Track substrate products and enzyme reuse
Keep the catalytic protein separate from the molecules converted in the reaction.
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
What happens to the catalyst overall in a completed reaction?
What you will learn
- Keep the catalytic protein separate from the molecules converted in the reaction.
- Apply the explanation to the stated evidence and recognise the limits of the model.
Follow the same enzyme
In a simple breakdown model, one substrate molecule binds at the active site and is converted into two product molecules. Once those products leave, the same enzyme can take part in another suitable reaction cycle. It is unchanged overall by its catalytic reaction.
The products contain material from the substrate. The enzyme is not split into the products. Not every enzyme reaction is a breakdown reaction: some help build molecules, so use the reaction model specified in the question.
Count only what the model supplies
If a stated model releases two products per completed cycle, four completed cycles release eight products. This counts completed events; it does not establish a real enzyme’s speed or the number of cycles per second.
An enzyme is reusable in suitable conditions, but it can become inactive or denatured, and substrate can run out. “Not consumed by the reaction” does not mean an enzyme functions forever in every environment.
Pause and explain
Does the breakdown drawing apply to every enzyme reaction?
Worked example
In the supplied breakdown model, one enzyme completes four cycles, each converting one substrate into two products. Count substrates and products.
Show the worked solution
- Four cycles convert four substrates.
- Two products per cycle gives 4 × 2 = 8.
- The enzyme is unchanged overall and remains distinct.
Answer Four substrates converted and eight products formed; the enzyme is not one of the products.
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.
A supplied cycle converts one substrate into two products. How many products follow three completed cycles?
Give me a hint
Multiply products per cycle.
Compare my reasoning
- 3 × 2 = 6.
Six products.
Look for these in your work
- Use the stated two-product model.
- Count completed cycles.
Eight completed cycles each convert one substrate into two products. Count both kinds of changed molecule.
Give me a hint
Keep reactant and product totals separate.
Compare my reasoning
- Eight substrates are converted.
- 8 × 2 = 16 products.
Eight substrates converted; 16 products formed.
Look for these in your work
- State both totals.
- Do not count enzyme as product.
Why can the phrase “enzymes are reusable” coexist with loss of activity after severe heating?
Give me a hint
Separate catalytic consumption from damage.
Compare my reasoning
- The catalytic reaction does not consume the enzyme overall.
- Severe heating can alter its structure independently of that reaction.
Reuse describes suitable catalytic cycles; heating can denature the protein and prevent further activity.
Look for these in your work
- Distinguish reaction from damage.
- Avoid an indestructible-enzyme claim.
Common mistakes
- Count completed cycles, not an invented time rate.
- Reusable does not mean indestructible.
What can you explain now?
Does the breakdown drawing apply to every enzyme reaction?
Compare with the explanation
No; use the stated reaction model.
Some enzyme reactions build molecules.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Trace one enzyme through two stated reaction cycles without turning it into a product.
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.