The Carbon Cycle

What is Carbon?

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

A teal square representing the periodic table element for Carbon. At the top left corner is the atomic number "6". In the centre is the large white letter "C", and below it, the word "Carbon" is written. The atomic weight "12.011" is displayed at the bottom.

The carbon atom has a nucleus containing 6 protons and 6 neutrons, surrounded by 6 electrons that orbit the nucleus in shells.

A visual representation of a Carbon atom. On the left, there's a detailed atomic model with concentric circles representing electron orbits, containing blue electron dots. Inside the nucleus, there are red and dark-coloured spheres representing protons and neutrons, respectively. To the right, the word "CARBON" is displayed prominently, followed by an equation "6 neutrons + 6 protons = 12". Adjacent to this, there's a small red circle with the atomic symbol "C" and the number "12" on it.

At room temperature, carbon is a black solid. However, it can also be found in different compounds, such as:

  • CO2 – Carbon dioxide
  • CH4 – Methane
  • C6H12O6 – Glucose

Molecules are sometimes displayed like this:

A structural diagram of ethane, showing two central Carbon atoms bonded together, each surrounded by three Hydrogen atoms connected with single bonds.

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

Carbon Cycle

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

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

Representation of the carbon cycle against a blue sky. At the top, CO2 is emitted from transportation, factories and animal respiration, and is absorbed by trees through photosynthesis under sunlight. Organic carbon is shown on the land and below the soil, there is root respiration, fossils, fossil fuel, and remnants of dead organisms. Arrows indicate the flow of carbon throughout the cycle.

Photosynthesis

Carbon is taken in from the surroundings by a process called photosynthesis, as seen 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 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 a plant (as part of a glucose molecule).

Respiration

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

Glucose + OxygenCarbon dioxide + Water

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

Both plants and animals respire. During respiration, glucose reacts with oxygen to produce carbon dioxide as a byproduct. Animals obtain glucose by eating plants.

Also, microorganisms break down dead plants and animals. As they do so, they respire, which releases carbon dioxide into the atmosphere.

Combustion

The last main process in the carbon cycle is combustion. Whenever fossil fuels burn, they release carbon dioxide, as seen in the equation below:

CH4 + 2O2CO2 + 2H2O

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

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

Carbon compounds can be trapped inside fossil fuels, such as coal, oil and natural gas. This process takes millions of years and involves intense pressure and heat.

These fossil fuels are extracted and used as sources of energy for many purposes, for example, in factories and vehicles. When we combust (burn) these fuels, they release carbon dioxide (CO2).

Linking Processes Together

Let’s look at the carbon cycle diagram again. A cow is an example of an organism that releases carbon atoms through respiration.

When the cow breathes out, it releases carbon dioxide into the air. Plants, like trees, take in that carbon dioxide and use it to make glucose and oxygen, through photosynthesis. So, what was once a carbon atom for the cow is now a carbon atom that forms part of a tree.

Remember!

The carbon cycle is the process by which carbon moves around in nature through living things, the air and the earth.

  • The combustion of fossil fuels such as coal, oil and natural gas releases carbon dioxide into the atmosphere.
  • When plants photosynthesise, they take in carbon dioxide
  • When plants and animals respire, they release carbon dioxide into the atmosphere

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