Enzymes and their role in digestion

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  • Enzymes & their role in digestion
    • 1. The Buccal Cavity
      • Food enters the buccal cavity (mouth) and mechanical digestion occurs through the action of teeth.
      • This mechanical breakdown of food into smaller pieces increases their surface area for they subsequent chemical breakdown by enzymes.
      • In the mouth, chemical digestion of starch takes place by the action of salivary amylase. Salivary amylase is produced and released from the salivary glands.
    • 2. The Stomach
      • The food is swallowed and travels down the oeso-phagus by peristalsis.
      • In the stomach, gastric juice is released which contains proteases which break down proteins into amino acids.
      • The stomach also produces HCL acid lowering the pH creating an optimum pH for the protease enzymes to work.
        • HCL acid also destroys any micro-organisms ingested.
      • The stomach is a muscular sac and helps to solubilise the food by mixing it with gastric juice.
    • 3. The Liver
      • The liver produces bile (a mixture of cholesterol salts), which is stored in the gall bladder.
      • When food enters the first compart-ment of the small intestine (the duodenum)bile is released from the gall bladder (passing down the bile duct and joining the pancreatic duct) into the duodenum.
      • Bile neutralises the stomach acid and raises the pH of the food to approx-imately pH 8.3. (slightly alkaline)
      • This provides an optimum pH for the enzymes (lipase, carbo-hydrases, proteases) produced by the walls of the duodenum (intestinal juices) and the pancreas (pancreatic juices) to complete digestion.
      • Bile also emulsifies fates.
        • Emuls-ification is the breakdown of large fat globules into smaller  fat globules, which increases the surface area of the fat making its digestion by lipase faster.
    • 4. The Duodenum (small intestine)
      • This compart-ment, which follows the stomach, is the main site of enzyme action.
      • Additionally pancreatic juices also contain lipase which breaks down fats or lipids into fatty acids and glycerol and carbo-hydrases break down a range of carbo-hydrates.
      • Pancreatic juice secreted from the pancreas via the pancreatic duct, contains pancreatic amylase to complete the digestion of starch and proteases to complete the digestion of proteins which began in the stomach.
      • The walls of the duodenum also produce intestinal juice containing amylase, protease, lipase and carbo-hydrases.
    • 5. The Ileum (small intestine)
      • This is the main site for the absorption of the digested food products (although there is still some digestion here too.)
      • It is adapted for the absorption of food in a number of ways.
      • The presence of millions of micro-scopic villi (villus sing.) on the inner surface of the ileum also increase the surface area. The epithelial cells (or epithelium is a cellular covering) on each villus contain microvilli further increasing the surface area.
        • Villi help absorption in a number of ways.
    • 6. The Colon (large intestine)
      • In this part of the alimentary canal, water is absorbed from the gut into the blood.
      • The colon is adapted by having a large surface area.
    • 7. The Rectum and Anus
      • The regions where the storage and removal/ egestion of faeces takes place respectively.


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