The Carbon Cycle
Revise The Carbon Cycle for GCSE Biology with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. Follow carbon as it moves through photosynthesis, respiration, decomposition and combustion in the carbon cycle.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
The carbon cycle returns carbon from organisms to the atmosphere as carbon dioxide, to be used again by plants in photosynthesis.
Carbon dioxide is removed from the atmosphere by photosynthesis in green plants and algae. The carbon becomes part of the carbohydrates, fats and proteins that make up their bodies, and passes along food chains when animals eat plants.
Carbon is returned to the atmosphere in three ways: respiration by plants, animals and microorganisms; decomposition of dead organisms by microorganisms; and combustion when wood and fossil fuels are burned. Understanding that the carbon is constantly recycled rather than used up is the key idea.
Revision notes
Removing carbon from the atmosphere
Green plants and algae remove carbon dioxide from the atmosphere by photosynthesis.
The carbon is used to make carbohydrates, fats and proteins that form the plants' bodies. Animals obtain this carbon by eating plants or other animals.
Returning carbon to the atmosphere
Respiration by plants, animals and microorganisms releases carbon dioxide.
Decomposition: microorganisms break down dead organisms and waste, releasing carbon dioxide as they respire. Combustion: burning wood and fossil fuels releases carbon dioxide.
Why the cycle matters
The carbon cycle returns carbon to the atmosphere so it can be used again by plants.
Materials are constantly recycled through an ecosystem, providing the building blocks for future organisms. Without decomposition, carbon would remain locked in dead material.
Key points
- Photosynthesis removes carbon dioxide from the atmosphere.
- Carbon becomes part of carbohydrates, fats and proteins.
- Respiration returns carbon dioxide to the atmosphere.
- Decomposition releases carbon dioxide.
- Combustion of fuels releases carbon dioxide.
- Materials are constantly recycled in an ecosystem.
Worked examples
Example 1
Name the process that removes carbon dioxide from the atmosphere. [1 mark]
Model answer
Photosynthesis (1 mark).
Example 2
Describe three ways carbon dioxide is returned to the atmosphere. [3 marks]
Model answer
Respiration by plants, animals and microorganisms (1 mark). Decomposition of dead organisms and waste by microorganisms (1 mark). Combustion when wood and fossil fuels are burned (1 mark).
Example 3
Explain the role of microorganisms in the carbon cycle. [2 marks]
Model answer
Microorganisms break down dead organisms and waste material (1 mark), releasing carbon dioxide back into the atmosphere as they respire, so the carbon can be used again by plants (1 mark).
Common mistakes
Forgetting decomposition.
It is one of the three main routes returning carbon to the atmosphere.
Saying only animals respire.
Plants and microorganisms respire too, and all release carbon dioxide.
Saying carbon is used up.
It is recycled continuously through the ecosystem.
Confusing combustion with respiration.
Combustion is burning; respiration is a reaction inside cells.
Exam tips
- Learn the one removal process and the three return processes.
- Always include decomposition among the return routes.
- Emphasise that carbon is recycled, not consumed.
- Name microorganisms as decomposers.
Key terms
- Carbon cycle
- The recycling of carbon between organisms and the atmosphere.
- Photosynthesis
- The process removing carbon dioxide from the atmosphere.
- Decomposition
- The breakdown of dead material by microorganisms.
- Combustion
- The burning of fuels, releasing carbon dioxide.
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Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.