Question

Plants cannot absorb molecular nitrogen from the atmosphere because

• A. It has double bonds making it highly stable.
• B. It has triple bonds making it highly stable.
• C. Its abundance in atmosphere inhibits absorption.
• D. It has double bonds making it highly unstable.

Hint:

Remember the structure of molecular nitrogen (\(N \equiv N\)) and the enzyme responsible for its fixation (nitrogenase). The stability of this triple bond is the central reason for the existence of the biological nitrogen cycle.

Answer 1

The Correct Option is B

Step 1: Concept Overview:
The problem focuses on the chemical reason plants cannot directly utilize atmospheric nitrogen gas (\(N_2\)). Nitrogen is crucial for plant development.
Step 2: In-Depth Analysis:
Atmospheric nitrogen is present as a diatomic molecule, \(N_2\).
The \(N_2\) molecule contains two nitrogen atoms joined by a very strong triple covalent bond (\(N \equiv N\)).
This triple bond possesses a high bond dissociation energy (\(\sim 945\) kJ/mol), rendering the \(N_2\) molecule chemically inert and exceptionally stable.
Plants, and almost all eukaryotes, lack the enzymes required to cleave this strong triple bond.
The conversion of atmospheric \(N_2\) to a usable form, such as ammonia (\(NH_3\)), is termed nitrogen fixation. This process is mainly performed by specific prokaryotic microorganisms (e.g., Rhizobium) that have the nitrogenase enzyme complex, capable of breaking the triple bond.
Step 3: Conclusion:
The high stability of the triple bond in molecular nitrogen prevents its direct uptake by plants.