LIPIDES 



93 



the rest of their structure. Some sterols are saturated, some are un- 

 saturated; some contain one hydroxyl group, whereas others contain 

 several. In addition, sterols differ from each other with regard to the 

 complexity of the side chains attached to the nucleus. Finally, sterols 

 may occur free in nature, or combined with a fatty acid or a carbo- 

 hydrate. 



Cholesterol is probably the best-known member of the sterols. It 

 may be obtained in pure form by extracting human gallstones with 

 hot alcohol containing some potassium alcoholate. The cholesterol 

 crystallizes from the alcoholic solution in rhombic crystals, which, in 

 chloroform solution, are levorotatory. When heated z?i vacuo at about 

 300°C., cholesterol may be distilled or sublimed without decompo- 

 sition. It is easily dissolved in fats and in bile. On account of the fact 

 that it is an alcohol and not an ester, cholesterol cannot be saponified; 

 consequently it can be separated from fats following saponification 

 by adding a suitable fat solvent such as ethyl ether. The water-soluble 

 soaps remain in the water solution, while the ether-soluble sterol is 

 found in the supernatant ether layer. 



CH3 



GH3 



H2C 



H.,C 

 H2 GH3 



C 



C H — C H2 — G H2- 



H2 GH3J 



G 



-GH2— GH 



G 



I 

 G- 



G 



GH, 



G 



GH 



' \h/h 



G 



GH2 



I 

 -GH2 



HO— G G 



G G 



H2 



GH2 



H 



cholesterol 



Ergosterol, a sterol which is the precursor of one of the D vitamins 

 (sec page 355), was fust solated from ergot, a fungus which grows on 

 plants, especially rye. This sterol occurs in a number of food materials 

 in amounts so small that its presence is difficult to detect. When 

 exposed to ultraviolet light, ergosterol gives rise to several new com- 

 pounds. One of these, calciferol, has been shown to possess antirachitic 

 properties. In chemical structure ergosterol differs fiom cholesterol 

 as shown by the formula on page 94. 



