Chapter 3 



CARBOHYDRATES 



Composition and definition 



The word "carbohydrate" hterally means carbon combined with water 

 and originates in the fact that many carbohydrates have the composition 

 Ca^HoO) j;. Tlie values of x and y may range from three to many thousand. 

 Although several carbohydrates do not have this composition, and some 

 other substances do, e.g., lactic acid (CsHgOs) , the term nevertheless fits 

 the great majority of carbohydrates and is in common use. 



Carbohydrates may be cliemically defined as simple sugars, or more 

 complex substances which yield simple sugars on hydrolysis.^ Simple 

 sugars are either aldehydes or ketones which contain at least two, and 

 usually several, hydroxyl groups. The aldehyde-alcohol type is called 

 an aldose, e.g., glucose, and the ketone-alcohol type a ketose, e.g., fructose. 



Occurrence and importance 



The plant world is the great source of the carbohydrates. The dry 

 matter of plants (excepting certain oily seeds) is from 60 to 90 per cent 

 carbohydrate. These compounds are constituents of most materials that 

 satisfy the primary needs of human life. 



Our food is made up principally of carbohydrates — approximately 70 

 per cent by weight of the food in the average diet. Much of our clothing 

 is made from carbohydrates — cotton, rayon, and linen. In the United 

 States probably more houses are built of wood than of all other materials 

 combined. Even in many buildings of brick and stone, wood enters into 

 the construction of walls, floors, stairways, and windows. The great fuel 

 materials, wood and coal, are either carbohydrates or derived from carbo- 

 hydrates. The carbohydrates are at the very foundation of the economic 

 structure of society. 



The importance of the carbohydrates is shown by Table 3-1. The 

 carbohydrate industries listed employ nearly as many people and turn 

 out products of greater value than the combined machinery and chemical 

 and drug industries. 



^ Hydrolysis consists in the cleavage of a complex molecule into smaller fragments 

 with the simultaneous addition of water. 



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