Digestion and absorption

• What is an endopeptidase?

  An endopeptidase is an enzyme which acts in the middle of a polypeptide and hydrolyses peptide bonds to produce shorter polypeptides.
• What is an exopeptidase?

  An exopeptidase is an enzyme which acts at the end of a polypeptide and hydrolyses peptide bonds to produce dipeptides and amino acids
• dipeptidase is an enzyme which hydrolyses peptide bonds to produce amino acids. 

   
• Digestion of Lipids

  • The digestion of lipids takes place solely in the lumen of the small intestine
  • Lipase enzymes break down lipids (fats) to glycerol and fatty acids
  • Lipase enzymes are produced in the pancreas and secreted into the small intestine by the pancreas
• Emulsification

  • In stomach, solid lipids are turned into a fatty liquid consisting of fat droplets - but this is not digestion
  • When the fatty liquid arrives in the small intestine, bile containing bile salts is secreted
  • The bile salts bind to the fatty liquid and breaks the fatty droplets into smaller ones via emulsification
  • Emulsification helps to increase the surface area of the fatty droplets for action of digestive enzymes
• How do amino acids and monosaccharides enter an epithelial cell?

  They enter via co-transport proteins that only function when sodium ions are present
• How is the sodium ion gradient between the lumen and epithelial cell maintained?

  The sodium ion gradient between the lumen and epithelial cell is maintained by the sodium-potassium pump
• Amino acids and monosaccharides move from an epithelial cell into the capillaries by facilitated diffusion.
• By which process do monoglycerides and fatty acids enter epithelial cells?

  Glycerol and fatty acids enter epithelial cells by the process of diffusion
• What happens to monoglycerides and fatty acids in epithelial cells?

  In epithelial cells, the endoplasmic reticulum turns them into triglycerides.

   

  Then, the Golgi apparatus packages them into lipoproteins.
• How do amino acids and monosaccharides enter an epithelial cell?

  they enter via co-transport proteins that only function when sodium ions are present. 
• Explain how amino acids are transported from the lumen of the ileum into the blood

  The majority of amino acids move from the lumen to epithelial cells via co-transport using sodium ions

  When the concentration of amino acids is greater in the lumen than the epithelial cells, they can move into the cell via facilitated diffusion

  Once the amino acids are inside the epithelial cell, they enter the blood via facilitated diffusion
• Explain the advantages of lipid droplet and micelle formation

  Droplets increase surface areas for lipase / enzyme action

  So faster hydrolysis / digestion of triglycerides / lipids

  Micelles carry fatty acids and glycerol / monoglycerides to / through membrane / to (intestinal epithelial) cell
• Explain how cells lining the ileum of mammals absorb the monosaccharide glucose by co-transport with sodium ions

  1. Sodium ions are actively transported from ileum cell to blood
  2. This forms a diffusion gradient for sodium to enter cells from gut
  3. glucose enters via facilitated diffusion with sodium ions