SOME PHOTOCHEMICAL CONSIDERATIONS 31 



light. This seems incomprehensible insofar as the light absorbed by chloro- 

 phyll should have damaging properties. In certain Chlorella mutants the 

 chlorophyll content — by comparison with the content of carotenoids — is so 

 low that only a small fraction of the blue light could have been absorbed by 

 the chlorophyll. These mutants are, nevertheless, extremely sensitive to 

 illumination. It must therefore be concluded that in this case the photo- 

 oxidation is catalyzed by a substance having an absorption maximum in blue. 

 Thus, the carotenoids and not chlorophyll must be photodynamically 

 active. Kandler assumes that an equilibrium normally exists between both 

 pigments guaranteeing a physiologically optimal use of absorbed radiation 

 energy. Under abnormal conditions the photodynamic action depends 

 either upon chlorophyll or upon the carotenoids. In extreme cases — when 

 one of the pigments is lacking — the remaining" pigment undergoes photo- 

 oxidation. This has been shown by Claes on carotenoid-less mutants of 

 Chlorella and by Stanier on mutants of purple bacteria. From these con- 

 siderations it follows that photosensitization is not necessarily connected with 

 lack of carotenoids. The strong action of blue light shows that carotenoids 

 can also be photooxidized. 



It seems that energy absorption by various pigments may be of importance 

 in photosynthesis. They all transfer the energy absorbed to chlorophyll a 

 which is the pigment responsible for photosynthesis (4). The other pigments 

 serve to render light of various wave-lengths useful for chlorophyll a (67). 

 Teale (53) succeeded in showing in artificial pigment systems that transfer 

 of energy may equally take place between the molecules of chlorophyll a 

 themselves. Duysens (17), as well as Arnold and Meek (3), reached the same 

 conclusion. 



Warburg et al. (60) discovered in Chlorella a labile oxygenase system which 

 in the dark transfers molecular Oo to the carotenoids. This reaction is 

 inhibited upon illumination and it disappears after short heating at 65 ° C 

 (see § 66) . It is not inhibited by 1 -•'' A^ HCN and by 1 "^ A^ phenanthroline 

 but it is highly dependent upon the O2 pressures. During this enzymatic 

 oxidation of the carotenoids no oxidation of chlorophyll can be observed. 



§ 14 Some Final Remarks 



In this chapter the principles of photochemistry and some modern views 

 on electron transport and excited molecules have been discussed. It is 

 self-evident that the scope of this book permits only a few observations and 

 hypotheses dealing with the excitation of chlorophyll molecules to be treated. 

 It may well be that some work of interest has been passed over in silence. 



The question arises whether the many hypotheses discussed — often of 

 rather a speculative nature notwithstanding some experimental evidence — 

 have seriously contributed to the understanding of the photosynthetical 

 process. The main problems of photosynthesis have always been and still 

 are energetics and chemistry. The great amount of work done on the excita- 



