Digestion and absorption of carbohydrates and proteins

The pathways used for the digestion and absorption of carbohydrates and proteins share several important common features. Proteins and starch, one of the major dietary carbohydrates, are both polymers that are initially broken down into smaller compounds by enzymes secreted into the intestinal lumen, principally by the pancreas. Further digestion occurs by a variety of enzymes present in the small intestinal brush-border membrane, generating the small molecules (monosaccharides, amino acids, di- and tripeptides) that are capable of being absorbed by the enterocytes. Transport of these molecules through the intestine occurs mainly via various membrane carrier proteins. In some cases, there is an active transport mechanism by which the movement of the nutrient through the membrane is coupled directly or indirectly to a source of metabolic energy, permitting concentrative transport against an electrochemical gradient. Other carrier systems accelerate the rate of movement of a nutrient through the membrane, but they are not linked to an energy source and therefore do not result in concentration against an electrochemical gradient. This type of transport process is known as facilitated diffusion. Simple passive diffusion either through the plasma membrane or between the enterocytes (the paracellular pathway) is quantitatively important only when the luminal concentration of nutrients is very high.