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Eugene Rabinowitch 



photosynthesis, since photosynthesis is known to proceed, at 

 high rate, in red algae, in 540 in^i-light, absorbed mostly 

 by phycoerythrin. Two possible explanations may be given. 

 In the first place, light absorption in system 2 could lead 

 to resonance transfer of energy to system 1, thus achieving 

 the required co-excitation of the two systems ("spilling 

 over"). Secondly, the presence of some phycoerythrin in 

 system 1 (suggested by Duysens) and the contribution of 

 chlorophyll a to the absorption at 540 m;i, may be sufficient 

 to achieve co-excitation of the two systems even without 

 energy transfer. If the latter is true, the suggestion, pre- 

 sented further below, that the two systems may be contained 

 in separate layers, would become more plausible. 



Experiments by S. Brody, and by W^. Butler and coworkers 

 suggest that the fluorescence band of chlorophyll a i^ vivo, 

 lies, at ordinary temperatures, at 680 m)i and belongs to 

 chlorophyll a 670. It is thus associated with pigment 

 system 2. (At the liquid air temperature, however, fluor- 

 escence bands appear also beyond 700 nui — and must be attri- 

 buted to chlorophyll a in system l). The primary distinc- 

 tion between the two systems probably consists in their 

 different location in the chloroplasts, implying association 

 with different cellular components. Since chlorophyll mole- 

 cules are known to fluoresce (in solution) only when assoc- 

 iated with polar molecules, and to be non-fluorescent in 

 dry hydrocarbons, one can suggest that pigment system 1 is 

 associated with hydrophobic organic molecules, and system 2 

 with hydrophilic proteins. 



It is known that chloroplasts consist of alternating 

 lamellae of more hydrophilic and more hydrophobic nature. 

 It is tempting to associate the fluorescent pigment system 2 

 with the hydrophilic, and the non-fluorescent pigment system 

 1 with the hydrophobic layers in the chloroplasts. One 

 could, for example, postulate a bimolecular sheet of pig- 

 ments, with one leaf turned tov/ards the hydrophilic, aqueous 

 layer, and the other towards the hydrophobic, lipoid layer; 

 the two leafs could be separated by an enzymatic layer, 

 containing e. ^. , the two cytochromes. 



Another alternative is to postulate a more uniform dis- 

 tribution of pigments in the two layers. The first altern- 

 ative is supported, however, by Goedheer's optical findings, 

 which suggest that chlorophyll in the chloroplasts forms 

 separate lamellae less than 10 A thick, (The proteidic and 



