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Boundless Biology: 9.3: Response to the Cellular Signal

Boundless Biology
9.3: Response to the Cellular Signal
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table of contents
  1. 1: The Study of Life
    1. 1.1: The Science of Biology
      1. 1.1.0: Introduction to the Study of Biology
      2. 1.1.1: Scientific Reasoning
      3. 1.1.2: The Scientific Method
      4. 1.1.3: Basic and Applied Science
      5. 1.1.4: Publishing Scientific Work
      6. 1.1.5: Branches and Subdisciplines of Biology
    2. 1.2: Themes and Concepts of Biology
      1. 1.2.0: Properties of Life
      2. 1.2.1: Levels of Organization of Living Things
      3. 1.2.2: The Diversity of Life
  2. 2: The Chemical Foundation of Life
    1. 2.1: Atoms, Isotopes, Ions, and Molecules
      1. 2.1.0: Overview of Atomic Structure
      2. 2.1.1: Atomic Number and Mass Number
      3. 2.1.2: Isotopes
      4. 2.1.3: The Periodic Table
      5. 2.1.4: Electron Shells and the Bohr Model
      6. 2.1.5: Electron Orbitals
      7. 2.1.6: Chemical Reactions and Molecules
      8. 2.1.7: Ions and Ionic Bonds
      9. 2.1.8: Covalent Bonds and Other Bonds and Interactions
      10. 2.1.9: Hydrogen Bonding and Van der Waals Forces
    2. 2.2: Water
      1. 2.2.0: Water’s Polarity
      2. 2.2.1: Water’s States: Gas, Liquid, and Solid
      3. 2.2.2: Water’s High Heat Capacity
      4. 2.2.3: Water’s Heat of Vaporization
      5. 2.2.4: Water’s Solvent Properties
      6. 2.2.5: Water’s Cohesive and Adhesive Properties
      7. 2.2.6: pH, Buffers, Acids, and Bases
    3. 2.3: Carbon
      1. 2.3.0: The Chemical Basis for Life
      2. 2.3.1: Hydrocarbons
      3. 2.3.2: Organic Isomers
      4. 2.3.3: Organic Enantiomers
      5. 2.3.4: Organic Molecules and Functional Groups
  3. 3: Biological Macromolecules
    1. 3.1: Synthesis of Biological Macromolecules
      1. 3.1.0: Types of Biological Macromolecules
      2. 3.1.1: Dehydration Synthesis
      3. 3.1.2: Hydrolysis
    2. 3.2: Carbohydrates
      1. 3.2.0: Carbohydrate Molecules
      2. 3.2.1: Importance of Carbohydrates
    3. 3.3: Lipids
      1. 3.3.0: Lipid Molecules
      2. 3.3.1: Waxes
      3. 3.3.2: Phospholipids
      4. 3.3.3: Steroids
    4. 3.4: Proteins
      1. 3.4.0: Types and Functions of Proteins
      2. 3.4.1: Amino Acids
      3. 3.4.2: Protein Structure
      4. 3.4.3: Denaturation and Protein Folding
    5. 3.5: Nucleic Acids
      1. 3.5.0: DNA and RNA
      2. 3.5.1: The DNA Double Helix
      3. 3.5.2: DNA Packaging
      4. 3.5.3: Types of RNA
  4. 4: Cell Structure
    1. 4.1: Studying Cells
      1. 4.1.0: Cells as the Basic Unit of Life
      2. 4.1.1: Microscopy
      3. 4.1.2: Cell Theory
      4. 4.1.3: Cell Size
    2. 4.2: Prokaryotic Cells
      1. 4.2.0: Characteristics of Prokaryotic Cells
    3. 4.3: Eukaryotic Cells
      1. 4.3.0: Characteristics of Eukaryotic Cells
      2. 4.3.1: The Plasma Membrane and the Cytoplasm
      3. 4.3.2: The Nucleus and Ribosomes
      4. 4.3.3: Mitochondria
      5. 4.3.4: Comparing Plant and Animal Cells
    4. 4.4: The Endomembrane System and Proteins
      1. 4.4.0: Vesicles and Vacuoles
      2. 4.4.1: The Endoplasmic Reticulum
      3. 4.4.2: The Golgi Apparatus
      4. 4.4.3: Lysosomes
      5. 4.4.4: Peroxisomes
    5. 4.5: The Cytoskeleton
      1. 4.5.0: Microfilaments
      2. 4.5.1: Intermediate Filaments and Microtubules
    6. 4.6: Connections between Cells and Cellular Activities
      1. 4.6.0: Extracellular Matrix of Animal Cells
      2. 4.6.1: Intercellular Junctions
  5. 5: Structure and Function of Plasma Membranes
    1. 5.1: Components and Structure
      1. 5.1.0: Components of Plasma Membranes
      2. 5.1.1: Fluid Mosaic Model
      3. 5.1.2: Membrane Fluidity
    2. 5.2: Passive Transport
      1. 5.2.0: The Role of Passive Transport
      2. 5.2.1: Selective Permeability
      3. 5.2.2: Diffusion
      4. 5.2.3: Facilitated transport
      5. 5.2.4: Osmosis
      6. 5.2.5: Tonicity
      7. 5.2.6: Osmoregulation
    3. 5.3: Active Transport
      1. 5.3.0: Electrochemical Gradient
      2. 5.3.1: Primary Active Transport
      3. 5.3.2: Secondary Active Transport
    4. 5.4: Bulk Transport
      1. 5.4.0: Endocytosis
      2. 5.4.1: Exocytosis
  6. 6: Metabolism
    1. 6.1: Energy and Metabolism
      1. 6.1.0: The Role of Energy and Metabolism
      2. 6.1.1: Types of Energy
      3. 6.1.2: Metabolic Pathways
      4. 6.1.3: Metabolism of Carbohydrates
    2. 6.2: Potential, Kinetic, Free, and Activation Energy
      1. 6.2.0: Free Energy
      2. 6.2.1: The First Law of Thermodynamics
      3. 6.2.2: The Second Law of Thermodynamics
      4. 6.2.3: Activation Energy
    3. 6.3: ATP: Adenosine Triphosphate
      1. 6.3.0: ATP: Adenosine Triphosphate
    4. 6.4: Enzymes
      1. 6.4.0: Enzyme Active Site and Substrate Specificity
      2. 6.4.1: Control of Metabolism Through Enzyme Regulation
  7. 7: Cellular Respiration
    1. 7.1: Energy in Living Systems
      1. 7.1.0: Transforming Chemical Energy
      2. 7.1.1: Electrons and Energy
      3. 7.1.2: ATP in Metabolism
    2. 7.2: Glycolysis
      1. 7.2.0: Importance of Glycolysis
      2. 7.2.1: The Energy-Requiring Steps of Glycolysis
      3. 7.2.2: The Energy-Releasing Steps of Glycolysis
      4. 7.2.3: Outcomes of Glycolysis
    3. 7.3: Oxidation of Pyruvate and the Citric Acid Cycle
      1. 7.3.0: Breakdown of Pyruvate
      2. 7.3.1: Acetyl CoA to CO2
      3. 7.3.2: Citric Acid Cycle
    4. 7.4: Oxidative Phosphorylation
      1. 7.4.0: Electron Transport Chain
      2. 7.4.1: Chemiosmosis and Oxidative Phosphorylation
      3. 7.4.2: ATP Yield
    5. 7.5: Metabolism without Oxygen
      1. 7.5.0: Anaerobic Cellular Respiration
    6. 7.6: Connections of Carbohydrate, Protein, and Lipid Metabolic Pathways
      1. 7.6.0: Connecting Other Sugars to Glucose Metabolism
      2. 7.6.1: Connecting Proteins to Glucose Metabolism
      3. 7.6.2: Connecting Lipids to Glucose Metabolism
    7. 7.7: Regulation of Cellular Respiration
      1. 7.7.0: Regulatory Mechanisms for Cellular Respiration
      2. 7.7.1: Control of Catabolic Pathways
  8. 8: Photosynthesis
    1. 8.1: Overview of Photosynthesis
      1. 8.1.0: The Purpose and Process of Photosynthesis
      2. 8.1.1: Main Structures and Summary of Photosynthesis
      3. 8.1.2: The Two Parts of Photosynthesis
    2. 8.2: The Light-Dependent Reactions of Photosynthesis
      1. 8.2.0: Introduction to Light Energy
      2. 8.2.1: Absorption of Light
      3. 8.2.2: Processes of the Light-Dependent Reactions
    3. 8.3: The Light-Independent Reactions of Photosynthesis
      1. 8.3.0: CAM and C4 Photosynthesis
      2. 8.3.1: The Calvin Cycle
      3. 8.3.2: The Carbon Cycle
  9. 9: Cell Communication
    1. 9.1: Signaling Molecules and Cellular Receptors
      1. 9.1.0: Signaling Molecules and Cellular Receptors
      2. 9.1.1: Forms of Signaling
      3. 9.1.2: Types of Receptors
      4. 9.1.3: Signaling Molecules
    2. 9.2: Propagation of the Cellular Signal
      1. 9.2.0: Binding Initiates a Signaling Pathway
      2. 9.2.1: Methods of Intracellular Signaling
    3. 9.3: Response to the Cellular Signal
      1. 9.3.0: Termination of the Signal Cascade
      2. 9.3.1: Cell Signaling and Gene Expression
      3. 9.3.2: Cell Signaling and Cellular Metabolism
      4. 9.3.3: Cell Signaling and Cell Growth
      5. 9.3.4: Cell Signaling and Cell Death
    4. 9.4: Signaling in Single-Celled Organisms
      1. 9.4.0: Signaling in Yeast
      2. 9.4.1: Signaling in Bacteria
  10. 10: Cell Reproduction
    1. 10.1: Cell Division
      1. 10.1.0: The Role of the Cell Cycle
      2. 10.1.1: Genomic DNA and Chromosomes
      3. 10.1.2: Eukaryotic Chromosomal Structure and Compaction
    2. 10.2: The Cell Cycle
      1. 10.2.0: Interphase
      2. 10.2.1: The Mitotic Phase and the G0 Phase
    3. 10.3: Control of the Cell Cycle
      1. 10.3.0: Regulation of the Cell Cycle by External Events
      2. 10.3.1: Regulation of the Cell Cycle at Internal Checkpoints
      3. 10.3.2: Regulator Molecules of the Cell Cycle
    4. 10.4: Cancer and the Cell Cycle
      1. 10.4.0: Proto-oncogenes
      2. 10.4.1: Tumor Suppressor Genes
    5. 10.5: Prokaryotic Cell Division
      1. 10.5.0: Binary Fission
  11. 11: Meiosis and Sexual Reproduction
    1. 11.1: The Process of Meiosis
      1. 11.1.0: Introduction to Meiosis
      2. 11.1.1: Meiosis I
      3. 11.1.2: Meiosis II
      4. 11.1.3: Comparing Meiosis and Mitosis
    2. 11.2: Sexual Reproduction
