40 



Bessel Kok 



cyt. c 



hi/ 1 

 chlorophyll 



Figure k 



cyt, f. In many chloropiast preparations plastocyanin sus- 

 tained much higher photo-oxidation rates than cyt. f, indica- 

 ting, that it bypasses cyt. f and reacts directly with P700. 

 Our preliminary data indicate that the kinetics of plasto- 

 cyanin photo-oxidation do not essentially differ from those 

 of cyt. f oxidation (cf. Fig. 3). This suggests that not 

 only cyt. f, but also plastocyanin has a fixed locus in the 

 chloropiast matrix and can operate in a complex with P700. 

 This intimate cooperation between the photoreceptor, a haem 

 and a copper enzyme, all of high potential, present in equal 

 amounts in the chloroplasts, has prompted some speculations 

 which will be described in the following section. 



IV. Discussion 



The first photoact yields besides X~ (re-oxidized by sub- 

 strate) a weak oxidant P700'''. The second (short wave sen- 

 sitized) photosystem is left with the tasks to evolve oxygen 

 (at least to assist this process) and at the same time re- 

 reduce P700"*". One of the products of the second photoact, 

 therefore, must be a reductant "Y~" of a potential lower than 

 +0.^3 volt. Just how much lower, is presently a point of 

 discussion since the chemical nature of Y is unknown. For 

 instance, as mentioned by Dr. Hoch in this symposium, the 

 possibility is not excluded that Y~ is as strong a reductant 

 as X~, This would mean a "parallel" operation of the two 

 photoacts and require an energetic coupling between them by 

 dismutation reactions in the dark. 



The other extreme is the assumption that the potential of 



