PHOTOSYNTHESIS OF CARBON COMPOUNDS 463 



photosynthesis appears to produce one molecule of PGA and one molecule of some 

 other three carbon compound. 



Steady state Expt. 28 gav^e very similar results, from 10 sec to saturation (see 

 Table VI for comparison at 40 sec). 



From these experiments alone we cannot identify this three carbon compound. 

 It could be merely a small pool of PGA itself, tightly bound to an enzyme, or in some 

 other way kept apart from the principal PGA pool. Such a pool of PGA molecules, 

 if sufficiently small (> o.i /xmole), would not be distinguishable from the other 

 PGA pool by our methods. 



Alternatively, the six carbon product of the carboxylation reaction may be 

 reductively split to one molecule of 3-PGA and one molecule of triose phosphate. 

 In either case, the requirement for the reaction leading to PGA and triose phosphate 

 must be light (or cofactors derived from the light reaction), and the intact chloroplast, 

 or some intact sub-unit of the chloroplast, as it occurs naturally in the living cell. 



One cannot say at the present time whether or not any of the chloroplasts or 

 chloroplast fragments isolated from broken cells retain the capacity to carry out such 

 a reductive splitting of the six carbon intermediate of the carbon reduction cycle. In 

 such cell-free systems, the carbon reduction cycle may well operate only via the 

 carboxylation reaction leading to two molecules of free 3-PGA. Recently Park^" 

 has prepared electron micrographs of chloroplast and chloroplast fragments which 

 had been found by him to have about as high a rate of photosynthetic COg reduction 

 as any such rates reported for cell-free systems. When compared with electron micro- 

 graphs of chloroplasts in intact cells, these isolated fragments appear to have under- 

 gone considerable physical change, particularly in regard to the apparent density 

 of the stroma and spacing between lamellae. It is possible that the reductive carboxy- 

 lation pathway, if correct, operates only in the unaltered lamellar system by means of 

 some rather direct transfer of photochemically-produced reducing power from the 

 pigmented layer to the carbon reduction cycle. 



If two different three carbon compounds are formed in vivo in the light by the 

 carboxylation of RuDP, and if these two products are kept separate until they have 

 been converted to triose phosphate, and react with each other to give hexose, then 

 the resulting hexose molecule might be dissimilarly labeled in its two halves, nameyl 

 carbon atoms i, 2, and 3, and carbon atoms 4, 5, and 6. Such asymmetry has been 

 reported by Gibbs and Kandler^^-^^. However, other explanations of the phenom- 

 enon are also consistent with the carbon reduction cycle^. 



ACKNOWLEDGEMENT 



The work described in this paper was sponsored by the United States Atomic Energy 

 Commission, University of California, Berkeley, Calif. (U.S.A.). 



REFERENCES 



1 J. A. Bassham, a. a. Benson, L. D. Kay, A. Z. Harris, A. T. Wilson and M. Calvin, /. Am. 



Chetn. Soc, 76 (1954) 1760. 

 * M. Calvin, /. Chem. Soc, {1956) 1895. 

 ' J. A. Bassham and M. Calvin, The Path of Carbon in Photosynthesis, Prentice-Hall, Englewood 



Cliffs, New Jersey, 1957. 



119 



