X. 



THE CHEMICAL CONSTITUENTS OF PLANTS 



181 



H C = 



H C OH 

 HO C H 

 H C OH 

 H C OH 



H C = O H C = O 



H C OH H C OH 



HC OH HO C H 



CH 2 OH 

 C = O 



HO C H 

 HO C H 



)H 2 OH 



d-glucose. 



)H 2 OH 



Z-mannose. 



HO C H 

 HC OH 

 CH 2 OH 



d-galactose. 



HO C 



H-L 



H 



OH 



HC OH 



H 2 OH 



d-fructose. 



In all cases the dextro- and laevo- forms are enantiomorphs, i.e., 

 the formula of one is the mirror-image of that of the other. 



This use of the terms leads to several unfortunate contradictions 

 thus d-fructose is strongly laevo-rotatory, while Z-arabinose is strongly 

 dextro-rotatory. 



According to recent investigations by Armstrong, Lowry and 

 others, it appears probable that there are two isomeric forms of 

 d-glucose which possess the following constitution : 



HC OH 



I 



H 



CH 9 OH 



i-glucose. 



JH 2 OH 



/3-glucose. 



The only difference here, is in the relative positions of the 

 hydroxyl and hydrogen attached to the uppermost carbon atom. 



II. Disaccharoses, C 12 H 22 O n . These compounds contain twelve 

 atoms of carbon and consist of a combination of two hexoses. By 

 hydrolysis brought about by acids or enzymes they split up into 

 their component hexoses, which may be either aldoses or ketoses. 



In some disaccharoses, the aldehyde or ketone groups retain their 

 functions and the sugar is then capable of reducing copper salts ; in 

 others, these groups become inactive and the sugar is then devoid of 

 reducing power. 



(a) Reducing sugars : 



Maltose glucose a-glucoside. 

 Gentiobiose glucose /?-glucoside. 

 Lactose glucose /?-galactoside. 

 Melibiose glucose galactoside. 

 Turanose glucose and fructose. 



