Nitrogen fixation: Microorganisms use ATP and a powerful reductant to reduce atmospheric nitrogen to ammonia.

Microorganisms use ATP and reduced ferredoxin, a powerful reductant, to reduce N₂ to NH₃. An iron-molybdenum cluster in nitrogenase deftly catalyzes the fixation of N₂, a very inert molecule. Higher organisms consume the fixed nitrogen to synthesize amino acids, nucleotides, and other nitrogen-containing biomolecules. The major points of entry of NH⁴⁺ into metabolism are glutamine or glutamate.