Correct Option
The correct option is (b) 2 only.
Explanation
Nitrogen is the most abundant gas in the atmosphere, comprising approximately 78% of its volume. Despite its abundance, atmospheric nitrogen ($N_2$) is chemically inert due to the strong triple bond between its atoms. Consequently, most organisms cannot utilize it directly from the air and rely on the nitrogen cycle to convert it into usable forms.
Statement-wise Analysis
- Statement 1 is Incorrect. Plants do not absorb nitrogen directly from the atmosphere through their leaves. Instead, they absorb nitrogen primarily from the soil through their root systems in the form of soluble ions such as nitrates ($NO_3^-$) and ammonium ($NH_4^+$).
- Statement 2 is Correct. The conversion of atmospheric nitrogen into forms usable by plants is primarily conducted by microorganisms. Bacteria such as Rhizobium (symbiotic in root nodules of legumes) and free-living bacteria like Azotobacter and blue-green algae fix atmospheric nitrogen into ammonia and nitrates, which plants can then assimilate.
- Statement 3 is Incorrect. Humans do not utilize atmospheric nitrogen through respiration. Although nitrogen constitutes the majority of inhaled air, it is not absorbed or metabolized by the lungs. The nitrogen inhaled is exhaled back into the atmosphere unchanged. Humans obtain necessary nitrogen for protein synthesis exclusively through dietary sources.
Key Takeaway: Atmospheric nitrogen is inaccessible to most life forms until it undergoes nitrogen fixation, a process primarily driven by soil bacteria that converts inert gas into reactive compounds like nitrates and ammonia.