Digestion and absorption

• During digestion large biological molecules are hydrolysed to smaller molecules that can be absorbed across cell membranes.
• Carbohydrates by amylases and membrane-bound disaccharides

  Lipids by lipase, including action of bile salts

  Proteins by endopeptidases, exopeptidases and membrane-bound dipeptidases.
• Mechanisms for the absorption of the products of digestion by cells lining the ileum of mammals include:

  • Co-transport mechanisms for the absorption of amino acids and monosaccharides
  • The role of micelles in the absorption of lipids
• Digestive system

  A) Salivary gland

  B) Epiglottis

  C) Oesophagus

  D) Stomach

  E) Liver

  F) Gall bladder

  G) Duodenum

  H) Pancreas

  I) Ileum (small intestine)

  J) Colon (large intestine)

  K) Appendix

  L) Rectum

  M) Anus

  
• Physical breakdown - the breakdown of large pieces of food into smaller pieces of food, starts in the mouth with slicing and chewing action of the teeth. Increases surface area for enzyme action.

  Continues in the stomach by the churning action of stomach wall contracting.
• Chemical digestion - the chemical breakdown of food molecules into smaller more soluble molecules by enzymes. By breaking a chemical bond by adding water.
• • amylase in saliva from salivary glands in mouth
  • amylase in pancreatic juice acts in the duodenum

  Membrane-bound disaccharides break down disaccharides into monosaccharides for easier absorption:

  • maltase hydrolyses maltose to glucose and glucose
  • sucrase hydrolyses sucrose into glucose and fructose
  • lactase hydrolyses lactose into glucose and galactose

  
• • Endopeptidases hydrolyse proteins
  • Exopeptidases hydrolyse polypeptides

  
• • Dipeptides are hydrolysed by dipeptidases

  
• • Bile (Secreted into small intestine) emulsifies large fat droplets to small droplets called micelles, to increase surface area for pancreatic lipase action
  • Lipase is found in the pancreas juices (pancreatic Lipase)
  • The lipid is hydrolysed into 3 fatty acids and glycerol or two fatty acids and a monoglyceride

  
• Absorption of hydrolysed products of digestion occurs in the ileum which is adapted to its function:

  1. Villi increase surface area for diffusion
  2. Thin walls reducing diffusion path
  3. Muscle contraction maintains diffusion gradient
  4. Blood vessels carry away absorbed material maintaining diffusion gradient
  5. Epithelial cells lining villi contain microvilli, increase surface area.
• Lipid absorption

  1. Fatty acids and monoglycerides leave micelles and enter epithelial cell
  2. Fatty acid leave micelle and is absorbed
  3. Chylomicron exits via exocytosis into the lacteal