Exam 20: The Gastrointestinal System

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The carbohydrates that enter the mouth are typically complex polymers of glucose (2/3 of dietary carbohydrates)or disaccharides like sucrose (glucose and fructose), lactose (glucose and galactose), and maltose (2 glucose). These glucose polymers can be derived from plants (starch)or from animals (glycogen). The digestion of carbohydrates begins within the mouth with the secretion of salivary amylase, which begins to digest glucose polymers. Those polymers are broken into maltose or limit dextrin (short, branched polysaccharides). However, they must be broken into monosaccharides before being absorbed in the small intestines. Once the chyme reaches the small intestines, pancreatic amylase begins to work again on the glucose polymers. However, there are other enzymes attached to the brush border that include dextrinase, glucoamylase, sucrase, maltase, and lactase that will further degrade the carbohydrates into monosaccharides. Once the carbohydrates are broken down, the constituents glucose and galactose are absorbed via sodium- linked glucose transporters on the apical membrane of the epithelial cells. Fructose enters those cells through facilitated diffusion. Each of the carbohydrates exits the epithelial cells through facilitated diffusion into capillaries to be transported systemically.