A major source of metabolic energy for most, if not all, mammalian cells is the oxidation of D-glucose. The lipid-rich membranes of such cells, however, are relatively impermeable to hydrophilic polar molecules such as glucose. Special transport processes have been developed to allow the cellular entry and exit of glucose. Carrier proteins located in the plasma membranes of cells can bind glucose and allow it to traverse the lipid membrane barrier, releasing the hexose into the cellular cytoplasm or body fluids.
Two distinct classes have been described: (a) a family of facilitative glucose transporters (TableS.^) and (b) Na+-glucose cotransporters (symporters). The former class are membrane integral proteins found on the surface of all cells. They transport D-glucose down its concentration gradient (from high to low), a process described as facilitative diffusion. The energy for the transfer comes from dissipation of the concentration difference of the glucose. Such glucose transporters allow glucose to enter cells readily, but they can also allow it to exit from cells according to the prevailing concentration difference. In contrast, the Na +-glucose cotransporters participate in the uphill movement of D-glucose against its concentration difference, that is, they perform active transport. They are especially expressed in the specialized brush borders of the enterocytes of the small intestine and the epithelial cells of the kidney (proximal) tubule. They occur at lower levels in the epithelial cells lining the lung and in the liver ( 8). Cooperation between the two classes of glucose transporters, together with the hormones involved in carbohydrate metabolism, allows a fine control of glucose concentration in the plasma, maintaining a continuous supply of the body's main source of cellular energy.
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