PROTOPLASM 19 



one being linked on to the amidogen of the other with the 

 giving off of H2O, thus : — 



H O I OH H I H H 



h\ I II t t I I il 



> N— C— — — N— C— C— OH 



h/ I I 



H H 



Amino-acetic acid. Amino-acetic acid. 



Glycin. Glycin. 



Glycyl-glycin. 



This he calls glycyl-gh'cin, — the amino acids which have 

 lost the OH of the acid being designated by the terminal 



Such compounds he calls peptides, characterising them, 

 according to the number of molecules linked, as di-, tri-, 

 tetra-, and poly-peptides. 



Some of the higher of these give the biuret test for 

 proteins from the presence of the linked CO. NHo group ; and 

 if an acid with the benzene ring is in the chain, they also 

 give the xantho-proteic test. 



He has also succeeded in building the pyrrol derivative, 

 pyrrolidine-carboxylic-acid, or 2^'i"olin, into polypeptides. 



2. Fats and Lipoids. 



In addition to ordinary fats (see p. 41), protoplasm also 

 contains a group of substances which are, like the fats, 

 soluble in alcohol and ether, and which have been called 

 Lipoids. These are generally most abundant in the outer 

 layers of protoplasmic units, where they help to form a 

 covering membrane. 



One of the most important is Cholesterol. This is a mono- 

 hydric unsaturated alcohol, which, when dissolved in hot 

 alcohol, tends to crystallise out on cooling in fiat square 

 plates, generally with a notch out of a corner. 



Some of the lipoids contain phosphorus, and have been 

 grouped as Phosphatides. The most important of these is 

 Lecithin. 



This is a fat in which one of the acid radicles is replaced 



