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Key concept
As temperature increases, enzyme activity increases because molecules have more kinetic energy and collide more frequently with the active site. Activity peaks at the optimum temperature (about 37°C for human enzymes). Above the optimum, the enzyme denatures: the active site changes shape and the substrate can no longer fit.
Temperature is the most commonly examined factor affecting enzyme activity. You must be able to explain the shape of the rate-temperature graph, define the optimum temperature, and explain denaturation. This appears in Paper 2, Paper 4, and Paper 6 practical questions.
Below the Optimum Temperature
As temperature increases from 0°C to the optimum:
Enzyme and substrate molecules gain kinetic energy.
They move faster and collide more frequently.
More enzyme-substrate complexes form per unit time.
The rate of reaction increases.
Rule of thumb (Q₁₀): For every 10°C rise in temperature, the rate of an enzyme-catalysed reaction approximately doubles: until the optimum is reached.
At and Above the Optimum Temperature
The optimum temperature is the temperature at which the enzyme works fastest. For most human enzymes, this is approximately 37°C (body temperature).
Above the optimum:
The increased thermal energy causes vibrations that break the bonds holding the enzyme's 3D shape.
The active site changes shape: it is no longer complementary to the substrate.
The substrate cannot bind, so enzyme-substrate complexes do not form.
The enzyme is denatured. This is a permanent change.
The rate of reaction decreases rapidly to zero.
The graph shape: Rate increases steeply, peaks at the optimum, then drops sharply. The graph is NOT symmetrical: the drop-off above the optimum is much steeper than the rise below it.
Interpreting Rate-Temperature Graphs
Examiners often give you a graph and ask you to:
Identify the optimum temperature: read the temperature at the peak of the curve.
Explain the shape: describe what happens below, at, and above the optimum.
Compare two enzymes: enzymes from different organisms have different optimum temperatures. Thermophilic bacteria (found in hot springs) have enzymes with optimum temperatures of 70–80°C.
Key exam point: At 0°C, the enzyme is NOT denatured: it is just inactive because molecules have too little kinetic energy for collisions. If you warm it back up, activity resumes. Denaturation is permanent and occurs only at high temperatures.
Try a focused question (3 marks)
The graph shows the effect of temperature on the rate of an enzyme-catalysed reaction. The rate peaks at 40°C and drops to zero by 60°C. Explain why the rate decreases above 40°C. (3 marks)
Show the answer and marking guidance
Above 40°C (the optimum), the enzyme begins to denature.
The bonds holding the 3D shape of the active site break.
The active site changes shape and is no longer complementary to the substrate, so enzyme-substrate complexes cannot form.
Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.
Common mix-ups to avoid
Check these points against your own answer before moving on.
Saying the enzyme "dies" at high temperature: enzymes are not alive; they denature.
Saying enzymes are denatured at low temperatures: they are inactive (too little kinetic energy), not denatured.
Drawing a symmetrical graph: the decline above the optimum is steeper than the rise below it.
Practical connection
Enzyme Rate Investigation With Amylase and Starch
Mix amylase and starch under controlled conditions, sample at timed intervals into iodine and record when starch is no longer detected. Change one factor, repeat each value and calculate rate as 1 ÷ time.