Enzymes: Question 8
Syllabus 5.1
A group of students investigates how the rate of reaction of a human digestive enzyme changes with temperature, testing temperatures from 0°C to 80°C. This enzyme has an optimum temperature of 37°C, the normal temperature inside the human body.
Which description best matches the general shape of the graph obtained when the rate of reaction (y-axis) is plotted against temperature (x-axis) across this range?
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Worked solution
Step 1: Recall how temperature affects enzyme activity below the optimum
As temperature rises towards the optimum (37°C for this human enzyme), reacting molecules gain more kinetic energy, collide more often, and more of these collisions are effective. So the rate of reaction rises as temperature increases up to 37°C. This rules out any description where the rate is flat, falling, or jumps suddenly rather than rising gradually.
Step 2: Recall what happens above the optimum
Once the temperature rises past the optimum, the enzyme starts to become denatured: the shape of its active site is permanently changed, so it becomes less and less able to bind its substrate. This means the rate of reaction falls again as the temperature continues to increase beyond 37°C, eventually reaching a very low rate at high temperatures such as 80°C.
Step 3: Match this pattern to the options
Only one description captures both stages correctly: a rise up to a peak at the optimum temperature (37°C), followed by a fall as the enzyme becomes increasingly denatured at higher temperatures.
- A ignores denaturation completely, describing an ever-increasing rate.
- B describes an unrealistic sudden on/off switch rather than a gradual rise.
- D inverts the whole relationship.
Final answer
The graph rises to a peak at the optimum temperature and then falls as the enzyme is denatured, option C.