Describe the complementary active site and substrate
Use a shape model to connect a matching substrate with product formation.
Cross-referenced to the 2028–2029 0653 section using original teaching from the related single-science courses. A 0653-specific objective-by-objective and Core/Supplement review is in progress. Written practice is self-checked and does not demonstrate supervised laboratory competence or award an official Cambridge grade.
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 is a substrate in this context?
What you will learn
- Use a shape model to connect a matching substrate with product formation.
- Apply the explanation to the stated evidence and recognise the limits of the model.
Identify the interaction region
The active site is the region of an enzyme where its substrate interacts. A substrate is the reactant on which that enzyme acts. In the required shape model, the active site has a complementary shape to the substrate, allowing the appropriate fit.
Complementary means the shapes fit together, not that the entire enzyme and substrate have identical shapes. A large enzyme can have a much smaller active-site region. Label the interaction region rather than calling every part of the enzyme an active site.
Connect fit to change
After suitable interaction, the enzyme catalyses conversion of substrate into product or products. Products are released and the enzyme is available again. Core descriptions should name active site, complementary substrate shape and product formation.
A two-dimensional cut-out drawing is a schematic, not an atom-by-atom protein structure. Shape is useful for this explanation, but a mere visual resemblance without the stated reaction evidence does not identify an actual enzyme or substrate.
Sources for this lesson
Pause and explain
Where does the matching substrate interact?
Worked example
A drawing labels a notched enzyme region, a fitting reactant and two released fragments. Describe the action.
Show the worked solution
- The notch represents the active site.
- The substrate fits its complementary shape.
- The substrate is converted and products are released.
Answer A complementary substrate interacts at the active site and is converted into released 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.
Name the enzyme region with a shape complementary to its substrate.
Give me a hint
Identify the interaction site.
Compare my reasoning
- The region where substrate interacts is the active site.
The active site.
Look for these in your work
- Name the site.
- Relate it to substrate fit.
A substrate fits a small enzyme region and is converted into two products. State a Core explanation using all three required ideas.
Give me a hint
Include active site, fit and products.
Compare my reasoning
- Identify the active site.
- Describe complementary substrate shape.
- Describe conversion and product release.
The substrate fits the complementary active site; the enzyme catalyses conversion to products, which are released.
Look for these in your work
- Include all three ideas.
- Keep enzyme and substrate distinct.
A student labels the whole surrounding square in a molecular diagram “active site”. Correct the interpretation.
Give me a hint
Distinguish the drawn border from the protein region.
Compare my reasoning
- The active site is a particular region of the enzyme.
- A surrounding diagram frame is not part of the molecule.
Label the substrate-interacting enzyme region, not the drawing frame.
Look for these in your work
- Identify the biological region.
- Reject a literal reading of the frame.
Common mistakes
- Complementary does not mean identical whole-molecule shape.
- Products are formed from substrate, not by using up the enzyme.
What can you explain now?
Where does the matching substrate interact?
Compare with the explanation
At the active site.
That is the specified enzyme region.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Draw a schematic enzyme, its active site, a complementary substrate and products.
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.