7.6 The carbon cycle
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Name the process that removes carbon dioxide from the air.
Mark scheme
photosynthesis
Model answer
Photosynthesis.
Name three processes that return carbon dioxide to the air.
Mark scheme
- respiration (1)
- decomposition / decay (by microorganisms) (1)
- combustion (1)
Model answer
Respiration, decay by decomposers, and combustion.
Describe how carbon in a plant can end up in carbon dioxide in the air, without the plant being burned.
Mark scheme
- plant is eaten by an animal / carbon passes along food chain (1)
- animal respires, releasing carbon dioxide (1)
- or plant / animal dies and decomposers break it down (1)
- decomposers respire, releasing carbon dioxide (1)
Model answer
The plant is eaten by an animal, so the carbon compounds pass into the animal, which releases carbon dioxide when it respires. When the plant or animal dies, decomposers break it down, and they release carbon dioxide as they respire.
Explain why decomposers are important in the carbon cycle.
Mark scheme
- break down dead organisms / waste (1)
- release carbon dioxide (by respiration) / return mineral ions to the soil (1)
Model answer
Decomposers break down dead material and waste. As they respire they return carbon dioxide to the air, and they return mineral ions to the soil.
Explain how burning fossil fuels is increasing the concentration of carbon dioxide in the air.
Mark scheme
- combustion releases carbon (stored for millions of years) as carbon dioxide (1)
- faster than it is removed by photosynthesis (1)
Model answer
Burning fossil fuels releases carbon that was locked away for millions of years as carbon dioxide, faster than photosynthesis can remove it.
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