In the process of photosynthesis, plants use water (H2O), carbon dioxide (CO2), and energy from sunlight to produce sugar. In the process, they give off oxygen (O2) as a waste product. Does the oxygen come from the water or the carbon dioxide?
During photosynthesis, the water molecules are broken down. The hydrogen is combined with the carbon dioxide to make sugar, and the oxygen is given off as a waste product. The oxygen you are breathing came from water molecules taken apart by plants.