thesizing chloroplasts form phosphorylated sugars as inter- 

 mediates in the carbon reduction cycle, since there is an abun- 

 dance of ATP in the chloroplasts and since most known 

 transformations of monosaccharides require phosphorylated 

 forms of the sugars. Transformation of the phosphorylated 

 sugars to the free sugars would for the most part result in 

 a waste of chemical energy, for the sugar would then usually 

 have to be phosphorylated again in reactions requiring ATP 

 or UTP. Only when it becomes necessary to form a mole- 

 cule that can be transported through the chloroplast mem- 

 brane is it likely that a free sugar of relatively small molec- 

 ular weight such as sucrose would be produced. 



A listing of various enzyme systems that appear to be 

 responsible for the carbon reduction cycle has been delayed 

 until now, since many of these biochemical steps are of in- 

 terest in a discussion of carbohydrate synthesis. In Table 2 

 there are listed the enzymes reported in the literature which 

 appear to be responsible for steps of the carbon reduction 

 cycle (Figure 2). Table 3 lists other enzymes which could 

 account for subsequent steps in the synthesis of carbohydrates 

 found to be labeled following relatively short periods of 

 photosynthesis of algae with C^*02. 



We wish to emphasize that the finding of an enzyme in 

 plant tissue does not, of course, prove that that particular re- 

 action goes on in the photosynthesizing chloroplast either at 

 all or in precisely the same way that it has been found to 

 occur in vitro. Moreover, we would not consider the isolation 

 of an enzyme with high catalytic activity a necessary condi- 

 tion for believing that a given reaction may occur in vivo. 

 The organization of the intact chloroplast inside the living 

 cell and replete with all necessary natural cofactors and en- 

 zymes is such that some steps which occur in vivo may prove 

 extremely difficult to demonstrate in cell-free systems. None- 

 theless, the isolation of a cell-free system, capable of carrying 

 out a reaction that has been suspected on the basis of in vivo 

 studies, is important corroborative evidence. 



The various enzymes listed in Tables 2 and 3, if present 



50 



