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.