      1. 11.2.0: Advantages and Disadvantages of Sexual Reproduction
      2. 11.2.1: Life Cycles of Sexually Reproducing Organisms
  12. 12: Mendel's Experiments and Heredity
    1. 12.1: Mendel’s Experiments and the Laws of Probability
      1. 12.1.0: Introduction to Mendelian Inheritance
      2. 12.1.1: Mendel’s Model System
      3. 12.1.2: Mendelian Crosses
      4. 12.1.3: Garden Pea Characteristics Revealed the Basics of Heredity
      5. 12.1.4: Rules of Probability for Mendelian Inheritance
    2. 12.2: Patterns of Inheritance
      1. 12.2.0: Genes as the Unit of Heredity
      2. 12.2.1: Phenotypes and Genotypes
      3. 12.2.2: The Punnett Square Approach for a Monohybrid Cross
      4. 12.2.3: Alternatives to Dominance and Recessiveness
      5. 12.2.4: Sex-Linked Traits
      6. 12.2.5: Lethal Inheritance Patterns
    3. 12.3: Laws of Inheritance
      1. 12.3.0: Mendel's Laws of Heredity
      2. 12.3.1: Mendel's Law of Dominance
      3. 12.3.2: Mendel's Law of Segregation
      4. 12.3.3: Mendel's Law of Independent Assortment
      5. 12.3.4: Genetic Linkage and Violation of the Law of Independent Assortment
      6. 12.3.5: Epistasis
  13. 13: Modern Understandings of Inheritance
    1. 13.1: Chromosomal Theory and Genetic Linkage
      1. 13.1.0: Chromosomal Theory of Inheritance
      2. 13.1.1: Genetic Linkage and Distances
      3. 13.1.2: Identification of Chromosomes and Karyotypes
    2. 13.2: Chromosomal Basis of Inherited Disorders
      1. 13.2.0: Disorders in Chromosome Number
      2. 13.2.1: Chromosomal Structural Rearrangements
      3. 13.2.2: X-Inactivation
  14. 14: DNA Structure and Function
    1. 14.1: Historical Basis of Modern Understanding
      1. 14.1.0: Discovery of DNA
      2. 14.1.1: Modern Applications of DNA
    2. 14.2: DNA Structure and Sequencing
      1. 14.2.0: The Structure and Sequence of DNA
      2. 14.2.1: DNA Sequencing Techniques
    3. 14.3: DNA Replication
      1. 14.3.0: Basics of DNA Replication
      2. 14.3.1: DNA Replication in Prokaryotes
      3. 14.3.2: DNA Replication in Eukaryotes
      4. 14.3.3: Telomere Replication
    4. 14.4: DNA Repair
      1. 14.4.0: DNA Repair
  15. 15: Genes and Proteins
    1. 15.1: The Genetic Code
      1. 15.1.0: The Relationship Between Genes and Proteins
      2. 15.1.1: The Central Dogma: DNA Encodes RNA and RNA Encodes Protein
    2. 15.2: Prokaryotic Transcription
      1. 15.2.0: Transcription in Prokaryotes
      2. 15.2.1: Initiation of Transcription in Prokaryotes
      3. 15.2.2: Elongation and Termination in Prokaryotes
    3. 15.3: Eukaryotic Transcription
      1. 15.3.0: Initiation of Transcription in Eukaryotes
      2. 15.3.1: Elongation and Termination in Eukaryotes
    4. 15.4: RNA Processing in Eukaryotes
      1. 15.4.0: mRNA Processing
      2. 15.4.1: Processing of tRNAs and rRNAs
    5. 15.5: Ribosomes and Protein Synthesis
      1. 15.5.0: The Protein Synthesis Machinery
      2. 15.5.1: The Mechanism of Protein Synthesis
      3. 15.5.2: Protein Folding, Modification, and Targeting
  16. 16: Gene Expression
    1. 16.1: Regulation of Gene Expression
      1. 16.1.0: The Process and Purpose of Gene Expression Regulation
      2. 16.1.1: Prokaryotic versus Eukaryotic Gene Expression
    2. 16.2: Prokaryotic Gene Regulation
      1. 16.2.0: The trp Operon: A Repressor Operon
      2. 16.2.1: Catabolite Activator Protein (CAP): An Activator Regulator
      3. 16.2.2: The lac Operon: An Inducer Operon
    3. 16.3: Eukaryotic Gene Regulation
      1. 16.3.0: The Promoter and the Transcription Machinery
      2. 16.3.1: Transcriptional Enhancers and Repressors
      3. 16.3.2: Epigenetic Control: Regulating Access to Genes within the Chromosome
      4. 16.3.3: RNA Splicing
      5. 16.3.4: The Initiation Complex and Translation Rate
      6. 16.3.5: Regulating Protein Activity and Longevity
    4. 16.4: Regulating Gene Expression in Cell Development
      1. 16.4.0: Gene Expression in Stem Cells
      2. 16.4.1: Cellular Differentiation
      3. 16.4.2: Mechanics of Cellular Differentation
      4. 16.4.3: Establishing Body Axes during Development
      5. 16.4.4: Gene Expression for Spatial Positioning
      6. 16.4.5: Cell Migration in Multicellular Organisms
      7. 16.4.6: Programmed Cell Death
    5. 16.5: Cancer and Gene Regulation
      1. 16.5.0: Altered Gene Expression in Cancer
      2. 16.5.1: Epigenetic Alterations in Cancer
      3. 16.5.2: Cancer and Transcriptional Control
      4. 16.5.3: Cancer and Post-Transcriptional Control
      5. 16.5.4: Cancer and Translational Control
  17. 17: Biotechnology and Genomics
    1. 17.1: Biotechnology
      1. 17.1.0: Biotechnology
      2. 17.1.1: Basic Techniques to Manipulate Genetic Material (DNA and RNA)
      3. 17.1.2: Molecular and Cellular Cloning
      4. 17.1.3: Reproductive Cloning
      5. 17.1.4: Genetic Engineering
      6. 17.1.5: Genetically Modified Organisms (GMOs)
      7. 17.1.6: Biotechnology in Medicine
      8. 17.1.7: Production of Vaccines, Antibiotics, and Hormones
    2. 17.2: Mapping Genomes
      1. 17.2.0: Genetic Maps
      2. 17.2.1: Physical Maps and Integration with Genetic Maps
    3. 17.3: Whole-Genome Sequencing
      1. 17.3.0: Strategies Used in Sequencing Projects
      2. 17.3.1: Use of Whole-Genome Sequences of Model Organisms
      3. 17.3.2: Uses of Genome Sequences
    4. 17.4: Applying Genomics
      1. 17.4.0: Predicting Disease Risk at the Individual Level
      2. 17.4.1: Pharmacogenomics, Toxicogenomics, and Metagenomics
      3. 17.4.2: Genomics and Biofuels
    5. 17.5: Genomics and Proteomics
      1. 17.5.0: Genomics and Proteomics
      2. 17.5.1: Basic Techniques in Protein Analysis
      3. 17.5.2: Cancer Proteomics
  18. 18: Evolution and the Origin of Species
    1. 18.1: Understanding Evolution
      1. 18.1.0: What is Evolution?
      2. 18.1.1: Charles Darwin and Natural Selection
      3. 18.1.2: The Galapagos Finches and Natural Selection
      4. 18.1.3: Processes and Patterns of Evolution
      5. 18.1.4: Evidence of Evolution
      6. 18.1.5: Misconceptions of Evolution
    2. 18.2: Formation of New Species
      1. 18.2.0: The Biological Species Concept
      2. 18.2.1: Reproductive Isolation
      3. 18.2.2: Speciation
      4. 18.2.3: Allopatric Speciation
      5. 18.2.4: Sympatric Speciation
    3. 18.3: Hybrid Zones and Rates of Speciation
      1. 18.3.0: Hybrid Zones
      2. 18.3.1: Varying Rates of Speciation
    4. 18.4: Evolution of Genomes
      1. 18.4.0: Genomic Similiarities between Distant Species
      2. 18.4.1: Genome Evolution
      3. 18.4.2: Whole-Genome Duplication
      4. 18.4.3: Gene Duplications and Divergence
      5. 18.4.4: Noncoding DNA
      6. 18.4.5: Variations in Size and Number of Genes
    5. 18.5: Evidence of Evolution
      1. 18.5.0: The Fossil Record as Evidence for Evolution
      2. 18.5.1: Fossil Formation
      3. 18.5.2: Gaps in the Fossil Record
      4. 18.5.3: Carbon Dating and Estimating Fossil Age
      5. 18.5.4: The Fossil Record and the Evolution of the Modern Horse
      6. 18.5.5: Homologous Structures
      7. 18.5.6: Convergent Evolution
      8. 18.5.7: Vestigial Structures
      9. 18.5.8: Biogeography and the Distribution of Species
  19. 19: The Evolution of Populations
    1. 19.1: Population Evolution
      1. 19.1.0: Defining Population Evolution
      2. 19.1.1: Population Genetics
      3. 19.1.2: Hardy-Weinberg Principle of Equilibrium
    2. 19.2: Population Genetics
      1. 19.2.0: Genetic Variation
      2. 19.2.1: Genetic Drift
      3. 19.2.2: Gene Flow and Mutation
      4. 19.2.3: Nonrandom Mating and Environmental Variance
    3. 19.3: Adaptive Evolution
      1. 19.3.0: Natural Selection and Adaptive Evolution
      2. 19.3.1: Stabilizing, Directional, and Diversifying Selection
      3. 19.3.2: Frequency-Dependent Selection
      4. 19.3.3: Sexual Selection
      5. 19.3.4: No Perfect Organism
  20. 20: Phylogenies and the History of Life
    1. 20.1: Organizing Life on Earth
      1. 20.1.0: Phylogenetic Trees
      2. 20.1.1: Limitations of Phylogenetic Trees
      3. 20.1.2: The Levels of Classification
    2. 20.2: Determining Evolutionary Relationships
      1. 20.2.0: Distinguishing between Similar Traits
      2. 20.2.1: Building Phylogenetic Trees
    3. 20.3: Perspectives on the Phylogenetic Tree
      1. 20.3.0: Limitations to the Classic Model of Phylogenetic Trees
      2. 20.3.1: Horizontal Gene Transfer
      3. 20.3.2: Endosymbiotic Theory and the Evolution of Eukaryotes
