Carbon Cycle IGCSE Biology 0610

The Carbon Cycle: Recycling Carbon Through Ecosystems

Biology editorial review is supported by the named review panel. Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.

Key concept

Carbon dioxide is removed from the atmosphere by photosynthesis and returned by respiration, combustion of fossil fuels and decomposition. Carbon passes through food chains as organic molecules and is recycled continuously.

The carbon cycle is one of the most commonly tested diagrams in ecology. Students must be able to identify each process that adds or removes carbon dioxide from the atmosphere and trace carbon through the different stores.

Carbon dioxide removal from the atmosphere

Photosynthesis is the main process that removes carbon dioxide from the atmosphere. Green plants and algae absorb CO₂ and use it, along with water, to produce glucose and other organic molecules. The carbon becomes part of the plant's body (in carbohydrates, fats and proteins).

Carbon is also removed from the atmosphere when it dissolves in oceans to form carbonates, and by marine organisms that build shells and skeletons from calcium carbonate. Over millions of years, these can form limestone rocks, storing carbon in the lithosphere.

Carbon dioxide return to the atmosphere

Three main processes return CO₂ to the atmosphere:

Respiration: All living organisms (plants, animals and microorganisms) respire, releasing CO₂ as a waste product. This is the reverse of photosynthesis in terms of gas exchange.

Combustion: Burning fossil fuels (coal, oil, natural gas) and wood releases the carbon stored in these materials as CO₂. Fossil fuels were formed millions of years ago from dead organisms that were buried and compressed.

Decomposition: When organisms die, decomposers (bacteria and fungi) break down the organic molecules, releasing CO₂ through their respiration.

Carbon through food chains

When a herbivore eats a plant, carbon in the plant's organic molecules is transferred to the herbivore. The herbivore uses some carbon compounds in respiration (releasing CO₂), some for growth (building new body tissues) and some is lost in faeces.

When a carnivore eats the herbivore, carbon transfers to the next trophic level. At every level, respiration returns CO₂ to the atmosphere. When any organism dies, decomposers complete the cycle by breaking down the carbon compounds.

Fossil fuels and the imbalance

Over millions of years, some dead organisms were buried in conditions where decomposition could not occur (e.g. In anaerobic mud). These became fossil fuels (coal from plants, oil and gas from marine organisms). This carbon was effectively locked away from the cycle.

Burning fossil fuels releases this stored carbon back into the atmosphere as CO₂ much faster than it was stored. This creates an imbalance, increasing atmospheric CO₂ concentration and contributing to the enhanced greenhouse effect and climate change.

Try a focused question (3 marks)

Describe how carbon in a leaf can end up as carbon dioxide in the atmosphere.

Show the answer and marking guidance

Carbon is in organic molecules (e.g. Glucose) in the leaf.
The plant may respire these molecules, releasing CO2.
Alternatively, an animal eats the leaf, and the carbon is released as CO2 when the animal respires, or when the animal dies and decomposers break down the body and respire.

Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.

Common mix-ups to avoid

Check these points against your own answer before moving on.

Saying only animals respire (plants also respire, continuously).
Forgetting combustion as a process that returns carbon to the atmosphere.
Drawing the carbon cycle with arrows pointing in the wrong direction.
Confusing the carbon cycle with the nitrogen cycle.