Thousands of different chemical reactions must happen inside a cell every second in order for it to function. If these cellular reactions happen too slowly, the cell may die. If the cell is part of a larger multicellular organism then this might harm the whole individual.
For example, in liver cells the toxic chemical hydrogen peroxide must be broken down into the harmless products, water and oxygen. If this reaction happens too slowly then hydrogen peroxide can build up and poison the cell.
Chemical reactions can be speeded up by increasing the temperature because more molecules will have sufficient energy to react with each other. However, increasing temperature will cause damage to cells.
Living cells solve this problem by producing proteins called enzymes. Enzymes speed up cellular reactions.
Liver cells produce the enzyme catalase to speed up the breakdown of hydrogen peroxide.
The chemicals that enzymes act on are called substrates and the substances produced are called products. In this example, the enzyme catalase acts on the substrate hydrogen peroxide to make the products water and oxygen.
An enzyme is a biological catalyst produced by all living cells. Catalysts are chemicals that speed up reactions, but are left unchanged after the reaction has been completed.
Enzymes can speed up the break down of complex molecules into simpler molecules or the building up of simple molecules into more complex molecules.