Which gas is primarily incorporated into plant biomass during photosynthesis?

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

Answer: (A)

During photosynthesis, plants fix carbon from carbon dioxide into organic molecules using energy captured from light. The carbon backbone of plant biomass—the sugars, starches, cellulose, and other organic matter—comes mainly from CO2 that is reduced and assembled in the Calvin cycle. In contrast, the oxygen released by the plant comes from splitting water, not from the carbon in CO2. Nitrogen, while essential for making proteins and nucleic acids, is incorporated into biomass as nitrogenous compounds after being absorbed as nitrate or ammonium, not as the primary carbon source. Methane isn’t used as a source of carbon in standard photosynthesis. So the gas that is primarily incorporated into plant biomass is carbon dioxide.