What Is the Carbon Cycle?
Carbon is one of the most important elements on Earth. It is the building block of life — found in every living organism, from bacteria to blue whales. But carbon doesn't stay locked inside organisms forever. It constantly moves between living things, the atmosphere, the oceans, and the ground in a process called the carbon cycle.
The continuous movement of carbon atoms through living organisms, the atmosphere, oceans, soil, and rocks via biological, chemical, and geological processes.
The carbon cycle is essential because:
- It regulates the amount of carbon dioxide (CO₂) in the atmosphere
- It provides carbon for building organic molecules (like glucose, proteins, and DNA)
- It connects all living organisms through shared chemical processes
In MYP Sciences, the carbon cycle is a perfect example of a system — it has stores (where carbon is held), fluxes (rates of movement between stores), inputs, outputs, and feedback loops. Understanding it as a system helps you see how changes in one part ripple through the whole.
Think of carbon atoms like players in a never-ending relay race. They pass from the air into plants, then into animals, then into the soil, and eventually back into the air — the race never stops, it just keeps cycling.
Figure 1: A simplified overview of the carbon cycle, showing carbon moving between the atmosphere, living organisms, soil, oceans, and fossil fuel stores.
Carbon in the Atmosphere and Living Things
Carbon exists in many forms and locations, known as carbon reservoirs or carbon stores.
Any place where carbon is stored for a period of time. Examples include the atmosphere, oceans, living organisms, soil, and fossil fuels.
The main carbon stores include:
| Carbon Store | Form of Carbon | Example |
|---|---|---|
| Atmosphere | Carbon dioxide (CO₂) | Atmospheric air (~0.04% CO₂) |
| Living organisms | Organic compounds | Glucose, proteins, fats |
| Soil | Organic matter, humus | Dead leaves, roots |
| Oceans | Dissolved CO₂, carbonate | Shells, coral |
| Fossil fuels | Hydrocarbons | Coal, oil, natural gas |
| Rocks | Calcium carbonate | Limestone, chalk |
Carbon moves between these stores through a series of biological and physical processes. The key biological processes you need to understand are photosynthesis, respiration, decomposition, and combustion.
The rate at which carbon moves between stores is called the carbon flux. When fluxes are balanced — as much carbon entering a store as leaving — the system is in equilibrium. Human activities have disrupted these fluxes, pushing the system out of balance.
Carbon in the atmosphere is mainly in the form of CO₂. Methane (CH₄) is also present in much smaller quantities (~1.9 ppm compared to ~420 ppm for CO₂), but it is still climatically significant as a potent greenhouse gas. Even though CO₂ makes up only about 0.04% of the atmosphere, it plays a massive role in regulating Earth's temperature through the greenhouse effect.
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