396 FAT METABOLISM 



Waxes. — These compounds are usually fatty acid esters of saturated 

 monohydroxy (rarely dihydroxy) alcohols such as cetyl alcohol 

 (C10H33OH), ceryl alcohol (C26H53OH) and myricyl alcohol 

 (C3iH(;30H). Some waxes, however, are fatty acid esters of the sterols 

 (see below). Waxes are of widespread occurrence in both plants and 

 animals. Examples are beeswax, poppy wax, and the wax of the bayberry 

 from which candles are made. 



Sterols. — These are complex, cyclic {i.e. containing ring groupings) al- 

 cohols of high molecular weight. Cholesterol (C27H45OH) is the best 

 known of these compounds and is apparently present in all animal cells, being 

 especially abundant in the brain and nervous tissue. Cholesterol is not 

 known to occur in the higher plants, but a number of similar compounds, 

 known as the phytosterols, have been isolated from plant tissues. One of 

 the most interesting of the sterols is ergosterol (C28H43OH). This was 

 first discovered in ergot, but is now known to be widely distributed in plants 

 and animals. It is especially abundant in yeast which serves as its commer- 

 cial source. Especial interest attaches to this compound, since it is a pre- 

 cursor of the anti-rachitic vitamin D. Upon irradiation ergosterol is con- 

 verted through a series of intermediate compounds into this vitamin. 



Cutin and Suberin. — The chemistry of both of these substances is very 

 imperfectly known, although it has long been recognized that their chemical 

 affinities are with the lipids. 



Cutin apparently is a mixture composed principally of free fatty acids 

 (often in oxidized form) and condensation products of the fatty acids such 

 as waxes and soaps. The fatty acids present appear to be preponderantly 

 hydroxy-fatty acids, i.e. those which contain one or more hydroxyl groups in 

 the molecule. 



Suberin appears to be a mixture of substances consisting principally of con- 

 densation products and other modifications of phellonic {Q\i^- (CH2)i9' 

 CHOHCOOH), phloionic (Ci8H340e) and other similar acids. The 

 principal distinction between cutin and suberin is that the constituent 

 fatty acids are different in the two materials, and that glycerol is one of 

 the hydrolytic products of suberin, but not of cutin. 



Soaps. — Fats react with inorganic bases as illustrated in the following 

 representative reaction : 

 CisHaiCOOv 



C15H31COO-AC3H5-I- SNaOH ^SCisHsiCOONa +C3H5(OH)3 



/ sodium palmitate glycerol 



C15H31CO0/ 

 palmitin 



