342 



LIGHT AND LIFE 



As in the model systems, these signals must be due to the generation 

 of unpaired electrons somewhere in the system. The fact that they 

 can be induced by red light, and induced just as rapidly at — 150°C 

 as they are at -[-25 °C, suggests that this is not due to an ordinary 

 chemical reaction which requires any kind of activation energy. If 

 that were the case, the formation reactions at — 150°C should have 

 very different rates than the reactions at -|-25°C, and they do not. 

 In Fig. 21 you can see the growth and decay of these signals, as far 

 as they have been measured. At room temperature the signal rises 



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T = 25»C 



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I 30 SEC I 



LIGHT ON LIGHT OFF 



LIGHT ON LIGHT OFF 



30 SEC 



WHOLE SPINACH CHLOROPLASTS 

 Fig. 21. Growth and decay curves ot whole spinach chloroplasts. 



just as fast as the instrument can measure it and part of the signal 

 falls very rapidly when the light is turned off, thus indicating the 

 presence of at least two different kinds of unpaired electrons. At 

 — 150°C the rise of the signal is just as fast as the instrument can 

 follow it, but in the case of the chloroplasts it does not fall at all 

 at low temperatures, indicating that the back-reactions in this case 

 do indeed have a temperature coefficient. 



Fig. 22. opposite. Left, ESR signals from Rhndnspirillum rubrum, .5 minutes 

 continuous illumination. Right, rise and decay of E.SR signals from Rhodospirillum 

 rubrum. 



