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Chapter 1 Introduction to Cells1

Cells Under the Microscope1

The Invention of the Light Microscope Led to the Discovery of Cells2

Cells，Organelles，and Even Molecules Can Be Seen Under the Microscope3

The Eucaryotic Cell9

The Nucleus Is the Information Store of the Cell9

Mitochondria Generate Energy from Food to Power the Cell10

Chloroplasts Capture Energy from Sunlight12

Internal Membranes Create Intracellular Compartments with Different Functions13

The Cytosol Is a Concentrated Aqueous Gel of Large and Small Molecules15

The Cytoskeleton Is Responsible for Cell Movements16

Unity and Diversity of Cells17

Cells Vary Enormously in Appearance and Function19

Living Cells All Have a Similar Basic Chemistry21

All Present-Day Cells Have Apparently Evolved from the Same Ancestor21

Bacteria Are the Smallest and Simplest Cells22

Molecular Biologists Have Focused on E.coli25

Giardia May Represent an Intermediate Stage in the Evolution of Eucaryotic Cells25

Brewer’s Yeast Is a Simple Eucaryotic Cell26

Single-celled Organisms Can Be Large，Complex，and Fierce：The Protozoans27

Arabidopsis Has Been Chosen Out of 300，000 Species as a Model Plant28

The World of Animals Is Represented by a Fly，a Worm，a Mouse，and Homo Sapiens29

Cells in the Same Multicellular Organism Can Be Spectacularly Different31

Essential Concepts34

Questions35

Chapter 2 Chemical Components of Cells37

Chemical Bonds37

Cells Are Made of Relatively Few Types of Atoms38

The Outermost Electrons Determine How Atoms Interact39

Ionic Bonds Form by the Gain and Loss of Electrons42

Covalent Bonds Form by the Sharing of Electrons43

There Are Different Types of Covalent Bonds45

Water Is the Most Abundant Substance in Cells48

Some Polar Molecules Form Acids and Bases in Water49

Molecules in Cells52

A Cell Is Formed from Carbon Compounds52

Cells Contain Four Major Families of Small Organic Molecules52

Sugars Are Energy Sources for Cells and Subunits of Polysaccharides53

Fatty Acids Are Components of Cell Membranes55

Amino Acids Are the Subunits of Proteins60

Nucleotides Are the Subunits of DNA and RNA61

Macromolecules Contain a Specific Sequence of Subunits65

Noncovalent Bonds Specify the Precise Shape of a Macromolecule69

Noncovalent Bonds Allow a Macromolecule to Bind Other Selected Molecules72

Essential Concepts73

Questions74

Chapter 3 Energy，Catalysis，and Biosynthesis79

Catalysis and the Use of Energy by Cells79

Biological Order Is Made Possible by the Release of Heat Energy from Cells79

Photosynthetic Organisms Use Sunlight to Synthesize Organic Molecules82

Cells Obtain Energy by the Oxidation of Biological Molecules83

Oxidation and Reduction Involve Electron Transfers84

Enzymes Lower the Barriers That Block Chemical Reactions85

How Enzymes Find Their Substrates：The Importance of Rapid Diffusion86

The Free-Energy Change for a Reaction Determines Whether It Can Occur89

The Concentration of Reactants Influences △G89

For Sequential Reactions，△G° Values Are Additive93

Activated Carrier Molecules and Biosynthesis94

The Formation of an Activated Carrier Is Coupled to an Energetically Favorable Reaction95

