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.

Copyright © 2009 Pearson Education, Inc. 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 © 2010 Pearson Education, Inc. Figure Activation of pancreatic proteases in the small intestine. Stomach Pancreas Epithelial cells Trypsinogen (inactive) Chymotrypsinogen (inactive) Procarboxypeptidase (inactive) Trypsin Chymotrypsin Carboxypeptidase Membrane-bound enteropeptidase

Copyright © 2010 Pearson Education, Inc. Protein digestion + Amino acids enter the capillary blood in the villi, and are transported to the liver via the hepatic portal vein. Small intestine Small intestine Stomach Foodstuff Protein Large polypeptides Pepsin (stomach glands) in presence of HCl Small polypeptides, small peptides Pancreatic enzymes (trypsin, chymotrypsin, carboxypeptidase) Amino acids Brush border enzymes (small intestine) Path of absorption Enzymes and source Site of action

Copyright © 2010 Pearson Education, Inc. Figure Protein digestion and absorption in the small intestine. Absorptive epithelial cell Apical membrane (microvilli) Amino acid carrier Capillary Lumen of intestine Pancreatic proteases Amino acids of protein fragments Brush border enzymes 1 Protein fragments (peptides) are digested to amino acids by brush border enzymes of mucosal cells. 2 The amino acids are then absorbed by active transport into the absorptive cells 3 The amino acids leave the villus epithelial cell by facilitated diffusion and enter the capillary via intercellular clefts. Active transport Passive transport

Copyright © 2010 Pearson Education, Inc. Carbohydrate digestion All monosaccharides enter the capillary blood in the villi, and are transported to the liver via the hepatic portal vein. Starch and disaccharides Oligosaccharides and disaccharides LactoseMaltoseSucrose Glucose Fructose Salivary amylase Mouth Pancreatic amylase Brush border enzymes in small intestine Small intestine Small intestine Foodstuff Galactose Path of absorption Enzyme(s) and source Site of action

Copyright © 2010 Pearson Education, Inc. Fat digestion Small intestine Small intestine Foodstuff Unemulsified fats Emulsification by the detergent action of bile salts from the liver Pancreatic lipases fatty acidsglycerol Path of absorption Enzyme(s) and source Site of action 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 proteins within the cells to make 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. Note: fats are NOT made of a long chain of monomers (unlike proteins and polysaccharides)

Copyright © 2010 Pearson Education, Inc. Figure Emulsification, digestion, and absorption of fats. Epithelial cells of small intestine Fat droplets coated with bile salts Fat globule Lacteal Bile salts Fatty acids and bile salts 1 Large fat globules are emulsified (physically broken up into smaller fat droplets) by bile salts in the duodenum. 2 Digestion of fat by the pancreatic enzyme lipase yields free fatty acids and glycerol, still associated with 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 by exocytosis. The chylomicrons enter lacteals. They are carried away from the intestine by lymph.

Copyright © 2010 Pearson Education, Inc. Nucleic acid digestion Subunits are absorbed into capillary blood in the villi and transported to the liver via the hepatic portal vein. Small intestine Small intestine Foodstuff Nucleic acids Pancreatic ribo- nuclease and deoxyribonuclease Brush border enzymes (nucleosidases and phosphatases) Pentose sugars, N-containing bases, phosphate ions Path of absorption Enzyme(s) and source Site of action

Copyright © 2009 Pearson Education, Inc. 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