A student is investigating the effect of light intensity on the rate of photosynthesis in a aquatic plant. Explain how the student could ensure the light intensity is the only variable being changed during the investigation.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The student has access to a laboratory with a range of light sources and measuring equipment.
Model answer (4 marks)
1. Use a thermostat or a water bath to keep the temperature constant for all trials.
2. Employ a sealed system or a CO₂ scrubber to maintain constant CO₂ levels.
3. Use a variable light source or several light sources to provide different light intensities.
4. Measure the light intensity with a light meter for each trial.
2. Employ a sealed system or a CO₂ scrubber to maintain constant CO₂ levels.
3. Use a variable light source or several light sources to provide different light intensities.
4. Measure the light intensity with a light meter for each trial.
Examiner tips
- Use precise terminology (thermostat, CO₂ scrubber, light meter).
- Show that each control directly addresses a potential confounding variable.
- Explain why each control is necessary for a valid comparison.
Mark scheme (4 marks)
- The student controls for temperature by using a thermostat or a water bath
- The student controls for carbon dioxide levels by using a sealed system or a CO2 scrubber
- The student uses multiple light sources or a variable light source to achieve different light intensities
- The student measures light intensity using a suitable device such as a light meter
Related
- All OCR A-Level Biology A (H420) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →