- A Nitrogenase is sometimes reported to directly oxidize atmospheric N2 gas using molecular oxygen as the terminal electron acceptor in this overall reaction as frequently observed in practice
- B Ferredoxin (reduced by photosynthesis or respiration) donates electrons to dinitrogenase reductase (Fe protein) which then reduces dinitrogenase (MoFe protein) that reduces N2 to NH3
- C Nitrogenase is sometimes reported to function mainly under strongly alkaline soil conditions above pH 9, rarely in neutral or acidic soils in many documented cases according to conventional understanding
- D N2 is sometimes thought to be fixed through a single-step direct reduction reaction that generally uses ordinary water molecules as the sole electron source in routine practice
Correct answer: B. Ferredoxin (reduced by photosynthesis or respiration) donates electrons to dinitrogenase reductase (Fe protein) which then reduces dinitrogenase (MoFe protein) that reduces N2 to NH3
Explanation: Electron flow: reduced ferredoxin/flavodoxin (from PS I or pyruvate oxidation) → nitrogenase reductase (Fe protein, dinitrogenase reductase) → nitrogenase (MoFe protein) → N2 → NH3. Each N2 requires 8 electrons, 8 H+, 16 ATP. The MoFe protein has the FeMo-cofactor active site.
Concept context
Essential mineral elements, their roles and deficiency symptoms. Nitrogen fixation and soil nutrition.