Nitrogen biological

Atmospheric monitoring, 23 139 Atmospheric nitrogen, biological fixation of, 11 111-112... 

The atmosphere contains about 3.9 x lO tons of elemental nitrogen. Biological nitrogen fixation and production of NO in combustion and thunderstorms are the major natural sinks. Finally, NO deposits as HNO3 in rainwater. In the Haber process N2 fixes industrially. 

Ammonia (NH ) is utilized by organisms, and it evenmaUy turns back to nitrogen (N ) or nitrate (NOj"). Nitrate is also used by organisms. However, the quantity that undergoes these reactions is miniscule compared with the entire nitrogen (N ) present in the atmosphere, which is 4x 10 kg. The amount of nitrogen biologically fixed is estimated to be 2 x 10" kg/year. 

The incorporation of carbon atoms into biological molecules is dependent upon the fixation of carbon dioxide during photosynthesis (Chapter 14). The nitrogen atoms of amino acids, nucleotides and other nitrogenous biological molecules are also derived from the atmosphere. However, the gaseous form of the element cannot be utilized directly by plants and animals. 

Howard J B and Rees D C 1996 Structural basis of biological nitrogen fixation Chem. Rev. 96 2965-82... 

Cobalt compounds have been in use for centuries, notably as pigments ( cobalt blue ) in glass and porcelain (a double silicate of cobalt and potassium) the metal itself has been produced on an industrial scale only during the twentieth century. Cobalt is relatively uncommon but widely distributed it occurs biologically in vitamin B12 (a complex of cobalt(III) in which the cobalt is bonded octahedrally to nitrogen atoms and the carbon atom of a CN group). In its ores, it is usually in combination with sulphur or arsenic, and other metals, notably copper and silver, are often present. Extraction is carried out by a process essentially similar to that used for iron, but is complicate because of the need to remove arsenic and other metals. 

Nitrogen monoxide ( nitnc oxide ) is another stable free radical Although known for hundreds of years NO has only recently been discovered to be an extremely important biochemical messenger and moderator of so many biological processes that it might be better to ask Which ones is it not involved m ... 

Our experience conditions us to focus on the organic components of the reaction—l arginine and l citrul line—and to give less attention to the inorganic one—nitric oxide (nitrogen monoxide NO) To do so however would lead us to overlook one of the most important discoveries in biology in the last quarter of the twentieth century... 

MISCELLANEOUS FLUORINATED NITROGEN HETEROGYGLIGS Two reviews (1981, 1990) include nitrogen heterocychcs not covered in the present survey (482,483). The 1990 review dealing with four-, five-, and six-membered ring heterocycHc compounds emphasizes biological properties (482). 

Biological value (BV) = % of absorbed nitrogen retained in body tissue complete retention = 100. Data in parentheses for control group, with no single-ceU protein in diet. 

Fig. 1. Biological pathways and processes iavolved ia the nitrogen cycle.

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