Active Transport and the Sodium-Potassium Pump

Diffusion moves things downhill, from high concentration to low, for free. But a cell needs sodium low inside and potassium high inside, which is backwards from where plain diffusion would ever take them. Keeping ions stacked up against their gradients takes constant work, and that work is active transport: using energy to move a substance against its concentration gradient. The sodium-potassium pump is the main engine for this job. It is a protein embedded in the membrane that binds 3 sodium ions from inside the cell, then binds ATP. Hydrolyzing that ATP transfers a phosphate group onto the pump, and that phosphate triggers the pump to change shape. The new shape releases the 3 sodium ions outside and exposes a binding site that grabs 2 potassium ions from outside instead. Losing the phosphate flips the pump back to its original shape, releasing the potassium inside and resetting the cycle.

Because the pump moves 3 positive charges out for every 2 it brings in, every cycle leaves the inside of the cell slightly more negative than before. This steady net export of positive charge is what keeps the resting membrane potential negative, typically around -70 millivolts in a neuron, and why nerve cells spend so much ATP running these pumps nonstop just to stay ready to fire.

The sodium gradient the pump builds does not just sit there, it gets spent. Other membrane proteins use the steep downhill rush of sodium back into the cell to drag other molecules along with it, a strategy called secondary active transport. Glucose, amino acids, and other nutrients often enter cells this way, riding sodium's gradient instead of burning their own ATP. Poison the pump and that gradient drains away through leak channels, so both the voltage and the free ride for other transporters disappear together.