Digestion and Absorption

• Carbohydrates:

  • glycosidic bonds are hydrolysed
  • Starch is hydrolysed by amylase into maltose
  • Maltose is hydrolysed into alpha glucose
• Amylase is produced in the salivary glands and the pancreas, it acts in the mouth and the small intestine

  Maltase is produced in the pancreas and is embedded in the cell membrane of the epithelial cells lining the ileum
• Proteins:

  • hydrolysis of peptide bonds
  • endopeptidase hydrolyses the bond in the centre of the polypeptide chain to create 2 shorter chains and increase surface area for exopeptidase
  • exopeptidase hydrolyses bonds at the ends of the chain to form dipeptides and some amino acids
  • dipeptidase splits dipeptides into amino acids.
• Endopeptidase is produced in the stomach ad the pancreas, it acts in the stomach and the small intestine. Exopeptidase is produced in the pancreas and acts in the small intestine. Dipeptidase is produced in the pancreas and is embedded in the cell membrane of the epithelial cells lining the ileum.
• Lipids:

  • hydrolysis of ester bonds
  • triglycerides are emulsified by bile salts to form lipid droplets which have a higher surface area for quicker lipase action.
  • Lipase hydrolyses triglycerides into monoglycerides and fatty acids.
• Lipase is produced in the pancreas and acts in the small intestine. Bile salts are made by the liver and stored in the gall bladder
• Adaptations of the small intestine for absorption:

  • surface area- many villi made up of epithelial cells wit many microvilli to increase the surface area.
  • the small intestine is separated from the blood by a single layer of epithelial cells in order to decrease the diffusion pathway
  • there is a rich blood supply removing absorbed products and peristalsis replaces absorbed nutrients in order to maintain the concentration gradient.
• Absorption of carbohydrates and proteins:

  • co-transport
  • Na ions pumped from epithelial cell into the blood
  • this creates a Na concentration gradient between the lumen ileum and the epithelial cell
  • Na ions diffuse through the channel protein whilst attached to glucose/amino acid - co-transport into the cell
  • Na and molecule separate. Na ion are pumped out of the cell and the glucose/amino acid facilitatedly diffuse through carrier proteins into the blood
• Absorption of Lipids:

  • monoglycerides and fatty acids from digestion combine with bile salts to form micelles
  • this increases their solubility and allows them to be transported to the epithelial cell membrane
  • the micelles separate out and the bile salts leave the monoglycerides and fatty acids to diffuse into the epithelial cell membrane and into the cell; by simple diffusion
  • In the SER they form triglycerides
  • In the Golgi apparatus, the triglycerides are modified, by addition of proteins, cholesterol and phospholipids to form chylomicrons
  • these are exocytosed into the lymph vessel