188 'fixation of carbon dioxide chap. 8 



of the donor, RH, or water). (Reactions of type 8.30 were postulated 

 by Willstatter and Stoll in 1918 as the main photochemical steps in 

 photosynthesis.) 



To sum up, it seems more logical to reserve the term "carbon dioxide 

 reduction" for reactions in which hydrogen atoms (or electrons) are 

 transferred from "donor" molecules to the carbon atom in carbon diox- 

 ide, and not to apply it to carboxylations and similar additive reactions. 

 True, in dealing with metabolic processes, it is often not clear whether 

 an observed consumption of carbon dioxide is caused by addition or 

 reduction; but the distinction should be kept in mind and applied 

 whenever possible. 



B. Carbon Dioxide Fixation by Living Cells* 



The review of different reversible carbon dioxide addition processes 

 in vitro in the preceding section illustrates the variety of reactions which 

 may occur when carbon dioxide comes in contact with living organisms. 



This interaction has been studied in detail only in the case of blood; 

 observations of the absorption of carbon dioxide by other tissues, animal 

 or vegetable, have for the most part been qualitative. However, the 

 work of Spoehr and Smith on sunflower leaves has opened the way to a 

 more quantitative treatment, which is a prerequisite for the complete 

 understanding of the fate of carbon dioxide in photosynthesis. 



From the studies of Smith, water, phosphate, and alkaline earth car- 

 bonates emerge as the three main factors determining the carbon dioxide 

 balance of nonilluminated leaves under high partial pressures of carbon 

 dioxide. It was mentioned above that the carbon dioxide balance of 

 blood was originally attributed exclusively to the conversion of carbonates 

 into bicarbonates. Later, it was found that carbamination also plays a 

 limited, but not negligible, part. A similar development may possibly 

 occur in the theory of the carbon dioxide absorption by plants; but the 

 suggestion of Willstatter and Stoll that carbamination is the inain factor 

 in this absorption is not borne out by the analysis of Spoehr and Smith. 



While dissolution in water and bicarbonate formation (and possibly 

 carbamination) determine the carbon dioxide balance of plants under 

 high partial pressures of this gas, the carbon dioxide binding in the 

 complex, {CO2J — which is probably a carboxylation — comes into greater 

 prominence under low pressures, for instance, in the free atmosphere. 

 Under these conditions, the {CO2} complex may account for carbon 

 dioxide quantities of the same order of magnitude (1 to 5 X 10~^% of 

 the dry weight of the leaves) as those absorbed by conversion into 

 bicarbonate. 



* Bibliography, page 211. 



