208 FIXATION OF CARBON DIOXIDE CHAP. 8 



require for completion 30 to 60 minutes). The implications of this fact 

 have been mentioned before (page 203). 



5. Carbon Dioxide Fixation by Heterotrophants 



We found in the preceding section that, despite the difficulties 

 encountered in attempts to reverse the decarboxylation of carboxyhc 

 acids in vitro in neutral media, carboxylation represents the most probable 

 primary carbon dioxide fixation mechanism in photosynthesis. In 

 addition to the direct evidence in favor of this theory, obtained in 

 experiments with radioactive carbon, we will now mention indirect argu- 

 ments provided by an increasing variety of nonphotochemical metabolic 

 processes. In these processes, carbon dioxide plays an unexpectedly 

 active part, and they are best explained by the incorporation of carbon 

 dioxide into carboxylic groups. Until a few years ago, carbon dioxide 

 was considered as an "inert" gas for all heterotrophants (even though 

 it was known that many life processes, such as the germination of seeds, 

 are inhibited by the total absence of this gas); but lately, examples of 

 nonphotochemical carbon dioxide assimilation have multiplied rapidly, 

 and have spread from the world of bacteria into those of the higher plants 

 and animals. 



The simplest case of carbon dioxide fixation by carboxylation is the 

 synthesis of formic acid, e. g. (in alkaline solution) : 



(8.34) H2 + HCO3- . HCOO- + H2O + 3 kcal 



The standard free energy of this reaction is AF = +0.9 kcal according 

 to table 8. VIII ( — 0.4 kcal according to the calculations of Woods 

 1936). Of all carboxylations, it has the most valid claim to being 

 considered as a reduction of carbon dioxide because it involves a complete 

 hydrogenation of a C=0 double bond, with one hydrogen atom becoming 

 bound to oxygen and another to carbon. 



In a formate solution which is in contact with air, reaction (8.34) 

 will proceed in the direction of decarboxylation, but under sufficiently 

 high partial pressures of hydrogen and carbon dioxide, this process can 

 be reversed. Escherichia coli, which was shown by Stephenson and 

 Stickland (1932, 1933) to bring about decarboxylation of formic acid, 

 was proved by Woods (1936) to be capable of catalyzing the reverse 

 reaction as well. Woods measured both the absorption of hydrogen 

 and the formation of formic acid. The reaction is poisoned by cyanide 

 (from 10~^ moles/1, on) and by narcotics (toluene, chloroform). It 

 tends to an equilibrium, from whose position a standard free energy of 

 — 0.17 kcal was calculated by Woods, in satisfactory agreement with 

 the values quoted above (+ 0.9 or — 0.4 kcal) derived from nonbio- 

 logical measurements. 



