4L 



Bessel Kok 



Y/Y" is only slightly lower than that of P700, i.e. the sec- 

 ond photoact is strictly in "series" with the first one. We 

 have mentioned this possibility on an earlier occasion {k) 

 and illustrate it again in scheme Fig. 5* 



These two extremes share one feature, namely, that the 

 quantum requirement of the overall process could be less 

 than 2 per electron (<8 per O2): V\/e feel the data of Fig. 1 

 and Table 1 definitely show a requirement of one quantum to 

 generate X" and P"*". In the parallel formulation the extra 

 energy required to evolve O2 starting from P does not neces- 

 sarily have to come in one quantum "packages" and in the 

 series formulation the energy of 2 (short wave) quanta might 

 suffice to evolve oxygen from a preformed complex. 



A minimum requirement of 2 hv/el. is more rigidly demanded 

 by the part-parallel part-series scheme [cf. Hill and Bendall 

 (11)3 which is presently accepted by most workers in the field. 

 In this formulation the reductant of the second photoact 



hvl 



hvn 



TPNH, 



Figure 5 



should be able to reduce cytochrome bg or plastoquinone (po- 

 tential of Y/Y~£0.0 volt). The reaction chain connecting 

 the two photoacts: 



provides an attractive site for photophosphorylation--coupled 

 to the main stream of electron transport. If one does not 



