How Does Photosynthesis Produce Oxygen?

Photosynthesis occurs in two stages in the chloroplasts of plant cells. In the light-dependent reactions (occurring in the thylakoid membranes), chlorophyll and other pigments absorb sunlight and use the energy to split water molecules in a process called photolysis: 2H2O -> 4H+ + 4e- + O2. The oxygen is released as a waste product through the stomata (pores) in leaves. The freed electrons are passed through an electron transport chain (generating ATP — cellular energy currency) and used to reduce NADP+ to NADPH. In the light-independent reactions (Calvin cycle, in the stroma), ATP and NADPH power the fixation of CO2 into glucose (C6H12O6). The overall equation is: 6CO2 + 6H2O + light energy -> C6H12O6 + 6O2. All the oxygen released comes from water splitting, not from CO2 — this was proven in 1941 using oxygen-18 isotope tracing. About 50-70% of Earth atmospheric oxygen comes from marine phytoplankton, with the rest from land plants.