The human digestive system

• The digestive system is an example of an organ system in which several organs work together to digest and absorb food.
• Enzymes catalyse specific reactions in living organisms due to the shape of their active site.
• Digestive enzymes convert food into small soluble molecules that can be absorbed into the bloodstream.
• Carbohydrases break down carbohydrates to simple sugars. Amylase is a carbohydrase which breaks down starch.
• Proteases break down proteins to amino acids.
• Lipases break down lipids (fats) to glycerol and fatty acids
• The products of digestion are used to build new carbohydrates, lipids and proteins. Some glucose is used in respiration.
• Bile is 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 which increases the surface area. The alkaline conditions and large surface area increase the rate of fat breakdown by lipase
• Enzymes are biological catalysts - they speed up chemical reactions in living organisms
• Enzymes have a number of properties:

  • they are all large proteins
  • there is a space within 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
• Th 'lock and key theory' is a model used to explain how enzymes work: the chemical that reacts is called the substrate (key) and it fits into the enzyme's active site (lock).
• High temperatures and extreme pH can cause an active site to change shape
• High temperatures cause an irreversible and permanent change. This is called denaturing.
• If the active site (the lock) has temporarily changed shape or is denatured, the enzyme will not work. The substrate (the key) will no longer fit into the active site (the lock).
• Digestive enzymes are produced by specialised cells in glands and in the lining of the gut:

  1. the enzymes pass out of the cells into the digestive system
  2. they come into contact with food molecules
  3. they catalyse the breakdown of large insoluble food molecules into smaller molecules
• The digestive enzymes, protease, lipase and carbohydrase, digest proteins, lipids (fats and oils) and carbohydrates to produce smaller molecules that can be easily absorbed into the bloodstream.
• The products of digestion are then transported and used as building blocks for new carbohydrates, lipids and proteins
• Some of the glucose is used in respiration
• The 'lock and key theory' is an example of how models are used in science to try and explain observations
• Amylase:

  • is produced in the salivary glands and the pancreas
  • is a carbohydrase that breaks down starch into maltose (a sugar)
  • maltose is later broken down into glucose
• Protease:

  • is produced in the stomach, pancreas and small intestine
  • breaks down proteins into amino acids
• Lipase:

  • is produced in the pancreas and small intestine
  • breaks down lipids (fats) into fatty acids and glycerol