are open, they merely allow solutes to diffuse down their concentration gradients rather than picking them up and transporting them against their gradients. Active transport enables a cell to maintain internal concentrations of small solutes that differ from concentrations in its environment. For example, compared with its

Figure 7.15  The sodium-potassium pump: a specific case of active transport. This transport system pumps ions against steep concentration gradients: Sodium ion concentration ([Na+]) is high outside the cell and low inside, while potassium ion concentration ([K+]) is low outside the cell and high inside. The pump oscillates between two shapes in a cycle that moves three Na+ out of the cell (steps 1  –  3  ) for every two K+ pumped into the cell (steps 4  –  6  ). The two shapes have different binding affinities for Na+ and K+. ATP hydrolysis powers the shape change by transferring a phosphate group to the transport protein (phosphorylating the protein).

EXTRACELLULAR FLUID

[Na+] high [K+] low

Na+ Na+

Na+

Na+

CYTOPLASM +

K K+

6 K+ is released; affinity for Na+ is high again, and the cycle repeats.

Animation: Active Transport

Na+

Na+

[Na+] low [K+] high

ATP

P

1 Cytoplasmic Na+ binds to the sodium-potassium pump. The affinity for Na+ is high when the protein has this shape.

ADP

2 Na+ binding stimulates phosphorylation by ATP. Na+

Na+ Na+ +

K K+

K+

K+

P

P

5 Loss of the phosphate group restores the protein’s original shape, which has a lower affinity for K+.

surroundings, an animal cell has a much higher concentration of potassium ions (K+) and a much lower concentration of sodium ions (Na+). The plasma membrane helps maintain these steep gradients by pumping Na+ out of the cell and K+ into the cell. As in other types of cellular work, ATP hydrolysis supplies the energy for most active transport. One way ATP can power active transport is when its terminal phosphate group is transferred directly to the transport protein. This can induce the protein to change its shape in a manner that translocates a solute bound to the protein across the membrane. One transport system that works this way is the sodium-potassium pump, which exchanges Na+ for K+ across the plasma membrane of animal cells (Figure 7.15). The distinction between passive transport and active transport is reviewed in Figure 7.16.

How Ion Pumps Maintain Membrane Potential All cells have voltages across their plasma membranes. Voltage is electrical potential energy (see Concept 2.2)—a separation of opposite charges. The cytoplasmic side of the membrane is negative in charge relative to the extracellular side because of an unequal distribution of anions and cations on the two sides. The voltage across a membrane, called a membrane potential, ranges from about -50 to -200 millivolts (mV). (The minus sign indicates that the inside of the cell is negative relative to the outside.)

P

i

4 The new shape has a high affinity for K+, which binds on the extracellular side and triggers release of the phosphate group.

Figure 7.16  Review: passive and active transport. Passive transport. Substances diffuse spontaneously down their concentration gradients, crossing a membrane with no expenditure of energy by the cell. The rate of diffusion can be greatly increased by transport proteins in the membrane.

Diffusion. Hydrophobic molecules and (at a slow rate) very small uncharged polar molecules can diffuse through the lipid bilayer.

Facilitated diffusion. Many hydrophilic substances diffuse through membranes with the assistance of transport proteins, either channel proteins (left) or carrier proteins (right).

3 Phosphorylation leads to a change in protein shape, reducing its affinity for Na+, which is released outside.

Active transport. Some transport proteins act as pumps, moving substances across a membrane against their concentration (or electrochemical) gradients. Energy is usually supplied by ATP hydrolysis.

ATP

VISUAL SKILLS

For each solute in the right panel, describe its direction of movement, and state whether it is moving with or against its concentration gradient.

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The membrane potential acts like a battery, an energy source that affects the traffic of all charged substances across the membrane. Because the inside of the cell is negative compared with the outside, the membrane potential favors the passive transport of cations into the cell and anions out of the cell. Thus, two forces drive the diffusion of ions across a membrane: a chemical force (the ion’s concentration gradient, which has been our sole consideration thus far in the chapter) and an electrical force (the effect of the membrane potential on the ion’s movement). This combination of forces acting on an ion is called the electrochemical gradient. In the case of ions, then, we must refine our concept of passive transport: An ion diffuses not simply down its concentration gradient but, more exactly, down its electrochemical gradient. For example, the concentration of Na+ inside a resting nerve cell is much lower than outside it. When the cell is stimulated, gated channels open that facilitate Na+ diffusion. Sodium ions then “fall” down their electrochemical gradient, driven by the concentration gradient of Na+ and by the attraction of these cations to the negative side (inside) of the membrane. In this example, both electrical and chemical contributions to the electrochemical gradient act in the same direction across the membrane, but this is not always so. In cases where electrical forces due to the membrane potential oppose the simple diffusion of an ion down its concentration gradient, active transport may be necessary. In Concepts 48.2 and 48.3, you’ll learn about the importance of electrochemical gradients and membrane potentials in the transmission of nerve impulses. Some membrane proteins that actively transport ions contribute to the membrane potential. An example is the sodiumpotassium pump. Notice in Figure 7.15 that the pump does not translocate Na+ and K+ one for one, but pumps three sodium ions out of the cell for every two potassium ions it pumps into the cell. With each “crank” of the pump, there is a net transfer of one positive charge from the cytoplasm to the extracellular fluid, a process that stores energy as voltage. A transport protein that generates voltage across a membrane is called an electrogenic pump. The sodium-potassium pump appears to be the major Figure 7.17  A proton pump. Proton pumps are electrogenic pumps that store energy by generating voltage (charge separation) across membranes. A proton pump translocates positive charge in the form of hydrogen ions. The voltage and H+ concentration gradient represent a dual energy source that can drive other processes, such as the uptake of nutrients. Most proton pumps are powered by ATP hydrolysis. –

ATP

+

– + H+

Proton pump

CYTOPLASM

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H+

electrogenic pump of animal cells. The main electrogenic pump of plants, fungi, and bacteria is a proton pump, which actively transports protons (hydrogen ions, H+) out of the cell. The pumping of H+ transfers positive charge from the cytoplasm to the extracellular solution (Figure 7.17). By generating voltage across membranes, electrogenic pumps help store energy that can be tapped for cellular work. One important use of proton gradients in the cell is for ATP synthesis during cellular respiration, as you will see in Concept 9.4. Another is a type of membrane traffic called cotransport.

Cotransport: Coupled Transport by a Membrane Protein A solute that exists in different concentrations across a membrane can do work as it moves across that membrane by diffusion down its concentration gradient. This is analogous to water that has been pumped uphill and performs work as it flows back down. In a mechanism called cotransport, a transport protein (a cotransporter) can couple the “downhill” diffusion of the solute to the “uphill” transport of a second substance against its own concentration gradient. For instance, a plant cell uses the gradient of H+ generated by its ATP-powered proton pumps to drive the active transport of amino acids, sugars, and several other nutrients into the cell. In the example shown in Figure 7.18, a cotransporter couples the return of H+ to the transport of sucrose into the cell. This protein can translocate sucrose into the cell against its concentration gradient, but only if the sucrose molecule travels in the company of an H+. Figure 7.18  Cotransport: active transport driven by a concentration gradient. A carrier protein, such as this H+/sucrose cotransporter in a plant cell (top), is able to use the diffusion of H+ down its electrochemical gradient into the cell to drive the uptake of sucrose. (The cell wall is not shown.) Although not technically part of the cotransport process, an ATP-driven proton pump is shown here (bottom), which concentrates H+ outside the cell. The resulting H+ gradient represents potential energy that can be used for active transport—of sucrose, in this case. Thus, ATP hydrolysis indirectly provides the energy necessary for cotransport.

+

H+/sucrose cotransporter

– H+

ATP

H+



EXTRACELLULAR FLUID H+

Diffusion of H+ +

H+

H+ H+

Sucrose

Sucrose

H+ +

+



H+

H+

Proton pump –

H+

+

+

H+

H+

H+

The H+ uses the transport protein as an avenue to diffuse down its own electrochemical gradient, which is maintained by the proton pump. Plants use H+/sucrose cotransport to load sucrose produced by photosynthesis into cells in the veins of leaves. The vascular tissue of the plant can then distribute the sugar to roots and other nonphotosynthetic organs that do not make their own food. What we know about cotransport proteins in animal cells has helped us find more effective treatments for diarrhea, a serious problem in developing countries. Normally, sodium in waste is reabsorbed in the colon, maintaining constant levels in the body, but diarrhea expels waste so rapidly that reabsorption is not possible, and sodium levels fall precipitously. To treat this life-threatening condition, patients are given a solution to drink containing high concentrations of salt (NaCl) and glucose. The solutes are taken up by sodiumglucose cotransporters on the surface of intestinal cells and passed through the cells into the blood. This simple treatment has lowered infant mortality worldwide. BioFlix® Animation: Active Transport

Concept Check 7.4 1. Sodium-potassium pumps help nerve cells establish a voltage across their plasma membranes. Do these pumps use ATP or produce ATP? Explain. 2. VISUAL SKILLS Compare the sodium-potassium pump in Figure 7.15 with the cotransporter in Figure 7.18. Explain why the sodium-potassium pump would not be considered a cotransporter. 3. MAKE CONNECTIONS Review the characteristics of the lysosome in Concept 6.4. Given the internal environment of a lysosome, what transport protein might you expect to see in its membrane? For suggested answers, see Appendix A.

Concept  7.5 Bulk transport across the plasma membrane occurs by exocytosis and endocytosis Water and small solutes enter and leave the cell by diffusing through the lipid bilayer of the plasma membrane or by being pumped or moved across the membrane by transport proteins. However, large molecules—such as proteins and polysaccharides, as well as larger particles—generally cross the membrane in bulk, packaged in vesicles. Like active transport, these processes require energy. BioFlix® Animation: Exocytosis and Endocytosis

Exocytosis As you saw in Figure 6.15, the cell secretes certain molecules by the fusion of vesicles with the plasma membrane; this process

is called exocytosis. A transport vesicle that has budded from the Golgi apparatus moves along microtubules of the cytoskeleton to the plasma membrane. When the vesicle membrane and plasma membrane come into contact, specific proteins rearrange the lipid molecules of the two bilayers so that the two membranes fuse. The contents of the vesicle spill out of the cell, and the vesicle membrane becomes part of the plasma membrane (see Figure 7.9, step 4). Many secretory cells use exocytosis to export products. For example, cells in the pancreas that make insulin secrete it into the extracellular fluid by exocytosis. In another example, nerve cells use exocytosis to release neurotransmitters that signal other neurons or muscle cells. When plant cells are making cell walls, exocytosis delivers some of the necessary proteins and carbohydrates from Golgi vesicles to the outside of the cell.

Endocytosis In endocytosis, the cell takes in molecules and particulate matter by forming new vesicles from the plasma membrane. Although the proteins involved in the processes are different, the events of endocytosis look like the reverse of exocytosis. First, a small area of the plasma membrane sinks inward to form a pocket. Then, as the pocket deepens, it pinches in, forming a vesicle containing material that had been outside the cell. Study Figure 7.19 carefully to understand the three types of endocytosis: phagocytosis (“cellular eating”), pinocytosis (“cellular drinking”), and receptor-mediated endocytosis. Human cells use receptor-mediated endocytosis to take in cholesterol for membrane synthesis and the synthesis of other steroids. Cholesterol travels in the blood in particles called low-density lipoproteins (LDLs), each a complex of lipids and a protein. LDLs bind to LDL receptors on plasma membranes and then enter the cells by endocytosis. In the inherited disease familial hypercholesterolemia, characterized by a very high level of cholesterol in the blood, LDLs cannot enter cells because the LDL receptor proteins are defective or missing: LDL

LDL receptor

Normal cell

Mild disease

Severe disease

Consequently, cholesterol accumulates in the blood, where it contributes to early atherosclerosis, the buildup of lipid deposits within the walls of blood vessels. This buildup narrows the space in the vessels and impedes blood flow, potentially resulting in heart damage and stroke.

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Figure 7.19

  Exploring Endocytosis in Animal Cells

Phagocytosis

Receptor-Mediated Endocytosis

Pinocytosis

EXTRACELLULAR FLUID Solutes Solutes Pseudopodium

Receptor Plasma membrane

Coat protein

Food or other particle

Coated vesicle with specific solutes (purple) bound to receptors (red)

Coated pit

Coated vesicle Food vacuole

CYTOPLASM

In phagocytosis, a cell engulfs a particle by extending pseudopodia (singular, pseudopodium) around it and packaging it within a membranous sac called a food vacuole. The particle will be digested after the food vacuole fuses with a lysosome containing hydrolytic enzymes (see Figure 6.13).

In pinocytosis, a cell continually “gulps” droplets of extracellular fluid into tiny vesicles, formed by infoldings of the plasma membrane. In this way, the cell obtains molecules dissolved in the droplets. Because any and all solutes are taken into the cell, pinocytosis as shown here is nonspecific for the substances it transports. In many cases, as above, the parts of the plasma membrane that form vesicles are lined on their cytoplasmic side by a fuzzy layer of coat protein; the “pits” and resulting vesicles are said to be “coated.”

Receptor-mediated endocytosis is a specialized type of pinocytosis that enables the cell to acquire bulk quantities of specific substances, even though those substances may not be very concentrated in the extracellular fluid. Embedded in the plasma membrane are proteins with receptor sites exposed to the extracellular fluid. Specific solutes bind to the receptors. The receptor proteins then cluster in coated pits, and each coated pit forms a vesicle containing the bound molecules. The diagram shows only bound molecules (purple triangles) inside the vesicle, but other molecules from the extracellular fluid are also present. After the ingested material is liberated from the vesicle, the emptied receptors are recycled to the plasma membrane by the same vesicle (not shown).

5 μm Green algal cell Plasma membrane

Pinocytotic vesicles forming (TEMs).

VISUAL SKILLS Use the scale bars to estimate the diameters of (a) the food vacuole that will form around the algal cell (left micrograph) and (b) the coated vesicle (lower right micrograph). (c) Which is larger, and by what factor? Animation: Exocytosis and Endocytosis

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0.25 μm

0.25 μm

Pseudopodium of amoeba

An amoeba engulfing a green algal cell via phagocytosis (TEM).

Coat protein

Top: A coated pit. Bottom: A coated vesicle forming during receptor-mediated endocytosis (TEMs).

Endocytosis and exocytosis also provide mechanisms for rejuvenating or remodeling the plasma membrane. These processes occur continually in most eukaryotic cells, yet the amount of plasma membrane in a nongrowing cell remains fairly constant. The addition of membrane by one process appears to offset the loss of membrane by the other. Energy and cellular work have figured prominently in our study of membranes. In the next three chapters, you will learn more about how cells acquire chemical energy to do the work of life.

Concept Check 7.5 1. As a cell grows, its plasma membrane expands. Does this involve endocytosis or exocytosis? Explain. 2. DRAW IT Return to Figure 7.9, and circle a patch of plasma membrane that is coming from a vesicle involved in exocytosis. 3. MAKE CONNECTIONS In Concept 6.7, you learned that animal cells make an extracellular matrix (ECM). Describe the cellular pathway of synthesis and deposition of an ECM glycoprotein. For suggested answers, see Appendix A.

BioFlix® Animation: Membrane Transport

7

Chapter Review

Summary of Key Concepts Concept  7.1

Cellular membranes are fluid mosaics of lipids and proteins (pp. 127–131) VOCAB In the fluid mosaic model, amphipathic proteins SELF-QUIZ goo.gl/6u55ks are embedded in the phospholipid bilayer. Phospholipids and some proteins move sideways within the membrane. The unsaturated hydrocarbon tails of some phospholipids keep membranes fluid at lower temperatures, while cholesterol helps membranes resist changes in fluidity caused by temperature changes. Membrane proteins function in transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, and attachment to the cytoskeleton and extracellular matrix. Short chains of sugars linked to proteins (in glycoproteins) and lipids (in glycolipids) on the exterior side of the plasma membrane interact with surface molecules of other cells. Membrane proteins and lipids are synthesized in the ER and modified in the ER and Golgi apparatus. The inside and outside faces of membranes differ in molecular composition.

?

a higher solute concentration (hypertonic) than the cytosol; water enters the cell if the solution has a lower solute concentration (hypotonic). If the concentrations are equal (isotonic), no net osmosis occurs. Cell survival depends on Passive transport: balancing water uptake Facilitated diffusion and loss. In facilitated diffusion, a transport protein speeds the movement of water or a solute across a membrane down its concenCarrier tration gradient. Ion Channel protein protein channels facilitate the diffusion of ions across a membrane. Carrier proteins can undergo changes in shape that translocate bound solutes across the membrane.

?

In what ways are membranes crucial to life?

Concept  7.2

Membrane structure results in selective permeability (pp. 131–132) A cell must exchange molecules and ions with its surroundings, a process controlled by the selective permeability of the plasma membrane. Hydrophobic substances are soluble in lipids and pass through membranes rapidly, whereas polar molecules and ions generally require specific transport proteins.

?

Go to MasteringBiology™ for Videos, Animations, Vocab Self-Quiz, Practice Tests, and more in the Study Area.

How do aquaporins affect the permeability of a membrane?

Concept  7.3

Passive transport is diffusion of a substance across a membrane with no energy investment (pp. 132–136) Diffusion is the spontaneous movement of a substance down its concentration gradient. Water diffuses out through the permeable membrane of a cell (osmosis) if the solution outside has

What happens to a cell placed in a hypertonic solution? Describe the free water concentration inside and out.

Concept  7.4

Active transport uses energy to move solutes Active transport against their gradients (pp. 136–139) Specific membrane proteins use energy, usually in the form of ATP, to do the work of active transport. Ions can have both a concentration (chemical) gradient and an electrical gradient (voltage). These gradients combine in the electrochemical gradient, which determines the net direction of ionic diffusion. Cotransport of two solutes occurs when a membrane protein enables the “downhill” diffusion of one solute to drive the “uphill” transport of the other.

?

ATP

ATP is not directly involved in the functioning of a cotransporter. Why, then, is cotransport considered active transport?

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Concept  7.5

Bulk transport across the plasma membrane occurs by exocytosis and endocytosis (pp. 139–141) In exocytosis, transport vesicles migrate to the plasma membrane, fuse with it, and release their contents. In endocytosis, molecules enter cells within vesicles that pinch inward from the plasma membrane. The three types of endocytosis are phagocytosis, pinocytosis, and receptor-mediated endocytosis.

?

Which type of endocytosis involves the binding of specific substances in the extracellular fluid to membrane proteins? What does this type of transport enable a cell to do?

Test Your Understanding Level 1: Knowledge/Comprehension 1. In what way do the membranes of a eukaryotic cell vary? (A) Phospholipids are found only in certain PRACTICE TEST membranes. goo.gl/CUYGKD (B) Certain proteins are unique to each membrane. (C) Only certain membranes of the cell are selectively permeable. (D) Only certain membranes are constructed from amphipathic molecules. 2. According to the fluid mosaic model of membrane structure, proteins of the membrane are mostly (A) spread in a continuous layer over the inner and outer surfaces of the membrane. (B) confined to the hydrophobic interior of the membrane. (C) embedded in a lipid bilayer. (D) randomly oriented in the membrane, with no fixed insideoutside polarity. 3. Which of the following factors would tend to increase membrane fluidity? (A) a greater proportion of unsaturated phospholipids (B) a greater proportion of saturated phospholipids (C) a lower temperature (D) a relatively high protein content in the membrane

Level 2: Application/Analysis 4. Which of the following processes includes all the others? (A) osmosis (B) diffusion of a solute across a membrane (C) passive transport (D) transport of an ion down its electrochemical gradient

(d) Will the artificial cell become more flaccid, more turgid, or stay the same? (e) Eventually, will the two solutions have the same or different solute concentrations?

”Cell” 0.03 M sucrose 0.02 M glucose

Level 3: Synthesis/Evaluation 7. EVOLUTION CONNECTION  Paramecium and other unicellular eukaryotes that live in hypotonic environments have cell membranes that limit water uptake, while those living in isotonic environments have membranes that are more permeable to water. Describe what water regulation adaptations might have evolved in unicellular eukaryotes in hypertonic habitats such as the Great Salt Lake and in habitats with changing salt concentration. 8. SCIENTIFIC INQUIRY  An experiment is designed to study the mechanism of sucrose uptake by plant cells. Cells are immersed in a sucrose solution, and the pH of the solution is monitored. Samples of the cells are taken at intervals, and their sucrose concentration is measured. The pH is observed to decrease until it reaches a steady, slightly acidic level, and then sucrose uptake begins. (a) Evaluate these results and propose a hypothesis to explain them. (b) Predict what would happen if an inhibitor of ATP regeneration by the cell were added to the beaker once the pH was at a steady level. Explain. 9. SCIENCE, TECHNOLOGY, AND SOCIETY  Extensive irrigation in arid regions causes salts to accumulate in the soil. (When water evaporates, salts that were dissolved in the water are left behind in the soil.) Based on what you learned about water balance in plant cells, explain why increased soil salinity (saltiness) might be harmful to crops. 10. WRITE ABOUT A THEME: INTERACTIONS  A human pancreatic cell obtains O2—and necessary molecules such as glucose, amino acids, and cholesterol—from its environment, and it releases CO2 as a waste product. In response to hormonal signals, the cell secretes digestive enzymes. It also regulates its ion concentrations by exchange with its environment. Based on what you have just learned about the structure and function of cellular membranes, write a short essay (100–150 words) to describe how such a cell accomplishes these interactions with its environment. 11. SYNTHESIZE YOUR KNOWLEDGE In the supermarket, lettuce and other produce are often sprayed with water. Explain why this makes vegetables crisp.

