What is the Carbon Cycle? Diagram, Definition, and Steps

What is the carbon cycle?

The carbon cycle is the biogeochemical cycle that circulates and exchanges carbon among the biosphere (the zone of life on earth), pedosphere (the outer surface layer of the Earth that is made up of soil and susceptible to soil formation processes), geosphere (the joint name for the lithosphere, hydrosphere, cryosphere, and atmosphere), hydrosphere (the cumulative mass of water found on, under, and above the surface of a planet, minor planet, or natural satellite), and atmosphere of the Earth ( commonly recognized as air or a layer of gases that is maintained by the gravity of the Earth).

Definition of the carbon cycle

The carbon cycle is the flow of carbon as it is recycled and reused across the biosphere, and it is also a lengthy process of carbon sequestration (the long-term disposal or capture of carbon dioxide from the atmosphere in order to slow or reverse atmospheric carbon dioxide pollution and to minimize or reverse climate change) and discharge from carbon sinks (any reservoir, natural or anything else, that gathers and stores carbon).

The carbon cycle is critical to the survival of life on Earth and needs to be kept at a certain level, hence, nature has a way of maintaining these levels in equilibrium on the Earth. This means that the amount of carbon naturally released from sinks equals the amount naturally absorbed by reservoirs. Maintaining this carbon balance ensures that the planet remains habitable for life.

Carbon cycle diagram
The carbon cycle diagram (The above carbon cycle diagram depicts how carbon atoms travel between different reservoirs in the Earth system. It is worth noting that carbon atoms are incorporated into various molecules as they move through the cycle; for instance, photosynthesis in plants and respiration in animals. Photo credit: UCAR

Steps of the carbon cycle

  1. CO2 emissions into the atmosphere
  2. Absorption by producers.
  3. Carbon compounds are transferred in the food chain
  4. The release of carbon into the atmosphere
  5. Human involvement

The above mentioned are the main carbon cycle steps and these steps are explained below;

CO2 emissions into the atmosphere

This carbon cycle step begins with the entry of carbon (in the form of carbon dioxide) into the atmosphere. Prior to this, animals undergo respiration (the process by which organisms release energy from their food) and plants pass through the process of combustion (the process of burning) which results in the release of carbon dioxide into the atmosphere. The role of respiration in the carbon cycle can be seen when animals respire, they release or emit carbon molecules back into the atmosphere.

Absorption by producers

The next step in the carbon cycle is the incorporation of carbon dioxide by photosynthetic organisms such as plants and algae (referred to as producers). These organisms convert carbon dioxide into sugars for energy and reproduction. During the absorption process of carbon dioxide by the producers, atmospheric CO2 is converted in the presence of solar energy to form carbon compounds that can be stored and used by the plants for their metabolic activities. Plants and other producer organisms, like cyanobacteria, are critical to Earth’s life because of their ability to convert atmospheric carbon into living matter.

Carbon compounds are transferred in the food chain

The carbon compounds from the plants (producers) enter the food chain after the preceding carbon cycle step (absorption by producers). Here, the carbon compounds are received by animals when they consume these plants. They use most of the carbon compounds in food to build their own bodies. For example, when plants are eaten by herbivores (deer) and the herbivores are in turn consumed by carnivores (lion) and further, along the food chain, the carnivores, herbivores, and producers are eaten by omnivores (humans). During the process, carbon compounds are transferred in the food chain.

The release of carbon into the atmosphere

While producers use the energy from the sun to form bonds between carbon atoms, animals break these bonds to release the energy contained within them, eventually converting sugars, lipids, and other carbon compounds into single-carbon units. The carbon compounds are eventually released into the atmosphere as CO2 as a result of decomposers (bacteria and fungi) eating animal carcasses and decomposing plant remains. Carbon is released back to the atmosphere because of the fact that the decomposition process emits carbon dioxide as a byproduct.

Human Involvement

Humans have recently made significant changes to the Earth’s carbon cycle. Humans have reduced the Earth’s ability to take carbon out of the atmosphere by burning massive amounts of fossil fuels and destroying roughly half of the world’s forests thereby releasing large amounts of carbon into the atmosphere that had been preserved in solid form as plant matter and fossil fuels. This explains how human activity can affect the carbon cycle.

The carbon cycle in biology

In explaining the carbon cycle in biology, a brief summary of what is carbon? and what is carbon in the biogeochemical cycle? is essential.

What is carbon?

Carbon is a necessary element for all life on Earth and carbon intake and output are components of all plant and animal life, whether they take in carbon to help manufacture food or release carbon as part of respiration. It is constantly moving from one location to another on the Earth and it is preserved in reservoirs and transferred between reservoirs through a variety of processes, including photosynthesis, the combustion of fossil fuels, and the simple release of breath from the lungs (respiration).

