CARBOHYDRATES 51 



up 2H forming an aldehyde, a hexose, which thus has been 

 built up from a pentose. 



CH 2 OH CHOH CHOH CHOH CHOH CHO 



The carbohydrates also may be "built down" step by step in 

 the laboratory. This method also has been of value in study- 

 ing their constitution. If treated with hydrogen peroxide in 

 the presence of a ferric salt as catalyzer, the elements of formic 

 acid are split off, leaving a carbohydrate with one less carbon 

 atom. Carbohydrates also may be built down by electrolysis 

 or by way of their oximes. 



Interconversion of Carbohydrates. Since the different mono- 

 saccharides are closely related compounds, differing only in the 

 arrangement of their hydrogen and hydroxyl groups around the 

 carbon atoms, it is not surprising to learn that certain of them 

 may very easily be converted into one another. Thus if a solu- 

 tion of glucose is made slightly alkaline and allowed to stand 

 for some time, it will be found to contain not glucose alone, 

 but also fructose and mannose. A portion of the glucose is 

 transformed into the other two sugars. It is immaterial which 

 of the three sugars is taken to start with, the result will be 

 the same and all three will be found in the solution. The inter- 

 conversion of the simple sugars is of interest physiologically, 

 for in the body such transformations are known to take place. 

 The lactose of milk is produced in the mammary gland from 

 glucose in the blood. Lactose is made up of equal parts of glu- 

 cose and galactose. A portion of the glucose from the blood 

 thus must be transformed into galactose in the gland. A second 

 instance of a similar transformation is the formation of the 

 polysaccharide glycogen which is stored up in the liver and 

 muscles as a reserve material. On hydrolysis glycogen yields 

 always glucose. It is well known, however, that glycogen will 

 be deposited in the body if an animal is fed fructose or various 

 other sugars. The glycogen formed under these circumstances 

 also yields glucose on hydrolysis, indicating the transformation 

 of the fructose, etc., into glucose. 



