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Boundless Biology: 36.1: Sensory Processes

Boundless Biology
36.1: Sensory Processes
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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

36.1: Sensory Processes

36.1.1: Reception

Reception is the first step in the processing of sensation and is dependent on the receptor type, stimulus, and receptive field.

Learning Objective

Explain the process of sensory reception

Key Points

  • Reception is the process of activating a sensory receptor by a stimuli.
  • Sensory transduction is the process of converting that sensory signal to an electrical signal in the sensory neuron.
  • The process of reception is dependent on the stimuli itself, the type of receptor, receptor specificity, and the receptive field, which can vary depending on the receptor type.

Key Terms

reception

the act or ability to receive signals from stimuli

somatosensation

general senses which respond to stimuli like temperature, pain, pressure, and vibration

Introduction to Sensation

In more advanced animals, the senses are constantly at work, making the animal aware of stimuli, such as light or sound or the presence of a chemical substance in the external environment, while monitoring information about the organism's internal environment. All bilaterally symmetric animals have a sensory system. The development of any species' sensory system has been driven by natural selection; thus, sensory systems differ among species according to the demands of their environments. For example, the shark, unlike most fish predators, is electrosensitive (i.e., sensitive to electrical fields produced by other animals in its environment). While it is helpful to this underwater predator, electrosensitivity is a sense not found in most land animals.

Senses provide information about the body and its environment. Humans have five special senses: olfaction (smell), gustation (taste), equilibrium (balance and body position), vision, and hearing. Additionally, we possess general senses, also called somatosensation, which respond to stimuli like temperature, pain, pressure, and vibration. Vestibular sensation, which is an organism's sense of spatial orientation and balance, proprioception (position of bones, joints, and muscles), and the sense of limb position that is used to track kinesthesia (limb movement) are part of somatosensation. Although the sensory systems associated with these senses are very different, all share a common function: to convert a stimulus (light, sound, or the position of the body) into an electrical signal in the nervous system. This process is called sensory transduction.

There are two broad types of cellular systems that perform sensory transduction. In one, a neuron works with a sensory receptor, a cell, or cell process that is specialized to engage with and detect a specific stimulus. Stimulation of the sensory receptor activates the associated afferent neuron, which carries information about the stimulus to the central nervous system. In the second type of sensory transduction, a sensory nerve ending responds to a stimulus in the internal or external environment; this neuron constitutes the sensory receptor. Free nerve endings can be stimulated by several different stimuli, thus showing little receptor specificity. For example, pain receptors in your gums and teeth may be stimulated by temperature changes, chemical stimulation, or pressure.

Reception

The first step in sensation is reception: the activation of sensory receptors by stimuli such as mechanical stimuli (being bent or squished, for example), chemicals, or temperature. The receptor can then respond to the stimuli. The region in space in which a given sensory receptor can respond to a stimulus, be it far away or in contact with the body, is that receptor's receptive field. Think for a moment about the differences in receptive fields for the different senses. For the sense of touch, a stimulus must come into contact with body. For the sense of hearing, a stimulus can be a moderate distance away. For vision, a stimulus can be very far away; for example, the visual system perceives light from stars at enormous distances .

Visual sensory system

Visual sensory system

This scheme shows the flow of information from the eyes to the central connections of the optic nerves and optic tracts, to the visual cortex. Area V1 is the region of the brain which is engaged in vision.

36.1.2: Transduction and Perception

Transduction is the process that converts a sensory signal to an electrical signal to be processed in a specialized area in the brain.

Learning Objective

Explain how stimuli are converted to signals that are carried to the central nervous system

Key Points

  • Sensory signals are converted to electrical signals via depolarization of sensory neuron membranes upon stimulus of the receptor, which causes opening of gated ion channels that cause the membrane potential to reach its threshold.
  • The receptor potentials are classified as graded potentials; the magnitude of these potentials is dependent on the strength of the stimulus.
  • The sensory system shows receptor specificity; although stimuli can be combined in processing regions of the brain, a specific receptor will only be activated by its specific stimulus.
  • The brain contains specific processing regions (such as the somatosensory, visual, and auditory regions) that are dedicated to processing the information which has previously passed through the thalamus, the 'clearinghouse and relay station' for both sensory and motor signals.
  • The four major components of encoding and transmitting sensory information include: the type of stimulus, the stimulus location within the receptive field, the duration, and the intensity of the stimulus.

Key Terms

action potential

a short term change in the electrical potential that travels along a cell

membrane potential

the difference in electrical potential across the enclosing membrane of a cell

transduction

the translation of a sensory signal in the sensory system to an electrical signal in the nervous system

Transduction

The most fundamental function of a sensory system is the translation of a sensory signal to an electrical signal in the nervous system. This takes place at the sensory receptor. The change in electrical potential that is produced is called the receptor potential. How is sensory input, such as pressure on the skin, changed to a receptor potential? As an example, a type of receptor called a mechanoreceptor possesses specialized membranes that respond to pressure. Disturbance of these dendrites by compressing them or bending them opens gated ion channels in the plasma membrane of the sensory neuron, changing its electrical potential . In the nervous system, a positive change of a neuron's electrical potential (also called the membrane potential), depolarizes the neuron. Receptor potentials are graded potentials: the magnitude of these graded (receptor) potentials varies with the strength of the stimulus. If the magnitude of depolarization is sufficient (that is, if membrane potential reaches a threshold), the neuron will fire an action potential. In most cases, the correct stimulus impinging on a sensory receptor will drive membrane potential in a positive direction, although for some receptors, such as those in the visual system, this is not always the case.

