Which gas is essential for photosynthesis?

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

Answer: (C)

Photosynthesis relies on carbon dioxide as the source of carbon to build the sugars that organisms use for energy and structure. In the light-dependent reactions and the Calvin cycle, plants take in carbon dioxide from the air and, using light energy, convert it into organic molecules like glucose. The overall equation shows carbon dioxide plus water producing glucose and releasing oxygen as a byproduct, so CO2 is the key gas that must be available for the process to proceed. Oxygen is not consumed in the synthesis of carbohydrates; it is produced and released. Nitrogen is essential for building proteins and chlorophyll but isn’t a reactant in the photosynthetic carbon-fixation step, and methane isn’t part of the process at all.