ATP Is the Most Widely Used Activated Carrier Molecule96

Energy Stored in ATP Is Often Harnessed to Join Two Molecules Together97

NADH and NADPH Are Important Electron Carriers98

There Are Many Other Activated Carrier Molecules in Cells100

The Synthesis of Biological Polymers Requires an Energy Input103

Essential Concepts105

Questions106

Chapter 4 How Cells Obtain Energy from Food108

The Breakdown of Sugars and Fats108

Food Molecules Are Broken Down in Three Stages to Produce ATP108

Glycolysis Is a Central ATP-producing Pathway110

Fermentations Allow ATP to Be Produced in the Absence of Oxygen114

Glycolysis Illustrates How Enzymes Couple Oxidation to Energy Storage114

Sugars and Fats Are Both Degraded to Acetyl CoA in Mitochondria118

The Citric Acid Cycle Generates NADH by Oxidizing Acetyl Groups to CO2119

Electron Transport Drives the Synthesis of the Majority of the ATP in Most Cells124

Storing and Utilizing Food125

Organisms Store Food Molecules in Special Reservoirs125

Many Biosynthetic Pathways Begin with Glycolysis or the Citric Acid Cycle127

Metabolism Is Organized and Regulated128

Essential Concepts129

Questions130

Chapter 5 Protein Structure and Function134

The Shape and Structure of Proteins134

The Shape of a Protein Is Specified by Its Amino Acid Sequence134

Proteins Fold into a Conformation of Lowest Energy139

Proteins Come in a Wide Variety of Complicated Shapes140

The α Helix and the β Sheet Are Common Folding Patterns141

Proteins Have Several Levels of Organization145

Few of the Many Possible Polypeptide Chains Will Be Useful147

Proteins Can Be Classified into Families147

Larger Protein Molecules Often Contain More Than One Polypeptide Chain148

Proteins Can Assemble into Filaments，Sheets，or Spheres149

A Helix Is a Common Structural Motif in Biological Structures152

Some Types of Proteins Have Elongated Fibrous Shapes152

Extracellular Proteins Are Often Stabilized by Covalent Cross-Linkages154

How Proteins Work154

Proteins Bind to Other Molecules155

The Binding Sites of Antibodies Are Especially Versatile156

Binding Strength Is Measured by the Equilibrium Constant157

Enzymes Are Powerful and Highly Specific Catalysts167

Lysozyme Illustrates How an Enzyme Works167

Vmax and KM Measure Enzyme Performance169

Tightly Bound Small Molecules Add Extra Functions to Proteins171

The Catalytic Activities of Enzymes Are Regulated172

Allosteric Enzymes Have Two Binding Sites That Interact173

A Conformational Change Can Be Driven by Protein Phosphorylation174

GTP-binding Proteins Can Undergo Dramatic Conformational Changes176

Motor Proteins Produce Large Movements in Cells176

Proteins Often Form Large Complexes That Function as Protein Machines178

Essential Concepts179

Questions180

Chapter 6 DNA184

The Structure and Function of DNA184

Genes Are Made of DNA185

A DNA Molecule Consists of Two Complementary Chains of Nucleotides185

The Structure of DNA Provides a Mechanism for Heredity188

DNA Replication189

DNA Synthesis Begins at Replication Origins190

New DNA Synthesis Occurs at Replication Forks191

The Replication Fork Is Asymmetrical193

DNA Polymerase Is Self-correcting194

Short Lengths of RNA Act as Primers for DNA Synthesis194

Proteins at a Replication Fork Cooperate to Form a Replication Machine196

DNA Repair198

Changes in DNA Are the Cause of Mutations198

A DNA Mismatch Repair System Removes Replication Errors That Escape from the Replication Machine200

DNA Is Continually Suffering Damage in Cells201

The Stability of Genes Depends on DNA Repair202

The High Fidelity with Which DNA Is Maintained Means That Closely Related Species Have Proteins with Very Similar Sequences205

