Section 4: Enzymes

Enzymes are organic catalysts - they speed up and control chemical reactions in living things.

Enzymes that work in the digestive system break down large food molecules into smaller nutrient molecules that can then be absorbed into the bloodstream.

How do enzymes work?

The enzyme that breaks down starch is called amylase. Part of the amylase molecule has a particular shape - this is called the active site. The shape of the starch molecule allows it to fit into the active site. The starch is then broken down.

Protein and fat molecules have different shapes to starch and will not fit into the active site of amylase. Proteins and fats therefore need different enzymes to break them down.

This is how enzymes work (Image may take a while to download - 63K)

Enzymes and temperature

Enzymes work best at around body temperature. At lower temperatures they work slowly and at higher temperatures stop working completely. This is because high temperatures cause the enzyme molecule to change shape and the active site will no longer accept the substance to be broken down. The enzyme is then described as 'denatured'.

Enzymes and pH

Enzymes also need a particular pH to work well. This is why some parts of the digestive system are acidic whereas other parts are alkaline. In the wrong pH the enzymes change shape and their active sites no longer accept the substance to be broken down.