5. Based on Figure 7.18, which of these experimental treatments would increase the rate of sucrose transport into a plant cell? (A) decreasing extracellular sucrose concentration (B) decreasing extracellular pH (C) decreasing cytoplasmic pH (D) adding a substance that makes the membrane more permeable to hydrogen ions 6. DRAW IT An artificial “cell” consisting of an aqueous solution enclosed in a selectively permeable membrane is immersed in a beaker containing a different solution, the “environment,” as shown in the accompanying diagram. The membrane is permeable to water and to the simple sugars glucose and fructose but impermeable to the disaccharide sucrose. (a) Draw solid arrows to indicate the net movement of solutes into and/or out of the cell. (b) Is the solution outside the cell isotonic, hypotonic, or hypertonic? (c) Draw a dashed arrow to show the net osmosis, if any.

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“Environment” 0.01 M sucrose 0.01 M glucose 0.01 M fructose

For selected answers, see Appendix A. For additional practice questions, check out the Dynamic Study Modules in MasteringBiology. You can use them to study on your smartphone, tablet, or computer anytime, anywhere!

An Introduction to Metabolism

8

Figure 8.1  What causes these breaking waves to glow?

Key Concepts 8.1

An organism’s metabolism transforms matter and energy, subject to the laws of thermodynamics

8.2

The free-energy change of a reaction tells us whether or not the reaction occurs spontaneously

8.3

ATP powers cellular work by coupling exergonic reactions to endergonic reactions

8.4

Enzymes speed up metabolic reactions by lowering energy barriers

8.5

Regulation of enzyme activity helps control metabolism

Firefly

The Energy of Life The living cell is a chemical factory in miniature, where thousands of reactions occur within a microscopic space. Sugars can be converted to amino acids that are linked together into proteins when needed. Conversely, when food is digested, proteins are dismantled into amino acids that can be converted to sugars. In multicellular organisms, many cells export chemical products that are used in other parts of the organism. The process called cellular respiration drives this cellular economy by extracting the energy stored in sugars and other fuels. Cells apply this energy to perform various types of work, such as the transport of solutes across the plasma membrane, which we discussed in Concept 7.4. In a more exotic example, the ocean waves shown in Figure 8.1 are brightly illuminated from within by free-floating, single-celled marine organisms called dinoflagellates. These dinoflagellates convert the energy stored in certain organic molecules to light, a process called bioluminescence. Most bioluminescent organisms are found in the oceans, but some exist on land, such as the bioluminescent firefly shown in the small photo. Bioluminescence and other metabolic activities carried out by a cell are precisely coordinated and controlled. In its complexity, its efficiency, and its responsiveness to subtle changes, the cell is peerless as a chemical factory. The concepts of metabolism that you learn in this chapter will help you understand how matter and energy flow during life’s processes and how that flow is regulated. When you see this blue icon, log in to MasteringBiology and go to the Study Area for digital resources.

Get Ready for This Chapter

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Concept  8.1 An organism’s metabolism transforms matter and energy, subject to the laws of thermodynamics The totality of an organism’s chemical reactions is called metabolism (from the Greek metabole, change). Metabolism is an emergent property of life that arises from orderly interactions between molecules.

Organization of the Chemistry of Life into Metabolic Pathways We can picture a cell’s metabolism as an elaborate road map of the thousands of chemical reactions that occur in a cell, arranged as intersecting metabolic pathways. A metabolic pathway begins with a specific molecule, which is then altered in a series of defined steps, resulting in a certain product. Each step of the pathway is catalyzed by a specific enzyme: Enzyme 1 A

Reaction 1

Starting molecule

Enzyme 2 B

Reaction 2

Enzyme 3 C

Reaction 3

D Product

There are mechanisms that regulate these enzymes, thus balancing metabolic supply and demand, analogous to the red, yellow, and green stoplights that control the flow of automobile traffic. Metabolism as a whole manages the material and energy resources of the cell. Some metabolic pathways release energy by breaking down complex molecules to simpler compounds. These degradative processes are called catabolic pathways, or breakdown pathways. A major pathway of catabolism is cellular respiration, in which the sugar glucose and other organic fuels are broken down in the presence of oxygen to carbon dioxide and water. (Pathways can have more than one starting molecule and/or product.) Energy that was stored in the organic molecules becomes available to do the work of the cell, such as ciliary beating or membrane transport. Anabolic pathways, in contrast, consume energy to build complicated molecules from simpler ones; they are sometimes called biosynthetic pathways. Examples of anabolism are the synthesis of an amino acid from simpler molecules and the synthesis of a protein from amino acids. Catabolic and anabolic pathways are the “downhill” and “uphill” avenues of the metabolic landscape. Energy released from the downhill reactions of catabolic pathways can be stored and then used to drive the uphill reactions of anabolic pathways. In this chapter, we will focus on mechanisms common to metabolic pathways. Because energy is fundamental to all metabolic processes, a basic knowledge of energy is necessary to understand how the living cell works. Although we will use 144

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some nonliving examples to study energy, the concepts demonstrated by these examples also apply to bioenergetics, the study of how energy flows through living organisms.

Forms of Energy Energy is the capacity to cause change. In everyday life, energy is important because some forms of energy can be used to do work—that is, to move matter against opposing forces, such as gravity and friction. Put another way, energy is the ability to rearrange a collection of matter. For example, you expend energy to turn the pages of this book, and your cells expend energy in transporting certain substances across membranes. Energy exists in various forms, and the work of life depends on the ability of cells to transform energy from one form to another. Energy can be associated with the relative motion of objects; this energy is called kinetic energy. Moving objects can perform work by imparting motion to other matter: A pool player uses the motion of the cue stick to push the cue ball, which in turn moves the other balls; water gushing through a dam turns turbines; and the contraction of leg muscles pushes bicycle pedals. Thermal energy is kinetic energy associated with the random movement of atoms or molecules; thermal energy in transfer from one object to another is called heat. Light is also a type of energy that can be harnessed to perform work, such as powering photosynthesis in green plants. An object not presently moving may still possess energy. Energy that is not kinetic is called potential energy; it is energy that matter possesses because of its location or structure. Water behind a dam, for instance, possesses energy because of its altitude above sea level. Molecules possess energy because of the arrangement of electrons in the bonds between their atoms. Chemical energy is a term used by biologists to refer to the potential energy available for release in a chemical reaction. Recall that catabolic pathways release energy by breaking down complex molecules. Biologists say that these complex molecules, such as glucose, are high in chemical energy. During a catabolic reaction, some bonds are broken and others are formed, releasing energy and resulting in lower-energy breakdown products. This transformation also occurs in the engine of a car when the hydrocarbons of gasoline react explosively with oxygen, releasing the energy that pushes the pistons and producing exhaust. Although less explosive, a similar reaction of food molecules with oxygen provides chemical energy in biological systems, producing carbon dioxide and water as waste products. Biochemical pathways, carried out in the context of cellular structures, enable cells to release chemical energy from food molecules and use the energy to power life processes. How is energy converted from one form to another? Consider Figure 8.2. The woman climbing the ladder to the diving platform is releasing chemical energy from the food she ate for lunch and using some of that energy to perform

Figure 8.2  Transformations between potential and kinetic energy.

derived from light energy absorbed by plants during photosynthesis. Organisms are energy transformers.

A diver has more potential energy on the platform than in the water.

The Laws of Energy Transformation

Diving converts potential energy to kinetic energy.

Climbing up converts the kinetic energy of muscle movement to potential energy.

A diver has less potential energy in the water than on the platform.

Animation: Energy Transformations

the work of climbing. The kinetic energy of muscle movement is thus being transformed into potential energy due to her increasing height above the water. The man diving is converting his potential energy to kinetic energy, which is then transferred to the water as he enters it, resulting in splashing, noise, and increased movement of water molecules. A small amount of energy is lost as heat due to friction. Now let’s consider the original source of the organic food molecules that provided the necessary chemical energy for these divers to climb the steps. This chemical energy was itself

The study of the energy transformations that occur in a collection of matter is called thermodynamics. Scientists use the word system to denote the matter under study; they refer to the rest of the universe—everything outside the system—as the surroundings. An isolated system, such as that approximated by liquid in a thermos bottle, is unable to exchange either energy or matter with its surroundings outside the thermos. In an open system, energy and matter can be transferred between the system and its surroundings. Organisms are open systems. They absorb energy—for instance, light energy or chemical energy in the form of organic molecules—and release heat and metabolic waste products, such as carbon dioxide, to the surroundings. Two laws of thermodynamics govern energy transformations in organisms and all other collections of matter.

The First Law of Thermodynamics According to the first law of thermodynamics, the energy of the universe is constant: Energy can be transferred and transformed, but it cannot be created or destroyed. The first law is also known as the principle of conservation of energy. The electric company does not make energy, but merely converts it to a form that is convenient for us to use. By converting sunlight to chemical energy, a plant acts as an energy transformer, not an energy producer. The brown bear in Figure 8.3a will convert the chemical energy of the organic molecules in its food to kinetic and other forms of energy as it carries out biological processes. What happens to this energy after it has performed work? The second law of thermodynamics helps to answer this question.

Figure 8.3  The two laws of thermodynamics. Heat CO2

+ H2O

Chemical energy in food

(a) First law of thermodynamics: Energy can be transferred or transformed but neither created nor destroyed. For example, chemical reactions in this brown bear will convert the chemical (potential) energy in the fish into the kinetic energy of running. MP3 Tutor: Basic Energy Concepts

Kinetic energy

(b) Second law of thermodynamics: Every energy transfer or transformation increases the disorder (entropy) of the universe. For example, as the bear runs, disorder is increased around its body by the release of heat and small molecules that are the by-products of metabolism. A brown bear can run at speeds up to 35 miles per hour (56 km/hr) —as fast as a racehorse.

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The Second Law of Thermodynamics If energy cannot be destroyed, why can’t organisms simply recycle their energy over and over again? It turns out that during every energy transfer or transformation, some energy becomes unavailable to do work. In most energy transformations, the more usable forms of energy are at least partly converted to thermal energy and released as heat. Only a small fraction of the chemical energy from the food in Figure 8.3a is transformed into the motion of the brown bear shown in Figure 8.3b; most is lost as heat, which dissipates rapidly through the surroundings. In the process of carrying out chemical reactions that perform various kinds of work, living cells unavoidably convert other forms of energy to heat. A system can put this energy to work only when there is a temperature difference that results in thermal energy flowing as heat from a warmer location to a cooler one. If temperature is uniform, as it is in a living cell, then the heat generated during a chemical reaction will simply warm a body of matter, such as the organism. (This can make a room crowded with people uncomfortably warm, as each person is carrying out a multitude of chemical reactions!) A consequence of the loss of usable energy as heat to the surroundings is that each energy transfer or transformation makes the universe more disordered. We are all familiar with the word “disorder” in the sense of a messy room or a rundown building. The word “disorder” as used by scientists, however, has a specific molecular definition related to how dispersed the energy is in a system, and how many different energy levels are present. For simplicity, we use “disorder” in the following discussion because our common understanding (the messy room) is a good analogy for molecular disorder. Scientists use a quantity called entropy as a measure of molecular disorder, or randomness. The more randomly arranged a collection of matter is, the greater its entropy. We can now state the second law of thermodynamics: Every energy transfer or transformation increases the entropy of the universe. Although order can increase locally, there is an unstoppable trend toward randomization of the universe as a whole. The physical disintegration of a system’s organized structure is a good analogy for an increase in entropy. For example, you can observe the gradual decay of an unmaintained building over time. Much of the increasing entropy of the universe is more abstract, however, because it takes the form of increasing amounts of heat and less ordered forms of matter. As the bear in Figure 8.3b converts chemical energy to kinetic energy, it is also increasing the disorder of its surroundings by producing heat and small molecules, such as the CO2 it exhales, that are the breakdown products of food. The concept of entropy helps us understand why certain processes are energetically favorable and occur on their own. It turns out that if a given process, by itself, leads to an increase in entropy, that process can proceed without requiring an input 146

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of energy. Such a process is called a spontaneous process. Note that as we’re using it here, the word spontaneous does not imply that the process would occur quickly; rather, the word signifies that it is energetically favorable. (In fact, it may be helpful for you to think of the phrase “energetically favorable” when you read the formal term “spontaneous,” the word favored by chemists.) Some spontaneous processes, such as an explosion, may be virtually instantaneous, while others, such as the rusting of an old car over time, are much slower. A process that, on its own, leads to a decrease in entropy is said to be nonspontaneous: It will happen only if energy is supplied. We know from experience that certain events occur spontaneously and others do not. For instance, we know that water flows downhill spontaneously but moves uphill only with an input of energy, such as when a machine pumps the water against gravity. Some energy is inevitably lost as heat, increasing entropy in the surroundings, so usage of energy means that a nonspontaneous process also leads to an increase in the entropy of the universe as a whole.

Biological Order and Disorder Living systems increase the entropy of their surroundings, as predicted by thermodynamic law. It is true that cells create ordered structures from less organized starting materials. For example, simpler molecules are ordered into the more complex structure of an amino acid, and amino acids are ordered into polypeptide chains. At the organismal level as well, complex and beautifully ordered structures result from biological processes that use simpler starting materials (Figure 8.4).

Figure 8.4  Order as a characteristic of life. Order is evident in the detailed structures of the biscuit star and the agave plant shown here. As open systems, organisms can increase their order as long as the order of their surroundings decreases, with an overall increase in entropy in the universe.

However, an organism also takes in organized forms of matter and energy from the surroundings and replaces them with less ordered forms. For example, an animal obtains starch, proteins, and other complex molecules from the food it eats. As catabolic pathways break these molecules down, the animal releases carbon dioxide and water—small molecules that possess less chemical energy than the food did (see Figure 8.3b). The depletion of chemical energy is accounted for by heat generated during metabolism. On a larger scale, energy flows into most ecosystems in the form of light and exits in the form of heat (see Figure 1.11). During the early history of life, complex organisms evolved from simpler ancestors. For instance, we can trace the ancestry of the plant kingdom from much simpler organisms called green algae to more complex flowering plants. However, this increase in organization over time in no way violates the second law. The entropy of a particular system, such as an organism, may actually decrease as long as the total entropy of the universe—the system plus its surroundings—increases. Thus, organisms are islands of low entropy in an increasingly random universe. The evolution of biological order is perfectly consistent with the laws of thermodynamics.

Concept Check 8.1 1. MAKE CONNECTIONS How does the second law of thermodynamics help explain the diffusion of a substance across a membrane? (See Figure 7.10.) 2. Describe the forms of energy found in an apple as it grows on a tree, then falls, then is digested by someone who eats it. 3. WHAT IF? If you place a teaspoon of sugar in the bottom of a glass of water, it will dissolve completely over time. Left longer, eventually the water will disappear and the sugar crystals will reappear. Explain these observations in terms of entropy. For suggested answers, see Appendix A.

Concept  8.2 The free-energy change of a reaction tells us whether or not the reaction occurs spontaneously The laws of thermodynamics that we’ve just discussed apply to the universe as a whole. As biologists, we want to understand the chemical reactions of life—for example, which reactions occur spontaneously and which ones require some input of energy from outside. But how can we know this without assessing the energy and entropy changes in the entire universe for each separate reaction?

Free-Energy Change, ∆G Recall that the universe is really equivalent to “the system” plus “the surroundings.” In 1878, J. Willard Gibbs, a professor

at Yale, defined a very useful function called the Gibbs free energy of a system (without considering its surroundings), symbolized by the letter G. We’ll refer to the Gibbs free energy simply as free energy. Free energy is the portion of a system’s energy that can perform work when temperature and pressure are uniform throughout the system, as in a living cell. Let’s consider how we determine the free-energy change that occurs when a system changes—for example, during a chemical reaction. The change in free energy, ∆G, can be calculated for a chemical reaction by applying the following equation: ∆G = ∆H - T∆S This equation uses only properties of the system (the reaction) itself: ∆H symbolizes the change in the system’s enthalpy (in biological systems, equivalent to total energy); ∆S is the change in the system’s entropy; and T is the absolute temperature in Kelvin (K) units (K = °C + 273; see Appendix C). Using chemical methods, we can measure ∆G for any reaction. (The value will depend on conditions such as pH, temperature, and concentrations of reactants and products.) Once we know the value of ∆G for a process, we can use it to predict whether the process will be spontaneous (that is, whether it is energetically favorable and will occur without an input of energy). More than a century of experiments has shown that only processes with a negative ∆G are spontaneous. For ∆G to be negative, ∆H must be negative (the system gives up enthalpy and H decreases) or T∆S must be positive (the system gives up order and S increases), or both: When ∆H and T∆S are tallied, ∆G has a negative value (∆G 6 0) for all spontaneous processes. In other words, every spontaneous process decreases the system’s free energy, and processes that have a positive or zero ∆G are never spontaneous. This information is immensely interesting to biologists, for it allows us to predict which kinds of change can happen without an input of energy. Such spontaneous changes can be harnessed to perform work. This principle is very important in the study of metabolism, where a major goal is to determine which reactions can supply energy for cellular work.

Free Energy, Stability, and Equilibrium As we saw in the previous section, when a process occurs spontaneously in a system, we can be sure that ∆G is negative. Another way to think of ∆G is to realize that it represents the difference between the free energy of the final state and the free energy of the initial state: ∆G = Gfinal state - Ginitial state Thus, ∆G can be negative only when the process involves a loss of free energy during the change from initial state to final state. Because it has less free energy, the system in its final state is less likely to change and is therefore more stable than it was previously.

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We can think of free energy as a measure of a system’s instability—its tendency to change to a more stable state. Unstable systems (higher G) tend to change in such a way that they become more stable (lower G). For example, a diver on top of a platform is less stable (more likely to fall) than when floating in the water; a drop of concentrated dye is less stable (more likely to disperse) than when the dye is spread randomly through the liquid; and a glucose molecule is less stable (more likely to break down) than the simpler molecules into which it can be split (Figure 8.5). Unless something prevents it, each of these systems will move toward greater stability: The diver falls, the solution becomes uniformly colored, and the glucose molecule is broken down into smaller molecules. Another term that describes a state of maximum stability is equilibrium, which you learned about in Concept 2.4 in connection with chemical reactions. There is an important relationship between free energy and equilibrium, including chemical equilibrium. Recall that most chemical reactions are reversible and proceed to a point at which the forward and backward reactions occur at the same rate. The reaction is then said to be at chemical equilibrium, and there is no further net change in the relative concentration of products and reactants. As a reaction proceeds toward equilibrium, the free energy of the mixture of reactants and products decreases. Free energy increases when a reaction is somehow pushed away from equilibrium, perhaps by removing some of the

products (and thus changing their concentration relative to that of the reactants). For a system at equilibrium, G is at its lowest possible value in that system. We can think of the equilibrium state as a free-energy valley. Any change from the equilibrium position will have a positive ∆G and will not be spontaneous. For this reason, systems never spontaneously move away from equilibrium. Because a system at equilibrium cannot spontaneously change, it can do no work. A process is spontaneous and can perform work only when it is moving toward equilibrium.

Free Energy and Metabolism We can now apply the free-energy concept more specifically to the chemistry of life’s processes.

Exergonic and Endergonic Reactions in Metabolism Based on their free-energy changes, chemical reactions can be classified as either exergonic (“energy outward”) or endergonic (“energy inward”). An exergonic reaction proceeds with a net release of free energy (Figure 8.6a). Because the chemical mixture loses free energy (G decreases), ∆G is negative for an exergonic reaction. Using ∆G as a standard for spontaneity, exergonic reactions are those that occur spontaneously. (Remember, the word spontaneous implies that

Figure 8.5  The relationship of free energy to stability, work capacity, and spontaneous change. Unstable systems (top) are rich in free energy, G. They have a tendency to change spontaneously to a more stable state (bottom), and it is possible to harness this “downhill” change to perform work.

• More free energy (higher G) • Less stable • Greater work capacity In a spontaneous change • The free energy of the system decreases (ΔG < 0) • The system becomes more stable • The released free energy can be harnessed to do work

• Less free energy (lower G) • More stable • Less work capacity

(a) Gravitational motion. Objects move spontaneously from a higher altitude to a lower one. MAKE CONNECTIONS

(b) Diffusion. Molecules in a drop of dye diffuse until they are randomly dispersed.

Compare the redistribution of molecules shown in (b) to the transport of hydrogen ions (H +) across a membrane by a proton pump, creating a concentration gradient, as shown in Figure 7.17. Which process(es) result(s) in higher free energy? Which system(s) can do work?

