Why do plants require assistance in converting atmospheric \(\mathrm{N_2}\) into a usable form, and which organisms typically provide this help?

• A. Because plants cannot break the strong triple bond in \(\mathrm{N_2}\); nitrogen-fixing bacteria in the soil or root nodules provide this help.
• B. Because plants lack chlorophyll; fungi in the soil convert \(\mathrm{N_2}\) for them.
• C. Because plants need \(\mathrm{N_2}\) for photosynthesis; viruses in the soil help convert \(\mathrm{N_2}\).
• D. Because plants cannot absorb \(\mathrm{N_2}\) gas directly; insects in the soil convert \(\mathrm{N_2}\) for them.

Answer: (A)

Verified step by step guidance

1. Understand the chemical structure of atmospheric nitrogen (\(\mathrm{N_2}\)): Atmospheric nitrogen exists as a diatomic molecule with a strong triple bond between the two nitrogen atoms. This bond requires significant energy to break, making \(\mathrm{N_2}\) chemically inert and unavailable for direct use by plants.
2. Recognize the biological limitation of plants: Plants lack the enzymes necessary to break the triple bond in \(\mathrm{N_2}\) and convert it into a biologically usable form, such as ammonia (\(\mathrm{NH_3}\)) or nitrate (\(\mathrm{NO_3^-}\)).
3. Identify the role of nitrogen-fixing bacteria: Certain bacteria, such as those in the genus *Rhizobium*, possess the enzyme nitrogenase, which can break the triple bond in \(\mathrm{N_2}\) and convert it into ammonia (\(\mathrm{NH_3}\)). These bacteria often form symbiotic relationships with plants, particularly legumes, by colonizing their root nodules.
4. Explain the symbiotic relationship: In this mutualistic interaction, nitrogen-fixing bacteria provide usable nitrogen compounds to the plant, while the plant supplies carbohydrates and a protective environment for the bacteria within its root nodules.
5. Clarify why other options are incorrect: Fungi, viruses, and insects do not play a role in converting \(\mathrm{N_2}\) into usable forms for plants. Additionally, \(\mathrm{N_2}\) is not directly needed for photosynthesis, and plants do not rely on chlorophyll for nitrogen fixation.