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Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key diagram
The carbon cycle shows how carbon moves between the atmosphere (as CO₂), living organisms, fossil fuels and the soil. Photosynthesis removes CO₂; respiration, combustion and decomposition return it.
The carbon cycle diagram is a high-value question that tests whether students can trace carbon through different processes. Every arrow must be labelled with the correct process, and the direction of carbon movement must be clear.
Drawing the cycle
Draw a central box or cloud labelled CO₂ in the atmosphere. Around it, draw boxes for plants, animals, dead organic matter / detritus and fossil fuels. Connect them with labelled arrows showing the processes that move carbon between them.
Processes that remove CO₂ from the atmosphere
Photosynthesis is the main process that removes carbon dioxide from the atmosphere. Plants absorb CO₂ and convert it into glucose (organic molecules). Draw an arrow from "CO₂ in atmosphere" to "Plants" and label it photosynthesis.
Processes that return CO₂ to the atmosphere
(1) Respiration by plants, animals and decomposers releases CO₂. Draw arrows from plants, animals and decomposers back to the atmosphere, all labelled "respiration". (2) Combustion of fossil fuels (coal, oil, gas) and wood releases CO₂. Draw an arrow from "fossil fuels" to the atmosphere labelled "combustion". (3) Decomposition: when decomposers (bacteria and fungi) break down dead organisms, they respire and release CO₂.
Fossil fuel formation and feeding
Draw an arrow from "dead organisms" to "fossil fuels" labelled fossilisation (incomplete decomposition over millions of years under heat and pressure). Draw an arrow from "plants" to "animals" labelled feeding. Draw an arrow from "plants" and "animals" to "dead organic matter" labelled death / excretion.
Label checklist
Check these labels are present and correctly positioned before finishing your drawing.
✓CO₂ in the atmosphere
✓Plants (producers)
✓Animals (consumers)
✓Dead organic matter / detritus
✓Fossil fuels
✓Decomposers (bacteria and fungi)
✓Photosynthesis (arrow)
✓Respiration (arrows)
✓Combustion (arrow)
✓Feeding (arrow)
✓Death / excretion (arrow)
✓Fossilisation (arrow)
Try a diagram question (4 marks)
Describe two processes in the carbon cycle that return carbon dioxide to the atmosphere. [4]
Show the answer and marking guidance
Respiration: organisms break down glucose, releasing carbon dioxide as a waste product.
Combustion: burning fossil fuels or wood releases the carbon stored in them as carbon dioxide.
Original Cambridge-style practice written for this site.
Common diagram mistakes
Check these against your own drawing before moving on.
Forgetting that plants respire as well as photosynthesise.
Omitting decomposers from the cycle.
Not labelling the arrows with process names: unlabelled arrows earn no marks.
Drawing arrows in the wrong direction (e.g. Photosynthesis arrow going toward the atmosphere).
Confusing the carbon cycle with the nitrogen cycle.