Other substances in digestion

Different enzymes work best at different pH values. The pH varies in the gut.

The mouth

Saliva is slightly alkaline. Enzymes in saliva convert starch into glucose:

  • amylase catalyses: starch → maltose
  • maltase catalyses: maltose → glucose
Part of a starch molecule is shown.  With the addition of specific enzymes the protein is broken down into separate glucose molecules.The digestion of starch to glucose needs two enzymes

In the stomach

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


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 hepatic duct travels from liver and gall bladder passing by pancreas where digestive enzymes are produced and flow bile duct. The bile duct connects to the small intestine.Bile and enzyme production in the liver and pancreas

Bile is secreted into the small intestine where it has two effects:

  • it neutralises the acid - providing the alkaline conditions needed in the small intestine
  • it emulsifies fats - providing a larger surface area over which the lipase enzymes can work