The oxygen released during photosynthesis comes from water molecules. During the light-dependent reactions of photosynthesis, water is split through a process called photolysis, which produces oxygen gas, hydrogen ions, and electrons. The oxygen generated from this splitting of water is then released into the atmosphere as a byproduct. Carbon dioxide absorbed by the plant is used to form glucose, not oxygen. This discovery was confirmed by experiments using radioactive isotopes that traced the source of oxygen to water, not carbon dioxide.
