53 



Bessel Kok. 



of substrate or the presence of DCMU which blocks alternate 

 path II), the variable fluorescence will be maximal, although 

 system I might still operate at full efficiency (cf. Fig. 6, 

 Table 1). 



Secondary collector C700 transfers its excitations to the 

 final trap of "photosystem I": P700. Upon excitation P700 

 produces a strong reductant ("H") and is left behind as a 

 weak oxidant. Trap II, after its photoconversion, is capable 

 of reducing PyOO"*" and producing O2. Except for the fact that 

 it should be (associated with) a chlorophyll type pigment, the 

 chemical nature of this trap and its mode of dark conversion 

 are immaterial for the present discussion. 



The data indicate that in green plants C7OC and trap II 

 occur in about equal concentration — but they seem to exclude 

 the identity of the two compounds. [System II can operate in 

 the absence of system I (cf. Section II)]. 



A peculiar feature of this scheme is that the "primary 

 collector unit" amounts to only ^0 pigment molecules--a sit- 

 uation also found in bacterial photosynthesis (I8). For op- 

 timal operation, each unit should, on the average, transfer 

 quanta to T II and C7OO in the proper ratio (e.g. 1:1), but 

 since the mechanism is self-regulating it does not require a 

 definite ratio between T II and C7OO or an association of 

 these with a distinct group of collector pigment. This self- 

 regulation is effected by the connecting electron transport 

 system mediated by P700--fed at both ends by some 10 small 

 units. The result of this "double focussing" is the classicsil 

 unit of 400 chlorophylls. 



The scheme provides for a second "switch" in the transfer 

 chain: quanta which fail to find P7OO can either be degraded 

 in C7OO itself, [at 77'*K re-emitted as F7OO (cf. Fig. k)] or 

 escape to C730 — v/hich also emits only at low temperature. 

 (Long wave light does not yield fluorescence at 300°K.) The 

 fact that at 77*K a considerable fraction of the absorbed 

 light ( 50%) can be re-emitted at 730 mu (cf. Fig. 5) suggests 

 that this is a funci^ionally significant process. One can as- 

 cribe an important task to pigment C730: the harmless degrad- 

 ation of excess quanta which might otherv;ise lead to photo- 

 inhibition. 



In green plants (C700) amounts to only 3-10% of (Chi.) and 

 causes a decline of the quantum yield of O2 evolution only 

 beyond 69O mu : Due to the impossibility of reversed quantum 

 flow traps II are not excited by light absorbed by G7OO 



