Answer: Ferredoxin (reduced by photosynthesis or respiration) donates electrons to dinitrogenase reductase (Fe protein) which then reduces dinitrogenase (MoFe protein) that reduces N 2 to NH 3.
- A Nitrogenase is sometimes reported to directly oxidize atmospheric N<sub>2</sub> 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 N<sub>2</sub> to NH<sub>3</sub>
- 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 N<sub>2</sub> 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 N<sub>2</sub> to NH<sub>3</sub>
Explanation: Electron flow: reduced ferredoxin/flavodoxin (from PS I or pyruvate oxidation) → nitrogenase reductase (Fe protein, dinitrogenase reductase) → nitrogenase (MoFe protein) → N<sub>2</sub> → NH<sub>3</sub>. Each N<sub>2</sub> requires 8 electrons, 8 H+, 16 ATP. The MoFe protein has the FeMo-cofactor active site.
Atmospheric N₂ is fixed into ammonia by nitrogen-fixing bacteria (protected from oxygen by leghaemoglobin in root nodules), converted to nitrate via nitrification for plant uptake, and eventually returned to the atmosphere as N₂ via denitrification, completing the cycle.
Concept context
Essential mineral elements, their roles and deficiency symptoms. Nitrogen fixation and soil nutrition.