The chemical reactions required to break them down would be too slow without enzymes.
Enzymes are biological catalysts - they speed up chemical reactions.
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. The substrate is a complementary shape to the active site. Once bound to the active site, the chemical reaction takes place.
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 to the enzyme being able to work. One enzyme is therefore specific to one substrate's chemical reaction or type of chemical reaction.
This theory for the way in which enzymes work is called the lock and key theory.