3.3 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 in pancreas hydrolyzes triglycerides into fatty acids and glycerol

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


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

  Faster hydrolysis by lipase
• Micelles


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

  1. Co-transport across epithelial cell membranes

  2. Facilitated diffusion
• Absorption of lipids


  1. Micelles deliver fatty acids and monoglycerides to epithelial cells

  2. Fatty acids and monoglycerides diffuse into epithelial cells

  3. Triglycerides reformed in endoplasmic reticulum 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
• Chylomicron


  when a fatty globule is combined with a protein
• Chylomicron are extruded from the epithelial cell to the lacteal
• lacteal


  lymph capillary