3.3.1Ecology and the Environment

Carbon Cycle — Edexcel International GCSE Science Double Award (Modular) - Biology Unit 2 Revision Notes & Topical Questions

5 min read 9 parts Lesson 8 of 12

📚 Edexcel International GCSE Science Double Award (Modular) - Biology Unit 2 Hub / Topical Revision

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Part 1 of 7Carbon is the basis of life

Carbon is found in all organic molecules: carbohydrates, proteins, lipids and nucleic acids.

The total amount of carbon on Earth is fixed, so it must be recycled.

2Carbon cycle

In the carbon cycle, carbon moves between the atmosphere (as carbon dioxide), living organisms (in organic compounds), the soil and the ocean, and fossil fuels.

The atmosphere holds only about 0.04 per cent carbon dioxide, so it is constantly exchanged with living things.

Part 2 of 7Processes that remove carbon dioxide

Photosynthesis is the main process that removes carbon dioxide from the air. Plants, algae and some bacteria use light energy to convert carbon dioxide and water into glucose and oxygen:

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

The carbon from carbon dioxide becomes part of the plant's carbohydrates, proteins and fats. When animals eat plants, this carbon passes along food chains as the consumers feed. Carbon dioxide also dissolves in the sea, where it is used by algae and by shell-forming organisms that lock carbon in calcium carbonate.

Part 3 of 7Processes that return carbon dioxide

1Respiration

Respiration all living organisms, plants, animals and microorganisms, release energy from glucose and release carbon dioxide into the air or water.

2Decomposition

Decomposition bacteria and fungi feed on dead plants and animals and on waste, and release carbon dioxide by respiration.

3Combustion

Combustion burning wood, and fossil fuels such as coal, oil and natural gas, releases carbon dioxide into the atmosphere quickly.

Part 4 of 7Fossil fuels and long-term storage

If dead plants and animals do not decompose fully because conditions are not suitable, such as in waterlogged, acidic or oxygen-poor places, the carbon remains in the material.

Fossil fuels and long-term storage

Over millions of years, heat and pressure turn this into fossil fuels: coal from plants, and oil and gas from marine organisms.

Limestone is formed from shells and skeletons of marine organisms.

Burning fossil fuels returns this stored carbon to the atmosphere within a short time, which has led to the increase in atmospheric carbon dioxide and enhanced global warming.

Process	Effect on atmospheric CO₂
Photosynthesis	Decreases
Respiration	Increases
Decomposition	Increases
Combustion	Increases
Ocean absorption	Decreases

Part 5 of 7Reading and drawing the cycle

In a carbon cycle diagram the main stores are the atmosphere, producers, consumers, decomposers, soil, and fossil fuels.

Arrows leaving the atmosphere should be labelled photosynthesis; arrows going back into the atmosphere from plants, animals and decomposers are respiration, while those from fuels and wood are combustion.

Arrows between organisms are feeding, and arrows from dead organisms to decomposers show death and waste.

In a balanced system the input and output of carbon dioxide are equal, so the concentration stays steady.

Human activities such as burning fossil fuels and cutting down forests have disrupted this balance, linking to climate change and to the effect on biodiversity.

Note that the processes in this cycle work in the same way as in earlier topics: photosynthesis is covered in plant biology and respiration is the release of energy from glucose in cells.

Part 6 of 7Worked example: a balanced and an unbalanced system

In a mature forest the carbon dioxide absorbed by photosynthesis during the day is roughly matched over a year by the carbon dioxide released by respiration of the trees, animals and decomposers, so the forest is close to carbon neutral.

Worked example: a balanced and an unbalanced system

If part of the forest is felled and burned, a large store of carbon is released as carbon dioxide by combustion, and the cleared area has fewer plants to absorb it afterwards, so atmospheric carbon dioxide rises.

Burning fossil fuels adds carbon that was locked away for millions of years, which is why it is not in balance with natural uptake.

Data questions may show a graph of the carbon dioxide concentration with a regular zig-zag pattern.

Worked example: a balanced and an unbalanced system

The yearly rise and fall reflects seasonal changes in photosynthesis in the northern hemisphere, which has more land and forests: concentration falls in summer when plants take in more carbon dioxide, and rises in winter.

The overall upward trend over the decades is due mainly to fossil fuel combustion and deforestation.

Carbon in the oceans

The oceans are a major store of carbon.

Carbon dioxide from the air dissolves in surface water, where phytoplankton use it in photosynthesis.

When marine organisms die their remains sink, and the carbon may be buried in sediment.

This slow route is one way in which carbon becomes locked away in limestone and fossil fuels.

Dissolved carbon dioxide also makes the sea more acidic, which can harm corals and shellfish.

Part 7 of 7Exam-style questions

Explain how carbon in the carbon dioxide of the air becomes part of the body of a fox. [4]4 marks
Describe two ways in which human activity has increased the concentration of carbon dioxide in the atmosphere. [2]2 marks

Key terms

Term	Meaning
Carbon cycle	The recycling of carbon between the atmosphere, living things, the soil and fossil fuels.
Photosynthesis	The process by which producers use light energy to make glucose from carbon dioxide and water.
Respiration	The release of energy from glucose in cells, producing carbon dioxide.
Combustion	Burning, which releases carbon dioxide from fuels.
Fossil fuel	A fuel such as coal, oil or gas formed over millions of years from dead organisms.
Decomposition	The breakdown of dead organisms and waste by microorganisms.
Deforestation	The clearing of forest for other uses.

Avoid these mistakes

Saying plants only photosynthesise and do not respire. Plants respire all the time, but in the light the rate of photosynthesis is usually higher.

Saying that the carbon cycle creates carbon. It is recycled; the total stays the same.

Confusing combustion with respiration. Combustion is burning; respiration is a cellular process releasing energy.

Forgetting that decomposers release carbon dioxide through their own respiration.

Last updated: 20 October 2026