The six named processes
Carbon is recycled between three stores — the atmosphere (as ), living organisms and fossil fuels — by six processes. Photosynthesis removes : producers use light energy absorbed by chlorophyll to build glucose, locking carbon into biomass. Respiration returns it, in producers, consumers and decomposers alike. Feeding carries carbon compounds from the organism eaten into the consumer, along the food chain.
Decomposition is fast, fossil-fuel formation is slow
Decomposition is the fast route: decomposers (bacteria and fungi) feed on dead organisms and waste — faeces, fallen leaves, bodies — and their own respiration releases that carbon as within a short time. Fossil-fuel formation happens instead, where remains are buried and oxygen is largely absent: over millions of years, heat and pressure convert them into coal (from ancient plant material) or oil and natural gas (from ancient marine organisms).
Combustion unlocks carbon the cycle had banked
Combustion of fossil fuels breaks down their carbon compounds, releasing in moments carbon that took geological time to bury. Left alone, the cycle is broadly balanced: photosynthesis removes about as much as respiration and decomposition add. Human burning of coal, oil and gas — together with less photosynthesis where forests are cleared — tips that balance and has raised the concentration of in the atmosphere.
Drawn from real examiner reports.
Fossil-fuel formation needs three ideas
Examiners record this as the least-known step, and vague answers such as "dead things turn into fuel" earn nothing. Credit needs three linked ideas: dead plant or animal remains are buried; oxygen is largely absent, so decomposers cannot break them down; and over millions of years heat and pressure convert them into coal, oil or natural gas.
Flagged Jun 2022 P42 Q4ai/aii — "fossil-fuel formation" the least-known step
Feeding arrows point to the eater
On a carbon-cycle diagram the feeding arrow runs from the organism being eaten TO the organism eating it — producer to primary consumer. Candidates reverse it, or confuse it with a respiration arrow, which instead runs from every organism up to the atmosphere. Check each feeding arrow points towards the consumer, the way carbon actually moves as one organism eats another.
Flagged Jun 2022 P42 Q4ai/aii — the feeding arrow direction
Combustion ≠ respiration
Both release , so candidates treat them as equivalent. Respiration returns carbon that photosynthesis removed days or years earlier, leaving the total carbon unchanged. Combustion of fossil fuels releases carbon locked out of the cycle for millions of years, adding it back far faster than it was buried — so it raises atmospheric .
Flagged Jun 2022 P42 Q4ai/aii — combustion raises atmospheric CO2
Decomposition ≠ fossilisation
These are alternative fates for the same dead matter, and oxygen decides which. With oxygen present, decomposers break the remains down and the carbon returns to the air as within a short time. Where oxygen is largely absent — waterlogged mud, sediment, peat — decomposition is prevented, and the carbon is locked away for millions of years instead.
Feeding ≠ photosynthesis
Asked how carbon enters an organism, candidates answer "photosynthesis" whatever the organism. Only a producer takes carbon straight from the atmosphere, building into glucose. A consumer never draws carbon from the air — it obtains carbon compounds from the biomass of whatever it eats. Match the process to the type of organism the question names.
Plants and decomposers respire too
Diagrams are commonly labelled with respiration arrows from the animals only. Every living organism respires — producers respire day and night as well as photosynthesising, and it is the decomposers' own respiration, not decay by itself, that returns the carbon in dead matter to the atmosphere as .
Sort each process: adds or removes
Split the six processes by direction. ONLY photosynthesis removes from the atmosphere; respiration and combustion add it. Feeding moves carbon between organisms, and decomposition releases it through the decomposers' own respiration.
Name the process AND the direction
Diagram questions carry one mark for the process and one for which way carbon moves, so "respiration" alone scores half. Respiration and combustion arrows point up to the atmosphere, photosynthesis arrows point down to the producer, and feeding arrows point to the consumer.
Human impact: name the process, then conclude
For deforestation or fuel-use questions, name the process that changes, then the consequence for atmospheric . Fewer trees means less photosynthesis removing it; more burning means more added. Where both happen, the rise is greater — removal falls as addition climbs.
Carbon cycle — the continual recycling of carbon between the atmosphere (as carbon dioxide, ), living organisms and fossil fuels, through photosynthesis, respiration, feeding, decomposition, formation of fossil fuels and combustion.
| Process | What happens to carbon | Direction of movement |
|---|---|---|
| Photosynthesis | Producers convert and water into glucose (using light energy absorbed by chlorophyll), storing carbon in their biomass | Removes from the atmosphere |
| Respiration | Every living organism breaks down nutrient molecules (glucose) in its cells to release energy | Adds to the atmosphere |
| Feeding | Carbon compounds in an organism's biomass are transferred to the consumer that eats it | Moves carbon between organisms (no direct atmosphere exchange) |
| Decomposition | Decomposers (bacteria, fungi) break down dead organisms/waste; their own respiration releases the carbon | Adds to the atmosphere (relatively quickly) |
| Formation of fossil fuels | Buried dead organisms are not fully decomposed (little/no oxygen); over millions of years, heat and pressure convert them into coal, oil or natural gas | Locks carbon away, out of the atmosphere, for a very long time |
| Combustion | Burning fossil fuels breaks down the stored carbon compounds | Adds to the atmosphere (releasing carbon stored for millions of years, rapidly) |
Photosynthesis (precise definition) — the process by which plants make glucose from carbon dioxide and water, using light energy absorbed by chlorophyll.
Respiration (precise definition) — the chemical reactions in living cells that break down nutrient molecules (glucose) to release energy for metabolism, occurring continuously in every living cell.
Only photosynthesis removes from the atmosphere; respiration and combustion both add to it. Over long timescales the cycle is normally kept roughly balanced by photosynthesis removing about as much as respiration, decomposition and (natural) combustion add back.
Define the carbon cycle, naming the six processes involved.
State the name of the process that (a) removes carbon dioxide from the atmosphere, and (b) is carried out by every living organism to release carbon dioxide back into the atmosphere. (2 marks)