      4. 20.3.3: Web, Network, and Ring of Life Models
  21. 21: Viruses
    1. 21.1: Viral Evolution, Morphology, and Classification
      1. 21.1.0: Discovery and Detection of Viruses
      2. 21.1.1: Evolution of Viruses
      3. 21.1.2: Viral Morphology
      4. 21.1.3: Virus Classification
    2. 21.2: Virus Infections and Hosts
      1. 21.2.0: Steps of Virus Infections
      2. 21.2.1: The Lytic and Lysogenic Cycles of Bacteriophages
      3. 21.2.2: Animal Viruses
      4. 21.2.3: Plant Viruses
    3. 21.3: Prevention and Treatment of Viral Infections
      1. 21.3.0: Vaccines and Immunity
      2. 21.3.1: Vaccines and Anti-Viral Drugs for Treatment
    4. 21.4: Prions and Viroids
      1. 21.4.0: Prions and Viroids
  22. 22: Prokaryotes: Bacteria and Archaea
    1. 22.1: Prokaryotic Diversity
      1. 22.1.0: Classification of Prokaryotes
      2. 22.1.1: The Origins of Archaea and Bacteria
      3. 22.1.2: Extremophiles and Biofilms
    2. 22.2: Structure of Prokaryotes
      1. 22.2.0: Basic Structures of Prokaryotic Cells
      2. 22.2.1: Prokaryotic Reproduction
    3. 22.3: Prokaryotic Metabolism
      1. 22.3.0: Energy and Nutrient Requirements for Prokaryotes
      2. 22.3.1: The Role of Prokaryotes in Ecosystems
    4. 22.4: Bacterial Diseases in Humans
      1. 22.4.0: History of Bacterial Diseases
      2. 22.4.1: Biofilms and Disease
      3. 22.4.2: Antibiotics: Are We Facing a Crisis?
      4. 22.4.3: Bacterial Foodborne Diseases
    5. 22.5: Beneficial Prokaryotes
      1. 22.5.0: Symbiosis between Bacteria and Eukaryotes
      2. 22.5.1: Early Biotechnology: Cheese, Bread, Wine, Beer, and Yogurt
      3. 22.5.2: Prokaryotes and Environmental Bioremediation
  23. 23: Protists
    1. 23.1: Eukaryotic Origins
      1. 23.1.0: Early Eukaryotes
      2. 23.1.1: Characteristics of Eukaryotic DNA
      3. 23.1.2: Endosymbiosis and the Evolution of Eukaryotes
      4. 23.1.3: The Evolution of Mitochondria
      5. 23.1.4: The Evolution of Plastids
    2. 23.2: Characteristics of Protists
      1. 23.2.0: Cell Structure, Metabolism, and Motility
      2. 23.2.1: Protist Life Cycles and Habitats
    3. 23.3: Groups of Protists
      1. 23.3.0: Excavata
      2. 23.3.1: Chromalveolata: Alveolates
      3. 23.3.2: Chromalveolata: Stramenopiles
      4. 23.3.3: Rhizaria
      5. 23.3.4: Archaeplastida
      6. 23.3.5: Amoebozoa and Opisthokonta
    4. 23.4: Ecology of Protists
      1. 23.4.0: Protists as Primary Producers, Food Sources, and Symbionts
      2. 23.4.1: Protists as Human Pathogens
      3. 23.4.2: Protists as Plant Pathogens
  24. 24: Fungi
    1. 24.1: Characteristics of Fungi
      1. 24.1.0: Characteristics of Fungi
      2. 24.1.1: Fungi Cell Structure and Function
      3. 24.1.2: Fungi Reproduction
    2. 24.2: Ecology of Fungi
      1. 24.2.0: Fungi Habitat, Decomposition, and Recycling
      2. 24.2.1: Mutualistic Relationships with Fungi and Fungivores
    3. 24.3: Classifications of Fungi
      1. 24.3.0: Chytridiomycota: The Chytrids
      2. 24.3.1: Zygomycota: The Conjugated Fungi
      3. 24.3.2: Ascomycota: The Sac Fungi
      4. 24.3.3: Basidiomycota: The Club Fungi
      5. 24.3.4: Deuteromycota: The Imperfect Fungi
      6. 24.3.5: Glomeromycota
    4. 24.4: Fungal Parasites and Pathogens
      1. 24.4.0: Fungi as Plant, Animal, and Human Pathogens
    5. 24.5: Importance of Fungi in Human Life
      1. 24.5.0: Importance of Fungi in Human Life
  25. 25: Seedless Plants
    1. 25.1: Early Plant Life
      1. 25.1.0: Early Plant Life
      2. 25.1.1: Evolution of Land Plants
      3. 25.1.2: Plant Adaptations to Life on Land
      4. 25.1.3: Sporophytes and Gametophytes in Seedless Plants
      5. 25.1.4: Structural Adaptations for Land in Seedless Plants
      6. 25.1.5: The Major Divisions of Land Plants
    2. 25.2: Green Algae: Precursors of Land Plants
      1. 25.2.0: Streptophytes and Reproduction of Green Algae
      2. 25.2.1: Charales
    3. 25.3: Bryophytes
      1. 25.3.0: Bryophytes
      2. 25.3.1: Liverworts and Hornworts
      3. 25.3.2: Mosses
    4. 25.4: Seedless Vascular Plants
      1. 25.4.0: Seedless Vascular Plants
      2. 25.4.1: Vascular Tissue: Xylem and Phloem
      3. 25.4.2: The Evolution of Roots in Seedless Plants
      4. 25.4.3: Ferns and Other Seedless Vascular Plants
      5. 25.4.4: The Importance of Seedless Vascular Plants
  26. 26: Seed Plants
    1. 26.1: Evolution of Seed Plants
      1. 26.1.0: The Evolution of Seed Plants and Adaptations for Land
      2. 26.1.1: Evolution of Gymnosperms
      3. 26.1.2: Evolution of Angiosperms
    2. 26.2: Gymnosperms
      1. 26.2.0: Characteristics of Gymnosperms
      2. 26.2.1: Life Cycle of a Conifer
      3. 26.2.2: Diversity of Gymnosperms
    3. 26.3: Angiosperms
      1. 26.3.0: Angiosperm Flowers
      2. 26.3.1: Angsiosperm Fruit
      3. 26.3.2: The Life Cycle of an Angiosperm
      4. 26.3.3: Diversity of Angiosperms
    4. 26.4: The Role of Seed Plants
      1. 26.4.0: Herbivory and Pollination
      2. 26.4.1: The Importance of Seed Plants in Human Life
      3. 26.4.2: Biodiversity of Plants
  27. 27: Introduction to Animal Diversity
    1. 27.1: Features of the Animal Kingdom
      1. 27.1.0: Characteristics of the Animal Kingdom
      2. 27.1.1: Complex Tissue Structure
      3. 27.1.2: Animal Reproduction and Development
    2. 27.2: Features Used to Classify Animals
      1. 27.2.0: Animal Characterization Based on Body Symmetry
      2. 27.2.1: Animal Characterization Based on Features of Embryological Development
    3. 27.3: Animal Phylogeny
      1. 27.3.0: Constructing an Animal Phylogenetic Tree
      2. 27.3.1: Molecular Analyses and Modern Phylogenetic Trees
    4. 27.4: The Evolutionary History of the Animal Kingdom
      1. 27.4.0: Pre-Cambrian Animal Life
      2. 27.4.1: The Cambrian Explosion of Animal Life
      3. 27.4.2: Post-Cambrian Evolution and Mass Extinctions
  28. 28: Invertebrates
    1. 28.1: Phylum Porifera
      1. 28.1.0: Phylum Porifera
      2. 28.1.1: Morphology of Sponges
      3. 28.1.2: Physiological Processes in Sponges
    2. 28.2: Phylum Cnidaria
      1. 28.2.0: Phylum Cnidaria
      2. 28.2.1: Class Anthozoa
      3. 28.2.2: Class Scyphozoa
      4. 28.2.3: Class Cubozoa and Class Hydrozoa
    3. 28.3: Superphylum Lophotrochozoa
      1. 28.3.0: Superphylum Lophotrochozoa
      2. 28.3.1: Phylum Platyhelminthes
      3. 28.3.2: Phylum Rotifera
      4. 28.3.3: Phylum Nemertea
      5. 28.3.4: Phylum Mollusca
      6. 28.3.5: Classification of Phylum Mollusca
      7. 28.3.6: Phylum Annelida
    4. 28.4: Superphylum Ecdysozoa
      1. 28.4.0: Superphylum Ecdysozoa
      2. 28.4.1: Phylum Nematoda
      3. 28.4.2: Phylum Arthropoda
      4. 28.4.3: Subphyla of Arthropoda
    5. 28.5: Superphylum Deuterostomia
      1. 28.5.0: Phylum Echinodermata
      2. 28.5.1: Classes of Echinoderms
      3. 28.5.2: Phylum Chordata
  29. 29: Vertebrates
    1. 29.1: Chordates
      1. 29.1.0: Characteristics of Chordata
      2. 29.1.1: Chordates and the Evolution of Vertebrates
      3. 29.1.2: The Evolution of Craniata and Vertebrata
      4. 29.1.3: Characteristics of Vertebrates
    2. 29.2: Fishes
      1. 29.2.0: Agnathans: Jawless Fishes
      2. 29.2.1: Gnathostomes: Jawed Fishes
    3. 29.3: Amphibians
      1. 29.3.0: Characteristics and Evolution of Amphibians
      2. 29.3.1: Modern Amphibians
    4. 29.4: Reptiles
      1. 29.4.0: Characteristics of Amniotes
      2. 29.4.1: Evolution of Amniotes
      3. 29.4.2: Characteristics of Reptiles
      4. 29.4.3: Evolution of Reptiles
      5. 29.4.4: Modern Reptiles
    5. 29.5: Birds
      1. 29.5.0: Characteristics of Birds
      2. 29.5.1: Evolution of Birds
    6. 29.6: Mammals
      1. 29.6.0: Characteristics of Mammals
      2. 29.6.1: Evolution of Mammals
      3. 29.6.2: Living Mammals
    7. 29.7: The Evolution of Primates
      1. 29.7.0: Characteristics and Evolution of Primates
      2. 29.7.1: Early Human Evolution
      3. 29.7.2: Early Hominins
      4. 29.7.3: Genus Homo
  30. 30: Plant Form and Physiology
    1. 30.1: The Plant Body
      1. 30.1.0: Plant Tissues and Organ Systems
    2. 30.2: Stems
      1. 30.2.0: Functions of Stems
      2. 30.2.1: Stem Anatomy
      3. 30.2.2: Primary and Secondary Growth in Stems
      4. 30.2.3: Stem Modifications
    3. 30.3: Roots
      1. 30.3.0: Types of Root Systems and Zones of Growth
      2. 30.3.1: Root Modifications
    4. 30.4: Leaves
      1. 30.4.0: Leaf Structure and Arrangment
      2. 30.4.1: Types of Leaf Forms
