Digestion

• Physical Digestion

  • mechanical break-up of food into small particles.
  • increases the surface area to volume ratio of ingested food so that digestive enzymes can act of the food particles more effectively
  • Examples:

  1. Food being chewed in the mouth
  2. Churning action of the stomach
  3. Emulsification of fats by bile salts
• Chemical Digestion


  • Breaking down of large molecules in food, such as proteins, starch and fats, into small, soluble molecules that can be absorbed.
  • Involves enzymes
• Processes in mouth


  1. Salivary glands are stimulated to secrete saliva.
  2. Saliva mixes with the food and softens it.
  3. Salivary amylase in saliva breaks down starch to maltose.
  4. Optimum pH for salivary amylase is at pH 7.
  5. The food is broken down into smaller pieces by chewing, which increases the surface area to volume ratio for salivary amylase to work on.
  6. The food is rolled into small round masses called boli.
  7. The boli are swallowed and passed down into the oesophagus via the pharynx
• Processes in the Stomach


  1. Gastric glands are stimulated to secrete gastric juice.
  2. Peristalsis churns and breaks up the food and mixes the food with gastric juice.
  3. Gastric juice contains hydrochloric acid, mucus and protease
  4. Hydrochloric acid (pH 2) stops action of salivary amylase by denaturing it, provides low pH environment in the stomach for protease to digest proteins and kills microorganisms in food.
  5. Protease breaks down proteins into polypeptides.
  6. Food remains in the stomach for 3 to 4 hours, becomes partially digested substance, chyme.
  7. Chyme passes into the duodenum in small amounts
• Processes in the Small Intestine


  1. Chyme enters duodenum, stimulates pancreas to secrete pancreatic juice.
  2. Pancreatic juice passes to the duodenum via the pancreatic duct.
  3. Pancreatic juice contains pancreatic amylase, protease and lipase.
  4. Gall bladder releases bile into the duodenum via the bile duct. Bile speeds up digestion of fats by breaking up large fat molecules into smaller fat molecules.
  5. Small intestines produces enzymes maltase, protease and lipase.
  6. Acidic chyme from stomach is neutralised by alkaline fluids from the pancreas, gall bladder and small intestine (pH 8)
• Carbohydrate Digestion


  • Examples of carbohydrates include starch, sucrose and cellulose.

  • Carbohydrates are digested by carbohydrases such as amylase and maltase.

  • Starch is broken down into maltose by amylase in the mouth and small intestine.

  • Maltose is broken down into glucose by maltase in the small intestine

  
• Protein Digestion


  • Proteins are digested by proteases.

  • Protein digestion starts in the stomach, where stomach protease digests proteins to polypeptides.

  • The undigested proteins that enter the small intestine are digested by intestinal protease to polypeptides.

  • The polypeptides produced are further digested to amino acids by intestinal protease.

  
• Fats Digestion


  • Bile is released into the duodenum, and the bile salts emulsify fats by lowering the surface tension of the fats.

  • Thus, the fats break into tiny fat droplets, increasing the surface area to volume ratio for enzyme action.

  • Emulsified fats are digested by lipases into fatty acids and glycerol in the small intestine.