C*02 FIXATION IN LIGHT 1645 



posure to 0*0^ in light. Plants which have been engaged in steady photo- 

 synthesis in ordinary carbon dioxide were briefly exposed to C*02 and then 

 killed by acid. The results of one of these experiments were shown above, 

 in the second part of Table 30. IV. This table indicates, first of all, that 

 the total C* fi.xation after 80-sec. exposure to C*02 in light is 3-6 times 

 larger than the total uptake in 1 min. darkness after "saturating" pre- 

 illumination. The distribution of the tracer, while again showing its pres- 

 ence in practically all fractions (including ether-extractable fatty acids, as 

 well as in a large amount of amino acids), was characterized by a marked in- 

 crease of C* in the sugar fraction (from < 1% to 5% of the total tracer), 

 and by strong accumulation of the tracer in the ether-insoluble anionic 

 fraction (specifically, in its sub-fraction elutable by ammonia from A-3 

 duolite resin). Together, the two anionic sub-fractions (the second one 

 contained substances elutable by NaOH, and not by NH4OH), accounted 

 for as much as 74% of total tracer in the material obtained from Chlorella 

 and for 71% of that obtained from Scenedesmus. 



The ether-soluble fraction of these materials (obtained after 1 min. exposure to C*02 

 in the dark, as well as after 30-sec. exposure in light), was found to contain mainly 

 malic acid (75% of the total activity of this fraction). The cationic fraction from dark 

 C*02 fixation by Chlorella contained up to 50% alanine; less than 15% alanine were 

 found in the material obtained by dark fixation in Scenedesmus, or by fixation in light 

 by either of the two species. 



A most significant finding was the identification by Calvin and Benson 

 (1947) of the bulk of the ammonia-elutable, anionically tagged compounds 

 (in other words, of the main part of tagged material formed in 30 sec. ex- 

 posure to C*02 in light), as phosphogly eerie acid. It was identified as such 

 by preparation of crystallized p-bromophenacylgly cerate after hydrolysis 

 by 1 A^ HCl. Up to 90% of the anionic subfraction not elutable with 

 ammonia was found to be convertible to phosphoglyceric acid by oxidation 

 in air, and it was suggested that it may be phosphoglyceric aldehyde. 



Calvin and Benson (1948) further fractionated the last-named fraction (obtained 

 from Scenedesvius by 30-60 sec. photosynthesis in C*02). It accounted for 44% of 

 total C* in the 30-sec. material, and 59% in the 60-sec. material. Its chemical behavior 

 showed that it could have contained hexose phosphates and triose phosphates, and per- 

 haps also gluconic and mucic acids. The eluted material was divided into subfractions 

 by vacuum concentration and readsorption. One subfraction was not readsorbable; 

 another, when readsorbed, could not be eluted again by ammonia. The second sub- 

 fraction could be interpreted as phosphoglyceric acid, formed by air oxidation of phos- 

 phoglyceraldehyde, while the nonreadsorbable subfraction could have originally consisted 

 of easily hydrolyzable hexose phosphates (e. g., glucose-6-phosphate). 



Calvin and Benson (1947) found no tagged intermediates or by-products 

 of the tricarboxylic acid cycle (citric, isocitric, glutamic acids) in cells 

 killed immediately after a brief period of photosynthesis in C*02; the 



