1152 THE LIGHT FACTOR. III. COLOR CHAP. 30 



in other words, that the fate of a large proportion of chlorophyll molecules 

 in state A is different from transfer into state Y {e.g., they may undergo 

 transition into a metastable state; cf. scheme 23.1). Whether this is true 

 not only of chlorophyll in solution but also of chlorophyll in the living 

 cell remains uncertain. 



In vivo, direct photochemical dissociation of chlorophyll by blue-violet 

 light appears even less likely than in vitro (in consideration of the known 

 photostability of chlorophyll in the living cell) . True, the yield of chloro- 

 phyll fluorescence in vivo, as a function of wave length of the exciting light 

 (represented in fig. 24.5A), shows a slight decline at the violet end of the 

 spectrum; but this decline is most likely due to the presence of carotenoids, 

 and not to a decreased efficiency of light absorbed by chlorophyll itself. 

 As a matter of fact, figure 24. 5A was interpreted on page 814 as an indica- 

 tion that the fluorescence of chlorophyll in Chlorella can be excited — al- 

 though with reduced efficiency — also by the light quanta absorbed by the 

 carotenoids; and this interpretation of the fluoresence curve is in agree- 

 ment with Emerson and Lewis' quantum yield curve of photosynthesis. 

 (However, the fluorescence curve in figure 24. 5A shows no minimum at 

 490 mn, which is so prominent in the quantum yield curve in figure 30.1.) 



To sum up, the results of the quantum yield studies of both photosyn- 

 thesis and fluorescence are best explained by the assumption that a con- 

 siderable fraction — of the order of one half — of the quanta absorbed by the carot- 

 enoids in green plants is passed over to chlorophyll, transferring the latter into 

 state Y. Therefore, these quanta can be used in the same way as those 

 absorbed directly by chlorophyll, either for photosynthesis or for fluor- 

 escence. It is furthermore likely that most or all blue-violet quanta 

 absorbed directly by chlorophyll are utilized for conversion into the fluore- 

 scent state Y. 



Quantum yield measurements of Noddack and Eichhoff (1939) and 

 Eichhoff (1939) covered only the range above 515 m/x and therefore 

 reveal nothing concerning the function of the carotenoids. Their only 

 interesting feature is the high yield in the infrared; therefore they will be 

 discussed in the next section. 



The action spectrum of the green alga Ulva taeniata, measured polaro- 

 graphically by Haxo and Blinks (1950), is represented in figure 30. 11 A, 

 and is in general agreement with the results of Emerson and Lewis. 



3. Photosynthesis of Green Plants in Ultraviolet and Infrared 



The exten.sion of the photosynthetically active region into the ultra- 

 ^'iolet has not been studied by systematic quantum yield measurements in 

 monochromatic light, although it seems to be generally assumed that the 



