unit 3 cell signaling

Chapter 11: Cell Signaling

1. The Big Picture of Cell Signaling

  • Signal Transduction Pathways: Cells communicate via signaling molecules that lead to a cascade of events within the cell.

  • Three Stages of Cell Signaling:

    1. Reception – The target cell detects a signaling molecule from outside the cell.

    2. Transduction – The receptor initiates a series of changes, creating a signal transduction pathway.

    3. Response – The transduced signal triggers a specific cellular activity.

2. Types of Signaling Molecules

  • Ligands: The molecule that binds specifically to a receptor; can be proteins, peptides, amino acids, steroids, or gases.

  • Hydrophobic Signals (e.g., steroid hormones): Cross the plasma membrane and bind to intracellular receptors.

  • Hydrophilic Signals: Bind to cell-surface receptors, as they cannot pass through the membrane.

3. Types of Receptors

  • G Protein-Coupled Receptors (GPCRs):

    • Structure: Span the membrane and work with G proteins.

    • Mechanism: Ligand binding activates the G protein, which then activates or inhibits downstream effectors.

  • Receptor Tyrosine Kinases (RTKs):

    • Structure: Enzyme-linked receptors that form dimers and autophosphorylate upon ligand binding.

    • Mechanism: Phosphorylation of tyrosine residues activates relay proteins.

  • Ion Channel Receptors:

    • Structure: Ligand-gated channels that open or close in response to ligand binding.

    • Mechanism: Allows ions like Na⁺ or Ca²⁺ to flow in/out, changing the cell’s potential and triggering responses.

  • Intracellular Receptors:

    • Located in the cytoplasm or nucleus and often influence gene expression directly by acting as transcription factors.

4. Signal Transduction Pathways

  • Phosphorylation Cascade: Series of protein phosphorylations that amplify the signal, involving protein kinases and phosphatases.

  • Second Messengers: Small, non-protein molecules like cAMP, Ca²⁺, and IP₃ that amplify and spread signals.

    • cAMP Pathway: Activated by GPCRs, triggering protein kinase A (PKA).

    • Calcium Ions and IP₃: Ca²⁺ is a major signaling molecule, often released via IP₃ pathways, important in muscle contraction and other responses.

5. Cellular Responses to Signaling

  • Gene Expression: Signals can activate transcription factors, altering protein synthesis.

  • Metabolic Pathways: Can activate or inhibit enzymes, changing metabolic activity.

  • Cell Movement and Shape Changes: Important in processes like chemotaxis and development.

  • Apoptosis (Programmed Cell Death): Signaling pathways control this, ensuring proper tissue maintenance and removal of damaged cells.

Chapter 43: The Immune System and Cell Signaling

1. Innate vs. Adaptive Immunity

  • Innate Immunity:

    • Characteristics: Rapid response, non-specific, present from birth.

    • Components:

      • Barrier Defenses: Skin, mucous membranes, secretions.

      • Internal Defenses: Phagocytic cells, natural killer (NK) cells, antimicrobial proteins, inflammation.

  • Adaptive Immunity:

    • Characteristics: Specific, slower response, memory component.

    • Components:

      • Humoral Response: Antibodies defend against infection in body fluids.

      • Cell-Mediated Response: Cytotoxic T cells defend against infection in body cells.

2. Innate Immunity: Cellular Responses

  • Phagocytic Cells: Includes neutrophils, macrophages, and dendritic cells that engulf pathogens.

  • Natural Killer (NK) Cells: Recognize infected or cancerous cells and induce apoptosis.

  • Inflammatory Response: Triggered by histamine and other chemicals that increase blood flow and attract immune cells to sites of injury/infection.

3. Adaptive Immunity: Lymphocytes and Antigen Recognition

  • Lymphocytes:

    • B Cells: Produce antibodies and are part of humoral immunity.

    • T Cells: Include helper T cells (coordinate the immune response) and cytotoxic T cells (kill infected cells).

  • Antigens and Antigen Receptors:

    • Each lymphocyte has unique receptors that bind specific antigens.

    • B Cell Receptors (BCRs): Bind directly to antigens.

    • T Cell Receptors (TCRs): Recognize antigens presented by MHC (Major Histocompatibility Complex) molecules on other cells.

4. Clonal Selection and Immunological Memory

  • Clonal Selection: Antigen binding triggers clonal expansion of B or T cells specific to that antigen.

  • Effector Cells and Memory Cells:

    • Effector Cells: Short-lived, active cells that combat the antigen.

    • Memory Cells: Long-lived cells that enable a faster, stronger response upon re-exposure (basis for vaccines).

5. Humoral vs. Cell-Mediated Response

  • Humoral Response (B Cells): Involves antibodies circulating in the blood and lymph.

    • Neutralization: Antibodies block pathogen binding.

    • Opsonization: Antibodies mark pathogens for phagocytosis.

    • Complement Activation: Antibody-pathogen complexes trigger the complement system, leading to cell lysis.

  • Cell-Mediated Response (T Cells):

    • Helper T Cells (CD4+): Bind to MHC II on antigen-presenting cells, secrete cytokines to activate other immune cells.

    • Cytotoxic T Cells (CD8+): Recognize MHC I on infected cells, release perforin and granzymes to induce apoptosis.

6. Cell Signaling in Immune Response

  • Cytokines: Proteins secreted by immune cells that act as signaling molecules.

    • Interleukins: Stimulate the proliferation of immune cells.

    • Interferons: Released in response to viral infection, helping nearby cells resist the virus.

  • Chemokines: Direct cell movement toward infection sites.

  • MHC Proteins: Present antigen fragments to T cells, key in activating adaptive immunity.

7. Immunological Disorders

  • Allergies: Overactive immune response to non-harmful substances.

  • Autoimmune Diseases: Immune system attacks own body cells (e.g., lupus, rheumatoid arthritis).

  • Immunodeficiency: Lack or failure of immune response (e.g., AIDS).