154 PRIMARY PHOTOCHEMICAL PROCESS CHAP. 7 



directly with either of the two reaction components. Photosynthesis 

 might require several such intermediates (Y', Y" • • •), or none at all. 

 It is even possible (although not very probable) that only a single 

 oxidation-reduction system lies between water and carbon dioxide, i. e., 

 that X and Z are identical (this single intermediary being the chloro- 

 phyll, cf. Chapter 19). 



Any one (or several) full arrows iji scheme 7.1 may represent the pri- 

 mary photochemical process (or processes) . If one assumes only one such 

 process, it appears that four quanta should be sujEficient to bring about 

 photosynthesis, since four hydrogen atoms are required for the reduction 

 of one molecule of carbon dioxide to the carbohydrate level. Earlier 

 quantum yield determinations seemed to support this conclusion (Vol. 

 II, Chap. 29); and, even though recent experiments have proved it to 

 be incorrect, it is still useful to begin our discussion with the considera- 

 tion of "four quanta theories," since they can be used afterwards as a 

 basis for theories in which a larger number of quanta are assumed to 

 contribute to the reduction of one molecule of carbon dioxide. We shall 

 initiate this discussion in section 2 (page 155) with the four quanta 

 theories which consider the primary photochemical process to be the 

 dehydrogenation of water (cf. Scheme 7. II). In section 3 (page 157), we 

 shall consider a similar theory which identifies this process with the 

 hydrogenation of carbon dioxide (cf. Scheme 7. Ill); and in section 4 (page 

 159), we shall make the least specific assumption that the primary process 

 is an exchange of hydrogen between two intermediates (cf. Scheme 7. IV). 



Thermochemical difficulties (Vol. II, Chapter 29) make four quanta 

 theories implausible, and recent redeterminations of the quantum yield 

 of photosynthesis have confirmed that at least eight quanta are required 

 for the reduction of one molecule of carbon dioxide. The next step in 

 our discussion will thus be the transition to "eight quanta theories," by 

 a combination of two different or identical four quanta processes. Two 

 "eight quanta theories" will be discussed in sections 5 and 6 (pages 160 

 and 164). In the first one, four hydrogen atoms take part in two different 

 photochemical transfers each (cf. Schemes 7.V and 7.VA), while in the 

 second, eight hydrogen atoms are transferred by eight identical photo- 

 chemical reactions, but the energy of four of them is used afterwards for 

 a second activation of the other four (cf. Scheme 7. VI). 



In these eight quanta schemes, too, the primary photochemical 

 processes may be located either at the "oxidation end" or at the "re- 

 duction end" of the reaction sequence (or in both places), or somewhere 

 in the middle. In schemes 7,V and 7. VI, the last alternative is used as 

 the least specific one. We consider these schemes the most appropriate 

 starting points in the quest for the true chemical mechanism of photo- 

 synthesis. Scheme 7.VA, suggested by Franck and Herzfeld (1941) 



