Beyond mechanical processing—mixing, maceration, transport—the main function of the digestive tract is to split large food components into small absorbable units (A).
During this enzymatic hydrolysis, H2O
is reinserted in those places from where it had previously been removed during dehydration synthesis in the animal or plant cells that built the compounds. For all nutrients, this hydrolysis follows the same pattern: first, they are split into larger fragments, and subsequently they are broken down into the smallest, absorbable subunits.
Digestion of foods begins in the buccal cavity. The enzyme amylase, produced mostly by the parotid glands is able to hydrolyze complex carbohydrates (starch). In infants, lingual lipase plays an extremely important role, initiating fat digestion. Its role in adults is controversial; in all likelihood, its role in lipid digestion is negligible as long as there is full pancreatic function. Since the buccal cavity is relatively small, the effectiveness of these enzymes depends on how long food is kept in the mouth: the longer it is chewed, the more time for the enzymes to act upon the food.
The macerated food, mixed with saliva, reaches the stomach after passing through the esophagus. At the beginning of a meal, the stomach lumen tends to have a neutral pH. Then, HCl is released and mixed with the mass. Once a critical level of acidity is reached, this inactivates the salivary enzymes. Only proteins are actively digested in the stomach, and an acidic pH is indispensable for this process. By the time the food leaves the stomach, it usually consists mostly of larger nutrient fragments. Since the same hydrolysis can also be achieved through pancreatic enzymes, digestion without a stomach is possible—within limits.
Small intestine. The most important pancreatic juices are bicarbonate—to neutralize stomach acids—and digestive enzymes for all major nutrients. Proteolytic enzymes and phospholipase A are secreted as inactive precursors, which are not activated until they reach the duodenum, thus preventing autodigestion of protein-containing pancreatic membranes. Lipase, amylase, and ribonucleases are secreted in their active form.
These enzymes prevail in pancreatic juice in constant ratios. Adaptation of these concentrations to specific nutrients occurs very slowly, as, for instance, to prolonged high fat intake.
Pancreatic enzymes begin their hydro-lytic work in the proximal jejunum, producing either directly absorbable units (lipids) or fragments, which are then further hydrolyzed by specific membrane-bound enzymes located in the intestinal mucosa. The end products of these specific enzymatic reactions are absorbed directly into blood, or into lymphatic fluid, as in the case of lipids.
Proximal small intestine: pancreatic enzymes
Mucosa of small intestine
Disaccharide maltose m
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