TWO DIFFERENT PRIMARY FOUR QUANTA PROCESSES 161 



per einstein) is scarcely sufficient to cover the net requirements of 

 photosynthesis (112 kcal per mole) and leave a sufficient margin for losses 

 involved in the stabilization of unstable intermediates (c/. Wohl 1935). 



If we assume, as a working hypothesis, that eight quanta are actually 

 utilized in photosynthesis (while quanta absorbed above this number are 

 lost by energy dissipation), we may ask how eight primary photochemical 

 processes can be utilized for the transfer of four hydrogen atoms. The 

 obvious answer is that each hydrogen atom can be activated twice, thus 

 enhancing its reducing power. 



This result can be achieved in two ways. One is to activate the same 

 four hydrogen atoms photochemically tvnce in succession, e. g., to combine 

 two of the different four quanta processes discussed in the preceding 

 sections. The other solution is to double the number of identical 

 primary photochemical processes, e. g., (7.2), (7.8a) or (7.10a), and to 

 allow four of the primary products to recombine, transferring their 

 recombination energy to the remaining four intermediates. This kind 

 of secondary reactions can be designated as "energy dismutations," 

 because of their analogy to chemical dismutations repeatedly mentioned 

 in this chapter. 



We will begin by exploring the first alternative, that is, by discussing 

 hypotheses which postulate two sets of different primary photochemical 

 reactions. We may designate one set — in which hydrogen atoms are 

 taken away from water (or from an intermediate donor), as photoxidations, 

 and those of the second set— in which the same hydrogen atoms are 

 transferred to carbon dioxide (or an intermediate acceptor), as photo- 

 reductions (using this term in a sense different from that assigned to it 

 by Gaffron, cf. Chapter 6). 



The hypothesis of two primary processes has often been associated 

 with the assumption that the intermediate hydrogen acceptor is chloro- 

 phyll, and that this pigment is capable of taking hydrogen atoms away 

 from water (or another donor), with the help of light, and transferring 

 them to carbon dioxide (or another acceptor), also with the help of light. 

 As announced before, we will postpone the question of the role of 

 chlorophyll in photosynthesis until chapter 19, and use in the following 

 schemes the symbols X or Z where the original papers may have used 

 Chi (= chlorophyll). However, we will retain the assumption that the 

 same catalyst whose oxidized form participates in the photoxidation of 

 water also participates, in the reduced form, in the photoreduction of 

 carbon dioxide. (A less specific assumption would be to consider the 

 photoxidation and photoreduction as separated by an unknown num- 

 ber of intermediate oxidation-reduction catalysts.) In other words, we 

 assume that only one of the intermediate catalysts in scheme 7.1, either 

 X, Y, or Z, is a "photocatalyst." We begin with the second alter- 



