The Products of Membranous-Phase Digestion Are Absorbed by Sodium Co-Transport
Sodium co-transport proteins for glucose and galactose are located in the apical membrane, in proximity to the membranous-phase digestive enzymes. Because these saccharide monomers are produced by the action of membranous- phase enzymes on polysaccharides, they move very short distances to binding sites on co-transport proteins.
When both the glucose-binding (or galactose-binding) sites and the sodium- binding sites on these proteins are occupied, absorption occurs as previously described in the description of transport proteins.In the initial phases of digestion of a starch-containing meal, the glucose concentration at the apical membrane is very high because there is ample substrate. Sodium is also readily available as a result of its presence in the various GI secretions. At this time, movement of both sodium and glucose into the enterocytes is down a concentration gradient. As digestion and absorption proceed, the glucose concentration at the apical membrane diminishes. Thus, toward the end of the digestive and absorptive process, the concentration of glucose at the Iuminal surface of the enterocyte apical membrane becomes small. At this point the concentration of glucose within the enterocyte can be higher than in the intestinal lumen, thus creating an unfavorable concentration gradient for glucose absorption. However, the transcellular sodium concentration gradient is maintained, driving the continued absorption of glucose (see Figure 30-14). The process of glucose absorption by this mechanism is very efficient, and little free glucose escapes the absorptive process.
To complete the process of carbohydrate absorption, the glucose must move through the basolateral membrane, into the lateral spaces, and then into the capillaries. Movement of glucose through the basolateral membrane occurs by facilitated diffusion, in which there is a transport pathway protein, but the direction of transport is driven only by the concentration gradient for glucose. As the intracellular glucose concentration in the enterocytes increases because of the action of sodiumglucose co-transport from the gut lumen, glucose diffuses from the cells into the lateral spaces. From the lateral spaces, it diffuses through the capillary basement membrane into the blood.
Absorption of the products of membranous-phase protein digestion occurs in a manner similar to that of carbohydrates. Sodium co-transport systems exist for free amino acids and might also exist for dipeptides and tripeptides. At least three co-transport proteins are necessary for absorption of free amino acids. The mechanism of transport for dipeptides and tripeptides might also involve sodium co-transport, but this issue is not established with certainty (Figure 30-l6).