Definition check: Nitrogen fixation refers to the direct conversion of atmospheric nitrogen gas (N2) into which biologically useful compound?

Introduction / Context:
Nitrogen fixation is the gateway that brings inert atmospheric N2 into the biosphere. Examinations often require precise identification of the immediate chemical product of this process.

Given Data / Assumptions:

• The substrate is atmospheric N2.
• The process is termed “nitrogen fixation.”
• We must name the direct product.

Concept / Approach:
Biological nitrogen fixation produces ammonia (NH3), which in aqueous environments largely exists as ammonium (NH4+). Plants and microbes can assimilate ammonium directly or after it is oxidized to nitrate by nitrifiers. Glucose and ATP are unrelated products of carbon/energy metabolism; nitrate is produced by nitrification, not fixation.

Step-by-Step Solution:
Recall the enzyme: nitrogenase reduces N2. Match the product: NH3/NH4+ is formed, not nitrate. Select “Ammonia” as the correct direct product.

Verification / Alternative check:
All nitrogen cycle schematics depict N2 → NH3 at the fixation step, before nitrification and assimilation.

Why Other Options Are Wrong:

• Glucose / ATP: Products of photosynthesis or cellular respiration, not fixation.
• Nitrate: Product of nitrification (oxidation of ammonia), not fixation.
• Urea: A metabolic excretion product in animals; not a fixation product.

Common Pitfalls:
Confusing nitrification (NH3 → NO3−) with fixation (N2 → NH3).

Final Answer:
Fixation converts N2 directly to ammonia.