Carbon cycle

Where it happens

The carbon cycle links the atmosphere, living organisms and fossil fuels, keeping carbon moving between its form as atmospheric carbon dioxide and its form in the organic compounds that make up living things.

The cycle has 4 main reservoirs that appear in SPM diagrams: the atmosphere (carbon dioxide gas), the bodies of producers and consumers (organic compounds such as glucose, starch, protein and lipid), dead organic matter in soil and water, and fossil fuels such as coal, petroleum and natural gas locked underground. Aquatic ecosystems also hold carbon as dissolved carbon dioxide, which algae and water plants use in the same way land plants use the gas in air.

Inputs and outputs

• Input to living things: atmospheric carbon dioxide, absorbed by producers through the stomata of leaves or from water by algae.
• Input to photosynthesis: light energy, which drives the fixing of carbon dioxide into glucose in the chloroplast.
• Output of producers: glucose, converted into starch, cellulose, protein and lipid that form plant tissue and later feed consumers.
• Output of respiration: carbon dioxide released by every producer, consumer and decomposer, day and night.
• Output of decomposition: carbon dioxide from the respiration of bacteria and fungi, plus simple carbon compounds returned to the soil.
• Output of combustion: carbon dioxide released when fossil fuels and wood burn, together with heat energy.

The steps

1. Photosynthesis: producers such as green plants and algae remove carbon dioxide from the atmosphere and fix it into glucose, using light energy in the chloroplast.
2. Building tissue: the glucose is converted into starch, cellulose, protein and lipid, so the carbon now forms part of the producer's body.
3. Feeding: carbon compounds pass along food chains as primary consumers eat producers and secondary consumers eat primary consumers.
4. Respiration: all living organisms, both producers and consumers, break down glucose in aerobic respiration and release carbon dioxide back into the atmosphere.
5. Death and waste: when organisms die or release faeces, the carbon compounds in their bodies and waste pass to decomposers.
6. Decomposition: bacteria and fungi break down dead organisms and waste, releasing carbon dioxide through their own respiration.
7. Fossilisation: where dead matter is buried without oxygen and decomposition is incomplete, the carbon is stored for millions of years as coal, petroleum or natural gas.
8. Combustion: burning fossil fuels and wood releases this stored carbon back into the atmosphere as carbon dioxide, closing the cycle.

Why it matters and how it is controlled

The carbon cycle keeps the element carbon, the basic building block of all organic compounds, moving between the atmosphere and living things. Photosynthesis and respiration are roughly balanced processes that keep atmospheric carbon dioxide levels relatively stable over the short term.

Control of the cycle rests on the balance between the one process that removes carbon dioxide, photosynthesis, and the 3 processes that release it: respiration, decomposition and combustion. Photosynthesis only runs in light and needs healthy producers, so deforestation reduces the rate of removal.

Respiration and decomposition run continuously and speed up at warm temperatures. Combustion is the only route that is not part of natural living activity, and its scale is set by human fuel use.

When release outpaces removal, carbon dioxide accumulates in the atmosphere. The syllabus links this imbalance to the greenhouse effect: extra carbon dioxide traps more heat radiated from the Earth's surface, raising average temperatures and driving climate change.

Planting trees, protecting forests and burning less fossil fuel each shift the balance back toward removal.

How it is examined

You may be asked to trace a carbon atom's journey from the atmosphere through a food chain and back, to explain the roles of photosynthesis and respiration in the cycle, or to explain how combustion adds carbon dioxide to the atmosphere.

Diagram-based items usually label arrows with letters and ask you to name the process each represents. The reliable test is direction: any arrow pointing toward the atmosphere box is respiration, decomposition or combustion; the single arrow pointing away from the atmosphere into producers is photosynthesis.

Essay items combine the cycle with the greenhouse effect and ask for 2 or 3 ways human activity increases atmospheric carbon dioxide and 2 or 3 ways it can be reduced. Each named process with its correct effect earns a mark, so write process, direction and effect together in one sentence.

Common misconceptions

Worked exam-style question

Question. Diagram Y shows a simplified carbon cycle. Arrow P points from the atmosphere to a tree, arrow Q points from a deer to the atmosphere, arrow R points from a layer labelled coal to the atmosphere, and arrow S points from dead leaves to a group of fungi.

(a) Name the processes represented by arrows P, Q and R. (b) Explain how the carbon in the dead leaves at S is eventually returned to the atmosphere.

(c) A forest is cleared and burned to build a road. Explain 2 effects of this on the amount of carbon dioxide in the atmosphere.

Model answer. (a) P is photosynthesis, Q is respiration and R is combustion of fossil fuel. (b) The fungi act as decomposers.

They secrete enzymes that break the complex organic compounds in the leaves into simple soluble substances, which they absorb. The fungi then use these substances in aerobic respiration, releasing carbon dioxide into the atmosphere.

(c) Fewer trees means less photosynthesis, so less carbon dioxide is removed from the atmosphere. Burning the trees is combustion, which releases the carbon stored in the wood as carbon dioxide.

Both effects increase atmospheric carbon dioxide, which contributes to the greenhouse effect.

Source:SRC-DSKP-EN

Frequently asked questions