PHYSIOLOGY OF CELLS 53 



The formulas of the acid and fat just given are not in themselves important 

 in this connection, but that fats are composed of glycerol and three mole- 

 cules of fatty acid, and the method of combining these molecules, should 

 be remembered. 



The glycerol component of fats is readily produced in the organism 

 from the carbohydrate glucose. The fatty acids are also undoubtedly de- 

 rived, directly or indirectly, from carbohydrates. The details of neither 

 process need be considered here. In animals both glycerol and fatty acids 

 may be derived from fatty food, since in digestion as pointed out in Chap- 

 ter VII the process of synthesis above described is reversed, and by hy- 

 drolysis (addition of water) fats are broken up into their components. 



Foods for Plants. — Carbohydrates, fats, and proteins constitute the 

 foods of plants. For purposes of building up protoplasm plants cannot 

 make use of carbon dioxide, oxygen, water, and inorganic salts until 

 these have first been combined within the plant into organic compounds. 

 These inorganic substances which the plant takes in and uses as raw 

 materials in the process of food manufacture may be called nutrient sub- 

 stances hut not foods. The term foods must be reserved for the organic 

 substances which the plant uses in building up protoplasm. Ordinarily 

 the plant manufactures much more food than it can use and reserves are 

 accumulated. These reserves of food are stored in fleshy roots, stems, 

 leaves and seeds where they may be of use to the plant during the next 

 growing season, or serve as nourishment for the young plant. Plants of 

 the lower orders store very little food. 



Foods for Animals. — The reserves of foods stored away by plants are 

 of the utmost importance to animals since most of the latter do not have 

 the ability to manufacture their food from the raw materials. Animals 

 can use only the organic foods stored away by plants. This dependence 

 on plant reserves in the case of herbivorous animals is obvious. It is not 

 less real in exclusively flesh-eating animals which are dependent, through 

 plant-eating animals, upon the activities of plants. 



In addition to the carbohydrates, proteins or albuminous substances 

 of various sorts resembling proteins, and oils or fats, all of which are se- 

 cured directly or indirectly from plants, animals get from plants certain 

 salts necessary to the well-being of the animal body. They also secure 

 from plants certain substances of unknown chemical nature which are 

 called vitamines. These substances are comparatively abundant in 

 fresh vegetables, particularly leaf vegetables. They are less abundant 

 in the cereals and are lacking in the vegetable oils. That these vitamines 

 are of importance in the diet of animals is shown by experimental feeding. 

 Animals which get foods deficient in vitamines suffer from malnutrition 

 although they may get food of sufficient calorific value. If the diet of 

 such experimental animals be changed so as to include sufficient vita- 

 mines the animals may recover. Flesh-eating animals get vitamines 



