Which gas from the air do plants primarily use for photosynthesis?

• A. Argon
• B. Oxygen
• C. Neon
• D. Carbon dioxide

Answer: (D)

Plants use carbon dioxide from the air as the key starting material for photosynthesis. In the chloroplasts, carbon dioxide is fixed in the Calvin cycle, with RuBisCO catalyzing the conversion of CO2 into organic molecules that eventually form glucose. This process requires water and light energy, and it yields oxygen as a by-product, which is released from the leaves. The other gases mentioned—argon and neon—are inert and not involved in photosynthesis, while oxygen is not used as a reactant in making the carbohydrate; it is produced. So the gas the plant primarily uses is carbon dioxide.