THE CHLOROPLAST REACTION 267 



ene blue, and can be supplied with reduced coenzyme either in dark 

 by addition of such acids as citric or malic or in light in presence of 

 chloroplasts. 



Two cytochromes, cytochrome / and cytochrome b^ have been 

 shown to be present in the green leaf (9) and appear to be peculiar to 

 green tissue. Analogous cytochromes are present in the photo- 

 synthetic bacteria. Spectral changes (observed as difference spec- 

 tra) indicate that in the bacteria and in the green plant these cyto- 

 chromes are relatively more oxidized in light. (See contributions of 

 Duysens, Chance, and others.) However reduced cytochrome / is 

 not oxidized by oxygen in presence of chloroplast preparations; 

 dismissing for the moment the problem of accessibility it may be that 

 the plant has no corresponding oxidase and that oxidation in light is 

 to be attributed to a photochemical product. 



Recently light-induced phosphorylation in plastid preparations 

 has been observed which appears to differ from oxidative phosphoryl- 

 ation in at least some respects. Frenkel showed that illumination of 

 Rhodospirillum fragments in absence of oxygen resulted in phosphoryl- 

 ation of added ADP; further this reaction was not inhibited by DNP 

 (10-* M) or by iodoacetamide (10"^ M) but was inhibited rather 

 weakly by cyanide (IQ-^ M). Whatley, Allen, and Arnon similarly 

 found with "whole" chloroplast preparations from chard a light- 

 induced phosphorylation which was inhibited by oxygen; it w^as stim- 

 ulated by addition of vitamin K, ascorbic acid, FMN, and Mg++ but 

 not by addition of coenzyme. Furthermore addition of Krebs cycle 

 acid substrates in dark did not result in phosphorylation. Arnon 

 was thus led to describe this reaction as "photophosphorylation." 

 He was able to prepare from the same leaves a particulate fraction of 

 smaller average size which did not show " photophosphorylation" in 

 light but was able to phosphorylate in dark when supplied with suit- 

 able acids. This dark phosphorylation was dependent on oxygen and 

 inhibited by the usual inhibitors for oxidative phosphorylation. Oh- 

 mura in attempting to repeat the work of Arnon et al. at first (10) 

 obtained preparations of chloroplast fragments which gave oxidative 

 phosphorylation in the dark when supplied with malic or citric acids 

 but did not photophosphorylate. In later work he isolated the par- 

 ticles in presence of ascorbic acid (11). He then obtained a prepara- 

 tion of larger particles ("whole" chloroplasts) which showed "photo- 

 phosphorylation" and no oxidative phosphorylation and another of 



