Nitrogenase complex:

Biological  nitrogen  fixation  is carried  out  through  the  nitrogenase  complex  which consists  of two proteins: a reductase,   that gives electrons with high reducing power, and a nitrogenase,  that uses these electrons  to decrease N₂ to NH₃. The reductase is a 64 kDa dimer of identical subunits which hold one iron-sulfur cluster and two ATP binding sites. The nitrogenase is a bigger protein of 220 kDa which consists of two α- and two β-subunits (α2β2) and contains an iron-molybdenum complex.  The transport of electrons from the reductase to the nitrogenase protein is coupled to the hydrolysis of ATP through the reductase.

Figure:  The flow of electrons in the nitrogenase-catalyzed reduction of N2.

while the reduction of N₂ to NH₃ is only a six-electron process:

N₂ +6 e^- + 6H⁺→ 2NH₃

the reductase  is imperfect  and H₂  is also establish. Therefore two additional electrons are also needed:

N₂ + 8 e^- +8 H⁺→ 2NH₃ +H₂

The   eight   high-potential   electrons   come   from   reduced   ferredoxin   which   is produced either in chloroplasts by the action of photosystem I or in oxidative electron transport. The whole reaction of biological nitrogen fixation:

N² +8 e^- +16 ATP +16H₂O → 2NH₃+H₂+16ADP+16P_i+8H⁺

highlights  which  it is energetically  very  expensive,  with  at least  16  ATP  molecules being hydrolyzed.