SECTION III. 



THE CARBOHYDRATES. 



The importance of carbohydrates becomes obvious when once 

 it is reahzed that the metabolism of the green plant is essentially 

 a carbohydrate metabolism. Carbohydrate, an essential in 

 the food of the plant and of the animal, is synthesized from 

 raw inorganic material only by the green plant, wherefore the 

 maintenance of life is entirely dependent on the plant. Glucose, 

 perhaps the simplest carbohydrate expression, is the all-im- 

 portant respirable material both in the animal and in the plant 

 and it, together with other simple sugars, forms a raw material 

 for the making of more complex carbohydrates, of proteins, 

 of fats, and of other substances. With but few exceptions, 

 carbohydrate in the form of cane sugar, starch, inulin, 

 glycogen, and hemicellulose are the most significant reserve 

 food materials of plants, whilst in the animal, glycogen alone 

 forms a temporary reserve. In the plant, carbohydrate in 

 the shape of cellulose, ligno-cellulose, hemicellulose, and pento- 

 sanes, form entirely or in part the structural basis of the cell 

 wall when present,* and thus plays an important role in the 

 structural mechanism. In the animal, on the other hand, 

 carbohydrates are seldom thus employed ; chitin, spongin, and 

 chondro-mucoid, which to a limited extent enters into the 

 composition of muscle, may be cited. Indeed, the synthesis of 

 carbohydrate in the animal is for the most part restricted to 

 the production of lactose, or milk sugar, from pre-existing 

 glucose. 



Notwithstanding the differences in the physiological 



* It will be remembered that the Myxomycetes, many Chrysophyceae, 

 Euglenineae, and other members of the lower Protophyta, together with 

 the gametes of the majority of plants, both high and low, are naked 

 structures with no cell wall. 



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