CHLOROPHYLL CONTENT AND YIELD IN FLASHING LIGHT 1275 



mechanisms in which the limiting catalyst will have to act only once (or, 

 more generally, less than four times) in the reduction of one molecule of 

 carbon dioxide and the production of one molecule of oxygen. 



Even with [x]o = 8[Chl]oT, the concentration [x]o remains much too 

 low for x to be identified with the carbon dioxide acceptor, A. It must 

 thus be another component of the catalytic mechanism. According to the 

 classification we have used often before, it may be either a preparatory 

 or a finishing catalyst. The preparatory catalysts work on stable sub- 

 strates ; therefore nothing seems to prevent them from accumulating more 

 and more products as the dark intervals between flashes increase in length ; 

 and this should permit the maximum yield per flash to increase indefinitely. 

 The kind of preparatory catalysts to which this consideration does not apply 

 are "acceptors" (similar to the repeatedly discussed carbon dioxide ac- 

 ceptor, A) that have a limited capacity and, once "filled," can be emptied 

 only by, or in consequence of, the light reaction. We can postulate, ad hoc, 

 such an acceptor, e. g., on the "reduction side" as an additional intermediate 

 between the first carbon dioxide compound, ACO2, and the chlorophyll 

 complex, or, on the "oxidation side," where a water acceptor compound, 

 A'H20 (or, more generally, an intermediate hydrogen donor, RH2), was 

 postulated on previous occasions. Another, and more plausible possibility 

 is, however, to relate the maximum yield per flash to the limited availability 

 of a finishing catalyst, i. e., a catalyst acting on the products of the primary 

 photochemical process. Of course, if these intermediary photoproducts 

 were stable (meaning by "stability" that their life-time is much longer than 

 the working period, t^, of the catalyst, x), then the catalyst could continue 

 working, in the dark, until it has completely exhausted the products of the 

 preceding flash; in this case, the yield per flash would again become capable 

 of increasing indefinitely with the intensity of the flash, if the dark intervals 

 between flashes are increased correspondingly. However, it is plausible to 

 assume that the intermediary photoproducts are unstable; and, if their ex- 

 istence is so fleeting that a "second batch" cannot wait until the "first 

 batch" had been processed and stabilized by the catalyst, the maximum 

 yield per flash will be equal to the size of this first batch, and therefore equiv- 

 alent to the available amount of the stabilizing catalyst (more exactly, it 

 win be equal to this amount, [x], divided by the number of times the cata- 

 lyst has to operate in the reduction of one molecule of carbon dioxide to 

 carbohydrate). No useful purpose will be served, in this case, by further 

 increasing the energy of the flash, or prolonging the dark intervals. 



One could suggest that the figure obtained in this way (3.2 X 10~'[Chl]o) is not 

 necessarily equal to the concentration of the catalyst x, but may represent a multiple 

 of it— the product of [x]o and the number of batches the catalyst is allowed to process 

 after a flash. We have assumed that this number is 1, i. e., that the Ufe-time of the 



