454 PROTOPLASM 



the formula C6H12O6; both have the same molecular weight; and 

 both would, on analysis, yield 40 per cent of carbon, 6.6 per cent 

 of hydrogen, and 53.3 per cent of oxygen, yet they taste different 

 and have different optical properties. These differences are 

 due not to chemical composition but to structure. The indi- 

 viduals are said to be isomers of each other. The structural 

 isomerism responsible for the difference is illustrated in glucose, 

 which has a CHO group and is known as an aldehyde (or aldose 

 sugar), while its isomer fructose has a CO group and is known as 

 a ketone (or ketose sugar). 



The pentose sugars, with five carbon atoms, are not often 

 found free in nature; they are usually associated with mucilages, 

 such as the pentosans. They seem to be of some physiological 

 importance, since they always occur with nucleic acid which 

 constitutes a large part of nuclear material. 



Glucose and fructose are the only two common and free hexose 

 sugars. The rest occur associated as constituents of higher 

 compounds such as the glucosides, an important group of sub- 

 stances among which are saponin and digitalin. Glucose is the 

 most widely distributed of the sugars and is the energy-producing 

 sugar oxidized in the bodies of higher animals — probably also 

 in those of the lower animals and in plants. It is the universal 

 fuel sugar, which means that other sugars when taken in as food 

 are ultimately converted into glucose before being oxidized or 

 broken down into carbon dioxide and water. Glucose occurs 

 in blood to the extent of about 0.14 per cent. It is abundant in 

 fruits; the brown powder on the surface of raisins consists of 

 glucose crystals; molasses is one-third glucose. Honey contains 

 glucose, but the chief constituent here is fructose, the sweetest 

 of sugars. It is usually found wherever glucose occurs. Glucose 

 is commercially prepared from starch, the following series of 

 changes taking place: Starch -^ dextrin ^ maltose ^ glucose. 

 The importance of sugars as food has led to attempts to produce 

 them from cheap raw material such as sawdust. This seems to 

 be already a commercial process in Germany. Work done at 

 the Forest Products Laboratory in Madison, Wisconsin, suggests 

 that the method may be some day widely used. It appears that 

 cellulose (wood) may, by hydrolysis, be converted into sugar; 

 that is to say, by combining water chemically with cellulose, 

 glucose results. 



