Cell Signaling and Apoptosis

Chapter Review

SUMMARY OF KEY CONCEPTS

CONCEPT 11.1: External Signals Are Converted to Responses Within the Cell (pp. 213-217)

  • Importance of Signal Transduction Pathways:

    • Crucial for many biological processes, including yeast mating, which shows evolutionary similarities with multicellular organisms, indicating early origins of signaling mechanisms.
    • Bacterial cells engage in quorum sensing, a method to sense local densities of bacterial cells.

  • Local and Long-Distance Signaling:

    • Local signaling among animal cells involves:

    • Direct contact

    • Secretion of local regulators.

    • For long-distance signaling:

    • Animal and plant cells utilize hormones.

    • Animals also use electrical signals to convey messages.

  • Hormonal Interaction Example:

    • Like epinephrine, many hormones bind to membrane receptors, triggering a three-stage signaling pathway:
    1. Reception:
      • Binding of the signaling molecule (ligand) to a receptor.
    2. Transduction:
      • Relaying the signal through a cascade.
    3. Response:
      • Activation of a cellular response.

CONCEPT 11.2: Reception (pp. 217-221)

  • Specificity of Binding:

    • Binding between a signaling molecule (ligand) and its receptor is highly specific. A shape change in the receptor often occurs, representing the initial transduction.

  • Types of Cell-Surface Transmembrane Receptors:

    1. G Protein-Coupled Receptors (GPCRs):
    • Work alongside cytoplasmic G proteins.
    • Ligand binding activates the receptor which then activates a specific G protein that in turn activates another protein, propagating the signal.
    1. Receptor Tyrosine Kinases (RTKs):
    • Binding leads to dimer formation and phosphorylation of tyrosines on the monomer's cytoplasmic part.
    • Relay proteins in the cell can then bind to different phosphorylated tyrosines triggering multiple pathways simultaneously.
    1. Ligand-Gated Ion Channels:
    • Open or close in response to specific signaling molecules, thus regulating ion flow across membranes.

  • Abnormal Function and Diseases:

    • Dysfunctional GPCRs and RTKs can lead to various human diseases.

  • Intracellular Receptors:

    • Found in the cytoplasm or nucleus.
    • Bind signaling molecules that are hydrophobic or small enough to penetrate the plasma membrane.

CONCEPT 11.3: Transduction (pp. 221-225)

  • Signal Cascades:

    • Each step in a transduction pathway converts the signal into a different form, commonly involving shape changes in proteins.

  • Phosphorylation Cascades:

    • Many pathways involve series of protein kinases adding phosphate groups to the subsequent proteins, activating them.
    • Enzymes known as protein phosphatases remove these phosphate groups.

  • Balancing Activity:

    • The balance between phosphorylation (active) and dephosphorylation (inactive) regulates protein activity.

  • Second Messengers:

    • Molecules like cyclic AMP (cAMP) and Ca2+ diffuse through the cytosol to broadcast signals quickly.
    • G proteins can activate adenylyl cyclase, producing cAMP from ATP.
    • Ca2+ is utilized in both GPCR and RTK pathways.
    • Tyrosine kinase pathways can also involve other second messengers such as diacylglycerol (DAG) and inositol trisphosphate (IP3), which can increase Ca2+ levels.

CONCEPT 11.4: Response (pp. 226-229)

  • Nuclear Responses:

    • Some signaling pathways lead to nuclear responses where specific genes are turned on or off via activated transcription factors.
    • Other pathways result in cytoplasmic regulation instead.

  • Regulation of Cellular Responses:

    • Cellular responses are regulated at multiple points; each protein in signaling pathways amplifies the signal by activating multiple copies of the subsequent component, sometimes leading to overall amplification exceeding one millionfold.
    • Protein combinations within a cell provide specificity in signal detection and ensuing responses.

  • Efficient Signaling:

    • Scaffolding proteins enhance signaling efficiency and coordination through pathway branching.
    • Signal termination can occur rapidly due to reversible ligand binding.

CONCEPT 11.5: Apoptosis Integrates Multiple Cell-Signaling Pathways (pp. 229-231)

  • Apoptosis:

    • A programmed cell death process where cellular components are disposed of in an orderly manner.
    • Studies on the model organism Caenorhabditis elegans have elucidated molecular details of apoptosis signaling pathways.

  • Triggering Apoptosis:

    • Multiple apoptotic pathways exist in mammals which can be activated by various signals originating either from outside or inside the cells.

  • Genetic Similarities:

    • There are notable genetic similarities in apoptosis genes across yeast, nematodes, and mammals that suggest conserved mechanisms in these organisms.