carbon cycle

1. Ecol. the circulation of carbon atoms in the biosphere as a result of photosynthetic conversion of carbon dioxide into complex organic compounds by plants, which are consumed by other organisms: the carbon returns to the atmosphere in the form of carbon dioxide as a result of respiration, decay by fungi, bacteria, etc., and combustion of fossil fuels.

2. Astrophysics. a cycle of nuclear transformations in the interiors of the stars by means of which hydrogen is gradually converted into helium with the release of nuclear energy.

[1910-15]

* * *

Circulation through nature of carbon in the form of the simple element and its compounds.

The source of carbon in living things is carbon dioxide (CO₂) from air or dissolved in water. Algae and green plants (producers) use CO₂ in photosynthesis to make carbohydrates, which in turn are used in the processes of metabolism to make all other compounds in their tissues and those of animals that consume them. The carbon may pass through several levels of herbivores and carnivores (consumers). Animals and, at night, plants return the CO₂ to the atmosphere as a by-product of respiration. The carbon in animal wastes and in the bodies of organisms is released as CO₂ in a series of steps by decay organisms (decomposers), chiefly bacteria and fungi (see fungus). Some organic carbon (the remains of organisms) has accumulated in Earth's crust in fossil fuels, limestone, and coral. The carbon of fossil fuels, removed from the cycle in prehistoric times, is being returned in vast quantities as CO₂ via industrial and agricultural processes, some accumulating in the oceans as dissolved carbonates and some staying in the atmosphere (see greenhouse effect).

* * *

▪ ecology

  in biology, circulation of carbon in various forms through nature. Carbon is a constituent of all organic compounds, many of which are essential to life on Earth. The source of the carbon found in living matter is carbon dioxide (CO₂) in the air or dissolved in water. Algae and terrestrial green plants (producers) are the chief agents of carbon dioxide fixation through the process of photosynthesis, through which carbon dioxide and water are converted into simple carbohydrates. These compounds are used by the producers to carry on metabolism, the excess being stored as fats and polysaccharides. The stored products are then eaten by consumer animals, from protozoans to man, which convert them into other forms. All animals return CO₂ directly to the atmosphere as a by-product of their respiration. The carbon present in animal wastes and in the bodies of all organisms is released as CO₂ by decay, or decomposer, organisms (chiefly bacteria and fungi) in a series of microbial transformations.

      Part of the organic carbon—the remains of organisms—has accumulated in the Earth's crust as fossil fuels (fossil fuel) (e.g., coal, gas, and petroleum), limestone, and coral. The carbon of fossil fuels, removed from the cycle in prehistoric time, is now being released in vast amounts as CO₂ through industrial and agricultural processes, much of it quickly passing into the oceans and there being “fixed” as carbonates. If oxygen is scarce (as in sewage, marshes, and swamps) some carbon is released as methane gas.

▪ nuclear fusion

also called  carbon–nitrogen cycle 

      sequence of thermonuclear reactions that provides most of the energy radiated by the hotter stars. It is only a minor source of energy for the Sun and does not operate at all in very cool stars. Four hydrogen nuclei are in effect converted into one helium nucleus, a fraction of the mass being released as energy (according to the law of mass–energy equivalence, E = mc²). The German-U.S. physicist Hans Bethe (Bethe, Hans), in 1938, first described the process.

      The reactions are as follows: a carbon-12 (¹²C) nucleus captures a hydrogen nucleus (¹H, a proton) to form a nucleus of nitrogen-13 (¹³N); a gamma ray (γ) is emitted in the process. The nitrogen-13 nucleus emits a positive electron (positron, e⁺) and becomes carbon-13 (¹³C). This nucleus captures another proton, becomes nitrogen-14 (¹⁴N), and emits another gamma ray. The nitrogen-14 captures a proton to form oxygen-15 (¹⁵O); the resulting nucleus ejects a positron as above and is thereby transformed to nitrogen-15 (¹⁵N). Eventually, the nitrogen-15 nucleus captures a fast-moving proton and breaks down into a carbon-12 nucleus plus a helium nucleus (alpha particle) of mass 4 (⁴He).

      In symbols:

 

* * *