      3. 30.4.2: Leaf Structure, Function, and Adaptation
    5. 30.5: Plant Development
      1. 30.5.0: Meristems
      2. 30.5.1: Genetic Control of Flowers
    6. 30.6: Transport of Water and Solutes in Plants
      1. 30.6.0: Water and Solute Potential
      2. 30.6.1: Pressure, Gravity, and Matric Potential
      3. 30.6.2: Movement of Water and Minerals in the Xylem
      4. 30.6.3: Transportation of Photosynthates in the Phloem
    7. 30.7: Plant Sensory Systems and Responses
      1. 30.7.0: Plant Responses to Light
      2. 30.7.1: The Phytochrome System and Red Light Response
      3. 30.7.2: Blue Light Response
      4. 30.7.3: Plant Responses to Gravity
      5. 30.7.4: Auxins, Cytokinins, and Gibberellins
      6. 30.7.5: Abscisic Acid, Ethylene, and Nontraditional Hormones
      7. 30.7.6: Plant Responses to Wind and Touch
    8. 30.8: Plant Defense Mechanisms
      1. 30.8.0: Plant Defenses Against Herbivores
      2. 30.8.1: Plant Defenses Against Pathogens
  31. 31: Soil and Plant Nutrition
    1. 31.1: Nutritional Requirements of Plants
      1. 31.1.0: Plant Nutrition
      2. 31.1.1: The Chemical Composition of Plants
      3. 31.1.2: Essential Nutrients for Plants
    2. 31.2: The Soil
      1. 31.2.0: Soil Composition
      2. 31.2.1: Soil Formation
      3. 31.2.2: Physical Properties of Soil
    3. 31.3: Nutritional Adaptations of Plants
      1. 31.3.0: Nitrogen Fixation: Root and Bacteria Interactions
      2. 31.3.1: Mycorrhizae: The Symbiotic Relationship between Fungi and Roots
      3. 31.3.2: Nutrients from Other Sources
  32. 32: Plant Reproduction
    1. 32.1: Plant Reproductive Development and Structure
      1. 32.1.0: Plant Reproductive Development and Structure
      2. 32.1.1: Sexual Reproduction in Gymnosperms
      3. 32.1.2: Sexual Reproduction in Angiosperms
    2. 32.2: Pollination and Fertilization
      1. 32.2.0: Pollination and Fertilization
      2. 32.2.1: Pollination by Insects
      3. 32.2.2: Pollination by Bats, Birds, Wind, and Water
      4. 32.2.3: Double Fertilization in Plants
      5. 32.2.4: Development of the Seed
      6. 32.2.5: Development of Fruit and Fruit Types
      7. 32.2.6: Fruit and Seed Dispersal
    3. 32.3: Asexual Reproduction
      1. 32.3.0: Asexual Reproduction in Plants
      2. 32.3.1: Natural and Artificial Methods of Asexual Reproduction in Plants
      3. 32.3.2: Plant Life Spans
  33. 33: The Animal Body: Basic Form and Function
    1. 33.1: Animal Form and Function
      1. 33.1.0: Characteristics of the Animal Body
      2. 33.1.1: Body Plans
      3. 33.1.2: Limits on Animal Size and Shape
      4. 33.1.3: Limiting Effects of Diffusion on Size and Development
      5. 33.1.4: Animal Bioenergetics
      6. 33.1.5: Animal Body Planes and Cavities
    2. 33.2: Animal Primary Tissues
      1. 33.2.0: Epithelial Tissues
      2. 33.2.1: Connective Tissues: Loose, Fibrous, and Cartilage
      3. 33.2.2: Connective Tissues: Bone, Adipose, and Blood
      4. 33.2.3: Muscle Tissues and Nervous Tissues
    3. 33.3: Homeostasis
      1. 33.3.0: Homeostatic Process
      2. 33.3.1: Control of Homeostasis
      3. 33.3.2: Homeostasis: Thermoregulation
      4. 33.3.3: Heat Conservation and Dissipation
  34. 34: Animal Nutrition and the Digestive System
    1. 34.1: Digestive Systems
      1. 34.1.0: Digestive Systems
      2. 34.1.1: Herbivores, Omnivores, and Carnivores
      3. 34.1.2: Invertebrate Digestive Systems
      4. 34.1.3: Vertebrate Digestive Systems
      5. 34.1.4: Digestive System: Mouth and Stomach
      6. 34.1.5: Digestive System: Small and Large Intestines
    2. 34.2: Nutrition and Energy Production
      1. 34.2.0: Food Requirements and Essential Nutrients
      2. 34.2.1: Food Energy and ATP
    3. 34.3: Digestive System Processes
      1. 34.3.0: Ingestion
      2. 34.3.1: Digestion and Absorption
      3. 34.3.2: Elimination
    4. 34.4: Digestive System Regulation
      1. 34.4.0: Neural Responses to Food
      2. 34.4.1: Hormonal Responses to Food
  35. 35: The Nervous System
    1. 35.1: Neurons and Glial Cells
      1. 35.1.0: Neurons and Glial Cells
      2. 35.1.1: Neurons
      3. 35.1.2: Glia
    2. 35.2: How Neurons Communicate
      1. 35.2.0: Nerve Impulse Transmission within a Neuron: Resting Potential
      2. 35.2.1: Nerve Impulse Transmission within a Neuron: Action Potential
      3. 35.2.2: Synaptic Transmission
      4. 35.2.3: Signal Summation
      5. 35.2.4: Synaptic Plasticity
    3. 35.3: The Nervous System
      1. 35.3.0: The Nervous System
    4. 35.4: The Central Nervous System
      1. 35.4.0: Brain: Cerebral Cortex and Brain Lobes
      2. 35.4.1: Brain: Midbrain and Brain Stem
      3. 35.4.2: Spinal Cord
    5. 35.5: The Peripheral Nervous System
      1. 35.5.0: Autonomic Nervous System
      2. 35.5.1: Sensory-Somatic Nervous System
    6. 35.6: Nervous System Disorders
      1. 35.6.0: Neurodegenerative Disorders
      2. 35.6.1: Neurodevelopmental Disorders: Autism and ADHD
      3. 35.6.2: Neurodevelopmental Disorders: Mental Illnesses
      4. 35.6.3: Other Neurological Disorders
  36. 36: Sensory Systems
    1. 36.1: Sensory Processes
      1. 36.1.0: Reception
      2. 36.1.1: Transduction and Perception
    2. 36.2: Somatosensation
      1. 36.2.0: Somatosensory Receptors
      2. 36.2.1: Integration of Signals from Mechanoreceptors
      3. 36.2.2: Thermoreception
    3. 36.3: Taste and Smell
      1. 36.3.0: Tastes and Odors
      2. 36.3.1: Reception and Transduction
    4. 36.4: Hearing and Vestibular Sensation
      1. 36.4.0: Sound
      2. 36.4.1: Reception of Sound
      3. 36.4.2: Transduction of Sound
      4. 36.4.3: The Vestibular System
      5. 36.4.4: Balance and Determining Equilibrium
    5. 36.5: Vision
      1. 36.5.0: Light
      2. 36.5.1: Anatomy of the Eye
      3. 36.5.2: Transduction of Light
      4. 36.5.3: Visual Processing
  37. 37: The Endocrine System
    1. 37.1: Types of Hormones
      1. 37.1.0: Hormone Functions
      2. 37.1.1: Lipid-Derived, Amino Acid-Derived, and Peptide Hormones
    2. 37.2: How Hormones Work
      1. 37.2.0: How Hormones Work
      2. 37.2.1: Intracellular Hormone Receptors
      3. 37.2.2: Plasma Membrane Hormone Receptors
    3. 37.3: Regulation of Body Processes
      1. 37.3.0: Hormonal Regulation of the Excretory System
      2. 37.3.1: Hormonal Regulation of the Reproductive System
      3. 37.3.2: Hormonal Regulation of Metabolism
      4. 37.3.3: Hormonal Control of Blood Calcium Levels
      5. 37.3.4: Hormonal Regulation of Growth
      6. 37.3.5: Hormonal Regulation of Stress
    4. 37.4: Regulation of Hormone Production
      1. 37.4.0: Humoral, Hormonal, and Neural Stimuli
    5. 37.5: Endocrine Glands
      1. 37.5.0: Hypothalamic-Pituitary Axis
      2. 37.5.1: Thyroid Gland
      3. 37.5.2: Parathyroid Glands
      4. 37.5.3: Adrenal Glands
      5. 37.5.4: Pancreas
      6. 37.5.5: Pineal Gland and Gonads
      7. 37.5.6: Organs with Secondary Endocrine Functions
  38. 38: The Musculoskeletal System
    1. 38.1: Types of Skeletal Systems
      1. 38.1.0: Functions of the Musculoskeletal System
      2. 38.1.1: Types of Skeletal Systems
      3. 38.1.2: Human Axial Skeleton
      4. 38.1.3: Human Appendicular Skeleton
    2. 38.2: Bone
      1. 38.2.0: Bone
      2. 38.2.1: Cell Types in Bones
      3. 38.2.2: Bone Development
      4. 38.2.3: Growth of Bone
      5. 38.2.4: Bone Remodeling and Repair
    3. 38.3: Joints and Skeletal Movement
      1. 38.3.0: Classification of Joints on the Basis of Structure and Function
      2. 38.3.1: Movement at Synovial Joints
      3. 38.3.2: Types of Synovial Joints
      4. 38.3.3: Bone and Joint Disorders
    4. 38.4: Muscle Contraction and Locomotion
      1. 38.4.0: Structure and Function of the Muscular System
      2. 38.4.1: Skeletal Muscle Fibers
      3. 38.4.2: Sliding Filament Model of Contraction
      4. 38.4.3: ATP and Muscle Contraction
      5. 38.4.4: Regulatory Proteins
      6. 38.4.5: Excitation–Contraction Coupling
      7. 38.4.6: Control of Muscle Tension
  39. 39: The Respiratory System
    1. 39.1: Systems of Gas Exchange
      1. 39.1.0: The Respiratory System and Direct Diffusion
      2. 39.1.1: Skin, Gills, and Tracheal Systems
      3. 39.1.2: Amphibian and Bird Respiratory Systems
      4. 39.1.3: Mammalian Systems and Protective Mechanisms
    2. 39.2: Gas Exchange across Respiratory Surfaces
      1. 39.2.0: Gas Pressure and Respiration
      2. 39.2.1: Basic Principles of Gas Exchange
      3. 39.2.2: Lung Volumes and Capacities
