THE CLASSIFICATION OF THE PROTEINS 



445 



hence these compounds probably play a part in the transmission of hereditary 

 factors from cell to cell and from generation to generation. 



Nucleoproteins are formed by combinations between proteins and nucleic 

 acids. Nucleic acids are complex compounds which upon hydrolysis yield 

 phosphoric acid, a carbohydrate (usually ^-ribose), two purine bases, and 

 two pyrimidine bases. The molecules of both the latter classes of compounds 

 contain nitrogen and are cyclic in structure. 



The various steps in protein synthesis are indicated diagrammatically in 



Fig. 98. 



Fig. 98. Diagram illustrating stages in the process of protein synthesis in plants. 



The Classification of the Proteins.— The enormous size and complexity 

 of protein molecules together with the fact that their molecular structure 

 is only imperfectly understood has made it impossible to classify them upon 

 a strictly chemical basis. It has been necessary to use certain physical 

 properties such as solubility ^ in acids, alkalies or salt solutions and coagulation 

 or precipitability as a basis for their classification. The "American" system 

 of classification recognizes three main groups of natural proteins: 



I. The Simple Proteins.— These are proteins which yield only amino acids 

 upon hydrolysis with acids or enzymes. The simple proteins are classified 

 into sub-groups upon the basis of their solubilities : 



1 The great size of protein molecules makes it probable that most protein 

 "solutions" are actually colloidal sols. The term "dispersibility" might describe 

 their behavior more accurately than solubility. 



