ACTIVE TRANSPORT

Active Transport Since the cell membrane is somewhat permeable to sodium ions, simple diffusion would result in a net movement of sodium ions into the cell, until the concentrations on the two sides of the membrane became equal. Sodium actually does diffuse into the cell rather freely, but as fast as it does so, the cell actively pumps it out again, against the concentration difference. The mechanism by which the cell pumps the sodium ions out is called active transport. Active transport requires the expenditure of energy for the work done by the cell in moving molecules against a concentration gradient. Active transport enables a cell to maintain a lower concentration of sodium inside the cell, and also enables a cell to accumulate certain nutrient inside the cell at concentrations much higher than the extracellular concentrations. The exact mechanism of active transport is not known. It has been proposed that a carrier molecule is involved, which reacts chemically with the molecule that is to be actively transported. This forms a compound which is soluble in the lipid portion of the membrane and the carrier compound then moves through the membrane against the concentration gradient to the other side. The transported molecule is then released, and the carrier molecule diffuses back to the other side of the membrane where it picks up another molecule. This process requires energy, since work must done in transporting the molecule against a diffusion gradient. The energy is supplied in the form of ATP. The carrier molecules are thought to be integral proteins; proteins which span the plasma membrane. These proteins are specific for the molecules they transport. Chemiosmosis Populating the inner membrane of the mitochondrion are many copies of a protein complex called an ATP synthase, the enzyme that actually makes ATP! It works like an ion pump running in reverse.