Relate enzyme and substrate concentrations to reaction rate
Rate usually increases with enzyme concentration when substrate is available; substrate concentration raises rate until enzyme capacity limits it.
Section 3.2 · Factors that affect enzyme action
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
Why can more enzyme increase reaction rate when substrate is available?
What you will learn
- Explain the effect of enzyme concentration when substrate is not limiting
- Explain why increasing substrate concentration eventually gives a plateau in rate
More enzyme provides more active sites
At fixed substrate concentration and other conditions, increasing enzyme concentration provides more active sites. If sufficient substrate is available, more enzyme–substrate complexes can form per unit time and the rate can increase.
If substrate is scarce, adding enzyme may have little effect because many active sites remain without substrate. State the conditions of a comparison rather than claiming enzyme concentration always has a proportional effect.
Substrate eventually saturates the enzyme
At low substrate concentration, increasing substrate makes productive encounters more likely, so rate rises. At high substrate concentration, most active sites are occupied much of the time; adding further substrate has little effect and rate approaches a maximum.
The plateau reflects enzyme capacity under the particular conditions. It does not mean substrate disappears or that the enzyme has been used up. Use initial rates when comparing substrate concentrations so depletion and product effects are limited.
Pause and explain
Why does rate approach a plateau at high substrate concentration?
Worked example
Rate increases as substrate concentration rises from 1 to 6 mmol dm⁻³, then changes very little from 6 to 10 mmol dm⁻³. Explain the plateau.
Show the worked solution
- At low concentration, more substrate increases productive binding.
- At high concentration, active sites approach saturation.
Answer At lower substrate concentration, more substrate increases the frequency of productive enzyme–substrate complex formation. At higher concentration, most active sites are occupied much of the time, so rate approaches the enzyme’s maximum under these conditions and extra substrate has little effect.
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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.
State why increasing enzyme concentration can increase initial rate when substrate is in excess.
Give me a hint
Relate enzyme amount to active-site number.
Compare my reasoning
- More enzyme provides more active sites.
- More substrate can be converted per unit time.
When substrate is in excess, more enzyme provides more active sites that can form enzyme–substrate complexes, increasing the amount of product formed per unit time.
Look for these in your work
- Active sites linked to enzyme amount
- Rate linked to product per time
Substrate concentration doubles, but measured rate is unchanged at a high concentration. Give the likely explanation.
Give me a hint
Consider whether the enzyme is saturated.
Compare my reasoning
- Most active sites may already be occupied.
- Enzyme concentration limits the maximum rate under these conditions.
At high substrate concentration, most active sites may already be occupied much of the time. The enzyme concentration limits the rate, so doubling substrate has little further effect.
Look for these in your work
- Saturation explained
- Condition-specific limit stated
A student increases enzyme concentration but also dilutes the substrate to half its starting concentration. Why is the result difficult to interpret?
Give me a hint
Two independent rate factors changed.
Compare my reasoning
- More enzyme may increase available active sites.
- Lower substrate may reduce productive binding, so the effects oppose each other.
The result combines two changes: more enzyme can increase active-site availability, while lower substrate concentration can reduce productive binding. Keep substrate concentration constant when testing enzyme concentration.
Look for these in your work
- Both variables identified
- Fair-test correction stated
Common mistakes
- Saying substrate concentration must increase rate without limit
- Explaining a substrate plateau as enzyme denaturation
- Ignoring whether substrate is limiting when discussing enzyme concentration
What can you explain now?
Why can more enzyme increase reaction rate when substrate is available?
Compare with the explanation
It provides more active sites for enzyme–substrate complexes
More available sites can support more reactions per time.
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Make it stick
How do enzyme concentration and substrate concentration affect rate, and why can either comparison have a plateau or limit?
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.