The carbon cycle in biology refers to the movement of carbon from one biological reservoir to another biological reservoir. The carbon cycle starts when plants obtain some of the carbon in the atmosphere and use it to make food (through photosynthesis). This carbon can then be ingested and stored by the animals that consume the plants. When animals die, their remains decompose, capturing the stored carbon in layers that inevitably turn into rock or minerals. Some of this sediment may form fossil fuels, which include coal, oil, or natural gas, which emit carbon into the atmosphere when combusted.

Carbon reservoir

Carbon can be deposited in a variety of reservoirs, such as plants and animals, which are classified as carbon life forms. Plants use carbon to develop leaves and stems, which are then eaten and absorbed by animals. These animals use the absorbed carbon for cellular growth.

In addition, carbon is preserved in the atmosphere as gases like carbon dioxide. and it is also preserved in the oceans, where it is captured by a variety of marine life forms. For example, carbon is used by clams and corals, to form shells and skeletons.

The carbon cycle processes

  1. Short-term or fast carbon cycle processes
  2. Long-term or slow carbon cycle processes

Short-term or fast carbon cycle processes

This type of process is concerned with the yearly changes that happen in the atmosphere, terrestrial ecosystems, and marine ecosystems. It is so named because the transformation of carbon across reservoirs occurs in a relatively short period of time (minutes, hours, days, months, or years). The biosphere is governed by the fast cycle because of the ability to transfer carbon from the atmosphere to the biosphere and back to the atmosphere in a matter of years. The fast carbon process entails comparatively short biogeochemical processes in the biosphere between the environment and living organisms. These short processes consist of the annual photosynthetic cycles and annual or seasonal vegetative growth and decomposition cycles.

Long-term or slow carbon cycle processes

This type of process takes a long period of time to move because carbon has to be transferred through the Earth’s crust between rocks, soil, ocean, and atmosphere which takes a lot of time for the cycle to be completed. For example, surplus carbons from short-term cycling are preserved in the long-term reservoir before they are removed after a long period of time.

The slow carbon cycle is comprised of medium to long-term geochemical processes associated with the rock cycle.  The exchange of energy between the ocean and the atmosphere can take thousands of years, and rock weathering can take eons.

Another slow carbon cycle process is mountain building (the geological processes that support mountain formation) processes that result in the return of geologic carbon to the Earth’s surface. The rocks are weathered there, and carbon is released back to the atmosphere via gas and to the rivers and ocean via evaporation, and other geologic carbon is returned to the ocean via hydrothermal calcium ion emission. This slow cycle moves between 10 and 100 million tonnes of carbon when they occur. Volcanoes, for example, can return geologic carbon directly to the atmosphere in the form of carbon dioxide. Although, it represents less than 1% of the carbon dioxide emitted into the atmosphere when compared to the emitted carbon dioxide as a result of the combustion of fossil fuels.

Importance of carbon cycle

  • As previously stated in the beginning that all living organisms are made up of carbon in some form or another; thus, its mere presence is very important for life itself. When the process fails, life may become disrupted, and it may even come to an end.
  • The process of the carbon cycle is very important in the maintenance of balance in various ecosystems due to the movement of carbon across different reservoirs. If an imbalance occurs, severe environmental disasters such as global warming may occur.
  • The carbon cycle is critical to the food chain as their close relationship stems from the fact that all living organisms are composed of carbon. The carbon present in the producers migrates to the consumers who eat them via food chains (and food webs).
  • Carbon dioxide and methane, two carbon-based gases that significantly contribute to global warming, are vital for climate regulation. Because they are composed of carbon, the carbon cycle naturally determines the amount of these gases in the atmosphere. For example, when a large amount of carbon is emitted into the atmosphere, the level of greenhouse gases rises, trapping more heat in the Earth.

The above-listed reasons show why the carbon cycle is important.

What role does cellular respiration play in the carbon cycle?

The carbon cycle includes cellular respiration and photosynthesis. The carbon cycle refers to the processes by which carbon is recycled in the biosphere. While cellular respiration emits carbon dioxide into the environment, photosynthesis removes it from the atmosphere. The interchange of carbon dioxide and oxygen during photosynthesis and cellular respiration aid in the maintenance of stable oxygen and carbon dioxide levels in the atmosphere.

Ecosystem components in which the carbon cycle occur

  • Atmosphere
  • Lithosphere
  • Biosphere
  • Oceans

The above are some examples of ecosystem components that can absorb carbon, convert carbon into living matter, or release carbon back into the atmosphere. These systems work together to keep the Earth’s carbon cycle fairly stable.


Carbon dioxide in the Earth’s atmosphere is a significant carbon repository (a place where carbon is found in plenty quantity). In the presence of two oxygen atoms, carbon forms a stable, gaseous molecule. In nature, this gas is released by volcanic activity and animal respiration, which attaches carbon molecules from the food they eat to oxygen molecules before exhaling it.

