Enzymes are biological catalysts - they speed up chemical reactions. The reactions of photosynthesis, and many other biological processes, are controlled by enzymes.

Enzymes are required for most of the chemical reactions that occur in organisms. These reactions occur in the breakdown of chemical molecules, which we see in the digestive system.

Enzymes are also involved in the building up of chemical molecules elsewhere in the body.

Enzymes are proteins that have a complex 3D-shape. Each enzyme has a region called an active site.

The substrate - the molecule or molecules taking part in the chemical reaction - fits into the active site. Once bound to the active site, the chemical reaction takes place.

Diagram showing how enzymes work in the body

In an organism, the active site of each enzyme is a different shape. It is a perfect match to the shape of the substrate molecule, or molecules. This is essential for the enzyme to work. One enzyme is therefore specific to one substrate in a chemical reaction, or type of chemical reaction.

This model for the way in which enzymes work is called the lock and key theory. Enzymes are specific for their substrates like keys are for their locks.