METABOLISM 121 



(C'eHioOs)!!. Because of its comparative indigestibility cellulose 

 is not usually available as a food, though in certain plants a layer 

 of it is deposited on the inside of the cell-wall during periods of 

 food-storage and is later absorbed and used by the plant. Such 

 reserve cellulose is in some cases an important source of food. 

 Of far more significance than its use as a food, however, is the 

 fact that cellulose is the material out of which the cell-wall, and 

 therefore the entire skeleton of the plant, is constructed. 



B. Fats. — Fats resemble carbohydrates in being compo.sed only 

 of carbon, hydrogen, and oxygen, but differ from them in the 

 relative proportion of these elements. The hydrogen atoms are 

 approximately twice as numerous as those of carbon, but in 

 comparison with these two, the amount of oxygen is very small. 

 Three common plant fats well illustrate the chemical composi- 

 tion of this type of food. These are palmitin, C3H5(C02Ci5H3i)3; 

 stearin, C3H5(C02Ci7H35)3; and olein, C3H5(C02Ci7H33)3. Some 

 fats are liquid and others solid at ordinary temperatures. Fats 

 are insoluble in water and when moved about from cell to cell 

 by diffusion a fat must therefore be broken down into its simpler 

 and soluble components, glycerine and a fatty acid. In nature, 

 fats are readily converted into sugars and sugars into fats. 

 Fats are not as common a type of food as carbohydrates among 

 plants, but are nevertheless of frequent occurrence in certain 

 situations, particularly in seeds and other regions where food in 

 concentrated form is advantageous. They are apparently not 

 produced by any special plastids but appear in the form of minute 

 droplets in the cytoplasm. Fats are chiefly important as sources 

 of energy. 



C. Proteins. — Proteins are composed not only of the basic 

 carbon, hydrogen, and oxygen of the carbohydrates and fats, but 

 include nitrogen also, and generally a small amount of sulphur. 

 They are exceedingly variable, both in chemical composition and 

 in physical properties, and are often very unstable. Protein 

 molecules are large and complex, as is illustrated by the calcu- 

 lated formulas of two common plant proteins, zein of corn, C736 

 H1161N184O208S3; and (jliadin of wheat, C685H1068N196O211S5. We 

 know less alxnit prot(Mns than a])out any other class of organic 

 compounds. They are of particular interest from the fact that 

 protoplasm itself is a mixture of complex proteins. 



Proteins undoubtedly are formed by the union of a simple 

 carbohydrate, such as the glucose which has been produced in 



