Active Transport and Cotransport

Active transport mechanisms involve the expenditure of energy. The process of diffusion describes particle move­ment from an area of higher concentration to one of lower concentration, resulting in an equal distribution across the cell membrane. Sometimes, however, different concentra-

tions of a substance are needed in the intracellular and extracellular fluids. For example, the intracellular func­tioning of the cell requires a much higher concentration of potassium than is present in the extracellular fluid while maintaining a much lower concentration of sodium than in the extracellular fluid. In these situations, energy is re­quired to pump the ions "uphill" or against their concen­tration gradient. When cells use energy to move ions against an electrical or chemical gradient, the process is called active transport.

The active transport system studied in the greatest de­tail is the sodium-potassium (Na⁺/K⁺) ATPase pump (see Fig. 4-16). This pump moves sodium from inside the cell to the extracellular region, where its concentration is approx­imately 14 times greater than inside; the pump also returns potassium to the inside, where its concentration is approx­imately 35 times greater than it is outside the cell. Energy used to pump sodium out of the cell and potassium into the cell is obtained by splitting and releasing energy from the high-energy phosphate bond in ATP by the enzyme ATPase. Were it not for the activity of the sodium-potassium pump, the osmotically active sodium particles would accu­mulate in the cell, causing cellular swelling because of an accompanying influx of water (see Chapter 5).

Two types of active transport systems exist: primary active transport and secondary active transport. In primary active transport, the source of energy (e.g., ATP) is used di­rectly in the transport of a substance. Secondary active trans­port mechanisms harness the energy derived from the primary active transport of one substance, usually sodium ions, for the cotransport of a second substance. For exam­ple, when sodium ions are actively transported out of a cell by primary active transport, a large concentration gradient develops (i.e., high concentration on the outside and low on the inside). This concentration gradient represents a large storehouse of energy because sodium ions are always attempting to diffuse into the cell. Similar to facilitated dif­fusion, secondary transport mechanisms use membrane transport proteins. These proteins have two binding sites, one for sodium ions and the other for the substance under­going secondary transport. Secondary transport systems are classified into two groups: cotransport or symport sys­tems, in which the sodium ion and solute are transported in the same direction, and countertransport or antiport sys­tems, in which sodium ions and the solute are transported in the opposite direction (Fig. 4-17). An example of co­transport occurs in the intestine, where the absorption of glucose and amino acids is coupled with sodium transport.