Correct Option
The correct option is (a).
Explanation
The Nitrogen Cycle is a critical biogeochemical cycle that circulates nitrogen through the atmosphere, soil, and living organisms. It involves the transformation of nitrogen into various chemical forms, making it accessible to plants and animals while maintaining atmospheric stability.
Statement-wise Analysis
- Statement 1 is Correct: Nitrogen gas (N2) is the most abundant component of Earth's atmosphere, constituting approximately 78% by volume. However, in this gaseous form, it is largely inert and cannot be used directly by most life forms.
- Statement 2 is Incorrect: Nitrogen is a fundamental building block of life. It is an essential constituent of amino acids (which form proteins), nucleic acids (DNA and RNA), vitamins, and chlorophyll. It is strictly required for the synthesis of these biomolecules.
- Statement 3 is Incorrect: The nitrogen cycle operates as a closed system in terms of mass balance. While nitrogen fixation removes nitrogen from the atmosphere to the soil, processes like denitrification return nitrogen gas back to the atmosphere. This cycle ensures that the percentage of nitrogen in the atmosphere remains more or less constant, rather than constantly increasing.
Key Takeaway: Nitrogen is essential for the formation of proteins and genetic material. The nitrogen cycle maintains a dynamic equilibrium, ensuring the atmospheric concentration of nitrogen remains stable at approximately 78%.