673 



Martin Gibbs 



Fructose-6-P, D-erythrose-U-P, D-glucose-6-P, glycolaldehyde-P 

 and D-ribose-5-P had no inhibitory effects on any of the dehy- 

 drogenases. 



Sedoheptulose-T-P may act as a controlling agent of the carbon 

 reduction cycle. Accumulation of this compound could effectively 

 block the cycle in the reconstituted systems. Inorganic phos- 

 phate was suggested earlier to have a similar function^ ■'■'. 



Conclusion 



The reductive pentose- P scheme as postulated by Calvin and 

 his associates can account for most in vivo and in vitro obser- 

 vations sximmarized here. While the role of asymmetry in the 

 labeling pattern is not unique to autotrophic systems, its signif- 

 icance is still unknown in the photosynthetic process. The appar- 

 ent absence of fructose-l,6-diP aldolase from extracts of the 

 Cyanophyceae poses a problem. Either the correct conditions for 

 detecting this enzyme have not been employed or the more primi- 

 tive photosynthetic cell may have a reductive carbon cycle which 

 differs from the higher plants. A perplexing problem is the low 

 rate of COo fixation by the isolated higher plant chloroplasts, 

 especially, when the same preparations reduce TPN with concomi- 

 tant ATP formation to satisfy the demands of intact cell photo- 

 synthesis. It may be that in vitro preparations presently used 

 may or may not duplicate the carbon cycle of the in vivo process. 

 There are no reports on the isolation of particles from unicellu- 

 lar material capable of assimilating CO2 at a rate equivalent to 

 or higher than that reported for the higher plant chloroplast. 

 The photoelectron transport system is apparently so intimately 

 connected with the carbon reduction cycle that physical separa- 

 tion may not be possible. Perhaps the connecting link between 

 the two systems is more than ATP and TPNH. 



ACKNOWLEDGMENTS 



This investigation was aided by grants from the National 

 Science Foxindation and from the United States Air Force through 

 the Air Force Office of Scientific Research of the Air Research 

 and Development Command, under contract No. AF ^9(638)T98« 



REFERENCES 



1. Bassham, J. A., and Calvin, M., Path of carbon in photo- 

 synthesis, Printice Hall, Inc., Englewood Cliffs, New 

 Jersey, 1957- 



