A student is investigating the effect of substrate concentration on the rate of catalase activity using hydrogen peroxide and potato tissue. Describe how the student should carry out this investigation safely and accurately, including how they would control variables to make it a fair test.

OCR A-Level Biology B — Advancing Biology (H422) — 1.2 Practical skills assessed in the practical endorsement · Describe · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Catalase is an enzyme found in potato tissue that breaks down hydrogen peroxide into water and oxygen. A student wants to investigate how changing the concentration of hydrogen peroxide affects the rate of the reaction.

Model answer (5 marks)

1. Use a 10 mL graduated cylinder or syringe to measure the same volume of each H₂O₂ solution.
2. Keep the temperature constant – e.g. perform the experiment in a water bath at 25 °C.
3. Use equal masses or equal surface areas of potato tissue for each trial.
4. Measure the rate of reaction by collecting the volume of O₂ produced in a graduated cylinder over a fixed time (e.g. 30 s) or by timing how long it takes to collect a set volume.
5. Repeat each concentration at least three times and calculate the mean rate.
6. Wear safety goggles and handle glassware carefully to avoid breakage; H₂O₂ is an irritant.

(Each point is worth one mark.)

Examiner tips

  • Use the exact wording from the mark scheme – e.g. "measure the same volume" and "keep the temperature constant". Show the sequence of steps clearly and include the safety precaution. Remember to mention the mean of repeated trials for reliability.

Common mistakes

  • Using different volumes of H₂O₂ for each concentration. Failing to control temperature or potato mass. Not repeating trials or not calculating a mean.

Mark scheme (5 marks)

  1. Measure a fixed/equal volume of hydrogen peroxide at each concentration using a measuring cylinder / syringe
  2. Control a named variable — e.g. keep the temperature constant / use equal masses or surface areas of potato tissue / keep pH constant
  3. Measure the rate of reaction — e.g. collect / measure the volume of oxygen gas produced over a set time / measure time for a set volume of oxygen to be produced
  4. Repeat the experiment at each concentration and calculate a mean to improve reliability
  5. Safety precaution appropriate to the investigation — e.g. wear safety goggles because hydrogen peroxide is an irritant / handle glassware carefully to avoid breakage

Key terms in this question

catalase · substrate concentration

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