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Eugene Pabinowitch 



lower en/ymatic reaction chain), with a high oxidation po- 

 tential (approximately +0.8 V) , to an acceptor (an inter- 

 mediate in the upper enzymatic reaction chain) with a high 

 reduction potential (approximately -0.4 V). It thus leads 

 to the storage of about 1.2 eV of chemical energy per elec- 

 tron (or hydrogen atom) transferred. Since the reduction of 

 carbon dioxide to the carbohydrate level requires the trans- 

 fer of four hydrogen atoms, the total storage in the reduc- 

 tion of one carbon dioxide molecule is 4.8 eV, or about 110 

 kcal. per mole. Energy storage in a form other than oxida- 

 tion-reduction energy can play only an auxiliary role in 

 photosynthesis; this applies, in particular, to the forma- 

 tion of high energy phosphate (ATP). 



2. 



Experiments by Emerson (and others) have established that 

 in normal photosynthesis, about 8 light quanta are needed to 

 transfer 4 hydrogen atoms (or electrons) "from ZTT to X" (in 

 figure l). As suggested already in 1947, this could be 

 achieved in two ways: either by eight parallel one-electron 

 transfers, followed by four dismutations ; or by two consec- 

 utive sets of four transfers each. Various recent studies 

 support the second alternative. They suggest that the first 

 transfer leads from ZTI to an intermediate acceptor roughly 

 halfway between XR and Z--in other words, with an oxidation 

 potential of the order of +0.2 Vr-and the second from this 

 to the ultimate acceptor, X. 



The finding of cytochromes in chl oroplasts , due to T?obin 

 Hill and coworkers, led to speculations on the role of these 

 catalytic proteins in photosynthesis. At first, they have 

 been assigned--by Arnon, among others--to positions on one 

 or the other end of the photochemical sequence. This re- 

 quired, however, ascribing to them an extreme (positive or 

 negative) potential, not characteristic of known cyto- 

 chromes. Much more plausible is Hill's and Bendel's recent 

 suggestion that the chloroplast cytochromes serve as inter- 

 mediates between the two photochemical reactions. Of the 

 two cytochromes found by Hill in chl oroplasts , one belongs 

 to the group of cytochromes bi, ("cytochrome bf,")> and has 

 a n.ormal potential of approximately 0.0 V, while the other, 

 designated as "cytochrome i_" (of cytochrome c_ type), has a 

 potential of +0.37 V. It has been suggested that they 

 serve in series, as indicated in figure 2. 



