132 PLANT PHYSIOLOGY 



The difference lies in the location of the atoms in the chain. In 

 glucose, the oxygen not associated with a hydroxyl (OH) group 

 is on the end carbon atom, while in fructose it is on the one next 

 to the end, which is the difference between the aldehyde and the 

 ketone mentioned above. Similarly the oxygen could occur in 

 other places, producing in each case a different sugar. 



Stereoisomers may be illustrated by d-glucose and 1-glucose: 



d-glucose 



CH 2 OH 



I 

 H— C— OH 



I 

 H— C— OH 



I 

 HO— C— H 



I 

 H— C— OH 



I 

 CHO 



Here it will be seen that 1-glucose has the same kind of atoms on 

 each carbon atom in the chain, but the arrangement is such that 

 one is the mirror image of the other. With both stereo and struc- 

 tural isomerism possible, it is easy to see how so many different 

 sugars with the same formula are obtained. 



Rotatory Power. — The carbohydrates which are soluble will ro- 

 tate the plane of polarized light passed through the solution. The 

 degree of the rotation is measured by a polarimeter (an instrument 

 of the physical chemist) and depends upon the kind of carbo- 

 hydrate, the concentration, and the length of the column of 

 solution through which the beam of polarized light has to pass. 

 But the same length of the same concentration of the same sugar 

 at the same temperature will always produce the same effect, i. e., 

 the sugar has a specific rotatory power. Sugars which rotate the 

 beam to the right are called dextrorotSLry and those which rotate 

 it to the left are called feyorotary. 



Reducing Action.— Because of the aldehyde or ketone group, 

 the monosaccharides and some other sugars are reducing agents. 

 The test for sugar with Fehling's solution, a standard solution of 

 copper sulphate, depends upon this reducing action, the solution 

 being reduced by the sugar to cuprous oxide. The amount of the 

 oxide produced bears a definite relation to the amount of sugar 

 present. 



