Cell Membrane Physiology and Transport covers cell membrane physiology and transport for INI CET (AIIMS PG).
Cell Membrane Structure (Fluid Mosaic Model — Singer & Nicolson, 1972):
- Phospholipid bilayer: Hydrophilic heads face aqueous environment; hydrophobic tails face inward
- Membrane proteins: Integral (span bilayer) or peripheral (attached to surface)
- Cholesterol: Intercalated between phospholipids — modulates membrane fluidity
- Glycocalyx: Carbohydrate chains on outer surface (glycolipids, glycoproteins) — cell recognition, protection
Membrane Transport Mechanisms:
Passive Transport (no energy — along electrochemical gradient):
- Simple diffusion: Non-polar molecules (O₂, CO₂, N₂), small uncharged polar molecules (H₂O, urea)
- Facilitated diffusion: Ion channels or carrier proteins; stereospecific; saturable (unlike simple diffusion)
- Ion channels: Selectivity based on size and charge; some are ligand-gated or voltage-gated
- Carrier proteins: Conformational change to transport; examples: GLUT transporters (glucose), ion pumps
Active Transport (requires energy — against gradient):
- Primary active transport: Direct ATP hydrolysis — e.g., Na⁺/K⁺-ATPase (3Na⁺ out, 2K⁺ in per ATP)
- Secondary active transport: Uses electrochemical gradient created by primary active transport
- Symport (cotransport): Both substances in same direction — e.g., SGLT (glucose + Na⁺ reabsorption in proximal tubule)
- Antiport (exchange): Substances in opposite directions — e.g., Na⁺/Ca²⁺ exchanger (NCX) in cardiac myocytes (3Na⁺ in, 1Ca²⁺ out)
Key Membrane Transporters:
| Transporter | Location | Function |
|---|---|---|
| Na⁺/K⁺-ATPase | All animal cells | Maintains [Na⁺]i low, [K⁺]i high |
| Ca²⁺-ATPase (PMCA) | Plasma membrane | Low [Ca²⁺]i (10⁻⁷ M) |
| H⁺/K⁺-ATPase | Parietal cells | Gastric acid secretion |
| GLUT 1-4 | Various tissues | Glucose uptake (insulin-independent vs dependent) |
| SGLT 1 & 2 | Renal tubule | Glucose reabsorption |
Osmosis and Tonicity:
- Osmosis: Water movement across semipermeable membrane from low solute → high solute concentration
- Tonicity (clinical relevance):
- Isotonic: Same osmolality as plasma (~280-300 mOsm/kg) — 0.9% NaCl (saline), 5% glucose
- Hypotonic: Lower osmolality — causes cell swelling (lysis if severe)
- Hypertonic: Higher osmolality — causes cell shrinkage (crenation)
- Osmolality vs Osmolarity: osmolality = per kg solvent (more accurate); osmolarity = per liter solution
Donnan Equilibrium:
- Unequal distribution of diffusible ions across a membrane when one species is non-diffusible
- Non-diffusible anion (protein, ATP) on one side → attracts cations, repels anions → establishes electrochemical gradient
- Example: Plasma proteins (negative charge) → more cations in plasma than IF
⚡ Exam Tip for INI CET (AIIMS PG): Na⁺/K⁺-ATPase consumes ~25-30% of total cellular ATP — crucial for maintaining cell volume, resting membrane potential, and secondary active transport. Ouabain and digoxin inhibit Na⁺/K⁺-ATPase (↑[Na⁺]i → reduces NCX activity → ↑[Ca²⁺]i in cardiac myocytes → positive inotropy).