Mechanoreceptor activation

Mechanoreceptor activation

(a) Mechanosensitive ion channels are gated ion channels that respond to mechanical deformation of the plasma membrane. A mechanosensitive channel is connected to the plasma membrane and the cytoskeleton by hair-like tethers. When pressure causes the extracellular matrix to move, the channel opens, allowing ions to enter or exit the cell. (b) Stereocilia in the human ear are connected to mechanosensitive ion channels. When a sound causes the stereocilia to move, mechanosensitive ion channels transduce the signal to the cochlear nerve.

Sensory receptors for the various senses work differently from each other. They are specialized according to the type of stimulus they sense; thus, they have receptor specificity. For example, touch receptors, light receptors, and sound receptors are each activated by different stimuli. Touch receptors are not sensitive to light or sound; they are sensitive only to touch or pressure. However, stimuli may be combined at higher levels in the brain, as happens with olfaction, contributing to our sense of taste.

Encoding and Transmission of Sensory Information

Four aspects of sensory information are encoded by sensory systems: the type of stimulus, the location of the stimulus in the receptive field, the duration of the stimulus, and the relative intensity of the stimulus. Thus, action potentials transmitted over a sensory receptor's afferent axons encode one type of stimulus. This segregation of the senses is preserved in other sensory circuits. For example, auditory receptors transmit signals over their own dedicated system. The electrical activity in the axons of the auditory receptors will be interpreted by the brain as an auditory stimulus: a sound .

The intensity of a stimulus is often encoded in the rate of action potentials produced by the sensory receptor. Thus, an intense stimulus will produce a more rapid train of action potentials. Reducing the stimulus will likewise slow the rate of production of action potentials. A second way in which intensity is encoded is by the number of receptors activated. An intense stimulus might initiate action potentials in a large number of adjacent receptors, while a less intense stimulus might stimulate fewer receptors. Integration of sensory information begins as soon as the information is received in the central nervous system.

Perception

Perception is an individual's interpretation of a sensation. Although perception relies on the activation of sensory receptors, perception happens, not at the level of the sensory receptor, but at the brain level. The brain distinguishes sensory stimuli through a sensory pathway: action potentials from sensory receptors travel along neurons that are dedicated to a particular stimulus.

All sensory signals, except those from the olfactory system, are transmitted though the central nervous system: they are routed to the thalamus and to the appropriate region of the cortex. The thalamus is a structure in the forebrain that serves as a clearinghouse and relay station for sensory (as well as motor) signals. When the sensory signal exits the thalamus, it is conducted to the specific area of the cortex dedicated to processing that particular sense .

Sensation processing

Sensation processing

The brain has dedicated areas to the processing of stimuli, including: (a) thalamus and (b) the auditory, visual and somatosensory processing regions.

Attributions

  • Reception
    • "Boundless." http://www.boundless.com/. Boundless Learning CC BY-SA 3.0.
    • "reception." http://en.wiktionary.org/wiki/reception. Wiktionary CC BY-SA 3.0.
    • "OpenStax College, Biology. October 23, 2013." http://cnx.org/content/m44754/latest/?collection=col11448/latest. OpenStax CNX CC BY 3.0.
    • "OpenStax College, Biology. October 17, 2013." http://cnx.org/content/m44754/latest/?collection=col11448/latest. OpenStax CNX CC BY 3.0.
    • "somatosensation." http://en.wiktionary.org/wiki/somatosensation. Wiktionary CC BY-SA 3.0.
    • "Gray722." http://en.wikipedia.org/wiki/File:Gray722.png. Wikipedia Public domain.
  • Transduction and Perception
    • "Boundless." http://www.boundless.com/. Boundless Learning CC BY-SA 3.0.
    • "action potential." http://en.wiktionary.org/wiki/action_potential. Wiktionary CC BY-SA 3.0.
    • "membrane potential." http://en.wiktionary.org/wiki/membrane_potential. Wiktionary CC BY-SA 3.0.
    • "OpenStax College, Biology. October 23, 2013." http://cnx.org/content/m44754/latest/?collection=col11448/latest. OpenStax CNX CC BY 3.0.
    • "OpenStax College, Biology. October 17, 2013." http://cnx.org/content/m44754/latest/?collection=col11448/latest. OpenStax CNX CC BY 3.0.
    • "transduction." http://en.wiktionary.org/wiki/transduction. Wiktionary CC BY-SA 3.0.
    • "OpenStax College, Sensory Processes. October 17, 2013." http://cnx.org/content/m44754/latest/Figure_36_01_01f.jpg. OpenStax CNX CC BY 3.0.
    • "OpenStax College, Sensory Processes. October 17, 2013." http://cnx.org/content/m44754/latest/Figure_36_01_02.jpg. OpenStax CNX CC BY 3.0.

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