Chapter 7 



THE PRIMARY PHOTOCHEMICAL PROCESS* 



1. The Problem of the Primary Process 



Observations with isolated chloroplasts, bacteria, and hydrogen- 

 adapted algae, described in chapters 4, 5 and 6, as well as kinetic meas- 

 urements (to be described in Volume II), indicate that photosynthesis 

 is not a direct reaction between carbon dioxide and water, but a com- 

 plicated sequence of physical, chemical and photochemical processes. 

 One of the most important problems in the study of the mechanism of 

 photosynthesis is the identification of the primary 'photochemical reaction 

 (or reactions), and its separation from the nonphotochemical processes, 

 which may precede or follow it in the reaction sequence. 



In the third chapter, the photosynthesis of green plants was described 

 as a hydrogen transfer from water to carbon dioxide; and in the fifth 

 chapter, bacterial photosynthesis was characterized as a transfer of 

 hydrogen to the same acceptor, from reductants other than water. 

 Although these hydrogen transfers may be associated with reactions of a 

 different type, e. g., carboxylations, hydrations, phosphorylations or 

 dismutations, we feel safe to assume that the primary photochemical 

 process is a stage in the main oxidation-reduction process. 



One suggestion of a different kind (c/. Ruben 1943, and Emerson, 

 Stauffer and Umbreit 1944) was that the absorbed light energy (or, at 

 least, a part of it) is used for the synthesis of high energy phosphate esters, 

 whose subsequent degradation is coupled with endergonic oxidation- 

 reductions. This theory, derived from observations on the mechanism 

 of energy utilization in respiration and fermentation, will be discussed in 

 chapter 9 (page 226), and found improbable. 



Even less plausible is the hypothesis of Kautsky (1932) that the light energy is first 

 stored in metastable oxygen molecules {cf. Chapter 18, page 514), later modified by the 

 substitution of a dissociable oxygen complex for free oxygen (cf. Kautsky and Franck 

 1943). 



Another suggestion, which we consider quite implausible, was made by Seybold 

 (1941). He thought that the Ught energy absorbed by chlorophyll b is used for polym- 

 erization of sugars to starch rather than for the reduction of carbon dioxide. 



* BibUography, page 170. 



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