A student is investigating the rate of transpiration in a plant using a potometer. They measure the distance a bubble moves along a capillary tube over a 10-minute period. Describe how the student could ensure the reliability of the measurements taken during this investigation.

OCR A-Level Biology A (H420) — 1 Development of practical skills in biology · Describe · 4 marks · View as Markdown

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

Model answer (4 marks)

1. Repeat the measurement several times under the same conditions for each plant.
2. Record the distance travelled by the bubble in each trial.
3. Identify any out‑lying results that are clearly inconsistent with the others and exclude them.
4. Calculate the mean distance per 10 min from the remaining trials.
5. Keep all controlled variables (light, temperature, humidity, pot size, water supply, plant species, time of day) constant during the whole set of trials.

Examiner tips

  • Use the word ‘repeat’ to show you understand reliability; include a brief note on excluding outliers; mention calculating a mean; keep controlled variables constant.
  • Show the sequence of steps in order; use bullet points or numbered list to match the marking scheme.

Common mistakes

  • Failing to mention exclusion of anomalous data; Not stating that controlled variables must remain constant; Calculating a mean without first checking for outliers.

Mark scheme (4 marks)

  1. Repeat the measurement multiple times for the same conditions/plant.
  2. Calculate a mean/average from the repeated measurements.
  3. Identify and discard/ignore any anomalous results before calculating the mean.
  4. Ensure all relevant controlled variables are kept constant throughout the experiment.

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