645 



J. A. Bassham 



4.8- 

 4.4- 

 4.0- 

 3.6- 



< 32- 



lO 



^ 



en 



2.8 

 24 



2.0 



1.6- 



o 



=1. 1.2 



.8 



CO2-AIR 



_L 



_L 



N2 



2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 

 TIME (MIN) 



Fig. 5. Effects of sudden ranoval and readditlon of CO? on levels of PSCR cycle. 

 Sugar monophosphates. # Glucose monophosphate, O sedoheptulose monophosphate, 

 ■ fructose monophosphate. 



suggested transfer of Ci phosphate groups from fructose diphosphate and sedo- 

 heptulose diphosphate to ribulose monophosphate. This transfer which might 

 initially be inhibited by the high level of ribulose diphosphate would accelerate 

 as the level of ribulose diphosphate decreases. 



In summary, while recent kinetic studies of the carbon reduction cycle of 

 photosynthesis confirm the cycle in essentially the form in which it was pro- 

 posed ten years ago, there is a scattering of evidence which suggests that the 

 in vivo cycle might vary somewhat in its mechanistic details from the cycle 

 which was proposed at that time. First there is a variety of kinetic evidence 

 which suggests the direct conversion of the carboxylation intermediate compound 

 to the level of sugar phosphate without complete equilibration with the free 

 pool of PGA. Secondly, there is some indication that when the light is turned 

 off there is only a partial brief reoxidation of sugar phosphate via triose 

 phosphate dehydrogenase and that this period is followed by a block on this 

 reoxidation. Third, some experiments suggest the possibility of direct transfer 

 of phosphate groups from the no. 1 carbon atoms of hexose and heptose diphos- 

 phates to ribulose monophosphate. Fourth, the dramatic effects of disulfide 

 compounds on the carbon reduction patterns, and other biochemical considerations 

 argue for a role of disulfide compounds, perhaps including ferredoxin, in med- 

 iating a reductive carboxylation of ribulose diphosphate to give sugar phos- 

 phates directly. A separate but related possibility is that some of the newly 

 incorporated carboxyl carbon is also reduced to the level of sugar phosphate 

 without an opportunity for equilibration with the free PGA pool. Such a direct 



