The Carbon Cycle

Life on Earth is based on chemicals that contain carbon. The carbon cycle is nature’s way of recycling this, ensuring that life continues.

Carbon cycle.

1. Plants, Animals and the Atmosphere.

Since this is a cycle there is no start or end. But we must start somewhere so let’s begin with an atom of carbon in a molecule of carbon dioxide in the air. This is then taken in by a plant in a garden in London and used in photosynthesis. The carbon dioxide reacts with water and forms a molecule of glucose. Since glucose is a solid, our carbon atom is now part of the plant. At night, when there is no light, the plant uses the glucose molecule for energy. It breaks it back down into water and carbon dioxide. The carbon dioxide is released from the plant and ends up back in the air, where it is blown away by the wind.


A plant in a field in Italy takes in our carbon dioxide molecule and turns it into a glucose molecule. But this time, instead of using the glucose for energy, it links it to lots of other glucose molecules to form cellulose. Cellulose forms the wall of new cells of plants. So the carbon atom is once again part of a plant.

This plant just happens to be in a field of cows. A cow eats the plant and digests it. It uses the cellulose to release enery in the process of respiration. Respiration produces water and carbon dioxide which are breathed out. So once again our carbon atom ends up right back where it started as carbon dioxide – in the air!

Guess what? It is blown around for a while and a few months later ends up in a plant in India. The plant uses it to make starch which is stored in the leaves for later use (plants can break down starch into glucose for respiration). This time, the plant happens to have been in a field ploughed by a farmer. The plant is killed and goes into the soil. It gradually rots away. When a plant rots, one of the substances that is formed is … you guessed it – carbon dioxide. Our carbon atom ends up back in the atmosphere.

So carbon can be locked away inside of a plant or animal for a few minutes or many years, but it normally ends up back in the air as carbon dioxide. Living creatures form what we term the ‘biosphere‘ of the Earth and we say the biosphere is a ‘carbon sink‘. Other carbon sinks are the hydrosphere (oceans, seas, lakes etc) and lithosphere (rocks) as you will see shortly.

2. Oceans, Rocks and the Atmosphere

Carbon cycle - limestone

Limestone cliffs in South Wales [photo: M Hunter]

OK, the carbon dioxide with our carbon atom gets blown up towards

the North Pole but it never gets there because it dissolves in the sea. The oceans and seas of the world act as gigantic carbon sinks. After a while, it gets used by a mussel, a type of shellfish. The mussel uses our carbon atom to make calcium carbonate which it needs to make its shell. When the mussel dies, its dead shell becomes buried in sediments and turns into rock. The calcium carbonate of its shell helps to make a rock called limestone. Rocks take millions of years to form so our atom of carbon will be locked away for millions of years this time.

So how will it get back into the atmosphere now? Eventually, the limestone will be uplifted to the surface of the Earth. Rain is slightly acidic and will weather the limestone. One of the products of this weathering is … carbon dioxide. Another way that our carbon atom could end up back in the air is because of volcanic action. If the magma of a volcano passes through the limestone, it decomposes the limestone and you should by now be able to guess what gas is given off and blasted into the air by the volcano!

3. Fossil fuels

Carbon cycle - tree ferns.

Tree ferns, like those which died to form coal [photo: Mokkie]

Let’s leave our carbon atom now and consider fossil fuels. These are made from the remains of dead plants (coal) and sea creatures (oil and gas) that lived and died in the carboniferous era. They are therefore a carbon sink but the carbon is very concentrated, in some cases, coal can be ninety percent carbon. Since fossil fuels are in the rocks, this carbon was taken from the air millions of years ago. When these are burned, they release their carbon into the atmosphere as carbon monoxide and carbon dioxide. We have been digging up and coal and oil now for several hundred years, releasing carbon from the distant past into our present-day atmosphere. Many scientists believe this is a bad thing and say it is leading to Global Warming.

The carbon cycle seems a bit complicated at first but all you have to do to understand it is to think about how plants, animals, oceans, rocks and fossil fuels interact with the atmosphere. Could you draw a carbon cycle? And finally, because of the carbon cycle, some of the carbon in your body could once have been part of dinosaur poo … nice!

Taught science for 16 years at a secondary school in the East Midlands.