3.3 digestion and absorption

• What is the function of endopeptidases?

  Hydrolyse peptide bonds between specific amino acids in the middle of a polypeptide.
• What is the function of exopeptidases?

  Hydrolyse peptide bonds between specific amino acids at the terminal ends of a polypeptide.
• What is the function of dipeptidases?

  Hydrolyse the peptide bonds in a dipeptide.
• What is digestion?

  The process in which larger insoluble molecules are hydrolysed by enzymes into smaller soluble molecules, which can be absorbed and assimilated.
• How are carbohydrates digested?

  • saliva enters the mouth via salivary glands, and contains salivary amylase
  • amylase hydrolyses starch into maltose at a neutral pH
  • food is swallowed and enters the stomach; acidic conditions denature the amylase preventing further hydrolysis
  • food reaches the small intestine and is mixed its pancreatic juice
  • pancreatic amylase continues the hydrolysis of starch to maltose at a neutral pH
  • food enters the ileum; maltase is a membrane-bound enzyme and hydrolyses maltose to alpha-glucose
• How are proteins digested?

  • digested by peptidases (proteases)
  • endopeptidases hydrolyse peptide bonds between specific amino acids in the middle of a polypeptide, producing smaller polypeptides
  • exopeptidases hydrolyse peptide bonds between specific amino acids at the terminal ends of a polypeptide, producing dipeptides/amino acids
  • membrane-bound dipeptidases hydrolyse the peptide bonds in a dipeptide, producing single amino acids
• Why is the combined action of endopeptidases and exopeptidases more efficient than exopeptidases alone?

  The action of endopeptidases to split up the one polypeptide chain into multiple polypeptides increases the number of ends for exopeptidases to work on. This increases the rate of digestion.
• How are lipids (triglycerides) digested?

  • bile salts emulsify lipids into lipid droplets
  • lipid droplets contains triglycerides increase the surface area for lipases, so faster hydrolysis can occur
  • in the lumen of the small intestine, lipase hydrolyses ester bonds to form fatty acids and monoglycerides
• How are lipids (triglycerides) digested?

  • micelles are formed, micelles are vesicles containing: bile salts, monoglycerides, fatty acids
  • micelles carry fatty acids and monoglycerides to the epithelial cell membrane where they enter the cell via simple diffusion
  • triglycerides are reformed in the Golgi body where they are modified and proteins are sometimes added forming a chylomicron
  • golgi body forms vesicles
  • vesicles containing triglyceride/chylomicron are released and move towards the cell membrane and are released by exocytosis and enter the lymph capillary (lacteal)
• Why do fatty acids and monoglycerides enter the epithelial cell via simple diffusion?

  They are non-polar (lipid soluble)
• How are monosaccharides absorbed?

  • sodium ions are actively transported out of the epithelial cells by the sodium-potassium pump, into the ileum into the blood, creating and maintaining a high concentration gradient between the lumen of the ileum and the epithelial cell
  • co-transport of sodium ions and glucose via a carrier or co-transport protein from the lumen to the epithelial cell, so the concentration of glucose inside the cell increases
  • glucose enters the blood from the epithelial cell via facilitated diffusion through a protein
• How are amino acids absorbed?

  • sodium ions are actively transported out of the epithelial cells in the ileum into the blood, creating and maintaining a concentration gradient between the lumen of the ileum and the epithelial cell
  • co-transport of sodium ions and amino acids via a carrier protein from the lumen to the epithelial cell, so the concentration of amino acids inside the cell increases
  • amino acids enter the blood from the epithelial cell via facilitated diffusion through a protein