"Facilitated diffusion" is an unfortunate term since the process it denotes does not involve diffusion. The term arose because the end results of both diffusion and facilitated diffusion are the same. In both processes, the net flux of an uncharged molecule across a membrane always proceeds from higher to lower concentration and continues until the concentrations on the two sides of the membrane become equal. At this point in facilitated diffusion, equal numbers of molecules are binding to the transporter at the outer surface of the cell and moving into the cell as are binding at the inner surface and moving out. Neither diffusion nor facilitated diffusion is coupled to energy derived from metabolism, and thus they are incapable of moving solute from a lower to a higher concentration across a membrane.
Among the most important facilitated-diffusion systems in the body are those that move glucose across plasma membranes. Without such glucose transporters, cells would be virtually impermeable to glucose, which is a relatively large, polar molecule. One might expect that as a result of facilitated diffusion the glucose concentration inside cells would become equal to the extracellular concentration. This does not occur in most cells, however, because glucose is metabolized to glucose 6-phosphate almost as quickly as it enters. Thus, the intracellular glucose concentration remains lower than the extracellular concentration, and there is a continuous net flux of glucose into cells.
Several distinct transporters are known to mediate the facilitated diffusion of glucose across cell membranes. Each transporter is coded by a different gene, and these genes are expressed in different types of cells. The transporters differ in the affinity of their binding sites for glucose, their maximal rates of transport when saturated, and the modulation of their transport activity by various chemical signals, such as the hormone insulin. As discussed in Chapter 18, although glucose enters all cells by means of glucose transporters, insulin affects only the type of glucose transporter expressed primarily in muscle and adipose tissue. Insulin increases the number of these glucose transporters in the membrane and, hence, the rate of glucose movement into cells.
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