Nitrogen Metabolism: Molecular Nitrogen and Nitrogen Fixation: Abiological and Biological Nitrogen Fixation (For CBSE, ICSE, IAS, NET, NRA 2022)

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In atmosphere nitrogen is present freely as dinitrogen or nitrogen gas. Molecular Nitrogen or diatomic nitrogen is triple bonded so is highly stable and due to this stability, molecular nitrogen is not very reactive in the atmosphere under normal conditions. For living being՚s nitrogen is essential constituent and nitrogenous bases are part of nucleic acids. Proteins are made up of amino acids of which nitrogen is an important constituent. Air has but most of the living beings cannot utilize this atmospheric Nitrogen and it has very low boiling point which is even lower than oxygen. Nitrogen cycle converts this nitrogen into a usable form. Lighting fixes nitrogen in NH3 and nitrogen fixing bacteria like Rhizobium also convert N2 into NH3. Most plants absorb nitrates from soil and reduce it to NH3 in the cells for further metabolic reactions. Dead organisms and their excreta like urea are decomposed by bacteria into NH3 and by a different set of bacteria into nitrates. These are in the soil for use by plants.

Image Showing Nitrogen Cycle

Nitrogen Fixation (Biological and Abiological)

Nitrogenous compounds which are unreactive in nature are highly essential for plants. These compounds are responsible for the formation of proteins in plants. The process of conversion of atmospheric nitrogen into useful nitrogenous compounds is called nitrogen fixation. Nitrogen fixation is a reductive process i.e.. , nitrogen fixation will stop if there is no reducing condition or if oxygen is present. This nitrogen fixation may take place by two different methods – a biological and biological.

Abiological Nitrogen Fixation

  • In a biological nitrogen fixation, the nitrogen is reduced to ammonia without involving any living cell. Abiological fixation can be of two types: industrial and natural.
  • For example, ammonia gas is prepared by Haber՚s process by the combination of nitrogen and hydrogen and through at a very high temperature and pressure. The reaction is reversible, and the production of ammonia is exothermic.

  • This is industrial fixation and nitrogen reduced to ammonia.
  • In natural process, the lightning and ultraviolet radiation in the atmosphere favor combination of gaseous nitrogen and oxygen to form nitric oxide (NO) , which are again oxidized with to form nitrogen peroxide (NO2) .

  • Nitrogen peroxide can combine with water during rain to form nitrous acid (HNO2) and nitric acid (HNO3) . These acids fall on the ground (earth surface) along with rainwater and reacts with alkaline radicals to form water soluble nitrates (NO3-) and nitrites (NO2-) .

Biological Nitrogen Fixation

Biological nitrogen fixation is reduction of molecular nitrogen to ammonia by a living cell in the presence of enzymes called nitrogenases.

Transformation of atmospheric gaseous nitrogen by micro-organisms and certain photosynthetic bacteria and blue green algae (Myxophyceae) into the combined form of NH4 + nourishing the bacteria and green plants is generally known as biological nitrogen fixation. This process is accomplished largely by three sets of organisms – the first one consisting of microorganisms living in endosymbiotic association with a number of plants, the second consisting of organisms living in the soil independent of host plants and the third consisting of a group of photosynthetic bacteria and blue green algae.

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