PROBLEM OF THE PRIMARY PROCESS 153 



than the energy-storing photochemical reaction. In other words, the problem of 

 oxidation and reduction by light-activated molecules is one of reaction kinetics rather 

 than thermodynamics. 



We will now describe the different specific interpretations of the 

 photochemical oxidation-reduction process in photosynthesis, using a 

 logical rather than historic approach. It was stated above that the 

 photochemical stage may be located at the "oxidation end" or at the 

 "reduction end," or in the middle, of the sequence of reactions by which 

 hydrogen atoms are transferred from water to oxygen. We can represent 

 this "hydrogen bucket brigade" by the following scheme: 



CO2 O2 



i I 



!C02i < HX Y < HZ {OHl 



{HC02I X< HY Z< {HoOl 



I ! 



i : 



{CH2O} H20 



Scheme 7.1. — Photosynthesis as an oxidation-reduction, coupled with preparatory 

 and finishing catalytic reactions (represented by dotted and dashed arrows respec- 

 tively). Full arrows symbolize hydrogen transfers (or electron transfers) between 

 adjacent oxidation-reduction systems. 



Full arrows in scheme 7.1 symbolize hydrogen transfers between adjacent 

 oxidation-reduction systems (e. g., the second full arrow from the left 



represents the reaction HY + X > Y + HX). For the sake of 



simplicity, all these systems are assumed to be "monovalent" (c/. 

 Chapter 9). Broken arrows represent "finishing" catalytic reactions 

 (dismutations, polymerizations, etc.) by which the first reduction product 

 of carbon dioxide, {HCO2}, is converted into a carbohj^drate, and the 

 first oxidation product of water, {OHj, into free oxygen, while dotted 

 arrows symbolize the "preparatory" reactions by which the reactants 

 (CO2 and H2O) are "fixed" prior to their participation in the oxidation- 

 reduction proper. Braces in scheme 7.1 — as throughout this book — 

 indicate that the components are supposed to be present, not in the free 

 state, but as parts of larger molecules or complexes. 



The catalytic system which serves as the immediate hydrogen donor 

 to carbon dioxide (or to a carbon dioxide-acceptor complex, cj. Chapter 

 8), is designated in scheme 7.1 by X, and the system which serves as the 

 immediate hydrogen acceptor from water (or a water-acceptor complex), 

 by Z, while Y stands for an intermediate catalyst which does not react 



