Original application practice · 13 indicative marks

Enzymes: rates, plateaus and conclusions

Answer without the guided prompts, then review the science and working.

These are our practice questions and indicative marks, not examination-board questions or an awarded grade. The data are shared with guided practice; a case you have already studied is not an unseen test.

Context and data

An enzyme assay uses the same enzyme amount, pH and temperature (30 °C) at each substrate concentration. Initial rates are measured over a short interval. Each value is a mean of three independent assays; their spread is not supplied.

View the rate graph ↓

Substrate concentration and initial product-formation rate
Substrate concentration / mmol dm⁻³	Initial rate / µmol min⁻¹
0.5	0.12
1	0.22
2	0.38
4	0.50
8	0.52

Initial rate against substrate concentration. Lines connect supplied observations; they are not additional measurements.

Questions and data only. Answer guidance is omitted.

Return to exam conditions

Five exam-condition questions

1. Describe the relationship between substrate concentration and initial rate.

3 indicative marks

2. At 4 mmol dm⁻³, assume the measured rate stays constant for 120 s. Calculate the amount of product formed.

2 indicative marks

3. Suggest why doubling substrate concentration from 4 to 8 mmol dm⁻³ has little effect on initial rate.

2 indicative marks

4. At 8 mmol dm⁻³, predict the initial rate if the amount of active enzyme is halved and rate remains proportional to active enzyme amount. Explain your prediction.

2 indicative marks

5. A student concludes that the enzyme has been permanently damaged at 8 mmol dm⁻³. Evaluate the conclusion and describe a comparison that could test it.

4 indicative marks

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Selected OCR A H420 connections: 2.1.4. Use official papers for examination-board assessment.