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.
(c) 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