During photosynthesis, which gas is released as a byproduct?

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

Answer: (D)

Photosynthesis uses light energy to split water in the light-dependent reactions, and the splitting of water releases oxygen gas as a byproduct. Water is oxidized to provide electrons for the photosynthetic electron transport chain, and the oxygen atoms form O2 that escapes the plant. Carbon dioxide is a feedstock used to make sugars, not a byproduct, while methane and nitrogen aren’t produced by this process in plants. So the gas released is oxygen.