THE LIVING CELL AND ITS FOOD 



155 



Cell Energy and Cell Food. — The living cell, whether plant 

 or animal, performs its function of life through the agency of 

 energy which is set free by its utilization of food supplied to it J 

 from without. When we study later the peculiar processes or 

 functions of plants which differentiate them from the animals 

 we shall find that the ultimate source of this energy is the sun. 

 For the present, however, and so far as the cell itself is con-^ 

 cerned, the source of the cell's energy is its food. We have just 

 stated that the cell is composed of carbohydrates, fats and pro- 

 teins and inorganic salts. It is clear, therefore, that the food 

 of the cell must contain these same substances. So far as we 

 know the inorganic constituents of the cell or of the cell food 

 have nothing to do, at least directly, with the energy of the cell. 

 In its energy relationship, therefore, the food of the cell is 

 wholly organic of the three groups of compounds mentioned. 

 This is borne out by the fact that the seeds of plants which 

 contain the germ of a new plant have stored within them all 

 of these constituents, and these are used by the developing 

 embryo as food. Also all animals require as food, not one 

 only, but all of these groups of compounds. 



Food as Energy Material. — How then do these organic 

 substances which we may term energy food yield this energy to 

 the living cell and thus to the whole living organism? If we 

 recall the chemical composition and constitution of these com- i 

 pounds we see that they are alike in being complex compounds 

 of carbon, hydrogen and oxygen, with nitrogen in addition to 

 these in the proteins. The percentage composition of the three 

 groups of compounds is : ^. 



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