CHAPTER III. 



THE CARBOHYDRATES. 



IT has been pointed out in the preceding chapter that while 

 plants are capable of effecting from relatively simple compounds the 

 synthesis of those complex albumins which are found in their 

 various tissues and organs, animals do not possess this power, 

 and are therefore dependent for their supply of nitrogen upon the 

 albuminous food-stuffs that have been elaborated by plants. The 

 carbohydrate supply of animals is also derived from plants, but for 

 the maintenance of life it is not necessary that the carbohydrates 

 should be furnished as such, as animals are not only capable of 

 splitting off the carbohydrate radicle of the albuminous molecule as 

 occasion demands, but, as we shall see later, they can also form carbo- 

 hydrates directly from the fats which are stored in their tissues. The 

 carbohydrates cannot therefore be regarded as essential food-stuffs, 

 and we see, as a matter of fact, that carnivorous animals, at least, 

 are capable of existing on albuminous food exclusively. Carbo- 

 hydrates are important, however, as the stored energy which is thus 

 supplied to animals represents a considerable caloric value, and they 

 can hence protect the albumins from undue destruction. The im- 

 portance of the carbohydrates as food-stuffs is thus only secondary, 

 and they are totally unable to take the place of the albumins. All 

 living matter requires a definite amount of nitrogen so that life may 

 be maintained, and if this is withdrawn death inevitably results. 

 It is to be noted, however, that whereas animals can exist without 

 carbohydrate food, and whereas the albumins largely predominate in 

 its tissues, the reverse holds good for plants. Here the carbo- 

 hydrates prevail, while the albumins are much less abundant. Con- 

 sequently we may expect to find a far greater diversity of carbo- 

 hydrates in the vegetable than in the animal world. This is 

 actually the case. As it would lead too far, in a work of this 

 scope, however, to consider all those carbohydrates which occur in 

 the vegetable world, we shall confine our attention in the subsequent 

 pages to those forms which may be regarded as common food-stuffs, 

 or those which are more or less peculiar to the animal body. 



All carbohydrates consist of carbon, hydrogen, and oxygen, and 

 in most members of the group the elements hydrogen and oxygen 

 are present in such proportion as to form water. In others, how- 

 ever, this is not the case ; and there are substances, such as lactic 

 acid and acetic acid which likewise contain hydrogen and oxygen in 

 this proportion, but which are manifestly not carbohydrates. As 



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