46 INTRODUCTION TO GENERAL PHYSIOLOGY 



At this point we may note that the only set of amino-acids of 

 use to protoplasm is that in which the NH 2 is attached in the 

 a-position as regards the carboxyl. Thus amino-butyric acid 

 might be 



CH 3 CH 2 -CH(NH 2 ) COOH or CH 3 CH(NH 2 )CH 2 COOH 



but it is only the first that is of value. The nitrogenous constituents 

 of the cell structures are all of this a-series. 



Practically all organic acids and some of them have very 

 complex structure can form amino-derivatives. In some com- 

 pounds we find that another hydrogen atom has been lost from 

 ammonia, and we have the group NH=, the bivalent imino- 

 group. 



When we try to feed an animal on amino-acids only, as source 

 of nitrogen, we find that it can be done if we take a sufficient variety, 

 but that one kind alone is insufficient. There are some particular 

 ones that are necessary, because the animal cannot make them out 

 of the appropriate fatty acid and ammonia as it is able to do in 

 the cases of others. The complex one known as tryptophane, in 

 which a ring containing nitrogen is united with amino-propionic 

 acid, or alanine, is one of these. If we set about making an electric 

 motor, we discover that certain parts of it must be made of 

 substances with definite properties, different from those of other 

 parts. While the wire must be a conductor, the segments of the 

 commutator must be separated from one another by a material 

 which is an insulator, and any conductor would be useless for this 

 purpose. It is possible also that some special chemical groups may 

 be required for the manufacture of substances of importance as 

 regards their action on protoplasmic processes, not as actual 

 components of the machinery. For the lubrication of the bearings 

 of our motor some oil is wanted, and water would not suffice. 



With regard to the general properties of amino-acids, it is to be 

 noted that, while the carboxyl group confers those of an acid, the 

 NH 2 group is basic. Thus, these acids are what are called 

 "amphoteric," being both acids and bases. But it must be 

 remembered that the acidic and basic properties are potential only. 

 Amino acids in which the two characters are almost balanced, as 

 when there is one acidic and one basic group, are unable to combine 

 with neutral salts, nor even with weak acids or bases. The 

 probable explanation of this behaviour is that such acids exist, 

 even in solution, in a closed ring form. Thus glycine : 



H.,C COOH 



"I 

 H 2 N 



