Photosynthesis In Higher Plants

The light-dependent reactions are explained in NCERT Class 11, Chapter 13 (Photosynthesis in Higher Plants). When photosystem II absorbs light, its reaction-centre chlorophyll loses excited electrons, creating an electron deficit. These electrons are replenished by the splitting of water molecules, a light-driven reaction known as photolysis, which occurs on the inner side of the thylakoid membrane. Photolysis splits water into protons, electrons and molecular oxygen; the oxygen released is the source of atmospheric oxygen, and the protons contribute to the proton gradient used for ATP synthesis. Photophosphorylation is the process of making ATP using light energy, not the splitting of water itself. Carboxylation refers to the fixation of carbon dioxide in the Calvin cycle, a dark-reaction step unrelated to water splitting. Photorespiration is a wasteful oxygen-consuming pathway in C3 plants and does not produce oxygen from water. A plausibility check supports the answer: since the oxygen evolved during photosynthesis has been shown by isotope studies to originate from water rather than carbon dioxide, the oxygen-releasing reaction at photosystem II must be the photolysis of water.