THE MECHANISM OF PHOTOSYNTHESIS 



309 



behavior, fluorescing much more strongly with light absorbed by chloro- 

 phyll a than with that absorbed by the biUns. This suggests that chloro- 

 phyll a in the red algae has a low photosyuthetie activity because part of 

 it is connected with the 725-m^t-fluorescent pigment, which easily takes 

 over its energy. Duysens obtained some indications that this unknown 

 pigment might be chlorophyll d. Since the absorption of chlorophyll d 

 in the organisms used is not very evident in comparison with its fluo- 

 rescence, the fluorescence must have an unusually high yield. 



100 



80 



' 50 



Q. 

 (E 

 O 



S 40 

 < 



20 



400 440 



480 



680 720 



760 



520 560 600 640 

 WAVE LENGTH, m/i 



Fig. 5-8. Absorption spectra of pigments, and spectral sensitivity of photosynthesis in 

 a red alga. (From Haxo and Blinks, 1950.) 



Duysens concludes, from his cjuantitative estimates of the yield of 

 chlorophyll a fluorescence, that in red algae phycoerythrin and phyco- 

 cyanin transfer their excitation energy to chlorophyll a with an efficiency 

 probably above 80 per cent. This emphasizes the dual character of the 

 chlorophyll a in these plants. Part of it is active in photosynthesis and 

 accepts the bulk of energy absorbed by the other pigments; the other 

 part is inactive in photosynthesis, being coupled with the unknown, 

 strongly fluorescent pigment and inaccessible to the energy absorbed by 

 other pigments. 



Duysens holds the view that excitation energy flows to the chemical 

 stage of the photosyuthetie process only via chlorophyll a and, in bac- 

 teria, via the bacteriochlorophyll complex with the lowest excitation level. 



Green bacteria have not been studied. 



The importance of fluorescence spectra and action spectra for fluo- 

 rescence in the study of the mechanism of energy transfer, so clearly 

 demonstrated in these investigations, was theoretically discussed earlier 

 by Wassink (1948b) (cf. also Sect. 5). 



Haxo and Blinks (1950) found that the greater part of the carotenoids 



