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Question 8

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

Answer

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 leg-haemoglobin

(c) Non-haem iron protein, ferrodoxin as the electron-carrier

(d) 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+ + 16 ATP→ 2 NH3 + 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 leg-haemoglobin, which protects nitrogenase from oxygen.

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