CH.,OH 



O. H 



+ 



OPO;f 



H OH 



glucose-l- phosphate 



PLANT METABOLISM 



CH.OH 



O, H 



H OH 



polyglucose unit 



phosphor>'lase 



0-- 



221 



HO 



HPO4- 



H OH 



lengthened chain 



The mannose-6-phosphate discussed above is converted to man- 

 nose- 1 -phosphate by a mechanism analogous to that of pages 168 to 

 170 and for gIucose-6-phosphate. Mannose-1 -phosphate is probably 

 used in synthesizing the mannans of plants by a mechanism similar 

 to that for starch. 



Glucose-l -phosphate is believed to serve in the synthesis of cellulose, 

 the most abundant carbohydrate. Since the acetal linkage is (3 rather 

 than a as in starch, another enzyme system than phosphorylase would 

 be expected and has been isolated from bacteria. 



Utilization 



Plants respire in the dark and during all other periods when photo- 

 synthesis is not occurring, except perhaps at subfreezing temperatures 

 when the multitude of reactions requiring water might be stopped or 

 greatly retarded. In fact, many investigators feel that plants respire 

 even during photosynthesis. 



Much of this respiration utilizes carbohydrates as substrates includ- 

 ing sugars, their derivatives, and polysaccharide reserves. Cellulose 

 seems to be solely a structural substance as far as plants are concerned 

 and is not metabolized once formed. The hemicelluloses of some 

 plants may be metabolized under some conditions, but the primary 

 cleavage reactions are unknown. Starches serve as the principal reserve 

 carbohydrates and are converted by means of the 4,6-transglucosidase 

 and phosphorylase to glucose-l -phosphate. 



Plants probably metabolize free sugars of various kinds to some 

 extent. But the comparative importance of mono- and disaccharides 

 in the energy supply is unknown. Therefore, discussion of the mech- 

 anism by which the individual sugars are attacked is delayed, pending 