      4. 39.2.3: Gas Exchange across the Alveoli
    3. 39.3: Breathing
      1. 39.3.0: The Mechanics of Human Breathing
      2. 39.3.1: Types of Breathing
      3. 39.3.2: The Work of Breathing
      4. 39.3.3: Dead Space: V/Q Mismatch
    4. 39.4: Transport of Gases in Human Bodily Fluids
      1. 39.4.0: Transport of Oxygen in the Blood
      2. 39.4.1: Transport of Carbon Dioxide in the Blood
  40. 40: The Circulatory System
    1. 40.1: Overview of the Circulatory System
      1. 40.1.0: The Role of the Circulatory System
      2. 40.1.1: Open and Closed Circulatory Systems
      3. 40.1.2: Types of Circulatory Systems in Animals
    2. 40.2: Components of the Blood
      1. 40.2.0: The Role of Blood in the Body
      2. 40.2.1: Red Blood Cells
      3. 40.2.2: White Blood Cells
      4. 40.2.3: Platelets and Coagulation Factors
      5. 40.2.4: Plasma and Serum
    3. 40.3: Mammalian Heart and Blood Vessels
      1. 40.3.0: Structures of the Heart
      2. 40.3.1: Arteries, Veins, and Capillaries
      3. 40.3.2: The Cardiac Cycle
    4. 40.4: Blood Flow and Blood Pressure Regulation
      1. 40.4.0: Blood Flow Through the Body
      2. 40.4.1: Blood Pressure
  41. 41: Osmotic Regulation and the Excretory System
    1. 41.1: Osmoregulation and Osmotic Balance
      1. 41.1.0: Introduction to Osmoregulation
      2. 41.1.1: Transport of Electrolytes across Cell Membranes
      3. 41.1.2: Concept of Osmolality and Milliequivalent
      4. 41.1.3: Osmoregulators and Osmoconformers
    2. 41.2: Nitrogenous Wastes
      1. 41.2.0: Nitrogenous Waste in Terrestrial Animals: The Urea Cycle
      2. 41.2.1: Nitrogenous Waste in Birds and Reptiles: Uric Acid
    3. 41.3: Excretion Systems
      1. 41.3.0: Contractile Vacuoles in Microorganisms
      2. 41.3.1: Flame Cells of Planaria and Nephridia of Worms
      3. 41.3.2: Malpighian Tubules of Insects
    4. 41.4: Human Osmoregulatory and Excretory Systems
      1. 41.4.0: Kidney Structure
      2. 41.4.1: Nephron: The Functional Unit of the Kidney
      3. 41.4.2: Kidney Function and Physiology
    5. 41.5: Hormonal Control of Osmoregulatory Functions
      1. 41.5.0: Epinephrine and Norepinephrine
      2. 41.5.1: Other Hormonal Controls for Osmoregulation
  42. 42: The Immune System
    1. 42.1: Innate Immune Response
      1. 42.1.0: Innate Immune Response
      2. 42.1.1: Physical and Chemical Barriers
      3. 42.1.2: Pathogen Recognition
      4. 42.1.3: Natural Killer Cells
      5. 42.1.4: The Complement System
    2. 42.2: Adaptive Immune Response
      1. 42.2.0: Antigen-presenting Cells: B and T cells
      2. 42.2.1: Humoral Immune Response
      3. 42.2.2: Cell-Mediated Immunity
      4. 42.2.3: Cytotoxic T Lymphocytes and Mucosal Surfaces
      5. 42.2.4: Immunological Memory
      6. 42.2.5: Regulating Immune Tolerance
    3. 42.3: Antibodies
      1. 42.3.0: Antibody Structure
      2. 42.3.1: Antibody Functions
    4. 42.4: Disruptions in the Immune System
      1. 42.4.0: Immunodeficiency
      2. 42.4.1: Hypersensitivities
  43. 43: Animal Reproduction and Development
    1. 43.1: Reproduction Methods
      1. 43.1.0: Methods of Reproducing
      2. 43.1.1: Types of Sexual and Asexual Reproduction
      3. 43.1.2: Sex Determination
    2. 43.2: Fertilization
      1. 43.2.0: External and Internal Fertilization
      2. 43.2.1: The Evolution of Reproduction
    3. 43.3: Human Reproductive Anatomy and Gametogenesis
      1. 43.3.0: Male Reproductive Anatomy
      2. 43.3.1: Female Reproductive Anatomy
      3. 43.3.2: Gametogenesis (Spermatogenesis and Oogenesis)
    4. 43.4: Hormonal Control of Human Reproduction
      1. 43.4.0: Male Hormones
      2. 43.4.1: Female Hormones
    5. 43.5: Fertilization and Early Embryonic Development
      1. 43.5.0: Fertilization
      2. 43.5.1: Cleavage, the Blastula Stage, and Gastrulation
    6. 43.6: Organogenesis and Vertebrate Formation
      1. 43.6.0: Organogenesis
      2. 43.6.1: Vertebrate Axis Formation
    7. 43.7: Human Pregnancy and Birth
      1. 43.7.0: Human Gestation
      2. 43.7.1: Labor and Birth
      3. 43.7.2: Contraception and Birth Control
      4. 43.7.3: Infertility
  44. 44: Ecology and the Biosphere
    1. 44.1: The Scope of Ecology
      1. 44.1.0: Introduction to Ecology
      2. 44.1.1: Organismal Ecology and Population Ecology
      3. 44.1.2: Community Ecology and Ecosystem Ecology
    2. 44.2: Biogeography
      1. 44.2.0: Biogeography
      2. 44.2.1: Energy Sources
      3. 44.2.2: Temperature and Water
      4. 44.2.3: Inorganic Nutrients and Other Factors
      5. 44.2.4: Abiotic Factors Influencing Plant Growth
    3. 44.3: Terrestrial Biomes
      1. 44.3.0: What constitutes a biome?
      2. 44.3.1: Tropical Wet Forest and Savannas
      3. 44.3.2: Subtropical Deserts and Chaparral
      4. 44.3.3: Temperate Grasslands
      5. 44.3.4: Temperate Forests
      6. 44.3.5: Boreal Forests and Arctic Tundra
    4. 44.4: Aquatic Biomes
      1. 44.4.0: Abiotic Factors Influencing Aquatic Biomes
      2. 44.4.1: Marine Biomes
      3. 44.4.2: Estuaries: Where the Ocean Meets Fresh Water
      4. 44.4.3: Freshwater Biomes
    5. 44.5: Climate and the Effects of Global Climate Change
      1. 44.5.0: Climate and Weather
      2. 44.5.1: Causes of Global Climate Change
      3. 44.5.2: Evidence of Global Climate Change
      4. 44.5.3: Past and Present Effects of Climate Change
  45. 45: Population and Community Ecology
    1. 45.1: Population Demography
      1. 45.1.0: Population Demography
      2. 45.1.1: Population Size and Density
      3. 45.1.2: Species Distribution
      4. 45.1.3: The Study of Population Dynamics
    2. 45.2: Environmental Limits to Population Growth
      1. 45.2.0: Exponential Population Growth
      2. 45.2.1: Logistic Population Growth
      3. 45.2.2: Density-Dependent and Density-Independent Population Regulation
    3. 45.3: Life History Patterns
      1. 45.3.0: Life History Patterns and Energy Budgets
      2. 45.3.1: Theories of Life History
    4. 45.4: Human Population Growth
      1. 45.4.0: Human Population Growth
      2. 45.4.1: Overcoming Density-Dependent Regulation
      3. 45.4.2: Age Structure, Population Growth, and Economic Development
    5. 45.5: Community Ecology
      1. 45.5.0: The Role of Species within Communities
      2. 45.5.1: Predation, Herbivory, and the Competitive Exclusion Principle
      3. 45.5.2: Symbiosis
      4. 45.5.3: Ecological Succession
    6. 45.6: Innate Animal Behavior
      1. 45.6.0: Introduction to Animal Behavior
      2. 45.6.1: Movement and Migration
      3. 45.6.2: Animal Communication and Living in Groups
      4. 45.6.3: Altruism and Populations
      5. 45.6.4: Mating Systems and Sexual Selection
    7. 45.7: Learned Animal Behavior
      1. 45.7.0: Simple Learned Behaviors
      2. 45.7.1: Conditioned Behavior
      3. 45.7.2: Cognitive Learning and Sociobiology
  46. 46: Ecosystems
    1. 46.1: Ecology of Ecosystems
      1. 46.1.0: Ecosystem Dynamics
      2. 46.1.1: Food Chains and Food Webs
      3. 46.1.2: Studying Ecosystem Dynamics
      4. 46.1.3: Modeling Ecosystem Dynamics
    2. 46.2: Energy Flow through Ecosystems
      1. 46.2.0: Strategies for Acquiring Energy
      2. 46.2.1: Productivity within Trophic Levels
      3. 46.2.2: Transfer of Energy between Trophic Levels
      4. 46.2.3: Ecological Pyramids
      5. 46.2.4: Biological Magnification
    3. 46.3: Biogeochemical Cycles
      1. 46.3.0: Biogeochemical Cycles
      2. 46.3.1: The Water (Hydrologic) Cycle
      3. 46.3.2: The Carbon Cycle
      4. 46.3.3: The Nitrogen Cycle
      5. 46.3.4: The Phosphorus Cycle
      6. 46.3.5: The Sulfur Cycle
  47. 47: Conservation Biology and Biodiversity
    1. 47.1: The Biodiversity Crisis
      1. 47.1.0: Loss of Biodiversity
      2. 47.1.1: Types of Biodiversity
      3. 47.1.2: Biodiversity Change through Geological Time
      4. 47.1.3: The Pleistocene Extinction
      5. 47.1.4: Present-Time Extinctions
    2. 47.2: The Importance of Biodiversity to Human Life
      1. 47.2.0: Human Health and Biodiversity
      2. 47.2.1: Agricultural Diversity
      3. 47.2.2: Managing Fisheries
    3. 47.3: Threats to Biodiversity
      1. 47.3.0: Habitat Loss and Sustainability
      2. 47.3.1: Overharvesting
      3. 47.3.2: Exotic Species
      4. 47.3.3: Climate Change and Biodiversity
    4. 47.4: Preserving Biodiversity
      1. 47.4.0: Measuring Biodiversity
      2. 47.4.1: Changing Human Behavior in Response to Biodiversity Loss
      3. 47.4.2: Ecological Restoration

