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The Carbon Cycle

All green plants "trap" carbon dioxide (CO2) from the atmosphere. They use the energy from the sun in the process known as photosynthesis. Plants use carbon dioxide (CO2) to build carbohydrates such as glucose.

Some carbon is converted into starch and cellulose and other compounds that make up the plant. Chemical molecules that contain carbon are said to be "organic". When first discovered, these organic chemicals were thought to be unique to living organisms. They are the basis of all life.

When a plant is eaten this organic material is digested and the carbon atoms of the plant become atoms of the animal. Animals use the carbon substances either for energy of for building their own molecules. Thus carbon travels up the food chain.

Plants, like animals, breathe. Carbon dioxide is breathed out as carbon substances are burnt for energy. This is called respiration and provides the energy for body processes. It goes on continually. At night more CO2 goes back into the atmosphere than is absorbed by the plant's photosynthesis during daylight. This is why plants are removed from hospital wards at night

Click for Carbon Cycle Graphic

Overall plants trap much more carbon than they exhale. That is why forests are called "the lungs of the planet", as they keep breathing in the carbon dioxide giving out oxygen, thus providing life for animals. The USA proposes that the way to deal with excess carbon dioxide in the atmosphere is to plant vast areas of land with trees to absorb the excess carbon dioxide. These are variously called "managed forests" or "sinks", as they take up the carbon dioxide.

Too much carbon dioxide and methane is being released ending up in the upper atmosphere. This could and should be trapped by plants and made of more use in the living systems. As forests are being destroyed, particularly rain forests, which contain more stored dry mass than other systems, all carbon and nutrients are stored in the forest not the soil, huge quantities of CO2, on combustion of the wood, are released into the atmosphere.

The ability of the planet to absorb this excess CO2 is jeopardised, the very plants that are needed to assimilate this into their mass are destroyed so creating a vicious circle, an imbalance between what enters and exits any system. More on Climate change.

When plant or animal material dies, the carbon goes to the soil. here is it is decomposed by organisms in the soil, which incorporate the carbon into other forms which help the soil to live. Where there is not enough air and oxygen, the decomposition of carbon leads to the production of methane. Methane is also produced by grazing animals. Methane is a greenhouse gas (GhG).

Plants may be entombed in boggy areas and be compressed to form peat. Ultimately this can become encased in the earths crust to form coal or oil. On combustion the coal releases energy as heat and the trapped carbon, as carbon dioxide.

The cycle becomes out of balance when the amount of heat generated and the resultant carbon released do not balance with the amount of radiation from the sun reaching the earth and the amount of uptake of carbon dioxide by plants to make new plant growth, ultimately to decay or form fossil fuel store.

This type of cycling applies to nearly all the elements of the planet. No new matter is created but all matter is repeatedly rearranged. Everything is in constant flux. The science that analyses these cycles and systems and measures which organisms are doing what to maintain balance and order is called ecology.

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2002 Edition