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Carbon cycle

Part V of "Matter cycles": The carbon cycle

Carbon is a very important element, as it makes up organic matter, which is a part of all life. Carbon follows a certain route on earth, called the carbon cycle. Through following the carbon cycle we can also study energy flows on earth, because most of the chemical energy needed for life is stored in organic compounds as bonds between carbon atoms and other atoms.
The carbon cycle naturally consists of two parts, the terrestrial and the aquatic carbon cycle. The aquatic carbon cycle is concerned with the movements of carbon through marine ecosystems and the terrestrial carbon cycle is concerned with the movement of carbon through terrestrial ecosystems.

The carbon cycle is based on carbon dioxide (CO2), which can be found in air in the gaseous form, and in water in dissolved form. Terrestrial plants use atmospheric carbon dioxide from the atmosphere, to generate oxygen that sustains animal life. Aquatic plants also generate oxygen, but they use carbon dioxide from water.
The process of oxygen generation is called photosynthesis. During photosynthesis, plants and other producers transfer carbon dioxide and water into complex carbohydrates, such as glucose, under the influence of sunlight. Only plants and some bacteria have the ability to conduct this process, because they possess chlorophyll; a pigment molecule in leaves that they can capture solar energy with.


The overall reaction of photosynthesis is:
carbon dioxide + water + solar energy -> glucose + oxygen
6 CO2 + 6 H2O + solar energy -> C6H12O6 + 6 O2

The oxygen that is produced during photosynthesis will sustain non-producing life forms, such as animals, and most micro organisms. Animals are called consumers, because they use the oxygen that is produced by plants. Carbon dioxide is released back into the atmosphere during respiration of consumers, which breaks down glucose and other complex organic compounds and converts the carbon back to carbon dioxide for reuse by producers.

Carbon that is used by producers, consumers and decomposers cycles fairly rapidly through air, water and biota. But carbon can also be stored as biomass in the roots of trees and other organic matter for many decades. This carbon is released back into the atmosphere by decomposition, as was noted before.
Not all organic matter is immediately decomposed. Under certain conditions dead plant matter accumulates faster than it is decomposed within an ecosystem. The remains are locked away in underground deposits. When layers of sediment compress this matter fossil fuels will be formed, after many centuries. Long-term geological processes may expose the carbon in these fuels to air after a long period of time, but usually the carbon within the fossil fuels is released during humane combustion processes.
The combustion of fossil fuels has supplied us with energy for as long as we can remember. But the human population of the world has been expanding and so has our demand for energy. That is why fossil fuels are burned very extensively. This is not without consequences, because we are burning fossil fuels much faster than they develop. Because of our actions fossil fuels have become non-renewable recourses.

Although the combustion of fossil fuels mainly adds carbon dioxide to air, some of it is also released during natural processes, such as volcanic eruptions.

In the aquatic ecosystem carbon dioxide can be stored in rocks and sediments. It will take a long time before this carbon dioxide will be released, through weathering of rocks or geologic processes that bring sediment to the surface of water.
Carbon dioxide that is stored in water will be present as either carbonate or bicarbonate ions. These ions are an important part of natural buffers that prevent the water from becoming too acidic or too basic. When the sun warms up the water carbonate and bicarbonate ions will be returned to the atmosphere as carbon dioxide.

Schematic representations of the aquatic and terrestrial part of the carbon cycle are shown here:

1) The aquatic carbon cycle





2) The terrestrial carbon cycle

Carbon dioxide (greenhouse gas)

As many people know carbon dioxide is a greenhouse gas, which basically means that too much carbon dioxide in air causes the earth to warm up.
Humans emit great amounts of carbon dioxide during combustion processes and because of this, the greenhouse effect consisted. The greenhouse effect means that the climate is affected by the concentrations of greenhouse gasses on earth.
In the past few decades a warmer climate has developed, because of the large amounts of carbon dioxide and other greenhouse gases that we emit. This warmer climate can cause problems, such as the melting of large ice formations at the Arctic's.

For more information on CO2, move to the carbon dioxide page

For more information on carbon, move to the periodic chart

Back to main page of matter cycles

To the matter cycles pollution page

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