PHOTOSYNTHETIC PHOSPHORYLATION AND THE ENERGY CONVERSION PROCESS 345 



cult, therefore, to visualize how oxidative phosphorylation by mitochondria 

 could generate enough ATP in leaf tissues that are noted for their vigorous 

 photosynthetic activity. 



The difficulty was removed in 1954, when isolated chloroplasts were 

 found to synthesize ATP in light without the aid of mitochondria [13]. 

 When conditions were so arranged that CO2 assimilation was excluded, 

 isolated chloroplasts used light energy for the esterification of inorganic 

 phosphate in accord with the overall reaction : 



n-P + n-ADP-^^n-ATP (i) 



Light-induced ATP formation in chloroplasts raised at once the 

 question whether this process is analogous to oxidative phosphorylation by 

 mitochondria. At least two fundamental differences were apparent. ATP 

 formation by illuminated chloroplasts occurred without the consumption 

 of molecular oxvgen and without the addition of a chemical substrate to 

 supplv free energy needed for the formation of the pyrophosphate bonds 

 of ATP. The term photosynthetic phosphorylation [13, 14] was therefore 

 given to the light-induced ATP formation by chloroplasts to distinguish 

 it from oxidative (respiratory) phosphorylation by mitochondria and the 

 anaerobic phosphorylations at substrate level that occur in glycolysis. In both 

 of these processes ATP formation occurs at the expense of energy liberated 

 bv the oxidation of a chemical substrate, whereas the only "substrate" 

 which is being consumed in photosynthetic phosphorylation is light. 



4. Photosynthetic phosphorylation in chloroplasts and bacteria 



Although there was no net consumption (as measured by manometric 

 pressure change) of molecular oxygen in photosynthetic phosphorylation, 

 the process when first discovered, proceeded at a sustained rate only in the 

 presence of oxygen [13, Fig. 2 (b)]. Oxvgen seemed to act as a catalyst in 

 photosynthetic phosphorylation, not as a substrate, as it does in oxidative 

 phosphorylation. A decisive difference between photosynthetic and 

 oxidative phosphorvlation was the inability of chloroplasts to form ATP 

 in the dark by oxidizing hvdrogen donors of oxidative phosphorylation 

 with molecular oxvgen [50]. 



Further investigation of photosynthetic phosphorylation by spinach 

 chloroplasts soon resulted in the identification of FMX and ^-itamin K as 

 catalysts in the process [51, p. 6326; 52, 53]. At optimal (but still catalytic) 

 concentrations of either F^MX [>,!,] or vitamin K (Fig. 3), photosynthetic 

 phosphorylation became independent of external oxygen and proceeded 

 vigorously in an atmosphere of nitrogen or argon. At a much lower, 

 " microcatalytic ", concentration of the added cofactors, photosynthetic 

 phosphorylation remained dependent on oxvgen, although still showing no 

 net oxygen consumption. 



