Season Atmospheric CO2 / ppm Mean rate of photosynthesis / arbitrary units spring 415 60 summer 408 95 autumn 414 45 winter 420 20 Scientists measured the c...

This question is about the carbon cycle, using seasonal photosynthesis and CO₂ data.

(a) The process that removes carbon dioxide from the atmosphere is photosynthesis [1].

(b) As the rate of photosynthesis increases, the concentration of carbon dioxide decreases [1]; this is a negative correlation, for example photosynthesis is highest in summer (95 units) when CO₂ is lowest (408 ppm) [1].

(c) The atmospheric carbon dioxide concentration is highest in winter because in winter the rate of photosynthesis is lowest (only 20 units) [1]; so very little carbon dioxide is removed from the air [1]; meanwhile respiration by living organisms and combustion of fuels continue to release carbon dioxide, so it builds up [1].

(d) Any two processes, other than photosynthesis, that involve carbon dioxide [1 each]: respiration; combustion (burning) of fuels (accept decomposition / decay).

(e) Carbon locked in fossil fuels is returned to the atmosphere when the fossil fuels are burned / combusted [1], releasing carbon dioxide [1].

(f) Large-scale deforestation could increase atmospheric carbon dioxide because there are fewer trees, so less photosynthesis takes place and less CO₂ is removed from the air [1]; the felled trees are often burned, releasing CO₂ [1]; and the decay of dead wood by decomposers also releases CO₂ [1].

(g) A decomposer is an organism (a bacterium or fungus) [1] that breaks down dead organisms and waste material [1], releasing nutrients back into the environment.

Exam tip: the winter CO₂ peak is a balance argument, removal (photosynthesis) falls while release (respiration and combustion) carries on, so the net amount rises.