A researcher investigates how the colour of light affects photosynthesis in cress seedlings. Seedlings under blue light are kept at 24 ∘C24\ ^\circ\text{C}24 ∘C and seedlings under green light are kept at 16 ∘C16\ ^\circ\text{C}16 ∘C. Both groups receive the same light intensity, water and carbon dioxide concentration.
After 10 days, the seedlings under blue light show a greater increase in dry mass. The researcher concludes that blue light alone caused the greater increase.
Why is this conclusion not valid?
Dry mass cannot be used to compare rates of photosynthesis in plants.
Temperature also changed, and temperature affects the enzyme-controlled rate of photosynthesis.
Chlorophyll absorbs green light more strongly than it absorbs blue light.
Carbon dioxide is only used in the dark, so light colour cannot affect dry mass.
117 exam-style questions on OCR GCSE Biology 1.4 Photosynthesis, covering 1.4.1 Photosynthesisers as biomass producers, 1.4.2 Factors affecting the rate of photosynthesis, and 1.4.3 Limiting factors in photosynthesis. Each one has a worked solution and a mark scheme showing where the marks go.