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ESSENTIAL
CELL BIOLOGY
FIFTH EDITION
FI F T H
ED I T I O N
ESSENTIAL
CELL BIOLOGY
Bruce Alberts
UNIVERSIT Y OF CALIFORNIA, SAN FRANCISCO
Karen Hopkin
SCIENCE WRITER
Alexander Johnson
David Morgan
Martin Raff
UNIVERSIT Y COLLEGE LONDON (EMERITUS)
Keith Roberts
UNIVERSIT Y OF EAST ANGLIA (EMERITUS)
Peter Walter
n
W . W . N O R T O N & C O M PA N Y
NE W YORK • LONDON
W. W. Norton & Company has been independent since its founding in 1923, when William
Warder Norton and Mary D. Herter Norton first published lectures delivered at the Peo-
ple’s Institute, the adult education division of New York City’s Cooper Union. The firm soon
expanded its program beyond the Institute, publishing books by celebrated academics
from America and abroad. By midcentury, the two major pillars of Norton’s publishing
program—trade books and college texts—were firmly established. In the 1950s, the Norton
family transferred control of the company to its employees, and today—with a staff of four
hundred and a comparable number of trade, college, and professional titles published
each year—W. W. Norton & Company stands as the largest and oldest publishing house
owned wholly by its employees.
Copyright © 2019 by Bruce Alberts, Dennis Bray, Karen Hopkin, Alexander Johnson, the Estate of Julian
Lewis, David Morgan, Martin Raff, Nicole Marie Odile Roberts, and Peter Walter
All rights reserved

Printed in Canada
Editors: Betsy Twitchell and Michael Morales

Associate Editor: Katie Callahan
Editorial Consultant: Denise Schanck
Senior Associate Managing Editor, College: Carla L. Talmadge
Editorial Assistants: Taylere Peterson and Danny Vargo
Director of Production, College: Jane Searle
Managing Editor, College: Marian Johnson
Managing Editor, College Digital Media: Kim Yi
Media Editor: Kate Brayton
Associate Media Editor: Gina Forsythe
Media Project Editor: Jesse Newkirk
Media Editorial Assistant: Katie Daloia
Ebook Production Manager: Michael Hicks
Content Development Specialist: Todd Pearson
Marketing Manager, Biology: Stacy Loyal
Director of College Permissions: Megan Schindel
Permissions Clearer: Sheri Gilbert
Composition: Emma Jeffcock of EJ Publishing Services
Illustrations: Nigel Orme
Design Director: Hope Miller Goodell
Designer: Matthew McClements, Blink Studio, Ltd.
Indexer: Bill Johncocks
Manufacturing: Transcontinental Interglobe—Beauceville, Quebec
Permission to use copyrighted material is included alongside the appropriate content.
Library of Congress Cataloging-in-Publication Data
Names: Alberts, Bruce, author.

Title: Essential cell biology / Bruce Alberts, Karen Hopkin, Alexander
Johnson, David Morgan, Martin Raff, Keith Roberts, Peter Walter.
Description: Fifth edition. | New York : W.W. Norton & Company, [2019] |
Includes index.
Identifiers: LCCN 2018036121 | ISBN 9780393679533 (hardcover)
Subjects: LCSH: Cytology. | Molecular biology. | Biochemistry.
Classification: LCC QH581.2 .E78 2019 | DDC 571.6—dc23 LC record available at
https://lccn.loc.gov/2018036121
W. W. Norton & Company, Inc., 500 Fifth Avenue, New York, NY 10110
wwnorton.com
W. W. Norton & Company Ltd., 15 Carlisle Street, London W1D 3BS
1 2 3 4 5 6 7 8 9 0
PREFACE
Nobel Prize–winning physicist Richard Feynman once noted that nature

has a far, far better imagination than our own. Few things in the universe
illustrate this observation better than the cell. A tiny sac of molecules
capable of self-replication, this marvelous structure constitutes the fun-
damental building block of life. We are made of cells. Cells provide all
the nutrients we consume. And the continuous activity of cells makes
our planet habitable. To understand ourselves—and the world of which
we are a part—we need to know something of the life of cells. Armed
with such knowledge, we—as citizens and stewards of the global
community—will be better equipped to make well-informed decisions
about increasingly sophisticated issues, from climate change and food
security to biomedical technologies and emerging epidemics.
In Essential Cell Biology we introduce readers to the fundamentals of
cell biology. The Fifth Edition introduces powerful new techniques that
allow us to examine cells and their components with unprecedented
precision—such as super-resolution fluorescence microsocopy and
cryoelectron microscopy—as well as the latest methods for DNA
sequencing and gene editing. We discuss new thinking about how cells
organize and encourage the chemical reactions that make life possible,
and we review recent insights into human origins and genetics.
With each edition of Essential Cell Biology, its authors re-experience the
joy of learning something new and surprising about cells. We are also
reminded of how much we still don’t know. Many of the most fascinat-
ing questions in cell biology remain unanswered. How did cells arise on
the early Earth, multiplying and diversifying through billions of years of
evolution to fill every possible niche—from steaming vents on the ocean
floor to frozen mountaintops—and, in doing so, transform our planet’s
entire environment? How is it possible for billions of cells to seamlessly
cooperate and form large, multicellular organisms like ourselves? These
are among the many challenges that remain for the next generation of
cell biologists, some of whom will begin a wonderful, lifelong journey
with this textbook.
Readers interested in learning how scientific inquisitiveness can fuel break-
throughs in our understanding of cell biology will enjoy the stories of dis-
covery presented in each chapter’s “How We Know” feature. Packed with
experimental data and design, these narratives illustrate how biologists
tackle important questions and how experimental results shape future
ideas. In this edition, a new “How We Know” recounts the discoveries that
first revealed how cells transform the energy locked in food molecules into
the forms used to power the metabolic reactions on which life depends.
As in previous editions, the questions in the margins and at the end of
each chapter not only test comprehension but also encourage careful
thought and the application of newly acquired information to a broader
biological context. Some of these questions have more than one valid
v
vi Preface
answer and others invite speculation. Answers to all of the questions

are included at the back of the book, and many provide additional
information or an alternative perspective on material presented in the
main text.
More than 160 video clips, animations, atomic structures, and high-
resolution micrographs complement the book and are available online.
The movies are correlated with each chapter and callouts are highlighted
in color. This supplemental material, created to clarify complex and critical
concepts, highlights the intrinsic beauty of living cells.
For those who wish to probe even more deeply, Molecular Biology of
the Cell, now in its sixth edition, offers a detailed account of the life of
the cell. In addition, Molecular Biology of the Cell, Sixth Edition: A Prob-
lems Approach, by John Wilson and Tim Hunt, provides a gold mine of
thought-provoking questions at all levels of difficulty. We have drawn
upon this tour-de-force of experimental reasoning for some of the ques-
tions in Essential Cell Biology, and we are very grateful to its authors.
Every chapter of Essential Cell Biology is the product of a communal effort:

both text and figures were revised and refined as drafts circulated from
one author to another—many times over and back again! The numer-
ous other individuals who have helped bring this project to fruition are
credited in the Acknowledgments that follow. Despite our best efforts, it
is inevitable that errors will have crept into the book, and we encourage
eagle-eyed readers who find mistakes to let us know, so that we can
correct them in the next printing.
Acknowledgments
The authors acknowledge the many contributions of professors and
students from around the world in the creation of this Fifth Edition. In
particular, we received detailed reviews from the following instructors
who had used the fourth edition, and we would like to thank them for
their important contributions to our revision:
Delbert Abi Abdallah, Thiel College, Pennsylvania
Ann Aguanno, Marymount Manhattan College
David W. Barnes, Georgia Gwinnett College
Manfred Beilharz, The University of Western Australia
Christopher Brandl, Western University, Ontario
Marion Brodhagen, Western Washington University
David Casso, San Francisco State University
Shazia S. Chaudhry, The University of Manchester, United Kingdom
Ron Dubreuil, The University of Illinois at Chicago
Heidi Engelhardt, University of Waterloo, Canada
Sarah Ennis, University of Southampton, United Kingdom
David Featherstone, The University of Illinois at Chicago
Yen Kang France, Georgia College
Barbara Frank, Idaho State University
Daniel E. Frigo, University of Houston
Marcos Garcia-Ojeda, University of California, Merced
David L. Gard, The University of Utah
Adam Gromley, Lincoln Memorial University, Tennessee
Elly Holthuizen, University Medical Center Utrecht, The Netherlands
Harold Hoops, The State University of New York, Geneseo
Bruce Jensen, University of Jamestown, North Dakota
Andor Kiss, Miami University, Ohio
Annette Koenders, Edith Cowan University, Australia
Arthur W. Lambert, Whitehead Institute for Biomedical Research
Denis Larochelle, Clark University, Massachusetts
David Leaf, Western Washington University
Esther Leise, The University of North Carolina at Greensboro
Bernhard Lieb, University of Mainz, Germany
Preface vii
Julie Lively, Louisiana State University

Caroline Mackintosh, University of Saint Mary, Kansas
John Mason, The University of Edinburgh, Scotland
Craig Milgrim, Grossmont College, California
Arkadeep Mitra, City College, Kolkata, India
Niels Erik Møllegaard, University of Copenhagen
Javier Naval, University of Zaragoza, Spain
Marianna Patrauchan, Oklahoma State University
Amanda Polson-Zeigler, University of South Carolina
George Risinger, Oklahoma City Community College
Laura Romberg, Oberlin College, Ohio
Sandra Schulze, Western Washington University
Isaac Skromne, University of Richmond, Virginia
Anna Slusarz, Stephens College, Missouri
Richard Smith, University of Tennessee Health Science Center
Alison Snape, King’s College London
Shannon Stevenson, University of Minnesota Duluth
Marla Tipping, Providence College, Rhode Island
Jim Tokuhisa, Virginia Polytechnic Institute and State University
Guillaume van Eys, Maastricht University, The Netherlands
Barbara Vertel, Rosalind Franklin University of Medicine and Science, Illinois
Jennifer Waby, University of Bradford, United Kingdom
Dianne Watters, Griffith University, Australia
Allison Wiedemeier, University of Louisiana at Monroe
Elizabeth Wurdak, St. John’s University, Minnesota
Kwok-Ming Yao, The University of Hong Kong
Foong May Yeong, National University of Singapore
We are also grateful to those readers who alerted us to errors that they
found in the previous edition.
Working on this book has been a pleasure, in part due to the many people
who contributed to its creation. Nigel Orme again worked closely with
author Keith Roberts to generate the entire illustration program with his
usual skill and care. He also produced all of the artwork for both cover
and chapter openers as a respectful digital tribute to the “squeeze-bottle”
paintings of the American artist Alden Mason (1919–2013). As in previ-
ous editions, Emma Jeffcock did a brilliant job in laying out the whole
book and meticulously incorporated our endless corrections. We owe a
special debt to Michael Morales, our editor at Garland Science, who co-
ordinated the whole enterprise. He oversaw the initial reviewing, worked
closely with the authors on their chapters, took great care of us at numer-
ous writing meetings, and kept us organized and on schedule. He also
orchestrated the wealth of online materials, including all video clips
and animations. Our copyeditor, Jo Clayton, ensured that the text was
stylistically consistent and error-free. At Garland, we also thank Jasmine
Ribeaux, Georgina Lucas, and Adam Sendroff.
For welcoming our book to W. W. Norton and bringing this edition to
print, we thank our editor Betsy Twitchell, as well as Roby Harrington,
Drake McFeely, Julia Reidhead, and Ann Shin for their support. Taylere
Peterson and Danny Vargo deserve thanks for their assistance as
the book moved from Garland to Norton and through production.
We are grateful to media editor Kate Brayton and content develop-
ment specialist Todd Pearson, associate editors Gina Forsythe and
Katie Callahan, and media editorial assistant Katie Daloia whose
coordination of electronic media development has resulted in an un-
matched suite of resources for cell biology students and instructors
alike. We are grateful for marketing manager Stacy Loyal’s tireless
enthusiasm and advocacy for our book. Megan Schindel, Ted Szczepanski,
and Stacey Stambaugh are all owed thanks for navigating the permis-
sions for this edition. And Jane Searle’s able management of produc-
tion, Carla Talmadge’s incredible attention to detail, and their shared
knack for troubleshooting made the book you hold in your hands
a reality.
viii Preface
Denise Schanck deserves extra special thanks for providing continuity

as she helped shepherd this edition from Garland to Norton. As always,
she attended all of our writing retreats and displayed great wisdom in
orchestrating everything she touched.
Last but not least, we are grateful, yet again, to our colleagues and our
families for their unflagging tolerance and support. We give our thanks
to everyone in this long list.
Resources for Instructors

and Students
INSTRUCTOR RESOURCES
wwnorton.com/instructors
Smartwork5
Smartwork5 is an easy-to-use online assessment tool that helps stu-
dents become better problem solvers through a variety of interactive
question types and extensive answer-specific feedback. All Smartwork5
questions are written specifically for the book, are tagged to Bloom’s
levels and learning objectives, and many include art and animations.
Get started quickly with our premade assignments or take advantage
of Smartwork5’s flexibility by customizing questions and adding your
own content. Integration with your campus LMS saves you time by al-
lowing Smartwork5 grades to report right to your LMS gradebook, while
individual and class-wide performance reports help you see students’
progress.
Interactive Instructor’s Guide

