9.4: Photosystem II
The multi-protein complex photosystem II (PS II) harvests photons and transfers their energy through its bound pigments to its reaction center, and ultimately to photosystem I (PSI) through the electron transport chain. The pigments responsible for caputirng the light energy in photosystems include chlorophyll a, chlorophyll b, and carotenoids.
The pigment molecules are arranged across two photosystem domains — the antenna complex and the reaction center. The main aim of the pigment molecules distributed in the antenna complex is to absorb light in the form of photons and funnel them to the special chlorophyll pair of the reaction center. However, carotenoids also serve a protective function. They help dissipate the vast amount of absorbed energy to avoid damage to the plant tissue.
The PSII reaction center, also known as P680, absorbs a photon that excites an electron in the chlorophyll. The high-energy electrons break free and are passed on to the primary electron acceptor. Oxidized P680's missing electrons are replaced by extracting low-energy electrons from the splitting of water molecules. Splitting of one H2O molecule releases two electrons, two hydrogen atoms, and one atom of oxygen. Two oxygen atoms from two H2O molecules combine to form a molecule of oxygen that is released into the environment. In contrast, the hydrogen ions released play a critical role in establishing a proton gradient across the thylakoid membrane that is essential for the synthesis of ATP in the chloroplast.