carbon-nitrogen cycle

Did you mean: carbon-nitrogen cycle (in biology), Carbon-nitrogen-oxygen cycles

Dictionary: car·bon-ni·tro·gen cycle (kär'bən-nī'trə-jən)

A chain of thermonuclear reactions in which nitrogen isotopes are formed in intermediate stages and carbon acts essentially as a catalyst to convert four hydrogen atoms into one helium atom with the emission of two positrons. The entire sequence is thought to generate significant amounts of energy in the sun and certain other stars. Also called carbon cycle, nitrogen cycle.

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Britannica Concise Encyclopedia: nitrogen cycle

Circulation of nitrogen in various forms throughout nature. Nitrogen is essential to life, but in the atmosphere it is in a form (the diatomic molecule N₂) unavailable to most organisms. Nitrogen fixation by microbes turns this nitrogen into nitrates and other compounds, which plants or algae assimilate into their tissues. Animals that eat plants in turn incorporate the compounds into their own tissues. Microbes decompose the remains and waste of all living things into ammonia (ammonification); the ammonia may leave the soil through vaporization into the air or leaching into water. Ammonia remaining in soil may be transformed by bacteria into nitrates (nitrification), which then can be reassimilated into living organisms, or into free nitrogen (denitrification), which reenters the atmosphere. Hence, once fixed from air, some nitrogen goes through the cycle repeatedly without returning to the gaseous state.

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Columbia Encyclopedia: nitrogen cycle

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nitrogen 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, which form proteins of nucleic acids, and of many other organic materials.

Nitrogen Fixation

Although the earth's atmosphere is 78% nitrogen, free gaseous nitrogen cannot be utilized by animals or by higher plants. They depend instead on nitrogen that is present in the soil. To enter living systems, nitrogen must be “fixed” (combined with oxygen or hydrogen) into compounds that plants can utilize, such as nitrates or ammonia. A certain amount of atmospheric nitrogen is fixed by lightning and by some cyanobacteria (blue-green algae). But the great bulk of nitrogen fixation is performed by soil bacteria of two kinds: those that live free in the soil and those that live enclosed in nodules in the roots of certain leguminous plants (e.g., alfalfa, peas, beans, clover, soybeans, and peanuts). Among the free-living forms are species of Clostridium, discovered c.1893 by Sergei Winogradsky, and Azotobacter, discovered c.1901 by M. W. Beijerinck. Both Clostridium and Azotobacter are generally present in agricultural soils, and both are saprophytes, i.e., they use the energy from decaying organic matter in the soil to fuel soil processes, including nitrogen fixation.

Bacteria that live in the roots of legumes are of the genus Rhizobium, first isolated c.1888 by Beijerinck. These rod-shaped bacteria enter the roots chiefly through the root hairs and then work their way to the inner root tissues. There they stimulate the growth of tumorlike nodules. Within the nodules the bacteria develop into forms called bacteroids, which live in a symbiotic (mutually beneficial) relationship with the green plant. The bacteroids take carbohydrates from the plant for energy to fix nitrogen and synthesize amino acids; the plants take the amino acids elaborated in the nodule to build plant tissue. Animals in turn consume the plants and convert plant protein into animal protein. Rhizobia can be found free-living in the soil, but they cannot fix nitrogen in the free state, nor can the legume root fix nitrogen without Rhizobia.

The exact biochemistry of nitrogen fixation within the nodule is not yet understood. It is estimated that more than 300 lbs of nitrogen per acre (340 kg per hectare) can be fixed by fields of alfalfa and other legumes. After a harvest legume roots left in the soil decay, returning organic nitrogen compounds to the soil for uptake by the next generation of plants. For this reason crop rotation in which a leguminous crop is rotated with a nonleguminous one is a common practice for maintaining soil fertility.

Other Aspects of the Nitrogen Cycle

Decomposing animal remains and animal wastes also return organic nitrogen to the soil as ammonia. Many different kinds of decay microorganisms participate in ammonification. The nitrifying bacteria of the genus Nitrosomonas oxidize the ammonia to nitrites, and Nitrobacter oxidize the nitrites to nitrates. The nitrates can then be taken up again by the green plant. The cycle of fixation-decay-nitrification-fixation can proceed indefinitely without any nitrogen being returned to a gaseous state. But still another group of microorganisms, the denitrifying bacteria, can reduce nitrates all the way to molecular nitrogen. Denitrification occurs only in the absence of oxygen and is not common in well-cultivated soils.

Effects of Artificial Fixation

Nitrogen fixation can also be accomplished artificially by various methods (see nitrogen). Humans annually fix vast amounts of nitrogen for industrial purposes and for use as fertilizer. Unfortunately, large-scale legume cultivation and artificial fixation may be upsetting the natural nitrogen cycle in the biosphere. There is some question whether natural denitrification can keep pace with fixation. For one thing, run-off of nitrate fertilizer can cause eutrophication of lakes and streams (see water pollution) and can foul drinking supplies. Another environmental problem is that inorganic fertilizers tend to depress legume fixation. As a consequence, root tissue remaining after harvest is poorer, and thus more fertilizer must be applied the following year.

Medical Dictionary: carbon-nitrogen cycle

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Wikipedia: CNO cycle

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Overview of the CNO-I Cycle.