An all-in-one resource for instructors who want to integrate active
learning into their course. Searchable by chapter, phrase, topic, or
learning objective, the Interactive Instructor’s Guide compiles the many
valuable teaching resources available with Essential Cell Biology. This
website includes activities, discussion questions, animations and videos,
lecture outlines, learning objectives, primary literature suggestions,
medical topics guide, and more.
Coursepacks
Easily add high-quality Norton digital media to your online, hybrid, or
lecture course. Norton Coursepacks work within your existing learning
management system. Content is customizable and includes chapter-
based, multiple-choice reading quizzes, text-based learning objectives,
access to the full suite of animations, flashcards, and a glossary.
Test Bank
Written by Linda Huang, University of Massachusetts Boston, and Cheryl
D. Vaughan, Harvard University Division of Continuing Education,
the revised and expanded Test Bank for Essential Cell Biology includes
65–80 questions per chapter. Questions are available in multiple-choice,
matching, fill-in-the-blank, and short-answer formats, with many using
art from the textbook. All questions are tagged to Bloom’s taxonomy
level, learning objective, book section, and difficulty level, allowing in-
structors to easily create meaningful exams. The Test Bank is available in
ExamView and as downloadable PDFs from wwnorton.com/instructors.
Preface ix
Animations and Videos

Streaming links give access to more than 130 videos and animations,
bringing the concepts of cell biology to life. The movies are correlated
with each chapter and callouts are highlighted in color.
Figure-integrated Lecture Outlines

All of the figures are integrated in PowerPoint, along with the section
and concept headings from the text, to give instructors a head start
creating lectures for their course.
Image Files
Every figure and photograph in the book is available for download in
PowerPoint and JPG formats from wwnorton.com/instructors.
STUDENT RESOURCES
digital.wwnorton.com/ecb5
Animations and Videos

Streaming links give access to more than 130 videos and animations,
bringing the concepts of cell biology to life. Animations can also be
accessed via the ebook and in select Smartwork5 questions. The movies
are correlated with each chapter and callouts are highlighted in color.
Student Site
Resources for self-study are available on the student site, including
multiple-choice quizzes, cell explorer slides, challenge and concept
questions, flashcards, and a glossary.
ABOUT THE AUTHORS
BRUCE ALBERTS received his PhD from Harvard University and is a

professor in the Department of Biochemistry and Biophysics at the
University of California, San Francisco. He was the editor in chief of
Science from 2008 to 2013 and served as president of the U.S. National
Academy of Sciences from 1993 to 2005.
KAREN HOPKIN received her PhD from the Albert Einstein College of
Medicine and is a science writer. Her work has appeared in various
scientific publications, including Science, Proceedings of the National
Academy of Sciences, and The Scientist, and she is a regular contributor to
Scientific American’s daily podcast, “60-Second Science.”
ALEXANDER JOHNSON received his PhD from Harvard University and
is a professor in the Department of Microbiology and Immunology at the
University of California, San Francisco.
DAVID MORGAN received his PhD from the University of California, San
Francisco, where he is a professor in the Department of Physiology and
vice dean for research in the School of Medicine.
MARTIN RAFF received his MD from McGill University and is emeritus
professor of biology at the Medical Research Council Laboratory for
Molecular Cell Biology at University College London.
KEITH ROBERTS received his PhD from the University of Cambridge and
was deputy director of the John Innes Centre. He is emeritus professor at
the University of East Anglia.
PETER WALTER received his PhD from The Rockefeller University in New
York and is a professor in the Department of Biochemistry and Biophysics
at the University of California, San Francisco, and an investigator of the
Howard Hughes Medical Institute.
x
Preface xi
LIST OF CHAPTERS and
SPECIAL FEATURES
CHAPTE R 1 Cells: The Fundamental Units of Life 1

PANEL 1–1 Microscopy 12
TABLE 1–1 Historical Landmarks in Determining Cell Structure 24
PANEL 1–2 Cell Architecture 25
How We Know: Life’s Common Mechanisms 30
TABLE 1–2 Some Model Organisms and Their Genomes 35
CHAPTE R 2 Chemical Components of Cells 39

TABLE 2–1 Length and Strength of Some Chemical Bonds 48
TABLE 2–2 The Chemical Composition of a Bacterial Cell 52
How We Know: The Discovery of Macromolecules 60
PANEL 2–1 Chemical Bonds and Groups 66
PANEL 2–2 The Chemical Properties of Water 68
PANEL 2–3 The Principal Types of Weak Noncovalent Bonds 70
PANEL 2– 4 An Outline of Some of the Types of Sugars 72
PANEL 2–5 Fatty Acids and Other Lipids 74
PANEL 2– 6 The 20 Amino Acids Found in Proteins 76
PANEL 2–7 A Survey of the Nucleotides 78
CHAPTE R 3 Energy, Catalysis, and Biosynthesis 81

PANEL 3–1 Free Energy and Biological Reactions 94
TABLE 3–1 Relationship Between the Standard Free-Energy Change, G°, and the Equilibrium Constant 96
How We Know: “High-Energy” Phosphate Bonds Power Cell Processes 102
TABLE 3–2 Some Activated Carriers Widely Used in Metabolism 109
CHAPTE R 4 Protein Structure and Function 117

PANEL 4 –1 A Few Examples of Some General Protein Functions 118
PANEL 4 –2 Making and Using Antibodies 140
TABLE 4 –1 Some Common Functional Classes of Enzymes 142
How We Know: Measuring Enzyme Performance 144
TABLE 4 –2 Historical Landmarks in Our Understanding of Proteins 160
PANEL 4 –3 Cell Breakage and Initial Fractionation of Cell Extracts 164
PANEL 4 – 4 Protein Separation by Chromatography 166
PANEL 4 –5 Protein Separation by Electrophoresis 167
PANEL 4 – 6 Protein Structure Determination 168
CHAPTE R 5 DNA and Chromosomes 173

How We Know: Genes Are Made of DNA 193
xi
xii List of Chapters and Special Features
CHAPTE R 6 DNA Replication and Repair 199

How We Know: The Nature of Replication 202
TABLE 6 –1 Proteins Involved in DNA Replication 213
TABLE 6 –2 Error Rates 218
CHAPTE R 7 From DNA to Protein: How Cells Read the Genome 227
TABLE 7–1 Types of RNA Produced in Cells 232
TABLE 7–2 The Three RNA Polymerases in Eukaryotic Cells 235
How We Know: Cracking the Genetic Code 246
TABLE 7–3 Antibiotics That Inhibit Bacterial Protein or RNA Synthesis 256
TABLE 7– 4 Biochemical Reactions That Can Be Catalyzed by Ribozymes 261
CHAPTE R 8 Control of Gene Expression 267

How We Know: Gene Regulation—The Story of Eve 280
CHAPTE R 9 How Genes and Genomes Evolve 297

TABLE 9–1 Viruses That Cause Human Disease 318
TABLE 9–2 Some Vital Statistics for the Human Genome 322
How We Know: Counting Genes 324
CHAPTE R 10 Analyzing the Structure and Function of Genes 333

How We Know: Sequencing the Human Genome 348
CHAPTE R 11 Membrane Structure 365

TABLE 11–1 Some Examples of Plasma Membrane Proteins and Their Functions 375
How We Know: Measuring Membrane Flow 384
CHAPTE R 12 Transport Across Cell Membranes 389

TABLE 12–1 A Comparison of Ion Concentrations Inside and Outside a Typical Mammalian Cell 391
TABLE 12–2 Some Examples of Transmembrane Pumps 403
How We Know: Squid Reveal Secrets of Membrane Excitability 412
TABLE 12–3 Some Examples of Ion Channels 419
CHAPTE R 13 How Cells Obtain Energy from Food 427

TABLE 13–1 Some Types of Enzymes Involved in Glycolysis 431
PANEL 13–1 Details of the 10 Steps of Glycolysis 436
PANEL 13–2 The Complete Citric Acid Cycle 442
How We Know: Unraveling the Citric Acid Cycle 444
CHAPTE R 14 Energy Generation in Mitochondria and Chloroplasts 455

TABLE 14 –1 Product Yields from Glucose Oxidation 469
PANEL 14 –1 Redox Potentials 472
How We Know: How Chemiosmotic Coupling Drives ATP Synthesis 476
CHAPTE R 15 Intracellular Compartments and Protein Transport 495

TABLE 15 –1 The Main Functions of Membrane-enclosed Organelles of a Eukaryotic Cell 497
TABLE 15 –2 The Relative Volumes and Numbers of the Major Membrane-enclosed Organelles
in a Liver Cell (Hepatocyte) 498
List of Chapters and Special Features xiii
TABLE 15 –3 Some Typical Signal Sequences 502

TABLE 15 – 4 Some Types of Coated Vesicles 513
How We Know: Tracking Protein and Vesicle Transport 520
CHAPTE R 16 Cell Signaling 533

TABLE 16 –1 Some Examples of Signal Molecules 536
TABLE 16 –2 Some Foreign Substances That Act on Cell-Surface Receptors 544
TABLE 16 –3 Some Cell Responses Mediated by Cyclic AMP 550
TABLE 16 – 4 Some Cell Responses Mediated by Phospholipase C Activation 552
How We Know: Untangling Cell Signaling Pathways 563
CHAPTE R 17 Cytoskeleton 573

TABLE 17–1 Drugs That Affect Microtubules 584
How We Know: Pursuing Microtubule-associated Motor Proteins 588
TABLE 17–2 Drugs That Affect Filaments 594
CHAPTE R 18 The Cell-Division Cycle 609

TABLE 18–1 Some Eukaryotic Cell-Cycle Durations 611
How We Know: Discovery of Cyclins and Cdks 615
TABLE 18–2 The Major Cyclins and Cdks of Vertebrates 617
PANEL 18–1 The Principal Stages of M Phase in an Animal Cell 628
CHAPTE R 19 Sexual Reproduction and Genetics 651

PANEL 19–1 Some Essentials of Classical Genetics 675
How We Know: Using SNPs to Get a Handle on Human Disease 684
CHAPTE R 20 Cell Communities: Tissues, Stem Cells, and Cancer 691

TABLE 20 –1 A Variety of Factors Can Contribute to Genetic Instability 721
TABLE 20 –2 Examples of Cancer-critical Genes 728
How We Know: Making Sense of the Genes That Are Critical for Cancer 730
Preface xv
CONTENTS
Preface v
About the Authors x
CHAPTER 1
Cells: The Fundamental Units of Life 1

UNITY AND DIVERSITY OF CELLS 2
Cells Vary Enormously in Appearance and Function 2
Living Cells All Have a Similar Basic Chemistry 3
Living Cells Are Self-Replicating Collections of Catalysts 4
All Living Cells Have Apparently Evolved from the Same Ancestral Cell 5
Genes Provide Instructions for the Form, Function, and Behavior of Cells and Organisms 6
CELLS UNDER THE MICROSCOPE 6
The Invention of the Light Microscope Led to the Discovery of Cells 7
Light Microscopes Reveal Some of a Cell’s Components 8
The Fine Structure of a Cell Is Revealed by Electron Microscopy 9
THE PROKARYOTIC CELL 11
Prokaryotes Are the Most Diverse and Numerous Cells on Earth 14
The World of Prokaryotes Is Divided into Two Domains: Bacteria and Archaea 15
THE EUKARYOTIC CELL 16
The Nucleus Is the Information Store of the Cell 16
Mitochondria Generate Usable Energy from Food Molecules 17
Chloroplasts Capture Energy from Sunlight 18
Internal Membranes Create Intracellular Compartments with Different Functions 19
The Cytosol Is a Concentrated Aqueous Gel of Large and Small Molecules 21
The Cytoskeleton Is Responsible for Directed Cell Movements 22
The Cytosol Is Far from Static 23
Eukaryotic Cells May Have Originated as Predators 24
MODEL ORGANISMS 27
Molecular Biologists Have Focused on E. coli 27
Brewer’s Yeast Is a Simple Eukaryote 28
Arabidopsis Has Been Chosen as a Model Plant 28
Model Animals Include Flies, Worms, Fish, and Mice 29
Biologists Also Directly Study Humans and Their Cells 32
Comparing Genome Sequences Reveals Life’s Common Heritage 33
Genomes Contain More Than Just Genes 35
ESSENTIAL CONCEPTS 36
QUESTIONS 37
xv
xvi Contents
CHAPTER 2
Chemical Components of Cells 39

