Enzymes and digestion

Enzyme action

  • Enzymes are proteins that act as biological catalysts - this means they speed up reactions without being used up.
  • An enzyme works on the substrate, forming products.
  • An enzyme’s active site and its substrate are complementary in shape.
  • An enzyme will only work on one substrate - it is substrate specific.
  • Enzymes and substrates collide to form enzyme-substrate complexes.
  • The substrates are broken down (or in some cases built up).
  • The products are released.
  • The enzyme is free to act again.
  • This theory is known as the ‘lock and key model’.
  • It explains why each enzyme will only work on one substrate.
  • For example, the active site of amylase is only complementary to starch and will therefore only break down starch, not protein or fat.
CarbohydraseCarbohydrate Simple sugar, glucose
AmylaseStarch Simple sugar, glucose
ProteaseProtein Amino acid
LipaseFat (lipid)Glycerol and fatty acids


Inhibitors are molecules that partially fit into an enzyme’s active site but are not broken down.

They inhibit the reaction.

As long as they are in the active site the substrate cannot enter to be broken down, thus reducing the rate of reaction.