500 LIGHT AND LIFE 



closely bound to the chlorophyll system, or whether ATP is formed 

 by mitochondria, by the process of oxidative phosphorylation. 



The only cytoplasmic particles known to form ATP were mitochon- 

 dria, and oxidative phosphorylation by them has usually been in- 

 voked in explaining the source of ATP used in photosynthesis [see, 

 for example, ref. 24, Fig. 7; also review, ref. 7]. In early models 

 of ATP formation in photosynthesis it was proposed that the reduc- 

 tion of pyridine nucleotide was carried out by illuminated chloro- 

 plasts and the resulting reduced pyridine nucleotide was re-oxidized 

 with molecular oxygen by mitochondria (163). This coupled chloro- 

 plast-mitochondrial system differed from conventional oxidative phos- 

 phorylation only in the source of the reduced pyridine nucleotide. 

 In one case the pyridine nucleotide was reduced by light and in the 

 other by a respiratory substrate. The phosphorylation reactions proper 

 leading to the synthesis of ATP were in both cases supposed to be 

 dependent on enzymes localized in mitochondria. 



This model for the generation of ATP in photosynthesis posed a 

 serious difficulty. The most specialized photosynthetic tissue, the 

 mesophyll of leaves, is noted for its paucity of mitochondria. Within 

 the mesophyll cells, especially in the palisade parenchyma, chloro- 

 plasts are the dominant cytoplasmic bodies; mitochondria are few 

 (48, 79) . It was difficult, therefore, to visualize how oxidative phos- 

 phorylation by mitochondria could generate enough ATP in tissues 

 noted for their vigorous photosynthetic activity. 



The difficulty was removed by the discovery in 1954 that isolated 

 chloroplasts are capable of synthesizing ATP in light without the aid 

 of mitochondria (11). When conditions were so arranged that COo 

 assimilation was excluded, isolated chloroplasts used light energy for 

 the esterification of inorganic phosphate in accord with the over-all 

 reaction: 



light 

 n-P + 77-ADP > 77-ATP (2) 



Light-induced ATP formation in chloroplasts raised at once the 

 question whether this process is analogous to oxidative phosphoryla- 

 tion by mitochondria. Two fundamental differences were apparent. 

 ATP formation by illuminated chloroplasts occurred without the 

 consumption of molecular oxygen and without the addition of a 

 chemical substrate to supply free energy needed for the formation 

 of the pyrophosphate bonds of ATP. The term photosynthetic phos- 

 phorylation (11, G) was therefore given to the light-induced ATP 



