Plants source elements differently: carbon from air, others from soil

Steps:

• Identify elements essential to all organisms: carbon, magnesium, iron, nitrogen.
• Recall plant nutrient uptake: minerals like magnesium, iron, nitrogen absorbed via roots from soil.
• Note carbon acquisition: plants fix CO2 from atmosphere through photosynthesis in leaves.
• Conclude carbon is the exception, not sourced from soil.

Why A is correct:

• Carbon is obtained via photosynthesis (6CO2 + 6H2O → C6H12O6 + 6O2), using atmospheric CO2, not soil.

Why the others are wrong:

• B. Magnesium: Absorbed from soil as Mg2+ ions for chlorophyll synthesis.
• C. Iron: Taken up from soil as Fe2+ or Fe3+ for enzyme function.
• D. Nitrogen: Obtained from soil as nitrates (NO3-) or ammonium (NH4+) for proteins.

Final answer: A