9.3: Response to the Cellular Signal

9.3.1: Termination of the Signal Cascade

Signal cascades convey signals to the cell through the phosphorylation of molecules by kinases.

Learning Objective

Describe the process by which the signal cascade in cell communication is terminated

Key Points

  • The chain of events that conveys the signal through the cell is called a signaling pathway or cascade.
  • Phosphorylation, a major component of signal cascades, adds a phosphate group to proteins, thereby changing their shapes and activating or inactivating the protein.
  • Degrading or removing the ligand so it can no longer access its receptor terminates the signal.
  • Enzymes like phosphotases can remove phosphate groups on proteins during dephosphorylation and reverse the cellular modifications produced by signaling cascades.

Key Terms

signaling cascade

the chain of events that conveys the signal through the cell

phosphorylation

the addition of a phosphate group to a compound; often catalyzed by enzymes

dephosphorylation

the removal of phosphate groups from a compound; often catalyzed by enzymes

Termination of the Signal Cascade

Ligand binding to the receptor allows for signal transduction through the cell. The chain of events that conveys the signal through the cell is called a signaling pathway or cascade. Signaling pathways are often very complex because of the interplay between different proteins. A major component of cell signaling cascades is the phosphorylation of molecules by enzymes known as kinases. Phosphorylation adds a phosphate group to serine, threonine, and tyrosine residues in a protein, changing their shapes, and activating or inactivating the protein .

Phosphorylation

Phosphorylation

In protein phosphorylation, a phosphate group is added to residues of the amino acids serine, threonine, and tyrosine.

The aberrant signaling often seen in tumor cells is proof that the termination of a signal at the appropriate time can be just as important as the initiation of a signal. One method of terminating or stopping a specific signal is to degrade or remove the ligand so that it can no longer access its receptor. One reason that hydrophobic hormones like estrogen and testosterone trigger long-lasting events is because they bind carrier proteins. These proteins allow the insoluble molecules to be soluble in blood, but they also protect the hormones from degradation by circulating enzymes.

