A student is investigating the effect of light intensity on the rate of photosynthesis in pondweed. Explain how the student should make their investigation reliable and what they should do to control variables.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student places a piece of pondweed in a beaker of water and moves a lamp to different distances from the beaker. They count the number of oxygen bubbles produced per minute as a measure of the rate of photosynthesis.
Model answer (4 marks)
1. Repeat each measurement at every lamp distance and calculate a mean rate of photosynthesis.
2. Keep the temperature of the water constant throughout the experiment.
3. Keep the concentration of CO₂ constant, for example by using the same volume and concentration of sodium hydrogen carbonate solution each time.
4. Ensure that the lamp distance (light intensity) is the only variable that changes; all other factors must remain unchanged.
2. Keep the temperature of the water constant throughout the experiment.
3. Keep the concentration of CO₂ constant, for example by using the same volume and concentration of sodium hydrogen carbonate solution each time.
4. Ensure that the lamp distance (light intensity) is the only variable that changes; all other factors must remain unchanged.
Examiner tips
- Use the word ‘repeat’ and ‘mean’ to show you understand statistical reliability. Mention temperature and CO₂ explicitly – examiners look for these controls. State that the lamp distance is the independent variable and all else is controlled.
- common_mistakes
- :
- Forgetting to mention temperature or CO₂ as controlled variables. Treating the lamp distance as a dependent variable instead of independent. Not stating that the mean should be calculated from repeated trials.
Mark scheme (4 marks)
- Repeat each measurement (at each distance / light intensity) and calculate a mean
- Keep the temperature constant / controlled throughout the investigation
- Keep the concentration of carbon dioxide constant (e.g. by using the same volume / concentration of sodium hydrogen carbonate solution)
- The independent variable (distance of lamp / light intensity) must be the only variable that changes, so all other variables must remain the same
Key terms in this question
Related
- All OCR GCSE Biology A: Gateway Science (J247) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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