Cell Structure and Function

Cell Membrane - The Dynamic Gatekeeper

• Structure & Composition: Fluid mosaic model. Phospholipid bilayer (amphipathic: polar heads, non-polar tails). Cholesterol: stability & fluidity. Proteins: integral (transmembrane; channels, carriers, receptors), peripheral. Glycocalyx (glycolipids/glycoproteins): cell recognition, protection, adhesion.
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• Key Functions: Selective barrier controlling passage. Regulates nutrient uptake, waste excretion. Facilitates cell signaling (via receptors) and cell-cell interactions/adhesion. Establishes & maintains resting membrane potential (RMP), typically -70 mV to -90 mV.
• Transport Mechanisms:

⭐ The Na+/K+ ATPase is an electrogenic pump: 3 Na+ ions out & 2 K+ ions in per ATP. Vital for maintaining Na+/K+ gradients, RMP, cell volume, and driving secondary active transport. Consumes ~25% cellular ATP (up to 70% in neurons).

Cytoplasmic Organelles - Cellular Factories

Nucleus & Cytoskeleton - Command & Scaffold

• Nucleus: Cell's command center; stores genetic blueprint (DNA).
  • Nuclear Envelope: Double membrane with pores for regulated transport.
  • Nucleolus: Ribosome (rRNA) synthesis.
  • Chromatin: DNA + histone proteins; Euchromatin (active), Heterochromatin (condensed).
• Cytoskeleton: Internal scaffold for shape, movement, & transport. 📌 MIM:
  • Microfilaments (Actin, ~7 nm): Cell cortex, motility, cleavage furrow.
  • Intermediate Filaments (~8-12 nm): Mechanical strength (e.g., keratins, lamins).
  • Microtubules (Tubulin, ~25 nm): Tracks for motor proteins (kinesin, dynein); cilia, flagella, mitotic spindle.

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⭐ Drugs like Vincristine & Paclitaxel target microtubules, inhibiting cell division; crucial in cancer chemotherapy.

Cell Cycle & Signaling - Growth & Gossip

• Cell Cycle: Orderly sequence for cell duplication.
  • Phases: G1 (growth), S (DNA synthesis), G2 (prep for mitosis), M (mitosis). 📌 "Go Sally Go, Make Children!"
  • Mitosis Stages: PMAT (Prophase, Metaphase, Anaphase, Telophase).
  • Checkpoints: Critical control at G1/S (restriction point) & G2/M (DNA integrity). Regulated by Cyclins & CDKs.
• Cell Signaling: Cells communicate via chemical signals (ligands).
  • Ligand binds receptor → intracellular cascade → cellular response.
  • Key Pathways & Second Messengers:

-   Common second messengers: cAMP, cGMP, $Ca^{2+}$, IP3, DAG.

⭐ p53 ("guardian of the genome") is a crucial tumor suppressor, halting cell cycle at the G1/S checkpoint if DNA damage is detected, preventing proliferation of damaged cells. This is a frequent exam topic regarding cancer biology and cell cycle control.

High‑Yield Points - ⚡ Biggest Takeaways

• Plasma membrane: Fluid mosaic model; critical for selective permeability and cell integrity.
• Mitochondria: Powerhouse of the cell; primary site of ATP synthesis (oxidative phosphorylation).
• Rough ER: With ribosomes; essential for protein synthesis and initial modification.
• Smooth ER: Key for lipid synthesis, steroid production, and detoxification.
• Golgi apparatus: Modifies, sorts, and packages proteins and lipids for transport.
• Lysosomes: Contain hydrolytic enzymes for intracellular digestion; "suicidal bags".
• Nucleus: Houses DNA (genetic material); controls cell growth and reproduction.