Inside the cell, many different enzymes reverse the cellular modifications that result from signaling cascades. For example, phosphatases are enzymes that remove the phosphate group attached to proteins by kinases in a process called dephosphorylation. Cyclic AMP (cAMP) is degraded into AMP by phosphodiesterase, and the release of calcium stores is reversed by the Ca2+ pumps that are located in the external and internal membranes of the cell.

9.3.2: Cell Signaling and Gene Expression

Gene expression, vital for cells to function properly, is the process of turning on a gene to produce RNA and protein.

Learning Objective

Describe the regulation of gene expression

Key Points

  • Each cell controls when and how its genes are expressed.
  • Malfunctions in the control of gene expression are detrimental to the cell and can lead to the development of many diseases, such as cancer.
  • In prokaryotic cells, the control of gene expression is mostly at the transcriptional level.
  • In eukaryotic cells, the control of gene expression is at the epigenetic, transcriptional, post-transcriptional, translational, and post-translational levels.

Key Terms

translation

a process occurring in the ribosome in which a strand of messenger RNA (mRNA) guides assembly of a sequence of amino acids to make a protein

gene expression

the transcription and translation of a gene into messenger RNA and, thus, into a protein

transcription

the synthesis of RNA under the direction of DNA

Gene Expression

For a cell to function properly, necessary proteins must be synthesized at the proper time. All cells control or regulate the synthesis of proteins from information encoded in their DNA. The process of turning on a gene to produce RNA and protein is called gene expression.

Whether in a simple unicellular organism or a complex multi-cellular organism, each cell controls when and how its genes are expressed. For this to occur, there must be a mechanism to control when a gene is expressed to make RNA and protein; how much of the protein is made; and when it is time to stop making that protein because it is no longer needed. The regulation of gene expression conserves energy and space. It would require a significant amount of energy for an organism to express every gene at all times, so it is more energy efficient to turn on the genes only when they are required. In addition, only expressing a subset of genes in each cell saves space because DNA must be unwound from its tightly-coiled structure to transcribe and translate the DNA. Cells would have to be enormous if every protein were expressed in every cell all the time. The control of gene expression is extremely complex. Malfunctions in this process are detrimental to the cell and can lead to the development of many diseases, including cancer.

Prokaryotic versus Eukaryotic Gene Expression

To understand how gene expression is regulated, we must first understand how a gene codes for a functional protein in a cell. The process occurs in both prokaryotic and eukaryotic cells, just in slightly different manners . Prokaryotic organisms are single-celled organisms that lack a cell nucleus; their DNA floats freely in the cell cytoplasm. To synthesize a protein, the processes of transcription and translation occur almost simultaneously. When the resulting protein is no longer needed, transcription stops. As a result, the primary method to control what type of protein and how much of each protein is expressed in a prokaryotic cell is the regulation of DNA transcription. All of the subsequent steps occur automatically. When more protein is required, more transcription occurs. Therefore, in prokaryotic cells, the control of gene expression is mostly at the transcriptional level.

Prokaryotic & Eukaryotic Gene Expression

Prokaryotic & Eukaryotic Gene Expression

Prokaryotic transcription and translation occur simultaneously in the cytoplasm; regulation occurs at the transcriptional level. Eukaryotic gene expression is regulated during transcription and RNA processing, which take place in the nucleus, and during protein translation, which takes place in the cytoplasm. Further regulation may occur through post-translational modifications of proteins.

Eukaryotic cells, in contrast, have intracellular organelles that add to their complexity. In eukaryotic cells, the DNA is contained inside the cell's nucleus where it is transcribed into RNA. The newly-synthesized RNA is then transported out of the nucleus into the cytoplasm where ribosomes translate the RNA into protein. The processes of transcription and translation are physically separated by the nuclear membrane: transcription occurs only within the nucleus, and translation occurs only outside the nucleus in the cytoplasm. The regulation of gene expression can occur at all stages of the process. Regulation may occur when the DNA is uncoiled and loosened from nucleosomes to bind transcription factors (epigenetic level); when the RNA is transcribed (transcriptional level); when the RNA is processed and exported to the cytoplasm after it is transcribed (post-transcriptional level); when the RNA is translated into protein (translational level); or after the protein has been made (post-translational level).

9.3.3: Cell Signaling and Cellular Metabolism

The rush of adrenaline that leads to greater glucose availability is an example of an increase in metabolism.

Learning Objective

Explain how cellular metabolism can be altered

Key Points

  • The activation of β-adrenergic receptors in muscle cells by adrenaline leads to an increase in cyclic AMP.
  • Cyclic AMP activates PKA (protein kinase A), which phosphorylates two enzymes.
  • Phophorylation of the first enzyme promotes the degradation of glycogen by activating intermediate GPK that in turn activates GP, which catabolizes glycogen into glucose.
  • Phosphorylation of the second enzyme, glycogen synthase (GS), inhibits its ability to form glycogen from glucose.
  • The inhibition of glucose to form glycogen prevents a futile cycle of glycogen degradation and synthesis, so glucose is then available for use by the muscle cell.

Key Terms

protein kinase A

a family of enzymes whose activity is dependent on cellular levels of cyclic AMP (cAMP)

epinephrine

(adrenaline) an amino acid-derived hormone secreted by the adrenal gland in response to stress

cyclic adenosine monophosphate

cAMP, a second messenger derived from ATP that is involved in the activation of protein kinases and regulates the effects of adrenaline

Increase in Cellular Metabolism

As the environments of most organisms are constantly changing, the reactions of metabolism must be finely regulated to maintain a constant set of conditions within cells. Metabolic regulation also allows organisms to respond to signals and interact actively with their environments. Two closely-linked concepts are important for understanding how metabolic pathways are controlled. Firstly, the regulation of an enzyme in a pathway is how its activity is increased and decreased in response to signals. Secondly, the control exerted by this enzyme is the effect that these changes in its activity have on the overall rate of the pathway. For example, an enzyme may show large changes in activity (i.e. it is highly regulated), but if these changes have little effect on the rate of a metabolic pathway, then this enzyme is not involved in the control of the pathway.

The result of one such signaling pathway affects muscle cells and is a good example of an increase in cellular metabolism. The activation of β-adrenergic receptors in muscle cells by adrenaline leads to an increase in cyclic adenosine monophosphate (also known as cyclic AMP or cAMP) inside the cell . Also known as epinephrine, adrenaline is a hormone (produced by the adrenal gland attached to the kidney) that prepares the body for short-term emergencies. Cyclic AMP activates PKA (protein kinase A), which in turn phosphorylates two enzymes. The first enzyme promotes the degradation of glycogen by activating intermediate glycogen phosphorylase kinase (GPK) that in turn activates glycogen phosphorylase (GP), which catabolizes glycogen into glucose. (Recall that your body converts excess glucose to glycogen for short-term storage. When energy is needed, glycogen is quickly reconverted to glucose. ) Phosphorylation of the second enzyme, glycogen synthase (GS), inhibits its ability to form glycogen from glucose. In this manner, a muscle cell obtains a ready pool of glucose by activating its formation via glycogen degradation and by inhibiting the use of glucose to form glycogen, thus preventing a futile cycle of glycogen degradation and synthesis. The glucose is then available for use by the muscle cell in response to a sudden surge of adrenaline—the "fight or flight" reflex.

Formation of Cyclic AMP

Formation of Cyclic AMP

This diagram shows the mechanism for the formation of cyclic AMP (cAMP). cAMP serves as a second messenger to activate or inactivate proteins within the cell.

9.3.4: Cell Signaling and Cell Growth

Cell growth is promoted by ligands known as growth factors.

Learning Objective

Explain how cell growth is affected by growth factors.

Key Points

  • Normally, cells do not divide unless they are stimulated by signals from other cells.
  • Most growth factors, which promote cell growth, bind to cell-surface receptors that are linked to tyrosine kinases.
  • MAP kinase stimulates the expression of proteins that interact with other cellular components to initiate cell division.
  • Uncontrolled cell growth leads to cancer.

Key Terms

receptor

a protein on a cell wall that binds with specific molecules so that they can be absorbed into the cell in order to control certain functions

growth factor

a naturally-occurring substance capable of stimulating cellular growth, proliferation, and cellular differentiation

oncogene

any gene that contributes to the conversion of a normal cell into a cancerous cell when mutated or expressed at high levels

Cell Growth

Cell signaling pathways play a major role in cell division. Cells do not normally divide unless they are stimulated by signals from other cells. The ligands that promote cell growth are called growth factors. Most growth factors bind to cell-surface receptors that are linked to tyrosine kinases. These cell-surface receptors are called receptor tyrosine kinases (RTKs). Activation of RTKs initiates a signaling pathway that includes a G-protein called RAS, which activates the MAP kinase pathway described earlier. The enzyme MAP kinase then stimulates the expression of proteins that interact with other cellular components to initiate cell division. In addition, uncontrolled cell growth leads to cancer; mutations in the genes encoding protein components of signaling pathways are often found in tumor cells .

