1276 THE PIGMENT FACTOR CHAP. 32 



photoproduct is small compared with the working period of the catalyst. But what if 

 this life-time is long enough to enable each molecule of the catalyst to work an average 

 of two or three times before it has to stop because the substrate has disintegrated? If 

 this were so, the necessary number of catalyst molecules would be only one half or one 

 third of the above-calculated figures. However, the duration of the dark interval neces- 

 sary to obtain the maximum yield per flash, would then change with increasing intensity 

 of the flash. After a weak flash, a single working period would be sufficient to process 

 all photoproducts; after a stronger flash, two such periods would be required, to permit 

 a second "round" of catalytic activity, and so on, until the full life-time of the photo- 

 products is utilized. No such dependence of the length of dark intervals on flash in- 

 tensity required to obtain the maximum yield per flash has been noted by Emerson and 

 co-workers. 



The attribution of the maximum flash yield to a back reaction destroy- 

 ing all photoproducts that cannot be immediately stabilized by a catalyst 

 was suggested by Franck, and the concept was developed in detail by 

 Franck and Herzfeld (1941) in conjunction with the reaction mechanism 

 illustrated by scheme 7.VA, in which the "stabilizing" catalyst, Eb, is as- 

 sumed to provide the limiting influence. However, the same hypothesis 

 can be combined also with the other reaction schemes discussed in chapters 

 7, 9, 24 and 28, insofar as these schemes, too, contain "finishing" catalytic 

 reactions that follow the primary photochemical process (or processes). 



In chapter 28 we concluded, from an analysis of the phenomena of light 

 saturation, and of changes in chlorophyll fluorescence in strong light, that 

 the hght saturation of photosynthesis is caused (usually, but probably not 

 always) by the bottleneck of a finishing catalytic reaction. "Flash satura- 

 tion" may thus be due to the same limiting agent as saturation in continu- 

 ous Hght; and this hypothesis is strengthened by quantitative comparison 

 of the two saturation yields. 



In making this comparison, we assume that the maximum velocity of 

 the rate-hmiting catalytic reaction can be represented by a product of a con- 

 centration factor, such as [^b], and a velocity constant, /cb (i. e., that this 

 reaction is a monomolecular transformation of a catalyst-substrate com- 

 plex). If the maximum yield in continuous light is hmited by the same 

 "bottleneck" as the maximum yield per flash, the concentration of the 

 catalyst limiting the rate in continuous light must have the value derived 

 above from the ratio r, namely (assuming n = 4), [x] = 0.0016 [Chl]o. 

 Since PJSax: is of the order of 0.05 [Chl]o (^a = about 20 seconds), we can 

 write 0.0016 [Chljo X k = P^H] = 0.05 [Chl]o, and thus obtain k = 

 30 sec. -\ When the yield per flash was measured as function of the length 

 of the dark interval between flashes, a value of about 50 sec.-^ was in fact 

 found for the monomolecular rate constant of the reaction by which photo- 

 synthesis is completed after the flash. (For the description of these experi- 

 ments, see chap. 34, section B2.) In other words, the product k\E] de- 



