Photosynthesis and light intensity — required practical

AQA GCSE Biology

Investigate the effect of light intensity on the rate of photosynthesis in pondweed.

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

Method

  1. Place pondweed in sodium hydrogencarbonate solution (a CO₂ source) in a boiling tube.
  2. Put a lamp a set distance away and count the bubbles of oxygen given off per minute (or measure the volume of gas collected).
  3. Repeat at different distances of the lamp from the pondweed.

Variables

  • Independent variable: Light intensity (varied by moving the lamp — intensity ∝ 1/distance²)
  • Dependent variable: Rate of photosynthesis (bubbles per minute or volume of O₂)
  • Control variables: Temperature, CO₂ concentration, same piece of pondweed, time

Results & analysis

Rate increases as light intensity increases, then levels off when another factor (CO₂ or temperature) becomes limiting. Light intensity is proportional to 1/distance², so halving the distance quadruples the intensity.

Risk assessment

  • The lamp gets hot — allow it to cool; keep water away from the electrics.

Improving accuracy & reliability

  • Use a water bath / heat shield so the lamp does not warm the water (temperature must be controlled).
  • Leave time to acclimatise at each distance; repeat and take a mean.
Exam tip: Control temperature with a beaker of water (heat shield) between the lamp and tube — otherwise the lamp is changing two variables at once.

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