Absorption of Monosaccharides
Simple sugars are far and away the predominant carbohydrate absorbed in the digestive tract, and in many animals the most important source of energy. Monosaccharides, however, are only rarely found in normal diets. Rather, they are derived by enzymatic digestion of more complex carbohydrates within the digestive tube.
Particularly important dietary carbohydrates include starch and disaccharides such as lactose and sucrose. None of these molecules can be absorbed for the simple reason that they cannot cross cell membranes unaided and, unlike the situation for monosaccharides, there are no transporters to carry them across.
This section will focus on understanding the processes involved in assimilation of three important carbohydrates: starch, lactose and sucrose. The key concepts involved in all three cases are that:
- the final enzymatic digestion that liberates monosaccharides is conducted by enzymes that are tethered in the lumenal plasma membrane of absorptive enterocytes (so-called "brush border hydrolyases").
- glucose generated by digestion of starch or lactose is absorbed in the small intestine only by cotransport with sodium, a fact that has exceptionally important implications in medicine.
Brush Border Hydrolases Generate Monosaccharides
Polysaccharides and disaccharides must be digested to monosaccharides prior to absorption and the key players in these processes are the brush border hydrolases, which include maltase, lactase and sucrase. Dietary lactose and sucrose are "ready" for digestion by their respective brush border enzymes. Starch, as discussed previously, is first digested to maltose by amylase in pancreatic secretions and, in some species, saliva.
Dietary lactose and sucrose, and maltose derived from digestion of starch, diffuse in the small intestinal lumen and come in contact with the surface of absorptive epithelial cells covering the villi where they engage with brush border hydrolases:
- maltase cleaves maltose into two molecules of glucose
- lactase cleaves lactose into a glucose and a galactose
- sucrase cleaves sucrose into a glucose and a fructose
At long last, we're ready to actually absorb these monosaccharides. Glucose and galactose are taken into the enterocyte by cotransport with sodium using the same transporter. Fructose enters the cell from the intestinal lumen via facilitated diffusion through another transporter.
Absorption of Glucose and Other Monosaccharides: Transport Across the Intestinal Epithelium
Absorption of glucose entails transport from the intestinal lumen, across the epithelium and into blood. The transporter that carries glucose and galactose into the enterocyte is the sodium-dependent hexose transporter, known more formally as SGLUT-1. As the name indicates, this molecule transports both glucose and sodium ion into the cell and in fact, will not transport either alone.
The essence of transport by the sodium-dependent hexose transporter involves a series of conformational changes induced by binding and release of sodium and glucose, and can be summarized as follows:
- the transporter is initially oriented facing into the lumen - at this point it is capable of binding sodium, but not glucose
- sodium binds, inducing a conformational change that opens the glucose-binding pocket
- glucose binds and the transporter reorients in the membrane such that the pockets holding sodium and glucose are moved inside the cell
- sodium dissociates into the cytoplasm, causing glucose binding to destabilize
- glucose dissociates into the cytoplasm and the unloaded transporter reorients back to its original, outward-facing position
Fructose is not co-transported with sodium. Rather it enters the enterocyte by another hexose transporter (GLUT5).
Once inside the enterocyte, glucose and sodium must be exported from the cell into blood. We've seen previously how sodium is rapidly shuttled out in exchange for potassium by the battery of sodium pumps on the basolateral membrane, and how that process maintains the electrochemical gradient across the epithelium. The energy stored in this gradient is actually what is driving glucose entry through the sodium-dependent hexose transporter described above. Recall also how the massive transport of sodium out of the cell establishes the osmotic gradient responsible for absorption of water.
Glucose, galactose and fructose are tranported out of the enterocyte through another hexose transporter (called GLUT-2) in the basolateral membrane. These monosaccharides then diffuse "down" a concentration gradient into capillary blood within the villus.
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