Cell Membrane Structure and Function

Membrane Structure - The Fluidic Fence

• Fluid Mosaic Model (Singer & Nicolson, 1972): Dynamic, fluid structure.
  • Lipids & proteins exhibit lateral mobility.
• Composition:
  • Lipids (≈50%):
    • Phospholipids: Amphipathic; form bilayer. Hydrophilic head, 2 hydrophobic tails.
    • Cholesterol: Modulates fluidity (↓fluidity at ↑temp; maintains fluidity at ↓temp).
    • Glycolipids: Outer leaflet; cell recognition (e.g., ABO antigens).
  • Proteins (≈50%):
    • Integral (transmembrane): Span membrane; e.g., channels, carriers, receptors.
    • Peripheral: Surface-bound (inner/outer); e.g., enzymes, signal transducers.
  • Carbohydrates (<10%):
    • Glycocalyx (cell coat): Outer surface; glycoproteins & glycolipids. Functions: recognition, adhesion, protection, neg. charge.
• Key Properties:
  • Fluidity: Crucial for cell movement, fusion, division.
  • Asymmetry: Distinct inner & outer leaflet compositions.
  • Selective Permeability.

⭐ The fluidity of the cell membrane is primarily determined by the degree of unsaturation of fatty acid chains in phospholipids and the amount of cholesterol.

Membrane Proteins - Versatile VIPs

• ~50% of membrane mass; execute specific functions.
• Types:
  • Integral: Embedded/transmembrane. Amphipathic. (e.g., channels, carriers, receptors).
  • Peripheral: Surface-bound to proteins/lipids. Loosely attached. (e.g., spectrin, ankyrin).
• Functions (📌 Mnemonic: TRACIE):
  • Transport: Carriers (GLUTs), Pumps (Na⁺/K⁺-ATPase for active transport).
  • Receptors: Signal transduction (GPCRs, Tyrosine Kinase Receptors).
  • Anchorage/Adhesion: Cytoskeleton/ECM links (Integrins, Cadherins).
  • Channels: Selective ion passage (Na⁺, K⁺, Ca²⁺ channels).
  • Enzymes: Membrane-bound catalysis (Adenylyl cyclase, ATPase).
  • Identity: Cell recognition (Glycoproteins, MHC molecules).

⭐ The Na⁺/K⁺-ATPase pump is an electrogenic carrier protein, moving 3 Na⁺ out and 2 K⁺ in per ATP, crucial for nerve impulses and maintaining cell volume_

Membrane Transport - Cellular Customs

• Passive Transport (No ATP): Substances move down electrochemical gradient.
  • Simple Diffusion: Small, nonpolar, lipid-soluble (O₂, CO₂, steroids) & some small uncharged polar (H₂O, urea) cross unaided. Rate proportional to gradient. Fick's Law: $J = -D \frac{dC}{dx}$.
  • Facilitated Diffusion: Uses membrane proteins (channels/carriers) for polar molecules/ions (e.g., glucose via GLUT, ion channels). Saturable, specific.
  • Osmosis: Net water movement from low to high solute concentration.
• Active Transport (ATP used): Moves substances against electrochemical gradient.
  • Primary Active Transport: Direct ATP hydrolysis.
    • Na⁺/K⁺ ATPase pump: 📌 3 Na⁺ Out, 2 K⁺ In. Maintains Na⁺/K⁺ gradients, RMP, cell volume.
    • Ca²⁺ ATPase (SERCA, PMCA). H⁺/K⁺ ATPase.
  • Secondary Active Transport: Uses ion gradient (usually Na⁺ from primary active transport).
    • Symport (Cotransport): Both substances same direction (e.g., SGLT1: Na⁺-glucose in intestine/kidney).
    • Antiport (Countertransport): Substances opposite directions (e.g., Na⁺/Ca²⁺ exchanger, Na⁺/H⁺ exchanger).

, secondary active transport (SGLT1))

⭐ SGLT1 (Sodium-Glucose Linked Transporter 1) is a key example of secondary active transport (symport), reabsorbing glucose against its concentration gradient in the small intestine and renal tubules, coupled with Na⁺ entry.

High‑Yield Points - ⚡ Biggest Takeaways

• Fluid Mosaic Model by Singer & Nicolson: describes the dynamic arrangement of lipids and proteins.
• Phospholipid Bilayer: Forms the core; amphipathic with hydrophobic tails and hydrophilic heads, creating a semipermeable barrier.
• Membrane Proteins: Integral (transmembrane) and peripheral; crucial for transport, signaling (receptors), and enzymatic activity.
• Cholesterol: Modulates membrane fluidity; acts as a bidirectional regulator, ensuring stability across temperatures.
• Selective Permeability: Controls substance passage; small, nonpolar molecules (O₂, CO₂) diffuse freely, while ions and polar molecules require transport mechanisms.
• Glycocalyx: Carbohydrate layer on the outer surface (cell coat); vital for cell recognition, adhesion, and protection.
• Lipid Rafts: Specialized microdomains rich in sphingolipids and cholesterol; function as platforms for signal transduction and protein trafficking.