The carbon cycle diagram looks intimidating because it has arrows going everywhere. It is not. Carbon comes out of the air by one process, gets passed along by feeding, and goes back to the air by two. Learn those four moves and every arrow on any version of the diagram becomes readable.
📚 What you need to know
Carbon is finite and constantly recycled, so the total in the biosphere stays roughly constant.
Organic stores hold carbon in living things and fossil fuels; inorganic stores hold it in the atmosphere, soils and oceans.
Carbon leaves the atmosphere by photosynthesis and returns by respiration and combustion.
It moves between organisms by feeding, and into the soil by defecation, death and decomposition.
Fossilisation traps carbon underground over millions of years, forming peat, coal, oil and gas.
Carbon sequestration is capturing atmospheric carbon dioxide and storing it, naturally or artificially.
An ecosystem can act as a store, sink or source depending on the balance of its inputs and outputs.
Where the carbon sits
Type of store
Where it is
What form the carbon takes
Organic
Living organisms, crude oil, natural gas, coal
Carbohydrates in organisms; hydrocarbons in fossil fuels
A store is in equilibrium when uptake is balanced by release. The carbon a tree takes in by photosynthesis is balanced, over its adult life, by the carbon it releases through respiration and eventually through decay.
The carbon cycle
Cover the labels and try to name every arrow. If you can do that, you can read any version of this diagram an exam throws at you.
🧩 What each process does
Photosynthesis (transformation) — plants absorb carbon dioxide and convert it into carbohydrates.
Respiration (transformation) — plants, animals and decomposers release carbon dioxide.
Feeding (transfer) — carbon in organic compounds passes along the food chain.
Defecation (transfer) — carbon returns to the soil in waste products.
Death and decomposition (transfer) — decomposers break down dead organisms and release carbon into the soil and air.
Fossilisation — where decomposers are absent, heat and pressure turn buried remains into peat, coal, oil and gas over millions of years.
Combustion — burning fossil fuels combines that locked carbon with oxygen, releasing carbon dioxide.
Transformation or transfer? A transformation changes the carbon into a different chemical form, as photosynthesis and respiration do. A transfer just moves it somewhere else without changing what it is, as feeding and defecation do. Exam questions do ask you to sort them.
Carbon sequestration
Sequestration means capturing carbon dioxide from the atmosphere and storing it in a solid or liquid form. Trees do it naturally every day, locking carbon into wood and into the soil beneath them. Over geological time, fossilisation did it on an enormous scale, which is why fossil fuels exist at all.
Sequestration in one line
atmospheric CO2 → captured → stored in biomass, soil, rock or underground
Store, sink or source: the same forest, three answers
Planting trees creates a sink for a few decades. Once the forest matures it becomes a store — still valuable, but no longer removing extra carbon each year.
Worked examples
WORKED EXAMPLE
Name the process represented by each arrow: (a) atmosphere to plants, (b) plants to animals, (c) fossil fuels to atmosphere.
(a) carbon dioxide taken out of the airphotosynthesis(b) carbon passed along the food chainfeeding(c) burning releases the locked carboncombustionOnly one arrow leaves the atmosphere in a normal diagram, and photosynthesis is it.
WORKED EXAMPLE
Explain why fossil fuels have such a long residence time, and why that matters.
Step 1: how they formed
Organisms died in conditions without decomposers, so their carbon was buried instead of being recycled, and heat and pressure turned it into coal, oil and gas.
Step 2: why the carbon stays
Underground and out of reach of living organisms, the carbon has no route back into the cycle. Its residence time is hundreds of millions of years.
Step 3: why it matters
Burning them opens a route that did not exist naturally, moving that carbon to the atmosphere in seconds.
A very slow sink turned into a very fast sourceThe phrase “a store that had no natural way back” is what earns the explanation mark.
WORKED EXAMPLE
Classify each as a transformation or a transfer: photosynthesis, feeding, respiration, defecation.
Transformations — the carbon changes chemical form
photosynthesis, respiration
Transfers — the carbon just moves
feeding, defecation
Two of eachAsk yourself: has the carbon become a different substance? If yes, it is a transformation.
💡 Exam tip
Practise labelling a blank carbon cycle. Most questions on this topic are exactly that task in disguise.
Say which store to which store when naming a process: “respiration, from plants to the atmosphere”.
Remember decomposers respire too — that arrow from soil back to the atmosphere is easy to forget.
Keep organic and inorganic stores separate in your head. Fossil fuels are organic even though they are underground.
For store, sink or source questions, always compare uptake with release before answering.
⚠ Common mix-up
Thinking only animals respire. Plants and decomposers respire constantly and return carbon to the air.
Calling fossil fuels an inorganic store. They came from living things, so they are organic.
Mixing up combustion and respiration. Both release carbon dioxide, but combustion is burning fuel, not a process in cells.
Assuming a forest is always a sink. A mature forest is usually a store; a burning one is a source.
Forgetting the ocean. It is one of the largest carbon stores on the planet and exchanges gas with the atmosphere constantly.
Saying the carbon cycle is “broken”. It still works — we have simply sped up one flow enormously.
Up next: How Humans Alter the Carbon Cycle — fossil fuels, farming, forestry and what rising carbon dioxide is doing to the oceans.
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