Where do plants primarily obtain the nitrogen necessary for synthesizing amino acids and DNA?

• A. Directly from atmospheric nitrogen gas ([LaTeX]N_2[LaTeX])
• B. By absorbing nitrogen from sunlight
• C. From nitrate ([LaTeX]NO_3^-[LaTeX]) and ammonium ([LaTeX]NH_4^+[LaTeX]) ions in the soil
• D. From carbon dioxide ([LaTeX]CO_2[LaTeX]) during photosynthesis

Answer: (C)

Verified step by step guidance

1. Step 1: Understand the role of nitrogen in plants. Nitrogen is a critical element for plants as it is a key component of amino acids, which are the building blocks of proteins, and nucleotides, which make up DNA and RNA.
2. Step 2: Recognize that atmospheric nitrogen gas (N₂) is abundant but not directly usable by most plants. Plants cannot utilize nitrogen gas directly because they lack the enzymes necessary to break the strong triple bond in N₂ molecules.
3. Step 3: Learn how plants obtain nitrogen. Plants primarily absorb nitrogen in the form of nitrate (NO₃⁻) and ammonium (NH₄⁺) ions from the soil. These ions are produced through processes like nitrogen fixation (conversion of N₂ into NH₄⁺ by bacteria) and nitrification (conversion of NH₄⁺ into NO₃⁻ by bacteria).
4. Step 4: Eliminate incorrect options. Nitrogen is not absorbed from sunlight, nor is it obtained from carbon dioxide during photosynthesis. These processes are unrelated to nitrogen acquisition.
5. Step 5: Conclude that plants rely on nitrate (NO₃⁻) and ammonium (NH₄⁺) ions in the soil as their primary sources of nitrogen for synthesizing amino acids and DNA.