A student is investigating the effect of light intensity on the rate of photosynthesis in pondweed. The student places a lamp at different distances from the pondweed and counts the number of oxygen bubbles produced per minute. The student notices that when the lamp is very close to the pondweed, the rate of bubble production stops increasing even when the lamp is moved closer. Explain why the student must control variables in this investigation, and suggest two variables that must be kept constant and explain why each one must be controlled.

OCR A-Level Biology B — Advancing Biology (H422) — 1.1 Practical skills assessed in a written examination · Explain · 5 marks · View as Markdown

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

A student places a piece of pondweed in a boiling tube of water. A lamp is placed at measured distances from the boiling tube. At each distance, the student counts the number of oxygen bubbles produced per minute as a measure of the rate of photosynthesis.

Model answer (5 marks)

Variables must be controlled so that any change in the rate of photosynthesis can be attributed only to the change in light intensity, making the test fair.
1. Temperature – keep it constant (e.g. by using a water bath or a lamp of fixed wattage). Temperature influences enzyme activity; if it varied it would act as a limiting factor and alter the rate independently of light intensity.
2. Carbon‑dioxide concentration – keep it constant (e.g. by adding sodium hydrogen carbonate to the water). CO₂ is a reactant in photosynthesis; a change in its concentration would change the rate regardless of light intensity.

Examiner tips

  • Use the word ‘controlled’ and explain the effect of each variable on photosynthesis.
  • Show the causal link: variable → enzyme activity/CO₂ availability → rate of photosynthesis.
  • Keep the answer concise and use UK spelling.
  • Mention the method of keeping the variable constant if possible.

Common mistakes

  • Failing to explain why the variable matters, just stating it must be kept constant.
  • Using vague terms like ‘other factors’ instead of specific variables such as temperature or CO₂.
  • Not linking the variable to the rate of photosynthesis (e.g. saying temperature is important but not why).

Mark scheme (5 marks)

  1. Variables must be controlled so that any change in the rate of photosynthesis can be attributed only to the change in light intensity / so that it is a fair test
  2. Temperature must be kept constant (e.g. using a water bath or keeping the lamp wattage the same)
  3. Because temperature affects enzyme activity / the rate of photosynthesis, so if it changed it would act as a limiting factor / alter the rate independently of light intensity
  4. Carbon dioxide concentration must be kept constant (e.g. by adding sodium hydrogen carbonate to the water)
  5. Because carbon dioxide is a reactant in photosynthesis / a limiting factor, so if its concentration changed it would alter the rate independently of light intensity

Key terms in this question

photosynthesis · rate of photosynthesis

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