Mechanism Of Nitrogen Fixation
The reduction of atmospheric gaseous nitrogen to ammonia is called nitrogen fixation.
Nitrogen is a major component not only of proteins, but of nucleic acids, coenzymes, and many other cell constituents as well.
Nitrogen fixation is one the vital process for plants and animals. Plants cannot absorb atmospheric nitrogen, but they can intake ammonia and nitrate from the soil. This nitrogen is utilized to build protein. As nitrogen is a major component of proteins and nucleic acid it become important for life.
There is about 70% of atmospheric gases is nitrogen gas. But neither animals nor plants can utilized this atmospheric nitrogen. However, there are some microorganisms found in soil, work on atmospheric nitrogen and convert it to usable ammonia and nitrate form. A part of microbial activity there are some physical process like lightening occurs in atmosphere that add up in the conversion of atmospheric nitrogen to ammonia or nitrates.
Because ammonia and nitrate levels often are low only a few bacteria and archaea can carry out nitrogen fixation, the rate of this process limits plant growth in many situations. Nitrogen fixation occurs in (1) free-living bacteria and archaea (e.g. Azotobacter; Klebsiella, Clostridium, and Methanococcus), (2) bacteria living in symbiotic association with plants such as legumes (Rhizobium), and (3) cyanobacteria (Nostoc, Anabaena, and Trichodesmia).
The reduction of nitrogen to ammonia is catalyzed by the enzyme nitrogenase. Although the complete process of nitrogen fixation is not clear till now but it is believed that nitrogen is reduced by two-electron additions.
The reduction of molecular nitrogen to ammonia is quite exergonic, but reaction has a high activation energy because molecular nitrogen is an unreactive gas with a triple bond between the two nitrogen atoms. Therefore nitrogen reduction is expensive and requires a large ATP expenditure. At least 8 electrons and 16 ATP molecules, 4 ATPs per pair of electrons, are required.
