Investigate inhibitor concentration and enzyme activity
Inhibitor concentration can affect activity; use controlled concentration series and initial-rate measurements to compare fairly.
AS Level · Cambridge International Biology 9700, 2028–2030. These lessons teach enzyme concepts and experimental reasoning. Written practice is self-checked; it does not demonstrate supervised practical competence or replace teacher-reviewed assessment.
Before you begin
- Recognise that proteins have specific three-dimensional shapes and that reaction rates can be compared using controlled measurements.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is the key independent variable in a test of inhibitor concentration?
What you will learn
- Describe a fair investigation of inhibitor concentration
- Interpret how increasing reversible inhibitor concentration can affect enzyme activity
Change inhibitor concentration systematically
Prepare a range of inhibitor concentrations, including a zero-inhibitor control. Use the same enzyme and substrate concentrations, pH, temperature, total volume, mixing and timing in every trial.
Measure initial rate using the same progress method across trials. Repeat measurements where possible. If adding inhibitor changes total volume, compensate with solvent in the control so concentration differences are not confounded by dilution.
Interpret the pattern with mechanism in mind
Increasing inhibitor concentration can reduce enzyme activity because inhibitor molecules make productive enzyme–substrate interactions less likely. The size of the effect depends on inhibitor type, substrate concentration and other conditions.
A lower rate alone does not identify whether inhibition is competitive or non-competitive. Compare results across substrate concentrations or use other evidence before assigning a mechanism. Reversible inhibition does not mean the inhibitor permanently destroys the enzyme.
Pause and explain
A higher inhibitor concentration lowers initial rate. What is the strongest direct conclusion?
Worked example
Initial rates fall as reversible inhibitor concentration rises while other conditions are fixed. Give a supported conclusion and one limit.
Show the worked solution
- The inhibitor is associated with lower activity under the tested conditions.
- The rate pattern alone does not identify inhibitor type.
Answer Increasing inhibitor concentration reduced initial rate under the conditions tested, consistent with reversible inhibition. This result alone does not establish whether the inhibitor is competitive or non-competitive.
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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.
Outline a fair test of the effect of inhibitor concentration on an enzyme rate.
Give me a hint
Include a control, initial rate and controlled conditions.
Compare my reasoning
- Use a series of inhibitor concentrations plus zero inhibitor.
- Keep enzyme, substrate, pH and temperature constant and measure initial rate.
Test a series of inhibitor concentrations including a zero-inhibitor control. Keep enzyme and substrate concentrations, pH, temperature, volume and timing constant, then compare repeated initial-rate measurements.
Look for these in your work
- Concentration series and control included
- Other conditions and rate measurement controlled
A rate falls from 4.8 to 2.4 arbitrary units min⁻¹ as inhibitor rises. Calculate the percentage decrease.
Give me a hint
Find the change relative to the original rate.
Compare my reasoning
- Decrease = 4.8 − 2.4 = 2.4.
- Percentage decrease = 2.4 ÷ 4.8 × 100 = 50%.
The rate decreased by 2.4 units min⁻¹. Percentage decrease = (2.4 ÷ 4.8) × 100 = 50%.
Look for these in your work
- Change calculated
- Original value used as denominator
A student concludes that the inhibitor is competitive because one inhibitor concentration lowers the rate. Explain the evidence gap.
Give me a hint
Competitive and non-competitive inhibition can both lower rate.
Compare my reasoning
- A lower rate is compatible with more than one mechanism.
- Further comparisons are needed, such as varying substrate concentration.
A lower rate alone does not identify a competitive mechanism. Both competitive and non-competitive inhibitors can reduce activity. Compare effects across substrate concentrations or use other evidence to distinguish them.
Look for these in your work
- Alternative mechanisms acknowledged
- Further evidence identified
Common mistakes
- Claiming an inhibitor type from a single rate comparison
- Changing substrate concentration between inhibitor trials
- Calling reversible inhibition permanent denaturation
What can you explain now?
What is the key independent variable in a test of inhibitor concentration?
Compare with the explanation
Inhibitor concentration
This is the factor being changed.
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Make it stick
How would you test inhibitor concentration fairly, and what does a rate decrease alone not reveal?
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.