How do plants obtain the nitrogen they need for growth?

• A. By producing nitrogen through cellular respiration
• B. By absorbing nitrate and ammonium ions from the soil
• C. By directly absorbing nitrogen gas (\(N_2\)) from the atmosphere
• D. By photosynthesizing nitrogen compounds from sunlight

Answer: (B)

Verified step by step guidance

1. Understand that nitrogen is an essential nutrient for plants, as it is a key component of amino acids, proteins, and nucleic acids.
2. Recognize that plants cannot directly use nitrogen gas (\(N_2\)) from the atmosphere because it is highly stable and requires conversion into a usable form.
3. Learn that nitrogen fixation is a process carried out by certain bacteria in the soil or in symbiotic relationships with plants (e.g., legumes). These bacteria convert \(N_2\) into ammonia (\(NH_3\)), which can then be transformed into ammonium (\(NH_4^+\)) or nitrate (\(NO_3^-\)).
4. Understand that plants absorb nitrogen primarily in the form of nitrate (\(NO_3^-\)) and ammonium (\(NH_4^+\)) ions through their roots from the soil.
5. Recognize that photosynthesis is unrelated to nitrogen acquisition, as it is a process for converting sunlight into chemical energy, not for synthesizing nitrogen compounds.