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(c) Chemical reaction. In a cell, a glucose molecule is broken down into simpler molecules.

Figure 8.6  Free energy changes (𝚫 G) in exergonic and endergonic reactions. (a) Exergonic reaction: energy released, spontaneous Reactants

Free energy

Amount of energy released (ΔG < 0)

Energy Products

Progress of the reaction (b) Endergonic reaction: energy required, nonspontaneous

Free energy

Products

Reactants

Energy

Amount of energy required (ΔG > 0)

Progress of the reaction Animation: Exergonic and Endergonic Reactions

it is energetically favorable, not that it will occur rapidly.) The magnitude of ∆G for an exergonic reaction represents the maximum amount of work the reaction can perform.* The greater the decrease in free energy, the greater the amount of work that can be done. We can use the overall reaction for cellular respiration as an example: C6H12O6 + 6 O2 S 6 CO2 + 6 H2O ∆G = -686 kcal/mol (-2,870 kJ/mol) For each mole (180 g) of glucose broken down by respiration under what are called “standard conditions” (1 M of each reactant and product, 25°C, pH 7), 686 kcal (2,870 kJ) of energy is made available for work. Because energy must be conserved, the chemical products of respiration store 686 kcal less free energy per mole than the reactants. The products are,

in a sense, the spent exhaust of a process that tapped the free energy stored in the bonds of the sugar molecules. It is important to realize that the breaking of bonds does not release energy; on the contrary, as you will soon see, it requires energy. The phrase “energy stored in bonds” is shorthand for the potential energy that can be released when new bonds are formed after the original bonds break, as long as the products are of lower free energy than the reactants. An endergonic reaction is one that absorbs free energy from its surroundings (Figure 8.6b). Because this kind of reaction essentially stores free energy in molecules (G increases), ∆G is positive. Such reactions are nonspontaneous, and the magnitude of ∆G is the quantity of energy required to drive the reaction. If a chemical process is exergonic (downhill), releasing energy in one direction, then the reverse process must be endergonic (uphill), using energy. A reversible process cannot be downhill in both directions. If ∆G = -686 kcal/mol for respiration, which converts glucose and oxygen to carbon dioxide and water, then the reverse process—the conversion of carbon dioxide and water to glucose and oxygen—must be strongly endergonic, with ∆G = +686 kcal/mol. Such a reaction would never happen by itself. How, then, do plants make the sugar that organisms use for energy? Plants get the required energy—686 kcal to make a mole of glucose—from the environment by capturing light and converting its energy to chemical energy. Next, in a long series of exergonic steps, they gradually spend that chemical energy to assemble glucose molecules.

Equilibrium and Metabolism Reactions in an isolated system eventually reach equilibrium and can then do no work, as illustrated by the isolated hydroelectric system in Figure 8.7. The chemical reactions of metabolism are reversible, and they, too, would reach equilibrium if they occurred in the isolation of a test tube. Because systems at equilibrium are at a minimum of G and can do no work, a cell that has reached metabolic equilibrium is dead! The fact that metabolism as a whole is never at equilibrium is one of the defining features of life.

Figure 8.7  Equilibrium and work in an isolated hydroelectric system. Water flowing downhill turns a turbine that drives a generator providing electricity to a lightbulb, but only until the system reaches equilibrium. ΔG < 0

ΔG = 0

*The word maximum qualifies this statement, because some of the free energy is released as heat and cannot do work. Therefore, ∆G represents a theoretical upper limit of available energy.

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Like most systems, a living cell is not in equilibrium. The constant flow of materials in and out of the cell keeps the metabolic pathways from ever reaching equilibrium, and the cell continues to do work throughout its life. This principle is illustrated by the open (and more realistic) hydroelectric system in Figure 8.8a. However, unlike this simple system in which water flowing downhill turns a single turbine, a catabolic pathway in a cell releases free energy in a series of reactions. An example is cellular respiration, illustrated by analogy in Figure 8.8b. Some of the reversible reactions of respiration are constantly “pulled” in one direction—that is, they are kept out of equilibrium. The key to maintaining this lack of equilibrium is that the product of a reaction does not accumulate but instead becomes a reactant in the next step; finally, waste products are expelled from the cell. The overall sequence of reactions is kept going by the huge free-energy difference between glucose and oxygen at the top of the energy “hill” and carbon dioxide and water at the “downhill” end. As long as our cells have a steady supply of glucose or other fuels and oxygen and are able to expel waste products to the surroundings, their metabolic pathways never reach equilibrium and can continue to do the work of life. Stepping back to look at the big picture, we can see once again how important it is to think of organisms as open systems. Sunlight provides a daily source of free energy for an ecosystem’s plants and other photosynthetic organisms.

Animals and other nonphotosynthetic organisms in an ecosystem must have a source of free energy in the form of the organic products of photosynthesis. Now that we have applied the free-energy concept to metabolism, we are ready to see how a cell actually performs the work of life.

Concept Check 8.2 1. Cellular respiration uses glucose and oxygen, which have high levels of free energy, and releases CO2 and water, which have low levels of free energy. Is cellular respiration spontaneous or not? Is it exergonic or endergonic? What happens to the energy released from glucose? 2. Visual Skills How would the processes of catabolism and anabolism relate to Figure 8.5c? 3. WHAT IF? Some nighttime partygoers wear glow-inthe-dark necklaces. The necklaces start glowing once they are “activated” by snapping the necklace in a way that allows two chemicals to react and emit light in the form of chemiluminescence. Is the chemical reaction exergonic or endergonic? Explain your answer. For suggested answers, see Appendix A.

Concept  8.3 ATP powers cellular work by coupling exergonic reactions to endergonic reactions A cell does three main kinds of work:

Figure 8.8  Equilibrium and work in open systems.

(a) An open hydroelectric system. Water flowing through a turbine keeps driving the generator because intake and outflow of water keep the system from reaching equilibrium.



ΔG < 0



ΔG < 0 ΔG < 0 ΔG < 0

Chemical work, the pushing of endergonic reactions that would not occur spontaneously, such as the synthesis of polymers from monomers (chemical work will be discussed further here; examples are shown in Chapters 9 and 10) Transport work, the pumping of substances across membranes against the direction of spontaneous movement (see Concept 7.4) Mechanical work, such as the beating of cilia (see Concept 6.6), the contraction of muscle cells, and the movement of chromosomes during cellular reproduction

A key feature in the way cells manage their energy resources to do this work is energy coupling, the use of an exergonic process to drive an endergonic one. ATP is responsible for mediating most energy coupling in cells, and in most cases it acts as the immediate source of energy that powers cellular work. Animation: Energy Coupling

(b) A multistep open hydroelectric system. Cellular respiration is analogous to this system: Glucose is broken down in a series of exergonic reactions that power the work of the cell. The product of each reaction is used as the reactant for the next, so no reaction reaches equilibrium.

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The Structure and Hydrolysis of ATP ATP (adenosine triphosphate) was introduced when we discussed the phosphate group as a functional group (see Concept 4.3). ATP contains the sugar ribose, with the

nitrogenous base adenine and a chain of three phosphate groups (the triphosphate group) bonded to it (Figure 8.9a). In addition to its role in energy coupling, ATP is also one of the nucleoside triphosphates used to make RNA (see Figure 5.23). The bonds between the phosphate groups of ATP can be broken by hydrolysis. When the terminal phosphate bond is broken by addition of a water molecule, a molecule of P i throughout inorganic phosphate (HOPO32-, abbreviated ~ this book) leaves the ATP, which becomes adenosine diphosphate, or ADP (Figure 8.9b). The reaction is exergonic and releases 7.3 kcal of energy per mole of ATP hydrolyzed: ATP + H2O S ADP + ~ Pi ∆G = -7.3 kcal/mol (-30.5 kJ/mol) This is the free-energy change measured under standard conditions. In the cell, conditions do not conform to standard Figure 8.9  The structure and hydrolysis of adenosine triphosphate (ATP). Throughout this book, the chemical structure of the triphosphate group seen in (a) will be represented by the three joined yellow circles shown in (b). Adenine N O –O

P

O O

O–

P O–

HC

O O

P

O

CH2

O–

N O

H

Triphosphate group (3 phosphate groups)

C C

NH2 C

N

N

H

H

H OH

Ribose

OH

(a) The structure of ATP. In the cell, most hydroxyl groups of phosphates are ionized (—O – ).

P

P

P

Adenosine triphosphate (ATP) H2O

P

P

Adenosine diphosphate (ADP)

+

Pi

+

Energy

Inorganic phosphate

(b) The hydrolysis of ATP. The reaction of ATP and water yields ADP and inorganic phosphate ( P i ) and releases energy. Animation: The Structure of ATP Animation: Space-Filling Model of ATP Animation: Stick Model of ATP

CH

conditions, primarily because reactant and product concentrations differ from 1 M. For example, when ATP hydrolysis occurs under cellular conditions, the actual ∆G is about -13 kcal/mol, 78% greater than the energy released by ATP hydrolysis under standard conditions. Because their hydrolysis releases energy, the phosphate bonds of ATP are sometimes referred to as high-energy phosphate bonds, but the term is misleading. The phosphate bonds of ATP are not unusually strong bonds, as “high-energy” may imply; rather, the reactants (ATP and water) themselves have high energy relative to the energy P i ). The release of energy during of the products (ADP and ~ the hydrolysis of ATP comes from the chemical change of the system to a state of lower free energy, not from the phosphate bonds themselves. ATP is useful to the cell because the energy it releases on losing a phosphate group is somewhat greater than the energy most other molecules could deliver. But why does this hydrolysis release so much energy? If we reexamine the ATP molecule in Figure 8.9a, we can see that all three phosphate groups are negatively charged. These like charges are crowded together, and their mutual repulsion contributes to the instability of this region of the ATP molecule. The triphosphate tail of ATP is the chemical equivalent of a compressed spring.

How the Hydrolysis of ATP Performs Work When ATP is hydrolyzed in a test tube, the release of free energy merely heats the surrounding water. In an organism, this same generation of heat can sometimes be beneficial. For instance, the process of shivering uses ATP hydrolysis during muscle contraction to warm the body. In most cases in the cell, however, the generation of heat alone would be an inefficient (and potentially dangerous) use of a valuable energy resource. Instead, the cell’s proteins harness the energy released during ATP hydrolysis in several ways to perform the three types of cellular work—chemical, transport, and mechanical. For example, with the help of specific enzymes, the cell is able to use the energy released by ATP hydrolysis directly to drive chemical reactions that, by themselves, are endergonic. If the ∆G of an endergonic reaction is less than the amount of energy released by ATP hydrolysis, then the two reactions can be coupled so that, overall, the coupled reactions are exergonic. This usually involves phosphorylation, the transfer of a phosphate group from ATP to some other molecule, such as the reactant. The recipient molecule with the phosphate group covalently bonded to it is then called a phosphorylated intermediate. The key to coupling exergonic and endergonic reactions is the formation of this phosphorylated intermediate, which is more

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Figure 8.10  How ATP drives chemical work: Energy coupling using ATP hydrolysis. In this example, the exergonic process of ATP hydrolysis is used to drive an endergonic process—the cellular synthesis of the amino acid glutamine from glutamic acid and ammonia. (a) Glutamic acid conversion to glutamine. Glutamine synthesis from glutamic acid (Glu) by itself is endergonic (ΔG is positive), so it is not spontaneous.

Glu

+

NH3

Glu

ΔGGlu = +3.4 kcal/mol

Glutamine

Glutamic acid Ammonia

(b) Conversion reaction coupled with ATP hydrolysis. In the cell, glutamine synthesis occurs in two steps, coupled by a phosphorylated intermediate. 1 ATP phosphorylates glutamic + Glu acid, making it less stable, with more free energy. 2 Ammonia displaces the phosphate group, Glutamic acid forming glutamine.

NH2

NH3

P

1

ATP

Glu

2

+ ADP

Glu

Phosphorylated intermediate

NH2

+ ADP + P i

Glutamine

ΔGGlu = +3.4 kcal/mol (c) Free-energy change for coupled reaction. ΔG for the glutamic acid conversion to glutamine (+3.4 kcal/mol) plus ΔG for ATP hydrolysis (–7.3 kcal/mol) gives the free-energy change for the overall reaction (–3.9 kcal/mol). Because the overall process is exergonic (net ΔG is negative), it occurs spontaneously.

Glu

+

NH3

+

ATP

ΔGGlu = +3.4 kcal/mol

Glu

NH2

+ ADP +

Pi

ΔGATP = –7.3 kcal/mol

+ ΔGATP = –7.3 kcal/mol Net ΔG = –3.9 kcal/mol

MAKE CONNECTIONS

Referring to Figure 5.14, explain why glutamine (Gln) is diagrammed as a glutamic acid (Glu) with an amino group attached.

reactive (less stable, with more free energy) than the original unphosphory­lated molecule (Figure 8.10). Transport and mechanical work in the cell are also nearly always powered by the hydrolysis of ATP. In these cases, ATP hydrolysis leads to a change in a protein’s shape and often its ability to bind another molecule. Sometimes this occurs via a phosphorylated intermediate, as seen for the transport protein in Figure 8.11a. In most instances of mechanical work involving motor proteins “walking” along cytoskeletal

elements (Figure 8.11b), a cycle occurs in which ATP is first bound noncovalently to the motor protein. Next, ATP is P i. Another ATP molecule hydrolyzed, releasing ADP and ~ can then bind. At each stage, the motor protein changes its shape and ability to bind the cytoskeleton, resulting in movement of the protein along the cytoskeletal track. Phosphorylation and dephosphorylation promote crucial protein shape changes during many other important cellular processes as well.

Figure 8.11  How ATP drives transport and mechanical work. ATP hydrolysis causes changes in the shapes and binding affinities of proteins. This can occur either (a) directly, by phosphorylation, as shown for a membrane protein carrying out active transport of a solute (see also Figure 7.15), or (b) indirectly, via noncovalent binding of ATP and its hydrolytic products, as is the case for motor proteins that move vesicles (and other organelles) along cytoskeletal “tracks” in the cell (see also Figure 6.21). Transport protein

Solute

Vesicle

ATP

ADP + P i P

Solute transported

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ADP + P i

ATP

Pi

(a) Transport work: ATP phosphorylates transport proteins, causing a shape change that allows transport of solutes.

152

ATP

Cytoskeletal track

Motor protein

Protein and vesicle moved

(b) Mechanical work: ATP binds noncovalently to motor proteins and then is hydrolyzed, causing a shape change that walks the motor protein forward.

Figure 8.12  The ATP cycle. Energy released by breakdown reactions (catabolism) in the cell is used to phosphorylate ADP, regenerating ATP. Chemical potential energy stored in ATP drives most cellular work. ATP synthesis from ADP + P i requires energy.

ATP

Energy from catabolism (exergonic, energy-releasing processes)

+ H O 2

ADP + P i

ATP hydrolysis to ADP + P i yields energy.

Energy for cellular work (endergonic, energy-consuming processes)

Animation: Metabolism Overview

The Regeneration of ATP An organism at work uses ATP continuously, but ATP is a renewable resource that can be regenerated by the addition of phosphate to ADP (Figure 8.12). The free energy required to phosphorylate ADP comes from exergonic breakdown reactions (catabolism) in the cell. This shuttling of inorganic phosphate and energy is called the ATP cycle, and it couples the cell’s energy-yielding (exergonic) processes to the energyconsuming (endergonic) ones. The ATP cycle proceeds at an astonishing pace. For example, a working muscle cell recycles its entire pool of ATP in less than a minute. That turnover represents 10 million molecules of ATP consumed and regenerated per second per cell. If ATP could not be regenerated by the phosphorylation of ADP, humans would use up nearly their body weight in ATP each day. Because both directions of a reversible process cannot be downhill, the regeneration of ATP is necessarily endergonic: P i S ATP + H2O ADP + ~

∆G = +7.3 kcal/mol (+30.5 kJ/mol) (standard conditions) Since ATP formation from ADP and ~ P i is not spontaneous, free energy must be spent to make it occur. Catabolic (exergonic) pathways, especially cellular respiration, provide the energy for the endergonic process of making ATP. Plants also use light energy to produce ATP. Thus, the ATP cycle is a revolving door through which energy passes during its transfer from catabolic to anabolic pathways.

Concept Check 8.3 1. How does ATP typically transfer energy from an exergonic to an endergonic reaction in the cell? 2. Which of the following has more free energy: glutamic acid + ammonia + ATP or glutamine + ADP + ~ P i? Explain your answer. 3. MAKE CONNECTIONS Does Figure 8.11a show passive or active transport? Explain. (See Concepts 7.3 and 7.4.) For suggested answers, see Appendix A.

Concept  8.4 Enzymes speed up metabolic reactions by lowering energy barriers The laws of thermodynamics tell us what will and will not happen under given conditions but say nothing about the rate of these processes. A spontaneous chemical reaction occurs without any requirement for outside energy, but it may occur so slowly that it is imperceptible. For example, even though the hydrolysis of sucrose (table sugar) to glucose and fructose is exergonic, occurring spontaneously with a release of free energy (∆G = -7 kcal/mol), a solution of sucrose dissolved in sterile water will sit for years at room temperature with no appreciable hydrolysis. However, if we add a small amount of the enzyme sucrase to the solution, then all the sucrose may be hydrolyzed within seconds, as shown here: Sucrase O

+

+

H2O

Sucrose (C12H22O11)

OH Glucose (C6H12O6 )

HO Fructose (C6H12O6 )

How does the enzyme do this? An enzyme is a macromolecule that acts as a catalyst, a chemical agent that speeds up a reaction without being consumed by the reaction. In this chapter, we are focusing on enzymes that are proteins. (Some RNA molecules, called ribozymes, can function as enzymes; these will be discussed in Concepts 17.3 and 25.1.) Without regulation by enzymes, chemical traffic through the pathways of metabolism would become terribly congested because many chemical reactions would take such a long time. In the next two sections, we will see why spontaneous reactions can be slow and how an enzyme changes the situation.

The Activation Energy Barrier Every chemical reaction between molecules involves both bond breaking and bond forming. For example, the hydrolysis of sucrose involves breaking the bond between glucose and fructose and one of the bonds of a water molecule and then forming two new bonds, as shown above. Changing one molecule into another generally involves contorting the starting molecule into a highly unstable state before the reaction can proceed. This contortion can be compared to the bending of a metal key ring when you pry it open to add a new key. The key ring is highly unstable in its opened form but returns to a stable state once the key is threaded all the way onto the ring. To reach the contorted state where bonds can change, reactant molecules must absorb energy from their surroundings. When the new bonds of the product molecules form, energy is released as heat, and the molecules return to stable shapes with lower energy than the contorted state.

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The initial investment of energy for starting a reaction— the energy required to contort the reactant molecules so the bonds can break—is known as the free energy of activation, or activation energy, abbreviated EA in this book. We can think of activation energy as the amount of energy needed to push the reactants to the top of an energy barrier, or uphill, so that the “downhill” part of the reaction can begin. Activation energy is often supplied by heat in the form of thermal energy that the reactant molecules absorb from the surroundings. The absorption of thermal energy accelerates the reactant molecules, so they collide more often and more forcefully. It also agitates the atoms within the molecules, making the breakage of bonds more likely. When the molecules have absorbed enough energy for the bonds to break, the reactants are in an unstable condition known as the transition state. Figure 8.13 graphs the energy changes for a hypothetical exergonic reaction that swaps portions of two reactant molecules: AB + CD S AC + BD Reactants Products The activation of the reactants is represented by the uphill portion of the graph, in which the free-energy content of

Figure 8.13  Energy profile of an exergonic reaction. The “molecules” are hypothetical, with A, B, C, and D representing portions of the molecules. Thermodynamically, this is an exergonic reaction, with a negative ∆G, and the reaction occurs spontaneously. However, the activation energy (EA) provides a barrier that determines the rate of the reaction. The reactants AB and CD must absorb enough energy from the surroundings to reach the unstable transition state, where bonds can break.

A

B

C

D

After bonds have broken, new bonds form, releasing energy to the surroundings.

Free energy

Transition state

A

B

C

D

EA

Reactants A

B ΔG < 0

C

D

Products Progress of the reaction DRAW IT Graph the progress of an endergonic reaction in which EF and GH form products EG and FH, assuming that the reactants must pass through a transition state.

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the reactant molecules is increasing. At the summit, when energy equivalent to EA has been absorbed, the reactants are in the transition state: They are activated, and their bonds can be broken. As the atoms then settle into their new, more stable bonding arrangements, energy is released to the surroundings. This corresponds to the downhill part of the curve, which shows the loss of free energy by the molecules. The overall decrease in free energy means that EA is repaid with dividends, as the formation of new bonds releases more energy than was invested in the breaking of old bonds. The reaction shown in Figure 8.13 is exergonic and occurs spontaneously (∆G 6 0). However, the activation energy provides a barrier that determines the rate of the reaction. The reactants must absorb enough energy to reach the top of the activation energy barrier before the reaction can occur. For some reactions, EA is modest enough that even at room temperature there is sufficient thermal energy for many of the reactant molecules to reach the transition state in a short time. In most cases, however, EA is so high and the transition state is reached so rarely that the reaction will hardly proceed at all. In these cases, the reaction will occur at a noticeable rate only if energy is provided, usually by heat. For example, the reaction of gasoline and oxygen is exergonic and will occur spontaneously, but energy is required for the molecules to reach the transition state and react. Only when the spark plugs fire in an automobile engine can there be the explosive release of energy that pushes the pistons. Without a spark, a mixture of gasoline hydrocarbons and oxygen will not react because the EA barrier is too high.

