14.1 Digestion Prepares Large Biomolecules for Use in Metabolism
Digestion begins in the mouth, where food is homogenized into an aqueous slurry susceptible to enzyme digestion. The homogenized food then passes into the stomach, an acidic environment. The low pH of the stomach denatures proteins, thus preparing them for degradation.
14.2 Proteases Digest Proteins into Amino Acids and Peptides
Protein digestion begins in the stomach with the action of the proteolytic enzyme pepsin. The digestion products of pepsin stimulate the release of the hormone cholecystokinin from specialized cells in the upper intestine. Cholecystokinin stimulates the release of bile salts from the gall bladder and digestion enzymes from the pancreas in the form of zymogens or proenzymes. Enteropeptidase converts trypsinogen into trypsin, which, in turn, activates the other zymogens.
14.3 Dietary Carbohydrates Are Digested by Alpha-
Complex carbohydrates such as starch and glycogen are degraded by α-amylase, which cleaves the α-1,4 bonds of starch and glycogen. The products of α-amylase digestion are the disaccharide maltose and limit dextrin, a carbohydrate rich in α-1,6 bonds. Limit dextrin is digested by α-dextrinase. Sucrase and lactase digest the disaccharides sucrose and lactose, respectively.
14.4 The Digestion of Lipids Is Complicated by Their Hydrophobicity
Because dietary lipids, mostly triacylglycerols, are not water soluble, they must be converted into an emulsion, a mixture of lipid droplets and water, to be digested by lipases secreted by the pancreas. Bile salts, provided by the gall bladder, facilitate the formation of the emulsion. The products of lipase digestion—