AP Biology Unit IV Notes: Cell Communication, Homeostasis, and the Cell Cycle

Overview of Unit IV in AP Biology

  • Topics in Unit IV include:
    • Cell communication, signaling, and signal transduction
    • Feedback and homeostasis
    • Cell division and the cell cycle
    • Cell cycle regulation, cancer, and apoptosis

Cell Communication and Signaling

Importance of Cell Communication
  • Cells never operate in isolation; they communicate constantly.
  • Communication types:
    • Direct communication: Cells are connected via junctions, allowing molecule exchange.
    • Long-distance signaling: Signals (ligands) travel through the bloodstream (e.g., hormones).
    • Local signaling: Cells produce local regulators for close communication.
Ligands and Their Roles
  • Definition: Signaling molecules such as hormones and local regulators.
  • Hormones travel long distances through blood, whereas local regulators function over short distances.
  • Ligands bind to receptors based on complementary shapes, leading to cellular responses.
Quorum Sensing
  • A bacterial communication method where cells release signals to interact when a population reaches a critical density.
  • Example: Biofilm formation leading to plaque on teeth.
Phases of Cell Signaling
  1. Reception: Ligand binds to receptor molecules, initiating the signaling cascade.
  2. Signal Transduction: The message changes form; often involves amplification and uses second messengers.
  3. Cellular Response: The end result is a functional change in the target cell.
Types of Hormones
  • Steroid Hormones: Non-polar, diffuse through membranes, bind to cytoplasmic receptors, act in nuclei, and function as transcription factors.
  • Water-soluble Hormones: Bind to membrane-bound receptors and activate second messengers, generally leading to quicker but shorter responses.

Epinephrine and G Protein-Coupled Receptors (GPCR)

The Fight or Flight Response
  • Epinephrine (adrenaline) is a polar hormone released during stress, affecting various tissues specifically.
  • Causes include: increased heart rate, pupil dilation, decreased digestion, and glucose mobilization from glycogen.
Mechanism of Epinephrine Signaling
  1. Binding: Epinephrine binds to GPCRs on cell membranes, altering receptor shape.
  2. G Protein Activation: The G protein exchanges GDP for GTP, becoming active.
  3. Adenylyl Cyclase Activation: The activated G protein stimulates adenylyl cyclase to convert ATP into cyclic AMP (cAMP), a second messenger.
  4. Phosphorylation Cascade: cAMP activates a series of kinases, amplifying the initial signal and leading to the activation of glycogen phosphorylase to produce glucose.
Signal Termination
  • After the signal's purpose is served, the system needs to reset.
  • G protein hydrolyzes GTP back to GDP, inactivating it.
  • This reduces cAMP levels, stopping the downstream effects and returning the system to baseline.

Feedback Mechanisms and Homeostasis

Overview of Homeostasis
  • Homeostasis is the maintenance of internal conditions (e.g., temperature, glucose levels) at optimal levels.
  • Feedback mechanisms involve outputs returning as inputs to control systems.
Types of Feedback
  • Negative Feedback: Counteract changes to return systems to set points (e.g., blood glucose regulation with insulin and glucagon).
  • Positive Feedback: Intensifies changes; drives processes to completion (e.g., childbirth).
Blood Sugar Regulation Example
  • Negative feedback: Insulin lowers high blood glucose and glucagon raises low blood glucose.
  • In Type 2 diabetes, cells become resistant to insulin, failing to lower blood glucose levels.
  • In Type 1 diabetes, the immune system destroys insulin-producing cells in the pancreas.

The Cell Cycle

Mitosis Overview
  • Mitosis is crucial for growth, repair, and reproduction in multicellular organisms.
  • Key phases of the cell cycle:
    • G1 (growth), S (synthesis, DNA replication), G2 (prep for division), M (mitosis).
Mitosis Phases
  1. Prophase: Chromatin condenses, nuclear envelope disintegrates, spindle apparatus forms.
  2. Metaphase: Chromosomes align at the equator, spindle fibers attach to kinetochores.
  3. Anaphase: Sister chromatids are pulled apart to opposite poles.
  4. Telophase: Nuclear membranes re-form, chromosomes unwind, spindle fibers break down.
  5. Cytokinesis: Cytoplasm divides, forming two daughter cells.

Regulation of the Cell Cycle

Checkpoints
  • Checkpoints are regulatory points in the cell cycle assessing cell condition before proceeding to the next phase (G1, G2, M).
  • Poor conditions can divert cells to G0 (non-dividing state) or trigger apoptosis (programmed cell death).
Cyclins and CDKs
  • Cyclins fluctuate in concentration while CDKs remain constant. When activated, cyclin-CDK complexes push the cell past checkpoints and into division.

Cancer and Cell Cycle Dysregulation

Types of Cellular Mutations Leading to Cancer
  • Proto-oncogenes: Mutations can lead to excessive growth factor production, leading to uncontrolled division.
  • Tumor Suppressor Genes: Mutations can eliminate inhibitory signals, allowing unchecked cell proliferation.
Example Gene Mutations
  • ROS proto-oncogene: In normal cells, it's activated by external signals but in mutated forms, it's constitutively active, leading to excess cell division.
  • p53 Tumor Suppressor: Normally responds to DNA damage by stopping the cell cycle or initiating apoptosis. Mutations can lead cells with damaged DNA to continue dividing, increasing cancer risk.