72 PHOTOSYNTHESIS 



some intracellular substance not necessarily chlorophyll, (2) 

 the activation of this complex not directly by light but by 

 a product of the light reaction, the amount of this product 

 limiting the rate of supply of energy to the photosynthetic 

 system at high light intensities, and (3) the breakdown of the 

 activated complex w^ith the formation of reduced carbon 

 compounds. This mechanism differs from that of Will- 

 statter and Stoll in the introduction of an intermediate 

 between the light process and the carbon dioxide inter- 

 mediate. An estimate of the amount of this intermediate B 

 (in terms of chlorophyll) can be obtained from the results of 

 experiments with flashing light and its concentration is 

 found to be of the order of i/2,oooth that of chlorophyll. 

 The catalyst A has been shown to be present in a concentra- 

 tion approximately equal to that of the chlorophyll. So far 

 these substances are identified solely in terms of their 

 kinetic properties, their chemical nature being completely 

 unspecified. 



I ht+B ^ B' I ^ photosynthetic product 

 (via chlorophyll) 



Franck and Herzfeld (1941) put forward an essentially 

 similar theory but in which it was supposed that carbon 

 dioxide or a derivative of carbon dioxide formed a chemical 

 complex with chlorophyll. Three types of process were pro- 

 posed similar to those of the earlier formulations. The first 

 process involved the formation of a carbon dioxide complex 

 with a substance each molecule of which was presumed to 

 be associated with at least one molecule of chlorophyll. The 

 photochemical process was suggested as a two-step process, 

 the first quantum resulting in the transfer of a hydrogen 

 atom from chlorophyll to the carboxyl group of the com- 

 plex, the second resulting in the regaining of the hydrogen 

 atom by chlorophyll from a donor which was ultimately 

 water. Since four hydrogen atoms are transferred per mole- 

 cule of carbon dioxide reduced to carbohydrate such a 

 mechanism as this requires eight photochemical steps, i.e. 

 eight quanta as a minimum for the reduction of one molecule 



