Carbon cycle — one process removes CO2, three return it
Carbon dioxide () stays roughly constant because removal balances return. ONE process removes it: photosynthesis, where producers fix carbon into glucose and pass it along food chains by feeding. THREE processes return it: respiration (every living cell, including plants), decomposition (bacteria and fungi respiring as they break down dead matter) and combustion (burning wood or fossil fuels). Always state the DIRECTION of carbon movement, not just the process name.
Word equations and precise definitions
Photosynthesis (removes ): carbon dioxide + water glucose + oxygen, needing light and chlorophyll; balanced as . Respiration (returns ): the chemical release of energy from glucose. Decomposition: microorganisms break down dead organisms and waste and respire. Combustion: a fuel burns in oxygen, returning the stored carbon as .
Fossil fuels lock carbon away for millions of years
Fossil fuels (coal, oil, natural gas) formed from organisms that did NOT fully decompose, compressed over millions of years — a long-term carbon store. In an undisturbed ecosystem the removed by photosynthesis roughly equals that returned by respiration, decomposition and combustion, so atmospheric stays constant. Burning fossil fuels and deforestation return carbon faster than photosynthesis removes it, so it rises, enhancing the greenhouse effect.
Drawn from real examiner reports.
Wrong arrow on a carbon-cycle diagram
An arrow FROM the air INTO a plant is always photosynthesis — the only process that removes . Arrows FROM organisms or fuels INTO the air are respiration, decomposition or combustion. Candidates put a release process on the removal arrow. Fix: decide the direction first (into the air = release; out of the air = photosynthesis), then name the process.
Decay is not just "things rot away"
Decomposition is not credited as "things rot". The mark-scheme point is that microorganisms (decomposers — bacteria and fungi) break down dead material and RESPIRE, releasing . Always name the organisms and state that they respire; a vague "it decays" answer scores zero.
Combustion must release CO2, not just heat
"Burning gives out energy" is not enough. Combustion must be described as a fuel (wood or a fossil fuel) reacting with oxygen and returning its STORED CARBON to the air as . The examinable point is the carbon dioxide released, not the heat given out.
Wrong or unbalanced photosynthesis equation
Examiners penalise a wrong glucose formula or an unbalanced equation. Glucose is — not or . The balanced equation is . Check the same number of C, H and O atoms appears on both sides.
Forgetting that plants respire too
Plants do not only photosynthesise — they respire all the time, day and night, returning to the air. In bright light photosynthesis usually removes more than respiration returns, giving a net uptake, but respiration never stops. "Plants only take in " is wrong.
Fossil fuels are a store, not a process
Coal, oil and natural gas are where carbon is locked away — they are a store, not a carbon-cycle process. The PROCESS that releases their carbon to the air is combustion. Do not answer "fossil fuels" when the question asks for the named process that returns the carbon.
Annotate every arrow "in" or "out" first
Mark each arrow "in" (towards an organism) or "out" (towards the air). The one "in" arrow is photosynthesis; each "out" arrow is respiration, decomposition or combustion, chosen from its source box. This routine turns a confusing diagram into sure marks.
Explain means give the mechanism
"Describe" wants the pattern; "explain" wants the reason. For rising : extra combustion plus less photosynthesis from deforestation carbon returned faster than removed levels rise. Listing activities without the imbalance loses marks.
Name the process, not "the carbon cycle"
When a question asks which process, answer with the specific named process — photosynthesis, respiration, decomposition or combustion. "The carbon cycle" or a bare direction earns nothing; the examiner wants the process that matches that arrow.
4SD0 scope (statement 4.10): the stages of the carbon cycle — the four processes photosynthesis, respiration, decomposition and combustion. (The nitrogen cycle is single-science only and is NOT examined in the Double Award.)
Carbon is recycled between the atmosphere (as carbon dioxide, ), living organisms, dead material and fossil fuels. In a balanced ecosystem the removed from the air equals the returned to it.
| Process | Direction of carbon | What happens (mark-scheme form) |
|---|---|---|
| Photosynthesis | Removes from air | Plants make glucose from carbon dioxide + water using light energy absorbed by chlorophyll |
| Respiration | Returns to air | Chemical release of energy from glucose in living cells (plants, animals AND microorganisms) |
| Decomposition (decay) | Returns to air | Microorganisms (bacteria/fungi) break down dead organisms and waste and respire |
| Combustion (burning) | Returns to air | Burning wood or fossil fuels releases the stored carbon as |
Word equations (learn both exactly):
Balanced symbol equations:
How carbon passes along a food chain: photosynthesis fixes carbon into glucose in producers carbon becomes part of the producer's carbohydrates, fats and proteins animals gain carbon compounds by feeding all organisms return some carbon by respiration dead organisms and waste are broken down by decomposers, returning the rest.
Define photosynthesis and state its role in the carbon cycle.
The carbon cycle keeps the amount of carbon dioxide in the air roughly constant.
(a) Name the ONE process that removes carbon dioxide from the atmosphere. (1) (b) Name TWO processes that return carbon dioxide to the atmosphere. (2)