CHEMICAL BONDS 40
Cells Are Made of Relatively Few Types of Atoms 40
The Outermost Electrons Determine How Atoms Interact 41
Covalent Bonds Form by the Sharing of Electrons 43
Some Covalent Bonds Involve More Than One Electron Pair 44
Electrons in Covalent Bonds Are Often Shared Unequally 45
Covalent Bonds Are Strong Enough to Survive the Conditions Inside Cells 45
Ionic Bonds Form by the Gain and Loss of Electrons 46
Hydrogen Bonds Are Important Noncovalent Bonds for Many Biological Molecules 47
Four Types of Weak Interactions Help Bring Molecules Together in Cells 47
Some Polar Molecules Form Acids and Bases in Water 49
SMALL MOLECULES IN CELLS 50
A Cell Is Formed from Carbon Compounds 50
Cells Contain Four Major Families of Small Organic Molecules 51
Sugars Are both Energy Sources and Subunits of Polysaccharides 52
Fatty Acid Chains Are Components of Cell Membranes 54
Amino Acids Are the Subunits of Proteins 56
Nucleotides Are the Subunits of DNA and RNA 56
MACROMOLECULES IN CELLS 58
Each Macromolecule Contains a Specific Sequence of Subunits 59
Noncovalent Bonds Specify the Precise Shape of a Macromolecule 62
Noncovalent Bonds Allow a Macromolecule to Bind Other Selected Molecules 62
QUESTIONS 65
CHAPTER 3
Energy, Catalysis, and Biosynthesis 81

THE USE OF ENERGY BY CELLS 82
Biological Order Is Made Possible by the Release of Heat Energy from Cells 83
Cells Can Convert Energy from One Form to Another 84
Photosynthetic Organisms Use Sunlight to Synthesize Organic Molecules 85
Cells Obtain Energy by the Oxidation of Organic Molecules 86
Oxidation and Reduction Involve Electron Transfers 87
FREE ENERGY AND CATALYSIS 88
Chemical Reactions Proceed in the Direction That Causes a Loss of Free Energy 89
Enzymes Reduce the Energy Needed to Initiate Spontaneous Reactions 89
The Free-Energy Change for a Reaction Determines Whether It Can Occur 90
G Changes as a Reaction Proceeds Toward Equilibrium 92
The Standard Free-Energy Change, G°, Makes It Possible to Compare the Energetics of
Different Reactions 92
The Equilibrium Constant Is Directly Proportional to G° 96
In Complex Reactions, the Equilibrium Constant Includes the Concentrations of
All Reactants and Products 96
Contents xvii
The Equilibrium Constant Also Indicates the Strength of Noncovalent Binding Interactions 97
For Sequential Reactions, the Changes in Free Energy Are Additive 98
Enzyme-catalyzed Reactions Depend on Rapid Molecular Collisions 99
Noncovalent Interactions Allow Enzymes to Bind Specific Molecules 100
ACTIVATED CARRIERS AND BIOSYNTHESIS 101
The Formation of an Activated Carrier Is Coupled to an Energetically Favorable Reaction 101
ATP Is the Most Widely Used Activated Carrier 104
Energy Stored in ATP Is Often Harnessed to Join Two Molecules Together 106
NADH and NADPH Are Both Activated Carriers of Electrons 106
NADPH and NADH Have Different Roles in Cells 108
Cells Make Use of Many Other Activated Carriers 108
The Synthesis of Biological Polymers Requires an Energy Input 110
QUESTIONS 114
CHAPTER 4
Protein Structure and Function 117

THE SHAPE AND STRUCTURE OF PROTEINS 119
The Shape of a Protein Is Specified by Its Amino Acid Sequence 119
Proteins Fold into a Conformation of Lowest Energy 122
Proteins Come in a Wide Variety of Complicated Shapes 124
The a Helix and the b Sheet Are Common Folding Patterns 126
Helices Form Readily in Biological Structures 127
b Sheets Form Rigid Structures at the Core of Many Proteins 129
Misfolded Proteins Can Form Amyloid Structures That Cause Disease 129
Proteins Have Several Levels of Organization 129
Proteins Also Contain Unstructured Regions 130
Few of the Many Possible Polypeptide Chains Will Be Useful 131
Proteins Can Be Classified into Families 132
Large Protein Molecules Often Contain More than One Polypeptide Chain 132
Proteins Can Assemble into Filaments, Sheets, or Spheres 134
Some Types of Proteins Have Elongated Fibrous Shapes 134
Extracellular Proteins Are Often Stabilized by Covalent Cross-Linkages 135
HOW PROTEINS WORK 137
All Proteins Bind to Other Molecules 137
Humans Produce Billions of Different Antibodies, Each with a Different Binding Site 138
Enzymes Are Powerful and Highly Specific Catalysts 139
Enzymes Greatly Accelerate the Speed of Chemical Reactions 142
Lysozyme Illustrates How an Enzyme Works 143
Many Drugs Inhibit Enzymes 147
Tightly Bound Small Molecules Add Extra Functions to Proteins 148
HOW PROTEINS ARE CONTROLLED 149
The Catalytic Activities of Enzymes Are Often Regulated by Other Molecules 150
Allosteric Enzymes Have Two or More Binding Sites That Influence One Another 151
Phosphorylation Can Control Protein Activity by Causing a Conformational Change 152
Covalent Modifications Also Control the Location and Interaction of Proteins 153
Regulatory GTP-Binding Proteins Are Switched On and Off by the Gain and Loss of a Phosphate Group 154
xviii Contents
ATP Hydrolysis Allows Motor Proteins to Produce Directed Movements in Cells 154
Proteins Often Form Large Complexes That Function as Machines 155
Many Interacting Proteins Are Brought Together by Scaffolds 156
Weak Interactions Between Macromolecules Can Produce Large Biochemical
Subcompartments in Cells 157
HOW PROTEINS ARE STUDIED 158
Proteins Can Be Purified from Cells or Tissues 158
Determining a Protein’s Structure Begins with Determining Its Amino Acid Sequence 159
Genetic Engineering Techniques Permit the Large-Scale Production, Design, and Analysis of
Almost Any Protein 161
The Relatedness of Proteins Aids the Prediction of Protein Structure and Function 162
QUESTIONS 170
CHAPTER 5
DNA and Chromosomes 173

THE STRUCTURE OF DNA 174
A DNA Molecule Consists of Two Complementary Chains of Nucleotides 175
The Structure of DNA Provides a Mechanism for Heredity 176
THE STRUCTURE OF EUKARYOTIC CHROMOSOMES 178
Eukaryotic DNA Is Packaged into Multiple Chromosomes 179
Chromosomes Organize and Carry Genetic Information 180
Specialized DNA Sequences Are Required for DNA Replication
and Chromosome Segregation 181
Interphase Chromosomes Are Not Randomly Distributed Within the Nucleus 182
The DNA in Chromosomes Is Always Highly Condensed 183
Nucleosomes Are the Basic Units of Eukaryotic Chromosome Structure 184
Chromosome Packing Occurs on Multiple Levels 186
THE REGULATION OF CHROMOSOME STRUCTURE 188
Changes in Nucleosome Structure Allow Access to DNA 188
Interphase Chromosomes Contain both Highly Condensed
and More Extended Forms of Chromatin 189
QUESTIONS 196
Contents xix
CHAPTER 6
DNA Replication and Repair 199

DNA REPLICATION 200
Base-Pairing Enables DNA Replication 200
DNA Synthesis Begins at Replication Origins 201
Two Replication Forks Form at Each Replication Origin 201
DNA Polymerase Synthesizes DNA Using a Parental Strand as a Template 205
The Replication Fork Is Asymmetrical 206
DNA Polymerase Is Self-correcting 207
Short Lengths of RNA Act as Primers for DNA Synthesis 208
Proteins at a Replication Fork Cooperate to Form a Replication Machine 210
Telomerase Replicates the Ends of Eukaryotic Chromosomes 213
Telomere Length Varies by Cell Type and with Age 214
DNA REPAIR 215
DNA Damage Occurs Continually in Cells 215
Cells Possess a Variety of Mechanisms for Repairing DNA 217
A DNA Mismatch Repair System Removes Replication Errors That Escape Proofreading 218
Double-Strand DNA Breaks Require a Different Strategy for Repair 219
Homologous Recombination Can Flawlessly Repair DNA Double-Strand Breaks 220
Failure to Repair DNA Damage Can Have Severe Consequences for a Cell or Organism 222
A Record of the Fidelity of DNA Replication and Repair Is Preserved in Genome Sequences 223
QUESTIONS 225
CHAPTER 7
From DNA to Protein: How Cells Read

the Genome 227
FROM DNA TO RNA 228
Portions of DNA Sequence Are Transcribed into RNA 229
Transcription Produces RNA That Is Complementary to One Strand of DNA 230
Cells Produce Various Types of RNA 232
Signals in the DNA Tell RNA Polymerase Where to Start and Stop Transcription 233
Initiation of Eukaryotic Gene Transcription Is a Complex Process 235
Eukaryotic RNA Polymerase Requires General Transcription Factors 235
Eukaryotic mRNAs Are Processed in the Nucleus 237
In Eukaryotes, Protein-Coding Genes Are Interrupted
by Noncoding Sequences Called Introns 239
Introns Are Removed from Pre-mRNAs by RNA Splicing 239
RNA Synthesis and Processing Takes Place in “Factories” Within the Nucleus 242
Mature Eukaryotic mRNAs Are Exported from the Nucleus 242
mRNA Molecules Are Eventually Degraded in the Cytosol 242
FROM RNA TO PROTEIN 243
An mRNA Sequence Is Decoded in Sets of Three Nucleotides 244
tRNA Molecules Match Amino Acids to Codons in mRNA 245
xx Contents
Specific Enzymes Couple tRNAs to the Correct Amino Acid 249

The mRNA Message Is Decoded on Ribosomes 249
The Ribosome Is a Ribozyme 252
Specific Codons in an mRNA Signal the Ribosome Where to Start and to Stop Protein
Synthesis 253
Proteins Are Produced on Polyribosomes 255
Inhibitors of Prokaryotic Protein Synthesis Are Used as Antibiotics 255
Controlled Protein Breakdown Helps Regulate the Amount of Each Protein in a Cell 256
There Are Many Steps Between DNA and Protein 257
RNA AND THE ORIGINS OF LIFE 259
Life Requires Autocatalysis 259
RNA Can Store Information and Catalyze Chemical Reactions 260
RNA Is Thought to Predate DNA in Evolution 261
QUESTIONS 264
CHAPTER 8
Control of Gene Expression 267

AN OVERVIEW OF GENE EXPRESSION 268
The Different Cell Types of a Multicellular Organism Contain the Same DNA 268
Different Cell Types Produce Different Sets of Proteins 269
A Cell Can Change the Expression of Its Genes in Response to External Signals 270
Gene Expression Can Be Regulated at Various Steps from DNA to RNA to Protein 270
HOW TRANSCRIPTION IS REGULATED 271
Transcription Regulators Bind to Regulatory DNA Sequences 271
Transcription Switches Allow Cells to Respond to Changes in Their Environment 273
Repressors Turn Genes Off and Activators Turn Them On 274
The Lac Operon Is Controlled by an Activator and a Repressor 275
Eukaryotic Transcription Regulators Control Gene Expression from a Distance 276
Eukaryotic Transcription Regulators Help Initiate Transcription
by Recruiting Chromatin-Modifying Proteins 276
The Arrangement of Chromosomes into Looped Domains Keeps Enhancers in Check 278
GENERATING SPECIALIZED CELL TYPES 278
Eukaryotic Genes Are Controlled by Combinations of Transcription Regulators 279
The Expression of Different Genes Can Be Coordinated by a Single Protein 279
Combinatorial Control Can Also Generate Different Cell Types 282
The Formation of an Entire Organ Can Be Triggered by a Single Transcription Regulator 284
Transcription Regulators Can Be Used to Experimentally Direct the Formation of Specific Cell
Types in Culture 285
Differentiated Cells Maintain Their Identity 286
Contents xxi
POST-TRANSCRIPTIONAL CONTROLS 287

mRNAs Contain Sequences That Control Their Translation 288
Regulatory RNAs Control the Expression of Thousands of Genes 288
MicroRNAs Direct the Destruction of Target mRNAs 289
Small Interfering RNAs Protect Cells From Infections 290
Thousands of Long Noncoding RNAs May Also Regulate Mammalian Gene Activity 291
QUESTIONS 293
CHAPTER 9
How Genes and Genomes Evolve 297

