Unit 4 Review AP Biology

Unit 4 Overview: Cell Communication and the Cell Cycle

• Unit 4 covers 10-15% of the AP Biology exam.

• Focus on how cells communicate and respond to environmental stimuli through molecular variation.

• Importance of this variation for survival and reproduction.

Types of Cell Communication

Cell-to-Cell Contact

• Examples:

  • Immune system cells directly contacting each other, e.g., helper T cells and macrophages.

  • Plants: Plasmodesmata allow communication through channels between plant cells.

Short-Distance Signaling (Local Regulators)

• Local regulators communicate over short distances.

• Examples:

  • Cytokines released by helper T cells activate B cells and T cells.

  • Chemokines and histamines involved in inflammatory responses.

  • Growth factors such as platelet-derived growth factor.

  • Neurotransmitters in synapses between neurons.

  • Pheromones in bacterial quorum sensing.

Long-Distance Signaling

• Involves hormones traveling through the bloodstream to target cells.

• Target Cells: Specific receptors for hormones are present on the cell surface or within the cytoplasm.

Hormone Action and Signal Transduction

• Receptor Types:

  • Membrane Receptor: for peptide and protein hormones (hydrophilic); requires binding to a cell surface receptor to trigger a response.

  • Intracellular Receptor: for lipid-based hormones (hydrophobic) that can diffuse through membranes.

Signal Transduction Pathways

• Signal transduction starts when a hormone binds to its receptor, activating a signal transduction pathway.

• Protein Modifications:

  • Example of GDP transforming to GTP.

  • Receptor Tyrosine Kinases (RTK):

    • Two unphosphorylated polypeptides form a dimer; require relay proteins for signal propagation.

  • Kinases: Enzymes transferring phosphate groups to proteins, activating them through conformational changes.

  • Example of epinephrine activating adenylyl cyclase, which produces cyclic AMP as a second messenger and initiates a phosphorylation cascade.

  • This cascade ultimately results in the hydrolysis of glycogen, increasing blood sugar during stress responses (fight-or-flight).

Roles of Second Messengers

• Importance: Second messengers amplify the signal inside the cell when the first step is insufficient.

• Key second messengers include cyclic AMP and inositol trisphosphate (IP3).

Types of Receptors

• G-Protein Coupled Receptors (GPCR): Relay signals via G-proteins.

• Tyrosine Kinase Receptors: Respond to ligands and transmit signals via relay proteins.

• Ligand-Gated Ion Channels: Open upon ligand binding, allowing ions to flow, initiating action potentials in neurons.

Cellular Responses to Signal Transduction

• Growth Factors: Ligands stimulate cell growth/division; processes include activating transcription factors for necessary gene expression.

• Cell Cycle: Responses may lead to cell cycle progression or apoptosis (programmed cell death) under specific conditions, such as developmental processes or damaged cells.

Feedback Mechanisms

• Organisms utilize feedback mechanisms for internal and external environmental response, e.g., maintaining body temperature.

Implications of Mutations in Receptors

• Mutations in receptor proteins can disrupt signaling pathways.

• Example: Malfunction of Ras protein leading to uncontrolled cell growth despite the absence of growth factors.

• Drugs may act as agonists (activators) or antagonists (inhibitors) of receptors, affecting signaling pathways.

Medical Applications

• Understanding receptor mechanisms aids in drug design and therapeutic interventions, utilizing molecules that mimic or block natural ligands.