[Chap. XVI SYNTHESIS OF FATS AND PROTEINS 145 



This fact seems all the more remarkable when certain other aspects 

 of proteins are considered. First, since protoplasm is characterized 

 largely by proteins, there is a possibility of many different kinds of 

 protoplasms, even though the plants and animals do fall far short of 

 making all the different kinds of proteins that are mathematically pos- 

 sible. Second, the hereditary units of protoplasm, such as chromosomes 

 and genes, that determine different species and varieties of plants and 

 animals are composed mainly of proteins. The number of different kinds 

 of these units, therefore, may easily be great enough to account for many 

 different kinds of plants and animals, aside from the fact that they may 

 occur in countless different combinations. 



Finally, these hereditary units in one animal are constructed from 

 proteins obtained either from plants or from another animal. But this 

 picture is not complete until we fully realize that in all the individuals of 

 a certain species of plant or animal the construction of the same kinds of 

 proteins and hereditary units proceeds generation after generation. To 

 this part of the story we shall return in the chapters on heredity. 



Accumulation of proteins. The accumulation of proteins in plants, like 

 that of fats, is dependent upon the relative amount of sugar made. 

 Among other internal conditions upon which it is dependent are the 

 acidity and water content of the cells, and the presence of enzymes. 

 The more important environmental factors are light, temperature, mois- 

 ture, and certain salts. Protein accumulation is greatest in certain seeds, 

 though it may occur to some extent in all plant organs. The amount of 

 su2;ar made in one of the corn plants described in Chapter XVIII, and 

 the amount of "fats" and proteins that accumulated in it are as follows: 



Amount of sugar made in the plant .32.0 ounces 



" '' accumulated "fat" 0.7 



" " " protein 2.2 " 



The most expensive portion of the diet of human beings is the protein. 

 It may be seen from the accompanying diagram ( Fig. 61 ) , that soybeans 

 are the richest source of protein and why this bean is one of the most 

 important of foods in the Asiatic nations, where meat in the diet is 

 very limited. It is one of the least expensive sources of proteins. Even in 

 the United States, where meat is consumed in comparatively large 

 quantities, the principal source of protein in the diet is wheat. 



Summary. Proteins are the most complex foods. They are made from 

 sugar and certain salts of nitrogen and sulfur, and in some cases of 

 phosphorus also. Some of the intermediate steps in the synthesis of pro- 



