THE FATS 97 



are insoluble in water and cold alcohol, but are dissolved by acetone, hot 

 alcohol, benzol, chloroform, and ether. When shaken with water, protein 

 solutions, soap, or gum arabic, the fats assume a finely divided condition 

 known as an emulsion. The suspension in water is only temporary, while 

 the emulsions are permanent. 



The fats are hydrolyzed or saponified by superheated steam into glycerol 

 and the fatty acids, the reaction being the reverse of that indicated in the 

 equation above. On boiling with caustic alkalies, they are similarly saponi- 

 fied; the fatty acids are then combined with the bases to form salts or soaps. 



Lecithins are tri-glycerides in which the H atom of two instead of three 

 groups of the glycerol is replaced by a fatty acid radical; for the H of the 

 third hydroxyl (OH) group there is substituted an ester-like combination 

 of phosphoric acid with a nitrogen-containing organic base, choline. 



CH 2 OOCC 17 H 



35 



CHOOCC 17 H 35 C 2 H 4 OH 



CH 2 O - O P - O C 2 H 4 N=(CH 3 ) 3 



\ \ 



(CH 3 ) 3 =N OH 



HO 



Lecithin Choline 



On saponification the di-stearyl lecithin molecule above combines with 

 three molecules of water and is split into two molecules of stearic acid, one 

 of glycero-phosphoric acid and one of choline. 



The lecithins are soluble in alcohol, benzene, chloroform, and ether. 

 They are precipitated from chloroform or alcohol-ether solution by 

 acetone. 



The lecithins are found in nearly all animal and vegetable tissues, espe- 

 cially in nervous tissues. They are essential constituents of the cell. 

 Kephalin is of more than passing interest in that its presence hastens 

 blood clotting. It accomplishes this result by removing the restraints 

 of antithrombin on fibrin formation. 



Cholesterol is a complex alcohol with the elementary formula C 27 H 45 OH, 

 and related to the vegetable terpenes, being grouped with the fats solely 

 because of its physical properties. Accordingly, it cannot be saponified. It 

 crystallizes in the form of thin, colorless, transparent plates usually notched 

 in one corner. It exists in the tissues in part in the form of esters with the 

 complex fatty acids. Cholesterol is an essential cell constituent; it is present 

 in relatively large amounts in nervous tissue. It occurs also in wool fat, eggs, 

 milk, and blood plasma. 



Cholesterol and the lecithins are often termed lipoids or fat-like 

 substances. 