GENERATING GENETIC VARIATION 298
In Sexually Reproducing Organisms, Only Changes to the Germ Line
Are Passed On to Progeny 299
Point Mutations Are Caused by Failures of the Normal Mechanisms
for Copying and Repairing DNA 300
Mutations Can Also Change the Regulation of a Gene 302
DNA Duplications Give Rise to Families of Related Genes 302
Duplication and Divergence Produced the Globin Gene Family 304
Whole-Genome Duplications Have Shaped the Evolutionary History of Many Species 306
Novel Genes Can Be Created by Exon Shuffling 306
The Evolution of Genomes Has Been Profoundly Influenced by Mobile Genetic Elements 307
Genes Can Be Exchanged Between Organisms by Horizontal Gene Transfer 308
RECONSTRUCTING LIFE’S FAMILY TREE 309
Genetic Changes That Provide a Selective Advantage Are Likely to Be Preserved 309
Closely Related Organisms Have Genomes That Are Similar
in Organization as Well as Sequence 310
Functionally Important Genome Regions Show Up as Islands of Conserved DNA Sequence 310
Genome Comparisons Show That Vertebrate Genomes Gain and Lose DNA Rapidly 313
Sequence Conservation Allows Us to Trace Even the Most Distant Evolutionary Relationships 313
MOBILE GENETIC ELEMENTS AND VIRUSES 315
Mobile Genetic Elements Encode the Components They Need for Movement 315
The Human Genome Contains Two Major Families of Transposable Sequences 316
Viruses Can Move Between Cells and Organisms 317
Retroviruses Reverse the Normal Flow of Genetic Information 318
EXAMINING THE HUMAN GENOME 320
The Nucleotide Sequences of Human Genomes Show How Our Genes Are Arranged 321
Differences in Gene Regulation May Help Explain How Animals with Similar Genomes Can Be So Different 323
The Genome of Extinct Neanderthals Reveals Much about What Makes Us Human 326
Genome Variation Contributes to Our Individuality—But How? 327
QUESTIONS 329
xxii Contents
CHAPTER 10
Analyzing the Structure and Function of

Genes 333
ISOLATING AND CLONING DNA MOLECULES 334
Restriction Enzymes Cut DNA Molecules at Specific Sites 335
Gel Electrophoresis Separates DNA Fragments of Different Sizes 335
DNA Cloning Begins with the Production of Recombinant DNA 337
Recombinant DNA Can Be Copied Inside Bacterial Cells 337
An Entire Genome Can Be Represented in a DNA Library 339
Hybridization Provides a Sensitive Way to Detect Specific Nucleotide Sequences 340
DNA CLONING BY PCR 341
PCR Uses DNA Polymerase and Specific DNA Primers to Amplify
DNA Sequences in a Test Tube 342
PCR Can Be Used for Diagnostic and Forensic Applications 343
SEQUENCING DNA 346
Dideoxy Sequencing Depends on the Analysis of DNA Chains
Terminated at Every Position 346
Next-Generation Sequencing Techniques Make Genome Sequencing Faster and
Cheaper 347
Comparative Genome Analyses Can Identify Genes and Predict Their Function 350
EXPLORING GENE FUNCTION 350
Analysis of mRNAs Provides a Snapshot of Gene Expression 351
In Situ Hybridization Can Reveal When and Where a Gene Is Expressed 352
Reporter Genes Allow Specific Proteins to Be Tracked in Living Cells 352
The Study of Mutants Can Help Reveal the Function of a Gene 354
RNA Interference (RNAi) Inhibits the Activity of Specific Genes 354
A Known Gene Can Be Deleted or Replaced with an Altered Version 355
Genes Can Be Edited with Great Precision Using the Bacterial CRISPR System 358
Mutant Organisms Provide Useful Models of Human Disease 359
Transgenic Plants Are Important for both Cell Biology and Agriculture 359
Even Rare Proteins Can Be Made in Large Amounts Using Cloned DNA 361
QUESTIONS 363
Contents xxiii
CHAPTER 11
Membrane Structure 365

THE LIPID BILAYER 367
Membrane Lipids Form Bilayers in Water 367
The Lipid Bilayer Is a Flexible Two-dimensional Fluid 370
The Fluidity of a Lipid Bilayer Depends on Its Composition 371
Membrane Assembly Begins in the ER 373
Certain Phospholipids Are Confined to One Side of the Membrane 373
MEMBRANE PROTEINS 375
Membrane Proteins Associate with the Lipid Bilayer in Different Ways 376
A Polypeptide Chain Usually Crosses the Lipid Bilayer as an a Helix 377
Membrane Proteins Can Be Solubilized in Detergents 378
We Know the Complete Structure of Relatively Few Membrane Proteins 379
The Plasma Membrane Is Reinforced by the Underlying Cell Cortex 380
A Cell Can Restrict the Movement of Its Membrane Proteins 381
The Cell Surface Is Coated with Carbohydrate 382
QUESTIONS 387
CHAPTER 12
Transport Across Cell Membranes 389

PRINCIPLES OF TRANSMEMBRANE TRANSPORT 390
Lipid Bilayers Are Impermeable to Ions and Most Uncharged Polar Molecules 390
The Ion Concentrations Inside a Cell Are Very Different from Those Outside 391
Differences in the Concentration of Inorganic Ions Across a Cell Membrane
Create a Membrane Potential 391
Cells Contain Two Classes of Membrane Transport Proteins: Transporters
and Channels 392
Solutes Cross Membranes by Either Passive or Active Transport 392
Both the Concentration Gradient and Membrane Potential Influence the
Passive Transport of Charged Solutes 393
Water Moves Across Cell Membranes Down Its Concentration Gradient—a
Process Called Osmosis 394
TRANSPORTERS AND THEIR FUNCTIONS 395
Passive Transporters Move a Solute Along Its Electrochemical Gradient 396
Pumps Actively Transport a Solute Against Its Electrochemical Gradient 396
The Na+ Pump in Animal Cells Uses Energy Supplied by ATP to Expel Na+ and Bring in K+ 397
The Na+ Pump Generates a Steep Concentration Gradient of Na+ Across the Plasma Membrane 398
Ca2+ Pumps Keep the Cytosolic Ca2+ Concentration Low 399
Gradient-driven Pumps Exploit Solute Gradients to Mediate Active Transport 399
The Electrochemical Na+ Gradient Drives the Transport of Glucose Across the Plasma Membrane of Animal Cells 400
Electrochemical H Gradients Drive the Transport of Solutes in Plants, Fungi, and Bacteria
+
402
ION CHANNELS AND THE MEMBRANE POTENTIAL 403
Ion Channels Are Ion-selective and Gated 404
Membrane Potential Is Governed by the Permeability of a Membrane to Specific Ions 405
xxiv Contents
Ion Channels Randomly Snap Between Open and Closed States 407
Different Types of Stimuli Influence the Opening and Closing of Ion Channels 408
Voltage-gated Ion Channels Respond to the Membrane Potential 409
ION CHANNELS AND NERVE CELL SIGNALING 410
Action Potentials Allow Rapid Long-Distance Communication Along Axons 411
Action Potentials Are Mediated by Voltage-gated Cation Channels 411
Voltage-gated Ca2+ Channels in Nerve Terminals Convert an Electrical Signal into a Chemical
Signal 416
Transmitter-gated Ion Channels in the Postsynaptic Membrane Convert the Chemical Signal
Back into an Electrical Signal 417
Neurotransmitters Can Be Excitatory or Inhibitory 418
Most Psychoactive Drugs Affect Synaptic Signaling by Binding to Neurotransmitter
Receptors 419
The Complexity of Synaptic Signaling Enables Us to Think, Act, Learn, and Remember 420
Light-gated Ion Channels Can Be Used to Transiently Activate or Inactivate Neurons in Living
Animals 421
QUESTIONS 424
CHAPTER 13
How Cells Obtain Energy from Food 427

THE BREAKDOWN AND UTILIZATION OF SUGARS AND FATS 428
Food Molecules Are Broken Down in Three Stages 428
Glycolysis Extracts Energy from the Splitting of Sugar 430
Glycolysis Produces both ATP and NADH 431
Fermentations Can Produce ATP in the Absence of Oxygen 433
Glycolytic Enzymes Couple Oxidation to Energy Storage in Activated Carriers 434
Several Types of Organic Molecules Are Converted to Acetyl CoA
in the Mitochondrial Matrix 438
The Citric Acid Cycle Generates NADH by Oxidizing Acetyl Groups to CO2 438
Many Biosynthetic Pathways Begin with Glycolysis or the Citric Acid Cycle 441
Electron Transport Drives the Synthesis of the Majority of the ATP in Most Cells 446
REGULATION OF METABOLISM 447
Catabolic and Anabolic Reactions Are Organized and Regulated 447
Feedback Regulation Allows Cells to Switch from Glucose Breakdown to
Glucose Synthesis 447
Cells Store Food Molecules in Special Reservoirs to Prepare for Periods of Need 449
QUESTIONS 452
Contents xxv
CHAPTER 14
Energy Generation in Mitochondria

and Chloroplasts 455
Cells Obtain Most of Their Energy by a Membrane-based Mechanism 456
Chemiosmotic Coupling Is an Ancient Process, Preserved in Present-Day Cells 457
MITOCHONDRIA AND OXIDATIVE PHOSPHORYLATION 459
Mitochondria Are Dynamic in Structure, Location, and Number 459
A Mitochondrion Contains an Outer Membrane, an Inner Membrane,
and Two Internal Compartments 460
The Citric Acid Cycle Generates High-Energy Electrons Required for ATP Production 461
The Movement of Electrons Is Coupled to the Pumping of Protons 462
Electrons Pass Through Three Large Enzyme Complexes in the Inner
Mitochondrial Membrane 464
Proton Pumping Produces a Steep Electrochemical Proton Gradient
Across the Inner Mitochondrial Membrane 464
ATP Synthase Uses the Energy Stored in the Electrochemical Proton
Gradient to Produce ATP 465
The Electrochemical Proton Gradient Also Drives Transport Across
the Inner Mitochondrial Membrane 466
The Rapid Conversion of ADP to ATP in Mitochondria Maintains
a High ATP/ADP Ratio in Cells 467
Cell Respiration Is Amazingly Efficient 468
MOLECULAR MECHANISMS OF ELECTRON TRANSPORT AND PROTON PUMPING 469
Protons Are Readily Moved by the Transfer of Electrons 469
The Redox Potential Is a Measure of Electron Affinities 470
Electron Transfers Release Large Amounts of Energy 471
Metals Tightly Bound to Proteins Form Versatile Electron Carriers 471
Cytochrome c Oxidase Catalyzes the Reduction of Molecular Oxygen 474
CHLOROPLASTS AND PHOTOSYNTHESIS 478
Chloroplasts Resemble Mitochondria but Have an Extra Compartment—the Thylakoid 478
Photosynthesis Generates—and Then Consumes—ATP and NADPH 479
Chlorophyll Molecules Absorb the Energy of Sunlight 480
Excited Chlorophyll Molecules Funnel Energy into a Reaction Center 481
A Pair of Photosystems Cooperate to Generate both ATP and NADPH 482
Oxygen Is Generated by a Water-Splitting Complex Associated with Photosystem II 483
The Special Pair in Photosystem I Receives its Electrons from Photosystem II 484
Carbon Fixation Uses ATP and NADPH to Convert CO2 into Sugars 484
Sugars Generated by Carbon Fixation Can Be Stored as Starch or Consumed to Produce ATP 487
THE EVOLUTION OF ENERGY-GENERATING SYSTEMS 488
Oxidative Phosphorylation Evolved in Stages 488
Photosynthetic Bacteria Made Even Fewer Demands on Their Environment 489
The Lifestyle of Methanococcus Suggests That Chemiosmotic Coupling Is an Ancient Process 490
QUESTIONS 492
xxvi Contents
CHAPTER 15
Intracellular Compartments and Protein

