Digestion and Absorption

• Digestion


  The process of breaking down large insoluble molecules into smaller soluble molecules that can be absorbed across cell membranes and into the bloodstream
• Molecules digested

  • Carbohydrates
  • Lipids
  • Proteins
• Amylase


  Enzyme that hydrolyses carbohydrates
• Disaccharidases


  Membrane-bound enzymes that hydrolyse disaccharides into monosaccharides
• Carbohydrate digestion


  1. Amylase in saliva starts digestion in mouth

  2. Amylase and disaccharidases in small intestine complete digestion
• Protein digestion


  1. Endopeptidases hydrolyse peptide bonds within protein chain

  2. Exopeptidases hydrolyse peptide bonds at ends of chain

  3. Dipeptidases hydrolyse bonds between two amino acids
• Lipid digestion


  1. Lipase enzyme in pancreatic secretions hydrolyses triglycerides

  2. Bile salts emulsify lipids to increase surface area for lipase
• Bile salts


  • Produced in liver, stored in gallbladder, released into small intestine
  • Emulsify lipids to increase surface area for lipase
• Increased surface area from emulsification

  Faster hydrolysis by lipase
• Micelles


  Spheres/vesicles made of fatty acids, monoglycerides and bile salts that deliver digested lipids to epithelial cells
• Absorption of monosaccharides and amino acids

  1. Co-transport across epithelial cell membrane

  2. Some by facilitated diffusion
• Absorption of lipids


  1. Micelles deliver fatty acids and monoglycerides to epithelial cells

  2. Fatty acids and monoglycerides diffuse into cells

  3. Reassembled into triglycerides and packaged into chylomicrons

  4. Chylomicrons released by exocytosis and absorbed into lacteals
• Villi and microvilli in small intestine increase surface area for absorption
• Capillary network in villi maintains concentration gradient for absorption