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Chapter 9 Digestive system 9.3 Digestive Enzymes Digestive enzymes help break down the major components of food: carbohydrates, proteins, nucleic acids, and fats. 5. Carbohydrate Digestion by Enzymes The digestion of starch (a carbohydrate) begins in the mouth. • Salivary amylase (produced by the salivary glands) digests starch into maltose (a disaccharide) • Pancreatic amylase (produced by the pancreas) and maltase (produced by the small intestine) then convert maltose in the small intestine to glucose (a monosaccharide). Glucose can be absorbed by the small intestine. Fig 9.11 McGraw-Hill Carbohydrate Digestion by Enzymes Other disaccharides, such as lactose, have their own enzyme that digests them in the small intestine. • Lactase is an enzyme that digests lactose, a sugar found in milk. o Individuals who have a lactase deficiency often have symptoms of lactose intolerance (diarrhea, gas, cramps) caused by the fermentation of non-digested lactose by intestinal bacteria 8. Protein Digestion by Enzymes The digestion of proteins begins in the stomach. • Pepsin is an enzyme produced by gastric glands that acts on proteins to produce peptides. • Trypsin (produced by the pancreas) and peptidases (produced in the small intestine) break down peptides into amino acids Fig 9.11 McGraw Hill 10. Fat Digestion by Enzymes • Lipase (produced by the pancreas) acts in the small intestine and digests fat molecules in the fat droplets after they have been emulsified by bile salts • Glycerol and fatty acids enter the cells of the villi, where they are rejoined and repackaged as lipoprotein droplets (chylomicrons) before entering the lacteals Fig 9.11 Mc Graw-Hill 12 Regulation of Digestive Enzymes Enzymes function best at an optimum temperature and pH that helps maintain the proper shape to fit their substrate. • Since the digestive system is maintained at a constant 37oC, enzymatic activity is largely controlled by pH o The pH of the stomach is between 1 and 2 but can increase to around 7.4 to 7.8 when sodium bicarbonate in pancreatic juice is released from the pancreas o This increase in pH occurs after chyme enters the duodenum, and allows different digestive enzymes to be active depending on the pH Check Your Progress 1. Describe where in the digestive tract the chemical digestion of each of the following types of nutrients occurs: carbohydrates, proteins, and fats. 2. Identify the final molecule (monomer) resulting from the digestion of carbohydrates, proteins, and fats. 3. Explain how the structure of the digestive system assists in regulating the digestive enzymes.
