PHOTOS YNTHETIC AND RESPIRATORY FIXATION OF C*02 1633 



or of decarboxylations coupled with oxidation-reductions, such as 

 (36.2) CH3COCOOH + A + H2O (pyruvate + oxidant) . 



CH3COOH + CO2 + AH2 (acetate + carbon dioxide + reductant) 



The oxidant in (36.2) is, in vivo, phosphopyridine nucleotide. 



Some irreversible reactions of the latter type become reversible if ac- 

 companied by conversion of "low energy" into "high energy" phosphate. 

 (For this, the oxidant and the reductant must be in the form of phosphate 

 esters, and the ester of the reductant must release less energy on hydrolysis 

 than that of the oxidant; this is the case, e. g., when the reductant contains 

 a carbonyl and the oxidant a carboxyl group.) 



More recently, it has been found that the capacity for taking up radio- 

 active carbon from carbon dioxide and distributing the tagged carbon 

 atoms through a variety of metabolites is common to many animal and 

 plant cells. It may perhaps be considered an inevitable concomitant of 

 cellular respiration (and fermentation), since both processes involve revers- 

 ible decarboxylations such as (36.1) or (36.2) and associated reactions, 

 which, too, are either reversible in themselves, or can be reversed by cou- 

 pling with degradation or high-energy phosphates. More specifically, all 

 partial processes of glycolysis leading from sucrose to pyruvic acid, as well 

 as the "Krebs cycle," by which pyruvic acid is decarboxylated and dehy- 

 drogenated, are of this character. Therefore, what we observe as the net 

 production of carbon dioxide in respiration (or fermentation) is the excess 

 of reactions running in the direction of chain fission (and hydrogen trans- 

 fer to oxidized pyridine nucleotides) over reactions by which the carbon 

 chain is built up (and hydrogenated with hydrogen supplied by reduced 

 pyridine nucleotides). The presence of C*02 in the atmosphere in which 

 respiration or fermentation takes place must then lead to some C* atoms 

 "creeping back," first into the intermediates of the decarboxylation cycle, 

 and thence into those of glycolysis. Simultaneously, the C* atoms may 

 also penetrate into compounds (such as certain amino acids), whose respira- 

 tory breakdown is coupled with that of carbohydrates. 



Because of the existence of a C* uptake from C*02 which is unrelated to 

 photosynthesis, it is not permissible to conclude that a certain compound 

 is an intei-mediate (or an early product) of photosynthesis merely because it 

 appears C* tagged soon after the exposure of the plant to C*02. Rather, 

 specific evidence is needed to justify such a conclusion. In strong light, 

 when photosynthesis far exceeds respiration, C* uptake determined by 

 radioactive method can be compared mth the net CO2 uptake determined 

 by manometry (or other analytical methods), and if the former does not ex- 

 ceed markedly the latter, the hulk of the C*-tagged compounds must have 

 been produced by photosynthesis. Even in this case, however, the tagged 



