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The Carbon Cycle

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What is carbon?

Carbon is a non-metallic element that is a vital building block

in many biological molecules. Without carbon, life would be

very different or may even be impossible.

What things can you think of that contain carbon compounds?

Carbon is also present in the atmosphere as carbon dioxide.

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What is the carbon cycle?

Carbon is constantly recycled in the environment. There are

four main processes that are involved:

photosynthesis

respiration

decomposition

combustion.

Can you think of any examples of how carbon might be

recycled in these processes?

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What is the carbon cycle?

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Recap

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Plants and algae

Plants take in carbon dioxide from the atmosphere and use it

to make glucose during the process of photosynthesis.

During the day, plants are a carbon sink. This means that

they absorb more carbon dioxide than they give out.

Plants are a carbon source during the night.

They also release a small amount of

carbon dioxide as they respire.

During the day this is normally used

up by photosynthesis but at night,

when they cannot photosynthesize,

it gets released into the atmosphere.

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The role of animals

Animals are unable to make their own food and must feed on

plants, or on animals that have eaten plants.

Some of this carbon is used to

provide glucose. During respiration,

glucose reacts with oxygen to

release carbon dioxide, water

and energy.

This food contains carbon.

Carbon also provides the building blocks for fats, proteins and

carbohydrates that make up the bodies of the animals.

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Recap

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The role of animals

Animals release carbon back into the atmosphere in three

main ways:

Carbon may also be

locked up in the shells

of animals, such as

mussels or oysters.

through respiration, when carbon is released as

carbon dioxide

when their bodies or faeces decompose

when they are eaten by other animals.

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Dead plants and animals, and their

waste, have carbon still locked up

in the carbohydrates, proteins and

fats that they were formed from.

This dead matter provides food

for detritivores, such as

earthworms, and decomposers,

such as bacteria and fungi.

Decomposition

These break down the compounds in the dead matter and

release carbon dioxide through respiration. When the matter

is fully decomposed, all the energy that was originally locked

up during photosynthesis is returned to the environment.

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Over the course of millions of years, some plant and animal

matter can become fossilized. This can happen when organic

matter is buried underneath the ground.

Peat is sometimes used as a fuel as well. This is plant matter

that has been buried and compacted for many years, though

not necessarily for as long as fossil fuels.

Fossil fuels

Over time, pressure can cause the

organic matter to form into coal,

gas or oil. These fossil fuels can

then be extracted and burned (also

known as combusted).

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Combustion

Humans burn fossil fuels and wood in power stations,

vehicles, aeroplane engines and in fires.

Carbon that has been locked up in

fossil fuels for millions of years is

released back into the atmosphere.

oxygencarbon

dioxidecarbon + + water + heat

This releases carbon dioxide, water and heat through the

process of combustion:

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Where is carbon stored?

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How is carbon recycled?

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Glossary

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Multiple-choice quiz