During the light-dependent reactions of photosynthesis, what is the primary source of electrons that replace those lost by Photosystem II when it is photo-excited?

💡 Solution & Explanation

In oxygenic photosynthesis, Photosystem II absorbs light energy and expels energized electrons, which are replenished through the enzyme-mediated photolysis of water molecules occurring on the lumenal side of the thylakoid membrane. This photolytic splitting yields molecular oxygen, protons, and electrons that flow into the reaction center. Carbon dioxide is utilized subsequently during the Calvin cycle in the stroma. Robert Hill demonstrated that isolated chloroplasts evolve oxygen in the presence of an electron acceptor.