THE CARBOHYDRATES 133 



Formation of Glucosides. — It is possible to replace one of the 

 hydrogen atoms of the sugar molecule by compounds of many 

 different types thus producing a compound of a sugar with a sub- 

 stance entirely different. These compounds, which are called 

 glucosides, can in turn be broken down into the sugar and the other 

 radical. When hydrolyzed they are thus broken into glucose and 

 the hydroxyl of the accompanying substance. Although strictly 

 speaking the glucosides are compounds which yield only glucose, 

 the term is often used more loosely to include such compounds of 

 other sugars as well. 



Osazones. — Owing to the presence of the aldehyde or ketone 

 group in the molecule, the sugars are able to form with phenylhy- 

 drazine (C 6 H 8 N 2 ) characteristic products known as hydrazones 

 and osazones. The former are mostly colorless compounds soluble 

 in water and do not help much in the identification of sugars, but 

 the osazones, in which an extra molecule of phenylhydrazine is 

 added to the sugar, are not soluble in water and give crystals of 

 a specific melting point and characteristic shape, which are very 

 useful in identifying sugars. 



Formation of Acids.— When the sugars are oxidized they give 

 rise to three different types of acids varying with the conditions 

 of the process and the type of oxidizing agent employed. This 

 may be illustrated by glucose, which when oxidized gives gluconic, 

 glucuronic, or saccharic acid. 



Of these three, glucuronic is the most interesting physiologically 

 because it is often found in the urine of animals as a result of only 

 partial oxidation of glucose. Ordinarily glucose is oxidized down 

 to its final products of carbon dioxide and water, but when the 

 animal is given substances like camphor and turpentine, which 

 are difficult to oxidize, they are joined with glucose to form a 

 glucoside, which is then oxidized to glucuronic acid and eliminated 

 in the urine. Similar processes have been found in plants, and the 

 acids which have been derived from such oxidations have been 

 grouped together as "uronic acids" by recent investigators. 



