Active Transport Mechanisms

Active transport mechanisms use energy sources to "pump" ions against concentration gradients. It is estimated that cells expend about 25% of their ATP just on active transport. Three common active transport mechanisms are described below.

> Ion pumps - Directly couple ATP hydrolysis to transport. A well-studied example is the sodium-potassium pump of the plasma membrane (Figure 10.26). Note that in one turn of the multistep cycle, two potassiums are pumped in, three sodiums are pumped out, and one ATP is cleaved. The pump can be blocked by ouabain which, in the heart, stimulates contraction because sodium concentration increases and stimulates the sodium-calcium pump to remove sodium and import calcium. Increasing calcium leads to stronger muscular contraction.

> Cotransport Systems - The sodium-glucose cotransport system relies on the concentration gradient built up by the sodium-potassium pump to drive the import of glucose into cells. In this case, sodium outside the cell binds to the receptor and, upon binding of a glucose molecule, the sodium concentration gradient drives the sodium inward and glucose is carried with it (Figure 10.27).

> Transport by Modification - This system relies upon covalently modifying a molecule during (or shortly after) passive or facilitated transport so that it can no longer pass back through the membrane. For example, the phosphotransferase system of E. coli uses ATP to phosphorylate sugars as they are transported into the cell. The phosphorylated sugars cannot pass back out.

Important terminology for active transport mechanisms:

> Antiport - moves one or more molecules in as it moves one or more molecules out

> Synport - moves all molecules in same direction

> Electrogenic - causes change in charge as a result of transport

> Electroneutral - causes no change in charge as a result of transport

See also: Thermodynamics of Transport Across Membranes, Passive Versus Active Transport, Transport Mechanisms, Sodium-Potassium Pump

INTERNET LINK: Membrane Transport Mechanisms