ENERGY TRANSFER IN PHOTOCHEMICAL SYSTEMS 



59 



is an increase in the number of unpaired spins wlien the light is turned 

 on both at room temperature and at — 140°C. These signals represent 

 approximately 10^^ unpaired spins. The wavelengths of light effective 

 in exciting these signals are between 3500 A and 4500 A and between 

 6000 A and 7000 A, indicating absorption by chlorophyll. A rough 

 quantum yield measurement indicates a value lying between 0.1 and 1. 

 Figure 5 shows some results of growth and decay time measure- 



light on 



light off 



-I miH' 



-I40'C 



light on 



light off 



Fig. 5. Some results of growth and decay time measurements on wet, 

 large spinach chloroplast fragments. 



ments on the same samples. In this case, the curves represent power 

 absorption vs. time at constant magnetic field strength. The half-time 

 for the decay at 25 °C is of the order of 30 sec. At low light intensities 

 (about 10^^ quanta per sec), the rise time is about 30 sec, and at 

 higher light intensities (about 10^^ quanta per sec) the rise time is 

 about 6 sec. There is ^ood reason to believe that even the 6-sec figure 

 is light-limited. At — 140°C, essentially the same rise times are ob- 

 served, but the decay time is of the order of hours. This effect of cool- 

 ing is completely reversible. With dried chloroplasts at 25 °C, the rise 

 times are similar, but the decay times are of the order of hours. How- 



