PLANT METABOLISM 217 



There may or may not be phosphatases in plant cells capable of 

 attacking all the phosphate esters of the smaller monosaccharides 

 mentioned thus far. But as yet there is little evidence of their role 

 in nature except in the formation of glucose, fructose, and sedohep- 

 tulose. 



Many other monosaccharides occur as plant components, and their 

 biological syntheses are under investigation. Starting with the pentoses, 

 some of the interrelationships are summarized in Figure 9-4. Note 

 particularly the reaction connecting the d and i. families, both of 

 natural importance in the case of pentoses. Any small amounts of 

 free pentoses are probably formed by the action of phosphatases rather 

 than by reversal of the kinase-ATP reac tions. Ihe latter seem to be 

 irreversible in known cases of this kiml, but phosphatase action pro- 

 ^ides a potential mechanism for removal of the phosphate group and 

 the formation of the pentoses themselves. Isomerases then convert the 

 free ketopentoses into the related aldopentoses. This scheme provides 

 for many of the pentoses, although some isomers are missing and 

 lyxose is not considered. Lyxose does occia- naturally in heart muscle, 

 but little is known of its biochemistry. It differs from xylose in having 

 the opposite configuration about the second carbon atom. 



The compound central to the formation of many of the pentoses is 

 xylulose-5-phosphate, which may itself be formed subsequent to the 

 photosynthetic fixation of carbon dioxide (see Figure 9-3). Some 

 mechanism may exist also for combining 3-phosphoglyceraldehyde 

 with acetyl coenzyme A or the like to form xylulose-5-phosphate, 

 perhaps by simple reversal of the right-hand reaction of Figure 9-4. 

 Finally, this important intermediate in pentose synthesis probably 

 appears during the metabolism of hexoses by way of the pentose- 

 phosphate system (see Figure 9-6). 



As indicated on page 213, 3-phosphoglyceraldehyde and dihy- 

 droxyacetonephosphate arise during the photosynthetic fixation of 

 carbon dioxide. The two substances ultimately lead to glucose, fruc- 

 tose, mannose, and galactose as shown in Figure 9-5. The reactions 

 converting fructose-6-phosphate to glucose-6-phosphate and glucose-1- 

 phosphate are the reverse of those indicated on page 168 as equilib- 

 ria. The reaction yielding galactose-1 -phosphate is complex and has 

 been represented by the step shown on page 219. 



Thus the glucosyl and galactosyl residues are exchanged by this 

 reaction and galactose-1 -phosphate can be synthesized. Moreover, the 

 enzyme system transforms uridine diphosphoglucose into urdine di- 

 phosphogalactose. Hence, if the two glucose derivatives are formed, 



