Which gas is released by plants as a byproduct of photosynthesis?

• A. CO2
• B. O2
• C. N2
• D. H2

Answer: (B)

Photosynthesis releases oxygen as a byproduct when water is split during the light-dependent reactions. In the chloroplasts, light energy drives the photolysis of water, yielding electrons, protons, and oxygen gas. This oxygen diffuses out as a gas while the electrons and protons power the production of energy carriers used to fix carbon dioxide into sugars. So, the gas you observe coming off the plant is oxygen. The other gases listed aren’t produced by photosynthesis in this context: carbon dioxide is consumed to form sugars, nitrogen gas isn’t involved in this process, and hydrogen gas isn’t released as a byproduct.