Carbon cycle

Every living thing is built from carbon — it is part of the carbohydrates, proteins, fats and DNA inside cells. But the amount of carbon on Earth is fixed: it is not made or destroyed, it is recycled over and over.

The store of carbon in the air is carbon dioxide gas (CO₂). The carbon cycle describes how carbon atoms move:

• out of the air and into living organisms, then
• back into the air again.

Just four key processes drive the whole cycle, and they are exactly the ones the specification names:

Process	What it does to atmospheric CO₂
Photosynthesis	Removes CO₂ from the air
Respiration	Returns CO₂ to the air
Decomposition	Returns CO₂ to the air (decomposers respire)
Combustion	Returns CO₂ to the air (burning fuels)

Notice that only photosynthesis takes CO₂ out of the air — the other three all put it back. This balance is what keeps the level of CO₂ roughly steady in a natural ecosystem.

Exam tip. A common one-mark question is "name the process that removes carbon dioxide from the air". The answer is photosynthesis. Learn which way each process moves carbon — that single idea answers most carbon-cycle questions.

Two opposite processes do most of the carbon recycling.

Photosynthesis — carbon goes IN. Green plants and algae take in carbon dioxide from the air and use light energy to build glucose (and then other carbon compounds such as starch). This is how carbon from the air becomes part of a living thing. So photosynthesis removes CO₂ from the air.

Respiration — carbon comes OUT. All living organisms respire to release energy from glucose, and respiration produces carbon dioxide as a waste gas. This includes:

• plants (they respire as well as photosynthesise),
• animals (which eat plants or other animals to get their carbon), and
• decomposers (bacteria and fungi).

So respiration returns CO₂ to the air.

How does carbon get into animals? Animals cannot photosynthesise, so they obtain carbon by eating plants or other animals — carbon is passed along the food chain in the compounds that make up their food.

We can sum the two processes up like this:

CO₂ in air  --(photosynthesis)-->  carbon in plants  --(eaten)-->  carbon in animals
carbon in living things  --(respiration)-->  CO₂ back in air

Exam tip. Remember that plants respire too, not just photosynthesise. A frequent mistake is saying only animals respire. State that respiration by plants, animals and decomposers all returns CO₂.

When plants and animals die, their bodies still contain carbon. This carbon is returned to the air by decomposition.

Decomposers — mainly bacteria and fungi — feed on the dead remains and waste (such as droppings) of organisms. As they feed, they break down the dead material. Crucially, decomposers are living things, so they respire, and their respiration releases carbon dioxide back into the air.

So the chain of ideas is:

1. A plant or animal dies (or produces waste).
2. Decomposers (bacteria and fungi) feed on the dead remains.
3. As the decomposers respire, they release CO₂ to the air.

Decomposition is important because, without it, carbon would stay locked inside dead bodies and the cycle would slow down or stop. Decomposers therefore keep carbon moving and also recycle other nutrients in the soil.

Exam tip. Be precise: decomposition releases CO₂ because the decomposers respire. Marks are lost when students just say "the body rots" without linking it to respiration by decomposers.

Some carbon does not return to the air quickly — it gets stored.

Fossil fuels as a carbon store. Millions of years ago, some dead organisms did not fully decompose. Over long periods, under heat and pressure, their carbon-containing remains turned into fossil fuels: coal, oil and natural gas. So a large amount of carbon is stored (locked away) in fossil fuels for millions of years.

Combustion — carbon comes OUT. Combustion means burning. When we burn fossil fuels (in cars, power stations and factories) or burn wood, the carbon they contain reacts with oxygen and is released as carbon dioxide. So combustion returns CO₂ to the air.

Burning fuels does the same job as respiration in the carbon cycle — it puts CO₂ back — but it can release the carbon that was stored for millions of years very quickly. Because humans burn so much fossil fuel, extra CO₂ is being added to the air faster than photosynthesis can remove it, which raises atmospheric CO₂ levels.

Here is the whole carbon cycle as a labelled diagram:

Exam tip. "Combustion" and "burning" mean the same thing — use either. Link combustion clearly to burning fossil fuels (or wood) and to the release of CO₂. Mention that fossil fuels are a store of carbon if the question asks where carbon is locked away.

It helps to picture the journey of one carbon atom.

1. The atom starts as part of a CO₂ molecule in the air.
2. A green plant takes it in by photosynthesis and builds it into glucose / carbon compounds — carbon is now in a living thing.
3. The carbon can then take several routes back to the air:
   • the plant respires and releases it as CO₂, or
   • an animal eats the plant, then the animal respires and releases CO₂, or
   • the plant or animal dies, decomposers feed on it and respire, releasing CO₂ (decomposition), or
   • the remains form fossil fuels over millions of years and are later burned (combustion), releasing CO₂.

Either way, the carbon ends up back in the air as CO₂, ready to be taken in by photosynthesis again — so the cycle continues.

In a nutshell: Photosynthesis takes carbon out of the air; respiration, decomposition and combustion put it back. Fossil fuels are where carbon is stored for a very long time.

Exam tip. For a "describe the carbon cycle" question (often 4–6 marks), make sure you mention all four named processes — photosynthesis, respiration, decomposition and combustion — and say which way each one moves carbon. Including the fossil-fuel store is a reliable extra mark.