Dilute it, equilibrate it, collect every bubble. Then find the mean for each concentration.
Planning an enzyme investigation is a favourite 9700 Paper 5 Question 1. Here catalase in a potato extract breaks down hydrogen peroxide at five concentrations made by proportional dilution, at 30.0 °C and pH 7.0, and the oxygen collected in 2.0 minutes is read on a 100 cm³ gas syringe. Watch the set-up built step by step, then build it yourself and check it against the points the examiners look for.
Past-paper referenceOriginal experiment, wording and figures, written in the style of Cambridge AS & A Level Biology 9700 Paper 5, Question 1. It is not copied from a past paper. The “what the examiners said” notes are our paraphrase of the Principal Examiner Reports for Teachers.
Practise next: 9700/51 May/June 2023, Question 1 (catalase in carrot and yeast, gas syringe, proportional dilution, evaluating Vmax) · official past papers
Practise next: 9700/52 May/June 2023, Question 1 (planning an enzyme investigation over five substrate concentrations) · official past papers
Practise next: 9700/52 May/June 2025, Question 1 (equilibrating enzyme and substrate separately, dilution table, optimum pH) · official past papers
Examiner reports used: 9700 June 2023 (Papers 51 and 52), June 2025 (Papers 51 and 52) and November 2023 (Paper 51), with points from the June 2025 Paper 32 and November 2024 Paper 33 reports and the June 2023 Paper 51 and 52 mark schemes.
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What a full-mark diagram shows
Concentrations
At least five values of the independent variable, made by proportional dilution of a stock with distilled water, measured with graduated syringes.
Standardised variables
Each one with how it is kept constant: pH 7.0 buffer, the same volumes, a thermostatic water bath at a stated temperature, the same extract, swirled.
Collect and measure
A sealed flask with a bung and delivery tube to a gas syringe; the extract injected and the stopwatch started together; the volume collected in a set time.
Repeats, control, risk
At least three runs and a mean for each concentration, boiled extract as the control, and a hazard with its risk and precaution.
Habits the examiner reports keep asking for
Say the volume of gas collected, not the gas produced, when you name the dependent variable.
Equilibrate the enzyme and the substrate separately before mixing, and use the word equilibrate, not acclimatise.
Calculate a mean for each concentration, from at least three repeats. “Average” is not accepted for mean.
The control is boiled enzyme (or water in its place), not 0 % substrate.
Vmax needs a plateau. If the curve is still rising, extend the range of concentrations.