142 TEXTBOOK OF BOTANY 



( carbon, hydrogen, and oxygen ) derived from sugar. The molecules of 

 some proteins also contain phosphorus. But 85 per cent or more of the 

 protein molecule is made from sugar. These facts and several others are 

 represented by the empirical formulas of a few common proteins: 



Gliadin of wheat C685H1068O211N196S5 



Zein of corn C736H1161O208N184S3 



Egg albumin C696Hii25022nNi75S8 



Casein of milk C708H1130O224N 18084^4 



Among the facts exemplified by these fomiulas are the large size of the 

 molecules composed of more than 2000 atoms, the absence of phos- 

 phorus in some proteins, the relatively large amount of nitrogen in com- 

 parison to the amount of sulfur and phosphorus, and the fact that the 

 proportion of carbon to oxygen is higher in proteins than in sugar. This 

 last fact is evidence that oxidation-reduction processes occur in protein 

 synthesis. Lastly, one may infer that plants must obtain nitrogen, sulfur, 

 and phosphorus from external sources. 



Sources of nitrogen, sulfur, and phosphorus. With the exception of the 

 nitrogen-fixing bacteria, plants are dependent upon nitrogen compounds, 

 especially nitrate and ammonium salts, as a source of nitrogen. In the 

 life of ail other plants the free nitrogen of the air is merely an inert 

 gas. Most plants also are dependent upon sulfate salts as a source 

 of sulfur, and upon phosphates as a source of phosphorus. 



These salts of nitrogen, sulfur, and phosphorus are in the soil, and 

 they are all soluble in water. Those that are in solution in the water may 

 pass into the roots of land plants, and directly into all the cells of algae 

 and similar submerged plants. A summary of the major transformations 

 of nitrogen and sulfur compounds in the plant kingdom is reserved for 

 the chapters on non-green plants. For the present it is enough to recog- 

 nize that the relative amounts of salts of nitrogen, sulfur, and phosphorus 

 in the soil and in fertilizers may greatly affect the rate of growth of 

 plants, indirectly through their effects on the synthesis of proteins. Some 

 plants grow best when supplied with nitrate salts; others grow best when 

 supplied with ammonium salts. Some bacteria must have a supply of 

 organically bound nitrogen. 



Intermediate steps. The inorganic salts and sugar are not transformed 

 into large protein molecules in one simple process. Intermediate prod- 

 ucts consisting of relatively small molecules are formed first. The best 

 known of these products are called amino acids because they contain the 



