220 



PLANT BIOCHEMISTRY 



Work with strawberries suggests that an entirely different mechanism 

 may be operating, so a need for further work is obvious. 



Sucrose is one of the major end procUicts of photosyntlicsis and 

 for this reason is cliosen as illustrating the formation of disaccharides. 

 Two mechanisms are believed to operate, at least under some condi- 



Uridine diphospho- 

 glucose 



or 



Uridine diphospho- 

 glucose 



fructose-6- 

 phosphate 



fructose 



CH2OH 



sucrose 



CH2OPO3- 



OH HO H 



sucrose-6'-phosphate 



Sucrose-6'-phosphate 



sucrose + HOPO3 



tions. The uridine diphosphoglucose consumed in these reactions and 

 that of page 219 is thought to form according to 



Uridine diphosphate + ATP -^ uridine triphosphate + ADP 



(UTP) 



UTP + Glucosc-1 -phosphate — > 



O 



o 



uridine diphosphoglucose + O" 



— P— O— P— o- 



II I 



O OH 



pyrophosphate 



In this way the uridine diphosphoglucose is made available for further 

 reaction at the expense of ATP. 



As indicated in the scheme for glycolysis (page 168), the poly- 

 saccharides are formed from hexosephosphates. Phosphorylase cata- 

 lyzes the attachment of glucosyl groups to polyglucose units. These 

 latter materials appear necessary as starters since the enzyme does 

 not form maltose. The energy needed for the acetal bond comes 

 from the glucosephosphate bond. This process continues until the 

 molecules grow into amylose. Amylopectin, the other component of 

 starch, results from the action of 4,6-transglucosidase, which shifts 

 some of the glucose units on the ends from position 4 to 6 and forms 

 the branches. 



