8 NUTRITION OF FARM ANIMALS 



of carbon dioxid by chlorophyl-bearing plants, and the great 

 mass of vegetable tissue is composed either of carbohydrates or 

 of their nearly related derivatives. 



The more common carbohydrates have been known for a long 

 time. Starch is familiar to us in the mealy portion of grains and 

 in certain tubers, and cellulose in cotton and linen and, in im- 

 pure forms, in the woody fiber of plants. Of the sugars, cane 

 sugar has been known since almost prehistoric times, while the 

 presence of this and other sugars in plant juices, in sweet fruits, 

 honey, etc., is a familiar fact. The more common sugars were 

 separated and identified quite early in the history of chemistry. 



7. Classification. The carbohydrates contain hydrogen 

 and oxygen in exactly the proportions to form water, and their 

 name is derived from this fact, although compounds exist 

 which contain two atoms of hydrogen to one of oxygen and 

 yet are not carbohydrates, such, for example, as acetic acid, 

 C2H 4 O 2 . The simplest of the carbohydrates are the simple 

 sugars, more exactly designated as the monosaccharids. By 

 polymerization, with elimination of water, the monosaccharids 

 yield more complex carbohydrates which are conveniently classi- 

 fied as di-, tri-, and polysaccharids. 



Monosaccharids, or simple sugars 



8. Composition. The monosaccharids may be represented 

 by the general formula C n H 2n O n . Substances having this gen- 

 eral formula are known whose molecules contain from one to 

 nine carbon atoms and which, from a chemical point of view, 

 may be called carbohydrates. The simplest of these is formal- 

 dehyde, CH 2 O, which is believed by many to be the first step in 

 the synthesis of carbohydrates by the green plant. Only the 

 CG and C& compounds, however, known respectively as the 

 hexose and pentose carbohydrates, are of importance in their 

 relations to nutrition. 



9. Hexoses. The most important hexose monosaccharids 

 are dextrose, levulose, galactose and mannose. 



Dextrose, ^-glucose, or grape sugar, is generally regarded as 

 an aldose of the hexatomic alcohol sorbite. ' 



Sorbite : CH 2 OH- (CH - OH) 4 - CH 2 OH 

 Dextrose : CH 2 OH- (CH OH) 4 - CHO 