How Enzymes Speed Up Reactions Proteins, DNA, and other complex cellular molecules are rich in free energy and have the potential to decompose spontaneously; that is, the laws of thermodynamics favor their breakdown. These molecules only persist because at temperatures typical for cells, few molecules can make it over the hump of activation energy. The barriers for selected reactions must occasionally be surmounted, however, for cells to carry out the processes needed for life. Heat can increase the rate of a reaction by allowing reactants to attain the transition state more often, but this would not work well in biological systems. First, high temperature denatures proteins and kills cells. Second, heat would speed up all reactions, not just those that are needed. Instead of heat, organisms carry out catalysis, a process by which a catalyst (for example, an enzyme) selectively speeds up a reaction without itself being consumed. (You learned about catalysts in Concept 5.4.) An enzyme catalyzes a reaction by lowering the EA barrier (Figure 8.14), enabling the reactant molecules to absorb enough energy to reach the transition state even at moderate temperatures, as we’ll discuss shortly. It is crucial to note that an enzyme cannot change the ∆G for a reaction; it cannot make an endergonic reaction exergonic. Enzymes can only hasten reactions

Figure 8.14  The effect of an enzyme on activation energy. Without affecting the free-energy change (∆G) for a reaction, an enzyme speeds the reaction by reducing its activation energy (EA).

Free energy

Course of reaction without enzyme

EA without enzyme

EA with enzyme is lower

Reactants Course of reaction with enzyme

ΔG is unaffected by enzyme

substrate to the product (or products) of the reaction. The overall process can be summarized as follows: L

Enzyme + Substrate(s)

[email protected] substrate complex

L

Enzyme + Product(s)

Most enzyme names end in -ase. For example, the enzyme sucrase catalyzes the hydrolysis of the disaccharide sucrose into its two monosaccharides, glucose and fructose (see the diagram at the beginning of Concept 8.4): Sucrase + Sucrose + H2O

L

[email protected] [email protected] complex

L

Sucrase + Glucose + Fructose

Products

The reaction catalyzed by each enzyme is very specific; an enzyme can recognize its specific substrate even among closely related compounds. For instance, sucrase will act only on sucrose and will not bind to other disaccharides, such as Animation: How Enzymes Work maltose. What accounts for this molecular recognition? Recall that most enzymes are proteins, and proteins are macromolthat would eventually occur anyway, but this enables the cell ecules with unique three-dimensional configurations. The to have a dynamic metabolism, routing chemicals smoothly specificity of an enzyme results from its shape, which is a conthrough metabolic pathways. Also, enzymes are very specific sequence of its amino acid sequence. for the reactions they catalyze, so they determine which chemOnly a restricted region of the enzyme molecule actually ical processes will be going on in the cell at any given time. binds to the substrate. This region, called the active site, is typically a pocket or groove on the surface of the enzyme where Substrate Specificity of Enzymes catalysis occurs (Figure 8.15a; see also Figure 5.16). Usually, the The reactant an enzyme acts on is referred to as the enzyme’s active site is formed by only a few of the enzyme’s amino acids, substrate. The enzyme binds to its substrate (or subwith the rest of the protein molecule providing a framework strates, when there are two or more reactants), forming an that determines the shape of the active site. The specificity of an enzyme-substrate complex. While enzyme and substrate enzyme is attributed to a complementary fit between the shape are joined, the catalytic action of the enzyme converts the of its active site and the shape of the substrate. An enzyme is not a stiff structure locked into a given shape. In fact, recent work by biochemists has shown Figure 8.15  Induced fit between an enzyme and its substrate. that enzymes (and other proteins) seem to “dance” between subtly different shapes in Substrate a dynamic equilibrium, with slight differences in free energy for each “pose.” The shape that best fits the substrate isn’t necessarily the one with the lowActive site est energy, but during the very short time the enzyme takes on this shape, its active site can bind to the substrate. The active site itself is also not a rigid receptacle for the substrate. As the substrate enters the active site, the enzyme changes shape slightly due to interacEnzyme Enzyme-substrate tions between the substrate’s chemical complex groups and chemical groups on the side (a) In this space-filling model of the enzyme (b) When the substrate enters the active site, it chains of the amino acids that form the hexokinase (blue), the active site forms a forms weak bonds with the enzyme, groove on the surface. The enzyme’s inducing a change in the shape of the active site. This shape change makes substrate is glucose (red). protein. This change allows additional weak the active site fit even more snugly bonds to form, causing the active site to enfold the substrate and hold it in place. around the substrate (Figure 8.15b). Progress of the reaction

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The tightening of the binding after initial contact—called induced fit—is like a clasping handshake. Induced fit brings chemical groups of the active site into positions that enhance their ability to catalyze the chemical reaction.

Catalysis in the Enzyme’s Active Site In most enzymatic reactions, the substrate is held in the active site by so-called weak interactions, such as hydrogen bonds and ionic bonds. The R groups of a few of the amino acids that make up the active site catalyze the conversion of substrate to product, and the product departs from the active site. The enzyme is then free to take another substrate molecule into its active site. The entire cycle happens so fast that a single enzyme molecule typically acts on about 1,000 substrate molecules per second, and some enzymes are even faster. Enzymes, like other catalysts, emerge from the reaction in their original form. Therefore, very small amounts of enzyme can have a huge metabolic impact by functioning over and over again in catalytic cycles. Figure 8.16 shows a catalytic cycle involving two substrates and two products. Most metabolic reactions are reversible, and an enzyme can catalyze either the forward or the reverse reaction, depending on which direction has a negative ∆G. This in turn depends mainly on the relative concentrations of reactants and products. The net effect is always in the direction of equilibrium. Enzymes use a variety of mechanisms that lower activation energy and speed up a reaction (see Figure 8.16, step 3  ):







When there are two or more reactants, the active site provides a template on which the substrates can come together in the proper orientation for a reaction to occur between them. As the active site of an enzyme clutches the bound substrates, the enzyme may stretch the substrate molecules toward their transition state form, stressing and bending critical chemical bonds to be broken during the reaction. Because EA is proportional to the difficulty of breaking the bonds, distorting the substrate helps it approach the transition state and reduces the amount of free energy that must be absorbed to achieve that state. The active site may also provide a microenvironment that is more conducive to a particular type of reaction than the solution itself would be without the enzyme. For example, if the active site has amino acids with acidic R groups, the active site may be a pocket of low pH in an otherwise neutral cell. In such cases, an acidic amino acid may facilitate H+ transfer to the substrate as a key step in catalyzing the reaction. Amino acids in the active site directly participate in the chemical reaction. Sometimes this process even involves brief covalent bonding between the substrate and the

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Figure 8.16  The active site and catalytic cycle of an enzyme. An enzyme can convert one or more reactant molecules to one or more product molecules. The enzyme shown here converts two substrate molecules to two product molecules. 1 Substrates enter the active site; enzyme changes shape such that its active site enfolds the substrates (induced fit).

Substrates 6 Active site is available for two new substrate molecules.

2 Substrates are held in the active site by weak interactions, such as hydrogen bonds and ionic bonds.

Enzyme-substrate complex

3 The active site lowers EA and speeds up the reaction (see text).

Enzyme

45 Products are released. Products

4 Substrates are converted to products.

DRAW IT

The enzyme-substrate complex passes through a transition state (see Figure 8.13). Label the part of the cycle where the transition state occurs.

Animation: Enzymes: Steps in a Reaction

side chain of an amino acid of the enzyme. Subsequent steps of the reaction restore the side chains to their original states so that the active site is the same after the reaction as it was before. The rate at which a particular amount of enzyme converts substrate to product is partly a function of the initial concentration of the substrate: The more substrate molecules that are available, the more frequently they access the active sites of the enzyme molecules. However, there is a limit to how fast the reaction can be pushed by adding more substrate to a fixed concentration of enzyme. At some point, the concentration of substrate will be high enough that all enzyme molecules will have their active sites engaged. As soon as the product exits an active site, another substrate molecule enters. At this substrate concentration, the enzyme is said to be saturated, and the rate of the reaction is determined by the speed at which the active site converts substrate to product. When an enzyme population is saturated, the only way to increase the rate of product formation is to add more enzyme. Cells often increase the rate of a reaction by producing more enzyme molecules. You can graph the overall progress of an enzymatic reaction in the Scientific Skills Exercise.

Scientific Skills Exercise

Making a Line Graph and Calculating a Slope Does the Rate of Glucose 6-Phosphatase Activity Change over Time in Isolated Liver Cells?  Glucose 6-phosphatase, which is found in mammalian liver cells, is a key enzyme in control of blood glucose levels. The enzyme catalyzes the breakdown of glucose 6-phosphate into glucose and inorganic phosphate (  P  i ). These products are transported out of liver cells into the blood, increasing blood glucose levels. In this exercise, you will graph data from a time-course experiment that measured P  i concentration in the buffer outside isolated liver cells, thus indirectly measuring glucose 6-phosphatase activity inside the cells.

~

~

How the Experiment Was Done  Isolated rat liver cells were placed in a dish with buffer at physiological conditions (pH 7.4, 37°C). Glucose 6-phosphate (the substrate) was added so it could be taken up by cells and broken down by glucose 6-phosphatase. A sample of buffer was removed every 5 minutes and the concentration of P i that had been transported out of the cells was determined

~

Data from the Experiment Time (min)  0

Concentration of ~ P i (µmol/mL)   0

 5

 10

10

 90

15

180

20

270

25

330

30

355

35

355

40

355

Data from  S. R. Commerford et al., Diets enriched in sucrose or fat increase gluconeogenesis and G-6-Pase but not basal glucose production in rats, American Journal of Physiology - Endocrinology and Metabolism 283:E545–E555 (2002).

Interpret The Data 1. To see patterns in the data from a time-course experiment like this, it is helpful to graph the data. First, determine which set of

Effects of Local Conditions on Enzyme Activity The activity of an enzyme—how efficiently the enzyme functions—is affected by general environmental factors, such as temperature and pH. It can also be affected by chemicals that specifically influence that enzyme. In fact, researchers have learned much about enzyme function by employing such chemicals.

Effects of Temperature and pH Recall from Figure 5.20 that the three-dimensional structures of proteins are sensitive to their environment. As a consequence, each enzyme works better under some conditions than under other conditions, because these optimal conditions favor the most active shape for the enzyme.

data goes on each axis. (a) What did the researchers intentionally vary in the experiment? This is the independent variable, which goes on the x-axis. (b) What are the units (abbreviated) for the independent variable? Explain in words what the abbreviation stands for. (c) What was measured by the researchers? This is the dependent variable, which goes on the y-axis. (d) What does the units abbreviation stand for? Label each axis, including the units. 2. Next, you’ll want to mark off the axes with just enough evenly spaced tick marks to accommodate the full set of data. Determine the range of data values for each axis. (a) What is the largest value to go on the x-axis? What is a reasonable spacing for the tick marks, and what should be the highest one? (b) What is the largest value to go on the y-axis? What is a reasonable spacing for the tick marks, and what should be the highest one? 3. Plot the data points on your graph. Match each x-value with its partner y-value and place a point on the graph at that coordinate. Draw a line that connects the points. (For additional information about graphs, see the Scientific Skills Review in Appendix F.) 4. Examine your graph and look for patterns in the data. (a) Does the concentration of P  i increase evenly through the course of the experiment? To answer this question, describe the pattern you see in the graph. (b) What part of the graph shows the highest rate of enzyme activity? Consider that the rate of enzyme activity is related to the slope of the line, ∆y/∆x (the “rise” over the “run”), in µmol/(mL # min), with the steepest slope indicating the highest rate of enzyme activity. Calculate the rate of enzyme activity (slope) where the graph is steepest. (c) Can you think of a biological explanation for the pattern you see? 5. If your blood sugar level is low from skipping lunch, what reaction (discussed in this exercise) will occur in your liver cells? Write out the reaction and put the name of the enzyme over the reaction arrow. How will this reaction affect your blood sugar level?

~

Instructors: A version of this Scientific Skills Exercise can be assigned in MasteringBiology.

Temperature and pH are environmental factors important in the activity of an enzyme. Up to a point, the rate of an enzymatic reaction increases with increasing temperature, partly because substrates collide with active sites more frequently when the molecules move rapidly. Above that temperature, however, the speed of the enzymatic reaction drops sharply. The thermal agitation of the enzyme molecule disrupts the hydrogen bonds, ionic bonds, and other weak interactions that stabilize the active shape of the enzyme, and the protein molecule eventually denatures. Each enzyme has an optimal temperature at which its reaction rate is greatest. Without denaturing the enzyme, this temperature allows the greatest number of molecular collisions and the fastest conversion of the reactants to product molecules. Most human enzymes have optimal temperatures of about 35–40°C

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(close to human body temperature). The thermophilic bacteria that live in hot springs contain enzymes with optimal temperatures of 70°C or higher (Figure 8.17a). Just as each enzyme has an optimal temperature, it also has a pH at which it is most active. The optimal pH values for most Figure 8.17  Environmental factors affecting enzyme activity. Each enzyme has an optimal (a) temperature and (b) pH that favor the most active shape of the protein molecule.

enzymes fall in the range of pH 6–8, but there are exceptions. For example, pepsin, a digestive enzyme in the human stomach, works best at a very low pH. Such an acidic environment denatures most enzymes, but pepsin is adapted to maintain its functional three-dimensional structure in the acidic environment of the stomach. In contrast, trypsin, a digestive enzyme residing in the more alkaline environment of the human intestine would be denatured in the stomach (Figure 8.17b).

Cofactors Many enzymes require nonprotein helpers for catalytic activity, often for chemical processes like electron transfers that cannot easily be carried out by the amino acids in proteins. These adjuncts, called cofactors, may be bound tightly to the enzyme as permanent residents, or they may bind loosely and reversibly along with the substrate. The cofactors of some enzymes are inorganic, such as the metal atoms zinc, iron, and copper in ionic form. If the cofactor is an organic molecule, it is referred to, more specifically, as a coenzyme. Most vitamins are important in nutrition because they act as coenzymes or raw materials from which coenzymes are made.

Enzyme Inhibitors

Rate of reaction

Optimal temperature for typical human enzyme (37°C)

0

20

Optimal temperature for enzyme of thermophilic (heat-loving) bacteria (75°C)

40

60 80 100 120 Temperature (°C) (a) The photo shows thermophilic cyanobacteria (green) thriving in the hot water of a Nevada geyser. The graph compares the optimal temperatures for an enzyme from the thermophilic bacterium Thermus oshimai (75°C) and human enzymes (body temperature, 37°C).

Trypsin (intestinal enzyme)

Rate of reaction

Pepsin (stomach enzyme)

0

1

2

3

4

5 6 7 8 9 pH (b) This graph shows the rate of reaction for two digestive enzymes over a range of pH values.

10

INTERPRET THE DATA Looking at the graph in (b), what is the optimal pH for pepsin activity? Explain why natural selection might have resulted in the optimal pH for pepsin, a stomach enzyme (see Figure 3.11). What is the optimal pH for trypsin?

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Certain chemicals selectively inhibit the action of specific enzymes. Sometimes the inhibitor attaches to the enzyme by covalent bonds, in which case the inhibition is usually irreversible. Many enzyme inhibitors, however, bind to the enzyme by weak interactions, and when this occurs the inhibition is reversible. Some reversible inhibitors resemble the normal substrate molecule and compete for admission into the active site (Figure 8.18a and b). These mimics, called competitive inhibitors, reduce the productivity of enzymes by blocking substrates from entering active sites. This kind of inhibition can be overcome by increasing the concentration of substrate so that as active sites become available, more substrate molecules than inhibitor molecules are around to gain entry to the sites. In contrast, noncompetitive inhibitors do not directly compete with the substrate to bind to the enzyme at the active site (Figure 8.18c). Instead, they impede enzymatic reactions by binding to another part of the enzyme. This interaction causes the enzyme molecule to change its shape in such a way that the active site becomes much less effective at catalyzing the conversion of substrate to product. Toxins and poisons are often irreversible enzyme inhibitors. An example is sarin, a nerve gas. In the mid-1990s, terrorists released sarin in the Tokyo subway, killing several people and injuring many others. This small molecule binds covalently to the R group on the amino acid serine, which is found in the active site of acetylcholinesterase, an enzyme important in the nervous system. Other examples include the pesticides DDT and parathion, inhibitors of key enzymes in the nervous system. Finally, many antibiotics are inhibitors of specific

Figure 8.18  Inhibition of enzyme activity. (a) Normal binding Substrate Active site

A substrate can bind normally to the active site of an enzyme.

Enzyme

Figure 8.19  Mimicking evolution of an enzyme with a new function. Researchers tested whether the function of an enzyme called β-galactosidase, which breaks down the sugar lactose, could change over time in populations of the bacterium Escherichia coli. After seven rounds of mutation and selection in the lab, β-galactosidase evolved into an enzyme specialized for breaking down a sugar other than lactose. This ribbon model shows one subunit of the altered enzyme; six amino acids were different. Two changed amino acids were found near the active site.

Active site

(b) Competitive inhibition A competitive inhibitor mimics the substrate, competing for the active site.

Competitive inhibitor

Animation: Enzymes: Competitive Inhibition

Two changed amino acids were found in the active site.

(c) Noncompetitive inhibition A noncompetitive inhibitor binds to the enzyme away from the active site, altering the shape of the enzyme so that even if the substrate can bind, the active site functions less effectively, if at all. Animation: Enzymes: Noncompetitive Inhibition

Noncompetitive inhibitor

enzymes in bacteria. For instance, penicillin blocks the active site of an enzyme that many bacteria use to make cell walls. Citing enzyme inhibitors that are metabolic poisons may give the impression that enzyme inhibition is generally abnormal and harmful. In fact, molecules naturally present in the cell often regulate enzyme activity by acting as inhibitors. Such regulation—selective inhibition—is essential to the control of cellular metabolism, as we will discuss in Concept 8.5.

The Evolution of Enzymes Thus far, biochemists have identified more than 4,000 different enzymes in various species, most likely a very small fraction of all enzymes. How did this grand profusion of enzymes arise? Recall that most enzymes are proteins, and proteins are encoded by genes. A permanent change in a gene, known as a mutation, can result in a protein with one or more changed amino acids. In the case of an enzyme, if the changed amino acids are in the active site or some other crucial region, the altered enzyme might have a novel activity or might bind to a different substrate. Under environmental conditions where the new function benefits the organism, natural selection would tend to favor the mutated form of the gene, causing it to persist in the population. Evolution

Two changed amino acids were found on the surface.

This simplified model is generally accepted as the main way in which the multitude of different enzymes arose over the past few billion years of life’s history. Data supporting this model have been collected by researchers using a lab procedure that mimics evolution in natural populations (Figure 8.19).

Concept Check 8.4 1. Many spontaneous reactions occur very slowly. Why don’t all spontaneous reactions occur instantly? 2. Why do enzymes act only on very specific substrates? 3. WHAT IF? Malonate is an inhibitor of the enzyme succinate dehydrogenase. How would you determine whether malonate is a competitive or noncompetitive inhibitor? 4. DRAW IT A mature lysosome has an internal pH of around 4.5. Using Figure 8.17b as a guide, draw a graph showing what you would predict for the rate of reaction for a lysosomal enzyme. Label its optimal pH, assuming its optimal pH matches its environment. For suggested answers, see Appendix A.

Concept  8.5 Regulation of enzyme activity helps control metabolism Chemical chaos would result if all of a cell’s metabolic pathways were operating simultaneously. Intrinsic to life’s processes is a cell’s ability to tightly regulate its metabolic pathways by controlling when and where its various enzymes are active. It does this either by switching on and off the genes that encode specific enzymes (as we will discuss in Unit Three) or, as we discuss here, by regulating the activity of enzymes once they are made.

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Allosteric Regulation of Enzymes In many cases, the molecules that naturally regulate enzyme activity in a cell behave something like reversible noncompetitive inhibitors (see Figure 8.18c): These regulatory molecules change an enzyme’s shape and the functioning of its active site by binding to a site elsewhere on the molecule, via noncovalent interactions. Allosteric regulation is the term used to describe any case in which a protein’s function at one site is affected by the binding of a regulatory molecule to a separate site. It may result in either inhibition or stimulation of an enzyme’s activity.

