5 original questions · Application and evidence

Enzymes: rates, plateaus and conclusions

Use the context and data to draft a response, then compare your reasoning with an explanatory example.

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.

Original illustrative data. Use the stated assumptions and units; these are not examination-board questions.

This worksheet includes questions and data, with space for your responses. Answers are omitted.

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Five questions

1. Describe the change in initial rate as substrate concentration rises. Use the data to identify where the increase becomes small.

2. In a separate interval at 4 mmol dm⁻³, assume the rate remains 0.50 µmol min⁻¹ for 120 seconds. Calculate the product formed.

3. Explain one plausible reason why increasing substrate from 4 to 8 mmol dm⁻³ has little effect on initial rate.

4. At 8 mmol dm⁻³, the enzyme amount is approximately halved while other conditions remain comparable. Suggest the initial rate expected if rate is proportional to active enzyme amount here.