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Enzymes: biological catalysts and activation energy (OCR topic: Mode of action - biological catalysts) Enzymes as biological catalysts * A catalyst is a substance that increases the rate of a chemical reaction, and is unchanged at the end of the reaction. * Catalysts are effective in small amounts. * The catalysts of metabolism are a wide range of enzymes, produced by cells. * In enzyme-catalysed reactions, the starting substance is the substrate, which is converted to product. For example, sucrose is the substrate of the enzyme sucrase (also known as invertase), and the products are glucose and fructose. * Enzymes in cells ensure the reactions of metabolism happen at speed, in an organised manner. * Some enzymes (the extracellular enzymes) are released from cells, for example, many digestive enzymes. * Most enzymes remain within cells (the intracellular enzymes), occurring in organelles, in the cytosol and in the plasma membrane. Enzymes as protein catalysts * Most of the enzymes...

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