Q.51 Which one of the following statements is INCORRECT?
(A) Nitrogen fixation is aerobic process
(B) Dinitrogenase catalyzes reduction of nitrogen to ammonia
(C) Root nodules are found in Glycine max
(D) Nitrogen fixation is anaerobic process

The incorrect statement is (A) Nitrogen fixation is aerobic process; biological nitrogen fixation itself is fundamentally an anaerobic, oxygen-sensitive process, even when it occurs in aerobic organisms or tissues.​

Introduction

Nitrogen fixation is a key biochemical process that converts atmospheric nitrogen into ammonia, making nitrogen available to plants and sustaining the global nitrogen cycle. Exam questions on nitrogen fixation aerobic or anaerobic MCQ formats often test conceptual clarity about enzyme sensitivity to oxygen, root nodules, and the catalytic role of dinitrogenase.​

Option-by-option explanation

Option (A): Nitrogen fixation is aerobic process

This statement is incorrect. Biological nitrogen fixation, catalyzed by nitrogenase, is inherently an anaerobic process because the nitrogenase complex is highly oxygen-sensitive and is irreversibly inactivated by molecular oxygen. Even in aerobic nitrogen-fixing bacteria like Azotobacter or in legume nodules, the cell creates a micro-anaerobic environment (e.g., via high respiratory rates or leghemoglobin) so that nitrogenase operates effectively in the absence of free O₂.​

Why this is the wrong option in the MCQ

• The phrase “nitrogen fixation is aerobic process” implies that the enzymatic fixation itself requires or tolerates oxygen, which contradicts the oxygen-labile nature of nitrogenase.​

• Conceptually correct framing is: nitrogen fixation is an anaerobic process that can take place in aerobes, but only in protected, low-O₂ microenvironments.​

Option (B): Dinitrogenase catalyzes reduction of nitrogen to ammonia

This statement is correct. In the nitrogenase complex, the MoFe-protein (often called dinitrogenase) is the catalytic component that directly reduces atmospheric dinitrogen (N₂) to ammonia (NH₃). Electrons are transferred from the Fe-protein (dinitrogenase reductase) to dinitrogenase, which then performs stepwise reduction and protonation of N₂ to yield NH₃ under physiological conditions.​

Key points:

• Dinitrogenase is the site where N₂ binds and is converted to ammonia via multi-electron, multi-proton transfer steps.​

• The overall biological nitrogen fixation reaction requires at least eight electrons and a minimum of sixteen ATP molecules per N₂ reduced, with dinitrogenase executing the chemical reduction.​

Thus, option (B) correctly describes the biochemical role of dinitrogenase in nitrogen fixation.​

Option (C): Root nodules are found in Glycine max

This statement is correct. Glycine max is the cultivated soybean, a classic legume that forms nitrogen-fixing root nodules in symbiosis with rhizobia such as Bradyrhizobium species. These nodules harbor bacteria that differentiate into bacteroids, expressing nitrogenase and fixing atmospheric N₂ into ammonia, which the plant assimilates.​

Important facts:

• Soybean (Glycine max) plants routinely develop root nodules that are anatomically and physiologically specialized for nitrogen fixation.​

• Numerous studies on soybean nodulation, autoregulation of nodulation, and nodule physiology explicitly use Glycine max as the model legume species.​

Therefore, option (C) is a true statement consistent with standard plant physiology and microbiology.​

Option (D): Nitrogen fixation is anaerobic process

This statement is correct in the biochemical sense. Nitrogen fixation is called an anaerobic process because nitrogenase catalysis must occur in the absence of free oxygen; oxygen inactivates the enzyme. Even when nitrogen fixation occurs in aerobes (e.g., Azotobacter) or in well-aerated legume roots, the system uses protective strategies such as leghemoglobin, high respiratory rates, or specialized cell structures to maintain low internal O₂ around nitrogenase.​

Key points:

• Biological nitrogen fixation requires an oxygen-free or oxygen-limited microenvironment for nitrogenase stability and activity.​

• This is why standard texts describe biological nitrogen fixation as an anaerobic process, despite its occurrence in aerobically growing organisms.​

Thus, option (D) is the correct conceptual description, reinforcing that the process is anaerobic at the enzymatic level, making option (A) the only incorrect statement among the four.