42 



Bessel Kok 



accept this intermediate chain Y — ^ P (of. section II) the 

 only reasonable alternative to conceive a generation of ATP 

 coupled to substrate reduction is the locus indicated in 

 scheme Fig. 5 as X~ — ^ X^ (4), This hypothesis of "substrate 

 level" photophosphorylation requires, however, that the pri- 

 mary reductant of the first photoact has a quite low poten- 

 tial. 



Scheme Fig. 5 contains additional speculations which must 

 appear objectionable to many of you and might not survive 

 future evidence. However, it is not presented as the ulti- 

 mate truth but rather to illlustrate some possibly useful 

 thoughts. One feature is a "sharing" of traps by the two 

 photoreactions visualized by the identification of P700 with 

 the light collector of photoreaction II, i.e. ^uysens' "Q" 

 (12) and »'itt's "Q II" (7). The asymmetric shape of the 700 

 mu difference band (cf. 13, Fig. 2 and 13 ) suggests a shift 

 of the 700 mu absorption to a {-^^0%) weaker band at a ('-'13 

 mu) shorter wavelength. Photo-oxidized P700 (P690'*") thus 

 could conceivably again function as a light trap — compare 

 e.g. the photochromic back and forth shift of the plant 

 pigment phytochrorae (l4). The second photoact then would 

 be sensitized by all wavelengths except those beyond 69O mu 

 (of course, one must assume that P69O undergoes alternate 

 dark steps as well). 



Such a competitive sharing of trapping pigment by the two 

 photoacts allows a self-regulating sensitization mechanism 

 ("spill over"), in line with earlier thoughts of Franck (15) 

 and Myers (I6). Our fluorescence data, although supporting 

 this spill over from one photosystem to the other, do not 

 seem to favor an identification of the conversion centers 

 and indicate a more complex type of mechanism (this volume). 



A main argument for closely connecting the two traps i.e. 

 for a short path between the intermediate products of the 

 two photoacts — regardless of their nature--is the following: 

 The spectroscopic evidence such as obtained by Witt, ^uysens, 

 and ourselves for P700 operating in a charge transfer complex 

 with cyt. f and probably plastocyanin, subject to a push-pull 

 operation by two photoacts, is impressive. However, most of 

 the information originates from measurements under extrem-e 

 or abnormal circumstances. Under favorable conditions of ef- 

 ficient, steady state photosynthesis in whole algae, neither 

 P7OO nor cyt. f can be observed by our flashing light method 

 which is geared to the "time constant" of photosynthesis and 

 has a time resolution of a millisecond. With their much 