Essential Concepts206

Questions207

Chapter 7 From DNA to Protein212

From DNA to RNA212

Portions of DNA Sequence Are Transcribed into RNA212

Transcription Produces RNA Complementary to One Strand of DNA213

Several Types of RNA Are Produced in Cells215

Signals in DNA Tell RNA Polymerase Where to Start and Finish216

Eucaryotic RNAs Undergo Processing in the Nucleus218

Eucaryotic Genes Are Interrupted by Noncoding Sequences219

Introns Are Removed by RNA Splicing220

mRNA Molecules Are Eventually Degraded by the Cell222

The Earliest Cells May Have Had Introns in Their Genes223

From RNA to Protein224

An mRNA Sequence Is Decoded in Sets of Three Nucleotides224

tRNA Molecules Match Amino Acids to Codons in mRNA225

Specific Enzymes Couple tRNAs to the Correct Amino Acid227

The RNA Message Is Decoded on Ribosomes227

Codons in mRNA Signal Where to Start and to Stop Protein Synthesis230

Proteins Are Made on Polyribosomes232

Carefully Controlled Protein Breakdown Helps Regulate the Amount of Each Protein in a Cell232

There Are Many Steps Between DNA and Protein234

RNA and the Origins of Life234

Simple Biological Molecules Can Form Under Prebiotic Conditions235

RNA Can Both Store Information and Catalyze Chemical Reactions237

RNA Is Thought to Predate DNA in Evolution239

Essential Concepts240

Questions241

Chapter 8 Chromosomes and Gene Regulation246

The Structure of Eucaryotic Chromosomes246

Eucaryotic DNA Is Packaged into Chromosomes246

Chromosomes Exist in Different States Throughout the Life of a Cell247

Specialized DNA Sequences Ensure That Chromosomes Replicate Efficiently249

Nucleosomes Are the Basic Units of Chromatin Structure250

Chromosomes Have Several Levels of DNA Packing252

Interphase Chromosomes Contain Both Condensed and More Extended Forms of Chromatin253

Position Effects on Gene Expression Reveal Differences in Interphase Chromosome Packing256

Interphase Chromosomes Are Organized Within the Nucleus256

Gene Regulation257

Cells Regulate the Expression of Their Genes258

Transcription Is Controlled by Proteins Binding to Regulatory DNA Sequences259

Repressors Turn Genes Off and Activators Turn Them On261

Initiation of Eucaryotic Gene Transcription Is a Complex Process263

Eucaryotic RNA Polymerase Requires General Transcription Factors264

Eucaryotic Gene Regulatory Proteins Control Gene Expression from a Distance265

Packing of Promoter DNA into Nucleosomes Can Affect Initiation of Transcription266

Eucaryotic Genes Are Regulated by Combinations of Proteins267

The Expression of Different Genes Can Be Coordinated by a Single Protein268

Combinatorial Control Can Create Different Cell Types269

Stable Patterns of Gene Expression Can Be Transmitted to Daughter Cells271

The Formation of an Entire Organ Can Be Triggered by a Single Gene Regulatory Protein273

Essential Concepts274

Questions275

Chapter 9 Genetic Variation278

Genetic Variation in Bacteria278

The Rapid Rate of Bacterial Division Means That Mutation Will Occur Over a Short Time Period279

Mutation in Bacteria Can Be Selected by a Change in Environmental Conditions280

Bacterial Cells Can Acquire Genes from Other Bacteria281

Bacterial Genes Can Be Transferred by a Process Called Bacterial Mating282

Some Bacteria Can Take Up DNA from Their Surroundings284

Gene Exchange Occurs by Homologous Recombination Between Two DNA Molecules of Similar Nucleotide Sequence285

Genes Can Be Transferred Between Bacteria by Bacterial Viruses288

Transposable Elements Create Genetic Diversity289

Sources of Genetic Change in Eucaryotic Genomes291

Random DNA Duplications Create Families of Related Genes292

Genes Encoding New Proteins Can Be Created by the Recombination of Exons293

A Large Part of the DNA of Multicellular Eucaryotes Consists of Repeated，Noncoding Sequences294

About 105551300f the Human Genome Consists of Two Families of Transposable Sequences295

