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Photosystem II
The Reaction Center
The heart of photosystem II is the reaction center, where the energy of light is converted into the motion of energized electrons. At the center is a key chlorophyll molecule. When it absorbs light, one of its electrons is promoted to a higher energy. This energized electron then hops downward, through several other pigmented molecules, on to plastoquinone A, and finally over to plastoquinone B. When it gets enough electrons, this small quinone is released from the photosystem, and it delivers its electrons to the next link in the electron-transfer chain. Of course, this leaves the original chlorophyll without an electron. The upper half of the reaction center has the job of replacing this electron with a low-energy electron from water. The oxygen-evolving center strips an electron from water and passes it to a tyrosine amino acid, which then delivers it to the chlorophyll, making it ready to absorb another photon.Next: Harvesting Light
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Last changed by: A.Honegger,