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Question

What are the conditions necessary for fixation of atmospheric nitrogen by Rhizobium? What is their role in N2 - fixation?

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Solution

Rhizobium is a symbiotic bacteria present in the root nodules of leguminous plants.
The basic requirements for Rhizobium to carry out nitrogen fixation are as follows:
(a) Presence of the enzyme nitrogenase.
(b) Presence of leghaemoglobin.
(c) Non-haem iron protein, ferredoxin as the electron-carriers.
(d) The constant supply of ATP.
(e) Mg2+ ions as co-factors.

Rhizobium contains the enzyme nitrogenase – a Mo-Fe protein – that helps in the conversion of atmospheric free nitrogen into ammonia.
The reaction is as follows:

N2+8e+8H++16ATP2NH3+H2+16ADP+16Pi

The Rhizobium bacteria live as aerobes under free-living conditions, but require anaerobic conditions during nitrogen fixation. This is because the enzyme nitrogenase is highly sensitive to molecular oxygen. The nodules contain leghaemoglobin, which protects nitrogenase from oxygen.


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