114 ORIGIN AND NATURE OF LIFE 



Fat formation, as stated, is only a con- 

 tinuation of this reduction process with still 

 more storage of energy. 



Now the last group of the three great 

 organic divisions is approached ; this is the 

 central group in the life structure, and 

 absolutely indispensable in all living plants 

 and animals. It is the very citadel structure 

 of the cell's life. The group of the proteins 

 differs from fats and carbohydrates, which only 

 contain carbon, hydrogen, and oxygen, in 

 that it also contains nitrogen, and sometimes 

 also phosphorus, sulphur, iron, and a few 

 other common elements. Around these cen- 

 tral proteins the carbohydrates and fats and 

 all other constituents are united, in the 

 manner to be described in the next chapter, 

 to form the living machine. 



Enormous complexity and differences, such 

 as would require a large volume for their 

 description, arise in the formation of the 

 thousands of proteins known to the bio- 

 chemist. But if detail be abandoned a very 

 simple picture of the chemical building stones 

 from which the protein edifice is constructed, 

 and of the artifice by which these are built up 

 into the proteins, can easily be presented even 

 to the non-chemist. 



In describing the constitution of the fats, 



