A student is investigating the effect of light intensity on the rate of photosynthesis using pondweed. Describe how the student should carry out a fair and valid investigation, including how they would measure the rate of photosynthesis and control relevant variables.

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).

Pondweed releases oxygen bubbles during photosynthesis. A student places a lamp at different distances from the pondweed and counts the oxygen bubbles produced in one minute at each distance.

Model answer (5 marks)

1. Use a lamp as the light source and vary its distance from the pondweed to change light intensity.
2. Place the pondweed in a container of water at a constant temperature; keep the temperature the same for all trials (e.g. by using a water bath or by ensuring the surrounding water is at the same temperature).
3. Keep the concentration of CO₂ constant by adding a fixed amount of sodium hydrogen carbonate to the water before each trial.
4. For each lamp distance, count the number of oxygen bubbles produced in one minute – this is the rate of photosynthesis.
5. Repeat each trial at least three times and calculate the mean bubble count to improve reliability.

Examiner tips

  • Use the exact wording ‘count the number of oxygen bubbles per minute’ to match the mark scheme.
  • Show the sequence of steps clearly – lamp distance, temperature control, CO₂ control, counting, repetition.
  • Mention the mean calculation to demonstrate reliability.
  • Avoid extra details such as equipment names unless asked.

Common mistakes

  • Not specifying how to keep temperature constant.
  • Failing to mention CO₂ control or repeating trials.
  • Using a different measure of rate (e.g. time to produce a bubble) instead of bubble count per minute.

Mark scheme (5 marks)

  1. Count the number of oxygen bubbles produced (per minute) as a measure of the rate of photosynthesis
  2. Change the distance of the lamp from the pondweed to change light intensity
  3. Control temperature throughout the investigation (e.g. use a water bath or keep the surrounding water temperature constant)
  4. Control carbon dioxide concentration (e.g. add sodium hydrogen carbonate to the water to keep CO₂ levels constant)
  5. Repeat the experiment at each light intensity and calculate a mean to improve reliability

Key terms in this question

photosynthesis · rate of photosynthesis · light intensity

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