134 PROBLEMS OF PHOTOSYNTHESIS 



It does not matter whether assimilatory power is produced by the action of 

 Hght or is suppHed to the system from outside sources. In vivo, however, 

 photochemical generation of assimilatory power remains closely connected 

 with COo fixation. 



More recently, Krogmann (36) provided evidence of a third type of light- 

 dependent ATP synthesis — a type associated with the oxidation of the dye 

 trichlorophenol indophenol. Incubation of spinach chloroplasts with this 

 dye in its oxidized form, phosphate and ADP results in ATP synthesis only 

 in the presence of light and Oo. The oxidized dye is reduced by the action 

 of light and reoxidized by O.,. When the photoreduction of the dye is in- 

 hibited by certain chemicals (e.g. dichlorophenyl dimethylurea), no reoxida- 

 tion by O2 is possible and ATP synthesis stops. Upon addition of gluta- 

 thione, cysteine or ascorbic acid, the dye can be reduced. This chemical 

 reduction cannot be inhibited by dichlorophenyl dimethylurea. Light- 

 dependent reoxidation does occur but, under these circumstances, light must 

 have another site of action, as it cannot participate in a photoreduction 

 process. Nakamoto et al. (40) described the eff"ect of O2 on other chloroplastic 

 photophosphorylation systems. All these experiments clearly show how 

 complex phosphorylation must be in photosynthesis and support the finding 

 that photosynthetic phosphorylation may also be of the oxidative type. 



In § 69 we shall further examine the problem of light induced phosphoryla- 

 tion in connection with Warburg's findings concerning quinone catalysis. 

 According to Warburg's experiments light induced phosphorylation does not 

 exist. It is nothing but oxidative phosphorylation in the dark. 



§ 52 Some Final Remarks 



In this section as well as in Section C of Chapter 4 the hypothesis of water 

 photolysis forms the basis of many tentative approaches to the chemistry of 

 photosynthesis. Recent investigations of Warburg have shown that the 

 mechanism of the Hill reaction can be brought into accord with his findings 

 in living ChloreUa. The action of ferric ions and of quinone must be con- 

 sidered to be the same in isolated chloroplasts as in living ChloreUa. The 

 photodecomposition of water can thus be replaced by the photodecomposition 

 of the photolyte COo*. As a matter of fact, there is no evidence whatsoever 

 that light quanta of low energy can decompose water (46), though efforts 

 have been made to render this acceptable with the aid of excited states of 

 chlorophyll, free radicals, etc. It seems extremely abnormal that water, 

 which plays a most important but nevertheless more or less passive role in liv- 

 ing Nature, should exert such a highly "explosive" action in photosynthesis. 

 It must be admitted that nothing is impossible in Nature; nevertheless, it is 

 hardly permissible to accept this adage as the only evidence of reactions we 

 would like to see proceeding becauss they fit in well with our hypotheses. 



Part of the O2 of the photolyte certainly originates from water, due to the 

 intermediary formation of H2CO3. The li igin cl the photosynthetically pro- 



