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Thousands of chemical reactions take place inside your cells every second — breaking down food, building new molecules, releasing energy. At body temperature most of these reactions, left to themselves, would be far too slow to keep you alive. Cells solve this with enzymes: biological catalysts that speed reactions up enormously. This lesson, part of Topic B1 of OCR Gateway Combined Science, explains what enzymes are, how the lock-and-key model accounts for the way they work, and why each enzyme is so specific. The next lesson then looks at the factors that change how fast enzymes work.
By the end of this lesson you should be able to define an enzyme as a biological catalyst, describe the lock-and-key model using the terms active site and substrate, and explain why enzymes are specific.
This lesson builds AO1 (understanding of what enzymes are and the lock-and-key model) and AO2 (applying that model to explain why a given enzyme acts only on its own substrate).
An enzyme is a biological catalyst — a substance that speeds up the rate of a reaction without being used up in the process. Because it is not used up, a small amount of enzyme can catalyse a reaction over and over again.
Key facts:
Enzymes control both building-up reactions (small molecules joined into larger ones, such as making starch or proteins) and breaking-down reactions (large molecules split into smaller ones, such as digesting starch into sugars). Because almost every reaction inside a cell has its own enzyme, a single cell may contain thousands of different enzymes at once, each doing one precise job.
Exam Tip: The mark-scheme definition of an enzyme is "a biological catalyst that speeds up a reaction without being used up". Always include "without being used up" — it is what separates a catalyst from a reactant.
To see why enzymes matter so much, think about temperature. In a chemistry lab you can speed up a reaction simply by heating it — the higher temperature gives the molecules more energy so they collide harder and more often. A living cell cannot do this, because heating a human cell much above 37 °C would damage its proteins. So organisms cannot rely on heat to make their reactions fast enough for life. Instead they use enzymes, which speed reactions up at the cell's own gentle temperature.
Every chemical reaction needs a minimum amount of energy to get started — the energy to begin breaking and forming bonds. This is called the activation energy. A catalyst works by providing a different route for the reaction that has a lower activation energy, so a larger proportion of collisions are successful and the reaction goes far faster. Crucially, the catalyst itself is not changed: at the end it is exactly as it was at the start, ready to act again. That is why even a tiny quantity of enzyme can process an enormous number of substrate molecules over time.
| Property | Enzymes | Ordinary (inorganic) catalysts |
|---|---|---|
| Made of | Protein | Often metals (e.g. platinum) |
| Made by | Living cells | Manufactured |
| Specificity | Very specific — one substrate | Often work on many reactions |
| Conditions | Gentle (body temperature, mild pH) | Often need high temperatures |
Exam Tip: If a question asks how a catalyst speeds up a reaction, the idea worth stating is that it lowers the activation energy — the minimum energy the reaction needs to happen — without itself being used up.
To understand how an enzyme works, you need two terms:
The substrate fits into the active site rather as a key fits into a lock — which is where this model gets its name.
The diagram shows the three stages:
The lock-and-key model explains enzyme action like this: the active site has a specific shape that is complementary to the shape of its substrate, just as a lock accepts only the key that fits it. When the right substrate enters the active site, the enzyme catalyses the reaction; a molecule of the wrong shape cannot fit, so no reaction happens.
flowchart LR
A["Substrate<br/>(correct shape)"] --> B["Active site<br/>(complementary shape)"]
B --> C["Enzyme-substrate<br/>complex forms"]
C --> D["Reaction occurs"]
D --> E["Products released"]
E --> F["Enzyme unchanged,<br/>reused"]
This is exactly why enzymes can work at the gentle temperatures inside an organism: by holding the substrate in just the right position, the active site makes the reaction far easier, lowering the energy it needs to get started.
Exam Tip: Use the word complementary (not "the same") to describe the fit between substrate and active site — the active site is the complementary shape to the substrate, the way a lock is complementary to its key.
One crucial property follows directly from the lock-and-key model: each enzyme is specific. Because its active site has a particular shape, an enzyme will act on only one type of substrate (or one small group of very similar substrates). A molecule whose shape does not match the active site simply will not fit, so the enzyme cannot catalyse its reaction.
This is why your body needs many different enzymes — a different one for almost every reaction. For example:
| Enzyme | Substrate | Products |
|---|---|---|
| Carbohydrase (e.g. amylase) | Carbohydrate (e.g. starch) | Simple sugars (e.g. glucose) |
| Protease | Protein | Amino acids |
| Lipase | Lipid (fat) | Fatty acids and glycerol |
Notice the naming pattern: many enzymes are named after their substrate plus the ending -ase (carbohydrase acts on carbohydrate; lipase acts on lipids; protease acts on protein). Amylase is one particular carbohydrase, and it breaks down starch specifically. Notice too how these products link back to the previous lesson — proteins are broken down into amino acids, carbohydrates into sugars, and lipids into fatty acids (and glycerol), the biological building blocks a cell needs.
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