The Carbon Cycle: KS3 Biology Guide to How Carbon Moves Through Ecosystems

The carbon cycle is the continuous movement of carbon atoms between the atmosphere, living organisms, soil, oceans, and rocks. Carbon is essential to all life because it forms the backbone of organic molecules — carbohydrates, proteins, fats, and DNA. Understanding the cycle shows why maintaining a balance of carbon in the atmosphere matters, and what happens when that balance is disturbed.

Why carbon matters in living systems

Carbon is the fourth most abundant element in the universe and the defining element of all organic chemistry. In the atmosphere, carbon is mostly present as carbon dioxide (CO₂). In living organisms it forms the structures of:

Carbohydrates (sugars, starch, cellulose)

Proteins (enzymes, muscle, haemoglobin)

Lipids (fats, cell membranes)

DNA and RNA

When organisms die, decompose, or are burned, that carbon is released back into the atmosphere or oceans, completing the cycle.

The main processes in the carbon cycle

1. Photosynthesis — carbon in

Plants, algae, and some bacteria remove CO₂ from the atmosphere and convert it into organic molecules using energy from sunlight:

Carbon dioxide + water → glucose + oxygen

CO₂ + H₂O → C₆H₁₂O₆ + O₂ (simplified)

The carbon atoms in CO₂ are built into glucose and then into all the other organic compounds in the plant body. This is the primary route by which inorganic carbon becomes biological carbon.

2. Respiration — carbon out

All living organisms (plants, animals, bacteria, fungi) release CO₂ back into the atmosphere through aerobic respiration:

Glucose + oxygen → carbon dioxide + water + energy

C₆H₁₂O₆ + O₂ → CO₂ + H₂O

Plants photosynthesise during the day but respire all the time. Animals only respire (they cannot photosynthesise). The carbon released by respiration returns to the atmospheric CO₂ pool.

3. Feeding — carbon transfer between organisms

When animals eat plants, or when predators eat prey, carbon-containing molecules pass along the food chain. The carbon is used to build the consumer's own body or respired for energy, releasing CO₂.

4. Decomposition — carbon recycled from dead material

When organisms die, decomposers (bacteria and fungi) break down their bodies. This process — decomposition — releases CO₂ through the decomposers' own respiration, returning the carbon to the atmosphere or soil. In anaerobic (oxygen-poor) conditions such as waterlogged soil, decomposition is very slow, and carbon accumulates in the organic material (eventually forming coal and peat over millions of years).

5. Combustion — carbon from fossil fuels

When fossil fuels (coal, oil, natural gas) formed millions of years ago from the remains of ancient organisms, carbon was locked away underground. Combustion (burning) rapidly releases this stored carbon as CO₂:

Fuel + oxygen → carbon dioxide + water

Burning wood also releases carbon that trees had fixed from the atmosphere. The key difference with fossil fuels is that the carbon has been out of circulation for millions of years, so burning them adds carbon to the modern cycle that was not previously part of it.

A summary table of carbon cycle processes

Process | Direction | Organisms involved | Product released
Photosynthesis | CO₂ → organic carbon | Plants, algae | Oxygen (O₂)
Respiration | Organic carbon → CO₂ | All living things | Carbon dioxide (CO₂)
Decomposition | Organic carbon → CO₂ | Bacteria, fungi | Carbon dioxide (CO₂)
Combustion | Stored carbon → CO₂ | None (chemical reaction) | Carbon dioxide (CO₂)
Fossilisation | Organic carbon → fossil fuels | None (geological process) | —

The carbon cycle and climate change

In a balanced carbon cycle, the carbon fixed by photosynthesis roughly equals the carbon released by respiration, decomposition, and natural combustion (volcanic activity, wildfires). Burning fossil fuels adds extra CO₂ at a rate far greater than natural processes can remove. CO₂ is a greenhouse gas — it traps heat in the Earth's atmosphere. Rising CO₂ levels are the primary driver of current climate change.

Deforestation also disrupts the cycle: removing forests reduces the rate of photosynthesis (less carbon removed from air) and often involves burning the wood (releasing stored carbon). The net effect is a rapid increase in atmospheric CO₂.

How the carbon cycle fits the KS3 national curriculum

The Department for Education's KS3 science programme of study requires pupils to understand "the interdependence of organisms in an ecosystem, including food webs and insect pollinated crops" and how "the carbon cycle shows how matter is recycled." BBC Bitesize KS3 Biology covers photosynthesis, respiration, decomposition, and the carbon cycle as interconnected topics for Year 8 and Year 9 pupils, linking biology to chemistry and environmental science.

Common mistakes

Mistake 1 — Confusing photosynthesis and respiration. Photosynthesis takes in CO₂ and produces oxygen. Respiration does the reverse. A common exam error is reversing the word equations.

Mistake 2 — Saying plants only photosynthesise and do not respire. Plants carry out both processes. In daylight, net photosynthesis exceeds respiration (the plant gains carbon overall). At night, only respiration occurs.

Mistake 3 — Thinking decomposition only happens to animals. Decomposers break down ALL dead organic matter — plant material (leaves, wood) as well as animal bodies and excrement.

Mistake 4 — Stating that combustion "creates" CO₂ from nothing. Combustion re-releases carbon that was already stored in the fuel. Carbon atoms are conserved; they are not created or destroyed in the carbon cycle.

Frequently asked questions

What is the role of decomposers in the carbon cycle?

Decomposers (mainly bacteria and fungi) break down dead organic matter — dead plant material, animal carcasses, and waste products. They do this through their own respiration, releasing CO₂ into the atmosphere and releasing mineral nutrients (such as nitrates and phosphates) back into the soil for plants to reabsorb. Without decomposers, carbon would remain locked up in dead bodies and the cycle would grind to a halt. Decomposers are often called the "recyclers" of an ecosystem.

How does burning fossil fuels affect the carbon cycle?

Fossil fuels (coal, oil, natural gas) are the compressed remains of organisms that lived hundreds of millions of years ago. Over geological time, their carbon was removed from the active carbon cycle. When we burn them, we rapidly release this ancient carbon as CO₂, adding it to today's atmosphere. Natural processes such as photosynthesis and ocean absorption cannot absorb CO₂ at the rate we release it, so the concentration of CO₂ in the atmosphere rises, enhancing the greenhouse effect and warming the planet.

What is the difference between the carbon cycle and the water cycle?

Both are nutrient cycles — they describe how a substance moves continuously between the environment and living things and back again. The carbon cycle tracks carbon atoms moving between the atmosphere (CO₂), living organisms (organic molecules), soil (humus, organic matter), oceans (dissolved CO₂), and geological stores (fossil fuels, limestone). The water cycle tracks water molecules moving between the atmosphere (water vapour), land (rivers, lakes, groundwater), and oceans through evaporation, transpiration, condensation, and precipitation. They are closely linked: photosynthesis, a key step in the carbon cycle, requires water.

Why is photosynthesis important to the carbon cycle?

Photosynthesis is the only major biological process that removes CO₂ from the atmosphere and converts it into solid organic carbon (in plant tissues). Without photosynthesis, the other processes — respiration, decomposition, combustion — would steadily increase atmospheric CO₂ until it became lethal for most life. Photosynthesis also produces the oxygen that most living organisms need for respiration. It is, in effect, the engine that drives the flow of carbon through the living world.

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