Transport 495
MEMBRANE-ENCLOSED ORGANELLES 496
Eukaryotic Cells Contain a Basic Set of Membrane-enclosed Organelles 496
Membrane-enclosed Organelles Evolved in Different Ways 499
PROTEIN SORTING 500
Proteins Are Transported into Organelles by Three Mechanisms 500
Signal Sequences Direct Proteins to the Correct Compartment 502
Proteins Enter the Nucleus Through Nuclear Pores 503
Proteins Unfold to Enter Mitochondria and Chloroplasts 505
Proteins Enter Peroxisomes from both the Cytosol and the Endoplasmic Reticulum 506
Proteins Enter the Endoplasmic Reticulum While Being Synthesized 507
Soluble Proteins Made on the ER Are Released into the ER Lumen 508
Start and Stop Signals Determine the Arrangement of a Transmembrane Protein
in the Lipid Bilayer 509
VESICULAR TRANSPORT 511
Transport Vesicles Carry Soluble Proteins and Membrane Between Compartments 511
Vesicle Budding Is Driven by the Assembly of a Protein Coat 512
Vesicle Docking Depends on Tethers and SNAREs 514
SECRETORY PATHWAYS 515
Most Proteins Are Covalently Modified in the ER 516
Exit from the ER Is Controlled to Ensure Protein Quality 517
The Size of the ER Is Controlled by the Demand for Protein Folding 518
Proteins Are Further Modified and Sorted in the Golgi Apparatus 518
Secretory Proteins Are Released from the Cell by Exocytosis 519
ENDOCYTIC PATHWAYS 523
Specialized Phagocytic Cells Ingest Large Particles 523
Fluid and Macromolecules Are Taken Up by Pinocytosis 524
Receptor-mediated Endocytosis Provides a Specific Route into Animal Cells 525
Endocytosed Macromolecules Are Sorted in Endosomes 526
Lysosomes Are the Principal Sites of Intracellular Digestion 527
QUESTIONS 530
Contents xxvii
CHAPTER 16
Cell Signaling 533

GENERAL PRINCIPLES OF CELL SIGNALING 534
Signals Can Act over a Long or Short Range 534
A Limited Set of Extracellular Signals Can Produce a Huge Variety of Cell Behaviors 537
A Cell’s Response to a Signal Can Be Fast or Slow 538
Cell-Surface Receptors Relay Extracellular Signals via Intracellular Signaling Pathways 539
Some Intracellular Signaling Proteins Act as Molecular Switches 541
Cell-Surface Receptors Fall into Three Main Classes 543
Ion-Channel-Coupled Receptors Convert Chemical Signals into Electrical Ones 544
G-PROTEIN-COUPLED RECEPTORS 545
Stimulation of GPCRs Activates G-Protein Subunits 545
Some Bacterial Toxins Cause Disease by Altering the Activity of G Proteins 547
Some G Proteins Directly Regulate Ion Channels 548
Many G Proteins Activate Membrane-bound Enzymes That Produce Small
Messenger Molecules 549
The Cyclic AMP Signaling Pathway Can Activate Enzymes and Turn On Genes 549
The Inositol Phospholipid Pathway Triggers a Rise in Intracellular Ca2+ 552
A Ca2+ Signal Triggers Many Biological Processes 553
A GPCR Signaling Pathway Generates a Dissolved Gas That Carries a Signal to Adjacent Cells 554
GPCR-Triggered Intracellular Signaling Cascades Can Achieve Astonishing Speed,
Sensitivity, and Adaptability 555
ENZYME-COUPLED RECEPTORS 557
Activated RTKs Recruit a Complex of Intracellular Signaling Proteins 558
Most RTKs Activate the Monomeric GTPase Ras 559
RTKs Activate PI 3-Kinase to Produce Lipid Docking Sites in the Plasma Membrane 560
Some Receptors Activate a Fast Track to the Nucleus 565
Some Extracellular Signal Molecules Cross the Plasma Membrane and Bind to Intracellular Receptors 565
Plants Make Use of Receptors and Signaling Strategies That Differ from Those Used by Animals 567
Protein Kinase Networks Integrate Information to Control Complex Cell Behaviors 567
QUESTIONS 571
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When we returned we found that we had received a present from
the lady of the Ras in whose dominions we were. It consisted of
loaves of “teff” bread, ten fowls, sixty eggs and a jar of honey. All this
we distributed among the men. The Ras’s wife was owner of the soil
hereabouts; it had been given to her as dowry, and in such a case a
woman has practically absolute proprietorship, and the taxes are
collected for her benefit. But her authority does not extend to the
selection of officials; that power remains in the hands of the Ras. As
the Mussulmans had killed during the day the ox which we had
bought for twelve shillings and sixpence, the men had plenty of good
fare before them, and our own larder was not badly stocked. Vultures
of enormous size gathered round the spot where the animal was
slaughtered, and fought over the offal that was left on the ground.
Numbers of carrion crows had flocked to the feast, and had to be
spectators for the most part. But they slipped in among the big
combatants now and then, and filched away a morsel adroitly.
I have given rather fully my impressions of Korata, which is a
typical township of Western Abyssinia. It may interest the reader if I
supplement it by the brief note written by Stecker, who sojourned in
the place more than a fortnight. He wrote—
“On April 1” (1881) “we reached Korata, the most important, most
charmingly situated and largest town on Lake Tsana. Korata has
often been visited by Europeans before, most recently by Piaggia,
who stayed here about a year and a half and occupied himself with
ornithology. Korata is famous for the first-rate excellence of its
coffee, which, as I was able to satisfy myself, flourishes here
exceptionally. The place is almost the most important market on the
Tsana, but at the moment does not contain more than eight hundred
to one thousand inhabitants against three thousand in Theodore’s
time, as very many have either migrated from the town or died of
fever. At an earlier time the Mohammedans were numerous here, but
most of them emigrated to Gallabat after the order issued to them by
King John that they should one and all embrace Christianity. Only a
few families abandoned Islamism and adopted the Coptic faith.
There are also three Jewish families living here. Korata is divided
into ten districts whose names are Dengelteffa, Tukuwodeb,
Margeza, Kulomalfia, Siet biet Negus, Guaguata, Guwi, Adisamba,
Vof tschogevia, Gusudur, and Slam biet (or Slam modeb), which last
is at present entirely deserted. Korata is the abode of an
exceptionally numerous priesthood.” Dr. Stecker formed friendships
among the laity in the place, but “by no means lived in the best
harmony with the pretentious clergy.”[80]
CHAPTER IX
We remained two days at Korata in order to replenish our stock of

