CONSTITUENTS OF THE BIOSPHERE 15 



(a) Aldoses and Ketoses 



Sugars are of two types — aldoses, and ketoses. Amongst the C3 sugars, 

 for example, there is one ketose, dihydroxyacetone, and one aldose, gly- 

 ceraldehyde. The latter has an asymmetric carbon atom and exists in two 

 optically active forms. 



The two forms are designated D and l and the sign inside the parentheses 

 indicates the direction of rotation of the plane of polarized light. By using 

 the procedure of Kiliani (HCN reacts with sugars to form a nitrile-alcohol 

 which on hydrolysis yields an acid having one carbon atom more than the 

 starting material. By reduction of this acid a sugar is obtained having one 

 carbon atom more than the original sugar) one can prepare aldoses higher 

 up the series by inserting between (C-1) and (C-2), either a group of 

 configuration 



j-j Q OH or of configuration HO — C — H. 



I I 



In this way one obtains four tetroses, or C4 sugars : 



CHO CHO CHO CHO 



HO-C-H H— C-OH HO— C— H H— C— OH 



1 1 I I 



H— C— OH H— C-OH HO— C— H HO— C— H 



CH2OH CHoOH CHoOH CH.OH 



D-Threose D-Erythrose L-Erythrose L-Threose 



The letter d or l preceding the name of the sugar indicates its relation- 

 ship to one of the parent glyceraldehydes. This is a general rule in sugar 

 terminology. 



R R 



I I 



H— C— OH HO— C— H 



CH.OH CH2OH 



n— L— 



