The big idea: The Earth has a fixed amount of carbon. It is never made or destroyed — it is just recycled between the air, living things, the oceans and the rocks.
The same carbon atom might be CO₂ in the air one year, part of a leaf the next, then part of an animal, then CO₂ again when something respires.
This constant movement of carbon between living and non-living stores is called the carbon cycle.
The carbon cycle is a balance: photosynthesis removes CO₂ from the air, while respiration, decomposition and combustion return it.
Interactive diagram
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- Carbon cycle
- The continuous movement of carbon between the atmosphere, living organisms, the oceans and rocks.
- Photosynthesis
- The process in which producers use light energy to fix CO₂ from the air (or water) into organic carbon such as glucose.
- Respiration
- The process in which living things break down organic carbon for energy, releasing CO₂.
- Decomposition
- The breakdown of dead organisms and waste by decomposers; as they respire, they release the carbon as CO₂.
- Combustion
- The burning of carbon-containing material (wood, fossil fuels) which releases the stored carbon as CO₂.
One in, one out: Only one process removes CO₂ from the air: photosynthesis.
Three processes add CO₂ back: respiration, decomposition and combustion.
Hold on to that 'one out, three in' picture — it is the spine of every carbon-cycle answer.
To recycle carbon you need a way to take it out of the air and ways to put it back.
Photosynthesis takes carbon out: producers fix CO₂ into glucose, and that organic carbon then passes along the food chain when animals feed.
Respiration, decomposition and combustion all put carbon back as CO₂.
| Process | What it does to carbon | Effect on atmospheric CO₂ |
|---|---|---|
| Photosynthesis | Fixes CO₂ from the air into organic carbon (glucose) in producers | REMOVES CO₂ (lowers it) |
| Respiration | Breaks down organic carbon for energy in all living things | ADDS CO₂ (raises it) |
| Decomposition | Decomposers break down dead organisms and waste, respiring as they do | ADDS CO₂ (raises it) |
| Combustion | Burning of wood and fossil fuels releases their stored carbon | ADDS CO₂ (raises it) |
Photosynthesis — the only way carbon enters living things: Producers (plants, algae, some bacteria) use light energy to convert CO₂ into organic carbon (glucose).
This is the only route by which carbon from the air enters living things, so it is where the cycle 'starts'.
Where the carbon comes from depends on the producer: land plants take CO₂ gas from the air, while aquatic plants and algae take dissolved CO₂ and hydrogencarbonate (HCO₃⁻) ions from the water around them.
| Autotroph | Where it gets carbon | Form of the carbon |
|---|---|---|
| Land plant | From the air | Carbon dioxide (CO₂) gas |
| Aquatic plant / algae | From the surrounding water | Dissolved CO₂ and hydrogencarbonate (HCO₃⁻) ions |
Respiration, decomposition and combustion — putting carbon back: Respiration happens in every living thing (producers, consumers and decomposers) and releases CO₂ whenever organic carbon is broken down for energy.
Decomposition is decomposers (bacteria and fungi) feeding on dead matter and waste — as they respire, the carbon locked in the dead material is released as CO₂.
Combustion is burning: wood, peat and fossil fuels all store carbon, and burning them releases that carbon as CO₂.
Carbon sinks and carbon sources: A carbon sink stores carbon and takes it out of the air (forests, peat, limestone, the deep ocean, fossil fuels underground).
A carbon source puts carbon back into the air as CO₂ (respiration, decomposition, combustion).
Carbon becomes locked away in a sink when decomposition is slowed or stopped — in waterlogged, anaerobic, acidic conditions (peat bogs) or when carbon-rich remains are buried and compressed into limestone or fossil fuels over millions of years.
| Feature | Carbon sink | Carbon source |
|---|---|---|
| What it does | Stores carbon and removes it from the air | Releases carbon into the air as CO₂ |
| Example | Forests, peat bogs, limestone rock, the deep ocean, fossil fuels underground | Respiring organisms, decomposing material, burning fossil fuels and forests |
| Key process | Photosynthesis / slow decomposition / sediment formation | Respiration / decomposition / combustion |
| Net effect on CO₂ | Takes CO₂ out | Puts CO₂ in |
Removes CO₂ from the air
- Photosynthesis (the only one)
- Carried out by producers
- Fixes CO₂ into organic carbon (glucose)
- Builds up carbon sinks (forests, peat, etc.)
Adds CO₂ to the air
- Respiration — in all living things
- Decomposition — decomposers respiring
- Combustion — burning wood and fossil fuels
- These are the carbon sources
A memory hook: Photosynthesis Pulls carbon down (out of the air). Respiration, decomposition and combustion Return it.
If the question asks which arrow on a cycle diagram removes CO₂, it is always the photosynthesis arrow.
See how examiners mark answers
Access past paper questions with model answers. Learn exactly what earns marks and what doesn't.
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How this is tested: The headline task is a Paper 2 'Describe' or 'Explain' question (4-7 marks) asking how carbon is recycled within an ecosystem — you score by naming the processes and saying what each does to the carbon.
On Paper 1A a simplified cycle diagram with labelled arrows asks you to identify which arrows are respiration, or which process happens at a given arrow.
A 1-mark Identify can ask how aquatic autotrophs obtain carbon (dissolved CO₂ / HCO₃⁻), or what conditions form a carbon sink such as peat (waterlogged, anaerobic, acidic).
On Paper 3 a data question may give a CO₂-concentration graph and ask you to suggest what might happen to an ecosystem if CO₂ keeps rising.
IB-style question — describe how carbon is recycled
Describe how carbon is recycled within an ecosystem. [4]
How to score all four marks
- Photosynthesis fixes carbon. Producers use photosynthesis to take in CO₂ and fix it into organic carbon (glucose), removing it from the air.
- Feeding passes carbon on. Organic carbon passes from producers to consumers when they feed, so carbon moves along the food chain.
- Respiration returns carbon. All organisms respire, breaking down organic carbon and releasing CO₂ back into the air.
- Decomposition (and combustion) return the rest. When organisms die, decomposers break them down and respire, releasing CO₂; combustion of wood or fossil fuels also returns stored carbon as CO₂. (Award 1 mark per correct process up to 4.)
Final answer
Photosynthesis fixes CO₂ into organic carbon in producers; feeding passes carbon along the food chain; respiration in all organisms releases CO₂; and decomposition (plus combustion) releases the remaining carbon back to the air as CO₂.
✓ Why this scores full marks: Each marking point is a distinct process — photosynthesis, feeding, respiration, decomposition/combustion — and each says what happens to the carbon.
A common slip is to write 'plants take in CO₂ and animals breathe it out' — that is only two of the four scoring ideas. Name the processes and you can't miss the marks.
| Process | What it does to carbon | Effect on atmospheric CO₂ |
|---|---|---|
| Photosynthesis | Fixes CO₂ from the air into organic carbon (glucose) in producers | REMOVES CO₂ (lowers it) |
| Respiration | Breaks down organic carbon for energy in all living things | ADDS CO₂ (raises it) |
| Decomposition | Decomposers break down dead organisms and waste, respiring as they do | ADDS CO₂ (raises it) |
| Combustion | Burning of wood and fossil fuels releases their stored carbon | ADDS CO₂ (raises it) |