Allosteric Activation and Inhibition Most enzymes known to be allosterically regulated are constructed from two or more subunits, each composed of a polypeptide chain with its own active site. The entire complex oscillates between two different shapes, one catalytically active and the other inactive (Figure 8.20a). In the simplest kind of allosteric regulation, an activating or inhibiting regulatory molecule binds to a regulatory site (sometimes called an allosteric site), often located where subunits join. The binding of an activator to a regulatory site stabilizes the shape that has functional active sites, whereas the binding of an inhibitor stabilizes the inactive form of the enzyme. The subunits of an allosteric enzyme fit together in such a way that a shape change in one subunit is transmitted to all others. Through this interaction of subunits, a single activator or inhibitor molecule that binds to one regulatory site will affect the active sites of all subunits. Fluctuating concentrations of regulators can cause a sophisticated pattern of response in the activity of cellular enzymes. P i), for example, play The products of ATP hydrolysis (ADP and ~ a complex role in balancing the flow of traffic between anabolic and catabolic pathways by their effects on key enzymes. ATP binds to several catabolic enzymes allosterically, lowering their affinity for substrate and thus inhibiting their activity. ADP, however, functions as an activator of the same enzymes. This is logical because catabolism functions in regenerating ATP. If ATP production lags behind its use, ADP accumulates and activates the enzymes that speed up catabolism, producing more ATP. If the supply of ATP exceeds demand, then catabolism slows down as ATP molecules accumulate and bind to the same enzymes, inhibiting them. (You’ll see specific examples of this type of regulation when you learn about cellular respiration in the next chapter; see, for example, Figure 9.20.) ATP, ADP, and other related molecules also affect key enzymes in anabolic pathways. In this way, allosteric enzymes control the rates of important reactions in both sorts of metabolic pathways. In another kind of allosteric activation, a substrate molecule binding to one active site in a multisubunit enzyme triggers a shape change in all the subunits, thereby increasing catalytic activity at the other active sites (Figure 8.20b). Called cooperativity, this mechanism amplifies the response of enzymes to substrates: One substrate molecule 160

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Figure 8.20  Allosteric regulation of enzyme activity. (a) Allosteric activators and inhibitors Allosteric activator stabilizes active form. Allosteric enzyme with four subunits

Regulatory site (one of four)

Active site (one of four)

Activator Active form

Stabilized active form

Oscillation Allosteric inhibitor stabilizes inactive form.

Nonfunctional active site

Inactive form

Inhibitor Stabilized inactive form

At low concentrations, activators and inhibitors dissociate from the enzyme. The enzyme can then oscillate again. (b) Cooperativity: another type of allosteric activation Binding of one substrate molecule to active site of one subunit locks all subunits in active conformation. Substrate

Inactive form

Stabilized active form

The inactive form shown on the left oscillates with the active form when the active form is not stabilized by substrate.

primes an enzyme to act on additional substrate molecules more readily. Cooperativity is considered allosteric regulation because, even though substrate is binding to an active site, its binding affects catalysis in another active site. Although hemoglobin is not an enzyme (it carries O2 rather than catalyzing a reaction), classic studies of hemoglobin have elucidated the principle of cooperativity. Hemoglobin is made up of four subunits, each with an oxygen-binding site (see Figure 5.18). The binding of an oxygen molecule to one

binding site increases the affinity for oxygen of the remaining binding sites. Thus, where oxygen is at high levels, such as in the lungs or gills, hemoglobin’s affinity for oxygen increases as more binding sites are filled. In oxygen-deprived tissues, however, the release of each oxygen molecule decreases the oxygen affinity of the other binding sites, resulting in the release of oxygen where it is most needed. Cooperativity works similarly in multisubunit enzymes that have been studied.

Figure 8.22  Organelles and structural order in metabolism. Organelles such as the mitochondrion (TEM) contain enzymes that carry out specific functions, in this case the second and third stages of cellular respiration.

Mitochondrion

Feedback Inhibition

Figure 8.21  Feedback inhibition in isoleucine synthesis. Initial substrate (threonine)

Active site available; pathway can proceed

Threonine in active site Enzyme 1 (threonine deaminase)

Isoleucine used up by cell Intermediate A Feedback inhibition

Isoleucine binds to allosteric site.

Active site of enzyme 1 is no longer able to catalyze the conversion of threonine to intermediate A; pathway is halted.

Enzyme 2 Intermediate B Enzyme 3 Intermediate C

The matrix contains enzymes in solution that are involved in the second stage of cellular respiration. Enzymes for the third stage of cellular respiration are embedded in the inner membrane.

1 μm

Earlier, we discussed the allosteric inhibition of an enzyme in an ATP-generating pathway by ATP itself. This is a common mode of metabolic control, called feedback inhibition, in which a metabolic pathway is halted by the inhibitory binding of its end product to an enzyme that acts early in the pathway. Figure 8.21 shows an example of feedback inhibition operating on an anabolic pathway. Some cells use this fivestep pathway to synthesize the amino acid isoleucine from threonine, another amino acid. As isoleucine accumulates, it slows down its own synthesis by allosterically inhibiting the enzyme for the first step of the pathway. Feedback inhibition thereby prevents the cell from making more isoleucine than is necessary and thus wasting chemical resources.

Localization of Enzymes Within the Cell The cell is not just a bag of chemicals with thousands of different kinds of enzymes and substrates in a random mix. The cell is compartmentalized, and cellular structures help bring order to metabolic pathways. In some cases, a team of enzymes for several steps of a metabolic pathway are assembled into a multienzyme complex. The arrangement facilitates the sequence of reactions, with the product from the first enzyme becoming the substrate for an adjacent enzyme in the complex, and so on, until the end product is released. Some enzymes and enzyme complexes have fixed locations within the cell and act as structural components of particular membranes. Others are in solution within particular membraneenclosed eukaryotic organelles, each with its own internal chemical environment. For example, in eukaryotic cells, the enzymes for the second and third stages of cellular respiration reside in specific locations within mitochondria (Figure 8.22). In this chapter, you have learned about the laws of thermo­ dynamics that govern metabolism, the intersecting set of chemical pathways characteristic of life. We have explored the bioenergetics of breaking down and building up biological molecules. To continue the theme of bioenergetics, we will next examine cellular respiration, the major catabolic pathway that breaks down organic molecules and releases energy that can be used for the crucial processes of life.

Enzyme 4

Concept Check 8.5

Intermediate D Enzyme 5

End product (isoleucine)

1. How do an activator and an inhibitor have different effects on an allosterically regulated enzyme? 2. WHAT IF? Regulation of isoleucine synthesis is an example of feedback inhibition of an anabolic pathway. With that in mind, explain how ATP might be involved in feedback inhibition of a catabolic pathway.

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For suggested answers, see Appendix A.

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8

Chapter Review

Go to MasteringBiology™ for Videos, Animations, Vocab Self-Quiz, Practice Tests, and more in the Study Area.

Summary of Key Concepts Concept  8.1









? VOCAB SELF-QUIZ

goo.gl/6u55ks

Metabolism is the collection of chemical reactions that occur in an organism. Enzymes catalyze reactions in intersecting metabolic pathways, which may be catabolic (breaking down molecules, releasing energy) or anabolic (building molecules, consuming energy). Bioenergetics is the study of the flow of energy through living organisms. Energy is the capacity to cause change; some forms of energy do work by moving matter. Kinetic energy is associated with motion and includes thermal energy associated with random motion of atoms or molecules. Heat is thermal energy in transfer from one object to another. Potential energy is related to the location or structure of matter and includes chemical energy possessed by a molecule due to its structure. The first law of thermodynamics, conservation of energy, states that energy cannot be created or destroyed, only transferred or transformed. The second law of thermodynamics states that spontaneous processes, those requiring no outside input of energy, increase the entropy (molecular disorder) of the universe.

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A living system’s free energy is energy that can do work under cellular conditions. The change in free energy (∆G) during a biological process is related directly to enthalpy change (∆H) and to the change in entropy (∆S): ∆G = ∆H - T∆S. Organisms live at the expense of free energy. A spontaneous process occurs with no energy input; during such a process, free energy decreases and the stability of a system increases. At maximum stability, the system is at equilibrium and can do no work. In an exergonic (spontaneous) chemical reaction, the products have less free energy than the reactants (-∆G). Endergonic (nonspontaneous) reactions require an input of energy (+∆G). The addition of starting materials and the removal of end products prevent metabolism from reaching equilibrium.

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Explain the meaning of each component in the equation for the change in free energy of a spontaneous chemical reaction. Why are spontaneous reactions important in the metabolism of a cell?



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Unit two 

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EA with enzyme is lower

Reactants

ΔG is unaffected by enzyme

Course of reaction with enzyme

Progress of the reaction







Each enzyme has a unique active site that binds one or more substrate(s), the reactants on which it acts. It then changes shape, binding the substrate(s) more tightly (induced fit). The active site can lower an EA barrier by orienting substrates correctly, straining their bonds, providing a favorable micro­ environment, or even covalently bonding with the substrate. Each enzyme has an optimal temperature and pH. Inhibitors reduce enzyme function. A competitive inhibitor binds to the active site, whereas a noncompetitive inhibitor binds to a different site on the enzyme. Natural selection, acting on organisms with variant enzymes, is responsible for the diversity of enzymes found in organisms.

?

How do both activation energy barriers and enzymes help maintain the structural and metabolic order of life?

Concept  8.5

Regulation of enzyme activity helps control metabolism (pp. 159–161)

Concept  8.3

ATP is the cell’s energy shuttle. Hydrolysis of its terminal phosphate yields ADP and ~ P i and releases free energy. Through energy coupling, the exergonic process of ATP hydrolysis drives endergonic reactions by transfer of a phosphate group to specific reactants, forming a phosphorylated intermediate

EA without enzyme

Products

ATP powers cellular work by coupling exergonic reactions to endergonic reactions (pp. 150–153)

In a chemical reaction, the energy necessary to break the bonds of the reactants is the activation energy, EA. Enzymes lower the EA barrier: Course of reaction without enzyme



The free-energy change of a reaction tells us whether or not the reaction occurs spontaneously (pp. 147–150)



Concept  8.4

Enzymes speed up metabolic reactions by lowering energy barriers (pp. 153–159)

Explain how the highly ordered structure of a cell does not conflict with the second law of thermodynamics.

Concept  8.2



Describe the ATP cycle: How is ATP used and regenerated in a cell?

Free energy

An organism’s metabolism transforms matter and energy, subject to the laws of thermodynamics (pp. 144–147)

that is more reactive. ATP hydrolysis (sometimes with protein phosphorylation) also causes changes in the shape and binding affinities of transport and motor proteins. Catabolic pathways drive regeneration of ATP from ADP + ~ P i.



Many enzymes are subject to allosteric regulation: Regulatory molecules, either activators or inhibitors, bind to specific regulatory sites, affecting the shape and function of the enzyme. In cooperativity, binding of one substrate molecule can stimulate binding or activity at other active sites. In feedback inhibition, the end product of a metabolic pathway allosterically inhibits the enzyme for a previous step in the pathway. Some enzymes are grouped into complexes, some are incorporated into membranes, and some are contained inside organelles, increasing the efficiency of metabolic processes.

?

What roles do allosteric regulation and feedback inhibition play in the metabolism of a cell?

8. EVOLUTION CONNECTION  Some people argue that biochemical pathways are too complex to have evolved because all intermediate steps in a given pathway must be present to produce the final product. Critique this argument. How could you use the diversity of metabolic pathways that produce the same or similar products to support your case?

Test Your Understanding Level 1: Knowledge/Comprehension 1. Choose the pair of terms that correctly completes this sentence: Catabolism is to anabolism as _________ is to _________. (A) exergonic; spontaneous (B) exergonic; endergonic (C) free energy; entropy (D) work; energy

PRACTICE TEST

goo.gl/CUYGKD

2. Most cells cannot harness heat to perform work because (A) heat does not involve a transfer of energy. (B) cells do not have much thermal energy; they are relatively cool. (C) temperature is usually uniform throughout a cell. (D) heat can never be used to do work. 3. Which of the following metabolic processes can occur without a net influx of energy from some other process? (A) ADP + ~ P i S ATP + H2O (B) C6H12O6 + 6 O2 S 6 CO2 + 6 H2O (C) 6 CO2 + 6 H2O S C6H12O6 + 6 O2 (D) Amino acids S Protein 4. If an enzyme in solution is saturated with substrate, the most effective way to obtain a faster yield of products is to (A) add more of the enzyme. (B) heat the solution to 90°C. (C) add more substrate. (D) add a noncompetitive inhibitor.

9. SCIENTIFIC INQUIRY • DRAW IT  A researcher has developed an assay to measure the activity of an important enzyme present in pancreatic cells growing in culture. She adds the enzyme’s substrate to a dish of cells and then measures the appearance of reaction products. The results are graphed as the amount of product on the y-axis versus time on the x-axis. The researcher notes four sections of the graph. For a short period of time, no products appear (section A). Then (section B) the reaction rate is quite high (the slope of the line is steep). Next, the reaction gradually slows down (section C). Finally, the graph line becomes flat (section D). Draw and label the graph, and propose a model to explain the molecular events occurring at each stage of this reaction profile. 10. WRITE ABOUT A THEME: ENERGY AND MATTER Life requires energy. In a short essay (100–150 words), describe the basic principles of bioenergetics in an animal cell. How is the flow and transformation of energy different in a photosynthesizing cell? Include the role of ATP and enzymes in your discussion. 11. SYNTHESIZE YOUR KNOWLEDGE

5. Some bacteria are metabolically active in hot springs because (A) they are able to maintain a lower internal temperature. (B) high temperatures make catalysis unnecessary. (C) their enzymes have high optimal temperatures. (D) their enzymes are completely insensitive to temperature.

Level 2: Application/Analysis 6. If an enzyme is added to a solution where its substrate and product are in equilibrium, what will occur? (A) Additional substrate will be formed. (B) The reaction will change from endergonic to exergonic. (C) The free energy of the system will change. (D) Nothing; the reaction will stay at equilibrium.

Level 3: Synthesis/Evaluation 7. DRAW IT  Using a series of arrows, draw the branched metabolic reaction pathway described by the following statements, and then answer the question at the end. Use red arrows and minus signs to indicate inhibition. L can form either M or N. M can form O. O can form either P or R. P can form Q. R can form S. O inhibits the reaction of L to form M. Q inhibits the reaction of O to form P.

Explain what is happening in this photo in terms of kinetic energy and potential energy. Include the energy conversions that occur when the penguins eat fish and climb back up on the glacier. Describe the role of ATP and enzymes in the underlying molecular processes, including what happens to the free energy of some of the molecules involved. For selected answers, see Appendix A. For additional practice questions, check out the Dynamic Study Modules in MasteringBiology. You can use them to study on your smartphone, tablet, or computer anytime, anywhere!

S inhibits the reaction of O to form R. Which reaction would prevail if both Q and S were present in the cell in high concentrations? (A) L S M (B) M S O (C) L S N (D) O S P

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Superset

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Figure 9.1  How does food, like the sand eels captured by this puffin, power the work of life?

Key Concepts 9.1

Catabolic pathways yield energy by oxidizing organic fuels

9.2

Glycolysis harvests chemical energy by oxidizing glucose to pyruvate

9.3

After pyruvate is oxidized, the citric acid cycle completes the energy-yielding oxidation of organic molecules

9.4

During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis

9.5

Fermentation and anaerobic respiration enable cells to produce ATP without the use of oxygen

9.6

Glycolysis and the citric acid cycle connect to many other metabolic pathways

BioFlix® Animation: Introduction to Cellular Respiration

164

Life Is Work Living cells require transfusions of energy from outside sources to perform their many tasks—for example, assembling polymers, pumping substances across membranes, moving, and ­reproducing. The puffin in Figure 9.1 obtains energy for its cells by feeding upon sand eels and other aquatic organisms; many other animals obtain energy by feeding on photosynthetic organisms such as plants and algae. The energy stored in the organic molecules of food ultimately comes from the sun. Energy flows into an ecosystem as sunlight and exits as heat; in contrast, the chemical elements essential to life are recycled (Figure 9.2). Photosynthesis generates oxygen, as well as organic molecules used by the mitochondria of eukaryotes as fuel for cellular respiration. Respiration breaks this fuel down, using oxygen and generating ATP. The waste products of this type of r­ espiration, carbon dioxide and water, are the raw materials for photosynthesis. In this chapter, we consider how cells harvest the chemical energy stored in organic molecules and use it to generate ATP, the molecule that drives most cellular work. After presenting some basic information about respiration, we’ll focus on three key pathways of respiration: glycolysis, pyruvate oxidation and the citric acid cycle, and oxidative phosphorylation. We’ll also consider fermentation, a somewhat simpler pathway coupled to glycolysis that has deep evolutionary roots. When you see this blue icon, log in to MasteringBiology and go to the Study Area for digital resources.

Get Ready for This Chapter

Figure 9.2  Energy flow and chemical recycling in ecosystems. Energy flows into an ecosystem as sunlight and ultimately leaves as heat, while the chemical elements essential to life are recycled. Light energy ECOSYSTEM

CO2 + H2O

Photosynthesis in chloroplasts Cellular respiration in mitochondria

ATP

Organic + O2 molecules

ATP powers most cellular work

Heat energy Animation: Energy Flow and Chemical Recycling

Concept  9.1 Catabolic pathways yield energy by oxidizing organic fuels Metabolic pathways that release stored energy by breaking down complex molecules are called catabolic pathways (see Concept 8.1). Transfer of electrons from fuel molecules (like glucose) to other molecules plays a major role in these pathways. In this section, we consider these processes, which are central to cellular respiration.

Catabolic Pathways and Production of ATP Organic compounds possess potential energy as a result of the arrangement of electrons in the bonds between their atoms. Compounds that can participate in exergonic reactions can act as fuels. Through the activity of enzymes, a cell systematically degrades complex organic molecules that are rich in potential energy to simpler waste products that have less energy. Some of the energy taken out of chemical storage can be used to do work; the rest is dissipated as heat. One catabolic process, fermentation, is a partial degradation of sugars or other organic fuel that occurs without the use of oxygen. However, the most efficient catabolic pathway is aerobic respiration, in which oxygen is consumed as a reactant along with the organic fuel (aerobic is from the Greek aer, air, and bios, life). The cells of most eukaryotic and many prokaryotic organisms can carry out aerobic respiration. Some prokaryotes use substances other than oxygen as reactants in a similar process that harvests chemical energy without oxygen;

this process is called anaerobic respiration (the prefix an- means “without”). Technically, the term cellular respiration includes both aerobic and anaerobic processes. However, it originated as a synonym for aerobic respiration because of the relationship of that process to organismal respiration, in which an animal breathes in oxygen. Thus, cellular respiration is often used to refer to the aerobic process, a practice we follow in most of this chapter. Although very different in mechanism, aerobic respiration is in principle similar to the combustion of gasoline in an automobile engine after oxygen is mixed with the fuel (hydrocarbons). Food provides the fuel for respiration, and the exhaust is carbon dioxide and water. The overall process can be summarized as follows: Organic Carbon + Oxygen S + Water + Energy compounds dioxide Carbohydrates, fats, and protein molecules from food can all be processed and consumed as fuel, as we will discuss later in the chapter. In animal diets, a major source of carbohydrates is starch, a storage polysaccharide that can be broken down into glucose (C6H12O6) subunits. Here, we will learn the steps of cellular respiration by tracking the degradation of the sugar glucose: C6H12O6 + 6 O2 S 6 CO2 + 6 H2O + Energy (ATP + heat) This breakdown of glucose is exergonic, having a free-energy change of -686 kcal (2,870 kJ) per mole of glucose decomposed (∆G = -686 kcal/mol). Recall that a negative ∆G (∆G 6 0) ­indicates that the products of the chemical process store less energy than the reactants and that the reaction can happen spontaneously—in other words, without an input of energy. Catabolic pathways do not directly move flagella, pump solutes across membranes, polymerize monomers, or perform other cellular work. Catabolism is linked to work by a chemical drive shaft—ATP (see Concept 8.3). To keep working, the cell must regenerate its supply of ATP from ADP and P i (see Figure 8.12). To understand how cellular respiration ~ accomplishes this, let’s examine the fundamental chemical processes known as oxidation and reduction.

Redox Reactions: Oxidation and Reduction How do the catabolic pathways that decompose glucose and other organic fuels yield energy? The answer is based on the transfer of electrons during the chemical reactions. The relocation of electrons releases energy stored in organic molecules, and this energy ultimately is used to ­synthesize ATP.

