PLANT METABOLISM 399 



Carbohydrates. Although information is incomplete on the early prod- 

 ucts of photosynthesis and their mode of formation, the total materials 

 of the plant are the net final products of photosynthesis after modifica- 

 tion by other phases of plant metabolism. The carbohydrates are quanti- 

 tatively the most important group of compounds, and cellulose, starches, 

 and sugars are particularly abundant. 



Glucose and fructose commonly occur as free and combined sugars in 

 plants, whereas mannose and galactose are found only in combined form. 

 Among the disaccharides, sucrose is especially important. Maltose and 

 cellobiose do not occur free in demonstrable concentrations but arise 

 from the hydrolysis of starch and cellulose, respectively. Plants also 

 may yield the disaccharides gentiobiose, trehalose, and melibiose, but 

 they do not contain lactose. Raffinose, gentianose, and melizitose are 

 the most common plant trisaccharides, and stachyose is a tetrasaccharide 

 which has been isolated from lupine seeds. 



Particular interest is attached to the method by which the plant syn- 

 thesizes sucrose because the sugar is important to plants and man. It 

 has not as yet been synthesized chemically. Sucrose can be formed by 

 detached leaves when they are infiltrated with glucose, fructose, mannose, 

 galactose, or glyceraldehyde; the synthesis will not take place in the 

 absence of oxygen. The only controlled synthesis of sucrose by a cell- 

 free preparation has been achieved with an enzyme system from the 

 bacterium, Pseudamonas saccharophila. This organism produces a 

 sucrose phosphorylase (more properly termed a transglucosidase) , which 

 by phospliorolysis of sucrose produces fructose plus glucose-1-phosphate. 

 The equilibrium of the reaction is such that by a reversal of the reaction 

 a synthesis of about 5 per cent of sucrose from glucose- 1 -phosphate plus 

 fructose can be achieved. Though higher plants accumulate much sucrose, 

 there has been no success to date in demonstrating their synthesis of 

 sucrose by sucrose phosphorylase or by any other isolated enzyme system. 



Even less is known concerning the synthesis of cellulose. Although 

 bacterial preparations have been induced to form cellulose from a number 

 of precursors, some as simple as Co compounds, there is no detailed in- 

 formation on the pathway of its formation. 



Knowledge of starch synthesis is much more nearly complete. Peat, 

 Bourne, and co-workers (see Bernfeld for discussion) have shown that 

 potato tubers and other plant tissues contain a starch phosphorylase 

 which by phospliorolysis converts starch to glucose-1-phosphate. The 

 reversal of this reaction to yield starch from glucose-1-phosphate is 

 readily demonstrable. The first preparations used, synthesized only 

 amylose, the straight chain starch, but later preparations also yielded 

 amylopectin, the branched chain starch. The enzyme which synthesizes 

 amylose from glucose- 1 -phosphate is termed the P-enzyme. A separate 

 enzyme, termed the Q-enzyme, is needed to form the linkages at the 



