(pigment systems I and II, PSI and PSII) Two photochemical systems containing photosynthetic and accessory pigments and electron carriers that operate in sequence to perform the two light reactions of photosynthesis and so bring about photophosphorylation and reduction of NADP + (see diagram). PSII contains chlorophyll a (* P680 ) that absorbs light of 684 nm wavelength most efficiently. When activated by light it produces a strong oxidant that oxidizes water to oxygen, hydrogen ions, and electrons. The electrons are transferred via a primary electron acceptor (possibly C550) and plastoquinone and the cytochrome chain to PSI. The energy released when each electron is transferred is used to transform a molecule of ADP to ATP. PSI contains chlorophyll a (* P700 ) that absorbs light of 700 nm wavelength most efficiently. When P700 is activated a strong reductant is produced that reduces NADP + to NADPH.  |
