During photosynthesis, molecular oxygen (O2) is released as a byproduct. Based on the reaction stoichiometry and electron flow, which reactant is the primary source of the oxygen atoms found in O2, and what is the defining biochemical process that causes this release?

• A. Carbon dioxide (CO2) is the primary source of oxygen atoms in O2 because fixation of CO2 directly ejects oxygen atoms as gaseous O2 during formation of sugars in the Calvin cycle.
• B. Atmospheric O2 is recycled into O2 through a catalytic loop in chloroplasts independent of water and carbon dioxide, meaning photosynthesis does not alter water but only rearranges existing oxygen gas.
• C. Water (H2O) is the primary source of the oxygen atoms in O2; the defining process is water-splitting (photolysis) during the light reactions that liberates O2 and electrons.
• D. Both water and carbon dioxide contribute equally and randomly to the oxygen atoms in O2 because the photosynthetic electron transport chain exchanges oxygen atoms between CO2 and H2O without a preferred source.

Answer: (C)