carbon cycle

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carbon cycle,

in biology, the exchange of carbon between living organisms and the nonliving environment. Inorganic carbon dioxide in the atmosphere is converted by plants into simple carbohydrates, which are then used to produce more complex substances. Animals eat the plants and are then eaten by other animals. When these life forms die, they decay, breaking down into, among many other things, carbon dioxide, which returns to the atmosphere. Plants and animals also release carbon dioxide during respirationrespiration,
process by which an organism exchanges gases with its environment. The term now refers to the overall process by which oxygen is abstracted from air and is transported to the cells for the oxidation of organic molecules while carbon dioxide (CO2
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. Animals and some microorganisms require the carbon-containing substances from plants in order to produce energy and as a source of materials for many of their own biochemical reactions; this cycle is vital to them. The process of incorporating carbon dioxide into the molecules of living matter is called fixation. Nearly all carbon dioxide fixation is accomplished by means of photosynthesisphotosynthesis
, process in which green plants, algae, and cyanobacteria utilize the energy of sunlight to manufacture carbohydrates from carbon dioxide and water in the presence of chlorophyll. Some of the plants that lack chlorophyll, e.g.
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, in which green plants form carbohydrates from carbon dioxide and water, using the energy of sunlight to drive the chemical reactions involved. Green plants use carbohydrates to build the other organic molecules that make up their cells, such as cellulose, fats, proteins, and nucleic acids. Some of these compounds require the incorporation of nitrogen (see nitrogen cyclenitrogen cycle,
the continuous flow of nitrogen through the biosphere by the processes of nitrogen fixation, ammonification (decay), nitrification, and denitrification. Nitrogen is vital to all living matter, both plant and animal; it is an essential constituent of amino acids,
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). When carbohydrates are oxidized in cells they release the energy stored in their chemical bonds, and some of that energy is also used by the cell to drive other reactions. In the process of oxidation, or respiration, oxygen from the atmosphere (or from water) is combined with portions of the carbohydrate molecule, producing carbon dioxide and water, the compounds from which the carbohydrates were originally formed. However, not all of the carbon atoms incorporated by the plant can be returned to the atmosphere by its own respiration; some remain fixed in the organic materials that make up its cells. When the plant dies, its tissues are consumed by bacteria and other microorganisms, a process called decay. These microorganisms break down the organic molecules of the plant and use them for their own cell-building and energy needs; by their respiration more of the carbon is returned to the atmosphere. The carbon-containing molecules that an animal derives from consuming other organisms are reorganized to build its own cells or oxidized for energy by respiration, releasing carbon dioxide and water. When the animal dies it too is decayed by microorganisms, resulting in the return of more carbon to the atmosphere. Carbon-containing molecules in wood (or other dry, slow-decaying organic materials) may be oxidized by burning, or combustion, also producing carbon dioxide and water. Under conditions prevailing on earth at certain times, green plants have decayed only partially and have been transformed into fossil fuels—coal, peat, and oil. These materials are made of organic compounds formed by the plants; when burned, they too restore carbon dioxide to the atmosphere.

carbon cycle

(Bethe–Weizsächer cycle) A chain of nuclear fusion reactions by which energy may be generated in stars. The overall effect of the cycle is the transformation of hydrogen nuclei into helium nuclei with emission of gamma-ray photons (γ), positrons (e+), and neutrinos (ν). The major sequence of reactions is as follows:
12C + 1H → 13N + γ
13N → 13C + e+ + ν
1H + 13C → 14N + γ
1H + 14N → 15O + γ
15O → 15N + e+ + ν
1H + 15N → 12C + 4He

The carbon nucleus, 12C, reappears at the end of the cycle and can be regarded as a catalyst for the reaction:

1H → 4He + 2e+ + 2ν + 3γ

Because nitrogen (N) and oxygen (O) intermediates are involved, the cycle is often termed the carbon-nitrogen-oxygen cycle or CNO cycle.

The carbon cycle is very strongly temperature dependent and becomes the dominant energy-producing mechanism at core temperatures exceeding about 20 million K. It is therefore thought to be the major source of energy in hot massive stars of spectral types O, B, and A. The carbon cycle was proposed by Hans Bethe and independently by Carl von Weizsächer in 1938. See also proton-proton chain reaction.

carbon cycle

[′kär·bən ‚sī·kəl]
The cycle of carbon in the biosphere, in which plants convert carbon dioxide to organic compounds that are consumed by plants and animals, and the carbon is returned to the biosphere in the form of inorganic compounds by processes of respiration and decay.
(nuclear physics)
References in periodicals archive ?
What we have is a strong and robust coupling between seasonal variations in atmospheric CO2 growth and tropical temperatures over the past 50 years and this provides us with a key diagnostic tool to assist in our understanding of the global carbon cycle," he said.
This contributes to our understanding of the global carbon cycle and has implications for potential ways of mitigating rising levels of carbon dioxide - one of the major causes of climate change.
The research enabled by the TCS building will touch many areas of science and society-from research in astrophysics and nuclear reactor design to searching for cures for Parkinson's disease and better drugs to fight antibiotic resistant staph infections such as MRSA to long term climate and ecosystem simulations and to a better understanding of the global carbon cycle which underpins global climate," said Rick Stevens, Argonne's associate lab director for computing and life sciences.
These results, which were thoroughly analyzed before being published now, provide a valuable contribution to our better understanding of the global carbon cycle.
Ancillary research focusing on the photosynthetic properties of various plants and microbes, for instance, will add to the laboratory's growing expertise in understanding the conversion of sunlight to sugars, relevant not only to the production of new fuels from biomass but also essential to the global carbon cycle and carbon sequestration.
Washington, June 20 ( ANI ): We need to eat less meat and recycle our waste to rebalance the global carbon cycle and reduce our risk of dangerous levels of climate change, researchers say.
Washington, Feb 18 ( ANI ): Marine phytoplankton - the tiny, single-celled algae, which act as a natural sponge for carbon dioxide and are a critical part of the global carbon cycle - may play a key role in ensuring the health of the planet, say a Canadian scientist.
This had a dramatic impact on the global carbon cycle and subsequently on the climate.
Although a few are being replanted--often with "tree farms" or worse, the pace is entirely too slow and the overall impacts on biodiversity, wildlife habitat, water supplies and the global carbon cycle remain pernicious and immense.
In the marine environment the wiry, or 'filamentous', cyanobacteria play a vital role in the global carbon cycle.
Raymo studies the causes of climate change over Earth's history, in particular the role played by the global carbon cycle and Earth's orbital variations around the Sun.
Environmental Well-Being Subject: Future Global Carbon Cycle and Climate Projection:

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