The Carbon Cycle

The carbon cycle describes how nature reuses carbon atoms. It is a complex cycle that involves many living organisms.

Carbon is mostly distributed across five stores, which include:

  • The air – as carbon dioxide (CO2)
  • Soil – which contains a lot of microorganisms
  • Fossil fuels – Deep underground
  • Animals – Stored in biological molecules
  • Plants – Stored in biological molecules

Illustrative diagram of the carbon cycle. The image portrays sunlight aiding photosynthesis in trees, which contribute to organic carbon. Animals contribute through respiration. Human activities, such as transportation and factories, emit CO2. Below ground, roots respire, and as organisms die, they become part of the soil, potentially turning into fossils and fossil fuels. Arrows indicate the flow and cycle of carbon throughout the system.

Now let’s look at some of the main processes involved in the carbon cycle.

Processes Involved in the Carbon Cycle

Removing carbon dioxide from the atmosphere

During photosynthesis, plants and algae absorb carbon from the air in the form of carbon dioxide and convert it into glucose, which can be turned into carbohydrates, proteins and fats.

Carbon moves up the food chain

Plants release some of the carbon they’ve absorbed back into the atmosphere through respiration. This is the process of breaking down organic matter to produce energy. The equation for respiration is:

Glucose + Oxygen Water + Carbon dioxide

The carbon can also be passed on to animals and microorganisms when they eat plants and algae. The carbon from the plants or algae will be stored in the animals as biological molecules, such as carbohydrates, proteins and fats. If an animal eats another animal, they get the carbon from its food too.

Animals consume plants, algae, or other animals. Through respiration, they release carbon dioxide.

Decomposition in the carbon cycle

When plants and animals die, two things can happen:

1. Animals eat the dead organism and carbon dioxide is released when the animals respire. However, decomposing organisms, which thrive in warm, moist soil with plenty of oxygen, can consume these dead organisms.

This process breaks the organisms down into smaller pieces. Eventually, all of the trapped carbon will be released as carbon dioxide, during microbial respiration.

2. Sometimes, dead organisms decompose in conditions with limited oxygen. Over millions of years and significant pressure, their bodies can be converted into fossil fuels, such as:

  • Oil
  • Natural gas
  • Coal
Illustrative diagram showing the formation of oil and gas over time. On the left, 300-400 million years ago, small marine organisms are seen in blue waters above a seabed with plants. These organisms eventually settle and form remains on the seabed. In the middle, 100 million years ago, layers of sand, sediment, and rock accumulate over the remains. Over time and under pressure, these layers compress the remains. On the right, in the present day, the compressed layers have formed trapped oil and gas reservoirs beneath the earth, depicted by a drilling rig extracting the resources. The process is further simplified in small icons below, showing marine organisms turning into remains, which under time and pressure, become natural oil and gas.

When humans burn these fossil fuels for energy, they release the stored carbon back into the atmosphere as carbon dioxide.