Structure of reaction centres in photosynthesizing systems

In the excited singlet state the dimer of bacteriochlorophyll possesses a redox potential of 930 mV, which is sufficient to reduce the intermediate primary acceptor J. The rate of electron transfer from the excited state of bacteriochlorophyll dimer P to J is quite high (k 1011 s-1). The high rate of electron transfer ensures a high quantum yield (0 1) of the charge separation process 

The electron localized on the particle Q is subsequently used, through a complicated chain of chemical reactions, to reduce C02 to the carbohydrates (CH20)6, while the hole localized on the particle C+ is used to oxidize some certain substrate, say hydrogen sulphide to sulphur. This results in the regeneration of the active centre C-P-J-A-Q. The overall chemical reaction of photosynthesis in purple bacteria can be thus written as 

As distinct from the purple bacteria, plants, when photosynthesizing carbohydrates from C02, use water rather than H2S as an oxidized substrate. The oxidation of water requires far more energy than that of hydrogen sulphide. This circumstance plus the necessity for the sites of formation of oxidized and reduced particles to be spatially separated (in order to avoid their rapid recombination) seems to be the reason for the formation in plants 

Photosystem 1 is basically similar to the photosynthesizing system of bacteria just discussed. The difference between PSl and the photosystem of bacteria lies mainly in the fact that, instead of bacteriochlorophyll P890, the photochemical active centre of PSl contains chlorophyll a as a primary electron donor having the peak in the differential absorption spectrum at 700 nm and thus denoted as P700. In PS2 the primary donor of electrons is a chlorophyll molecule P680 with the peak in the differential optical spectrum at 680 nm. Photosystems 1 and 2 are located close to each other. Between them there is an electron transport chain containing molecules of plasto-quinones and cytochromes. 

Research on the electron transfer processes during bacterial photosynthesis is usually performed on chromatophores, i.e. extracts from photosyn-thesizing bacteria. These extracts are free from the cell walls but retain virtually all the contents of the cell membranes. These entities are convenient for research in that they scatter light much less than the bacteria themselves and, in addition, some portions of the electron transfer chain they contain can be acted upon chemically. 

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