332 



RADIATION BIOLOGY 



row. In the case of pigment-protein complexes with chlorophyll a and b, 

 a choice between these possibilities could be made, as the writer (1948a) 

 pointed out, by studying fluorescence spectra upon irradiation with light 

 of wave lengths absorbed preferably by either chlorophyll b or a (cf . Fig. 

 5-19). Duysens (1951) meanwhile carried out such experiments and 

 found a 100 per cent transfer of energy from chlorophyll b to a, i.e., 

 from the higher excitation (shorter wave length) to the lower. This 

 rules out the third possibility for the chlorophyll a-chlorophyll b chromo- 

 phylHns (see Fig. 5-19). In the case of the fucoxanthin-chlorophyll a 



FUCOXANTHIN CHLOROPHYLL FUCOXANTHIN CHLOROPHYLL FUCOXANTHIN CHLOROPHYLL 



a a a 



ACCEPTOR 



ACCEPTOR 



ACCEPTOR 



CHLOROPHYLL CHLOROPHYLc CHLOROPHYLL CHLOROPHYLL CHLOROPHYLL CHLOROPHYLL 



-rVV^ '•' I// /// 



ACCEPTOR 



ACCEPTOR 



CHLOROPHYLL CHLOROPHYLL 

 b a 



ACCEPTOR 



ACCEPTOR 



Fig. 5-19. Possible modes of energy transfer inside the fucoxanthin-chlorophyll a 

 chromophyllin of diatoms and inside the ohlorophyllin of green plants. (From 

 Wassink, 19486.) 



chromophyllins this cannot be decided, since fucoxanthin does not show 

 fluorescence. By analogy this now seems less probable than the one- 

 sided transfer. It could, however, not be excluded beforehand, since, 

 according to Moglich et at. (1942), the type of energy transfer designated 

 as "energy transport with the cooperation of various degrees of freedom" 

 may be especially important in large complexes. Such complexes might 

 provide an energy supply from their thermal motion to enable an energy 

 transport toward a higher level. So far as the evidence now goes, how- 

 ever, this mode of transport does not seem to be very important in photo- 

 synthesis. It should be borne in mind, however, that in living cells 

 thermal motion may be influenced by other sources of energy production. 

 Whether transfer of energy from one pigment of a chromophyllin to 

 another goes directly or via the protein cannot now be decided. Both 

 paths seem conceivable. Forster (1947) discussed the theoretical possi- 

 bilities of transport of energy from one chlorophyll molecule to another 



