PLANT MANUFACTURE OF FOOD 



135 



other carbohydrates, and all the fats and oils. Proteins, on 

 the other hand, are chemical compounds of the glucose elements 

 (C, H and O) with nitrogen and often sulphur in addition. Thus 

 the synthesis of proteins from carbohydrates (fig. 84) involves the 

 intake of additional elements from the environment; these come 

 from the nitrates, sulphates, phosphates and other minerals 

 dissolved in soil water. The first step in the synthesis of proteins 

 is the transformation of carbohydrates into amino acids by the 

 addition of nitrogen; this results from the activity of enzymes 

 similar to pepsin of the human gastric juice. The synthesis of 

 amino acids, which occurs only in plants, is dependent upon the 

 absorption of nitrates from the soil. Any cell of the plant can 



Fig. 84. — Proteins are synthesized from simple carbohydrates. 



carry on this synthesis since it is independent of light and 

 chlorophyll. Amino acids are later combined to form proteins 

 by the addition of sulphur and in some cases phosphorus. The 

 protein molecule is a huge and complex structure as can be seen 

 from the atomic composition of zein, a protein found in corn, 

 which is C736H1161N184O208S3. 



Since proteins contain carbon and hydrogen, they can be 

 oxidized and used as sources of energy. But they are far more 

 valuable to plants and animals as builders of protoplasm, because 

 of their nitrogenous character. Growth and reproduction in- 

 volves formation of new protoplasm, and new protoplasm can 

 be formed only from proteins. For this reason protein foods are 

 particularly abundant in growing cells and in reproductive 

 portions of plants. Plant proteins used as human food are secured 



