115. XII. 1952] 



M. Calvin and P. Massini: The Path of Carbon in Photosynthesis 



449 



TabU I 

 C^* Distribution in Photosynthetic Products of Barley and Scenedismus 



' Experiments are steady-state photosynthesis, 10,000 footcandles unless otherwise stated. ' 1000 footcandles. ' Alanine obtained from this 

 extract was 48% carboxyl-labeled. * Under the same conditions, Chlorella produced phosphoglycerate latelcd 93%, 3% and 2%, 

 respectively. ' In this extract, malic acid was labeled 6*5% and aspartic acid 4% in the non-carboxyl carbons. ' 3000 footcandles. 



" Malonate inhibited. 



lying between them and carbon dioxide will be the only 

 ones that will show a finite slope ; all others should start 

 with a zero slope. A finite slope is certainly the case for 

 phosphoglyceric acid and possibly for malic acid, in- 

 dicating at least two independent catrbon dioxide fixing 

 reactions, one leading to a three-carbon compound and 

 the other producing a four-carbon compound'. 



Since the hexose phosphates appear extremely early 

 in all of these photosynthesis experiments and because 

 of the known close relationship between the hexose 

 phosphates and phosphoglyceric acids in the glycolytic 

 sequence, it seemed most reasonable to suppose that 

 these hexose phosphates were formed from the phos- 

 phoglyceric acid by a combination of the two three- 

 carbon fragments derived from phosphoglyceric acid in 

 an overall process very similar to, if not identical with, 

 the reversal of glycolysis. 



One means of testing this suggestion would be a com- 

 parison of the distribution of radioactivity in the three 

 carbon atoms of glyceric acid with those in the hexose 

 as shown in Table I. It thus appears that the hexose is 

 indeed formed by the combination of two three-carbon 

 molecules derived from the glyceric acid in such a 

 manner that carbon atoms three and four of the hexose 

 correspond to the carboxyl-carbon of the glyceric acid ; 

 carbon atoms two and five with the alpha-carbon ; and 

 carbon atoms one and six with the beta-carbon of the 



* E. J. Badin and M. Calvin, J. Am. Chem. Soc. 7^, 5266 (1950). 

 - S. Kawaguchi, a. a. Benson, M.Calvin, and P. M. Hayes, J.Am. 

 Chem. Soc. 7i. 4477 (1952). 



glyceric acid. This correspondence is maintained when 

 the distribution in these two compounds (glyceric acid 

 and hexose) is compared for a wide variety of different 

 times. 



With this clear cut indication of the similarity be- 

 tween the path of hexose synthesis and the known path 

 of its breakdown, another means of testing how closely 

 this parallelism might be followed suggests itself. The 

 hexose derivative which is last in the sequence of 

 changes prior to the breakdown of the carbon skeleton 

 during glycolysis is the fructose-l,6-diphosphate. 

 Correspondingly, then, it presumably would be the 

 first hexose derivative to appear in the reverse direction. 

 If this is the, case and, furthermore, if the hexose deriv- 

 ative reservoirs involved in sucrose synthesis are more 

 or less isolated from those involved in storage and gly- 

 colysis, the radioactivity should appear in the fructose 

 half of the sucrose molecule prior to its appearance in 

 the glucose half. This is indeed the case'. However, 

 sucrose does not seem to be formed by the simple re- 

 versal of the sucrose phosphorylase system which was 

 described for certain bacteria^, since for this to be the 

 case, free fructose would have to be apparent in the 

 photosynthesizing organism, whereats it is never so 

 found, nor has the enzyme itself ever been isolated 

 from any green plant. 



* S. Kawaguchi, A. A. Benson, N. Calvin, and P. M. Hayes, 

 J. Am. Chem. Soc. 7i, 4477 (1952). 



' W. Z. Hassid, M. Doudoroff, and H. A. Barker, J. Am. Chem. 

 Soc. se, 1416 (1944). - M. Doudoroff, H. A. Barker, and W. Z. 

 Hassid, J. Biol. Chem. ISS, 725 (1947). 



83 



