Active transport

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

Primary

Primary active transport involves energy being directly used 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 this form of active transport, there is no direct coupling of ATP; instead the electrochemical potential difference created by pumping ions out of cells is utilized. The two main forms of this are counter and co-transport.

Counter transport involves pumping two species of ion or other solute in opposite directions across a membrane. One of these species is allowed to flow from high to low concentration, which yields theentropic 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 in order to transport one calcium out. The exchanger has a lower affinity for calcium than the (primary?) calcium pump, but its capacity to extrude calcium is greater (2000 ions per second, compared to 30 p/s for pump).

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 in this case 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