202 



FIXATION OF CARBON DIOXIDE 



CHAP. 8 



hypothesis. However, some quantitative discrepancies remain to be 

 clarified. In the first place, the amount of carbon dioxide taken up by 

 the bacteria appears to be at least one and perhaps two orders of magni- 

 tude larger than that absorbed by green plants. In the second place, the 

 amount of liberated phosphate is only one-fiftieth of that of absorbed 

 carbon dioxide. (To explain the latter fact, Vogler suggested that the 

 release of inorganic phosphate into the medium may be only a small token 

 of the large-scale transphosphorylation taking place inside the cell.) 



Direct evidence pertaining to the nature of the {CO2} complex is 

 scarce. Some important observations were, however, made with radio- 

 active carbon dioxide, C*02, by Ruben and coworkers. The fixation of 

 C*02 in the dark was first observed by Ruben, Hassid, and Kamen (1939) 

 in barley leaves and by Ruben, Kamen, Hassid, and Devault (1939) in 

 Chlorella. The properties of the compound formed by this "dark 

 fixation" of carbon dioxide were described in more detail by Ruben, 

 Kameii, and Hassid (1940). (They used the term ''reduction of carbon 

 dioxide," which we prefer to avoid for the reasons given on page 187.) 



The cell suspension was exposed 

 to radioactive carbon dioxide for sev- 

 eral minutes in the dark, acidified, 

 and boiled vigorously. The part of 

 the absorbed radioactive carbon di- 

 oxide which was not removed by this 

 treatment was obviously present in 

 an acid-resisting form, that is, not as 

 carbonate or carbamate. Most of the 

 radioactive carbon passed, upon boil- 

 ing, into the aqueous solution, showing 

 that the complex, {CO2}, was water- 

 soluble (at least, to a small extent — 

 the quantity of {CO2} produced in 

 these experiments was only of the 

 order of 10~^ mole in one milliliter of 

 algae). Figure 21 shows the uptake 

 of carbon dioxide in the dark as a 

 function of time. In this curve, an 

 apparent saturation corresponds to 

 about 0.2 ml. C*02 per ml. of algae, 

 or about 0.01 mole per liter of cell 

 volume (about five times the amount 

 of carbon dioxide dissolved in cell 

 water under the conditions of the experiment). The uptake of C*02 is 

 reversible: flushing the algae with inactive carbon dioxide removes 5 or 



1.0 



0.5 



40 

 Time, minutes 



80 



Fig. 21. — Radioactive carbon di- 

 oxide uptake by Chlorella in the dark. 

 Measured in arbitrary units, 1.5 units 

 corresponding to approximately 0.2 

 mm.3 carbon dioxide per mm.^ algae 

 (after Ruben, Kamen, and Hassid 1940). 