The Principle of Redox In many chemical reactions, there is a transfer of one or more electrons (e-) from one reactant to another. These ­electron transfers are called oxidation-reduction reactions,

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or redox reactions for short. In a redox reaction, the loss of electrons from one substance is called oxidation, and the addition of electrons to another substance is known as reduction. (Note that adding electrons is called reduction; adding negatively charged electrons to an atom reduces the amount of positive charge of that atom.) To take a simple, nonbiological example, consider the reaction between the elements sodium (Na) and chlorine (Cl) that forms table salt:

Figure 9.3  Methane combustion as an energy-yielding redox reaction. The reaction releases energy to the surroundings because the electrons lose potential energy when they end up being shared unequally, spending more time near electronegative atoms such as oxygen. Reactants becomes oxidized

Cl

+

Na+

+ becomes reduced (gains electron)

becomes oxidized

Y

Cl–

X

+ Ye –

becomes reduced

In the generalized reaction, substance Xe-, the electron donor, is called the reducing agent; it reduces Y, which accepts the donated electron. Substance Y, the electron acceptor, is the oxidizing agent; it oxidizes Xe- by removing its electron. Because an electron transfer requires both an electron donor and an acceptor, oxidation and reduction always go hand in hand. Not all redox reactions involve the complete transfer of electrons from one substance to another; some change the degree of electron sharing in covalent bonds. Methane combustion, shown in Figure 9.3, is an example. The covalent electrons in methane are shared nearly equally between the bonded atoms because carbon and hydrogen have about the same affinity for valence electrons; they are about equally electronegative (see Concept 2.3). But when methane reacts with oxygen, forming carbon dioxide, electrons end up shared less equally between the carbon atom and its new covalent partners, the oxygen atoms, which are very electronegative. In effect, the carbon atom has partially “lost” its shared electrons; thus, methane has been oxidized. Now let’s examine the fate of the reactant O2. The two atoms of the oxygen molecule (O2) share their electrons equally. But when oxygen reacts with the hydrogen from methane, forming water, the electrons of the covalent bonds spend more time near the oxygen (see Figure 9.3). In effect, each oxygen atom has partially “gained” electrons, so the oxygen molecule has been reduced. Because oxygen is so electronegative, it is one of the most powerful of all oxidizing agents. Energy must be added to pull an electron away from an atom, just as energy is required to push a ball uphill. The more electronegative the atom (the stronger its pull on electrons), the more energy is required to take an electron away from it. An electron loses potential energy when it shifts from a less electronegative atom toward a more electronegative one, just as a ball loses potential energy when it rolls downhill. A redox reaction that moves electrons closer to oxygen, 166

Unit two 

The Cell

2 O2

CO2 +

H

C

Energy + 2 H2O

becomes reduced

H

We could generalize a redox reaction this way: Xe – +

+

CH4

becomes oxidized (loses electron)

Na

Products

H

O

O

O

C

O H

O

H

H Methane (reducing agent)

Oxygen (oxidizing agent)

Carbon dioxide

Water

Animation: Redox Reactions

such as the burning (oxidation) of methane, therefore releases chemical energy that can be put to work.

Oxidation of Organic Fuel Molecules During Cellular Respiration The oxidation of methane by oxygen is the main combustion reaction that occurs at the burner of a gas stove. The combustion of gasoline in an automobile engine is also a redox reaction; the energy released pushes the pistons. But the energy-yielding redox process of greatest interest to biologists is respiration: the oxidation of glucose and other molecules in food. Examine again the summary equation for cellular ­respiration, but this time think of it as a redox process: becomes oxidized

C6H12O6

+ 6 O2

6 CO2 + 6 H2O + Energy

becomes reduced

As in the combustion of methane or gasoline, the fuel (glucose) is oxidized and oxygen is reduced. The electrons lose potential energy along the way, and energy is released. In general, organic molecules that have an abundance of hydrogen are excellent fuels because their bonds are a source of “hilltop” electrons, whose energy may be released as these electrons “fall” down an energy gradient during their transfer to oxygen. The summary equation for respiration indicates that hydrogen is transferred from glucose to oxygen. But the important point, not visible in the summary equation, is that the energy state of the electron changes as hydrogen (with its electron) is transferred to oxygen. In respiration, the oxidation of glucose transfers electrons to a lower energy state, liberating energy that becomes available for ATP synthesis. So, in general, we see fuels with multiple C—H bonds oxidized into products with multiple C—O bonds.

The main energy-yielding foods—carbohydrates and fats— are reservoirs of electrons associated with hydrogen, often in the form of C—H bonds. Only the barrier of activation energy holds back the flood of electrons to a lower energy state (see Figure 8.13). Without this barrier, a food substance like glucose would combine almost instantaneously with O2. If we supply the activation energy by igniting glucose, it burns in air, releasing 686 kcal (2,870 kJ) of heat per mole of glucose (about 180 g). Body temperature is not high enough to initiate burning, of course. Instead, if you swallow some glucose, enzymes in your cells will lower the barrier of activation energy, allowing the sugar to be oxidized in a series of steps.

Stepwise Energy Harvest via NAD1 and the Electron Transport Chain If energy is released from a fuel all at once, it cannot be harnessed efficiently for constructive work. For example, if a gasoline tank explodes, it cannot drive a car very far. Cellular respiration does not oxidize glucose (or any other organic fuel) in a single explosive step either. Rather, glucose is broken down in a series of steps, each one catalyzed by an enzyme. At key steps, electrons are stripped from the glucose. As is often the case in oxidation reactions, each electron travels with a proton—thus, as a hydrogen atom. The hydrogen atoms are not transferred directly to oxygen, but instead are usually passed first to an electron carrier, a coenzyme called nicotinamide adenine dinucleotide, a derivative of the vitamin niacin. This coenzyme is well suited as an electron carrier because it can cycle easily between its oxidized form, NAD1, and its reduced form, NADH. As an electron acceptor, NAD+ functions as an oxidizing agent during respiration. How does NAD+ trap electrons from glucose and the other organic molecules in food? Enzymes called dehydrogenases remove a pair of hydrogen atoms (2 electrons and 2 protons)

from the substrate (glucose, in the preceding example), thereby oxidizing it. The enzyme delivers the 2 electrons along with 1 proton to its coenzyme, NAD+, forming NADH (Figure 9.4). The other proton is released as a hydrogen ion (H+) into the surrounding solution: H C OH + NAD+

NAD+

CH2

O P

O–

O O

P O

O

NH2

N+ Nicotinamide (oxidized form)

H

+ 2[H] (from food)

Reduction of NAD+ Oxidation of NADH

H

H

O C

VISUAL SKILLS

NH2

+

H+

N Nicotinamide (reduced form)

Describe the structural differences between the oxidized form and the reduced form of nicotinamide.

H HO

O–

H+

NADH

C

O

2 e– + H+

Dehydrogenase

O

C O + NADH + H+

By receiving 2 negatively charged electrons but only 1 positively charged proton, the nicotinamide portion of NAD+ has its charge neutralized when NAD+ is reduced to NADH. The name NADH shows the hydrogen that has been received in the reaction. NAD+ is the most versatile electron acceptor in cellular respiration and functions in several of the redox steps during the breakdown of glucose. Electrons lose very little of their potential energy when they are transferred from glucose to NAD+. Each NADH ­molecule formed during respiration represents stored energy. This energy can be tapped to make ATP when the electrons complete their “fall” in a series of steps down an energy ­gradient from NADH to oxygen. How do electrons that are extracted from glucose and stored as potential energy in NADH finally reach oxygen? It will help to compare the redox chemistry of cellular ­respiration to a much simpler reaction: the reaction between hydrogen and oxygen to form water (Figure 9.5a). Mix H2 and O2, provide a spark for activation energy, and the gases combine explosively. In fact, combustion of liquid H2 and O2 is harnessed to help power the rocket engines that boost satellites into orbit and launch spacecraft. The explosion represents a release of energy as the electrons of hydrogen “fall” closer to the electronegative oxygen atoms. Cellular respiration also brings hydrogen and oxygen together to form water, but there are two important differences. First, in cellular respiration, the hydrogen that reacts with oxygen

2 e– + 2 H+

H

Dehydrogenase

OH

CH2

NH2 N

N

H N

O

H

H HO

OH

N

H

Figure 9.4  NAD1 as an electron shuttle. The full name for NAD+, nicotinamide adenine dinucleotide, describes its structure—the molecule consists of two nucleotides joined together at their phosphate groups (shown in yellow). (Nicotinamide is a nitrogenous base, although not one that is present in DNA or RNA; see Figure 5.23.) The enzymatic transfer of 2 electrons and 1 proton (H+) from an organic molecule in food to NAD+ reduces the NAD+ to NADH: Most of the electrons removed from food are transferred initially to NAD+, forming NADH.

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down the chain in an energy-yielding tumble analogous to gravity pulling objects downhill. In summary, during cellular respiration, most electrons travel the following “downhill” route: glucose S NADH S electron transport chain S oxygen. Later in this chapter, you will learn more about how the cell uses the energy released from this exergonic electron fall to regenerate its supply of ATP. For now, having covered the basic redox mechanisms of cellular respiration, let’s look at the entire process by which energy is harvested from organic fuels.

The Stages of Cellular Respiration: A Preview The harvesting of energy from glucose by cellular respiration is a cumulative function of three metabolic stages. We list them here along with a color-coding scheme we will use throughout the chapter to help you keep track of the big picture: 1. GLYCOLYSIS (color-coded blue throughout the chapter) 2. PYRUVATE OXIDATION and the CITRIC ACID CYCLE (color-coded light orange and dark orange)

3. OXIDATIVE PHOSPHORYLATION: Electron transport and chemiosmosis (color-coded purple)

Unit two 

The Cell

t spor tran tron ain ch

168

Free energy, G

Biochemists usually reserve the term cellular respiration for stages 2 and 3 together. In this text, however, we include glycolysis as a part of cellular respiration because most respiring cells deriving energy from glucose use glycolysis to produce the starting material for the citric acid cycle. As diagrammed in Figure 9.6, glycolysis and then pyruvate oxidation Figure 9.5  An introduction to electron transport chains. and the citric acid cycle are the catabolic 1 pathways that break down glucose and + /2 O2 H2 + 1/2 O2 2H other organic fuels. Glycolysis, which (from food via NADH) occurs in the cytosol, begins the degradaControlled release of tion process by breaking glucose into two 2 H+ + 2 e– energy for molecules of a compound called pyruvate. synthesis of ATP In eukaryotes, pyruvate enters the mitoATP chondrion and is oxidized to a compound Explosive ATP called acetyl CoA, which enters the citric release of acid cycle. There, the breakdown of heat and light ATP energy glucose to carbon dioxide is completed. (In prokaryotes, these processes take place 2 e– in the cytosol.) Thus, the carbon dioxide 12 O 2 2 H+ produced by respiration represents fragments of oxidized organic molecules. Some of the steps of glycolysis and the H2O H2O citric acid cycle are redox reactions in which dehydrogenases transfer electrons (a) Uncontrolled reaction. (b) Cellular respiration. In cellular respiration, the The one-step exergonic reaction same reaction occurs in stages: An electron from substrates to NAD+ or the related of hydrogen with oxygen to transport chain breaks the “fall” of electrons in this electron carrier FAD, forming NADH or form water releases a large reaction into a series of smaller steps and stores amount of energy in the form some of the released energy in a form that can be FADH2. (You’ll learn more about FAD of heat and light: an explosion. used to make ATP. (The rest of the energy is and FADH2 later.) In the third stage of released as heat.) respiration, the electron transport chain Elec

Free energy, G

is derived from organic molecules rather than H2. Second, instead of occurring in one explosive reaction, respiration uses an electron transport chain to break the fall of electrons to oxygen into several energy-releasing steps (Figure 9.5b). An electron transport chain consists of a number of molecules, mostly proteins, built into the inner membrane of the mitochondria of eukaryotic cells (and the plasma membrane of respiring prokaryotes). Electrons removed from glucose are shuttled by NADH to the “top,” higher-energy end of the chain. At the “bottom,” lower-energy end, O2 captures these electrons along with hydrogen nuclei (H+), forming water. (Anaerobically respiring prokaryotes have an electron acceptor at the end of the chain that is different from O2.) Electron transfer from NADH to oxygen is an exergonic reaction with a free-energy change of -53 kcal/mol (-222 kJ/mol). Instead of this energy being released and wasted in a single explosive step, electrons cascade down the chain from one carrier molecule to the next in a series of redox reactions, losing a small amount of energy with each step until they finally reach oxygen, the terminal electron acceptor, which has a very great affinity for electrons. Each “downhill” carrier is more electronegative than, and thus capable of oxidizing, its “uphill” neighbor, with oxygen at the bottom of the chain. Therefore, the electrons transferred from glucose to NAD+, which is thus reduced to NADH, fall down an energy gradient in the electron transport chain to a far more stable location in the electronegative oxygen atom. Put another way, oxygen pulls electrons

Figure 9.6  An overview of cellular respiration. During glycolysis, each glucose molecule is broken down into two molecules of pyruvate. In eukaryotic cells, as shown here, the pyruvate enters the mitochondrion. There it is oxidized to acetyl CoA, which will be further oxidized to CO2 in the citric acid cycle. The electron carriers NADH and FADH2 transfer electrons derived from glucose to electron transport chains. During oxidative phosphorylation, electron transport chains convert the chemical energy to a form used for ATP synthesis in the process called chemiosmosis. (During earlier steps of cellular respiration, a few molecules of ATP are synthesized in a process called substrate-level phosphorylation.) To visualize these processes in their cellular context, see Figure 6.32. Animation: Overview of Cellular Respiration

Electrons carried via NADH and FADH2

Electrons carried via NADH

GLYCOLYSIS Glucose

Pyruvate

CYTOSOL

PYRUVATE OXIDATION Acetyl CoA

CITRIC ACID CYCLE

OXIDATIVE PHOSPHORYLATION (Electron transport and chemiosmosis)

MITOCHONDRION

ATP

ATP

ATP

Substrate-level phosphorylation

Substrate-level phosphorylation

Oxidative phosphorylation

accepts electrons from NADH or FADH2 generated during the first two stages and passes these electrons down the chain. At the end of the chain, the electrons are combined with molecular oxygen and hydrogen ions (H+), forming water (see Figure 9.5b). The energy released at each step of the chain is stored in a form the mitochondrion (or prokaryotic cell) can use to make ATP from ADP. This mode of ATP synthesis is called oxidative phosphorylation because it is powered by the redox reactions of the electron transport chain. In eukaryotic cells, the inner membrane of the mitochondrion is the site of electron transport and another process called chemiosmosis, together making up oxidative phosphorylation. (In prokaryotes, these processes take place in the plasma membrane.) Oxidative phosphorylation accounts for almost 90% of the ATP generated by respiration. A smaller amount of ATP is formed directly in a few reactions of glycolysis and the citric acid cycle by a mechanism called substrate-level phosphorylation (Figure 9.7). This mode of ATP synthesis

Figure 9.7  Substrate-level phosphorylation. Some ATP is made by direct transfer of a phosphate group from an organic substrate to ADP by an enzyme. (For examples in glycolysis, see Figure 9.9, steps 7 and 10.)

occurs when an enzyme transfers a phosphate group from a substrate molecule to ADP, rather than adding an inorganic phosphate to ADP as in oxidative phosphorylation. “Substrate molecule” here refers to an organic molecule generated as an intermediate during the catabolism of glucose. You’ll see examples of substrate-level phosphorylation later in the chapter, in both glycolysis and the citric acid cycle. You can think of the whole process this way: When you withdraw a relatively large sum of money from an ATM, it is not delivered to you in a single bill of larger denomination. Instead, a number of smaller-denomination bills are dispensed that you can spend more easily. This is analogous to ATP production during cellular respiration. For each molecule of glucose degraded to carbon dioxide and water by respiration, the cell makes up to about 32 molecules of ATP, each with 7.3 kcal/mol of free energy. Respiration cashes in the large denomination of energy banked in a single ­molecule of glucose (686 kcal/mol under standard conditions) for the small change of many molecules of ATP, which is more ­practical for the cell to spend on its work. This preview has introduced you to how glycolysis, the citric acid cycle, and oxidative phosphorylation fit into the process of cellular respiration. We are now ready to take a closer look at each of these three stages of respiration.

Enzyme

Enzyme

Concept Check 9.1

ADP P

ATP

Substrate Product MAKE CONNECTIONS

Review Figure 8.9. Do you think the potential energy is higher for the reactants or the products in the reaction shown above? Explain.

1. Compare and contrast aerobic and anaerobic respiration, including the processes involved. 2. WHAT IF? If the following redox reaction occurred, which compound would be oxidized? Reduced? c 4h6o5 + nAD + S c 4h4o5 + nADh + h +

chapter 9 

For suggested answers, see Appendix A.

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169

Concept  9.2

Figure 9.8  The energy input and output of glycolysis. Animation: Glycolysis

Glycolysis harvests chemical energy by oxidizing glucose to pyruvate

GLYCOLYSIS

The word glycolysis means “sugar splitting,” and that is exactly what happens during this pathway. Glucose, a six-carbon sugar, is split into two three-carbon sugars. These smaller sugars are then oxidized and their remaining atoms rearranged to form two molecules of pyruvate. (Pyruvate is the ionized form of pyruvic acid.) As summarized in Figure 9.8, glycolysis can be divided into two phases: the energy investment phase and the energy payoff phase. During the energy investment phase, the cell actually spends ATP. This investment is repaid with interest during the energy payoff phase, when ATP is produced by substrate-level phosphorylation and NAD+ is reduced to NADH by electrons released from the oxidation of glucose. The net energy yield from glycolysis, per glucose molecule, is 2 ATP plus 2 NADH. The ten steps of the glycolytic pathway are shown in Figure 9.9. All of the carbon originally present in glucose is accounted for in the two molecules of pyruvate; no carbon is released as CO2 during glycolysis. Glycolysis occurs whether or not O2 is present. However, if O2 is present, the chemical energy stored in pyruvate and NADH can be extracted by pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation.

GLYCOLYSIS

OXIDATIVE PHOSPHORYLATION

CITRIC ACID CYCLE

PYRUVATE OXIDATION

OXIDATIVE PHOSPHORYLATION

CITRIC ACID CYCLE

PYRUVATE OXIDATION

ATP

Energy Investment Phase Glucose

2

ATP

2 ADP + 2 P

used

Energy Payoff Phase 4 ADP + 4 P

4

2 NAD+ + 4 e– + 4 H+

ATP

formed

2 NADH + 2 H+ 2 Pyruvate + 2 H2O

Net

Glucose

2 Pyruvate + 2 H2O

4 ATP formed – 2 ATP used 2

NAD+ +

4

e– +

4

2 ATP

H+

2 NADH + 2 H+

Figure 9.9  A closer look at glycolysis. Note that glycolysis is a source of ATP and NADH. GLYCOLYSIS: Energy Investment Phase WHAT IF? What would happen if you removed the dihydroxyacetone phosphate generated in step 4 as fast as it was produced?

ATP

Glyceraldehyde 3-phosphate (G3P) Glucose CH2OH O H H H OH H OH HO H

ADP

CH2O

H H OH HO Hexokinase

OH

1

H

Hexokinase transfers a phosphate group from ATP to glucose, making it more chemically reactive. The charge on the phosphate also traps the sugar in the cell.

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Unit two 

Fructose ATP 6-phosphate

Glucose 6-phosphate

ATP

The Cell

P O H

CH2O

H OH

OH

Phosphoglucoisomerase

2 Glucose 6phosphate is converted to fructose 6-phosphate.

O

P CH2OH

H

HO

HO

H

H

OH

HC

Fructose 1,6-bisphosphate ADP

Phosphofructokinase

P

OCH2

CH2O

O

H

HO

HO

H

H

3 Phosphofructokinase transfers a phosphate group from ATP to the opposite end of the sugar, investing a second molecule of ATP. This is a key step for regulation of glycolysis.

OH

O

CHOH CH2O

P

P

Isomerase Aldolase

4

5 Dihydroxyacetone phosphate (DHAP) CH2O

P

Aldolase cleaves the sugar CH2OH molecule into two different Conversion between DHAP three-carbon and G3P: This reaction sugars. never reaches equilibrium; G3P is used in the next step as fast as it forms. C

O

Concept Check 9.2

Figure 9.10  Oxidation of pyruvate to acetyl CoA, the step before the citric acid cycle. Pyruvate is a charged molecule, so in eukaryotic cells it must enter the mitochondrion via active transport, with the help of a transport protein. Next, a complex of several enzymes (the pyruvate dehydrogenase complex) catalyzes the three numbered steps, which are described in the text. The acetyl group of acetyl CoA will enter the citric acid cycle. The CO2 molecule will diffuse out of the cell. By convention, coenzyme A is abbreviated S-CoA when it is attached to a molecule, emphasizing the sulfur atom (S). (The gene encoding the transport protein was finally identified several years ago, after 40 years of research.)

1. VISUAL SKILLS During the redox reaction in glycolysis (see step 6 in Figure 9.9), which molecule acts as the oxidizing agent? The reducing agent? For suggested answers, see Appendix A.

Concept  9.3 After pyruvate is oxidized, the citric acid cycle completes the energy-yielding oxidation of organic molecules

OXIDATIVE PHOSPHORYLATION

CITRIC ACID CYCLE

PYRUVATE OXIDATION

GLYCOLYSIS

Glycolysis releases less than a quarter of the chemical energy in glucose that can be harvested by cells; most of the energy remains stockpiled in the two molecules of pyruvate. When O2 is present, the pyruvate in eukaryotic cells enters a mitochondrion, where the oxidation of glucose is completed. In aerobically respiring prokaryotic cells, this process occurs in the cytosol. (Later in the chapter, we’ll discuss what happens to pyruvate when O2 is unavailable or in a prokaryote that is unable to use O2.)