grain and other provisions. During this time I watched the birds of
prey as closely as I could. They are of the same species as those
found in the Soudan, and my rough and brief observations, so far as
they went, altogether confirmed the interesting remarks of Sir S.
Baker on the subject. The passage illustrates so well the brilliant
combination in that explorer of keen sportsmanship, and rare ability
in the study of natural history that I need offer no apology for quoting
it:—
“A question has been frequently discussed whether the vulture is
directed to his prey by the sense of smell, or by keenness of vision; I
have paid much attention to their habits, and, although there can be
no question that their power of scent is great, I feel convinced that all
birds of prey are attracted to their food principally by their acuteness
of sight. If a vulture were blind it would starve; but were the nostrils
plugged up with some foreign substance to destroy its power of
smell, it would not materially interfere with its usual mode of hunting.
Scent is always stronger near the surface of the ground: thus
hyenas, lions, and other beasts of prey will scent a carcase from a
great distance, provided they are to leeward; but the same animals
would be unaware of the presence of the body if they were but a
short distance to windward.
“If birds of prey trusted to their nostrils, they would keep as near
the ground as possible, like the carrion crow, which I believe is the
exception that proves the rule. It is an astonishing sight to witness
the sudden arrival of vultures at the death of an animal, when a few
moments before not a bird has been in sight in the cloudless sky. I
have frequently lain down beneath a bush after having shot an
animal, to watch the arrival of the various species of birds in regular
succession; they invariably appear in the following order:—
“No. 1, the black and white crow: this knowing individual is most
industrious is seeking for his food, and is generally to be seen either
perched upon rocks or upon trees; I believe he trusts much to his
sense of smell, as he is never far from the ground; at the same time
he keeps a vigilant look-out with a very sharp pair of eyes.
“No. 2 is the common buzzard; this bird, so well known for its
extreme daring, is omnipresent, and trusts generally to sight, as it
will stoop at a piece of red cloth in mistake for flesh; thus proving that
it depends more upon vision than smell.
“No. 3 is the red-faced small vulture.
“No. 4 is the large bare-throated vulture.
“No. 5 the marabou stork, sometimes accompanied by the
adjutant.
“When employed in watching the habits of these birds it is
interesting to make the experiment of concealing a dead animal
beneath a dense bush. This I have frequently done; in which case
the vultures never find it unless they have witnessed its death; if so,
they will already have pounced in their descent while you have been
engaged in concealing the body. They will then upon near approach
discover it by the smell. But, if an animal is killed in thick grass, eight
or ten feet high, the vultures will seldom discover it. I have frequently
known the bodies of large animals, such as elephants and buffaloes,
to lie for days beneath the shade of the dense nabbuk bushes,
unattended by a single vulture; whereas, if visible, they would have
been visited by these birds in thousands.
“Vultures and the marabou stork fly at enormous altitudes. I
believe that every species keeps to its own particular elevation, and
that the atmosphere contains regular strata of birds of prey, who,
invisible to the human eye at that enormous height, are constantly
resting upon their wide-spread wings, and, soaring in circles,
watching with telescopic sight the world beneath. At that great
elevation they are in an exceedingly cool temperature, therefore they
require no water; but some birds that make long flights over arid
deserts, such as the marabou stork and the bustard, are provided
with water-sacs; the former in an external bag a little below the
throat, the latter in an internal sac, both of which carry a large supply.
As the birds of prey that I have enumerated invariably appear at a
carcase in their regular succession, I can only suggest that they
travel from different distances or altitudes. Thus, the marabou stork
would be farthest from the earth; the large bare-necked vulture
would be the next below him, followed by the red-faced vulture, the
buzzard, and the crow that is generally about the surface. From their
immense elevation, the birds of prey possess an extraordinary field
of vision; and, although they are invisible from the earth, there can
be no doubt that they are perpetually hunting in circles within sight of
each other. Thus, should one bird discover some object upon the
surface of the earth below, his sudden pounce would be at once
observed and imitated by every vulture in succession. Should one
vulture nearest the earth perceive a body, or even should he notice
the buzzards collecting at a given point, he would at once become
aware of a prey; his rush towards the spot would act like a
telegraphic signal to others, that would be rapidly communicated to
every vulture at successive airy stations.
“If an animal be skinned, the red surface will attract the vultures in
an instant; this proves that their sight, and not their scent, has been
attracted by an object that suggests blood. I have frequently watched
them when I have shot an animal, and my people have commenced
the process of skinning. At first, not a bird has been in sight, as I
have lain on my back and gazed into the spotless blue sky; but
hardly has the skin been half withdrawn, than specks have appeared
in the heavens, rapidly increasing. ‘Caw, caw,’ has been heard
several times from the neighbouring bushes; the buzzards have
swept down close to my people, and have snatched a morsel of
clotted blood from the ground. The specks have increased to winged
creatures, at the great height resembling flies, when presently a
rushing sound behind me, like a whirlwind, has been followed by the
pounce of a red-faced vulture, that has fallen from the heavens in
haste with closed wings to the bloody feast, followed quickly by
many of his brethren. The sky has become alive with black specks in
the far distant blue, with wings hurrying from all quarters. At length a
coronet of steady soaring vultures forms a wide circle far above, as
they hesitate to descend, but continue to revolve around the object
of attraction. The great bare-necked vulture suddenly appears. The
animal has been skinned, and the required flesh secured by the
men; we withdraw a hundred paces from the scene. A general rush
and descent takes place; hundreds of hungry beaks are tearing at
the offal. The great bare-necked vulture claims respect among the
crowd; but another form has appeared in the blue sky, and rapidly
descends. A pair of long, ungainly legs, hanging down beneath the
enormous wings, now touch the ground, and Abou Seen (father of
the teeth or beak, the Arab name for the Marabou) has arrived, and
he stalks proudly towards the crowds, pecking his way with his long
bill through the struggling vultures, and swallowing the lion’s share of
the repast. Abou Seen, last but not least, had arrived from the
highest region, while others had the advantage of the start. This bird
is very numerous through the Nile tributaries of Abyssinia, and may
generally be seen perched upon the rocks of the waterside, watching
for small fish, or any reptile that may chance to come within his
reach. The well-known feathers are situated in a plume beneath the
tail.”[81]
On the night of January 23, the thermometer sank to 29° F. just
before sunrise, so no one turned out with alacrity, and we were later
than usual in starting.
I was surprised to find that the boy Zody sought to attach himself
to the expedition. While we were preparing to leave Korata he came
to us and said, “You are good people and I want to go with you.” We
raised no objection, and he joined our party. A little while afterwards
he declared his intention of accompanying us to the Soudan.
Thereupon he returned to Korata and “realized his capital” by selling
the cow which he possessed. Then he overtook the expedition, and
served throughout the journey to Gallabat, receiving the same pay
as the other boys. He was smart, willing, and useful, soon learned
from his comrades what our requirements were, and acted both as
indoor and outdoor servant. He had no Arabic, and all intercourse
between us was by signs, which he was extremely quick to
understand. The motive that prompted him to render us service was
undoubtedly his devoted loyalty to Crawley.
Most travellers have given the Habashes a thoroughly bad
character. It is said that they are vain, greedy, treacherous, and
cowardly. Though we Europeans had but few unpleasant
experiences of them while we were in the country, I cannot say that I
saw anything which would give me authority to dispute the
conclusions of writers who knew them better from longer
observation, and it is the more pleasant on this account to record an
instance of an Abyssinian’s fidelity and cheerful good-will.
We marched through the familiar dry grass for about half an hour
after we left Korata, and then reached the river Gelda at a place
where, according to the “tout,” it was fordable. This is a muddy, rapid
stream, flowing between earthy banks. Its bed here was soft and
yielding, and there was not the least possibility of getting the beasts
and baggage across. In fact, our guide had misled us. It was a most
annoying waste of time. While we were shooting on the previous day
we had seen a fording-place higher up the stream, and now struck
towards it in spite of the “tout’s” assertion that the water there would
be up to the men’s necks.
When we reached the spot, we found a shallow, fairly fast current
flowing over a rocky bed. The guide was entirely unabashed by the
fact that he had misdirected us and lied to us. After this we ceased to
consult natives and planned our course according to Stecker’s map,
and in this way managed the matter well enough. There are some
small inaccuracies in the doctor’s work, but it is entirely sufficient for
all practical purposes and a worthy record of the German traveller’s
high attainments and untiring patience.
The ledges of rock which form the bed of the river at the upper
ford are smooth and round, and in consequence very slippery. We
had a busy time getting the donkeys over. Men had to be posted in
the water to steady them as they crossed, but the real trouble was at
the further bank. It was high and steep, and the way up was by a
narrow cutting with a hard surface. The first wet donkey that passed
along this made it as insecure for foothold as a slide, and the next
donkey struggled and scrambled and fell back into the water. We
had to tell off boys to shovel relays of earth on to the slope and then
others grabbed the donkey’s load, its ear, its tail, or anything they
could get a grip of, and shoved it up bodily. Another detachment was
on duty on the top of the bank to prevent the animals from straying.
Beyond the river lies a steep hill about five hundred feet high. The
ascent is by a track of the usual kind, about a foot and a half wide.
The way down from the summit is very precipitous and was full of
loose stones. These the donkeys often dislodged, and many rolled
along and struck the animals that were moving in front, but we had
no trouble with them.
We passed a village which is called Selselima, consisting of the
usual tokhuls grouped about a round thatched church. Here I saw an
example of Abyssinian methods of administration. A lad of about
eighteen was standing in a field, watching us pass. Ras Gouksha’s
man, the “tout,” went up to him, grabbed him by his gown, and began
to question him. Soon another Abyssinian approached him, grasped
the boy’s shama in like manner, and also interrogated him. I inquired
what they were asking, and was told they were seeking directions to
guide them by the nearest way to the source of the Blue Nile, which
was then some four miles distant. Presently without apparent
reason, both our fellows began to belabour the lad with their staves. I
rode up and ordered them sharply to desist, and explain to me what
they were about. Thereupon they made off, and one of our escort
told me they had been beating the boy so as to force him to act as
their guide—without pay, of course—and accompany them to the
river, though he had already given them full directions for the road.
The end of the descent brought us to the lake shore again, and
we camped on the further bank of a little stream of clear water
running in a rocky bed. It is called the Tschembolo. This is near to
the village of Woreb, and is only two miles from the point where the
Blue Nile flows from Lake Tsana. We could not proceed further that
day, for the march had been a long one, and it was late already. So
we halted upon this spot by the brook, as it was a very suitable
camping-ground with a clean drinking supply, and determined to
move on the morrow to a more central position, if we could find one,
for the survey work which Crawley and Dupuis were to undertake.
Patients had followed me from Korata. They were people who had
reached our camp there in the morning, after I had packed up the
medicine-chest and mounted my mule. Some half a dozen had come
over to Woreb after us, and were now waiting to see me. But I was
dead tired that evening and refused to attend to them. There were no
serious cases, and I told those who wished to have advice to return
at nine o’clock the next morning. If I had made myself more
accessible than I was, I should have been mobbed not only by the
sick in Abyssinia, but by a host of people who fancied they had
ailments.
OUR GUIDE BETWEEN KORATA AND WOREB.
See p. 132.
Just before dinner there was a commotion close to our tents, and
we found that one of the soldiers of our escort had tried to stab
Walda Mariam,[82] the assistant of Johannes, with whom he had
come to meet us at Gallabat. We immediately disarmed the soldier,
and then discovered that both had been to a village near by to get
tedj, and were drunk. So we disarmed the deputy-interpreter also,
and I was sorry to see him in disgrace, for he had always been
willing, hard-working, and cheery on the journey. We placed the
soldier under arrest, and warned him that if he were found brawling
again he would be flogged. Then, order being restored, we went to
dinner.
In this part of Abyssinia the mountain air sharpened our appetites,
and I must own that we were sturdy trenchermen. Each would tackle
a whole guinea-fowl or duck for lunch, and be ready for another in
the evening. We had puddings “of sorts” too, and yet, like Oliver
Twist, we “asked for more.”
On the morning of January 26, the quarrelsome soldier, now
sober and crestfallen, was brought up for a formal reprimand. He
was told that his case would be reported at Gallabat, and that he
would be punished there according to military law. Then his arms
were returned to him. Walda Mariam also attended, looking sheepish
and sorry for himself, and his weapons too were restored to him.
After this the medicine-chest was unloaded, and I redeemed my
promise to patients. One family—father, mother, and two children—
had come from a distance to consult me at Korata. They had arrived
too late, and had followed us to Woreb. The father was suffering
from necrosis of the lower jaw, and I had to draw three teeth for him.
The mother had come to hear what instructions were given about the
children—a little boy and a little girl. They were suffering from
strumous, tuberculous glands of the neck. I had as “hospital orderly”
a soldier who understood Arabic and Amharic, and could therefore
put me en rapport with my patients. After an examination of the
children, I concluded that the right thing would be an operation with
the knife. I told this to the woman, and the soldier interpreted what I
said word for word. As soon as the little girl heard the word “knife,”
she gave vent to the loudest, most piercing scream that she could
utter, and fairly flew from the spot. The noise created a sensation
among the Habashes and boys, who seemed to think that I was
killing a patient. It would have been awkward in any case to operate
in camp, and on this occasion I had no opportunity. Nothing short of
brute force would have brought the little girl back, and I think the
word “knife” conveyed to her mother and brother the notion of the
butcher’s rather than the surgeon’s implement.
My companions had mounted their mules and started on an
exploring journey, intending to select a good site as a camping-
ground close to the river. But by the time they returned they had
decided that our present position could not be bettered, as the river-
banks were low ground covered with papyrus swamps, and any
halting-place there would be very unhealthy. They had also found
that all the points from which they wished to make observations were
easy of access from the spot that we had chosen.
I had a rather sharp touch of fever in the afternoon, and was
obliged to give up work and lie down. These attacks are unavoidable
at times in that country. They make one feel extremely helpless and
depressed while they last, but soon yield to quinine and a little rest.
The next morning, January 26, I was better, but felt shaky when I
got on my mule and started with my companions. At a short distance
from the camp we left the animals in a hollow and climbed a steep
hill about five hundred feet high. It was covered with tall grass and
the soil was stony, so that I was soon out of breath. But when we
reached the summit a lovely scene lay in front of us. The Blue Nile,
winding away from its outlet in the lake, was bright in the sunshine.
The green banks on either side were delightful to the eyes, and here
and there in the stream were little islands, vivid in colour, where the
papyrus grew to a height of twelve or fourteen feet. There were
many dark dots in the water—heads of hippopotami, which swarm in
these upper reaches of the Blue Nile. Mountains rise above the river-
valley on both sides, and the stream curves among the spurs at their
base, till it is lost to sight. The view from the high land is far-reaching,
and the clearness of the air makes even distant outlines very distinct.
I can scarcely imagine a scene at once so charming, tranquil, and
impressive as this prospect of lake, river, and mountains. It is
impossible to convey in words the effect of the simple, strong
colouring—the blue lake, the flashing stream, the verdurous islands
and swamps, the cloudless, lustrous sky, the chromes and grays and
purplish shadows among the ridges that sweep upward and recede
from the valley in splendid lines. And upon the peace of this
landscape at least, the personally conducted tripper will not intrude
just yet.
I noticed several cataracts, but none of any great extent. The
water in these rapids was rushing over and between rocks, and they
must be ugly places when the lake rises. In other parts the river was
smooth and still. In one pool we counted eighteen hippos taking the
air, just their snouts being out of water.
Almost at once we set to work and piled up a heap of stones, and
fixed in the centre of it a long pole as a landmark. This was a
straight, thin branch which we cut from a tree near at hand. After this
we set up three more “cairns” on selected spots, and then
descended to the low ground, where the mules were waiting. I
remained in the valley, for exertion soon tires one out after the fever.
My friends climbed other hills and erected more landmarks before
returning to lunch. In the meanwhile I tried my luck at angling in the
river, but caught nothing, and soon gave up the attempt, and lounged
under a palm-tree, reading.
My companions started on their mules after lunch to continue their
survey, and I went back to camp and was glad to be idle. The day’s
work was ended before evening, and I believe we were the first
Britishers who had five-o’clock tea beside the head waters of the
Blue Nile.
Next morning (January 27) I had instructions to set up a stone-
heap on a little hill about four miles from camp. I made my way to it
direct through a tangle of jungle-growths, and from the summit
obtained the photograph of the source of the river at the point of
outflow from the lake which forms the frontispiece of this book. I
believe this to be the only photo of the place in existence.
I descended the hill, and walked along the riverbank towards the
place where we had arranged to lunch. The soil was boggy, covered
in some parts by reed-grass and in others by jungle-growths. Hippo
spoor was everywhere. This tract is not well-wooded; there are a few
palms, and some scattered trees of other kinds, chiefly mimosas.
The bed of the river is hard rock, and the water was clear and
drinkable. The depth varied very greatly, from a few inches in the
rapids to some six metres in the pools. I saw the stream at its lowest.
Watermarks showed clearly that in flood time it rises sixteen to
eighteen feet above this level.
When I reached the place where we were to lunch, I saw a big
herd of hippos basking on the surface of the river. Crawley and I
walked towards them, and when they saw us, they sank, leaving only
their snouts visible. These offered an interesting target for rifle-fire,
and for awhile the hippos watched our practice with unconcern. Then
a shot told—there was a “general post”—and not a sign of the huge
beasts remained in sight.
After this I took my rod and tried for a fish in a pool below a rapid.
While I was standing there, I saw a large white eagle, a splendid
fellow, which had been watching me fishing, swoop from the branch
of a high tree. He circled twice above me, and then pounced upon a
young duck, that was hiding under a ledge in the river-bed, and bore
it off. The parent birds were close at hand, and I heard their loud,
frightened, and indignant cackling. I do not think wild duck are
plentiful hereabouts. No doubt they are harried by these eagles and
have to find safer breeding-places. We saw only a very few. I landed
a plump fish of the perch tribe, which weighed about four pounds. He
showed no fight, but came tamely into shallow water, where the boy
who was with me picked him up. My friends came late into camp
after a hard day’s work upon their survey.
The high ground beside the head waters of the Blue Nile is
pleasant and healthy and could support a large population. The
maximum temperature in the day, when we were there, was about
eighty-five degrees. Usually a cool, exhilarating breeze was blowing,
and we did not feel the heat disagreeably. If this place were more
easily accessible, it would be a perfect pleasure-resort and a most
valuable sanatorium for residents in the Soudan.
The event of January 28, was the appearance of potatoes at
table. I had often gazed at this unvarying factor in one’s diet with
indifference if not slightingly, and had wondered why the things were
served so constantly at European tables. It was not till we missed the
daily luxury that we appreciated it. The roast fowl was another bird
with this accompaniment. These were the only potatoes that we saw
in Abyssinia; they were very small. The cook had discovered them in
a remote hamlet. I can offer no explanation of their presence in that
place. He purchased a sackful for one salt and an empty lime-juice
bottle, and returned to camp in triumph, shouting at the top of his
voice that he had found “batatas” at last. Perhaps as he failed to
discover any more, the expedition denuded the country of its whole
stock.
CHAPTER X
On January 29 my companions and I started early for the little hill

