Describe pH effects from enzyme-activity data
Identify the best tested pH without treating pH 7 as a universal optimum.
Section 5.1 · Enzymes
Exam scope and assessment notes
Core and Extended · Biology 0970, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked; human subject-teacher review, supervised practical competence and cumulative assessment remain pending. Cambridge states that IGCSE 0970 has the same subject content as IGCSE 0610, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Recall protein building blocks and the distinction between a molecule and a living cell.
- Use the supplied observations, quantities and units rather than assuming an optimum for every enzyme.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
For the same reaction endpoint, which time indicates fastest activity?
What you will learn
- Identify the best tested pH without treating pH 7 as a universal optimum.
- Apply the explanation to the stated evidence and recognise the limits of the model.
Read pH as a condition
Enzyme activity depends on pH as well as temperature. An enzyme can have an optimum pH at which its measured activity is greatest under stated conditions. Activity often falls on either side of that region.
Different enzymes can work best at different pH values. Neutral pH is not automatically the optimum. Large pH changes may denature an enzyme, but a reduced rate at a non-optimal pH does not by itself prove irreversible denaturation.
Distinguish rate from completion time
For matched fixed endpoints, the shortest completion time indicates the fastest reaction. Supplied times at pH 4, 5, 6, 7 and 8 are 160, 80, 40, 80 and 160 s; pH 6 is fastest tested. A peak in rate corresponds to a trough in completion time.
Control temperature and the volumes and concentrations of enzyme, substrate and buffer. Testing additional pH values near the fastest region can refine the estimate. These sample values describe a supplied teaching model, not every amylase preparation.
Pause and explain
Does pH 7 always give maximum enzyme activity?
Worked example
Using the supplied endpoint times, which pH is fastest tested and how does its endpoint proxy compare with pH 5?
Show the worked solution
- 40 s at pH 6 is the shortest time.
- The proxy is 1/time for the same endpoint.
- (1/40) ÷ (1/80) = 2.
Answer pH 6 is fastest tested; its endpoint-rate proxy is twice the pH 5 proxy.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Matched endpoint times at pH 5, 6 and 7 are 80, 40 and 80 s. Which tested pH is fastest?
Give me a hint
Find the minimum time.
Compare my reasoning
- 40 s is shortest.
- It occurs at pH 6.
pH 6 is fastest tested.
Look for these in your work
- Read time in the correct direction.
- Retain the tested-condition limit.
A different enzyme has measured rates 7, 3 and 1 units/min at pH 2, 5 and 7. Which of those is fastest?
Give me a hint
Do not impose neutral pH.
Compare my reasoning
- 7 units/min is greatest.
- The value occurs at pH 2.
pH 2 is fastest among these supplied conditions.
Look for these in your work
- Use supplied rates.
- Avoid borrowing another enzyme’s optimum.
An enzyme is slower at pH 8 than pH 6. Why is permanent denaturation not yet established?
Give me a hint
Consider recovery and alternative explanations.
Compare my reasoning
- Reduced current activity is measured.
- A matched return-to-suitable-pH test is needed to examine lasting loss.
A slower rate alone does not show irreversible damage; test recovery under matched suitable conditions.
Look for these in your work
- Distinguish activity from irreversible damage.
- Suggest relevant recovery evidence.
Common mistakes
- Shortest endpoint time means fastest, not slowest.
- A non-optimal pH is not automatically irreversible denaturation.
What can you explain now?
Does pH 7 always give maximum enzyme activity?
Compare with the explanation
No; use the particular enzyme and conditions.
Optima can differ.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Sketch corresponding pH-rate and pH-completion-time patterns for the same endpoints.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.