42 



RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



It was then of interest to observe the rate of labeling of the individual 

 carbon atoms of each compound. Thus, if phosphoglyceric acid were 

 formed by a carboxylation reaction, the labeled carbon would probably 

 not be uniformly distributed among the three carbon atoms of the mol- 

 ecule after short exposures to C^^02. Actually, degradation studies of the 

 phosphoglyceric acid indicated that nearly all the C^"* was found in the 

 carboxyl position when the period of photosynthesis with C^^ was 5 sec. 

 Table 1-8 presents further data on the distribution of the label in the 

 molecule as a function of time. 



Table 1-8. Percentage of Distribution of Radiocarbon in Specific 

 Carbon Atoms of Compounds Labeled during Photosynthesis 



[From Melvin Calvin, Photosynthesis (The Path of Carbon in Photosynthesis and 

 the Primary Quantum Conversion Act of Photosynthesis), UCRL-2040, No. 22, 1952; 

 and J. G. Buchanan, J. A. Bassham, A. A. Benson, D. F. Bradley, M. Calvin, L. I. 

 Daus, M. Goodman, P. M. Hayes, V. H. Lynch, L. T. Norris, and A. T. Wilson, The 

 Path of Carbon in Photosynthesis. XVII. Phosphorus Compounds as Inter- 

 mediates in Photosynthesis, Phosphorus Metabolism, 2: 440-466 (1952).] 



From the data in Table 1-8 it is also clear that the phosphoglyceric acid 

 was formed by carboxylation of a two-carbon compound. This carbon 

 dioxide acceptor is labeled at an equal rate between the two positions, 

 which is slow compared with the rate of primary carboxylation. The 

 fact that no reservoirs of labeled two-carbon compounds have been found 

 after short periods of photosynthesis with C^^ is further evidence that the 

 two-carbon compound is not formed by the reduction and coupling of two 

 molecules of carbon dioxide. 



It is next possible to suggest and confirm a mechanism for the produc- 

 tion of a hexose from two molecules of phosphoglyceric acid. A reason- 

 able sequence would be the reduction of the glyceric acid to the aldehyde, 

 some of which would isomerize to give dihydroxyacetone, which could 

 condense to give the hexose. This may be represented schematically as 

 follows : 



