THE CARBOHYDRATES 



41 



H 

 O 



Hi 



HOCH2 



HOCH2 



H2COH 



H2COH 



H2 



O 

 H 



Formula S.IV. Inositol 

 (c) Disaccharides 



Glucose and fructose are the constituents of the most common dimeric 

 and polymeric sugars. The relationships between these compounds are 

 represented by the following scheme: 



Monosaccharides : 



Disaccharides: Maltose Sucrose 



Polysaccharides: 



Glucose- 



i 

 Maltose 



i 



Starch, 



Cellulose 



'Fructose 



Inulin 



Sucrose is the most common disaccharide in green leaves; its concen- 

 tration often exceeds that of the free hexoses (c/. Table 3. III). Sucrose 

 is a product of condensation of one molecule of a-glucose in the pyranoid 

 form and one molecule of fructose in the furanoid form: 



(3.4) 



a-Glucopyranose + fructofuranose 



-> sucrose + water 



H OH OH H 



Formula 3.V. Siicrose 



Since starch occurs regularly in a large proportion of leaves, one 

 would also expect to find maltose which is an intermediate between 

 starch and glucose. The molecule of maltose contains two glucopyranose 

 units bound by an oxygen "bridge." Brown and Morris (1893) found 

 0.7-5.3% and Gast (1917) up to 1% maltose in Tropaeolum leaves; but 

 Davis, Daish and Sawyer (1916) identified it as a product of hydrolysis 

 of starch during the slow drying of the leaves. If leaves are killed 

 rapidly, no maltose is found in them. Davis (1916) and Daish (1916) 

 showed that leaves contain maltase, which hydrolyzes maltose to glucose, 

 and attributed to this fact the absence of maltose in hving leaves. 

 However, the presence of diastase, which hydrolyzes starch, (c/. Brown 



