Carbon Cycle


Carbon is the most important element and forms the basis of all life found on Earth. Humans, animals, plants, invertebrates, prokaryotes, etc. are mainly made of carbon, hydrogen, and oxygen. It is the main component of DNA, RNA, and various primary and secondary metabolites found in higher and lower organisms. Excess of carbon or absence of carbon can cause a severe impact on the functioning of the entire environment.

What is the Carbon Cycle?

Carbon cycle is a process of recycling carbon atoms from the atmosphere to earth and then back to the atmosphere. Main reservoirs of carbon are sedimentary rocks and oceans, followed by the atmosphere, living beings, and fossil fuels. Carbon is the 4th most abundant gas in the atmosphere, and hence the slight shift in its concentration causes a heavy impact on both the biotic and the abiotic factors.

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Steps of Carbon Cycle

The carbon cycle occurs in the following places- the Atmosphere, the land, and finally, it is deposited into the ocean, from where it is recycled back to the atmosphere.

Carbon Cycle in the Atmosphere of the Earth

  • Carbon is released from the atmosphere in the form of rain.

  • Atmospheric CO2 reacts with water in the atmosphere and is converted into carbonic acid which falls on the earth.

  • Carbonic acid does chemical weathering of the rocks and releases various ions such as calcium, potassium, and sodium. etc.

  • Rivers carry these ions to the ocean and there they get deposited.

  • Carbon is released back into the atmosphere due to volcanic eruptions and hence the cycle begins again.

Carbon Cycle in ocean

  • These ions formed due to chemical weathering reach the ocean.

  • Calcium carbonate is formed due to a reaction between Calcium ions and bicarbonate ions. It is also formed by calcifying organisms such as corals and plankton.

  • When these organisms die they get buried on the ocean floor with their shells which causes accumulation of ${CaCO_3}$ on the seabed.

  • Over time due to the constant accumulation of shells and ${CaCO_3}$, an entire pile is formed which gets cemented together forming a rock which is the carbon reservoir. This rock is also known as Limestone.

  • On the surface of the ocean, carbon exchange takes place where ${CO_2}$ dissolves with the water, making the water more acidic.

  • This acidic water causes weathering of carbonate rocks and releases bicarbonate ions into the water.

  • In this way, Carbon exchange occurs between water and air.

Carbon Cycle on Land

  • Another way to incorporate carbon into the environment is by living beings.

  • Living organisms such as plants and phytoplanktons absorb ${CO_2}$ from the environment for the production of food.

  • In the presence of sunlight, photosynthesis takes place where carbohydrates and water are formed. And this way carbon gets incorporated in the biotic environment.

  • Plants and phytoplanktons are the producers of land and water ecosystems respectively and hence they are then eaten up by primary consumers, secondary consumers and finally humans or apex consumers.

  • As a result, carbon gets accumulated in the bodies of humans. Through the process of excretion, this consumed carbon is released back into the environment.

  • Decaying bacteria decay the organic waste generated and thus bring carbon back in chemical form.

  • During the process of respiration, plants and animals break these sugars into ${CO_2}$ and energy. Energy is used by living organisms to carry out their activities and ${CO_2}$ is released back into the atmosphere through exhalation.

  • When these plants and animals die they get buried in the soil, where decaying bacteria break complex organic compounds into simpler ones and carbon is released back into the atmosphere.

Significance of Carbon Cycle

The significance of the Carbon Cycle is as follows:

  • Carbon is the building block of life on earth, as all living beings are made mostly of carbon, hence the carbon cycle is of paramount importance.

  • The carbon cycle maintains the ${CO_2}$ levels of the environment.

  • It helps in maintaining the constant temperature of the earth, hence, supporting all living beings on earth.

  • An imbalance in this cycle can lead to various environmental problems such as greenhouse effects, pollution, loss of biodiversity, and ozone depletion. etc.

  • Also, excess ${CO_2}$ causes severe climatic changes leading to the melting of glaciers, tsunamis, quick increases in earth's temperatures, etc.

  • The carbon cycle ensures that all biotic and abiotic factors get their share of carbon so as to complete their own life processes.


Carbon is an important chemical that is required for maintaining life and the smooth functioning of the Earth. It is the main component of carbohydrates, proteins, lipids, and nucleotides which are required to sustain life on earth, hence it is of paramount importance that this element is used well and recycled properly so a balance is maintained.

Carbon is mostly stored in oceans, rocks, and living forms. Photosynthetic organisms are responsible for capturing atmospheric carbon into the living system and volcanoes, excretion, and decaying bacteria are responsible for bringing carbon back to the atmosphere. This constant exchange of carbon from one form to another and back to the environment is known as the Carbon cycle.


Q1. What is the Slow Carbon cycle?

Ans: The slow carbon cycle is where the movement of carbon between rock, soil, ocean, and atmosphere occurs at a very slow pace. Around 10-100 million metric tons of carbon move in this cycle. It is commonly seen when carbon is circulated from oceans to the atmosphere.

Q2. What is the Fast carbon cycle?

Ans: The fast carbon cycle is where the movement of carbon is at a quick pace. It mainly occurs when carbon is circulated amongst living beings such as plants and animals. Around 1000-100,000 million metric tons of carbon are recycled in a fast carbon cycle.

Q3. What is chemical weathering?

Ans: The breakdown of various materials such as rocks, wood, stones, minerals, etc. into small components is known as weathering. When weathering occurs due to chemical reactions then it is known as chemical weathering. Common chemical reactions that causes weathering are oxidation, reduction, carbonation, hydrolysis, etc.

Q4. How fossil fuels are formed?

Ans: Soil experiences lots of compression because of heat and pressure. Along with the soil, even organic content gets compressed leading to the formation of sedimentary rocks. When the death rate of living beings such as plants and animals increase, all the organic matter cannot be decomposed by bacteria, and hence this organic carbon content keeps on accumulating. It is a storehouse of reserved energy in the form of carbon and hydrogen. This process of compression of the organic matter over years leads to the formation of fossil fuels such as coal, oil, and natural gas.