V] CARBOHYDRATES 49 



Though many of the above sugars have been synthesized artificially, 

 only three are known to occur naturally, i.e. c?-glucose (dextrose or 

 grape-sugar), c?-mannose and d-galactose. 



Since compounds containing asymmetric carbon atoms are optically 

 active, i.e. can rotate a plane of polarized light, it follows that the sugars 

 under discussion are optically active. 



Glucose. This substance, which is also known as grape-sugar, is very 

 common and very widely distributed in plants. It occurs in the tissues 

 of leaves, stems, roots, flowers and fruits. It is produced as a result of 

 the hydrolysis of cane-sugar and maltose, and, in all probability, is the 

 first sugar synthesized from carbon dioxide and water. Its synthesis and 

 its relationships to other sugars will be discussed later (see p. 71). It 

 is a white crystalline substance, readily soluble in water and aqueous 

 alcohol, but only slightly soluble in absolute alcohol. 



c?-glucose is dextro-rotatoi-y. 



When either d- or Z-glucose is first dissolved in water, it is chemically 

 less active than would be expected of the aldehyde form depicted above. 

 This is explained by assuming that glucose, when first dissolved in water, 

 exists in the condition of a 7-lactone : 



HO— C— H 



CH2OH 



In the above state the carbon atom marked * is also asymmetric so 

 that two forms of glucose are possible, a- and y8-glucose : 



H— C— OH 



CH2OH 



iS-Glueose 



y 