The Evolution of Genomes Has Been Accelerated by Transposable Elements296

Viruses Are Fully Mobile Genetic Elements That Can Escape from Cells297

Retroviruses Reverse the Normal Flow of Genetic Information300

Retroviruses That Have Picked Up Host Genes Can Make Cells Cancerous302

Sexual Reproduction and the Reassortment of Genes304

Sexual Reproduction Gives a Competitive Advantage to Organisms in an Unpredictably Variable Environment304

Sexual Reproduction Involves Both Diploid and Haploid Cells305

Meiosis Generates Haploid Cells from Diploid Cells306

Meiosis Generates Enormous Genetic Variation307

Essential Concepts309

Questions310

Chapter 10 DNA Technology315

How DNA Molecules Are Analyzed315

Restriction Nucleases Cut DNA Molecules at Specific Sites315

Gel Electrophoresis Separates DNA Fragments of Different Sizes317

The Nucleotide Sequence of DNA Fragments Can Be Determined320

Nucleic Acid Hybridization320

DNA Hybridization Facilitates the Prenatal Diagnosis of Genetic Diseases321

In Situ Hybridization Locates Nucleic Acid Sequences in Cells or on Chromosomes323

DNA Cloning324

DNA Ligase Joins DNA Fragments Together to Produce a Recombinant DNA Molecule325

Bacterial Plasmids Can Be Used to Clone DNA326

Human Genes Are Isolated by DNA Cloning327

cDNA Libraries Represent the mRNA Produced by a Particular Tissue329

Hybridization Allows Even Distantly Related Genes to Be Identified331

The Polymerase Chain Reaction Amplifies Selected DNA Sequences332

DNA Engineering335

Completely Novel DNA Molecules Can Be Constructed335

Rare Cellular Proteins Can Be Made in Large Amounts Using Cloned DNA337

RNAs Can Be Produced by Transcription in Vitro338

Mutant Organisms Best Reveal the Function of a Gene339

Transgenic Animals Carry Engineered Genes340

Essential Concepts342

Questions343

Chapter 11 Membrane Structure348

The Lipid Bilayer348

Membrane Lipids Form Bilayers in Water349

The Lipid Bilayer Is a Two-dimensional Fluid352

The Fluidity of a Lipid Bilayer Depends on Its Composition353

The Lipid Bilayer Is Asymmetrical354

Lipid Asymmetry Is Generated Inside the Cell355

Lipid Bilayers Are Impermeable to Solutes and Ions356

Membrane Proteins357

Membrane Proteins Associate with the Lipid Bilayer in Various Ways358

A Polypeptide Chain Usually Crosses the Bilayer as an α Helix358

Membrane Proteins Can Be Solubilized in Detergents and Purified360

The Complete Structure Is Known for Very Few Membrane Proteins361

The Plasma Membrane Is Reinforced by the Cell Cortex363

The Cell Surface Is Coated with Carbohydrate364

Cells Can Restrict the Movement of Membrane Proteins366

Essential Concepts368

Questions368

Chapter 12 Membrane Transport372

The Ion Concentrations Inside a Cell Are Very Different from Those Outside372

Carrier Proteins and Their Functions373

Solutes Cross Membranes by Passive or ActiveTransport375

Electrical Forces as Well as Concentration Gradients Can Drive Passive Transport375

Active Transport Moves Solutes Against Their Electrochemical Gradients377

Animal Cells Use the Energy of ATP Hydrolysis to Pump Out Na＋378

The Na＋-K＋ Pump Is Driven by the Transient Addition of a Phosphate Group379

Animal Cells Use the Na＋ Gradient to Take Up Nutrients Actively380

The Na＋-K＋ Pump Helps Maintain the Osmotic Balance of Animal Cells381

Intracellular Ca 2＋ Concentrations Are Kept Low by Ca2＋ Pumps383

H＋ Gradients Are Used to Drive Membrane Transport in Plants，Fungi，and Bacteria384

