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What are enzymes, and how do they speed up reactions in living things?

Explain that enzymes are biological catalysts and describe their action using the lock and key model

A focused answer to the O-Level Biology outcome on enzymes. What a biological catalyst is, the lock and key model, the meaning of substrate, active site and specificity, and why enzymes matter in the body.

Generated by Claude Opus 4.88 min answer

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  3. Examples in context
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What this dot point is asking

SEAB wants you to explain that enzymes are biological catalysts that speed up reactions in living things without being used up, and to describe how they work using the lock and key model. You should use the terms substrate, active site, enzyme-substrate complex, product and specificity correctly, and explain why each enzyme acts on only one type of substrate.

The answer

What an enzyme is

An enzyme is a biological catalyst: a substance that speeds up the rate of a chemical reaction in a living organism without being used up or changed by the reaction. Because it is not used up, a small amount of enzyme can catalyse many reactions, again and again.

Enzymes are proteins, so their action depends on having a particular shape (this is why temperature and pH affect them, covered in the next dot point).

The lock and key model

Each enzyme has a region called the active site, with a specific shape. The substrate is the substance the enzyme acts on, and it has a shape that is complementary to the active site, fitting into it like a key fitting a lock. The steps are:

  1. The substrate fits into the active site, forming an enzyme-substrate complex.
  2. The enzyme catalyses the reaction: it breaks the substrate down (or joins substrates together) to form the products.
  3. The products, now a different shape, leave the active site.
  4. The enzyme is unchanged and is free to act on another substrate.

Specificity

Each enzyme is specific, meaning it acts on only one type of substrate. This is because only a substrate with the complementary shape can fit the active site. A substrate of the wrong shape will not fit, just as the wrong key will not turn a lock. This is why the body needs many different enzymes, one for each reaction.

Why enzymes matter

Without enzymes, the reactions of life would be far too slow at body temperature. Enzymes allow digestion, respiration, photosynthesis and the building of new molecules to happen quickly enough to keep an organism alive.

Examples in context

Example 1. Amylase in saliva. Amylase in the mouth catalyses the breakdown of starch into maltose. Because amylase is specific, it does not act on protein or fat; separate enzymes (protease and lipase) are needed for those.

Example 2. Catalase in cells. Catalase rapidly breaks down hydrogen peroxide, a toxic by-product of cell reactions, into harmless water and oxygen. A tiny amount of catalase clears a large amount of hydrogen peroxide because the enzyme is reused.

Try this

Q1. Define a biological catalyst. [2 marks]

  • Cue. A substance (enzyme) that speeds up the rate of a chemical reaction in a living organism without being used up or changed.

Q2. Name the part of the enzyme that the substrate fits into. [1 mark]

  • Cue. The active site.

Q3. Explain why an enzyme acts on only one type of substrate. [2 marks]

  • Cue. Only a substrate with a shape complementary to the active site can fit; a substrate of the wrong shape will not fit, so the enzyme is specific.

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original5 marksUsing the lock and key model, explain how an enzyme breaks down its substrate, and explain why one enzyme can only act on one type of substrate.
Show worked answer →

The enzyme has a region called the active site with a specific shape. The substrate has a complementary shape that fits into the active site, like a key fitting a lock. The substrate binds to the active site to form an enzyme-substrate complex. The enzyme then catalyses the reaction, breaking the substrate into products, which leave the active site. The enzyme is unchanged and can be used again.

One enzyme acts on only one substrate because only a substrate with the complementary shape can fit the active site. A substrate of the wrong shape will not fit, so the enzyme is specific.

Markers reward the active site, the complementary fit of the substrate (lock and key), the enzyme-substrate complex, the formation of products, and the explanation of specificity by shape.

Original3 marksState what is meant by a biological catalyst and give two ways enzymes are important in the human body.
Show worked answer →

A biological catalyst is a substance (an enzyme) that speeds up the rate of a chemical reaction in a living organism without being used up or changed by the reaction.

Two ways enzymes are important: they digest food (for example, amylase breaks down starch); they speed up reactions of respiration to release energy; they are needed for the many reactions that build and break down molecules in cells. Any two correct roles are accepted.

Markers reward a correct definition of a biological catalyst (speeds up a reaction, not used up) and two valid roles such as digestion and respiration.

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