Active transport

Active transport moves ions or molecules across a membrane from where they are less concentrated to where they are more concentrated, the opposite of the direction diffusion would take them. Working uphill like this requires an energy source and dedicated carrier proteins, often called pumps, so it stops when a cell runs short of ATP or is exposed to poisons that block ATP production.

In primary active transport, the pump uses ATP directly. The best-known example is the sodium-potassium pump, which moves three sodium ions out of the cell and two potassium ions in for each ATP it splits, keeping the inside of the cell low in sodium and electrically negative. In secondary active transport, a carrier uses the energy stored in such a gradient instead: sodium flowing back into the cell drags another substance with it, as when glucose is taken up against its own gradient.

Carriers that move two substances in the same direction are called symporters, and those that move them in opposite directions are antiporters. Active transport underlies nerve signaling, nutrient absorption in the gut and the kidney's control of salt.