The CNO cycle (for carbon-nitrogen-oxygen), or sometimes Bethe-Weizsäcker-cycle, is one of two sets of fusion reactions by which stars convert hydrogen to helium, the other being the proton-proton chain. Theoretical models show that the CNO cycle is the dominant source of energy in stars heavier than about 1.5 times the mass of the sun. The proton-proton chain is more important in stars the mass of the sun or less. This difference stems from temperature dependency differences between the two reactions; pp-chain reactions start occurring at temperatures around 4×10⁶ K, making it the dominant force in smaller stars. The CNO chain starts occurring at approximately 13×10⁶ K^{[citation needed]}, but its energy output rises much faster with increasing temperatures. At approximately 17×10⁶ K^{[citation needed]}, the CNO cycle starts becoming the dominant source of energy. The Sun has a core temperature of around 15.7×10⁶ K and only 1.7% of ⁴He nuclei being produced in the Sun are born in the CNO cycle. The CNO process was proposed by Carl von Weizsäcker^[1] and Hans Bethe^[2] independently in 1938 and 1939, respectively.

In the CNO cycle, four protons fuse using carbon, nitrogen and oxygen isotopes as a catalyst to produce one alpha particle, two positrons and two electron neutrinos. The positrons will almost instantly annihilate with electrons, releasing energy in the form of gamma rays. The neutrinos escape from the star carrying away some energy. The carbon, nitrogen, and oxygen isotopes are in effect one nucleus that goes through a number of transformations in an endless loop.

1 CNO-I
2 CNO-II
3 OF Cycle
4 Use in astronomy
5 See also
6 External links
7 References

CNO-I

The main reactions of the CNO cycle are ^[3]:

126C	+	11H	→	137N	+	γ			+	1.95 MeV
137N			→	136C	+	e⁺	+	ν_e	+	2.22 MeV
136C	+	11H	→	147N	+	γ			+	7.54 MeV
147N	+	11H	→	158O	+	γ			+	7.35 MeV
158O			→	157N	+	e⁺	+	ν_e	+	2.75 MeV
157N	+	11H	→	126C	+	42He			+	4.96 MeV

where the Carbon-12 nucleus used in the first reaction is regenerated in the last reaction.

CNO-II

In a minor branch of the reaction, occurring in the Sun's core just 0.04% of the time, the final reaction shown above does not produce carbon-12 and an alpha particle, but instead produces oxygen-16 and a photon and continues as follows:

157N	+	11H	→	168O	+	γ			+	12.13 MeV
168O	+	11H	→	179F	+	γ			+	0.60 MeV
179F			→	178O	+	e⁺	+	ν_e	+	2.76 MeV
178O	+	11H	→	147N	+	42He			+	1.19 MeV
147N	+	11H	→	158O	+	γ			+	7.35 MeV
158O			→	157N	+	e⁺	+	ν_e	+	2.75 MeV

Like the carbon, nitrogen, and oxygen involved in the main branch, the fluorine produced in the minor branch is merely catalytic and at steady state, does not accumulate in the star.

OF Cycle

This subdominant branch is significant only for massive stars. The reactions are started when one of the reactions in CNO-II results in fluorine-18 and gamma instead of nitrogen-14 and alpha:

178O	+	11H	→	189F	+	γ			+	5.61 MeV
189F			→	188O	+	e⁺	+	ν_e	+	1.656 MeV
188O	+	11H	→	199F	+	γ			+	7.994 MeV
199F	+	11H	→	168O	+	42He			+	8.114 MeV
168O	+	11H	→	179F	+	γ			+	0.60 MeV
179F			→	178O	+	e⁺	+	ν_e	+	2.76 MeV

Note that all these cycles have the same net result:

4 11H → 42He + 2 e⁺ + 2 ν_e + γ + 26.8 MeV

Use in astronomy

While the total number of "catalytic" CNO nuclei is conserved in the cycle, in stellar evolution the relative proportions of the nuclei are altered. When the cycle is run to equilibrium, the ratio of the carbon-12/carbon-13 nuclei is driven to 3.5, and nitrogen-14 becomes the most numerous nucleus, regardless of initial composition. During a star's evolution, convective mixing episodes bring material in which the CNO cycle has operated from the star's interior to the surface, altering the observed composition of the star. Red giant stars are observed to have lower carbon-12/carbon-13 and carbon-12/nitrogen-14 ratios than main sequence stars, which is considered to be convincing evidence for the operation of the CNO cycle.

The presence of the heavier elements carbon, nitrogen and oxygen places an upward bound on the maximum size of massive stars to approximately 150 solar masses. It is thought that the "metal-poor" early universe could have had stars up to 250 solar masses without interference from the CNO cycle.

External links

References

^ C. F. von Weizsäcker. Physik. Zeitschr. 39 (1938) 633.
^ H. A. Bethe. Physical Review 55 (1939) 434.
^ "Introductory Nuclear Physics", Kenneth S. Krane, John Wiley & Sons, New York, 1988, p.537

v • d • e Nuclear processes

Radioactive decay	Alpha decay · Beta decay · Gamma radiation · Cluster decay · Double beta decay · Double electron capture · Internal conversion · Isomeric transition · Spontaneous fission

Other processes	Emission processes: Neutron emission · Positron emission · Proton emission Capturing: Electron capture · Neutron capture

Stellar nucleosynthesis	pp-chain · CNO cycle · α process · Triple-α · Carbon burning · Ne burning · O burning · Si burning · R-process · S-process · P-process · Rp-process

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