Carbon Cycle

Why carbon must be recycled

Living organisms need carbon to make carbohydrates and other large molecules that form their bodies. As organisms grow, they need a continuing supply of these materials. The carbon cycle makes carbon available for reuse by moving it between living organisms and the non-living environment.

An ecosystem includes biotic components, its living organisms, and abiotic components, such as air, water and soil. Many materials cycle between these components. Carbon, for example, can move from carbon dioxide in the air into a plant, then into an animal, and eventually back into the air. The carbon atoms are transferred between different substances rather than being used up.

Where carbon is stored

A carbon store is a part of the environment in which carbon is held. Carbon occurs in several different chemical forms:

• In the atmosphere as carbon dioxide, $CO_2$.
• Dissolved in ocean water.
• In sea shells and limestone rocks as calcium carbonate, $CaCO_3$.
• In fossil fuels, including coal, oil and natural gas.
• In all living organisms as carbohydrates and other large molecules.

These stores are connected by processes that move carbon between them. Some carbon passes through living organisms relatively quickly, while carbon in rocks and fossil fuels can remain stored for very long periods.

Photosynthesis brings carbon into living organisms

Green plants and algae are photosynthetic producers: they make food using light energy. During photosynthesis, they use carbon dioxide and water to make glucose, a carbohydrate, and release oxygen. Chlorophyll absorbs the light needed for this process.

$$\text{carbon dioxide} + \text{water} \xrightarrow{\text{light}} \text{glucose} + \text{oxygen}$$

For a green plant on land, the carbon in the glucose comes from carbon dioxide in the air. The plant can use this glucose to make other substances needed for growth. Carbon that was in the atmosphere therefore becomes part of the plant's biomass, its living biological material.

Photosynthetic organisms are the main producers of food and biomass for life on Earth. They provide the carbon compounds on which food chains depend.

Feeding transfers carbon along food chains

When an animal eats a plant, it takes in the plant's carbon compounds. Some of that carbon becomes part of the animal's body. If another animal eats it, carbon compounds pass to the next organism in the food chain.

For example, carbon can pass from grass to a rabbit and then to a fox. Each feeding step transfers carbon already present in food; it does not create new carbon. Producers introduce carbon into the food chain through photosynthesis, while consumers obtain it by feeding.

Respiration returns carbon dioxide

Plants, animals and microorganisms all respire. Respiration transfers energy from food for processes in their cells. In aerobic respiration, glucose reacts with oxygen, producing carbon dioxide and water:

$$\text{glucose} + \text{oxygen} \rightarrow \text{carbon dioxide} + \text{water}$$

The carbon in the glucose becomes carbon dioxide, which can return to the atmosphere. Plants therefore have two different roles in the cycle: photosynthesis takes in carbon dioxide, while respiration releases it. Respiration occurs in plants during both day and night.

Decomposers recycle materials from dead organisms

When plants and animals die, their bodies still contain carbon compounds. Microorganisms, including bacteria and fungi, act as decomposers, breaking down dead material and waste products. This breakdown is called decay or decomposition.

Decomposers use some of these carbon compounds in respiration, releasing carbon dioxide back into the atmosphere. Decay also releases mineral ions into the soil, where plants can take them up again. Carbon dioxide and mineral ions have different destinations, but both become available for reuse. Without decomposition, many materials would remain locked in dead organisms rather than returning to the cycle.

Fuels and limestone also return carbon to the air

Combustion is the burning of a fuel. Burning carbon-containing fuels such as wood, coal, oil or natural gas releases carbon dioxide into the atmosphere. Burning wood returns carbon held in plant material; burning fossil fuels releases carbon from long-term stores.

Industrial processing of limestone also releases carbon dioxide. Limestone contains calcium carbonate. When it is strongly heated, for example during cement manufacture, it breaks down into calcium oxide and carbon dioxide:

$$CaCO_3 \rightarrow CaO + CO_2$$

This moves carbon from a rock store into the atmosphere. Human activities can therefore change how much carbon is held in different stores.

Reading and drawing the carbon cycle

In a carbon-cycle diagram, boxes represent stores and arrows represent transfers. Read each arrow from its starting store to its destination, using the process label to explain how the carbon moves.

For example, an arrow from atmospheric carbon dioxide to plants labelled ‘photosynthesis’ means that carbon from the air is incorporated into plant carbohydrates. An arrow from plants back to atmospheric carbon dioxide labelled ‘respiration’ represents a different process, not the reversal of the photosynthesis arrow.

To draw the main biological cycle, place atmospheric carbon dioxide above boxes for plants and animals. Add photosynthesis from the atmosphere to plants, feeding from plants to animals, and respiration from both plants and animals back to the atmosphere. Add dead material receiving carbon from organisms, then decomposers that break it down and return carbon dioxide through respiration. Finally, add fuel and limestone stores with arrows showing how combustion and industrial heating release carbon dioxide.

Trace a complete route, such as atmosphere → plant → animal → atmosphere. This shows why the cycle matters: carbon returned to the air can be taken up again by producers and used to support new growth.

Carbon stores

Carbon is found as:

• Atmospheric carbon dioxide and carbon dissolved in oceans.
• Calcium carbonate in sea shells and limestone.
• Carbon compounds in fossil fuels.
• Carbohydrates and other large molecules in living organisms.

Main transfers

• Photosynthesis: atmospheric carbon dioxide → carbohydrates in producers; provides food and biomass.
• Feeding: carbon compounds → consumers along food chains.
• plants, animals and microorganisms → carbon dioxide in the atmosphere.