The digestive system

• Muscle tissue


  Muscle cells can contract and contain special protein fibres which can change their length
• Tissue


  A group of cells with a similar structure and function
• Organ


  A group of tissues working together for a specific function e.g. stomach
• Main nutrients in food

  • Carbohydrates (e.g. starch)
  • Protein
  • Lipids (fats)
• All nutrients are large molecules and are too large to be absorbed into the bloodstream so have to be digested
• Digestion


  1. Large food molecules are broken down into small molecules by enzymes

  2. Small molecules can then be absorbed into the bloodstream
• Digestion in the mouth

  1. Food is chewed

  2. Enzymes in saliva begin to digest starch into smaller sugar molecules
• Digestion in the stomach

  1. Enzymes begin the digestion of proteins

  2. Stomach contains hydrochloric acid which helps the enzymes to digest proteins

  3. Stomach muscles churn the food into a fluid increasing the surface area for enzymes to digest
• Digestion in the small intestine

  1. Chemicals are released from the liver and pancreas

  2. Pancreas releases enzymes which continue the digestion of starch, protein and lipids

  3. Liver releases bile which speeds up the digestion of lipids and neutralises acid from the stomach

  4. Walls of the small intestine release enzymes to continue the digestion of protein and lipids

  5. Small food molecules produced by digestion are absorbed into the bloodstream by diffusion or active transport
• The products of digestion are used by the body to build new carbohydrates, lipids and proteins
• Some of the glucose produced is used in respiration