Explain enzyme specificity through complementary fit
Use the active site to explain why a different substrate may not be converted.
Exam scope and assessment notes
Extended only (Supplement) · 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 feature supplies specificity in this required model?
What you will learn
- Use the active site to explain why a different substrate may not be converted.
- Apply the explanation to the stated evidence and recognise the limits of the model.
Link structure and substrate choice
Enzyme specificity concerns which substrate and reaction an enzyme catalyses. In the required model, the substrate’s shape is complementary to the active site, allowing the appropriate fit and complex formation. A differently shaped molecule may fail to fit that active site.
Two substances can have the same element types without fitting the same enzyme. Molecule identity and structure matter. A common colour in a schematic is not evidence of a complementary shape.
Avoid absolute visual claims
Use the supplied shape and reaction evidence. Simplified diagrams can show a fitting substrate A and non-fitting molecule B, but they do not show all atoms or every possible related substrate. The syllabus shape model explains the stated case.
A greater quantity of a non-fitting molecule does not repair the incompatible active-site shape. For the stated simple model, lack of an appropriate fit prevents the required complex from forming and the specified reaction from being catalysed.
Pause and explain
Will adding more of a non-fitting molecule repair the active site in this model?
Worked example
A supplied enzyme has a triangular active-site notch. A triangular substrate fits; a round molecule does not. Explain the different predicted outcomes.
Show the worked solution
- The triangular substrate has complementary fit.
- It can form the stated enzyme-substrate complex.
- The round molecule lacks the fit needed for that reaction.
Answer The fitting substrate can be converted; the non-fitting molecule is not predicted to undergo that enzyme’s stated reaction.
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.
Explain why a substrate can fit one active site but not a differently shaped one.
Give me a hint
Compare the relevant interaction regions.
Compare my reasoning
- Only the first site has the supplied complementary shape.
- The second lacks that fit.
Complementary fit is specific to the supplied active-site structure.
Look for these in your work
- Refer to the active site.
- Avoid whole-enzyme identity claims.
Two reactants both contain C, H and O. Does this ensure the same enzyme catalyses both reactions?
Give me a hint
Element lists do not determine three-dimensional fit.
Compare my reasoning
- Molecular structures can differ.
- Active-site compatibility is not established by shared elements.
No; shared elements do not prove complementary fit or the same catalytic reaction.
Look for these in your work
- Separate composition from structure.
- State the missing fit evidence.
In a stated model, doubling non-fitting substrate B produces no product. Explain why this is consistent with specificity rather than a shortage of B.
Give me a hint
Use the supplied structural incompatibility.
Compare my reasoning
- B does not fit the active site.
- More B does not supply the missing complementary interaction.
The incompatible fit prevents the stated complex and reaction; increased amount alone does not fix it.
Look for these in your work
- Use shape and complex formation.
- Do not claim every high concentration restores activity.
Common mistakes
- Same colour does not prove complementary shape.
- Specificity is not explained by calling the enzyme alive.
What can you explain now?
Will adding more of a non-fitting molecule repair the active site in this model?
Compare with the explanation
No; quantity does not establish complementary fit.
Concentration and structure are different.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Describe a fitting and a non-fitting molecule using active site, complementarity and specificity.
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.