Cambridge O Level Biology · Syllabus 5090 · Enzymes
Catalyst
What is Catalyst?
A substance that increases the rate of a chemical reaction and is not changed by the reaction. It is not used up, does not appear among the products, and does not supply energy to the reaction; it only makes the reaction happen faster than it otherwise would.
This definition is part of the Enzymes chapter in Cambridge O Level Biology.
Catalyst in context
Enzymes are proteins that function as biological catalysts: each one increases the rate of a metabolic reaction and is not changed by that reaction, so a single enzyme molecule can catalyse the same reaction again and again. Cambridge O Level Biology (5090) Chapter 5 explains how an enzyme works through its active site, the enzyme-substrate complex and the lock-and-key model; why temperature and pH change the rate of an enzyme-catalysed reaction; what denaturation is and why it is not the same as being killed; and how to investigate amylase and catalase activity and interpret a product-time graph.
An enzyme is a protein that works as a biological catalyst: it increases the rate of a metabolic reaction and is not changed by that reaction. Each enzyme has an active site with a particular shape. A substrate whose shape is complementary to that active site fits into it and an enzyme–substrate complex forms; the reaction happens, the products no longer fit in the same way and leave, and the unchanged enzyme is free to work again. Raising the temperature towards the optimum gives molecules more kinetic energy, so collisions are more frequent and more of them are effective, and the rate rises. Above the optimum, increased vibration disrupts the bonds holding the enzyme's three‑dimensional shape, the active site changes shape, the substrate no longer fits, and the rate falls rapidly — the enzyme has been denatured. pH behaves the same way around an optimum pH that differs from enzyme to enzyme.
Common mistakes with Catalyst
- “Enzymes speed up reactions by giving them energy.” RepairAn enzyme increases the rate of the reaction. It does not supply energy to it. WhyThe definition of a catalyst says nothing about energy, and saying it does contradicts “not changed by the reaction”. A source of energy would be used up.
- “The enzyme is used up in the reaction.” RepairThe enzyme is unchanged and is free to catalyse the same reaction again. WhyThis is half of the catalyst definition. It is also why a small amount of enzyme can convert a large amount of substrate.
Questions students ask about Catalyst
What is an enzyme?
An enzyme is a protein that functions as a biological catalyst: it increases the rate of a metabolic reaction and is not changed by that reaction. Each enzyme has an active site with a particular shape. A substrate whose shape is complementary to that active site fits into it and an enzyme-substrate complex forms; the reaction happens, the products leave because they no longer fit in the same way, and the unchanged enzyme is free to work again.
Is it correct to say that heat kills an enzyme?
No. An enzyme is a protein molecule, not a living organism, so it cannot be killed. High temperature denatures it: the bonds holding its three-dimensional shape are disrupted and the active site changes shape. The same care applies to "the enzyme is used up" and "the enzyme gives the reaction energy": both contradict the definition of a catalyst, which is not changed by the reaction and does not supply energy to it.