beside the river on which I had set up a “cairn” the day before. This
is the first rising ground beyond the outlet of the Blue Nile. (It ought
to be named after some kingly, heroic, half-divine figure that looms in
the dawn of legend, but—with my modest compliments to future
geographers—it has since been called Hayes’s Hill.)
At the foot of it, just below the first cataract in the stream, is a
ferry. It is a primordial means of transit into Godjam. Two men are in
charge of a papyrus boat which they paddle across, using a pole in
the manner of a canoe-paddle, as the water is too deep for punting.
At this spot Dupuis had some survey work in hand which involved
taking a rope to the opposite bank. Our Berthon boat had been
brought down to the riverside, and Crawley and I rowed over towing
the line astern and made it fast to a tree. Crawley returned, and I
remained at leisure in Godjam and used my camera and fishing-rod
to wile away the time.
I saw some natives using a net like a seine in the shallow water
just below the rapids. They had some very fair catches, and among
the fishes which they hauled ashore I noticed in particular one of a
blue colour which had what appeared to me to be a red sucker on its
snout. I have no idea to what species it belonged.
My friends finished their mapping about one o’clock, and then we
lunched in the shade of a mimosa. While we sat there we had an
opportunity of watching the Habashes’ method of getting donkeys
across the stream. They were pushed into the water, then four to six
men would scramble on to the crazy tankoa, which looked as if it
must sink under their weight. Each man would now seize a donkey
by the ear, and then the boat would start guided by the old ferryman
in the stern. Thus the donkeys had to swim alongside the raft—it is
no more than that—and as their heads were held above water, they
could scarcely come to harm.
We saw that it would save us much time and trouble if our beasts
were towed across in this way, and, after some deliberation, we
asked the ferryman what his charge would be for transporting
seventy donkeys. He replied that he would do it for seven dollars.
This, on consideration, seemed a fair bargain, and we resolved to
make an early start on the following day. The baggage was to go
over in our own boat, which could be hauled across by means of a
line made fast on both banks.
After concluding this arrangement we went back to camp, carrying
the rope with us. It might have tempted Charon—I don’t know his
Abyssinian name—if we had left it. We had sent Walda Mariam as
an envoy to King Tecla Haimanot, whose lands we should enter after
crossing the river. The king was in residence not far off, and in the
evening our man returned with a satisfactory message giving us all
necessary permission. The jurisdiction of Tecla Haimanot extended
only ten or twelve miles northward beside the lake, and we found
that we should have to obtain the consent of Ras Mangousha,
whose dominions march here with those of the King of Godjam,
before we could complete the circuit of the lake.
THE DONKEYS CROSSING THE MULES SWIMMING THE
THE FERRY. FERRY.
See p. 140. See p. 140.
We made an early start on the morning of the 30th. I went to the

