98 THE CHEMICAL COMPOSITION OF THE BODY 



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 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. 



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

 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. Choles- 

 terol is an essential cell constitutent; 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. 



CARBOHYDRATES. 



The carbohydrates contain carbon and hydrogen and oxygen in the 

 proportion to form water. Other substances, such as acetic acid, CH 3 - 

 COOH, lactic acid, CH 3 CHOHCOOH, and inosit, (CHOH) 6 , which con- 

 tain hydrogen and oxygen in the proportion to form water, are not carbohy- 

 drates. Certain true carbohydrates also do not fulfill this condition. Chem- 

 ically, the carbohydrates are aldehyde or ketone derivatives of complex 

 alcohols; i.e., they have the structure 



1 <i R-CHOH-CHO or R-CO-CH 2 OH 



Aldose Ketose 



Accordingly, the carbohydrates are termed aldoses or ketoses. They 

 are commonly classified by the number of carbon atoms in the molecule; 

 e.g., pentoses are those containing five carbon atoms, and hexoses have six 

 carbons. Each member of the carbohydrate class, with the exception of 

 the pentoses, may be regarded as containing the saccharide group, C 6 H 10 O 5 . 

 The monosaccharides are then C 6 H 10 O 5 +H 2 O; the di-saccharides contain 

 two saccharide groups with water, (C 6 H 10 O 5 ) 2 +H 2 O, while the poly- 

 saccharides contain this group taken a large number of times, (C 6 H 10 O 5 ) n . 

 In general, the solubility of the saccharides varies inversely with the 

 number of saccharide groups present: the mono-saccharides, as a class, 

 being the most soluble and the poly-saccharides being the least so. On 

 boiling in the autoclave or with mineral acids, and by the action of amylo- 



