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
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 Composition and Function of Pancreatic Juice
Digestive enzymes that are produced by the small intestine are all brush border enzymes (membrane-bound) • Advantages and disadvantages of having membrane-bound enzymes
Figure 23.27 Activation of pancreatic proteases in the small intestine. Stomach Pancreas Epithelial cells Membrane-bound enteropeptidase Trypsinogen (inactive) Chymotrypsinogen (inactive) Procarboxypeptidase (inactive) Trypsin Chymotrypsin Carboxypeptidase
Protein digestion Enzymes and source Site of action Foodstuff Path of absorption Protein Pepsin (stomach glands) in presence of HCl Stomach Large polypeptides Pancreatic enzymes (trypsin, chymotrypsin, carboxypeptidase) + Small intestine Small polypeptides, small peptides • Amino acids enter the capillary blood in the villi, and are transported to the liver via the hepatic portal vein. Brush border enzymes (small intestine) Small intestine Amino acids
Figure 23.33 Protein digestion and absorption in the small intestine. Amino acids of protein fragments Lumen of intestine Brush border enzymes Apical membrane (microvilli) 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 villus epithelial cell by facilitated diffusion and enter the capillary via intercellular clefts. Active transport Capillary Passive transport
Carbohydrate digestion Enzyme(s) and source Site of action Foodstuff Path of absorption Starch and disaccharides Salivary amylase Mouth Pancreatic amylase Small intestine Oligosaccharides and disaccharides Brush border enzymes in small intestine • All monosaccharides enter the capillary blood in the villi, and are transported to the liver via the hepatic portal vein. Small intestine Lactose Maltose Sucrose Galactose Fructose Glucose
Note: fats are NOT made of a long chain of monomers (unlike proteins and polysaccharides) Fat digestion Enzyme(s) and source Site of action Foodstuff Path of absorption Unemulsified fats • Fatty acids and glycerol enter the intestinal cells via diffusion. Emulsification by the detergent action of bile salts from the liver Small intestine • 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. Pancreatic lipases Small intestine fatty acids glycerol • 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 2 Digestion of fat by the pancreatic enzyme lipase yields free fatty acids and glycerol, still associated with bile salts Fat droplets coated 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 by exocytosis. The chylomicrons enter lacteals. They are carried away from the intestine by lymph. Epithelial cells of small intestine Lacteal
Nucleic acid digestion Enzyme(s) and source Site of action Foodstuff Path of absorption Nucleic acids Pancreatic ribo- nuclease and deoxyribonuclease Small intestine • Subunits are absorbed into capillary blood in the villi and transported to the liver via the hepatic portal vein. Brush border enzymes (nucleosidases and phosphatases) Small intestine Pentose sugars, N-containing bases, phosphate ions
Other than digestion by enteric bacteria, no further digestion takes place Bacteria synthesize B complex vitamins and vitamin K water, and electrolytes are absorbed In the large intestine: