are formed in practically all protoplasm, are represented by two classes of 

 familiar compounds— fats and carbohydrates. We all know such fats as 

 butter, suet, lard, olive oil, peanut oil, and others. The carbohydrates in- 

 clude 'all the sugars and starches. When fats and carbohydrates oxidize, 

 water and carbon dioxide result. 



Proteins, fats and carbohydrates together are called "organic nutrients'' 

 because they occur in nature only in the bodies of living things, or or- 

 ganisms. Animals obtain their organic food from other animals or from 

 plants. Green plants are able, as we shall see, to build up carbohydrates 

 from water and carbon dioxide. They are able also to build up proteins out 

 of these carbohydrates when they have supplies of nitrogen, sulfur and 

 phosphorus salts. Both plants and animals are able to build fats out of 

 carbohydrates— fats and carbohydrates consisting of carbon, hydrogen and 

 oxygen in various proportions. 



Inorganic needs Plants accordingly must receive supplies of various 

 mineral substances, for these furnish elements used in building proteins. 

 We have not been considering these materials as "foods" chiefly because 

 most people, most of the time, are unaware of taking them into the body. 

 We get practically all we need in our fruits and meat and vegetables and 

 milk. The one great exception is common salt, which has to be added to 

 much of our food. But even so, people do not think of salt as "food", 

 perhaps because it is seldom that one eats salt by itself. 



At any rate, these minerals are quite as essential to maintaining proto- 

 plasm as are protein and the other "organic" substances. Salts and water 

 do not yield energy, but they make possible that complex of chemical 

 changes in protoplasm which we call metabolism. Some compounds ap- 

 parendy act indirectly, influencing special chemical processes, just as the 

 bromides used by the photographer slow the development of the negative. 



Animals and plants naturally absorb the various elements from their 

 surroundings, according to the composition of the sea water or of the par- 

 ticular soil. Calcium is more abundant in some regions, iodine is almost 

 entirely lacking in others, and so on. Such variations must influence what 

 the organisms take in, and may influence the way in which the protoplasm 

 actually grows and acts. 



The Soil and the Life It Sustains Studies made in Florida show that 

 variations in the character of the soil are reflected in the plants growing on 

 it, and that these in turn influence the cattle that feed upon them and the 

 human beings who depend upon the plants and animals. Plants grown 

 in some soils contain two to three times as much iron as plants of the same 

 species grown in a different soil. Cattle that range where the salt licks are 

 inadequate show defective bone formation and other nutritional defects. 

 The children in such areas also have defective bone formation and have low 



98 



