Dilute, equilibrate, mix, time. Say how, every time.
Planning an enzyme investigation followed by a colour or cloudiness change comes up again and again in 9700 Paper 5. Here trypsin clears a cloudy milk suspension, and you time how long it takes to match an end-point standard at five trypsin concentrations made by proportional dilution. 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: 9700/52 May/June 2023, Question 1 (polyphenol oxidase from bananas: dilution and a colorimeter method for substrate concentration) · official past papers
Practise next: 9700/51 May/June 2025, Question 1 (pepsin hydrolysing albumen: the effect of pH, followed with a colorimeter) · official past papers
Practise next: 9700/52 May/June 2025, Question 1 (phosphatase from mung beans: equilibration, proportional dilution and the optimum pH) · official past papers
Examiner reports used: 9700 June 2023 (Paper 52) and June 2025 (Papers 51 and 52).
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What a full-mark diagram shows
The independent variable
At least five values inside the range allowed, made by proportional dilution, with the volumes of stock and distilled water for each (a table is clearest).
Standardised variables, and how
Temperature with a water bath at a stated value and a thermometer; pH with a buffer of stated pH; the same volumes measured with graduated syringes.
The measurement
Enzyme and substrate equilibrated separately, mixed, timing started at once; the time to reach an end-point standard, or absorbance at set intervals from a zeroed colorimeter.
Reliability and safety
Three replicates and a mean for each concentration, a boiled-enzyme control, and a named hazard with its risk and a matching precaution.
Habits the examiner reports keep asking for
Equilibrate the enzyme and the substrate separately before mixing. “Acclimatise” is not accepted.
A 1 : 10 dilution is 1 part plus 9 parts, not 1 part plus 10.
“Use a water-bath” is not enough: give its temperature and say the tubes go in it.
The control is boiled enzyme (or water in place of the enzyme), not 0% substrate.
Write “mean” and “volume”, not “average” and “amount”, and spell out abbreviations.