1150 



THE LIGHT FACTOR. III. COLOR 



CHAP. 30 



One may argue that, if the absorption band of the carotenoids in vivo 

 is strongly broadened (as suggested above), the maximum of this band may 

 be much lower than in vitro (since the area of the band — which is pro- 

 portional to the probability of the corresponding electronic transition — 

 is not hkely to be much affected by the state of the pigment) . Consequently 

 the proportion of light absorbed by carotenoids in the region of the 7 mini- 

 mum may be smaller than estimated, thus reducing the discrepancy 

 between this proportion (supposedly, 60%) and the deficiency in 7 (25%). 



100 



80 



O 60 



I- 



Q- 



q: 



° 40 



m 



< 



20 







42 mm' 



40 mm 



39 mm'' 



400 440 480 520 560 600 640 680 720 



WAVE LENGTH, m,x 



Fig. 30.2. Comparison of total absorption of Chlorella with the absorption ac- 

 tive in photosynthesis (after Emerson and Lewis, 1943). 



It is unlikely, however, that this discrepancy can be completely elimi- 

 nated by this correction, i.e., that the assumption of a strongly broadened 

 and flattened absorption band of the carotenoids will permit the explana- 

 tion of the quantum yield curve on the basis of complete inefficiency of the 

 carotenoids. The average quantum yield deficiency between 400 and 580 

 mn according to figure 30.1, is about 15%; the average contribution of the 

 carotenoids to light absorption in the same region, according to figure 

 22.44, is close to 30%. Broadening of the absorption band is more 

 likely to increase than to decrease the latter value. Thus, the most likely 

 conclusion to be drawn from the results of Emerson and Lewis is that the 

 sensitizing efficiency of the carotenoids in Chlorella is not zero, but about 

 one half that of chlorophyll. 



It need hardly be stressed that all conclusions of this type are predicated 

 on the assumption of a uniform distribution of the pigments, and would 

 have to be revised if it were demonstrated that some pigments form "color 

 screens" between the external light source and the other pigments. 