ferry and had our Berthon boat put together. We found that our plan
answered excellently. Our boat, loaded with baggage, was easily
hauled over, and by a quarter to ten half a score of donkeys had
been carried across in tow of the ferrymen’s tankoa. Some of the
boys would put their arms under a donkey and lift it bodily into place
by the ferryboat at the starting-point, where the water was shallow. It
was a scrambling, pushing, splashing business, and they thoroughly
enjoyed standing in the stream and basking in the sunshine. A
number of Habashes from Woreb helped, and our crossing was a
merry, pleasant scene. The mules did not go “by ferry,” but swam
over with men beside them, as at other deep water passages. It was
all done and we had landed in Godjam without mishap of any kind by
a quarter-past one. The last load that I took over in our boat
consisted of nine of our men, rather more passengers, I fancy, than I
was “licensed to carry.” We scrambled up some rocks on the further
side, and then found ourselves on level ground, where travelling
would be easy.
Fully half of the donkeys had been reloaded by the time we
finished lunch, and we moved ahead with this detachment. The
country on the further side of the Nile seemed to me to be in a more
prosperous state. There was more cultivation of dhurra and grain.
The natives were fatter and looked, by comparison, “well-to-do.” I
heard afterwards that the ruler here is not so extortionate as certain
other chieftains. I believe that we were the first Europeans to
traverse this part of Godjam.
After a journey of three-quarters of an hour we approached the
village of Bahardar Georgis. Before entering the hamlet we had to
conform to a singular usage. Two men had stationed themselves
beside the track, one on each side of it, and they held a shama
across it. We asked Johannes what this meant, and he told us that it
was to protect the villagers from the power of the “evil eye.” This is
lost if the stranger who may possess it passes under the shama. We
had to move the greasy robe aside and go beneath it, hoping rather
than believing that it was not verminous. The Habashes are
extremely superstitious in this respect. It is customary to screen a
person of rank with a shama when he drinks, to safeguard him from
the same peril, and such persons are frequently kept from view
likewise while they eat their meals. Mr. Hormuzd Rassam, being a
high dignitary, was carefully shielded in this way to his no slight
disgust. He wrote: “As the generality of the garments spread out for
our seclusion had not been washed for months, and probably not
since they were first worn, the reader’s imagination may be left to
conceive the odour which surrounded us on these occasions. But
even if they had been washed no later than the previous day, the
disagreeable smell of rancid butter with which the natives besmear
their heads would suffice to render any such curtain almost
intolerable.”[83]
We had to use care in the selection of our camping-ground near
Bahardar Georgis. Much of the ground is covered by the papyrus
plant, and this shows dampness and, by consequence, risk of fever.
We finally selected a spot on some high ground, where there was a
dry, red soil. This happened to be close to a little settlement of the
curious people called Waitos, who are only found in this district.
They inhabited two or three huts near the village. Mr. Rassam has
given a concise and interesting account of them: “The Waitos are
Mussulmans of the Maliky sect, and although Mohammedanism
recognizes no castes among its adherents, nevertheless these
people, who subsist upon the flesh of the hippopotamus” (which is
considered unclean by all other Abyssinians), “are looked down upon
by their co-religionists, who consider it a degradation to associate
with them. A few among them cultivate a little grain, but the flesh of
the hippopotamus forms their staple food. . . . I was unable to obtain
any satisfactory account of the origin of this peculiar people. It is just
possible, however, that there may be some relationship between
them and the Watos, a tribe of Gallas inhabiting the banks of the
Hawash, south of Shoa, who are also said to live on the flesh of the
hippopotamus.”[84] Stecker, who was scrupulously accurate in
almost every particular which he mentioned, said that the Waitos
were, “strictly speaking, a Pagan sect (eigentlich Heiden-secte)”[85]
but in this he was mistaken. Oddly enough, these people, though
they are, in a sense, outcasts, are exclusive, and proud of their
isolation.
PUSHING THE DONKEYS INTO THE WATER
PREPARATORY TO THEIR BEING FERRIED
ACROSS.
See p. 141.
On the morning of January 31 my companions took the Berthon
boat and a whole cargo of surveyors’ gear, and started to complete
the work for which the expedition had been organized. A full account
of the results will be found in Sir William Garstin’s official Report on
the Sources of the Nile. My services were not requested, and I
certainly have had more practice in surveying symptoms than
ground. I stayed in camp for awhile, and then walked to the river with
angling-tackle, and landed a fair-sized fish with a spoon-bait. It was
of the perch family.
The country around Bahardar Georgis is flat, as I have said, and
in places swampy. The marshy tracts are overgrown by papyrus, and
it is not easy to distinguish them before one is both on and in them.
On this moist ground we saw flocks of wild geese and various
species of ibis. In the neighbourhood of the village I first observed
the bird known as buphaga Africana. It pitched upon the backs of
any of our donkeys that had sores, and caught the flies that settled
there. The bodies of these birds are gray, and scarcely
distinguishable from the donkey’s hide. Their beaks are red, and, at
a little distance, I noticed that they exactly resembled a sore on the
beasts which they infested. This colouring seems a remarkable
example of evolution in the direction of “protective mimicry.” They
worry cattle also, and it is well known that they peck the animals to
make raw places, which then attract the flies that are caught upon
them. I believe that they also use their beaks to mimic sores, for my
attention was first drawn to the birds by the semblance of abrasions
on the spinal area of some of the donkeys, where I knew that none
existed. No doubt the beaks, used thus, are a successful trap.
During the day I received a visit from an Abyssinian artist who
lived at Bahardar Georgis. He came to beg colours from us, if we
had any; I have no notion how the Habashes compound their
pigments. He told me that he had a commission to execute some
paintings for the church. I could only give him some pieces of red
and blue pencil. He was extremely grateful for these, and bestowed
upon me some samples of his art.
I presented him to Dupuis, who was greatly interested by his
pictures, and kindly took the trouble to discover, after much
rummaging, an old box of paints. This he gave to the youth, whose
delight was indescribable. He promised Dupuis works of art in his
best style. In the evening he came to our camp again, and I showed
him the Christmas Number of Pearson’s Magazine. This pleased him
highly, especially when he found that he was to be the possessor of
it. The three coloured plates caused him an ecstasy of wonder and
pleasure, and the respective artists—and the colour-printers—have a
venerating admirer by the lake-side.
My friends returned to camp rather later than they had expected,
but were well pleased by the result of the day’s work. They found
that at this season of the year the discharge from the lake into the
river was at the rate of forty-two cubic metres to the second. And
they brought some guinea-fowl to the larder. The future proprietor of
the Blue Nile Hotel and pension on Lake Tsana will always be able to
offer his guests fish and poultry.
On February 1 my companions started on foot, accompanied by
Johannes, a pack-mule and some boys, to see the first falls in the
course of the Blue Nile, and the ancient stone bridge which spans
the river at this point.[86] These falls are twenty-one miles below the
outlet of the stream from the lake.
I remained in charge of the camp, and, to wile away the time, took
our boat out upon the lake. This caused a great sensation among the
villagers, who had seen nothing of my friends’ excursion in it the
previous day. The Habashes flocked to the edge of the water, but
whenever I rested on the oars for a few moments, they rushed
screaming into their huts. Apparently they regarded me as a naval
force, and thought I meant to carry Bahardar Georgis by a coup de
main. Perhaps they remembered King Theodore’s descent upon the
island of Dek. “He was in pursuit, it appears, of a refractory chief
under Ras Ali, who had taken refuge on the island. In less than
twenty-four hours he had two hundred canoes constructed, in which
he suddenly appeared off the place with five hundred chosen
warriors.”[87] I should think that this was the only occasion on which
the tankoa was used in war, and have a feeling of compassion for
the five hundred warriors.
We noticed that at this place the children seemed especially
frightened of white men. Perhaps their mothers used us as bogeys to
terrify those that were troublesome. We found that we could soon
regain the confidence of the people by giving them any little picture,
no matter of what; prints from advertisements of furniture or clothes
served quite well. They received these with delight.
On the next day (February 2) my friends returned to camp, having
made short work of their tramp of forty-two miles. Crawley had shot
an oribi, a species of gazelle, on the way to the falls, and Dupuis
secured one on the march back. The former had served as tasty
rations to the party on the journey, the latter came into our larder,
and the prospect of venison after a long course of guinea-fowl was
very pleasant.
They had met a small body of Habashes on the road, who
attempted to turn them back, and these men loaded their rifles as a
menace. They were probably soldiers of the King of Godjam, but as
the Abyssinian troops wear no uniform it is difficult to distinguish
those who are “in the service” from those who are not. As among the
Boers, the combatant can at any time become a non-combatant if he
has time to hide his rifle and cartridges. My friends took no notice of
the hostile demonstration and rode on. Nothing came of it. Probably
the threat was intended to extort money. Usually we were well
received, and I attribute this to the sound judgment of our chief, who
took care to leave the villages near which we camped a little richer
than we found them.
The bridge is a most interesting relic of the times of Portuguese
ascendency in Abyssinia. I am indebted to Dupuis for permission to
reproduce his photograph of it. The following account of its
construction is taken from Dr. Johnson’s translation of Father Jerome
Lobo’s[88] “Voyage to Abyssinia.” I quote the passage in extenso
because it contains a reference to a question which has been much
discussed, though it has merely an academic interest, viz. whether
Lake Tsana, or the river which is its principal tributary, should be
regarded as the true source of the Blue Nile.
“The Nile, which the natives call Abavi” (Abai), “that is the father of
waters, rises first in Sacala, a province of the kingdom of Goiama”
(Godjam), “which is one of the most fruitful and agreeable of all the
Abyssinian dominions. This province is inhabited by a nation of the
Agaus, who call themselves Christians, but by daily intermarriages
they have allied themselves to the Pagan Agaus, and adopted all
their customs and ceremonies. These two nations are very
numerous, fierce, and unconquerable, inhabiting a country full of
mountains, which are covered with woods, and hollowed by nature
into vast caverns, many of which are capable of containing several
numerous families and hundreds of cows. To these recesses the
Agaus betake themselves when they are driven out of the plain,
where it is almost impossible to find them and certain ruin to pursue
them. This people increases extremely, every man being allowed so
many wives as he hath hundreds of cows; and it is seldom that the
hundreds are required to be complete.
“In the eastern part of this kingdom, on the declivity of a mountain,
whose descent is so easy that it seems a beautiful plain, is that
source of the Nile which has been sought after at so much expense
of labour, and about which such variety of conjectures hath been
formed without success. This spring, or rather these two springs, are
two holes, each about two feet diameter, a stone’s cast distant from
each other. The one is but about five feet and a half in depth, at least
we could not get our plummet farther, perhaps because it was
stopped by roots, for the whole place is full of trees. Of the other,
which is somewhat less, with a line of ten feet we could find no
bottom, and were assured by the inhabitants that none ever had
been found. It is believed here that these springs are the vent of a
great subterraneous lake; and they have this circumstance to favour
their opinion, that the ground is always moist, and so soft that the
water boils up underfoot as one walks upon it: this is more visible
after rains, for then the ground yields and sinks so much, that I
believe it is chiefly supported by the roots of trees that are
interwoven one with another. Such is the ground round about these
fountains. At a little distance to the south is a village named Guix,
through which the way lies to the top of the mountain, from whence
the traveller discovers a vast extent of land, which appears like a
deep valley, though the mountain rises so imperceptibly that those
who go up or down it are scarce sensible of any declivity.
“On the top of this mountain is a little hill, which the idolatrous
Agaus have in great veneration. Their priest calls them together at
this place once a year; and having sacrificed a cow, throws the head
into one of the springs of the Nile; after which ceremony every one
sacrifices a cow or more according to their different degrees of
wealth or devotion. The bones of these cows have already formed
two mountains of considerable height, which afford a sufficient proof
that these nations have always paid their adorations to this famous
river. They eat these sacrifices with great devotion, as flesh
consecrated to their deity. Then the priest anoints himself with the
grease and tallow of the cows, and sits down on a heap of straw on
the top and in the middle of a pile which is prepared. They set fire to
it, and the whole heap is consumed without any injury to the priest;
who, while the fire continues, harangues the standers by, and
confirms them in their present ignorance and superstition. When the
pile is burnt, and the discourse at an end, every one makes a large
present to the priest, which is the grand design of this religious
mockery.
“To return to the course of the Nile. Its waters, after the first rise,
run to the eastward for about a musket-shot; then turning to the
north, continue hidden in the grass and weeds for about a quarter of
a league, and discover themselves for the first time among some
rocks; a sight not to be enjoyed without some pleasure by those who
have read the fabulous accounts of this stream delivered by the
ancients, and the vain conjectures and reasonings which have been
formed upon its original, the nature of its water, its cataracts, and its
inundations, all which we are now entirely acquainted with, and eye
witnesses of.”[89]
The Nile “rolls away from its source with so inconsiderable a
current that it appears unlikely to escape being dried up by the hot
season, but soon receiving an increase from the Gemma, the Kelta,
the Bransu, and other less rivers, it is of such a breadth in the plain
of Boad, which is not above three days’ journey from its source, that
a ball shot from a musket will scarce fly from one bank to the other.
Here it begins to run northward, deflecting, however, a little towards
the east, for the space of nine or ten leagues; and then enters the so
much talked of lake of Dambia” (Tsana), “called by the natives
Barhar Sena, the resemblance of the sea, or Barhar Dambia, the sea
of Dambia. It crosses this lake only at one end, with so violent a
rapidity that the waves of the Nile” (Abai) “may be distinguished
through all the passage, which is six leagues.[90] Here begins the
greatness of the Nile. Fifteen miles farther, in the land of Alata, it
rushes precipitately from the top of a high rock, and forms one of the
most beautiful waterfalls in the world. I passed under it without being
wet, and resting myself there for the sake of the coolness, was
charmed with a thousand delightful rainbows which the sunbeams
painted on the water in all their shining and lively colours. The fall of
this mighty stream, from so great a height, makes a noise that may
be heard to a considerable distance. . . . The mist that rises from this
fall of water may be seen much farther than the noise can be heard.
After this cataract, the Nile again collects its scattered stream among
the rocks which seem to be disjoined in this place only to afford it a
passage. They are so near each other, that, in my time, a bridge of
beams, on which the whole Imperial army passed, was laid over
them. Sultan Segued[91] hath since built here a bridge of one arch in
the same place, for which purpose he procured masons from India”
(i.e. the Portuguese Indies).[92] “This bridge, which is the first the
Abyssinians have seen on the Nile, very much facilitates a
communication between the provinces, and encourages commerce
among the inhabitants of his Empire.”
Bruce’s description of the source of the Abai differs somewhat
from that given by Father Lobo. It is brief, and as an exceptional
interest attaches to the spot, it may not be superfluous to quote it.
“Half undressed as I was, by loss of my sash, and throwing my
shoes off” (as he had been directed to do in order to conform to a
superstitious usage of the natives), “I ran down the hill towards the
little island of green sods, which was about two hundred yards
distant; the whole side of the hill was thick grown with flowers, the
large bulbous roots[93] of which appearing above the surface of the
ground, and their skins coming off upon treading upon them,
occasioned me two very severe falls before I reached the brink of the
marsh. I after this came to the altar of green turf, which was in form
of an altar apparently the work of art, and I stood in rapture over the
principal fountain, which rises in the middle of it.”[94]
Bruce talked with “the Shum, the priest of the river, whose title
was Kefla Abai, or ‘Servant of the river.’ He was a man of about
seventy. The honourable charge which he possessed had been in
his family, he conceived, from the beginning of the world; and as he
was the happy father of eighty-four children, it appeared that his race
was likely to flow as long as the Nile itself. He had a long white
beard; round his body was wrapped a skin, which was fastened by a
broad belt. Over this he wore a cloak, the hood of which covered his
head, his legs were bare, but he wore sandals, which he threw off as
soon as he approached the bog from which the Nile” (Abai) “rises—a
mark of respect which Bruce and his attendants were also required
to perform.” The Shum very obligingly presented his comely and
sprightly daughter Irepone, aged sixteen, to the Scottish traveller as
housekeeper.
Bruce says that at this time the people of the place called the
Spirit of the River “The Everlasting God, Light of the World, Eye of
the World, God of Peace, Saviour and Father of the Universe.” He
asked the Shum if he had ever seen the Spirit, and the old man
answered without hesitation, “Yes, very frequently.”[95]
The Abai mentioned above is the main tributary of the Blue Nile,
but a glance at the map will show that Lake Tsana receives many
other rivers, and the surrounding mountains, of course, add the
volume of innumerable torrents and small streams to its waters in the
rainy season. For practical purposes the source of the Blue Nile is
the lake fed by all the affluents which collectively determine the
amount of the discharge into the river.
To return to the camp at Bahardar Georgis. The survey work of
the expedition was now completed, and our subsequent stages were
upon the homeward journey. On February 3 we visited the village
church. I obtained a clear photograph of some of the paintings in the
interior. Unfortunately, as I am no archæologist, I cannot pretend to
say whether the clothing of St. George, either in his combat or his
victorious return, or the conception of the other figures, gives an
indication of the origin of the Abyssinian school of ecclesiastical art. I
hope that some of my readers who are better informed may be able
to throw light upon the subject. It is, perhaps, of considerable interest
for the following reason: If the usages of the Abyssinian church,
which is strictly conservative, represent a really primitive form of
Christianity, they show that the observance of ritual ceremonies was,
in the early days, at least as much a matter of concern as the
condition of the individual conscience. And the style of the
Abyssinian pictures of sacred subjects may help to determine the
date when the accepted liturgy took its present form.
In the afternoon the guns of the party added some venison,
poultry, and game to the larder, and in the evening we were
serenaded—against our will and at our expense. A band of singers
and dancers from the village—both men and women—came into
camp. They had no intention of showing their skill without
remuneration, and as it would have caused ill-will among them and
disappointment to our own boys and escort if we had sent them
away, we endured their performance and paid for it. One of the
instruments which they use is a piece of board, over which strings
are stretched, so that it looks like a rude archaic forerunner of the
violin. They twang the strings with their fingers, but do not “stop”
them to obtain different notes from the same string. Our troupe also
played the tomtom and sang in the high nasal voice which is
characteristic of the race. We heard the last of them, thankfully, at
half-past nine.
While we were at Bahardar Georgis, the Waitos near our camp
drove a brisk trade in courbashes.[96] I bought of them some hippo
tusks, which they were glad to sell, as they live in great poverty. If I
had had some more small change (salt) I could have purchased a
quite considerable stock.
INTERIOR OF THE CHURCH AT BAHARDAR
GEORGIS.
See p. 153.
Insects are an annoying pest in this part of the lake side. Hosts of
mosquitoes and small beetles of a peculiar species appeared on the
flat ground at sunset, and swarmed throughout the neighbourhood
for an hour afterwards. The beetles settled upon us in throngs, and
crawled into our noses and ears and under our clothes, and we
could not even crush them on account of the unendurable smell
which their bodies then gave forth. When the first hour after sundown

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Essential Cell Biology Fifth

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Essential Cell Biology 5th Edition Bruce Alberts

Essential Cell Biology 5th Edition Bruce Alberts

Cell biology 3rd Edition Thomas D. Pollard

Neural cell biology 1st Edition Cheng Wang

Cell biology : translational impact in cancer biology

Biology of the Fungal Cell Dirk Hoffmeister

BRS Cell Biology Histology Leslie P. Gartner

Stem Cell Renewal and Cell Cell Communication Methods

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Editors: Betsy Twitchell and Michael Morales

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Names: Alberts, Bruce, author.

Nobel Prize–winning physicist Richard Feynman once noted that nature

answer and others invite speculation. Answers to all of the questions

Every chapter of Essential Cell Biology is the product of a communal effort:

Julie Lively, Louisiana State University

Denise Schanck deserves extra special thanks for providing continuity

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Interactive Instructor’s Guide

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BRUCE ALBERTS received his PhD from Harvard University and is a

LIST OF CHAPTERS and

CHAPTE R 1 Cells: The Fundamental Units of Life 1

CHAPTE R 2 Chemical Components of Cells 39

CHAPTE R 3 Energy, Catalysis, and Biosynthesis 81

CHAPTE R 4 Protein Structure and Function 117

CHAPTE R 5 DNA and Chromosomes 173

CHAPTE R 6 DNA Replication and Repair 199

CHAPTE R 8 Control of Gene Expression 267

CHAPTE R 9 How Genes and Genomes Evolve 297

CHAPTE R 10 Analyzing the Structure and Function of Genes 333

CHAPTE R 11 Membrane Structure 365

CHAPTE R 12 Transport Across Cell Membranes 389

CHAPTE R 13 How Cells Obtain Energy from Food 427

CHAPTE R 14 Energy Generation in Mitochondria and Chloroplasts 455

CHAPTE R 15 Intracellular Compartments and Protein Transport 495

TABLE 15 –3 Some Typical Signal Sequences 502

CHAPTE R 16 Cell Signaling 533

CHAPTE R 17 Cytoskeleton 573

CHAPTE R 18 The Cell-Division Cycle 609

CHAPTE R 19 Sexual Reproduction and Genetics 651

CHAPTE R 20 Cell Communities: Tissues, Stem Cells, and Cancer 691

Cells: The Fundamental Units of Life 1

Chemical Components of Cells 39

Energy, Catalysis, and Biosynthesis 81

Protein Structure and Function 117

DNA and Chromosomes 173

DNA Replication and Repair 199

From DNA to Protein: How Cells Read

Specific Enzymes Couple tRNAs to the Correct Amino Acid 249

Control of Gene Expression 267

POST-TRANSCRIPTIONAL CONTROLS 287

How Genes and Genomes Evolve 297