💎 Forms of Carbon

Carbon is an element. The symbol for carbon is C.

Carbon is a non-metal. It occurs naturally in different forms. (You will find out more about carbon in Topic 2.4.)

Diamonds are made of carbon.

The ‘lead’ in a pencil is not lead at all – it is another form of the element carbon, called graphite.

🌿 Carbon Use in Living Organisms

Living organisms do not need diamonds or pencil leads, but they do need carbon. Organisms cannot use carbon in the form of an element. They can only use it when it is part of a compound.

Carbon is part of many different compounds that make up cells. Carbohydrates, proteins and fats are all compounds that contain carbon.

🌱 Carbon in Plants and Nutrients

We rely on plants to make these substances. Plants take carbon dioxide from the air and use it in photosynthesis to make carbohydrates. Carbon dioxide is a compound that contains carbon atoms combined with oxygen atoms. (You will find out more about this in Unit 2.) The carbohydrates in plants contain carbon atoms that were originally part of the air.

Plants use the carbohydrates to make proteins and fats. All of these nutrients are compounds that contain carbon atoms.

🦌 Carbon Transfer in Food Chains

We are animals, so we get all of these carbon-containing nutrients when we eat plants or other animals. Decomposers get their carbon when they break down waste products from plants and animals.

🔁 Carbon Flow Diagram

We can show how carbon gets into the bodies of animals and decomposers using a flow diagram.

🌬️ Returning Carbon Dioxide to the Air

A lot of the carbon dioxide that plants take from the air eventually goes back into the air again. This happens when plants and animals respire. You may remember the respiration equation: glucose + oxygen → carbon dioxide + water.

When you breathe out, carbon dioxide that was produced in your cells, by respiration, goes into the air around you.

All organisms respire. Plants respire all the time. At night, when they cannot photosynthesise, they give out carbon dioxide, just as we do.

Decomposers respire, too. As they break down waste products from plants and animals, they release carbon dioxide into the air.

We can add this information to the flow diagram.

🔥 Fossil Fuels and the Carbon Cycle

There is one more very important set of processes to add to the diagram showing how carbon moves from the air, through organisms, and back to the air again.

⚙️ How Fossil Fuels Form

When organisms die, they are not always broken down quickly by decomposers. Sometimes, their bodies fall into places where there is no oxygen, such as deep in the ocean. In these places, the decomposers cannot respire, because there is not enough oxygen for them. Instead, the organisms’ bodies get gradually buried, as more and more sediment builds up on top of them. High pressure and heat change their remains into fossil fuels, including coal, oil or natural gas.

⏳ A Slow Transformation

Changing dead organisms to fossil fuels takes a very long time. Most of the fossil fuels that we use on Earth today were formed hundreds of millions of years ago.

🛢️ Oil and Gas from Marine Life

Oil and natural gas formed when tiny marine organisms died and fell to the sea bed. This oil rig, in the sea off West Africa, has pipes that go deep into the sea bed where deposits of liquid oil are present. The oil is brought up through the pipes, and taken ashore to be used as fuel.

🌋 Coal and Combustion

Coal was formed from the remains of plants that grew in huge swamps. Their remains were buried over millions of years, slowly turning into coal. Coal is dug out of the ground and then used as a fuel for cooking or heating homes, but most of it is used in power stations to generate electricity.

Fossil fuels contain carbon. This came from the carbohydrates, fats and proteins in the dead organisms. When we burn a fossil fuel, the carbon in it combines with oxygen from the air and forms carbon dioxide. This is called combustion.

♻️ Completing the Carbon Cycle

We can add the formation and combustion of fossil fuels to the flow diagram. The completed diagram is called the carbon cycle.

🧠 Fossils vs. Fossil Fuels

It is important to remember that fossil fuels are not the same as fossils. A fossil is the remains of an organism, or traces of it (such as its burrows) that have turned to rock. We can still see the shape of the organism in a fossil. But fossil fuels do not look like organisms at all, and oil and gas are not even rocks. Fossil fuels are given this name because – like fossils – they were formed a very long time ago and buried underground.