the electrons needed by photosystem 2 originate from splitting of water.
where do the electrons needed by photosystem 2 originate?
splitting of water
hill's finding was that the origin of oxygen in photosynthesis is water (ho) not carbon dioxide (co) as previously believed. hill's reaction also demonstrates that the light dependent redox reaction is the first reaction to take place in photosynthesis. the electrons thus freed by the splitting of water provides the electrons needed by the antenna system of photosystem ii where they are boosted in energy and then sent along the electron transport system (ets). those high energy electrons are able to move against a chemical potential gradient. hence, solar energy is converted to chemical energy by the reduction of nadp to nadph.
hill reaction
linear electron transport through a photosystem will leave the reaction center of that photosystem oxidized. elevating another electron will first require re-reduction of the reaction center. the excited electrons lost from the reaction center (p700) of photosystem i are replaced by transfer from plastocyanin, whose electrons come from electron transport through photosystem ii. photosystem ii, as the first step of the "z-scheme", requires an external source of electrons to reduce its oxidized chlorophyll "a" reaction center, called p680. the source of electrons for photosynthesis in green plants and cyanobacteria is water. two water molecules are oxidized by four successive charge-separation reactions by photosystem ii to yield a molecule of diatomic oxygen and four hydrogen ions. the electrons yielded are transferred to a redox-active tyrosine residue that then reduces the oxidized p680. this resets the ability of p680 to absorb another photon and release another photo-dissociated electron. the oxidation of water is catalyzed in photosystem ii by a redox-active structure that contains four manganese ions and a calcium ion; this oxygen-evolving complex binds two water molecules and contains the four oxidizing equivalents that are used to drive the water-oxidizing reaction (dolai's s-state diagrams). photosystem ii is the only known biological enzyme that carries out this oxidation of water. the hydrogen ions are released in the thylakoid lumen and therefore contribute to the transmembrane chemiosmotic potential that leads to atp synthesis. oxygen is a waste product of light-dependent reactions, but the majority of organisms on earth use oxygen for cellular respiration, including photosynthetic organisms.
photosynthesis