Photosynthesis limiting factors (6 marks)

A pondweed releases oxygen bubbles in light. Describe how you would investigate the effect of light intensity on the rate of photosynthesis, and explain why CO₂ concentration may also limit the rate.

Mark scheme (6):

• Place pondweed (e.g. Elodea) in sodium hydrogencarbonate solution at fixed temperature (1).
• Vary distance of lamp from beaker; count bubbles per minute / measure O₂ volume (1).
• Use inverse-square law: light intensity ∝ 1/d² (1).
• Repeat and calculate mean for reliability; control temperature with heat shield (1).
• At high light intensity, light is no longer limiting; CO₂ becomes the limiting factor (1).
• Increasing CO₂ would then increase rate until another factor (temp / chlorophyll) limits — Blackman's principle of limiting factors (1).