Uncontrolled Cell Growth

Uncontrolled Cell Growth

Colorectal cancer occurs after numerous mutations to a normal cell.

Cancer Biologists & Uncontrolled Cell Growth

Cancer biologists study the molecular origins of cancer with the goal of developing new prevention methods and treatment strategies that will inhibit the growth of tumors without harming the normal cells of the body. Signaling pathways control cell growth. These pathways are controlled by signaling proteins, which are, in turn, expressed by genes. Mutations in these genes can result in malfunctioning signaling proteins. This prevents the cell from regulating its cell cycle, triggering unrestricted cell division and cancer. The genes that regulate the signaling proteins are one type of oncogene: a gene that has the potential to cause cancer. The gene encoding RAS is an oncogene that was originally discovered when mutations in the RAS protein were linked to cancer. Further studies have indicated that 30 percent of cancer cells have a mutation in the RAS gene that leads to uncontrolled growth. If left unchecked, uncontrolled cell division can lead tumor formation and metastasis, the growth of cancer cells in new locations in the body.

Cancer biologists have been able to identify many other oncogenes that contribute to the development of cancer. For example, HER2 is a cell-surface receptor that is present in excessive amounts in 20 percent of human breast cancers. Cancer biologists realized that gene duplication led to HER2 overexpression in 25 percent of breast cancer patients and developed a drug called Herceptin (trastuzumab), a monoclonal antibody that targets HER2 for removal by the immune system. Herceptin therapy helps to control signaling through HER2. Its use, in combination with chemotherapy, has helped to increase the overall survival rate of patients with metastatic breast cancer.

9.3.5: Cell Signaling and Cell Death

When a cell is damaged, unnecessary, or dangerous to an organism, a cell can initiate the mechanism for cell death known as apoptosis.

Learning Objective

Describe how apoptosis is initiated

Key Points

  • Apoptosis allows a cell to die in a controlled manner by preventing the release of damaging molecules from inside the cell.
  • Internal checkpoints to monitor a cell's health exist; if abnormalities are observed, a cell can also spontaneously initiate the process of apoptosis.
  • In some cases, such as a viral infection or cancer, the cell's normal checks and balances fail.
  • External signaling can also initiate apoptosis.
  • Apoptosis is also essential for normal embryological development; unnecessary cells that appear during the early stages of development will eventually be eliminated through cell signaling.

Key Terms

apoptosis

a process of programmed cell death

glycoprotein

a protein with covalently-bonded carbohydrates

Apoptosis

When a cell is damaged, superfluous, or potentially dangerous to an organism, a cell can initiate a mechanism to trigger programmed cell death, or apoptosis. Apoptosis allows a cell to die in a controlled manner that prevents the release of potentially damaging molecules from inside the cell. 

Internal Signaling

There are many internal checkpoints that monitor a cell's health; if abnormalities are observed, a cell can spontaneously initiate the process of apoptosis. However, in some cases such as a viral infection or uncontrolled cell division due to cancer, the cell's normal checks and balances fail. 

External Signaling

External signaling can also initiate apoptosis. For example, most normal animal cells have receptors that interact with the extracellular matrix, a network of glycoproteins that provides structural support for cells in an organism. The binding of cellular receptors to the extracellular matrix initiates a signaling cascade within the cell. However, if the cell moves away from the extracellular matrix, the signaling ceases, and the cell undergoes apoptosis. This system keeps cells from traveling through the body and proliferating out of control, as happens with tumor cells that metastasize.

Another example of external signaling that leads to apoptosis occurs in T-cell development. T-cells are immune cells that bind to foreign macromolecules and particles, targeting them for destruction by the immune system. Normally, T-cells do not target "self" proteins (those of their own organism), a process that can lead to autoimmune diseases. In order to develop the ability to discriminate between self and non-self, immature T-cells undergo screening to determine whether they bind to so-called self proteins. If the T-cell receptor binds to self proteins, the cell initiates apoptosis to remove the potentially dangerous cell.

Apoptosis and Embryos

Apoptosis is also essential for normal embryological development. In vertebrates, for example, early stages of development include the formation of web-like tissue between individual fingers and toes . During the course of normal development, these unnecessary cells must be eliminated, enabling fully separated fingers and toes to form. A cell signaling mechanism triggers apoptosis, which destroys the cells between the developing digits.

Apoptosis

Apoptosis

The histological section of a foot of a 15-day-old mouse embryo, visualized using light microscopy, reveals areas of tissue between the toes which apoptosis will eliminate before the mouse reaches its full gestational age at 27 days.

Attributions

  • Termination of the Signal Cascade
    • "Boundless." http://www.boundless.com/. Boundless Learning CC BY-SA 3.0.
    • "OpenStax College, Biology. October 16, 2013." http://cnx.org/content/m44453/latest/?collection=col11448/latest. OpenStax CNX CC BY 3.0.
    • "phosphorylation." http://en.wiktionary.org/wiki/phosphorylation. Wiktionary CC BY-SA 3.0.
    • "dephosphorylation." http://en.wiktionary.org/wiki/dephosphorylation. Wiktionary CC BY-SA 3.0.
    • "OpenStax College, Propagation of the Signal. October 26, 2013." http://cnx.org/content/m44452/latest/. OpenStax CNX CC BY 3.0.
    • "OpenStax College, Propagation of the Signal. October 26, 2013." http://cnx.org/content/m44452/latest/. OpenStax CNX CC BY 3.0.
  • Cell Signaling and Gene Expression
    • "Boundless." http://www.boundless.com/. Boundless Learning CC BY-SA 3.0.
    • "gene expression." http://en.wiktionary.org/wiki/gene_expression. Wiktionary CC BY-SA 3.0.
    • "OpenStax College, Biology. October 16, 2013." http://cnx.org/content/m44453/latest/?collection=col11448/latest. OpenStax CNX CC BY 3.0.
    • "translation." http://en.wiktionary.org/wiki/translation. Wiktionary CC BY-SA 3.0.
    • "OpenStax College, Regulation of Gene Expression. October 26, 2013." http://cnx.org/content/m44534/latest/. OpenStax CNX CC BY 3.0.
    • "transcription." http://en.wiktionary.org/wiki/transcription. Wiktionary CC BY-SA 3.0.
    • "OpenStax College, Regulation of Gene Expression. October 26, 2013." http://cnx.org/content/m44534/latest/. OpenStax CNX CC BY 3.0.
  • Cell Signaling and Cellular Metabolism
    • "Boundless." http://www.boundless.com/. Boundless Learning CC BY-SA 3.0.
    • "cyclic adenosine monophosphate." http://en.wikipedia.org/wiki/cyclic%20adenosine%20monophosphate. Wikipedia CC BY-SA 3.0.
    • "protein kinase A." http://en.wikipedia.org/wiki/protein%20kinase%20A. Wikipedia CC BY-SA 3.0.
    • "Cellular metabolism." http://en.wikipedia.org/wiki/Cellular_metabolism%23Regulation_and_control. Wikipedia CC BY-SA 3.0.
    • "epinephrine." http://en.wiktionary.org/wiki/epinephrine. Wiktionary CC BY-SA 3.0.
    • "OpenStax College, Biology. October 16, 2013." http://cnx.org/content/m44453/latest/?collection=col11448/latest. OpenStax CNX CC BY 3.0.
    • "OpenStax College, Biology. October 26, 2013." http://cnx.org/content/m44422/latest/?collection=col11448/latest. OpenStax CNX CC BY 3.0.
    • "OpenStax College, Propagation of the Signal. November 7, 2013." http://cnx.org/content/m44452/latest/. OpenStax CNX CC BY 3.0.
  • Cell Signaling and Cell Growth
    • "Boundless." http://www.boundless.com/. Boundless Learning CC BY-SA 3.0.
    • "receptor." http://en.wiktionary.org/wiki/receptor. Wiktionary CC BY-SA 3.0.
    • "growth factor." http://en.wiktionary.org/wiki/growth_factor. Wiktionary CC BY-SA 3.0.
    • "oncogene." http://en.wiktionary.org/wiki/oncogene. Wiktionary CC BY-SA 3.0.
    • "OpenStax College, Biology. October 16, 2013." http://cnx.org/content/m44453/latest/?collection=col11448/latest. OpenStax CNX CC BY 3.0.
    • "Molecular Cancer | Full text | Role of APCand DNA mismatch repair genes in the development of colorectal cancers." http://www.molecular-cancer.com/content/2/1/41. Molecular Cancer Journal CC BY.
  • Cell Signaling and Cell Death
    • "Boundless." http://www.boundless.com/. Boundless Learning CC BY-SA 3.0.
    • "glycoprotein." http://en.wiktionary.org/wiki/glycoprotein. Wiktionary CC BY-SA 3.0.
    • "OpenStax College, Biology. October 16, 2013." http://cnx.org/content/m44453/latest/?collection=col11448/latest. OpenStax CNX CC BY 3.0.
    • "apoptosis." http://en.wiktionary.org/wiki/apoptosis. Wiktionary CC BY-SA 3.0.
    • "OpenStax College, Response to the Signal. October 16, 2013." http://cnx.org/content/m44453/latest/Figure_09_03_02.jpg. OpenStax CNX CC BY 3.0.

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