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: Understanding the Concept:
The question asks for the chemical reason why plants are unable to use the abundant nitrogen gas (\(N_2\)) directly from the atmosphere. Nitrogen is an essential nutrient for plant growth.
Step 2: Detailed Explanation:
Atmospheric nitrogen exists as a diatomic molecule, \(N_2\).
The two nitrogen atoms in the \(N_2\) molecule are linked by a very strong triple covalent bond (\(N \equiv N\)).
This triple bond has very high bond dissociation energy (\(\sim 945\) kJ/mol), which makes the \(N_2\) molecule chemically inert and extremely stable.
Plants, and indeed almost all eukaryotes, lack the necessary enzymes to break this strong triple bond.
The process of converting atmospheric \(N_2\) into a usable form like ammonia (\(NH_3\)) is called nitrogen fixation. This process is primarily carried out by certain prokaryotic microorganisms (like Rhizobium) that possess the enzyme complex called nitrogenase, which can break the triple bond.
Step 3: Final Answer:
The presence of a highly stable triple bond in molecular nitrogen makes it unavailable for direct absorption by plants.