7.4) Chemical digestion

7.4) Chemical digestion

Involves enzymes.

  • Breaking down large molecules to small molecules.
  • The large molecules are usually not soluble in water while the smaller ones are.
  • The small molecules can be absorbed through the epithelium of the alimentary canal, through the walls of the blood vessels and into the blood.


  • Amylase breaks down starch to simpler sugars.
  • Protease breaks down protein to amino acids.
  • Lipase breaks down fats to fatty acids and glycerol.


    Where digestion happens:

    • Proteases catalyse the breakdown of proteins into amino acids in the stomach and small intestine.
    • Lipases catalyse the breakdown of fats and oils into fatty acids and glycerol in the small intestine.
    • Amylase catalyses the breakdown of starch into maltose in the mouth and small intestine.
    • Maltase catalyses the breakdown of maltose into glucose in the small intestine.


    Digestion of protein:

    • Several proteases which break down proteins.
    • Pepsin is secreted in the stomach.
    • Pepsin acts on proteins and breaks them down into soluble compounds called peptides.
    • Trypsin is secreted by the pancreas in an inactive form, which is changed to an active enzyme in the duodenum.
    • Breaking down proteins to peptides.


    The stomach:

    • The stomach produces hydrochloric acid.
    • It kills many harmful microorganisms (bacteria) that might have been swallowed along with the food.
    • The enzymes in the stomach work best in acidic conditions – at a low pH.


    Functions of HCL in gastric juice:

    • Creates a very acid pH of 2.
    • This pH is important because it denatures enzymes in harmful organisms in food, such as bacteria
    • It provides the optimum pH for the protein-digesting enzyme pepsin to work.



    After it has been in the stomach, food travels to the small intestine. The enzymes in the small intestine work best in alkaline conditions – but the food is acidic after being in the stomach. Bile is a substance produced by the liver and stored in the gall bladder.

    • The bile emulsify the fat – they break them up into small droplets with a large surface area, which are more efficiently digested by lipase.
    • Bile is slightly alkaline and has the function of neutralising the acidic mixture of food and gastric juices as it enters the duodenum.
    • This is important because enzymes secreted into the duodenum need alkaline conditions to work at their optimum rate.