Salam Academy
Sign inCreate account
Learn0610Papers 2, 4 · ExtendedEnzymesEnzyme action
Enzymes · Subtopic 1 of 1

Enzymes

Enzymes are proteins that act as biological catalysts: they speed up reactions without being used up. Each enzyme has an active site with a shape complementary to its substrate (lock and key), forming an enzyme–substrate complex. Activity rises with temperature up to an optimum (about 37 °C for human enzymes) as molecules collide more often; above it the enzyme denatures: the active site changes shape and the substrate no longer fits. Each enzyme also has an optimum pH.

Cambridge 5090 Paper 1Cambridge 5090 Paper 2Cambridge 0610 Papers 2, 4Cambridge 0610 Papers 1, 3Edexcel 4BI1 Paper 1BEdexcel 4BI1 Paper 2B≈ 40 minRevision 1 · updated 2026-10-30
Section 1 of 14

Learning objectives

  • Describe enzymes as biological catalysts.
  • Explain enzyme action and specificity with the lock and key model.
  • Explain the effects of temperature and pH, including denaturation.
Pause and think

A high fever above 42 °C can be fatal. Why?

Section 2 of 14

Core idea

Start hereSpeeding up life

Without enzymes, digesting a single meal would take decades. Enzymes speed up reactions millions of times, at body temperature, without being used up.

The big idea

Each enzyme's active site fits one substrate. Temperature and pH change how fast enzymes work; too much heat or the wrong pH changes the active site's shape permanently.

1. How enzymes work

Lock and key

The enzyme is unchanged and can be reused.

2. Temperature and pH

Worked exampleInterpreting an enzyme graph

Amylase: rate 2 units at 20 °C, 6 at 37 °C, 1 at 60 °C.

Goal
Explain the pattern.
Strategy
Below optimum: collisions. Above: denaturation.
  1. 20→37 ∘C: rate↑20\rightarrow37\,^\circ\mathrm{C}:\ \text{rate}\uparrow
    More kinetic energy, more frequent successful collisions.
  2. 37→60 ∘C: rate↓37\rightarrow60\,^\circ\mathrm{C}:\ \text{rate}\downarrow
    Active site changes shape; substrate no longer fits.
AnswerOptimum about 37 °C; denatured at 60 °C.
✓ CheckHuman enzymes work best near body temperature ✓.
Common misconceptionKilled? Enzymes aren't alive, so they can't be killed. Use "denatured".
Pause and think

Why do we store food in a freezer rather than just a fridge for longer periods?

Summary

  • Biological catalysts.
  • Specific active site.
  • Optimum T and pH; denaturation.
Key words
Active site
Region of an enzyme where the substrate binds.
Substrate
Molecule an enzyme acts on.
Denatured
Active site permanently changed shape.
HistoryEmil Fischer. Emil Fischer proposed the lock and key model in 1894 after noticing that enzymes could tell apart sugars that were mirror images of each other.
Fun factHot-spring enzymes. Bacteria living in hot springs have enzymes that work at 70 °C. One of them, Taq polymerase, is used in PCR tests to copy DNA.
Keep reading for free

Create a free account to read the rest of this lesson: key formulae, worked examples, explore the graph, common mistakes, exam tip, guided practice and more. Your answers are checked and your progress is saved.

Create free accountSign in