Q5 — Photosynthesis investigation (6 marks)

Question: A student investigates the effect of light intensity on photosynthesis in Elodea by counting oxygen bubbles per minute at varying lamp distances. Describe how the student should ensure valid results, and predict the relationship between distance and rate.

Mark scheme (6):

• Control variables: same piece of pondweed / same volume + concentration of NaHCO₃ (CO₂ source) / constant temperature using water bath (any 2 = 2)
• Repeat readings at each distance and calculate a mean (1)
• Light intensity ∝ 1 / distance² (inverse square law) (1)
• As distance decreases, light intensity increases, so rate of photosynthesis increases (1)
• Eventually plateaus when another factor (CO₂ or temperature) becomes the limiting factor (1)