The Digestive System

• Food contains three main nutrients:

  • Carbohydrates (eg starch)
  • Protein
  • Lipids (fats)
• Large molecules have to be digested before they can be absorbed into the bloodstream.
• During digestion, large food molecules are broken down into small molecules by enzymes
• Food is chewed in the mouth, enzymes in the saliva digest the starch into smaller sugar molecules
• In the stomach, enzymes begin the digestion of proteins. The stomach also contains hydrochloric acid which helps the enzymes to digest proteins. The food spends several hours in the stomach. The churning action of the stomach muscles turn the food into a fluid increasing the surface area for enzymes to digest.
• The fluids pass into the small intestine, in which chemicals will be released from the liver and pancreas. In the small intestine, the small food molecules produced by digestion are absorbed into the bloodstream either by diffusion or by active transport.
• The pancreas releases enzymes which continue the digestion of starch and protein. They also start the digestion of lipids.
• The liver releases bile which helps to speed up the digestion of lipids. Bile also neutralises the acid released from the stomach.
• The fluid makes its way through the large intestine, where water is absorbed into the bloodstream.
• Digestion

  1. Food is chewed in the mouth

  2. The food then passes down the oesophagus into the stomach, in which the food spends several hours

  3. The food is turned into a fluid through enzymes in the stomach

  4. The fluid now passes into the small intestine

  5. Chemicals are released into the small intestine from the liver and the pancreas

  6. The walls of the small intestine release enzymes to continue digestion

  7. The small food molecules produced by digestion are absorbed into the bloodstream

  8. The fluid makes its way through the large intestine, where water is absorbed into the bloodstream

  9. Finally, the faeces is released from the body
• The products of digestion are used by the body to build new carbohydrates, lipids and proteins.
• Some of the glucose produced in digestion is used in respiration