Ion Channels and the Membrane Potential385

Ion Channels Are Ion Selective and Gated386

Ion Channels Randomly Snap Between Open and Closed States388

Voltage-gated Ion Channels Respond to the Membrane Potential390

The Membrane Potential Is Governed by Membrane Permeability to Specific Ions391

Ion Channels and Signaling in Nerve Cells394

Action Potentials Provide for Rapid Long-Distance Communication395

Action Potentials Are Usually Mediated by Voltage-gated Na＋ Channels395

Voltage-gated Ca2＋ Channels Convert Electrical Signals into Chemical Signals at Nerve Terminals397

Transmitter-gated Channels in Target Cells Convert Chemical Signals Back into Electrical Signals399

Neurons Receive Both Excitatory and Inhibitory Inputs400

Synaptic Connections Enable You to Think，Act，and Remember401

Essential Concepts404

Questions405

Chapter 13 Energy Generation in Mitochondria and Chloroplasts409

Cells Obtain Most of Their Energy by a Membrane-based Mechanism409

Mitochondria and Oxidative Phosphorylation410

A Mitochondrion Contains Two Membrane-bounded Compartments411

High-Energy Electrons Are Generated via the Citric Acid Cycle413

Electrons Are Transferred Along a Chain of Proteins in the Inner Mitochondrial Membrane414

Electron Transport Generates a Proton Gradient Across the Membrane415

The Proton Gradient Drives ATP Synthesis417

Coupled Transport Across the Inner Mitochondrial Membrane Is Driven by the Electrochemical Proton Gradient419

Proton Gradients Produce Most of the Cell’s ATP419

The Rapid Conversion of ADP to ATP in Mitochondria Maintains a High ATP：ADP Ratio in Cells421

Electron-Transport Chains and Proton Pumping421

Protons Are Readily Moved by the Transfer of Electrons422

The Redox Potential Is a Measure of Electron Affinities422

Electron Transfers Release Large Amounts of Energy423

Metals Tightly Bound to Proteins Form Versatile Electron Carriers425

Protons Are Pumped Across the Membrane by the Three Respiratory Enzyme Complexes427

Respiration Is Amazingly Efficient429

Chloroplasts and Photosynthesis430

Chloroplasts Resemble Mitochondria but Have an Extra Compartment430

Chloroplasts Capture Energy from Sunlight and Use It to Fix Carbon432

Excited Chlorophyll Molecules Funnel Energy into a Reaction Center433

Light Energy Drives the Synthesis of ATP and NADPH434

Carbon Fixation Is Catalyzed by Ribulose Bisphosphate Carboxylase436

Carbon Fixation in Chloroplasts Generates Sucrose and Starch438

The Genetic Systems of Mitochondria and Chloroplasts Reflect Their Procaryotic Origin438

Our Single-celled Ancestors439

RNA Sequences Reveal Evolutionary History439

Ancient Cells Probably Arose in Hot Environments440

Methanococcus Lives in the Dark，Using Only Inorganic Materials as Food441

Essential Concepts443

Questions444

Chapter 14 Intracellular Compartments and Transport448

Membrane-bounded Organelles448

Eucaryotic Cells Contain a Basic Set of Membrane-bounded Organelles449

Membrane-bounded Organelles Evolved in Different Ways450

Protein Sorting452

Proteins Are Imported into Organelles by Three Mechanisms453

Signal Sequences Direct Proteins to the Correct Compartment453

Proteins Enter the Nucleus Through Nuclear Pores455

Proteins Unfold to Enter Mitochondria and Chloroplasts457

Proteins Enter the Endoplasmic Reticulum While Being Synthesized458

Soluble Proteins Are Released into the ER Lumen459

Start and Stop Signals Determine the Arrangement of a Transmembrane Protein in the Lipid Bilayer461

