CHAPTER V 



THE TRANSFORMATION OF FOOD 



THE complex organic compounds found in Nature 

 are always, as we have seen, the products of animal 

 or plant activity, for although a few organic com- 

 pounds have been artificially manufactured, still as 

 an economic source of " food " these may be con- 

 sidered, at present, at all events, as insignificant. 

 Further, these organic substances are not, even then, 

 in forms capable of being made use of at once by 

 the protoplasm. They must be readjusted in their 

 composition and constitution before they can be 

 actually assimilated. For example, starch must be Digestion, 

 transformed into sugar, if for no other reason than to 

 render it soluble, so that it may penetrate the walls 

 of the cells ; grape-sugar, again, is a food-stuff to 

 yeast, but cane sugar is useless to it until it has been 

 altered into grape sugar. Readjustments and 

 alterations such as these, whether in the plant or 

 animal and many of them, as we shall see later on, 

 are exceedingly complicated in their nature are 

 collectively termed digestion. The two essentials 

 are (1) jthat the food shall have the appropriate 

 chemical composition, suited, that is to say, to the 

 wants of the organism, and (2) that it shall be in a 

 state of solution. 



Let us first of all consider digestion in one of the 

 higher animals. 



The "food" in the process of mastication is in 

 mixed with and affected by a secretion formed by animals - 



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