1278 THE PIGMENT FACTOR CHAP. 32 



tions (in fact, Eb can be a "mutase" bringing about the first of these dismu- 

 tations). The assumption of a stabilizing reaction on the oxidation side, 

 also catalyzed by Eb, seems unnecessary, since the limitation of the jdeld 

 of finished reduction products will automatically bring about also a limita- 

 tion of the yield of finished oxidation products. (Whenever the production 

 of AHCO2 exceeds the capacity of Eb, the intermediates AHCO2 will accu- 

 mulate, until their back reaction with A'HO will be able to compete with the 

 oxygen-liberating reaction, catalyzed by Eel the photosynthetic quotient 

 will thus be rapidly reduced, after an initial, unbalanced "induction" pe- 

 riod to the normal value of unity. Alternatively, if the stabilizing catalyst 

 is assumed to operate on an instable oxidation product (|0H} or A'OH), 

 there would seem to be no need to assume also a stabilizing reaction on the 

 reduction side. 



The postulate of a single photochemical oxidation-reduction step, fol- 

 lowed by dismutation, is found, in chemically more concrete form, in re- 

 cent speculations on the mechanism of carbon dioxide reduction in photo- 

 synthesis, derived from C(14) experiments (c/. chapter 36, section 12). 



We will now consider, from the point of view of the theory of the 

 "finishing bottleneck," the high quantum yield of photosynthesis and the 

 absence of an extended induction period in weak light — these being the two 

 kinetic observations that have played a decisive role in the development of 

 the theory of the "photosynthetic unit" (to be described in section 5). 



In the early discussions of this subject, Warburg's original value of the 

 quantum yield (7 = 0.25) was used; but even if we substitute the smaller 

 value (7 = 0.12-0.15), which now appears more plausible (c/. chapters 29 and 

 37D), it still remains true that probably all, or almost all, of the light quanta 

 absorbed by chlorophyll in weak light actually can be utilized in the photo- 

 synthetic process. 



The high quantum yield in weak fight (the explanation of which offers 

 some difficulty for a theory which would assume that only a small fraction 

 of chlorophyll molecules are associated with appropriate reduction sub- 

 strates) obviously does not embarrass a theory such as Franck's, which 

 postulates that, except for cases of carbon dioxide deficiency or specific 

 poisoning of the carboxylase, Ea, all chlorophyll molecules are associated 

 with photosensitive complexes, even when light saturation sets in. The 

 absence of an extended induction period in weak fight {cf. chapter 33) re- 

 quires, however, somewhat closer consideration. 



This fact indicates that the collection of the 4 or 8 quanta required for 

 the reduction of one molecule of carbon dioxide does not require that these 

 quanta be absorbed by the same molecule of chlorophyll. In chapter 25 

 (page 838) we derived a relationship between light intensity and the fre- 

 quency of absorption acts by a single molecule : 

 (32.4) n = 4 X 10-2iaAr^^sec.-i 



