Why is nitrogen fixation necessary for most living organisms?

• A. Because it converts ammonia into nitrate, which is the only usable form of nitrogen.
• B. Because nitrogen fixation produces oxygen required for cellular respiration.
• C. Because it removes excess nitrogen from the soil to prevent toxicity.
• D. Because atmospheric nitrogen (\(N_2\)) is inert and cannot be directly used by most organisms.

Answer: (D)

Verified step by step guidance

1. Understand the concept of nitrogen fixation: Nitrogen fixation is the process by which certain organisms, such as bacteria, convert atmospheric nitrogen (\(N_2\)), which is inert and unusable by most organisms, into biologically usable forms like ammonia (\(NH_3\)).
2. Recognize the importance of nitrogen: Nitrogen is a critical element for living organisms as it is a key component of amino acids, proteins, nucleic acids (DNA and RNA), and other biomolecules.
3. Identify the problem with atmospheric nitrogen: Atmospheric nitrogen (\(N_2\)) is highly stable due to its triple bond, making it chemically inert and unavailable for direct use by most organisms.
4. Learn how nitrogen fixation solves this issue: Nitrogen-fixing bacteria, such as those in the genus *Rhizobium*, use specialized enzymes like nitrogenase to break the triple bond in \(N_2\) and convert it into ammonia (\(NH_3\)), which can then be further processed into other usable forms like nitrate (\(NO_3^-\)).
5. Understand the ecological significance: Nitrogen fixation is essential for maintaining the nitrogen cycle, ensuring that nitrogen is available in forms that can be absorbed by plants and subsequently passed through the food chain to other organisms.