Digestion

• Define digestion 


  is the hydrolysis of large insoluble molecules into smaller more soluble molecules
• 2 types of digestion

  physical and chemical breakdown
• Describe how is starch digested (first stage mouth )

  Food containing starch is physically broken down by the teeth, this creates a large surface area for amylase to act on. Salivary glands release salivary amylase this hydrolyses the glycosidic bonds in starch forming maltose. The salivary amylase also contains mineral salts to maintain the PH at around neutral for amylase to work best at.
• Describe how starch is digested (stomach)

  the food is then passed into the stomach, here the conditions are acidic. They denature the enzyme amylase preventing further hydrolysis of starch
• Describe how starch is digested (small intestine)

  the food then passed into the small intestine, the pancreases releases pancreatic amylase. This hydrolyses any remaining starch into maltose. The muscles in the intestine wall push the food along the ileum. On the cell-surface membrane of the epithelial cells that line the ileum there are membrane-bound disaccharidases called maltase these hydrolyse the 1,4 glycosidic bonds in maltose producing alpha glucose/ glucose.
• describe how proteins are digested

  Endopeptidase, this enzyme hydrolyses the peptide bonds between amino acids in the centre of the polypeptide chain. Producing, smaller peptide molecules. Exopeptidases these hydrolyse the peptide bonds between amino acids on the ends of the polypeptide chain forming dipeptides and amino acids. Membrane bound disaccharidase known as dipeptidase hydrolyses the peptide bonds between 2 amino acids forming single amino acids.
• Describe emulsification of lipids (first stage)

  Bile salts are produced by the liver, they emulsify the lipids into tiny droplets called micelles. This increases the surface area for lipase to act on. This is a physical breakdown
• Describe digestion of lipids (small intestine-2nd stage)


  pancreases releases lipase into the small intestine this hydrolyses the ester bonds in the triglycerides forming monoglycerides, glycerol and fatty acids. This is a chemical breakdown. This creates micelles which are water soluble vesicles containing, glycerol, fatty acids, monoglycerides and bile salts
• Describe digestion of lipids (epithelial cells)

  the micelles then deliver the monoglycerides, triglycerides and glycerol to epithelial cells that line the ileum. When the micelles come into contact with the epithelial cells it breaks down releasing the glycerol, monoglycerides and fatty acids into the epithelial cells which diffuse straight across because they are non-polar. The monoglycerides and fatty acids are transported to the endoplasmic reticulum where they re-combine forming triglycerides.
• Digestion of lipids (golgi apparatus)

  1. Triglycerides transported to Golgi apparatus

  2. Cholesterol and lipoproteins added to form chylomicrons

  3. Chylomicrons leave epithelial cell by exocytosis

  4. Chylomicrons enter lymphatic system through lacteals

  5. Chylomicrons enter blood stream via lymphatic vessels

  6. Triglycerides in chylomicrons hydrolysed by enzyme in endothelial cells

  7. Triglycerides diffuse into cells