Multiple Transport Proteins Are Needed to Move Glucose and Amino Acids Across Epithelia

Figure 7-27 depicts the proteins that mediate absorption of glucose from the intestinal lumen into the blood. In the first stage of this process, a two-Na+/one-glucose symporter located in microvillar membranes imports glucose, against its concentration gradient, from the intestinal lumen across the apical surface of the epithelial cells. As noted above, this symporter couples the energetically unfavorable inward

▲ FIGURE 7-27 Transcellular transport of glucose from the intestinal lumen into the blood. The Na+/K+ ATPase in the basolateral surface membrane generates Na+ and K+ concentration gradients. The outward movement of K+ ions through nongated K+ channels (not shown) generates an inside-negative membrane potential. Both the Na+ concentration gradient and the membrane potential are used to drive the uptake of glucose from the intestinal lumen by the two-Na+/one-glucose symporter located in the apical surface membrane. Glucose leaves the cell via facilitated diffusion catalyzed by GLUT2, a glucose uniporter located in the basolateral membrane.

movement of one glucose molecule to the energetically favorable inward transport of two Na+ ions (see Figure 7-21). In the steady state, all the Na+ ions transported from the intestinal lumen into the cell during Na+/glucose symport, or the similar process of Na+/amino acid symport, are pumped out across the basolateral membrane, which faces the underlying tissue. Thus the low intracellular Na+ concentration is maintained. The Na+/K+ ATPase that accomplishes this is found exclusively in the basolateral membrane of intestinal epithelial cells. The coordinated operation of these two transport proteins allows uphill movement of glucose and amino acids from the intestine into the cell. This first stage in transcellular transport ultimately is powered by ATP hydrolysis by the Na+/K+ ATPase.

In the second stage, glucose and amino acids concentrated inside intestinal cells by symporters are exported down their concentration gradients into the blood via uniport proteins in the basolateral membrane. In the case of glucose, this movement is mediated by GLUT2 (see Figure 7-27). As noted earlier, this GLUT isoform has a relatively low affinity for glucose but increases its rate of transport substantially when the glucose gradient across the membrane rises (see Figure 7-3).

The net result of this two-stage process is movement of Na+ ions, glucose, and amino acids from the intestinal lumen across the intestinal epithelium into the extracellular medium that surrounds the basolateral surface of intestinal epithelial cells. Tight junctions between the epithelial cells prevent these molecules from diffusing back into the intestinal lumen, and eventually they move into the blood. The increased osmotic pressure created by transcellular transport of salt, glucose, and amino acids across the intestinal epithelium draws water from the intestinal lumen into the extracellular medium that surrounds the basolateral surface. In a sense, salts, glucose, and amino acids "carry" the water along with them.

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