BiologyCore18 min read

Enzymes

Biological catalysts, how they work, and what changes their rate

This topic appears in:

9th Class Biology · ch.71st Year Biology · ch.3

Practise the full question bank:

9th Class Biology — MCQs & exam Qs1st Year Biology — MCQs & exam Qs
01

Biological catalysts

Enzymes are proteins that act as biological catalysts — they speed up the chemical reactions of life without being used up. Without them, reactions like digestion and respiration would be far too slow to sustain life at body temperature. Like all catalysts, an enzyme works by lowering the activation energy of its reaction.

Each enzyme is specific: it catalyses only one reaction or type of reaction, because its shape fits only one substrate.

02

The lock and key

An enzyme has a precisely shaped pocket, the active site, into which only the matching substrate fits — like a key in a lock. The substrate binds, forming an enzyme–substrate complex; the reaction happens; the products leave; and the enzyme is free to work again.

This shape-matching is why enzymes are specific — and why anything that changes the shape of the active site stops the enzyme working.

Specificity comes from shape

Only a substrate whose shape is complementary to the active site can bind. Change the enzyme’s shape — with heat or the wrong pH — and the substrate no longer fits.

03

Temperature and pH

As temperature rises, enzyme activity increases — molecules move faster and collide more often — until the optimum, typically around 37 °C in humans. Beyond it, activity falls sharply as the enzyme is denatured: the heat breaks the bonds holding its shape, and the active site is destroyed.

Each enzyme also has an optimum pH. Most work best near neutral, but pepsin in the stomach works best in strong acid (about pH 2). Move too far from the optimum and the enzyme denatures.

04

Concentration and inhibitors

Increasing the substrate concentration speeds the reaction until all active sites are working flat out — then the rate levels off (the enzyme is saturated). Increasing the enzyme concentration speeds it as long as there is spare substrate.

Inhibitors slow enzymes: a competitive inhibitor blocks the active site by resembling the substrate, while a non-competitive inhibitor binds elsewhere and changes the active site’s shape. Many drugs and poisons work as enzyme inhibitors.

05

A worked example

Worked example

The rate of an enzyme reaction rises from 20 °C to 40 °C, then drops to almost zero by 60 °C. Explain this pattern.

  1. From 20 to 40 °C the rate rises.Warmer molecules move faster and collide more often, so more enzyme–substrate complexes form.
  2. The rate peaks near 40 °C — the optimum temperature.This is where activity is greatest.
  3. Above the optimum the rate falls steeply.The heat breaks the bonds holding the enzyme’s shape.
  4. By 60 °C the enzyme is denatured — the active site no longer fits the substrate.A denatured enzyme cannot bind substrate, so the reaction almost stops.

Rate rises to an optimum (~40 °C) as collisions increase, then falls as the enzyme denatures and its active site is destroyed.

06

Enzymes at work in the body

Enzymes are not just a lab curiosity — they run the body. Digestive enzymes break large food molecules into small soluble ones: amylase digests starch to sugars, protease digests protein to amino acids, and lipase digests fats to fatty acids and glycerol. Others build molecules up, copy DNA, and drive respiration. Each works best in the conditions of its own location — which is why stomach protease (pepsin) suits acid while enzymes in the small intestine suit slightly alkaline conditions.

EnzymeSubstrateProducts
Amylasestarchsugars (maltose)
Proteaseproteinamino acids
Lipasefatsfatty acids and glycerol
07

The induced-fit refinement

The simple lock-and-key model treats the active site as a rigid shape. The refined induced-fit model recognises that the active site is slightly flexible: as the substrate binds, the active site moulds a little more snugly around it, which helps the reaction along. Either way, the key point is unchanged — only a substrate whose shape is complementary to the active site can bind, and anything that distorts that shape (heat, extreme pH) stops the enzyme working.

Denaturing is permanent

Cooling a denatured enzyme does NOT restore it. Once heat or extreme pH has broken the bonds holding the active site's shape, the change is permanent — unlike simply slowing an enzyme by cooling it.

Practice questions

6 questions · 22 marks · full working on every one

Try each one on paper first, then open the working. The marks are shown where they are actually awarded, because that is where they are actually lost.

Short questions

4 · 10 marks

Two marks each, in the style of the short-question section of the paper. Answer in two or three lines.

SQ1[3 marks]
Explain, using the lock-and-key model, why an enzyme catalyses only one reaction.
Model answer

An enzyme has a specifically shaped active site. Only a substrate whose shape is complementary to the active site can fit and bind, forming an enzyme–substrate complex. Because only one substrate fits, the enzyme catalyses only that one reaction.

Examiner tip. Specificity = complementary shapes of active site and substrate.

SQ2[2 marks]
Explain why enzyme activity falls to almost zero at high temperatures.
Model answer

High temperature breaks the bonds that hold the enzyme's 3-D shape, denaturing it. The active site changes shape so the substrate can no longer bind, and the reaction almost stops.

Examiner tip. Heat denatures the enzyme, destroying the active site.

SQ3[3 marks]
Name the enzymes that digest carbohydrate, protein and fat, and give the products of each.
Model answer

Amylase digests starch (carbohydrate) to sugars; protease digests protein to amino acids; lipase digests fats to fatty acids and glycerol.

Examiner tip. Match each enzyme to its substrate and product.

SQ4[2 marks]
Explain why cooling slows an enzyme reaction but does not denature the enzyme.
Model answer

Cooling gives particles less kinetic energy, so there are fewer, less energetic collisions between enzyme and substrate — the reaction slows. But it does not break the bonds holding the enzyme's shape, so the active site is unchanged and the enzyme works normally again when warmed.

Examiner tip. Cold slows; heat (and extreme pH) denatures.

Long questions

2 · 12 marks

Theory and numerical together, as they appear in the long-question section.

LQ1[6 marks]
A student measures how the rate of an enzyme-controlled reaction changes with pH.
  1. Describe and explain the effect of increasing pH up to the enzyme's optimum. [2]
  2. Explain what happens beyond the optimum pH. [2]
  3. State two other factors that affect the rate of an enzyme reaction. [2]
Mark scheme
  1. As pH rises toward the optimum, the rate increases[1]
  2. conditions increasingly suit the enzyme's active site[1]
  3. Beyond the optimum the rate falls[1]
  4. the enzyme is denatured — the active site changes shape and substrate no longer binds[1]
  5. Temperature[1]
  6. substrate (or enzyme) concentration[1]

Rate rises to an optimum pH then falls as the enzyme denatures; other factors: temperature and substrate/enzyme concentration.

Examiner tip. Every enzyme has an optimum pH; moving away from it denatures the enzyme.

LQ2[6 marks]
A student measures the volume of gas produced as catalase breaks down hydrogen peroxide at different temperatures.
  1. Describe and explain the results expected from 10 °C up to the optimum. [3]
  2. Describe and explain the results above the optimum. [3]
Mark scheme
  1. As temperature rises to the optimum, the rate (gas produced) increases[1]
  2. molecules have more kinetic energy and collide more often[1]
  3. so more enzyme–substrate complexes form[1]
  4. Above the optimum the rate falls sharply[1]
  5. the heat breaks the bonds holding the enzyme's shape (denaturing)[1]
  6. the active site changes shape so the substrate no longer fits and the reaction stops[1]

Rate rises to an optimum (more successful collisions), then falls as the enzyme denatures and its active site is destroyed.

Examiner tip. Rise = more collisions; fall = denaturing of the active site.