Alimentary canal

• mechanical digestion - breakdown of large food molecules into small molecules without chemical change
• chemical digestion - breakdown of large, insoluble molecules into small soluble molecules with the use of enzymes, so that the products can be absorbed
• absorption - the movement of digested food molecules through the wall of the intestine into the blood
• assimilation - the movement of digested food molecules into the cells of the body where they are used, becoming part of the cells
• egestion - passing out of food that has not been digested, as faeces
• Peristalsis is involuntary waves of muscular contraction along the digestive canal that help to push substances along.
• Mouth and salivary glands:

  • Food is mechanically digested into the stomach
  • Salivary glands release saliva containing carbohydrase enzymes (amylase) that start the chemical digestion of starch into simple sugars
• Oesophagus allows food to pass through by peristalsis
• Stomach:

  • Protease enzymes break down proteins in chemical digestion
  • Churning of food aids in mechanical digestion
  • Contains hydrochloric acid to lower pH for enzyme action and kill bacteria by creating extremely acidic conditions, denaturing enzymes within them
  • Gastric juice contains the protease enzyme pepsin, which breaks down proteins into peptides and later into amino acids
  • Pepsin works fastest in a low pH environment
• Liver:

  • Produces bile, which is secreted into the small intestine
  • Bile neutralises acidic conditions from the stomach, emulsifies fats to increase their surface area for quicker digestion
• Gall bladder:

  • Stores bile made in the liver before secreting it into the small intestine
• Pancreas:

  • Secretes pancreatic juice into the small intestine, containing lipase, protease, and amylase for food absorption
• Small intestine:

  • Consists of duodenum and ileum
  • Duodenum neutralises stomach acidity and secretes protease, lipase, and amylase for breaking down molecules
  • Ileum absorbs final digestion products into the blood, chemically digesting fat
  • Cells in the ileum release enzymes to complete the breakdown of starch to sugars and protein to amino acids
• Large intestine:

  • Indigestible food passes through
  • Consists of colon, rectum, and anus where remaining salts and water are absorbed