Active Transport

Active transport is the movement of molecules across a cell membrane against their concentration gradient, from low to high concentration, using energy. It allows cells to maintain very different compositions inside and out, and it is one of the main uses of ATP in the body.

Primary transporters, or pumps, use the energy released by splitting ATP to move ions or molecules directly. The sodium-potassium pump, discovered by Jens Skou (Nobel Prize 1997), moves three sodium ions out and two potassium ions in for each ATP. It creates the ion gradients that underlie the Nernst potentials and the electrical signals of neurons. Other examples are calcium pumps in muscle and proton pumps that acidify the lysosome.

Secondary transporters do not use ATP directly. They couple the movement of one substance uphill to the downhill flow of another, usually sodium or protons, whose gradient was built by a primary pump. In the gut and kidney, the sodium-glucose cotransporter SGLT takes up glucose this way. The same principle powers the uptake of nutrients in bacteria, which depend on the proton gradient of chemiosmosis.

Large molecules and particles are moved by vesicles, in endocytosis and exocytosis, which also use energy. ABC transporters, a large family of ATP-driven pumps, export toxins and drugs, and their overactivity in cancer cells and in bacteria contributes to resistance. Active transport contrasts with passive transport, which moves substances down a gradient through the cell membrane without energy.

• Transmembrane Pump — the proteins that perform it
• ATP — its energy source
• Cell Membrane — the barrier crossed