THE CAKBOHYDRATES 17 



HONOSACCHARIDES 



Dextrose or Grape Sugar. This carbohydrate is found in fruits, 

 honey, and in minute quantities in the blood (0*12 per cent.) and 

 numerous tissues, organs, and fluids of the 

 body. It is the form of sugar found in 

 large quantities in the blood and urine 

 in the disease known as diabetes. 



Dextrose is soluble in hot and cold 

 water and in alcohol. It is crystalline 

 (see fig. 1), but not so sweet as cane sugar. 

 When heated with strong potash certain 

 complex acids are formed which have a 

 yellow or brown colour. This constitutes 



Moore's test for sugar. In alkaline solu- FIG. i. -Dextrose crystals. 



tions dextrose reduces salts of silver, 



bismuth, mercury, and copper. The reduction of cupric hydrate to 

 cuprous hydrate or oxide constitutes Trommer's test, which has been 

 already described at the head of the lesson. On boiling it with an 

 alkaline solution of picric acid, a dark red opaque solution due to reduc- 

 tion of the picric to picramic acid is produced. Another important 

 property of grape sugar is that under the influence of yeast it is con- 

 verted into alcohol and carbonic acid (C 6 H 12 O 6 =2C2H 6 O + 2CO 2 ). 



Dextrose may be estimated by the fermentation test, by the 

 polarimeter, and by the use of Fehlirig's solution. The last method 

 is the most important : it rests on the same principles as Trommer's 

 test, and we shall study it and other methods of estimating sugar in 

 connection with diabetic urine (see Lesson XII.). 



Levulose. When cane sugar is treated with dilute mineral acids 

 it undergoes a process known as inversion i.e. it takes up water and 

 is converted into equal parts of dextrose and levulose. The pre- 

 viously dextro-rotatory solution of cane sugar then becomes levo- 

 rotatory, the levo -rotatory power of the levulose being greater than 

 the dextro-rotatory power of the dextrose formed. Hence the term 

 inversion. The same hydrolytic change is produced by certain 

 ferments, such as the invert ferment of the intestinal juice, and of 



Pure levulose can be crystallised, but so great is the difficulty of 

 obtaining crystals of it that one of its names was ' uncrystallisable 

 sugar.' Small quantities of levulose have been found in blood, urine, 

 and muscle. It has been recommended as an article of diet in 

 diabetes in place of ordinary sugar; in this disease it does not 



c 



