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Major food groups • Carbohydrates = sugars = saccharides • Lipids = fats • Proteins • nucleic acids Carbohydrates, proteins and nucleic acids (but not fats) are long chains of smaller subunits, which are monomers. The combinations are polymers. Polymers in which the order of monomers provides information: proteins and nucleic acids. Nucleic acid digestion- pancreatic ribonuclease & deoxyribonuclease; nucleosidases & phosphatases; act in small intestines Copyright © 2009 Pearson Education, Inc. Composition and Function of Pancreatic Juice • Many enzymes are secreted in inactive form: Examples include: – Trypsinogen is activated to trypsin – Pepsinogen activated to pepsin by HCl – Procarboxypeptidase is activated to carboxypeptidase Copyright © 2009 Pearson Education, Inc. • Digestive enzymes that are produced by the small intestine are all brush border enzymes (membrane-bound) • Advantages and disadvantages of having membrane-bound enzymes Copyright © 2009 Pearson Education, Inc. Figure 23.27 Activation of pancreatic proteases in the small intestine. Stomach Pancreas Epithelial cells Membrane-bound enteropeptidase Trypsinogen Trypsin (inactive) Chymotrypsinogen Chymotrypsin (inactive) Procarboxypeptidase Carboxypeptidase (inactive) Copyright © 2010 Pearson Education, Inc. Protein digestion Foodstuff Protein Large polypeptides Small polypeptides, small peptides Amino acids Copyright © 2010 Pearson Education, Inc. Enzymes and source Pepsin (stomach glands) in presence of HCl Pancreatic enzymes (trypsin, chymotrypsin, carboxypeptidase) Brush border enzymes (small intestine) Site of action Path of absorption Stomach Small intestine + • Amino acids enter the capillary blood in the Small villi, and are transported intestine to the liver via the hepatic portal vein. Figure 23.33 Protein digestion and absorption in the small intestine. Amino acids of protein fragments Brush border enzymes Apical membrane (microvilli) Lumen of intestine Pancreatic proteases 1 Protein fragments (peptides) are digested to amino acids by brush border enzymes of mucosal cells. Absorptive epithelial cell 2 The amino acids are then absorbed by active transport into the absorptive cells Amino acid carrier 3 The amino acids leave the Active transport Passive transport Copyright © 2010 Pearson Education, Inc. Capillary villus epithelial cell by facilitated diffusion and enter the capillary via intercellular clefts. Carbohydrate digestion Foodstuff Enzyme(s) and source Site of action Path of absorption Starch and disaccharides Oligosaccharides and disaccharides Lactose Maltose Sucrose Galactose Glucose Fructose Copyright © 2010 Pearson Education, Inc. Salivary amylase Pancreatic amylase Brush border enzymes in small intestine Mouth Small intestine Small intestine • All monosaccharides enter the capillary blood in the villi, and are transported to the liver via the hepatic portal vein. Note: fats are NOT made of a long chain of monomers (unlike proteins and polysaccharides) Fat digestion Foodstuff Enzyme(s) and source Site of action Unemulsified fats Emulsification by the detergent action of bile salts from the liver Pancreatic lipases fatty acids glycerol Copyright © 2010 Pearson Education, Inc. Small intestine Path of absorption • Fatty acids and glycerol enter the intestinal cells via diffusion. • Fatty acids and monoglycerides are recombined to form triglycerides and then combined with other lipids and Small proteins within the cells to make intestine chylomicrons, which are extruded by exocytosis. • The chylomicrons enter the lacteals of the villi and are transported to the systemic circulation via the lymph in the thoracic duct. Figure 23.34 Emulsification, digestion, and absorption of fats. Fat globule 1 Large fat globules are emulsified (physically broken up into smaller fat droplets) by bile salts in the duodenum. Bile salts Fat droplets coated with bile salts 2 Digestion of fat by the pancreatic enzyme lipase yields free fatty acids and glycerol, still associated with bile salts Fatty acids and bile salts 3 Fatty acids and glycerol diffuse into epithelial cells. There they are recombined and packaged with other lipids and proteins to form chylomicrons. 4 Chylomicrons are extruded from the Epithelial cells of small intestine Copyright © 2010 Pearson Education, Inc. Lacteal epithelial cells by exocytosis. The chylomicrons enter lacteals. They are carried away from the intestine by lymph. Nucleic acid digestion Foodstuff Enzyme(s) and source Site of action Path of absorption Nucleic acids Pentose sugars, N-containing bases, phosphate ions Copyright © 2010 Pearson Education, Inc. Pancreatic ribonuclease and deoxyribonuclease Brush border enzymes (nucleosidases and phosphatases) Small intestine • Subunits are absorbed into capillary blood in the villi Small and transported to the intestine liver via the hepatic portal vein. In the large intestine: • Other than digestion by enteric bacteria, no further digestion takes place – Bacteria synthesize B complex vitamins and vitamin K • water, and electrolytes are absorbed Copyright © 2009 Pearson Education, Inc.