The Manufacture of Food 43 



phosphorus, in addition to carbon, hydrogen, and oxygen. In 

 protein synthesis the amount of sulfur and phosphorus consumed 

 is small, but a very large amount of nitrogen is required. Further- 

 more, nitrogen in the gaseous condition in which it occurs in the 

 air does not readily unite with other substances ; so, although it 

 makes up four fifths of the atmosphere, green plants cannot take 

 it directly from the air. For the nitrogen needed for protein 

 making, plants must depend, therefore, on the supply which comes 

 from the soil in the form of nitrates. This is carried to the cells 

 with the water that is absorbed by the roots. 



Protein synthesis, like the synthesis of fats, is probably 

 effected by the protoplasm. It may occur in nearly all parts of 

 a plant, but it takes place for the most part in the leaves where 

 the carbohydrates are being made and where their constituent 

 parts are in a condition to unite with the nitrogen, sulfur, and 

 phosphorus compounds. Light may be a factor in the process 

 when it takes place in the leaves, but it has been definitely proved 

 that it may also take place in the absence of light. Proteins, like 

 fats and starch, are mostly inert storage substances, and many of 

 them are insoluble in water. Because of their chemical composi- 

 tion they are especially used in building up protoplasm (Fig. 25). 



Steps in protein synthesis. The various steps in the building 

 of proteins are not fully known. It is probable, however, that 

 the nitrates derived from the soil are transformed into ammonia 

 (NH3) within the plant and that this unites with certain acids, 

 derived from the carbohydrates, forming amino acids. They 

 are called amino acids because the amino group (NH2) forms a 

 part of the molecule. 



The amino acids are comparatively simple substances, but, like 

 the simple sugars, they may be built together to form large and 

 very complex molecules. Just as many glucose molecules may 

 be joined together in the formation of starch, so amino acids may 

 be joined together to form protein. In fact, there is good evi- 

 dence that in some proteins the molecules are formed by the union 



