digestion and absorption

• Digestion

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

  • Carbohydrates
  • Lipids
  • Proteins
• Amylase

  Enzymes that hydrolyze carbohydrates
• Disaccharidases


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


  1. Amylase in saliva hydrolyzes starch into disaccharides

  2. Disaccharidases in small intestine hydrolyze disaccharides into monosaccharides
• Protein digestion


  1. Endopeptidases hydrolyze peptide bonds within the protein chain

  2. Exopeptidases hydrolyze peptide bonds at the ends of the protein chain

  3. Dipeptidases hydrolyze the bond between two amino acids
• Lipid digestion


  1. Lipase enzyme hydrolyzes triglycerides into fatty acids and glycerol

  2. Bile salts emulsify lipids into small droplets to increase surface area for lipase
• Emulsion


  Small droplets of lipids coated in bile salts, creating a large surface area for lipase to act
• Larger surface area of lipid droplets

  Faster hydrolysis by lipase
• Micelles


  Spheres or vesicles made up of fatty acids, monoglycerides, and bile salts
• Absorption of digested molecules

  1. Monosaccharides and amino acids absorbed by co-transport

  2. Fatty acids and monoglycerides diffuse into epithelial cells, re-formed into triglycerides, packaged into chylomicrons, and released into lacteals
• Villi and microvilli


  • Increase surface area for absorption
  • Capillary network maintains concentration gradient
• Absorption of monosaccharides and amino acids can be limited by co-transport capacity
• Lipids are absorbed as fatty acids, monoglycerides, and chylomicrons