3.3.3 Digestion and absorbtion

• Enzymes:

  Amylase: Hydrolyses glycosidic bond in starch --> maltose

  Lactase: Hydrolyses glycosidic bond in lactose --> alpha glucose and galactose

  Sucrase: Hydrolyses glycosidic bond in sucrose --> alpha glucose and fructose

  Maltase: Hydrolyses glycosidic bond in maltose --> alpha glucose

  Lipase: Hydrolyses ester bond in triglycerides --> 2 fatty acids and a monoglyceride or 3 fatty acids and a glycerol
• Salivary glands - Amylase

  Stomach - protease

  Liver - produces bile

  Gallbladder - stores and secretes bile

  Pancreas - Amylase, peptidase, lipase

  Small intestine - Absorbs small soluble molecules

  
• Where are disaccharides embedded?

  The cell surface membrane of epithelial cells
• What are bile salts used for?

  Large lipid droplets are emulsified into smaller droplets by bile salts, providing a larger surface area for lipase to act. Faster hydrolysis
• Digestion

  Larger insoluble molecules being hydrolysed into smaller soluble ones
• Digestion of proteins

  Endopeptidases - Hydrolyse internal peptide bonds creating smaller polypeptides, gives more ends for faster hydrolysis

  Exopeptidases - Hydrolyse end peptide bonds creating single amino acids

  Dipeptidases - combined action of previous enzymes leaves some dipeptides, these are hydrolysed by dipeptidases that are embedded on the cell surface membrane
• In the ileum - Blood flows away from the ileum to maintain the concentration gradient so products of digestion can continue to diffuse into the blood
• Co-transport of glucose and sodium:

  1. Sodium ions actively transported into the blood
  2. Keeping a low concentration of sodium ions in the cell
  3. So, sodium ions always have a concentration gradient to diffuse down into the cell
  4. Taking glucose with it
  5. Facilitated diffusion into the blood

  
• Co-transport of amino acids:

  1. Sodium ions actively transported into the blood
  2. Keeping a low concentration of sodium ions in the cell
  3. So, sodium ions always have a concentration gradient to diffuse down the cell
  4. Taking amino acids with it
• Absorption of fatty acids and monoglycerides:

  1. Digestion of triglycerides. fatty acids and monoglycerides are surrounded by bile salts to form micelles
  2. Fatty acids and monoglycerides are released and cross the cell surface membrane by simple diffusion - through the phospholipid bilayer
  3. Triglycerides are reformed through monoglycerides condensing
  4. Triglycerides are surrounded by protein to form chylomicrons
  5. Chylomicrons are secreted into the lymph vessel and transported into the blood