The Carbon Cycle

What is Carbon?

Carbon is a non-metal element in the periodic table, with the atomic symbol ‘C

The carbon atom consists of 6 neutrons and 6 protons within the nucleus, as well as 6 electrons around the outside of the nucleus.

At room temperature, carbon is a black solid. However, it is also found in different compounds. For example:

  • CO2 – This is carbon dioxide
  • CH4 – This is methane
  • C6H12O6 – This is glucose

Molecules are sometimes displayed like this:

Which is an ethane molecule (C2H6) that contains two carbon atoms.

Carbon Cycle

Let’s take a look at a diagram of the carbon cycle.

The carbon cycle describes how carbon is exchanged between different molecules, by different processes as it moves around the planet. Carbon is always constantly being recycled on Earth.


Carbon is taken in from the surroundings by a process called photosynthesis, In the equation:

Carbon dioxide + WaterGlucose + Oxygen

However, when using a symbol equation, it is easier to see what happens to carbon.

6CO2 + 6H2OC6H12O6 + 6O2

  • Carbon is in red in both the reactants and products of the photosynthesis reaction

Carbon is originally in the carbon dioxide (CO2) molecule, which is in the atmosphere. On the products side of the equation, carbon is present in the glucose molecule (C6H12O6). This is an example of a reaction in the carbon cycle that swaps carbon from one compound to another, in this case, the carbon that was previously in the atmosphere (as part of the carbon dioxide molecule) is now stored in the plants (as part of a glucose molecule).


Another process in the carbon cycle is respiration. Carbon is given out to the surroundings by respiration of animals and plants. The equation for respiration is:

Glucose + OxygenCarbon dioxide + Water

  • Respiration is the reverse of photosynthesis

Let’s take a look at the symbol equation for photosynthesis, so we can see the movement of carbon more closely.

C6H12O6 + 6O26CO2 + 6H2O

  • Carbon is in red in both the reactants and products of the respiration reaction

Both plants and animals respire. They take the glucose that they have and take in oxygen to produce carbon dioxide, a by-product of respiration. Animals take in glucose when they eat plants.

Another form of respiration is decay. Microorganisms decay dead plants and animals, this involves them breaking down the dead plants and animals. The microorganisms then respire, releasing carbon dioxide into the atmosphere. in the same equation:

C6H12O6 + 6O26CO2 + 6H2O


The last main process in the carbon cycle is combustion. Whenever fossil fuels burn, they release carbon dioxide. Take a look at the equation below:

CH4 + 2O2CO2 + 2H2O

  • Carbon is in red in both the reactants and products of the combustion reaction

The fuel used is in this example is methane. When the fuel burns, it reacts with oxygen in the air to release both carbon dioxide and water.

Carbon compounds can be locked up inside fossil fuels, in a process which takes millions of years. Then these fossil fuels are used as coal, oil and natural gas, which we use as fuel. For example, we use the fuel in factories and vehicles. When we combust these fuels, it will release carbon dioxide (CO2).


If we look at the cow in the carbon cycle diagram, a carbon atom that was originally part of the cow can be released through respiration (as CO2), then taken in by the tree (as CO2) through photosynthesis. So, what was once a carbon atom for the cow is now a carbon atom that forms part of a tree.


  • The process of the release and in-take of carbon is known as the carbon cycle
  • When fossil fuels burn (combustion) carbon dioxide is released into the atmosphere
  • When plants photosynthesise, carbon dioxide is taken in
  • When plants and animals respire, carbon dioxide is released into the atmosphere