Cells and organisation · GCSE Biology
Enzymes
Biological catalysts, the lock and key model, optimum conditions, denaturation and the amylase required practical.
An enzyme is a protein with a uniquely shaped active site. Temperature and pH change collision rate — until they change the shape, and then the enzyme is denatured.
The important bits
What you need to know
- 1
Enzymes are biological catalysts: protein molecules that speed up reactions without being used up.
- 2
The substrate fits the active site. This is the lock and key model. If the shape no longer matches, the enzyme cannot catalyse the reaction.
- 3
Each enzyme has an optimum temperature and an optimum pH. Digestive enzymes have different optima: pepsin in the stomach works in acid conditions; amylase in the mouth does not.
- 4
As temperature rises, particles collide more often so the rate increases, up to the optimum. Above that, the enzyme denatures: the active site changes shape permanently.
- 5
Extreme pH also denatures enzymes by disrupting the bonds that hold the active site’s shape.
- 6
Digestive enzymes break down large insoluble molecules: amylase (starch → sugars), protease (proteins → amino acids), lipase (lipids → fatty acids and glycerol). Bile emulsifies fats and neutralises stomach acid.
- 7
In the amylase practical, iodine stays blue-black while starch is present and turns orange-brown when starch has been broken down. Rate can be compared as 1/time.
Go deeper
Shape is the whole story
A catalyst lowers the activation energy so more collisions are successful. For an enzyme, that only happens if the substrate sits in the active site. Draw the enzyme as a shape with a gap, the substrate as the matching piece, and the products as two smaller pieces leaving. Denaturation is not “the enzyme dies”. The protein’s tertiary structure unravels, the active site no longer fits, and the reaction stops. Cooling an enzyme slows collisions but does not usually denature it, so the rate can recover on warming. Boiling it does not recover. That distinction is a classic six-mark split.
Go deeper
Reading enzyme graphs
A temperature graph climbs, peaks at the optimum, then falls steeply as denaturation takes over. A pH graph is a bell curve around the optimum. If a question gives two enzymes, compare their peaks rather than writing a generic sentence. In the amylase investigation, keep the starch and amylase volumes constant, use a water bath for temperature, and sample into iodine at timed intervals. The time at which the iodine no longer turns blue-black is the end-point. A shorter time means a faster rate. Buffer solutions hold pH still so you can test one independent variable at a time.
See the idea in action
Amylase and starch are kept at 10 °C, 20 °C, 37 °C and 60 °C. Iodine stays blue-black for 180 s, 90 s, 40 s and still blue-black after 300 s. Rate is fastest at 37 °C because that is closest to the enzyme’s optimum: frequent successful collisions and an intact active site. At 60 °C the enzyme is denatured, so starch remains. At 10 °C the enzyme is not denatured; collisions are just infrequent.
Exam technique
Turn knowledge into marks
Never write “the enzyme is killed”. Write “denatured: the active site changes shape so the substrate no longer fits”. For digestion questions, name the enzyme, the substrate and the product, then say where it is made.
Common mistakes
Do not give these marks away
- 01
Saying enzymes are killed by heat, or that cooling denatures them.
- 02
Forgetting that enzymes are proteins, so a change of shape is a change of function.
- 03
Mixing up bile (emulsifies fats, alkaline) with lipase (digests fats).
What happens to an enzyme when it is denatured?
AIt is used up in the reaction
BThe active site changes shape so the substrate no longer fits
CIt starts catalysing a different reaction
DIt becomes a carbohydrate
Show the answer
The active site changes shape so the substrate no longer fits. High temperature or extreme pH permanently alters the protein’s shape. The unique active site is lost, so the enzyme can no longer catalyse its reaction.
Quick questions
If this is the bit you searched
What is the lock and key model?
The substrate has a complementary shape to the enzyme’s active site, so they fit like a key in a lock. If the active site changes shape, the reaction cannot happen.
Why does enzyme activity fall after the optimum temperature?
The enzyme denatures. Bonds holding the active site’s shape break, so the substrate no longer fits, and the rate falls sharply.