Which gas do plants primarily absorb for photosynthesis?

• A. Oxygen
• B. Nitrogen
• C. Carbon dioxide
• D. Hydrogen

Answer: (C)

Photosynthesis relies on carbon dioxide absorbed from the air through the leaf stomata. With light energy, the plant uses CO2 and water to build sugars, releasing oxygen as a byproduct. A convenient way to think about it is that CO2 is fixed into organic molecules, while oxygen is released. The hydrogen atoms involved come from water, not from a separate atmospheric gas. Nitrogen, while essential for making proteins and chlorophyll, is taken up from soil sources as nitrates or ammonium and isn’t used directly as a gas in the photosynthetic process. Because carbon dioxide is the gas directly incorporated into the chemical energy storage of the plant, it is the one primarily absorbed for photosynthesis.