CARBON DIOXIDE-WATER EQUILIBRIUM 173 



which converts free radicals into saturated molecules. Franck designated 

 the catalysts involved in these three reactions as "catalyst A" (probably 

 a "carboxylase"), "catalyst B" (the "stabiHzing" catalyst, perhaps a 

 "mutase") and "catalyst C" (possibly a "catalase"); we have desig- 

 nated them, in chapters 6 and 7, as Ea, Eb and Ec, respectively. These 

 three catalysts are only a minimum; and the actual number of nonphoto- 

 chemical reactions in photosynthesis may be larger than three. The 

 evolution of oxygen, for example, may require two successive catalytic 

 reactions (c/. Schemes 6.1, etc.), while the reduction and polymerization of 

 the carbon dioxide-acceptor complex, {CO2}, to glucose, probably involves 

 a whole series of oxidoreductions, dismutations, and polymerizations, 

 requiring a complex catalytic system of which Franck's "catalyst B" 

 may be only the first component. 



This and the next four chapters will deal with these catalytic proc- 

 esses. We begin with the primary carbon dioxide fixation, represented 

 in chapter 7 by the formula: CO2 >• {CO2! . 



Evidence pertaining to the nature of the carbon dioxide-acceptor 

 complex in photosynthesis includes kinetic observations, experiments on 

 carbon dioxide absorption by plants in the dark, carbon dioxide fixation 

 by bacteria and other heterotrophic organisms, and the binding of carbon 

 dioxide by different absorbers in vitro. 



A. The Carbon Dioxide Fixation in vitro* 



1. The Carbon Dioxide-Water Equilibrium 



The primary absorber of carbon dioxide in all organisms is water, 

 which forms 70-80% of the average tissue. The absorption of carbon 

 dioxide by water is partly physical solution, partly chemical hydration, 

 determined by the constants of the following equihbria: 



^'s ^H20 ^Di 



(<S.l) C02(g.) V C02(aq.) v H2CO3 



^'d2 



H+ + HCOr V 2 H+ + CO3— 



The indices refer to solubility (S), hydration (H2O), first ionic dissociation 

 (Di), and second ionic dissociation (D2). The equiUbrium may be 

 further affected by cations whose carbonates have a small solubility 

 product, for example, Ca++ or Mg++. 



At a given pH and temperature, the equilibrium concentrations of all molecular 

 species of carbonic acid (CO2, H2CO3, HCO3-, and COj— ) are determined by that of 

 any one of them, and thus indirectly by the partial pressure of carbon dioxide in the at- 

 mosphere, or by the presence of a soUd carbonate. If the pK is allowed to adjust itself, 

 two parameters can be chosen, e. g., the concentrations [CO3 ] and [HCOs"]. 



The solubihty constant a' (the so-called Ostwald's distribution coefficient) of 



* Bibliography, page 209. 



