Chemical Processes in Photosynthesis

Photosynthesis is the reduction of C02 by electrons from water with the help of visible irradiation producing carbohydrate and oxygen. The outline of the electrone flow is expressed by Fig. 1. The electron from water is pumped up twice by photosystems II and I (PS II and I), where chlorophyll (Chi) molecules play the main role for the excitation, energy concentration, and charge separation. 

For water oxidation, a redox potential of E, = 2.33 V (at pH 7, vs. NHE) is needed in the first step to abstract one electron from a water molecule (Eq. (1)). When the intermediate is stabilized on a catalyst and four electrons of two molecules of H20 are oxidized without isolating the intermediates (so-called four-electron process), the required redox potential is only 0.82 V (Eq. (2)). 

The potential level of the 02 evolving site of the photosynthesis (see Fig. 1) ranging around 0.82 V shows that a four-electron process occurs in it. The water oxidation site of the photosynthesis contains more than four Mn ions interacting with each other, thus leading to the four-electron reaction of water to give 02, Such a multielectron reaction leads to the generation of H2 from proton reduction as described later in chapter 4 on water photolysis. 

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The role of quinones in photobiological reactions involving chlorophyll has also been investigated (405,406). Despite the great effort and the multidisciplinary approach, progress in this field is slow because of the enormous complexity involved in photosynthetic systems. Hopefully the recent advances in experimental ESR technique, including the coupling of rapid scan ESR to flash photolysis, will help to elucidate the nature of the physical and chemical processes in photosynthesis. 

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