Active transport

In cell biology, active transport is transport across a membrane that relies on chemical energy. In this form of transport, molecules move against either an electrical or concentration gradient (collectively termed an electrochemical gradient). There are two main types, primary and secondary. Primary transport involves the consumption of metabolic energy (often in the form of ATP) and is directly coupled to movement across a membrane, independent of any other species. Secondary transport concerns the diffusion of one species across a membrane to drive the transport of another. Transporters generally are membrane-spanning or "transmembrane" proteins.

Primary

Primary active transport directly uses energy to transport molecules across a membrane. Most of the enzymes that perform this type of transport are transmembrane ATPases. A primary ATPase universal to all cellular life is the sodium-potassium pump, which helps maintain the cell potential.

Secondary

In secondary active transport, there is no direct coupling of ATP; instead, the electrochemical potential difference created by pumping ions out of cells is used. The two main forms of this are counter- and co-transport.

In counter-transport two species of ion or other solute are pumped in opposite directions across a membrane. One of these species is allowed to flow from high to low concentration, which yields the entropic energy to drive the transport of the other solute from a low concentration region to a high one. An example is the sodium-calcium exchanger or "antiport", which allows three sodium ions into the cell to transport one calcium out.

Many cells also posses a calcium ATPase, which can operate at lower intracellular concentrations of calcium and sets the normal or resting concentration of this important second messenger. But the ATPase exports calcium ions more slowly: only 30 per second versus 2000 per second by the exchanger. The exchanger comes into service when the calcium concentration rises steeply or "spikes" and enables rapid recovery. This shows that a single type of ion can be transported by several enzymes, which need not be active all the time ("consitutively"), but may exist to meet specific, intermittant needs.

Co-transport also uses the flow of one solute species from high to low concentration to move another molecule against its preferred direction of flow; but here, both solutes move in the same direction across the membrane. An example is the glucose "symporter", which cotransports two sodiums for every molecule of glucose it imports into the cell.

See also Diffusion