THE CHEMICAL BASIS OP THE CELL. 23 



either precipitated or coagulated when solutions containing them 

 are boiled or have inorganic salts dissolved in them. 



Albumins are characterized chiefly by their great solubility in 

 water. Three forms are of importance: egg albumin, lactal- 

 bumin of milk, and serum albumin. 



Globulins occur principally in the muscle proteins, and are 

 insoluble in water, but soluble in dilute neutral salt solutions. 

 Many consider that the albumins and globulins are only nutri- 

 tive materials from which the protoplasm manufactures the 

 compound proteins which are the essential cellular proteins. 



Nucleoproteins, both in quantity and in relation to their activ- 

 ity, are probably the most important constituents of the cell. 

 They have a very complex structure, and occur in many varieties. 

 They consist of a combination between protein and a substance 

 called nucleic acid, which, on being broken up by chemical 

 means, yields phosphoric acid, a simple sugar called pentose, and 

 nitrogenous substances known as purine bases, and pyrimidines. 

 The purine bases are of great interest, because they are the ante- 

 cedents in the body of uric acid, which, being relatively insoluble, 

 may become deposited from the body fluids and cause gout or 

 gravel. That it is possible to have an enormous variety of nucleo- 

 proteins can be imagined when we consider that there exist differ- 

 ent sorts of purine bases, of carbohydrates, and of amino bodies. 

 The nucleus of the cell contains a nucleoprotein which is particu- 

 larly rich in purin bases and is often called nuclein. 



Phosphoproteins are compounds of phosphoric acid and simple 

 proteins, without any nucleic acid. An example is the casein of 

 milk (see p. 105). 



Glycoproteins are compound of carbohydrates with proteins. 

 The mucin of saliva is an example (see p. 46). 



Insoluble proteins resemble the coagulated proteins, and are 

 left behind after the extraction of the other proteins from the 

 cell. 



LIPOIDS. These include all the substances composing a coll 

 which are soluble in fat solvents. Besides fats and fatty acids, 

 the most important of these substances are lecithin and choles- 

 terol. 