Vesicular Transport462

Transport Vesicles Carry Soluble Proteins and Membrane Between Compartments463

Vesicle Budding Is Driven by the Assembly of a Protein Coat463

The Specificity of Vesicle Docking Depends on SNAREs465

Secretory Pathways467

Most Proteins Are Covalently Modified in the ER467

Exit from the ER Is Controlled to Ensure Protein Quality468

Proteins Are Further Modified and Sorted in the Golgi Apparatus469

Secretory Proteins Are Released from the Cell by Exocytosis470

Endocytic Pathways472

Specialized Phagocytic Cells Ingest Large Particles472

Fluid and Macromolecules Are Taken Up by Pinocytosis473

Receptor-mediated Endocytosis Provides a Specific Route into Animal Cells474

Endocytosed Macromolecules Are Sorted in Endosomes475

Lysosomes Are the Principal Sites of Intracellular Digestion476

Essential Concepts478

Questions479

Chapter 15 Cell Communication482

General Principles of Cell Signaling482

Signals Can Act over Long or Short Range482

Each Cell Responds to a Limited Set of Signals484

Receptors Relay Signals via Intracellular Signaling Pathways486

Some Signal Molecules Can Cross the Plasma Membrane488

Nitric Oxide Can Enter Cells to Activate Enzymes Directly489

There Are Three Main Classes of Cell-Surface Receptors490

Ion-Channel-linked Receptors Convert Chemical Signals into Electrical Ones491

Intracellular Signaling Cascades Act as a Series of Molecular Switches492

G-Protein-linked Receptors493

Stimulation of G-Protein-linked Receptors Activates G-Protein Subunits493

Some G Proteins Regulate Ion Channels495

Some G Proteins Activate Membrane-bound Enzymes496

The Cyclic AMP Pathway Can Activate Enzymes and Turn On Genes497

The Pathway Through Phospholipase C Results in a Rise in Intracellular Ca 2＋499

A Ca 2＋ Signal Triggers Many Biological Processes501

Intracellular Signaling Cascades Can Achieve Astonishing Speed，Sensitivity，and Adaptability：Photoreceptors in the Eye502

Enzyme-linked Receptors504

Activated Receptor Tyrosine Kinases Assemble a Complex of Intracellular Signaling Proteins505

Receptor Tyrosine Kinases Activate the GTP-binding Protein Ras506

Protein Kinase Networks Integrate Information to Control Complex Cell Behaviors508

