XVI . Apoproteins of pigment-proteins Complexes
In green plants, conversion of solar energy to chemical energy takes place in the thylakoid membranes of the chloroplast via the process of photosynthesis. Photosynthesis in turn is catalyzed by chlorophyll (Chl) coordinated to proteins (apoproteins). The Chl protein complexes are organized into two photosystems, namely photosystem I (PSI) and photosystem II (PSII) which function in series, and light harvesting antennas. Antenna pigments absorb light energy and channels it to the reaction centers of the photosystems (P680 in PSII and P700 in PSI) where the photochemical act takes places.
Photooxidation of P680 generates a strong oxidant that oxidizes water in the thylakoid lumen. The electrons from oxidized P680+, are transferred to plastoquinone via several electrons acceptors bound to PSII . From there the electrons are transferred to plastocyanin (in the thylakoid lumen) via the cytochrome b6/f Rieske complex.
The transfer of electrons from plastoquinone to plastocyanin is accompanied by proton translocation from the stroma to the thylakoid lumen. The resulting proton gradient results in the formation of ATP from ADP and inorganic phosphate. Plastocyanin reduces in turn Photooxidized P700+. Photooxidation and electron transfer in PSI results in the reduction of ferredoxin which in turn reduces NADP+ in the chloroplast stroma.
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