Define active transport. Classify it and describe the sodium–potassium pump with its functions. Add a note on secondary active transport.

Definition

Classification

1. Primary active transport — energy directly from ATP hydrolysis

2. Secondary active transport — energy from an ion gradient created by primary transport

3. Vesicular transport — endocytosis and exocytosis

Primary Active Transport

A. Sodium–potassium pump (Na⁺–K⁺ ATPase)

• Present in all cell membranes
• Structure — 2 α subunits (catalytic, bear the ATPase site) + 2 β subunits

3 Na+ pumped out → 2 K+ pumped IN → 1 ATP hydrolysed → Net loss of 1 positive charge from inside → electrogenic pump

Functions of the Na⁺–K⁺ pump

• Maintains low intracellular Na+ and high intracellular K+
• Contributes about −4 mV directly to the resting membrane potential
• Controls cell volume — prevents osmotic swelling
• Creates the Na+ gradient that drives all secondary active transport
• Consumes 20–30% of the cell's resting ATP (up to 70% in neurones)
• Inhibited by ouabain and digitalis

B. Other primary pumps

Characteristics of Active Transport

• Saturation — a maximum rate (Tmax) exists once all carriers are occupied
• Specificity — each carrier moves one substance or a closely related group
• Competitive inhibition — related solutes compete for the same carrier
• Temperature dependent — slows markedly with cooling
• Inhibited by metabolic poisons — cyanide, dinitrophenol, iodoacetate (all block ATP production)
• Stops in hypoxia — no ATP

Secondary Active Transport

Na+–K+ pump → Low intracellular Na+ → Steep inward Na+ gradient → Na+ re-entry drags glucose uphill → Glucose absorbed

Transcellular Transport — Glucose Absorption in the Gut

Apical membrane: SGLT-1 (secondary active, Na+-linked) → Glucose enters the enterocyte against its gradient → Basolateral membrane: GLUT-2 (facilitated diffusion) → Glucose leaves into blood down its gradient

• Na+ pumped out at the basolateral surface by the Na+–K+ pump → keeps the apical gradient going
• Same arrangement operates in the renal proximal tubule

Comparison of Transport Mechanisms

Vesicular Transport

Applied Aspects

• Digitalis → inhibits Na+–K+ ATPase → ↑ intracellular Na+ → ↓ Na+–Ca2+ exchange → ↑ intracellular Ca2+ → positive inotropic effect
• ORS in diarrhoea — works because Na+–glucose co-transport stays intact in cholera; glucose must be present for the sodium to be absorbed
• Familial hypercholesterolaemia — defective LDL receptor → failure of receptor-mediated endocytosis
• Cell swelling in hypoxia — no ATP → pump fails → Na+ and water enter the cell