46 BriLDING OF THE LIVING MACHINE 



form molecules of very considerable size and complexity. 

 The synthesis of proteins can most naturally be conceived 

 as constituting a series of increasingly elaborate com- 

 pounds, starting from a sugar and a nitrate as the sources 

 of the atoms to be combined. 



Physical Properties of Proteins. — These sub- 

 stances are difficult to define, both because their composi- 

 tion is only incompletely known, and also because they 

 are so diverse. They have, however, certain properties, 

 chemical and physical, in common; they are coagulated 

 by heat and by alcohol; they are similarly soluble in 

 water; they may be precipitated from aqueous solutions 

 by various salts. These compounds, consisting of very 

 large and very complex molecules containing invariably 

 carbon, hydrogen, oxygen, and nitrogen, and many of 

 them sulphur and phosphorus as well, either form con- 

 stituent parts of that living structure called protoplasm, 

 or are present in it as products or by-products. We may 

 perhaps the better conceive how protoplasm is built if 

 we attempt, through a classification of the organic nitro- 

 genous compounds of the series leading to the proteins, 

 to gain an idea of their relative composition and com- 

 plexity. Thus the simplest compounds in this series, 

 the amino-acids, and the nucloo-proteins, the most com- 

 plex, are apparently connected by compounds of inter- 

 mediate complexity and common properties. 



The Amino- Acids. — These substances, characterized 

 by consisting of one or more amino (NHj) groups and of 

 an organic acid radical (carboxyl group, COOH) are 

 much better known as the products of protein decomposi- 

 tion than as parts of an ascending synthetic series of 

 organic nitrogen compounds. Nevertheless, their (Com- 

 position and structure suggest means by which they may 

 come into existence. Thus, it is known that, from the 

 nitrates taken up from the soil, nitrites and ammonia 

 may be derived; from the latter, hydrocyanic acid — 

 common in plant tissues — is obtained. From the ac- 



