OBSERVATIONS ON THE FORMATION OF THE PHOTOSYNTHETIC APPARATUS 303 



The concept has developed in recent years that TPNH furnishes reducing 

 power to a great many synthetic reactions in metabohc pathways (ref. [19], 

 Table IV), while DPNH represents the prime energy source for oxidative 

 phosphorylation carried out by mitochondria, and thus only indirectly 

 supports and controls a great variety of synthetic metabolic reactions 

 through the production of ATP. 



In oxvgen-producing plants there is an obvious relation between the 

 TPN specificity of the PPXR and the TPN specific triosephosphate 

 dehydrogenase present in leaves [20 23], as has been pointed out by 

 Arnon [24]. Is it possible that the primacy of TPN reduction over DPN 

 reduction by green plant photosynthesis makes it feasible to channel 

 photosynthetic reducing power more effectively into many biosynthetic 

 pathwavs in addition to those of carbohydrate synthesis ? This primacy of 

 TPN photoreduction over that of DPX, in oxygen-producing plants, may 

 represent an important e\olutionary advance over the situation that exists 

 in Rhodospirilliim, where, at least in vitro, isolated chromatophores 

 specifically photoreduce DPX. 



It remains to be seen whether there is any relevance to the hypothesis 

 proposed. A beginning has been made in studies on the effect of light on 

 oxidized and reduced pyridine nucleotides in green plants [25], and on the 

 metabolic fate of hydrogen in illuminated algae [26], but comparisons with 

 photosynthetic bacteria are not yet available. One thing we do know is that 

 there appears to be a much closer relation between respiration and photo- 

 synthesis in the non-sulphur purple bacteria than there is in most oxygen- 

 producing plants [27 30]. While the reasons for this can be manifold, the 

 pvridine nucleotide specificity in light-induced reactions may represent an 

 important aspect in considerations of over-all metabolic control in photo- 

 synthetic organisms. 



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