NUTRITION 229 



C. FOOD AS THE MATERIAL FOR GROWTH, REPAIR, AND 



THE MANUFACTURE OF SPECIAL PRODUCTS OF 



CELL ACTIVITY 



15. Complexity of the chemical composition of living cells 

 and of the products of their manufacture. In the first sub- 

 division of this chapter we have considered food as fuel. 

 We are now in a position to consider some of the more 

 important features of the other great function of food, 

 namely as the material for the growth and repair of living 

 cells and for the manufacture of the special products of 

 cell life the secretions (internal and external), the hor- 

 mones, and all other substances produced by the body for 

 special purposes. 



The living cell is an extremely complex machine into the 

 construction of which enter numerous compounds of diverse 

 chemical nature. Formerly there was a tendency to regard 

 the cell as composed essentially of protein; but the increase 

 of our knowledge has shown that there are other essential 

 constituents, notably (in addition to water and inorganic 

 salts) a group of compounds known as lipoids, or lipins, 

 substances which more or less resemble fats in their physi- 

 cal characters, although not always in chemical structure. 

 The cell nucleus also contains special material of still dif- 

 ferent chemical composition. The chemical properties ' and 

 the physiological significance of these cell components are 

 far too complicated subjects for discussion in this work ; we 

 merely wish to emphasize the complexity of the mixture and 

 the variety of chemical compounds concerned. (See p. 42.) 



We are impressed with the same diversity of chemical 

 structure in the secretions, hormones, and other material 

 manufactured by the body for special purposes. The stu- 

 dent has only to recall the examples of these already men- 

 tioned the enzymes of the digestive juices ; the internal 

 secretions of the adrenals, thyroids, pituitary, and pancreas ; 



