AP Biology Unit 4: Cell Communication and Cell Cycle

Overview

• Focus on mechanisms of cell communication and the cell cycle.
• Highlighted as one of the easier and interesting units in AP Biology.

Mechanisms of Cell Communication

Direct Contact

• Gap Junctions: Allow molecules to pass directly from one cell to another.
• Cell-Cell Recognition: Receptors on one cell bind to molecules on another cell for identification.

Local Signaling

• Paracrine Signaling: Cells release local regulators that diffuse to nearby target cells.
  • Example: Synaptic signaling in neurons.

Long-Distance Signaling

• Endocrine Signaling: Hormones are released into the bloodstream to reach distant target cells.

Signal Transduction

• Involves a cascade of molecular interactions triggered by the binding of a signaling molecule to a receptor.
• Phosphorylation Cascade: Sequential activation of proteins by the addition of phosphate groups.
• Second Messengers: Small molecules like cAMP and calcium ions that spread the signal within the cell.

Receptors

• G-Protein Coupled Receptors (GPCRs): Activate G-proteins upon binding of a ligand.
• Receptor Tyrosine Kinases (RTKs): Dimerize and activate through phosphorylation.
• Ligand-Gated Ion Channels: Open to allow ions to pass through in response to a ligand.

Cell Junctions

• Desmosomes: Rivet-like structures that hold cells together.
• Tight Junctions: Prevent leakage of extracellular fluid across a layer of epithelial cells.
• Gap Junctions: Cytoplasmic channels between adjacent cells.

Cellular Responses

• Homeostasis: Maintaining a stable internal environment through feedback mechanisms.
  • Negative Feedback: Reduces the effect of a stimulus.
  • Positive Feedback: Amplifies a response.
• Apoptosis: Programmed cell death crucial for removing non-functional cells.

Cell Cycle

• Interphase: Cell growth and DNA replication (G1, S, G2 phases).
• Mitosis: Division of a cell into two daughter cells (P, M, A, T phases).
  • Prophase: Chromosomes condense, spindle fibers form.
  • Metaphase: Chromosomes align at the metaphase plate.
  • Anaphase: Sister chromatids are pulled apart.
  • Telophase: Nuclei reform around separated chromosomes.
• Cytokinesis: Division of the cytoplasm, resulting in two separate cells.

Checkpoints

• Ensure the correct progression through the cell cycle.
• Failures in checkpoints can lead to cancer.

Conclusion

• Understanding cell communication and the cell cycle is crucial for grasping how cells maintain homeostasis and replicate.
• These processes are foundational for advanced topics in biology, including cancer research.