44 



General Botany 



of a hundred or more amino-acid molecules. Just as starch 

 yields many molecules of glucose when it is digested, or broken 

 down, so when proteins are digested they yield many amino-acid 

 molecules. 



Fig. 25. Diagram of protein synthesis. 



The proteins are transported from the leaves in the food-con- 

 ducting tissue of the bundles, usually after they have been broken 

 down into simpler and more soluble substances (amino acids and 

 amides) . 



Importance of nitrogen in soil. Since the proteins make up 

 more than half of the living protoplasm, and since all of them 

 contain a considerable percentage of nitrogen, the need for abun- 

 dant nitrates in the soil is evident. Any kind of moist land would 

 furnish the raw materials for making carbohydrates and fats, 

 but to supply the necessary materials for protein manufacture, 

 the land must contain nitrogen, sulfur, and phosphorus. It is the 

 varying amounts of these three substances in the soil that make 

 the difference in agricultural land values when other conditions 

 are equally favorable. 



Sources of protein in the human diet. The most expensive 

 portion of the diet of human beings is the proteins. Figure 

 26 shows that in soy beans we possess the richest source of 

 protein. It also shows why the soy bean is one of the most 

 important of foods in the Asiatic nations, where animal foods 

 are very limited. One dollar will buy several times as much 

 protein in soy beans as it will in any other plant or animal 