Plants can eliminate carbon dioxide from the atmosphere by converting atmospheric carbon into sugars, proteins, lipids, and other essential molecules for life. It can also be eliminated from the atmosphere through absorption into the ocean, where the water molecules can bond with carbon dioxide to form carbonic acid.


The Earth’s crust, known as the lithosphere after the Greek words litho for stone and sphere for the globe, can also emit carbon dioxide into the atmosphere. Chemical reactions in the Earth’s crust and mantel can produce this gas. Volcanic activity can result in natural carbon dioxide emissions.

While the lithosphere has the ability to add carbon to the atmosphere, it also has the ability to remove it. Carbon-containing chemicals, like dead plants and animals, can be buried deep underground, where their carbon cannot be released back into the atmosphere. These underground reservoirs of organic matter liquefy over long periods of time to become coal, oil, and gasoline. Humans have started releasing most of this sequestered carbon back into the atmosphere in recent times by burning these materials to power vehicles, energy plants, and other human machinery.


Some living things remove carbon from the atmosphere, whereas others return it. Plants and animals are the most visible participants in this system. Plants absorb carbon dioxide from the atmosphere. The atmospheric carbon is the food that plants use to produce sugars, proteins, lipids, and other essential molecules for life. Plants create these organic compounds out of carbon dioxide and other trace elements using the energy of sunlight harvested through photosynthesis.

Animals consume plants (and other animals) and then disassemble the synthesized molecules in a nicely balanced set of chemical reactions. Animals get their energy from the chemical energy that plants store in the bonds that form between carbon atoms and other atoms during photosynthesis. Animal cells disassemble complex molecules like sugars, fats, and proteins all the way down to single-carbon units – molecules of CO2 produced by reacting carbon-containing food molecules with oxygen from the air.


The oceans on Earth have the capacity to absorb as well as release carbon dioxide. When atmospheric carbon dioxide interacts with ocean water molecules, it can create carbonic acid, a soluble liquid form of carbon.

When the amount of carbonic acid in the ocean exceeds the amount of carbon dioxide in the atmosphere, some of the carbonic acids may be released into the atmosphere as CO2. When there is more CO2 in the atmosphere, more CO2 is converted to carbonic acid, and ocean acidity levels rise, thereby maintaining the level of carbon.

Carbon cycle projects

Scientists are working day and night on different carbon cycle projects. For example, the global carbon project (GCP), that involved biologists, chemists, sociologists, economists, and, geologists. The GCP was to gain an understanding of the carbon cycle, its effects on climate, weather, and, global warming. In addition, the carbon cycle project was to answer some questions about the management strategies and the development of pathways to stabilize the carbon cycle in today’s world.

Another carbon cycle project is the global learning and observation to benefit the environment (GLOBE) program. The Globe carbon cycle project is an Earth System Science Projects (ESSPs) financially backed by NASA and the National Science Foundation (NSE) to create a practical experience, intermediate and secondary school-based science exercises for the GLOBE Program.

The GLOBE Carbon Cycle is dedicated to providing cutting-edge research and technological advancements in the field of terrestrial ecosystem carbon cycling into the classroom. It takes a systems-thinking approach to learn about the carbon cycle and how it relates to climate and energy.

Such kinds of carbon cycle projects can explain how human activity can affect the carbon cycle.


What is the carbon cycle definition?

The carbon cycle simple definition is the continuous flow of carbon atoms starting from the atmosphere to the Earth and then flowing or traveling back to the atmosphere.

Which processes are part of the carbon cycle?

  1. Respiration
  2. Photosynthesis
  3. Excretion
  4. Decomposition

How does diffusion in the carbon cycle process work?

Diffusion in the carbon cycle means the exchange of carbon between the atmosphere and the ocean. It works as carbon dioxide either flows from the ocean to the air or from the air to the ocean, depending on the concentration of carbon. Carbon is able to enter other phases of the carbon cycle as a result of this diffusion process, such as plant respiration and the combustion of fossil fuels.

What two processes fuel the carbon cycle processes?

  1. Cellular respiration
  2. Photosynthesis

What adds carbon to the atmosphere?

CO2 is naturally introduced into the atmosphere by organisms respiring or decomposing (decaying), carbonate rocks weathering, forest fires, and volcanoes erupting. Carbon dioxide is also released into the atmosphere as a result of human activities such as the combustion of fossil fuels and the destruction of forests, as well as the manufacture of cement.

What are the carbon cycle steps?

  1. CO2 emissions into the atmosphere
  2. Absorption by producers.
  3. Carbon compounds are transferred in the food chain
  4. The release of carbon into the atmosphere
  5. Human involvement

What is the carbon cycle equation?

The carbon cycle equation that is necessary for the release of carbon is CH2O+ O2 CO2 + H2O+ energy, which is the inverse of photosynthesis. Plants (and animals) discharge water and carbon dioxide through respiration.

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