Q4(a). Active transport; the Na⁺/K⁺ pump

Active transport

1. Definition: transport of an ion or molecule across a membrane against its concentration (electrochemical) gradient, so Δ G>0 for the movement; it must be coupled to an energy source.
2. Primary active transport uses ATP hydrolysis directly (Na⁺/K⁺-ATPase, Ca²+-ATPase, H⁺/K⁺-ATPase of the stomach). Secondary transport uses a gradient made by a primary pump (Na⁺-glucose symport).
3. Features: a specific carrier protein, saturation at high concentration, inhibition by specific poisons and stopping when ATP runs out. Passive diffusion and facilitated diffusion only move solutes downhill.

Mechanism of the Na⁺/K⁺ pump

1. E1, inside: the binding sites face the cytoplasm and have a high affinity for Na⁺; three Na⁺ bind (Mg²+-ATP also bound).
2. Phosphorylation: the terminal phosphate of ATP is transferred to an aspartate residue (E1–P), and the three Na⁺ are occluded (trapped inside the protein).
3. E1–P → E2–P: a conformational change turns the sites to the outside and lowers their Na⁺ affinity: 3Na⁺ are released to the extracellular fluid, and two K⁺ bind from outside.
4. Dephosphorylation: K⁺ binding triggers hydrolysis of the aspartyl phosphate; Pᵢ leaves and the two K⁺ are occluded (E2).
5. E2 → E1: ATP binds, the protein returns to the inward-facing form and 2K⁺ are released inside. The cycle repeats.
6. Net: 3Na+(in) + 2K+(out) + ATP → 3Na+(out) + 2K+(in) + ADP + Pᵢ. One positive charge leaves per cycle, so the pump is electrogenic and contributes to the negative resting potential. Cardiac glycosides (ouabain, digoxin) bind the E2–P form from outside and block it.
7. Selectivity: Na⁺ (102 pm) and K⁺ (138 pm) are told apart by the size of the binding cavities and by the dehydration energy each ion must pay to enter them.

Application. the Na⁺ gradient built by the pump drives secondary transport of glucose and amino acids into cells and restores the ion balance after each nerve impulse.