The Carbon Cycle: GCSE Biology

Why is carbon important and why does it need to cycle?

Carbon is the backbone of all organic molecules — carbohydrates, lipids, proteins, and DNA all contain carbon. Living organisms need a continuous supply of carbon compounds to build and maintain their cells. However, the total amount of carbon on Earth is fixed; it cannot be created or destroyed, only moved between carbon reservoirs (stores).

The main carbon reservoirs are:

• Atmosphere: carbon dioxide (CO₂) and methane (CH₄)
• Living organisms: carbon in organic molecules (biomass)
• Soil and sediment: dead organic matter, humus
• Oceans: dissolved CO₂, carbonate ions, marine organisms
• Rocks: limestone (CaCO₃), fossil fuels (coal, oil, natural gas)

The carbon cycle describes the pathways by which carbon moves between these reservoirs.

How does photosynthesis remove carbon from the atmosphere?

Photosynthesis in plants, algae, and cyanobacteria is the primary mechanism for removing CO₂ from the atmosphere and fixing it into organic compounds:

CO₂ + H₂O → C₆H₁₂O₆ + O₂ (requires light energy and chlorophyll)

The glucose produced is used to build cellulose (cell walls), starch (storage), fats, proteins, and nucleic acids. Carbon is thus transferred from the atmosphere into biomass — the carbon is said to be fixed or assimilated.

Aquatic plants and phytoplankton in the oceans perform photosynthesis on a massive scale — marine photosynthesis accounts for approximately 50% of all carbon fixation on Earth.

How does respiration return carbon to the atmosphere?

Aerobic respiration in all living organisms (plants, animals, fungi, bacteria) breaks down organic molecules and releases CO₂ back into the atmosphere:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (ATP)

Respiration occurs continuously — day and night — in all living cells. In plants, carbon is fixed by photosynthesis during daylight, but is also released by respiration around the clock. The net uptake of CO₂ by a plant is the difference between photosynthesis and respiration.

What is the role of decomposers in the carbon cycle?

When organisms die, the carbon in their tissues would remain locked up permanently if it were not for decomposers — bacteria and fungi that break down dead organic matter by secreting enzymes and absorbing the products.

Decomposers:

• Break down complex organic molecules (proteins, fats, carbohydrates) into simpler molecules including CO₂ and water.
• Release CO₂ into the atmosphere through their own respiration.
• Return mineral nutrients (as well as carbon) to the soil.

Detritivores such as earthworms, woodlice, and millipedes physically break dead material into smaller pieces, greatly increasing the surface area available for microbial decomposers and speeding up the process.

In waterlogged or acidic conditions, decomposition slows dramatically because the aerobic bacteria and fungi that drive it need oxygen. Dead plant material in waterlogged bogs accumulates as peat — carbon is stored for thousands of years rather than being returned to the atmosphere. This is why peat bogs are important carbon sinks.

How do combustion and fossil fuels fit into the cycle?

Combustion of organic material returns carbon to the atmosphere rapidly:

• Burning wood or biomass: carbon recently fixed by photosynthesis is returned to the atmosphere. This is approximately carbon-neutral if the trees are replanted, because the carbon released was taken up from the atmosphere recently.
• Burning fossil fuels (coal, oil, natural gas): these are the compressed remains of organisms that lived millions of years ago. Their carbon was removed from the active carbon cycle and stored underground. Burning them releases ancient carbon that has been out of the cycle for 300+ million years, making it a net addition to atmospheric CO₂ — not part of the natural cycle.

This distinction is crucial for understanding the enhanced greenhouse effect: burning fossil fuels is not balanced by any process removing the extra CO₂ on a relevant timescale.

Summary table: carbon cycle processes

How have humans disrupted the carbon cycle?

Two major human activities have tipped the balance of the carbon cycle:

1. Burning fossil fuels: releases CO₂ stored for millions of years, increasing atmospheric CO₂ concentration (from ~280 ppm pre-industrial to over 420 ppm today).
2. Deforestation: removing forests reduces the rate of photosynthesis globally (fewer trees to absorb CO₂) and also releases stored carbon when trees are burned or left to decompose.

Both processes increase atmospheric CO₂, strengthening the greenhouse effect and contributing to global climate change. The oceans absorb some of the extra CO₂ (acting as a carbon sink), but this increases ocean acidity (ocean acidification), damaging marine organisms with carbonate shells and skeletons.

Frequently asked questions

What is the difference between the carbon cycle covered at KS3 and at GCSE?

At KS3, the carbon cycle is taught as a simple loop: photosynthesis removes CO₂ from the air; respiration and combustion return it. At GCSE, the cycle is treated in greater depth: you need to understand decomposers and their role, the distinction between biomass burning and fossil fuel burning, carbon sinks (peat, oceans, forests), the disruption caused by human activity, and how all processes interconnect. You may also be asked to analyse data on changing atmospheric CO₂ concentrations and link these to specific human activities.

Why is peat important as a carbon store?

Peat forms in waterlogged, acidic bog environments where dead plant material (particularly sphagnum moss) accumulates faster than it can decompose. Without oxygen, aerobic decomposers cannot function, so the organic carbon remains locked in the peat. A peat bog can store thousands of years' worth of fixed carbon. When peat bogs are drained for agriculture or peat is extracted for gardening compost, the dried peat is exposed to oxygen and decomposes rapidly — or is burned — releasing its stored CO₂ and acting as a significant source of greenhouse gas rather than a carbon sink.

How do oceans act as a carbon sink?

The oceans absorb CO₂ from the atmosphere through two main mechanisms: (1) direct physical dissolution — CO₂ dissolves in cold surface water and is carried to the deep ocean by thermohaline circulation; (2) the biological pump — marine phytoplankton fix CO₂ through photosynthesis; when they die, their organic matter sinks to the ocean floor and is buried in sediment, effectively removing carbon from the active cycle. Currently the oceans absorb approximately 25–30% of the CO₂ humans emit. However, as the oceans warm (from climate change), CO₂ becomes less soluble and the ocean sink weakens.

How does deforestation affect the carbon cycle?

Deforestation affects the carbon cycle in two ways simultaneously: removing a carbon sink (trees that photosynthesise are cut down) and releasing stored carbon (trees are often burned or left to decompose, releasing their carbon as CO₂ or methane). The combined effect is a significant net release of carbon. Globally, land-use change (predominantly deforestation) accounts for approximately 10–15% of anthropogenic CO₂ emissions. Reforestation and preventing deforestation are therefore important strategies for managing atmospheric CO₂, though their effect takes decades to centuries to accumulate.

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