Energy transferring reactionsThe pigment molecules that capture energy from the sun are arranged in the thylakoid membranes of the chloroplasts in structurally separate units called photosystems. Hundreds of systems are present in each thylakoid. Each photosystem has a light collecting array of 200–300 molecules, the antenna complex, not unlike a satellite dish, which collects and focuses the photons into the reaction center where energy processing begins. The photons move from antenna molecule to antenna molecule without an energy loss, a resonance energy transfer. There are two kinds of photosystems, Photosystem I (PsI) and Photosystem II (PsII). In the reaction center of PsI the light-absorbing pigment is a specialized chlorophyll a molecule that absorbs red light of 700 nanometer wavelength most efficiently, hence the designation P700. PsII
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