MITOCHONDRION

CYTOSOL

Oxidation of Pyruvate to Acetyl CoA

1

C

O

C

O

3

S-CoA C

2

CH3 Pyruvate

Upon entering the mitochondrion via active transport, pyruvate is first converted to a compound called acetyl coenzyme A, or acetyl CoA (Figure 9.10). This step, linking glycolysis and the citric acid cycle, is carried out by a multienzyme complex that catalyzes three reactions: 1 Pyruvate’s carboxyl

Coenzyme A

CO2

O–

NAD +

O

CH3 NADH + H +

Acetyl CoA

Transport protein BioFlix® Animation: Acetyl CoA

The energy payoff phase occurs after glucose is split into two three-carbon sugars. Thus, the coefficient 2 precedes all molecules in this phase.

GLYCOLYSIS: Energy Payoff Phase

2 NAD +

2 ADP

+ 2 H+ 2

Triose phosphate dehydrogenase

6

ATP

2

2 NADH

P

OC

CH2O

P

1,3-Bisphosphoglycerate

Two sequential reactions: (1) G3P is oxidized by the transfer of electrons to NAD+, forming NADH. (2) Using energy from this exergonic redox reaction, a phosphate group is attached to the oxidized substrate, making a high-energy product.

Phosphoglycerokinase

7

2

O– C

CHOH

2 Pi

2

O

2 2 ADP

2 H2O

O

C

CHOH CH2 O

P

3-Phosphoglycerate

The phosphate group is transferred to ADP (substrate-level phosphorylation) in an exergonic reaction. The carbonyl group of G3P has been oxidized to the carboxyl group (—COO–) of an organic acid (3-phosphoglycerate).

H

Phosphoglyceromutase

8

2

O–

P

CO CH2OH

2-Phosphoglycerate

This enzyme relocates the remaining phosphate group.

Enolase

9

2

O– C

O

ATP

O

CO

P

Pyruvate kinase

CH2

Phosphoenolpyruvate (PEP)

Enolase causes a double bond to form in the substrate by extracting a water molecule, yielding phosphoenolpyruvate (PEP), a compound with a very high potential energy.

10

O– C

O

C

O

CH3

Pyruvate

The phosphate group is transferred from PEP to ADP (a second example of substrate-level phosphorylation), forming pyruvate.

BioFlix® Animation: Glycolysis chapter 9 

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171

group (—COO-), already somewhat oxidized and thus carrying little chemical energy, is now fully oxidized and given off as a molecule of CO2. This is the first step in which CO2 is released during respiration. 2 Next, the remaining two-carbon fragment is oxidized and the electrons transferred to NAD+, storing energy in the form of NADH. 3 Finally, coenzyme A (CoA), a sulfur-containing compound derived from a B vitamin, is attached via its sulfur atom to the two-carbon intermediate, forming acetyl CoA. Acetyl CoA has a high potential energy, which is used to transfer the acetyl group to a molecule in the citric acid cycle, a reaction that is therefore highly exergonic.

The Citric Acid Cycle The citric acid cycle functions as a metabolic furnace that further oxidizes organic fuel derived from pyruvate. Figure 9.11 summarizes the inputs and outputs as Figure 9.11  An overview of pyruvate oxidation and the citric acid cycle. The inputs and outputs per pyruvate molecule are shown. To calculate on a per-glucose basis, multiply by 2, because each glucose molecule is split during glycolysis into two pyruvate molecules.

PYRUVATE OXIDATION Pyruvate (from glycolysis, 2 molecules per glucose)

PYRUVATE OXIDATION

GLYCOLYSIS

CITRIC ACID CYCLE

OXIDATIVE PHOSPHORYLATION

ATP

CO2

NAD+

CoA NADH + H+

Acetyl CoA CoA

NADH + H+

CoA

NAD +

CITRIC ACID CYCLE

2 CO2 2 NAD+

FADH2

2 NADH

FAD

+ 2 H+ ADP + P i ATP

Animation: The Citric Acid Cycle

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Unit two 

The Cell

pyruvate is broken down to three CO2 molecules, including the molecule of CO2 released during the conversion of pyruvate to acetyl CoA. The cycle generates 1 ATP per turn by substrate-level phosphorylation, but most of the chemical energy is transferred to NAD+ and FAD during the redox reactions. The reduced coenzymes, NADH and FADH2, shuttle their cargo of high-energy electrons into the ­electron transport chain. The citric acid cycle is also called the tricarboxylic acid cycle or the Krebs cycle, the latter honoring Hans Krebs, the German-British scientist who was largely responsible for working out the pathway in the 1930s. Now let’s look at the citric acid cycle in more detail. The cycle has eight steps, each catalyzed by a specific enzyme. You can see in Figure 9.12 that for each turn of the citric acid cycle, two carbons (red) enter in the relatively reduced form of an acetyl group (step 1  ), and two ­different carbons (blue) leave in the completely oxidized form of CO2 molecules (steps 3 and 4  ). The acetyl group of acetyl CoA joins the cycle by combining with the compound oxaloacetate, forming citrate (step 1  ). Citrate is the ionized form of citric acid, for which the cycle is named. The next seven steps decompose the citrate back to oxaloacetate. It is this regeneration of oxaloacetate that makes the process a cycle. Referring to Figure 9.12, we can tally the energy-rich molecules produced by the citric acid cycle. For each ­acetyl group entering the cycle, 3 NAD+ are reduced to NADH (steps 3 , 4 , and 8  ). In step 6 , electrons are transferred not to NAD+, but to FAD, which accepts 2 electrons and 2 protons to become FADH2. In many animal tissue cells, the reaction in step 5 produces a ­guanosine triphosphate (GTP) molecule by substrate-level ­phosphorylation. GTP is a molecule similar to ATP in its structure and cellular function. This GTP may be used to make an ATP molecule (as shown) or directly power work in the cell. In the cells of plants, bacteria, and some animal tissues, step 5 forms an ATP molecule directly by substrate-level phosphorylation. The output from step 5   represents the only ATP generated during the citric acid cycle. Recall that each glucose gives rise to two molecules of acetyl CoA that enter the cycle. Because the numbers noted earlier are obtained from a single acetyl group entering the pathway, the total yield per glucose from the ­citric acid cycle turns out to be 6 NADH, 2 FADH2, and the ­equivalent of 2 ATP. Most of the ATP produced by respiration results later, from oxidative phosphorylation, when the NADH and FADH2 produced by the citric acid cycle and earlier steps relay the electrons extracted from food to the electron transport chain. In the process, they supply the necessary energy for the phosphorylation of ADP to ATP. We will explore this process in the next section.

GLYCOLYSIS

PYRUVATE OXIDATION

each time around. In eukaryotic cells, all the citric acid cycle enzymes are located in the mitochondrial matrix except for the enzyme that catalyzes step 6, which resides in the inner mitochondrial membrane. Carboxylic acids are represented in their ionized forms, as —COO-, because the ionized forms prevail at the pH within the mitochondrion.

be distinguished from each other.) Notice that the carbon atoms that enter the cycle from acetyl CoA do not leave the cycle in the same turn. They remain in the cycle, occupying a different location in the molecules on their next turn, after another acetyl group is added. Therefore, the oxaloacetate regenerated at step 8 is made up of different carbon atoms

Figure 9.12  A closer look at the citric acid cycle. In the chemical structures, red type traces the fate of the two carbon atoms that enter the cycle via acetyl CoA (step 1), and blue type indicates the two carbons that exit the cycle as CO2 in steps 3 and 4. (The red type goes only through step 5 because the succinate molecule is symmetrical; the two ends cannot

OXIDATIVE PHOSPHORYLATION

CITRIC ACID CYCLE

ATP

1 Acetyl CoA (from oxidation of pyruvate) adds its two-carbon acetyl group to oxaloacetate, producing citrate.

S-CoA C

O

CH3

Acetyl CoA

2 Citrate is converted to its isomer, isocitrate, by removal of one water molecule and addition of another.

8 The substrate

is oxidized, reducing NAD+ to NADH and regenerating oxaloacetate.

CoA-SH

NADH + H+

O

COO

8 COO

a water molecule rearranges bonds in the substrate.

Oxaloacetate

HO

C

COO– COO–

CH2

Malate

COO

H2O

CH2

COO

CH

H2O

COO–





CH2

7 Addition of

1

CH2

NAD +

HO

COO–

C

CH2

2

COO–

HC



HO

CH

Citrate

COO–

Isocitrate NAD +



CITRIC ACID CYCLE

7

CO2

COO– CH

NADH + H+

3

COO–

Fumarate

CoA-SH

HC

CH2 CH2

COO–

6

CoA-SH

FAD 6 Two

hydrogens are transferred to FAD, forming FADH2 and oxidizing succinate.

5

CH2 COO–

C

Succinate

Pi

GTP GDP

CO2

O

S-CoA

Succinyl CoA

NADH + H+

ADP ATP

BioFlix® Animation: The Citric Acid Cycle

NAD +

CH2

α-Ketoglutarate O

COO–

COO– CH2

CH2

FADH 2

C

4 COO–

3 Isocitrate is oxidized, reducing NAD+ to NADH. Then the resulting compound loses a CO2 molecule.

5 CoA is displaced by a phosphate group, which is transferred to GDP, forming GTP, a molecule with functions similar to ATP. GTP can also be used, as shown, to generate ATP.

chapter 9 

4 Another CO2 is lost, and the resulting compound is oxidized, reducing NAD+ to NADH. The remaining molecule is then attached to coenzyme A by an unstable bond.

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173

Concept Check 9.3 1. VISUAL SKILLS Name the molecules that conserve most of the energy from the redox reactions of the citric acid cycle (see Figure 9.12). How is this energy converted to a form that can be used to make ATP? 2. What processes in your cells produce the CO2 that you exhale? 3. VISUAL SKILLS The conversions shown in Figure 9.10 and step 4 of Figure 9.12 are each catalyzed by a large multienzyme complex. What similarities are there in the reactions that occur in these two cases?

Figure 9.13  Free-energy change during electron transport. The overall energy drop (∆G) for electrons traveling from NADH to oxygen is 53 kcal/mol, but this “fall” is broken up into a series of smaller steps by the electron transport chain. (An oxygen atom is represented here as 1⁄2 O2 to emphasize that the electron transport chain reduces molecular oxygen, O2, not individual oxygen atoms.)

GLYCOLYSIS

OXIDATIVE PHOSPHORYLATION

CITRIC ACID CYCLE

PYRUVATE OXIDATION

For suggested answers, see Appendix A. ATP

Concept  9.4

NADH

During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis

50 2 e–

NAD+ FADH2

40

2 e–

FAD

Fe•S

II

Q

174

Unit two 

The Cell

III

Cyt b Fe•S

30

Cyt c1

IV Cyt c

20

Cyt a Electron transport chain Cyt a3 Electrons (from NADH or FADH2) move from a less electronegative electron carrier (one with a lower affinity for electrons) to a more electronegative electron carrier down the chain, releasing free energy. 2 e–

10

The last electron carrier (Cyt a3) passes its electrons to oxygen, which is very electronegative.

The Pathway of Electron Transport The electron transport chain is a collection of molecules embedded in the inner membrane of the mitochondrion in eukaryotic cells. (In prokaryotes, these molecules reside in the plasma membrane.) The folding of the inner membrane to form cristae increases its surface area, providing space for thousands of copies of each component of the electron transport chain in a mitochondrion. The infolded membrane with its concentration of electron carrier molecules is well-suited for the series of sequential redox reactions that take place along the electron transport chain. Most components of the chain are proteins, which exist in multiprotein complexes numbered I through IV. Tightly bound to these proteins are prosthetic groups, nonprotein components such as cofactors and coenzymes essential for the catalytic functions of certain enzymes. Figure 9.13 shows the sequence of electron carriers in the electron transport chain and the drop in free energy as electrons travel down the chain. During this electron transport,

Complexes I-IV each consist of multiple proteins with electron carriers.

I

FMN Fe•S

Free energy (G) relative to O2 (kcal/mol)

Our main objective in this chapter is to learn how cells harvest the energy of glucose and other nutrients in food to make ATP. But the metabolic components of respiration we have examined so far, glycolysis and the citric acid cycle, produce only 4 ATP molecules per glucose molecule, all by substrate-level phosphorylation: 2 net ATP from glycolysis and 2 ATP from the citric acid cycle. At this point, molecules of NADH (and FADH2) account for most of the energy extracted from each glucose molecule. These electron escorts link glycolysis and the citric acid cycle to the machinery of oxidative phosphorylation, which uses energy released by the electron transport chain to power ATP synthesis. In this section, you will learn first how the electron transport chain works and then how electron flow down the chain is coupled to ATP synthesis.

(least electronegative)

2 H+ + 1 2 O2 (most electronegative)

0

Figure Walkthrough

H2O

electron carriers alternate between reduced and oxidized states as they accept and then donate electrons. Each component of the chain becomes reduced when it accepts electrons from its “uphill” neighbor, which has a lower affinity for electrons (in other words, is less electronegative). It then returns to its oxidized form as it passes electrons to its “downhill,” more electronegative neighbor. Now let’s take a closer look at the electrons as they drop in energy level, passing through the components of the electron transport chain in Figure 9.13. We’ll first describe the passage

of electrons through complex I in some detail as an illustration of the general principles involved in electron transport. Electrons acquired from glucose by NAD+ during glycolysis and the citric acid cycle are transferred from NADH to the first molecule of the electron transport chain in complex I. This molecule is a flavoprotein, so named because it has a prosthetic group called flavin mononucleotide (FMN). In the next redox reaction, the flavoprotein returns to its oxidized form as it passes electrons to an iron-sulfur protein (Fe #  S in complex I), one of a family of proteins with both iron and sulfur tightly bound. The iron-sulfur protein then passes the electrons to a compound called ubiquinone (Q in Figure 9.13). This electron carrier is a small hydrophobic molecule, the only member of the electron transport chain that is not a protein. Ubiquinone is individually mobile within the membrane rather than residing in a particular complex. (Another name for ubiquinone is coenzyme Q , or CoQ ; you may have seen it sold as a nutritional supplement.) Most of the remaining electron carriers between ubiquinone and oxygen are proteins called cytochromes. Their prosthetic group, called a heme group, has an iron atom that accepts and donates electrons. (The heme group in a cytochrome is similar to the heme group in hemoglobin, the protein of red blood cells, except that the iron in hemoglobin carries oxygen, not electrons.) The electron transport chain has several types of cytochromes, each named “cyt” with a letter and number to distinguish it as a different protein with a slightly different electron-carrying heme group. The last cytochrome of the chain, Cyt a3, passes its electrons to oxygen, which is very electronegative. Each oxygen atom also picks up a pair of hydrogen ions (protons) from the aqueous solution, neutralizing the -2 charge of the added electrons and forming water. Another source of electrons for the electron transport chain is FADH2, the other reduced product of the citric acid cycle. Notice in Figure 9.13 that FADH2 adds its electrons from within complex II, at a lower energy level than NADH does. Consequently, although NADH and FADH2 each donate an equivalent number of electrons (2) for oxygen reduction, the electron transport chain provides about one-third less energy for ATP synthesis when the electron donor is FADH2 rather than NADH. We’ll see why in the next section. The electron transport chain makes no ATP directly. Instead, it eases the fall of electrons from food to oxygen, breaking a large free-energy drop into a series of smaller steps that release energy in manageable amounts, step by step. How does the mitochondrion (or the plasma membrane in prokaryotes) couple this electron transport and energy release to ATP synthesis? The answer is a mechanism called chemiosmosis.

Chemiosmosis: The Energy-Coupling Mechanism Populating the inner membrane of the mitochondrion or the prokaryotic plasma membrane are many copies of a protein

Figure 9.14  ATP synthase, a molecular mill. Multiple ATP synthases reside in eukaryotic mitochondrial and chloroplast membranes and in prokaryotic plasma membranes. Inner mitochondrial membrane

Intermembrane space Mitochondrial matrix

1 H+ ions flowing down their gradient enter a channel in a stator, which is anchored in the membrane.

INTERMEMBRANE SPACE

H+

Stator

Rotor

2 H+ ions enter binding sites within a rotor, changing the shape of each subunit so that the rotor spins within the membrane. 3 Each H+ ion makes one complete turn before leaving the rotor and passing through a second channel in the stator into the mitochondrial matrix.

Internal rod

4 Spinning of the rotor causes an internal rod to spin as well. This rod extends like a stalk into the knob below it, which is held stationary by part of the stator.

Catalytic knob

ADP + Pi

ATP

MITOCHONDRIAL MATRIX

5 Turning of the rod activates catalytic sites in the knob that produce ATP from ADP and P i .

BioFlix® Animation: ATP Synthase Animation: Rotating ATP Synthase

complex called ATP synthase, the enzyme that makes ATP from ADP and inorganic phosphate (Figure 9.14). ATP synthase works like an ion pump running in reverse. Ion pumps usually use ATP as an energy source to transport ions against their gradients. Enzymes can catalyze a reaction in either direction, depending on the ∆G for the reaction, which is affected by the local concentrations of reactants and products (see Concepts 8.2 and 8.3). Rather than hydrolyzing ATP to pump protons against their concentration gradient, under the conditions of cellular respiration ATP synthase uses the energy of an existing ion gradient to power ATP synthesis. The power source for ATP synthase is a difference in the concentration of H+ on opposite sides of the inner mitochondrial membrane. This process, in which energy stored in the form of a hydrogen ion gradient across a membrane is used to drive cellular work such as the synthesis of ATP, is called chemiosmosis (from

chapter 9 

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175

Figure 9.15  Chemiosmosis couples the electron transport chain to ATP synthesis. 1 NADH and FADH2 shuttle highenergy electrons extracted from food during glycolysis and the citric acid cycle into an electron transport chain built into the inner mitochondrial membrane. The gold arrows trace the transport of electrons, which are finally passed to a terminal acceptor (O2, in the case of aerobic respiration) at the “downhill” end of the chain,

space than occurs with NADH. Chemical energy originally harvested from food is transformed into a proton-motive force, a gradient of H+ across the membrane. 2 During chemiosmosis, the protons flow back down their gradient via ATP synthase, which is built into the membrane nearby. The ATP synthase harnesses the protonmotive force to phosphorylate ADP, forming ATP. Together, electron transport and chemiosmosis make up oxidative phosphorylation.

forming water. Most of the electron carriers of the chain are grouped into four complexes (I–IV). Two mobile carriers, ubiquinone (Q) and cytochrome c (Cyt c), move rapidly, ferrying electrons between the large complexes. As the complexes shuttle electrons, they pump protons from the mitochondrial matrix into the intermembrane space. FADH2 deposits its electrons via complex II and so results in fewer protons being pumped into the intermembrane Intermembrane space

Inner mitochondrial membrane

Mitochondrial matrix GLYCOLYSIS

PYRUVATE OXIDATION

CITRIC ACID CYCLE

OXIDATIVE PHOSPHORYLATION

ATP

H+ Intermembrane space

H+

H+

H+

Cyt c

Protein complex of electron carriers

IV

Q III

I

II FADH2 FAD

Inner mitochondrial membrane NADH

2 H+ + 1 2 O2

NAD+

H2O ADP + P i

(carrying electrons from food) Mitochondrial matrix

ATP synthase

ATP H+

1 Electron transport chain Electron transport and pumping of protons (H+), which create an H+ gradient across the membrane

2 Chemiosmosis ATP synthesis powered by the flow of H+ back across the membrane

Oxidative phosphorylation WHAT IF

If complex IV were nonfunctional, could chemiosmosis produce any ATP, and if so, how would the rate of synthesis differ?

Animation: Electron Transport BioFlix® Animation: Electron Transport

the Greek osmos, push). We have previously used the word osmosis in discussing water transport, but here it refers to the flow of H+ across a membrane. From studying the structure of ATP synthase, scientists have learned how the flow of H+ through this large enzyme powers ATP generation. ATP synthase is a multisubunit complex with four main parts, each made up of multiple polypeptides (see Figure 9.14). Protons move one by one into binding sites on one of the parts (the rotor), causing it to spin in a way that catalyzes ATP production from ADP and inorganic phosphate. The flow of protons thus behaves somewhat like a rushing stream that turns a waterwheel. How does the inner mitochondrial membrane or the ­prokaryotic plasma membrane generate and maintain the H+

gradient that drives ATP synthesis by the ATP synthase protein complex? Establishing the H+ gradient is a major function of the electron transport chain, which is shown in its mitochondrial location in Figure 9.15. The chain is an energy converter that uses the exergonic flow of electrons from NADH and FADH2 to pump H+ across the membrane, from the mitochondrial matrix into the intermembrane space. The H+ has a tendency to move back across the membrane, diffusing down its gradient. And the ATP synthases are the only sites that provide a route through the membrane for H+. As we described previously, the ­passage of H+ through ATP synthase uses the exergonic flow of H+ to drive the phosphorylation of ADP. Thus, the energy stored in an H+ gradient across a membrane couples the redox reactions of the electron transport chain to ATP synthesis.

