36 



Bessel Kok 



short wavelengths but rises with increasing wavelength until 

 it approaches unity (1 eq./hV). Obviously, in long wave 

 light we observe only photosystem I, a process which occurs 

 with a quantum yield of one and can retain a considerable 

 fraction C^ ^0%) of the absorbed light as chemical energy. 

 The experiment described in Table 1 was made with our differ- 

 ence spectro-photometer (3) in which a sample is exposed to a 

 series of light flashes (l800 per minute). The dark periods 

 are long enough to allow dark conversion of the photoproducts 

 made in each flash. The apparatus measures cyclic absorbency 

 changes of intermediates as well as the net result of many 

 cycles, provided a color change accompanies the events. In 

 Table 1 we observed the repetitive bleaching (in each flash) 

 and re-reduction (in the dark) of P700. If for every P700 

 which is bleached, one electron is transferred to the primary 

 reductant (X) and from there to TPN, then the total amount of 

 TPNH2 accumulated should correspond to the sum of all P700 

 molecules which have turned over during the time the flashing 

 light was given. This indeed proved to be the case if we 

 assumed a (change of) molar extinction AE=8xlO^ L/mole. cm. 

 for P7OO -a value typical for the red absorption band of a 

 chlorophyll. This good agreement, together with the high 

 quantum yield observed in Fig. 1 yields quantitative support 

 to the proposal that F7OO is the photoconverter of photosys- 

 tem I in photosynthesis. 



II, Photoreduction and -oxidation of indophenol dye, site of 

 phosphorylation. 



In the long wave light or in the presence of DCMU the sec- 

 ond photoreaction of photosynthesis is inoperative. Earlier 

 (4) we have shown that DCMU does not really inhibit the photo- 

 reduction of DCPIP by chloroplasts, but only causes the 

 reduced dye to react back with photo-oxidized P700 (whose 



Reduction ot DCPIP by weak flashing light of two wavelengths. 



The atriount reduced per flash is compared to the net rate during one 



light-dark period. 

 725 mil values in parenthesis are corrected for absorbed intensity 



equal to the 674 light. 



Table 2 



