Sodium–potassium pump

The sodium–potassium pump, or Na+/K+ ATPase, is a carrier protein in the plasma membrane of nearly all animal cells. It is the classic example of primary active transport: it splits ATP and uses the energy to carry ions against their concentration gradients. Each cycle pushes three sodium ions out and brings two potassium ions in, so the cell interior stays low in sodium and rich in potassium.

The pump works by changing shape. Facing the cytoplasm, it binds three sodium ions; ATP then hands it a phosphate group, which flips it to face outward and weakens its hold on sodium. It next binds two potassium ions, loses the phosphate and returns to its first shape, releasing the potassium inside. Because it moves more positive charge out than in, it is called electrogenic and adds slightly to the negative resting membrane potential. The gradients it builds also power secondary active transport, such as glucose uptake in the gut and kidney, and help control cell volume.

The pump is a major energy user, by some estimates consuming about a third of the ATP the body makes at rest. Danish scientist Jens Skou discovered it in 1957 and shared the 1997 Nobel Prize in Chemistry. The heart drug digoxin acts by partly blocking it.