Cambridge IGCSE Biology 0610 · Topic 5.1
Enzymes
Every question in this chapter comes back to one relationship: an enzyme only works on a substrate whose shape fits its active site, and almost everything examinable here is really a question about what disturbs that fit. Once you can see temperature and pH as two different ways of interfering with the same active-site shape, this stops being five separate topics and becomes one idea applied in different directions.
Living cells depend on thousands of chemical reactions happening continuously, and most of those reactions would be far too slow to sustain life if left to run on their own. Enzymes solve that problem: they are proteins that act as biological catalysts, speeding up metabolic reactions without being used up or permanently changed by them. Cambridge groups this whole idea under a single syllabus point, 5.1 Enzymes, but it rewards understanding it as a chain — enzyme structure gives an active site its shape, that shape determines which substrate fits, and the fit determines whether, and how fast, a reaction happens.
Core candidates need to know what a catalyst and an enzyme are, how an active site and a substrate relate to one another, and how temperature and pH affect the rate of an enzyme-controlled reaction, including how that effect can be investigated. Supplement candidates go one level deeper into the same relationship: the enzyme–substrate complex, why active-site shape makes enzymes specific to particular substrates, why kinetic energy and effective collisions explain the rising part of a temperature graph, and why shape, fit and denaturation — without any reference to kinetic energy — explain what pH does instead.
What’s in this chapter
| Page | Level | What it covers |
|---|---|---|
| Enzyme action and specificity | Core & Supplement | What the active site actually does, why a substrate has to fit rather than just meet an enzyme, and why one enzyme cannot catalyse every reaction going. |
| Temperature and enzyme activity | Core & Supplement | Why rate rises with temperature right up until it doesn’t — and the two completely different reasons behind each half of that graph. |
| pH and enzyme activity | Core & Supplement | The same shape-and-fit logic as temperature, but without kinetic energy doing any of the explaining — and that difference is examinable on its own. |
| Investigating enzyme activity | Core · Paper 6 | How a temperature or pH investigation is actually built — variables, controls, endpoints — using the amylase–starch–iodine method as the working example. |
Core or Supplement?
There is no purely Supplement-only page in this chapter, because Cambridge builds the Supplement material directly on top of each Core idea rather than adding a separate topic. Every page below opens with the Core content — what you need regardless of tier — and then moves into the Supplement explanation of why that Core pattern happens. If you’re sitting Core only, you can stop at the point each page marks as Supplement without losing anything the Core paper would ask for.
Catalysts, enzymes and metabolic reactions
A catalyst is a substance that increases the rate of a chemical reaction and is not itself changed by that reaction. An enzyme is a protein that functions as a biological catalyst, and enzymes are involved in the metabolic reactions that take place throughout a living organism — chemical reactions occurring as part of the ordinary processes of life. Cells depend on many such reactions happening continuously, and most of them would happen far too slowly on their own to keep an organism functioning. Enzymes exist to close that gap: they raise the rate of a reaction to whatever speed life actually requires.
An enzyme speeds up a reaction without being consumed as a reactant, which is what lets a single enzyme molecule catalyse the same reaction again and again. Saying an enzyme is “not changed by the reaction” is often misread as meaning its structure can never change under any circumstances — that’s not the claim. Conditions such as an excessively high temperature or an unsuitable pH can still alter an enzyme’s shape and denature it; what the definition actually protects is that the enzyme isn’t consumed or transformed by the reaction it’s catalysing itself. That distinction — between what the reaction does to the enzyme and what the surrounding conditions can do to it — is exactly what the temperature and pH pages below are about.
Chapter connection
Reduced to one sentence, this whole chapter says: an enzyme’s activity depends on the shape of its active site, and anything that changes that shape changes how well — or whether — a substrate can fit it. A substrate with a complementary shape binds the active site, and for Supplement candidates that binding is named explicitly as an enzyme–substrate complex, before product is formed and released. Temperature changes the rate first by changing how much kinetic energy the particles have, and only later, past the optimum, by changing the active site’s shape through denaturation. pH changes activity by acting on shape directly, with no kinetic-energy stage involved at all. Once you can trace any exam question back through that one chain — enzyme structure → active-site shape → substrate fit → reaction rate — an unfamiliar enzyme or an unfamiliar graph stops being a problem, because the relationship being tested is always the same one.