Process of progressive reduction of one molecule of nitrogen during fixation in leguminous plants :
(a) Process of converting free atmospheric nitrogen
by living organisms (nitrogen fixers) into those form which plants can absorb, is called biological nitrogen fixation.
(b) N2 fixers may be free-living, e.g., Azotobader,
Clostridium and cyanobacteria like Nostoc, Anabaena and Aulosira or they may be symbionts such as Rhizobium living in the root nodules of leguminous plants.
© The nodules are the sites of nitrogen fixation,
Nodules contain leghaemoglobin (Lb) and the enzyme nitrogenase.
(d) Leghaemoglobin is a pink colour pigment similar
to haemoglobin of vertebrates and functions as an oxygen-scavenger and protects the nitrogenase from oxygen.
(e) During this process, a dinitrogen (N = N) molecule
is reduced by the addition of hydrogen atoms j into two molecules of ammonia catalysed by the nitrogenase enzyme.
(f) It requires (i) a strong reducing agent, (ii) energy in
the form of ATP to transfer the hydrogen atom and i
(ii) the enzyme systems. N2 + 8e_ + 8H+ + 16 ATP > 2NH3 + H2 + 16ADP + 16 Pi