106 TEXTBOOK OF BOTANY 



Fats, Fats and oils are likewise composed only of carbon, hydrogen, 

 and oxygen, but thev have comparatively little oxygen in proportion to 

 carbon and hydrogen. There is no general chemical distinction betw^een 

 fats and oils. Thev are distinguished by their melting points. At ordinary 

 room temperatures oils are liquids, while fats are solids. During digestion 

 fats and oils are hydrolvzed to glycerin and fatty acids. 



Proteins. The proteins are composed of carbon, hydrogen, oxygen, 

 nitrogen, sulfur, and sometimes phosphorus also. During digestion the 

 large complex molecules of proteins are hydrolyzed to simpler sub- 

 stances known as amino acids. 



When we consider all the substances that we have seen in the cells of 

 seeds and tubers, we readily recognize some of them as foods of man 

 and other animals. Four questions may now be considered. Which of 

 these substances are used as food in the animal body? When we buy a 

 pound of potatoes do we buy a pound of food? Is the food of green plants 

 and of animals identical? How does the green plant obtain food? 



Human foods. For our own bodies we recognize as foods those com- 

 pounds which can be transformed into the substances of which cells are 

 composed and from which our body obtains energy by oxidation. These 

 compounds, we have already seen, are carbohydrates, fats, and proteins, 

 together with their partially oxidized, reduced, or hydrolyzed deriva- 

 tives which may or may not be combined with certain ions of the mineral 

 salts. But as foods these compounds are subject to one more limitation: 

 they must be either in a soluble state, or capable of being digested by 

 appropriate enzymes in the alimentary tract. The sugars and some of 

 the derived compounds are already dissolved in water and may pass 

 from the alimentary canal into the blood stream. Our bodies produce 

 enzymes that digest starches, fats, and proteins, but not enzymes that 

 digest cellulose, pectic compounds, wood, cork, and inulin. If we were 

 entirely dependent upon our own enzvmes even a pound of dried pota- 

 toes would not be a pound of food. 



The non-green plants ( bacteria and fungi ) also produce enzymes that 

 digest starch, fats, and proteins; and a few kinds of non-green plants pro- 

 duce enzymes that digest cellulose, pectic compounds, inulin, and wood. 

 Thus the non-green plants as a group may utilize a wider range of 

 compounds as food than is used by animals. It is interesting to note that 

 some of the digestion that occurs in the alimentary tract of man and 

 other animals is due to the non-green plants that live there. 



The food of green plants. The food of the green plant may now be 



