ACTIVE TRANSPORT

Active-transport processes move molecules and other substances across a selectively permeable membrane against a concentration gradient—that is, from an area of lower concentration to one of higher concentration. This movement against the concentration gradient requires ATP energy. The active-transport process is similar to facilitated diffusion, except that the carrier protein in the plasma membrane must use energy to move the molecules against their concentration gradient. One active-transport mechanism, the sodium-potassium pump, helps maintain the high concentrations of potassium ions and low concentrations of sodium ions inside nerve cells that are necessary for the transmission of electrical impulses. Another active-transport mechanism, the calcium pump, keeps the calcium concentration hundreds of times lower inside the cell than outside.