34 CARBOHYDRATES 



panying diagrams. The glycolytic and pentose cycles are so well-known as to need little 

 clarification here. These two cycles are both widespread in higher plants, and neither 

 can be described as more important than the other. Their relative importance probably 

 varies from plant to plant, organ to organ, and time to time. In the diagrams the two 

 schemes are shown separately but with the compounds common to each circled to indicate 

 their many points of interconnection. 



A third group of pathways which has taken on importance in recent years is shown 

 in the third diagram which indicates the key importance of uridinediphosphoglucose as 

 the gateway to many important syntheses. In addition to the formation of UDP-sugars by 

 transformation of UDP-glucose, other sugars may enter the pathway directly by reaction 

 of their 1-phosphates with uridine triphosphate. The UDP-sugar intermediates are then 

 used for such diverse processes as oxidation, epimerization, glycosylation, etc. , (38). 

 Starch is probably formed chiefly by the UDP-glucose pathway rather than the classical 

 phosphorylase reaction (39). There appear to be two separate pathways for the formation 

 of ascorbic acid in plants. By one pathway the aldehyde group carbon becomes the car- 

 boxyl carbon of L-ascorbic acid; by the other the glucose chain is "inverted" so that C-6 

 of glucose becomes C-1 of ascorbic acid. The first pathway goes by way of galactose, 

 D-galacturonic acid and L-galactono-y-lactone. The second goes from 6-phosphogluco- 

 nate with inversion of configuration at C-5 (11, 40). Belkhode and Nath (41) have implicated 

 glucose cycloacetoacetate as an intermediate in the biosynthesis of ascorbic acid by mung 

 bean seedlings (Phaseolus mwigo) . The questioned pathway shown from 3-keto-L-gulono- 

 lactone occurs in mammals but has not so far been found in plants. As with ascorbic acid, 

 plants have two mechanisms for sucrose synthesis, the most important one using fructose- 

 6-phosphate and forming sucrose phosphate, the other using fructose and forming sucrose 

 immediately. There is a possibility that thymidine diphosphate sugar derivatives may in 

 some cases act the same way as UDP derivatives in sugar transformation (42). 



Details of the photosynthesis reaction are still under discussion (43, 44). Kandler 

 and Gibbs (45) have discussed the labelling patterns found in sugars resulting from in- 

 corporation of C'^Oj. Moses and Calvin (36) have ruled out the participation of 2-carboxyl 

 -4-ketopentitol-l, 5-diphosphate as an intermediate between ribulose-1, 5-diphosphate and 

 phosphoglyceric acid. 



Tracer studies by Sato et al. , (37) have shown that, as with other methyl groups, 

 the methyl ester found in pectin is derived from methionine. 



Biosynthesis pathways of the deoxy sugars, tartaric acid, the inositols, fructans, 

 mannans and galactomannans have been little investigated; but what evidence is available 

 indicates that these compounds all fit more closely into these schemes of carbohydrate 

 metabolism than with the pathways of other classes of compounds (48, 49). 



GENERAL REFERENCES 



Advances in Carbohydrate Chemistry, Vol. , 1, 1945 to present. 



Arnold, A. , editor, Formation , Storage , Mobilization , and Transformation of Carbohy- 

 drates , Ruhland^. 



Kertesz, Z. I. The Pectic Substances , Interscience Publishers, N. Y. , 1951. 



Mcllroy, R. J., The Plant Glycosides , Edward Arnold and Co. , London, 1951. 



Pigman, W. , ed. , The Carbohydrates , Academic Press, N. Y. , 1957. 



Smith, F. and Montgomery, R. , Chemistry of Plant Gums and Mucilages, Reinhold, 

 N. Y. , 1959. 



Whistler, R. L. and Smart, C. L. , Polysaccharide Chemistry , Academic Press, N. Y. , 

 1953. 



Whistler, R. L. and Wolfrom, M. L. , Methods in Carbohydrate Chemistry , Academic 

 Press, New York, 1962. 



Many articles in Paech and Tracey 2. 



