Na+/K+ ATPase

Meaning: The sodium-potassium pump is a transmembrane protein that is also an enzyme (an ATPase). Each cycle it moves 3 Na+ out and 2 K+ in. ATP phosphorylates the pump, which changes its conformation, and dephosphorylation resets it. Because ATP drives the transport directly, this is primary active transport. The result is high Na+ outside the cell and high K+ inside.

The pump is electrogenic, since each cycle removes one net positive charge from the cell. That adds slightly to the negative resting potential, but most of the resting potential comes from K+ leaking out through channels. The stored Na+ gradient also drives secondary active transport, such as Na+-glucose symporters and Na+/Ca2+ antiporters, and it helps control cell volume.

On the MCAT: A passage may describe a pump inhibitor (ouabain is a classic example) or ATP depletion. Predict that intracellular Na+ rises, the Na+ gradient shrinks, and Na+-coupled uptake of glucose or amino acids falls. Other questions ask which ion moves in which direction, or ask you to tell the pump apart from channels and facilitated diffusion, which are passive.

Common confusion: Students often treat it as a 1:1 exchange. The ratio is 3 Na+ out for 2 K+ in. Also, the pump moves ions against their gradients, so it needs energy. Ions flowing through channels move down their gradients and need no ATP.