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A variety of compounds including sugars and amino acids pass through the plasma membrane and into the cell at a much higher rate than would be expected on the basis of their size, charge, distribution coefficient, or magnitude of the concentration gradient.
The increased rate of transport through the membrane is believed to be facilitated by specific membrane carrier substances and is called facilitated (or mediated) diffusion.
During facilitated diffusion, the rate at which the solute permeates the membrane increases with increasing solute concentration up to a limit.
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Above this limiting concentration, no increase in the rate of transport across the membrane is observed. In other words, facilitated diffusion exhibits saturation kinetics (Fig. 15-38) and is therefore similar to the relationship between reaction rate and substrate concentration in enzyme-catalyzed reactions.
Other characteristics of facilitated diffusion are also similar to enzyme catalysis. Transport is specific; for example, in the erythrocyte, the inward diffusion of glucose, but not fructose or lactose, is facilitated. The rate of solute permeation can also be affected by the presence of structurally similar chemical compounds, much as in competitive enzyme inhibition. Facilitated diffusion exhibits pH dependency.
Although facilitated diffusion results in a more rapid attainment of concentration equilibrium across the membrane than passive diffusion, the normal equilibrium concentrations are not altered. Substances are not transported through the membrane against a concentration gradient. Although facilitated diffusion is not affected by chemicals that act as metabolic inhibitors, it is affected by enzyme inhibitors such as sulfhydryl blocking agents.
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Facilitated diffusion is believed to result from the interaction of solute with specific membrane molecules, presumably proteins, thereby forming, a carrier-solute complex. The complex undergoes a positional change within the membrane in such a way that the solute now faces the other membrane surface and is released from the carrier (Fig. 15-39).
An alternative suggestion is that the carrier may be a small molecule with the solute-carrier complex formed by an enzyme-catalyzed reaction within the membrane. Once formed, the solute-carrier complex diffuses to the other side of the membrane where the solute is released in a second reaction.
A good and well-defined example of facilitated diffusion occurs in the bacterium Escherichia coli. The sugar lactose does not readily permeate E. coli cells and cannot be hydrolyzed by cytoplasmic extracts of cells grown in the absence of lactose. However, when E. coli is in a medium containing lactose, the enzyme P-galactosidase, which hydrolyzes lactose, soon appears in the cell cytoplasm, and a specific transport protein appears in the membrane.
The presence of the substrate in the growth medium is said to induce the formation of the enzymes by the cells. Such cells are able to transport and subsequently hydrolyze the lactose thereby forming galactose and glucose. Certain mutants of E. coli can be induced in this way to form β-galactosidase even though lactose remains impermeable and cannot be. incorporated by the cells. Finally, other E. coli mutants can be found that are able to remove galactose from the medium but cannot metabolize it once it is inside the cell.
It is now clear from these studies that in wild-type cells the presence of lactose in the growth medium induces both the formation of the hydrolytic enzyme and a carrier system—called a permease or translocase. The sets of genes controlling the formation of the permease and the hydrolytic enzyme are coordinately induced in the presence of lactose. The loss or alteration of either set of genes (as in the E. coli mutants) results in a corresponding inability to induce the formation of the enzyme or the permease.
