The cyclic process in which carbon element is circulated continuously through the living and non-living components of the biosphere is called carbon cycle.
Carbon is a basic constituent of living organisms. In fact, carbon is the most essential constituent of all the organic compounds like carbohydrates, proteins, fats and nucleic acids present in the living organisms. Carbon dioxide present in the atmosphere is the main reservoir of carbon.
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The main steps of the carbon cycle in nature are:
i. Photosynthesis:
Carbon is present as carbon dioxide gas in the atmosphere. Green plants use this carbon dioxide and prepare their food (as carbohydrate) by the process of photosynthesis. When animals eat the plant, plant carbohydrate is converted into animal carbohydrate.
ii. Respiration:
When plants and animals respire by oxidising carbohydrates in their cells to release energy, they give out carbon dioxide, which is returned to the atmosphere.
When animals and plants die, their bodies are decomposed by decomposers and carbon dioxide is returned to the atmosphere.
iii. Burning of Fossil Fuels:
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Some of the dead plants and animals get buried deep under the earth. These change into fossil fuels like coal and petroleum through slow chemical changes. Petroleum gives us fuel like kerosene, petrol, diesel, petroleum gas, etc. When these fuels burn, they give out carbon dioxide which goes into the atmosphere.
iv. Weathering of Rocks:
Some carbon dioxide is present in the dissolved state in water. This gets converted into calcium carbonate (CaC03) in limestone and other carbonate rocks.
Weathering of carbonate containing rocks or treatment of their minerals gives out carbon dioxide. When acid rain falls on these rocks, then carbon dioxide is released.
v. Volcanic Eruptions:
Volcanic eruptions and hot springs also release carbon dioxide into the atmosphere.
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Thus, there is a continuous exchange of carbon dioxide between atmosphere, water bodies and living beings.
The different processes which help in the removal of carbon dioxide from the atmosphere are:
i. Photosynthesis by green plants.
ii. Formation of fossil fuels (like coal, petroleum, natural gas, etc.).
iii. Formation of carbonate rocks.
iv. Formation of carbonate shells, skeleton, etc. The different processes which help in the addition of carbon dioxide to the atmosphere are:
v. Respiration of plants and animals.
vi. Decay of dead plants and animals.
vii. Burning of fossil fuels.
viii. Action of acid rain on carbonate rocks, shells, etc.
ix. Volcanic eruptions.