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Unit two 

The Cell

At this point, you may be wondering how the electron transport chain pumps hydrogen ions. Researchers have found that certain members of the electron transport chain accept and release protons (H+) along with electrons. (The aqueous solutions inside and surrounding the cell are a ready source of H+.) At certain steps along the chain, electron transfers cause H+ to be taken up and released into the surrounding solution. In eukaryotic cells, the electron carriers are spatially arranged in the inner mitochondrial membrane in such a way that H+ is accepted from the mitochondrial matrix and deposited in the intermembrane space (see Figure 9.15). The H+ gradient that results is referred to as a proton-motive force, emphasizing the capacity of the gradient to perform work. The force drives H+ back across the membrane through the H+ channels provided by ATP synthases. In general terms, chemiosmosis is an energy-coupling mechanism that uses energy stored in the form of an H + gradient across a membrane to drive cellular work. In mitochondria, the energy for gradient formation comes from exergonic redox reactions along the electron transport chain, and ATP synthesis is the work performed. But chemiosmosis also occurs elsewhere and in other variations. Chloroplasts use chemiosmosis to generate ATP during photosynthesis; in these organelles, light (rather than chemical energy) drives both electron flow down an electron transport chain and the resulting H+ gradient formation. Prokaryotes, as already mentioned, generate H+ gradients across their plasma membranes. They then tap the proton-motive force not only to make ATP inside the cell but

also to rotate their flagella and to pump nutrients and waste products across the membrane. Because of its central importance to energy conversions in prokaryotes and eukaryotes, chemiosmosis has helped unify the study of bioenergetics. Peter Mitchell was awarded the Nobel Prize in 1978 for originally proposing the chemiosmotic model.

An Accounting of ATP Production by Cellular Respiration In the last few sections, we have looked rather closely at the key processes of cellular respiration. Now let’s take a step back and remind ourselves of its overall function: harvesting the energy of glucose for ATP synthesis. During respiration, most energy flows in this sequence: glucose S NADH S electron transport chain S protonmotive force S ATP. We can do some bookkeeping to calculate the ATP profit when cellular respiration oxidizes a molecule of glucose to six molecules of carbon dioxide. The three main departments of this metabolic enterprise are glycolysis, pyruvate oxidation and the citric acid cycle, and the electron transport chain, which drives oxidative phosphorylation. Figure 9.16 gives a detailed accounting of the ATP yield for each glucose molecule that is oxidized. The tally adds the 4 ATP produced directly by substrate-level phosphorylation during glycolysis and the citric acid cycle to the many more molecules of ATP generated by oxidative

Figure 9.16  ATP yield per molecule of glucose at each stage of cellular respiration. CYTOSOL

Electron shuttles span membrane

2 NADH

GLYCOLYSIS Glucose

2 Pyruvate

MITOCHONDRION

2 NADH or 2 FADH2 2 NADH

6 NADH

PYRUVATE OXIDATION

CITRIC ACID CYCLE

2 Acetyl CoA

+ 2 ATP

+ 2 ATP

by substrate-level phosphorylation

by substrate-level phosphorylation

Maximum per glucose:

2 FADH2

OXIDATIVE PHOSPHORYLATION (Electron transport and chemiosmosis)

+ about 26 or 28 ATP by oxidative phosphorylation, depending on which shuttle transports electrons from NADH in cytosol

About 30 or 32 ATP

Visual Skills

Explain exactly how the total of 26 or 28 ATP from oxidative phosphorylation was calculated (see the yellow bar in the figure).

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phosphorylation. Each NADH that transfers a pair of electrons from glucose to the electron transport chain contributes enough to the proton-motive force to generate a maximum of about 3 ATP. Why are the numbers in Figure 9.16 inexact? There are three reasons we cannot state an exact number of ATP molecules generated by the breakdown of one molecule of glucose. First, phosphorylation and the redox reactions are not directly coupled to each other, so the ratio of the number of NADH molecules to the number of ATP molecules is not a whole number. We know that 1 NADH results in 10 H+ being transported out across the inner mitochondrial membrane, but the exact number of H+ that must reenter the mitochondrial matrix via ATP synthase to generate 1 ATP has long been debated. Based on experimental data, however, most biochemists now agree that the most accurate number is 4 H+. Therefore, a single molecule of NADH generates enough proton-motive force for the synthesis of 2.5 ATP. The citric acid cycle also supplies electrons to the electron transport chain via FADH2, but since its electrons enter later in the chain, each molecule of this electron carrier is responsible for transport of only enough H+ for the synthesis of 1.5 ATP. These numbers also take into account the slight energetic cost of moving the ATP formed in the mitochondrion out into the cytosol, where it will be used. Second, the ATP yield varies slightly depending on the type of shuttle used to transport electrons from the cytosol into the mitochondrion. The mitochondrial inner membrane is impermeable to NADH, so NADH in the cytosol is segregated from the machinery of oxidative phosphorylation. The 2 electrons of NADH captured in glycolysis must be conveyed into the mitochondrion by one of several electron shuttle systems. Depending on the kind of shuttle in a particular cell type, the electrons are passed either to NAD+ or to FAD in the mitochondrial matrix (see Figures 9.15 and 9.16). If the electrons are passed to FAD, as in brain cells, only about 1.5 ATP can result from each NADH that was originally generated in the cytosol. If the electrons are passed to mitochondrial NAD+, as in liver cells and heart cells, the yield is about 2.5 ATP per NADH. A third variable that reduces the yield of ATP is the use of the proton-motive force generated by the redox reactions of respiration to drive other kinds of work. For example, the proton-motive force powers the mitochondrion’s uptake of pyruvate from the cytosol. However, if all the proton-motive force generated by the electron transport chain were used to drive ATP synthesis, one glucose molecule could generate a maximum of 28 ATP produced by oxidative phosphorylation plus 4 ATP (net) from substrate-level phosphorylation to give a total yield of about 32 ATP (or only about 30 ATP if the less efficient shuttle were functioning). Animation: Electron Transport Chain: Factors Affecting ATP Yield

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We can now roughly estimate the efficiency of respiration— that is, the percentage of chemical energy in glucose that has been transferred to ATP. Recall that the complete oxidation of a mole of glucose releases 686 kcal of energy under standard conditions (∆G = -686 kcal/mol). Phosphorylation of ADP to form ATP stores at least 7.3 kcal per mole of ATP. Therefore, the efficiency of respiration is 7.3 kcal per mole of ATP times 32 moles of ATP per mole of glucose divided by 686 kcal per mole of glucose, which equals 0.34. Thus, about 34% of the potential chemical energy in glucose has been transferred to ATP; the actual percentage is bound to vary as ∆G varies under different cellular conditions. Cellular respiration is remarkably efficient in its energy conversion. By comparison, even the most efficient automobile converts only about 25% of the energy stored in gasoline to energy that moves the car. The rest of the energy stored in glucose is lost as heat. We humans use some of this heat to maintain our relatively high body temperature (37°C), and we dissipate the rest through sweating and other cooling mechanisms. Surprisingly, perhaps, it may be beneficial under certain conditions to reduce the efficiency of cellular respiration. A remarkable adaptation is shown by hibernating mammals, which overwinter in a state of inactivity and lowered metabolism. Although their internal body temperature is lower than normal, it still must be kept significantly higher than the external air temperature. One type of tissue, called brown fat, is made up of cells packed full of mitochondria. The inner mitochondrial membrane contains a channel protein called the uncoupling protein that allows protons to flow back down their concentration gradient without generating ATP. Activation of these proteins in hibernating mammals results in ongoing oxidation of stored fuel (fats), generating heat without any ATP production. In the absence of such an adaptation, the buildup of ATP would eventually cause cellular respiration to be shut down by regulatory mechanisms that will be discussed later. In the Scientific Skills Exercise, you can work with data in a related but different case where a decrease in metabolic efficiency in cells is used to generate heat. BioFlix® Animation: Cellular Respiration

Concept Check 9.4 1. WHAT IF? What effect would an absence of O2 have on the process shown in Figure 9.15? 2. WHAT IF? In the absence of O2, as in question 1, what do you think would happen if you decreased the pH of the intermembrane space of the mitochondrion? Explain your answer. 3. MAKE CONNECTIONS Membranes must be fluid to function properly (as you learned in Concept 7.1). How does the operation of the electron transport chain support that assertion? For suggested answers, see Appendix A.

Scientific Skills Exercise

Making a Bar Graph and Evaluating a Hypothesis Does Thyroid Hormone Level Affect Oxygen Consumption in Cells?  Some animals, such as mammals and birds, maintain a relatively constant body temperature, above that of their environment, by using heat produced as a by-product of metabolism. When the core temperature of these animals drops below an internal set point, their cells are triggered to reduce the efficiency of ATP production by the electron transport chains in mitochondria. At lower efficiency, extra fuel must be consumed to produce the same number of ATPs, generating additional heat. Because this response is moderated by the endocrine system, researchers hypothesized that thyroid hormone might trigger this cellular response. In this exercise, you will use a bar graph to visualize data from an experiment that compared the metabolic rates (by measuring oxygen consumption) in mitochondria of cells from animals with different levels of thyroid hormone.

How the Experiment Was Done  Liver cells were isolated from sibling rats that had low, normal, or elevated thyroid hormone levels. The oxygen consumption rate due to activity of the mitochondrial electron transport chains of each type of cell was measured under controlled conditions.

Data from the Experiment Thyroid Hormone Level

Oxygen Consumption Rate [nmol O2/(min # mg cells)]

Low

4.3

Normal

4.8

Elevated

8.7

Data from  M. E. Harper and M. D. Brand, The quantitative contributions of mitochondrial proton leak and ATP turnover reactions to the changed respiration rates of hepatocytes from rats of different thyroid status, Journal of Biological Chemistry 268:14850–14860 (1993).

First, set up the axes. (a) What is the independent variable (intentionally varied by the researchers), which goes on the x-axis? List the categories along the x-axis; because they are discrete rather than continuous, you can list them in any order. (b) What is the dependent variable (measured by the researchers), which goes on the y-axis? (c) What units (abbreviated) should go on the y-axis? Label the y-axis, including the units specified in the data table. Determine the range of values of the data that will need to go on the y-axis. What is the largest value? Draw evenly spaced tick marks and label them, starting with 0 at the bottom. 2. Graph the data for each sample. Match each x-value with its y-value and place a mark on the graph at that coordinate, then draw a bar from the x-axis up to the correct height for each sample. Why is a bar graph more appropriate than a scatter plot or line graph? (For additional information about graphs, see the Scientific Skills Review in Appendix F.) 3. Examine your graph and look for a pattern in the data. (a) Which cell type had the highest rate of oxygen consumption, and which had the lowest? (b) Does this support the researchers’ hypothesis? Explain. (c) Based on what you know about mitochondrial electron transport and heat production, predict which rats had the highest, and which had the lowest, body temperature.

Interpret The Data 1. To visualize any differences in oxygen consumption between cell types, it will be useful to graph the data in a bar graph.

Concept  9.5 Fermentation and anaerobic respiration enable cells to produce ATP without the use of oxygen Because most of the ATP generated by cellular respiration is due to the work of oxidative phosphorylation, our estimate of ATP yield from aerobic respiration depends on an adequate supply of oxygen to the cell. Without the electronegative oxygen to pull electrons down the transport chain, oxidative phosphorylation eventually ceases. However, there are two general mechanisms by which certain cells can oxidize organic fuel and generate ATP without the use of oxygen: anaerobic respiration and fermentation. The distinction

Instructors: A version of this Scientific Skills Exercise can be assigned in MasteringBiology.

between these two is that an electron transport chain is used in anaerobic respiration but not in fermentation. (The electron transport chain is also called the respiratory chain because of its role in both types of cellular respiration.) We have already mentioned anaerobic respiration, which takes place in certain prokaryotic organisms that live in environments without oxygen. These organisms have an electron transport chain but do not use oxygen as a final electron acceptor at the end of the chain. Oxygen performs this function very well because it is extremely electronegative, but other, less electronegative substances can also serve as final electron acceptors. Some “sulfate-reducing” marine bacteria, for instance, use the sulfate ion (SO42 -) at the end of their respiratory chain. Operation of the chain builds up a protonmotive force used to produce ATP, but H2S (hydrogen sulfide)

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is made as a by-product rather than water. The rotten-egg odor you may have smelled while walking through a salt marsh or a mudflat signals the presence of sulfate-reducing bacteria. Fermentation is a way of harvesting chemical energy without using either oxygen or any electron transport chain—in other words, without cellular respiration. How can food be oxidized without cellular respiration? Remember, oxidation simply refers to the loss of electrons to an electron acceptor, so it does not need to involve oxygen. Glycolysis oxidizes glucose to two molecules of pyruvate. The oxidizing agent of glycolysis is NAD+, and neither oxygen nor any electron transfer chain is involved. Overall, glycolysis is exergonic, and some of the energy made available is used to produce 2 ATP (net) by ­substrate-level phosphorylation. If oxygen is present, then additional ATP is made by oxidative phosphorylation when NADH passes electrons removed from glucose to the electron transport chain. But glycolysis generates 2 ATP whether oxygen is present or not—that is, whether conditions are aerobic or anaerobic. As an alternative to respiratory oxidation of organic nutrients, fermentation is an extension of glycolysis that allows continuous generation of ATP by the substrate-level phosphorylation of glycolysis. For this to occur, there must be a sufficient supply of NAD+ to accept electrons during the oxidation step of glycolysis. Without some mechanism to recycle NAD+ from NADH, glycolysis would soon deplete the cell’s pool of NAD+ by reducing it all to NADH and would shut itself down for lack of an oxidizing agent. Under aerobic conditions, NAD+ is recycled from NADH by the transfer of electrons to the electron transport chain. An anaerobic alternative is to transfer electrons from NADH to pyruvate, the end product of glycolysis.

Types of Fermentation Fermentation consists of glycolysis plus reactions that regenerate NAD+ by transferring electrons from NADH to pyruvate or derivatives of pyruvate. The NAD+ can then be reused to oxidize sugar by glycolysis, which nets two molecules of ATP by substrate-level phosphorylation. There are many types of fermentation, differing in the end products formed from pyruvate. Two types are alcohol fermentation and lactic acid fermentation, and both are harnessed by humans for food and industrial production. In alcohol fermentation (Figure 9.17a), pyruvate is converted to ethanol (ethyl alcohol) in two steps. The first step releases carbon dioxide from the pyruvate, which is converted to the two-carbon compound acetaldehyde. In the second step, acetaldehyde is reduced by NADH to ethanol. This regenerates the supply of NAD+ needed for the continuation of glycolysis. Many bacteria carry out alcohol fermentation under anaerobic conditions. Yeast (a fungus), in addition to aerobic respiration, also carries out alcohol fermentation. For thousands of years, humans 180

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Figure 9.17  Fermentation. In the absence of oxygen, many cells use fermentation to produce ATP by substrate-level phosphorylation. NAD+ is regenerated for use in glycolysis when pyruvate, the end product of glycolysis, serves as an electron acceptor for oxidizing NADH. Two of the common end products formed from fermentation are (a) ethanol and (b) lactate, the ionized form of lactic acid.

2 ADP + 2 P i

Glucose

2

ATP

GLYCOLYSIS

O– C

O

C

O

CH3 2 Pyruvate 2 NAD+

2 NADH + 2 H+

H H

C

2 CO2 H

OH

CH3

NAD+ REGENERATION

2 Ethanol

C

O

CH3 2 Acetaldehyde

(a) Alcohol fermentation

2 ADP + 2 P i

Glucose

H

C

O

C

OH

CH3

ATP

GLYCOLYSIS

2 NAD+

O–

2

2 NADH + 2 H+

O– C

O

C

O

CH3 2 Pyruvate

NAD+ REGENERATION

2 Lactate (b) Lactic acid fermentation Animation: Fermentation

have used yeast in brewing, winemaking, and baking. The CO 2 ­b ubbles generated by baker’s yeast during alcohol fermentation allow bread to rise. During lactic acid fermentation (Figure 9.17b), pyruvate is reduced directly by NADH to form lactate as an end product, regenerating NAD+ with no release of CO2. (Lactate is the ionized form of lactic acid.) Lactic acid fermentation by certain fungi and bacteria is used in the dairy industry to make cheese and yogurt. Human muscle cells make ATP by lactic acid fermentation when oxygen is scarce. This occurs during strenuous

e­ xercise, when sugar catabolism for ATP production outpaces the muscle’s supply of oxygen from the blood. Under these conditions, the cells switch from aerobic respiration to fermentation. The lactate that accumulates was previously thought to cause the muscle fatigue and pain that occurs a day or so after intense exercise. However, evidence shows that within an hour, blood carries the excess lactate from the muscles to the liver, where it is converted back to pyruvate by liver cells. Because oxygen is available, this pyruvate can then enter the mitochondria in liver cells and complete cellular respiration. (Next-day muscle soreness is more likely caused by trauma to small muscle fibers, which leads to inflammation and pain.)

Figure 9.18  Pyruvate as a key juncture in catabolism. Glycolysis is common to fermentation and cellular respiration. The end product of glycolysis, pyruvate, represents a fork in the catabolic pathways of glucose oxidation. In a facultative anaerobe or a muscle cell, which are capable of both aerobic cellular respiration and fermentation, pyruvate is committed to one of those two pathways, usually depending on whether or not oxygen is present. Glucose

Pyruvate No O2 present: Fermentation

Comparing Fermentation with Anaerobic and Aerobic Respiration Fermentation, anaerobic respiration, and aerobic respiration are three alternative cellular pathways for producing ATP by harvesting the chemical energy of food. All three use glycolysis to oxidize glucose and other organic fuels to pyruvate, with a net production of 2 ATP by substrate-level phosphorylation. And in all three pathways, NAD+ is the oxidizing agent that accepts electrons from food during glycolysis. A key difference is the contrasting mechanisms for oxidizing NADH back to NAD+, which is required to sustain glycolysis. In fermentation, the final electron acceptor is an organic molecule such as pyruvate (lactic acid fermentation) or acetaldehyde (alcohol fermentation). In cellular respiration, by contrast, electrons carried by NADH are transferred to an electron transport chain, which regenerates the NAD+ required for glycolysis. Another major difference is the amount of ATP ­produced. Fermentation yields two molecules of ATP, produced by ­substrate-level phosphorylation. In the absence of an electron transport chain, the energy stored in pyruvate is unavailable. In cellular respiration, however, pyruvate is completely oxidized in the mitochondrion. Most of the chemical energy from this process is shuttled by NADH and FADH2 in the form of electrons to the electron transport chain. There, the electrons move stepwise down a series of redox reactions to a final electron acceptor. (In aerobic respiration, the final electron acceptor is oxygen; in anaerobic respiration, the final acceptor is another molecule that is electronegative, although less so than oxygen.) Stepwise electron transport drives oxidative phosphorylation, yielding ATP. Thus, cellular respiration harvests much more energy from each sugar molecule than fermentation can. In fact, aerobic respiration yields up to 32 molecules of ATP per glucose molecule—up to 16 times as much as does fermentation. Some organisms, called obligate anaerobes, carry out only fermentation or anaerobic respiration. In fact, these organisms cannot survive in the presence of oxygen, some forms of which can actually be toxic if protective systems are

Glycolysis

CYTOSOL

O2 present: Aerobic cellular respiration

MITOCHONDRION Ethanol, lactate, or other products

Acetyl CoA CITRIC ACID CYCLE

not present in the cell. A few cell types, such as cells of the vertebrate brain, can carry out only aerobic oxidation of pyruvate, not fermentation. Other organisms, including yeasts and many bacteria, can make enough ATP to survive using either fermentation or respiration. Such species are called facultative anaerobes. On the cellular level, our muscle cells behave as facultative anaerobes. In such cells, pyruvate is a fork in the metabolic road that leads to two alternative catabolic routes (Figure 9.18). Under aerobic conditions, pyruvate can be converted to acetyl CoA, and oxidation continues in the citric acid cycle via aerobic respiration. Under anaerobic conditions, lactic acid fermentation occurs: Pyruvate is diverted from the citric acid cycle, serving instead as an electron acceptor to recycle NAD+. To make the same amount of ATP, a facultative anaerobe has to consume sugar at a much faster rate when fermenting than when respiring.

The Evolutionary Significance of Glycolysis Evolution The role of glycolysis in both fermentation and respiration has an evolutionary basis. Ancient prokaryotes are thought to have used glycolysis to make ATP long before oxygen was present in Earth’s atmosphere. The oldest known fossils of bacteria date back 3.5 billion years, but appreciable quantities of oxygen probably did not begin to accumulate in the atmosphere until about 2.7 billion years ago. Cyanobacteria produced this O2 as a by-product of photosynthesis. Therefore,

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early prokaryotes may have generated ATP exclusively from glycolysis. The fact that glycolysis is today the most widespread metabolic pathway among Earth’s organisms suggests that it evolved very early in the history of life. The cytosolic location of glycolysis also implies great antiquity; the pathway does not require any of the membrane-enclosed organelles of the eukaryotic cell, which evolved approximately 1 billion years after the first prokaryotic cell. Glycolysis is a metabolic heirloom from early cells that continues to function in fermentation and as the first stage in the breakdown of organic molecules by respiration.

Figure 9.19  The catabolism of various molecules from food. Carbohydrates, fats, and proteins can all be used as fuel for cellular respiration. Monomers of these molecules enter glycolysis or the c