Digestion

• Enzymes are biological catalysts - they speed up chemical reactions in living organisms
• Enzymes:

  • large proteins
  • space between the protein molecule called the active site
  • each enzyme catalyses a specific reaction
  • they work best at a specific temperature and pH called the optimum
• the 'lock and key theory' is a model used to explain how enzymes work: the chemical that reacts is called the substrate and it fits into the enzyme's active site
• High temperatures and extremes of pH make enzymes change shape. This is called denaturing
• The enzyme cannot work once it is denatured, because the substrate cannot fit into the active site - the lock and key no longer fit together
• digestive enzymes are produced by specialised cells in glands and in the lining of the gut:

  • the enzymes pass out of the cells into the digestive system
  • they come into contact with food molecules
  • they catalyse the breakdown of large insoluble food molecules into smaller soluble molecules
• To test for sugars, add Benedict's reagent and heat in a water bath for two minutes. If sugar is present, it will turn red.
• To test for starch add iodine solution. If starch is present, it will turn blue-black
• To test for protein add buiret reagent. If protein is present, it will turn lilac
• Amylase:

  • is produced in the salivary glands and the pancreas
  • is a carbohydrase that breaks down starch into sugar (maltose)
  • starch -> maltose
• protease:

  • is produced in the stomach, pancreas and small intestine
  • breaks down proteins into amino acids
  • protein -> amino acids
• lipase:

  • is produced in the pancreas and small intestine
  • breaks down lipids into fatty acids and glycerol
  • fat -> fatty acids + glycerol
• bile is a liquid made in the liver and stored in the gall bladder. It is alkaline to neutralise hydrochloric acid from the stomach. It also emulsifies fat to form small droplets, increasing the surface area for enzymes to act on. The alkaline conditions and large surface area increase the rate at which fat is broken down by lipase
• bile doesn't contain any enzymes, so it doesn't digest fat molecules. It just breaks up fat droplets
• Amylase experiment:

  • put a test tube containing starch solution and a test tube containing amylase into a water bath at 37 degrees C
  • After 5 minutes add the amylase solution to the starch
  • every 30 seconds take a drop from the mixture and test it for starch using iodine solution
  • record how long it takes for the starch to be completely digested
  • repeat the experiment at different pH values using different buffer solutions