FOOD DIGESTION AND RESPIRATION 47 



becomes, by formation of an internal salt : 



H,C COO 



H.N> 



Before they can unite with acids or bases, they must be converted 

 into the hydrolysed form. This can be clone by strong acids and 

 bases only, not by weak ones, nor by neutral salts. 



There are some amino-acids which possess two carboxyls and 

 one NH 9 , while others have two basic groups to one acidic group. 

 The former, of course, are much stronger acids than those in which 

 the two functions are nearly balanced, while the latter are strong 

 bases. Both of these classes are good conductors of electricity, 

 whereas the mono-amino-mono-carboxylic acids are scarcely con- 

 ductors at all, being electrolytically dissociated only to a minute 

 degree. 



Although a sufficient variety of amino-acids, as said, suffice as 

 nitrogen supply to an animal, it is not in this separated form that 

 we take them in our food. In fact, if we wanted them so, we should 

 have to make them from the materials which we actually use, and 

 with great difficulty and expense. These materials are the "proteins? 

 of which there are a great variety, differing in the particular amino- 

 acids they contain and in the number of these combined together. 

 This number is always a large one, although there may be several 

 molecules of one kind of acid. Familiar examples are white of egg 

 and the lean of meat. 



The way in which amino-acids are combined together is by the 

 union of the amino-group of one acid with the carboxyl group of 

 another, water being eliminated in the way that is so common in 

 organic chemistry. The head of one molecule joins on to the tail 

 of another, as it were : 



HNH 



C C (OH H)NH 



II II I 



H., O C C OH 



II II- 

 H 2 O 



which represents the production of what is called a "dipeptide," 

 namely, glycyl-glycine. The union of OC and NH to 

 OC NH is known as the " peptide linkage." By continuing 

 the process, more and more acids can be united, forming " poly- 

 peptides," and ultimately proteins. 



Considering the large dimensions of their molecules, we naturally 

 expect proteins to behave as colloids. They have, in fact, the 