Essential Concepts510

Questions511

Chapter 16 Cytoskeleton514

Intermediate Filaments514

Intermediate Filaments Are Strong and Durable515

Intermediate Filaments Strengthen’ Cells Against Mechanical Stress516

Microtubules518

Microtubules Are Hollow Tubes with Structurally Distinct Ends519

Microtubules Are Maintained by a Balance of Assembly and Disassembly519

The Centrosome Is the Major Microtubule-organizing Center in Animal Cells521

Growing Microtubules Show Dynamic Instability522

Microtubules Organize the Interior of the Cell523

Motor Proteins Drive Intracellular Transport525

Organelles Move Along Microtubules526

Cilia and Flagella Contain Stable Microtubules Moved by Dynein527

Actin Filaments529

Actin Filaments Are Thin and Flexible530

Actin and Tubulin Polymerize by Similar Mechanisms531

Many Proteins Bind to Actin and Modify Its Properties532

An Actin-rich Cortex Underlies the Plasma Membrane of Most Eucaryotic Cells533

Cell Crawling Depends on Actin533

Actin Associates with Myosin to Form Contractile Structures536

During Muscle Contraction Actin Filaments Slide Against Myosin Filaments538

Muscle Contraction Is Triggered by a Sudden Rise in Ca 2＋539

Essential Concepts543

Questions544

Chapter 17 Cell Division549

Overview of the Cell Cycle549

The Eucaryotic Cell Cycle Is Divided into Four Phases549

The Cytoskeleton Carries Out Both Mitosis and Cytokinesis551

Some Organelles Fragment at Mitosis551

Mitosis552

The Mitotic Spindle Starts to Assemble in Prophase552

Chromosomes Attach to the Mitotic Spindle at Prometaphase553

Chromosomes Line Up at the Spindle Equator at Metaphase557

Daughter Chromosomes Segregate at Anaphase557

The Nuclear Envelope Re-forms at Telophase559

Cytokinesis560

The Mitotic Spindle Determines the Plane of Cytoplasmic Cleavage560

The Contractile Ring of Animal Cells Is Made of Actin and Myosin561

Cytokinesis in Plant Cells Involves New Cell-Wall Formation562

Meiosis563

Homologous Chromosomes Pair Off During Meiosis563

Meiosis Involves Two Cell Divisions Rather Than One564

Essential Concepts567

Questions568

Chapter 18 Cell-Cycle Control and Cell Death572

The Cell-Cycle Control System572

A Central Control System Triggers the Major Processes of the Cell Cycle572

The Cell-Cycle Control System Is Based on Cyclically Activated Protein Kinases574

MPF Is the Cyclin-Cdk Complex That Controls Entry into M Phase575

Cyclin-dependent Protein Kinases Are Regulated by the Accumulation and Destruction of Cyclin576

The Activity of Cdks Is Further Regulated by Their Phosphorylation and Dephosphorylation578

Different Cyclin-Cdk Complexes Trigger Different Steps in the Cell Cycle578

The Cell Cycle Can Be Halted in G1 by Cdk Inhibitor Proteins580

Cells Can Dismantle Their Control System and Withdraw from the Cell Cycle581

Control of Cell Numbers in Multicellular Organisms582

Cell Proliferation Depends on Signals from Other Cells582

Animal Cells Have a Built-in Limitation on the Number of Times They Will Divide584

Animal Cells Require Signals from Other Cells to Avoid Programmed Cell Death584

Programmed Cell Death Is Mediated by an Intracellular Proteolytic Cascade585

Cancer Cells Disobey the Social Controls on Cell Proliferation and Survival587

Essential Concepts589

Questions590

Chapter 19 Tissues594

Extracellular Matrix and Connective Tissues594

Plant Cells Have Tough External Walls594

Cellulose Fibers Give the Plant Cell Wall Its Tensile Strength596

Animal Connective Tissues Consist Largely of Extracellular Matrix600

Collagen Provides Tensile Strength in Animal Connective Tissues600

Cells Organize the Collagen That They Secrete602

Integrins Couple the Matrix Outside a Cell to the Cytoskeleton Inside It603

Gels of Polysaccharide and Protein Fill Spaces and Resist Compression604

Epithelial Sheets and Cell-Cell Junctions605

Epithelial Sheets Are Polarized and Rest on a Basal Lamina606

Tight Junctions Make an Epithelium Leak-proof and Separate Its Apical and Basal Surfaces607

Cytoskeleton-linked Junctions Bind Epithelial Cells Robustly to One Another and to the Basal Lamina609

Gap Junctions Allow Ions and Small Molecules to Pass from Cell to Cell612

Tissue Maintenance and Renewal，and Its Disruption by Cancer613

Different Tissues Are Renewed at Different Rates615

Stem Cells Generate a Continuous Supply of Terminally Differentiated Cells615

Mutations in a Single Dividing Cell Can Cause It and Its Progeny to Violate the Normal Controls618

Cancer Is a Consequence of Mutation and Natural Selection Within the Population of Cells That Form the Body619

Cancer Requires an Accumulation of Mutations620

Development621

Programmed Cell Movements Create the Animal Body Plan622

Cells Switch On Different Sets of Genes According to Their Position and Their History622

Diffusible Signals Can Provide Cells with Positional Information624

Studies in Drosophila Have Given a Key to Vertebrate Development626

Similar Genes Are Used Throughout the Animal Kingdom to Give Cells an Internal Record of Their Position627

Essential Concepts628

Questions629