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Carbon Cycle

Chemistry | 9-11 yrs | Interactive, Learning Pod


The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth. It is one of the most important cycles of the earth and allows for carbon to be recycled and reused throughout the biosphere and all of its organisms.

The annual movements of carbon, the carbon exchanges between reservoirs, occur because of various chemical, physical, geological, and biological processes. The ocean contains the largest active pool of carbon near the surface of the Earth.

In the Atmosphere

Carbon exists in the Earth’s atmosphere primarily as the gas carbon dioxide (CO2). Even though it is a small percentage of the atmosphere, it plays a vital role in supporting life. Other gases containing carbon in the atmosphere are methane and chlorofluorocarbons. Trees and other green plants such as grass convert carbon dioxide into carbohydrates during photosynthesis, releasing oxygen in the process. This process is most prolific in relatively new forests where tree growth is still rapid. The effect is strongest in deciduous forests during spring. Forests store 86% of the planet’s terrestrial above-ground carbon and 73% of the planet’s soil carbon.

Carbon is released into the atmosphere in several ways. They are :-

Through the respiration performed by plants and animals. This is an exothermic reaction and it involves the breaking down of glucose (or other organic molecules) into carbon dioxide and water.

Through the decay of animal and plant matter. Fungi and bacteria break down the carbon compounds in dead animals and plants and convert the carbon to carbon dioxide if oxygen is present, or methane if not.

Burning fossil fuels such as coal, petroleum products, and natural gas releases carbon that has been stored in the geosphere for millions of years. Burning agrofuels also releases carbon dioxide which has been stored for only a few years or less.

Production of cement. Carbon dioxide is released when limestone (calcium carbonate) is heated to produce lime (calcium oxide), a component of cement.

In the Biosphere

Carbon is an essential part of life on Earth. About half the dry weight of most living organisms is carbon. It plays an important role in the structure, biochemistry, and nutrition of all living cells. Living biomass holds about 575 gigatons of carbon, most of which is wood. Soils hold approximately 1,500 gigatons, mostly in the form of organic carbon.

Autotrophs are organisms that produce their own organic compounds using carbon dioxide from the air or water in which they live. To do this they require an external source of energy. Almost all autotrophs use solar radiation to provide this, and their production process is called photosynthesis. A small number of autotrophs exploit chemical energy sources in a process called chemosynthesis. The most important autotrophs for the carbon cycle are trees in forests.

Carbon is transferred within the biosphere as heterotrophs feed on other organisms or their parts (e.g., fruits). This includes the uptake of dead organic material by fungi and bacteria for fermentation or decay.

Most carbon leaves the biosphere through respiration. When oxygen is present, aerobic respiration occurs, which releases carbon dioxide into the surrounding air or water.

Burning of biomass (e.g. forest fires, wood used for heating, anything else organic) can also transfer substantial amounts of carbon to the atmosphere

Carbon may also be circulated within the biosphere when dead organic matter becomes incorporated in the geosphere.

In the Hydrosphere

The oceans contain around 36,000 gigatonnes of carbon, mostly in the form of bicarbonate ion. Extreme storms such as hurricanes and typhoons bury a lot of carbon, because they wash away so much sediment. Inorganic carbon, that is carbon compounds with no carbon-carbon or carbon-hydrogen bonds, is important in its reactions within water. This carbon exchange becomes important in controlling pH in the ocean. Carbon is readily exchanged between the atmosphere and ocean.

The flux or absorption of carbon dioxide into the world’s oceans is influenced by the presence of widespread viruses within ocean water, that infect many species of bacteria. The resulting bacterial deaths spawn a sequence of events that lead to greatly enlarged respiration of carbon dioxide, enhancing the role of the oceans as a carbon sink.

Further Research

Find an old empty glass jar and place a plant inside of it, keeping it covered. Store the jar in a dark place and leave it overnight. What changes will you see in the jar when it is removed next morning?

Take a glass and add some baking soda in it. Once that is done, pour some vinegar into it. You will see bubbles erupting from the glass. What is in these bubbles and why does this happen?

Find out about autotrophs and where they are present.

Head on to Chemistry for Kids for more such interesting chemistry videos and interactive articles.


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