FATS 463 



phosphatides (lecithin and cephalin); and the derived ones, 

 viz., the fatty acids and sterols (cholesterol). 



Dietary fats have risen from their former position as mere fuel 

 for the expenditure of energy to the more significant role of a 

 nutritive necessity. Fat components (unsaturated fatty acids 

 such as linoleic acid) are now classed by Burr and Evans as 

 nutritive needs of the same importance as amino acids (cystine, 

 lysine, etc.). The function, fate, and method of transport of 

 fats, which constitute about a third of our diet, are little 

 understood. 



Waxes are widely distributed in plants and animals, serving 

 mainly as protection. The wax coating on leaves and fruits 

 protects against freezing and a too rapid loss of water. 



Lipoids may be defined as ether-soluble constituents of tissues 

 without regard to their nature, or, as Leathes says, they are 

 substances that have any of the properties of oil so long as they 

 do not have them all. While biologists have included the 

 sterols (e.g., cholesterol) among the lipoids, and chemists have on 

 occasion used the term to include all the fats and fatlike sub- 

 stances (as lipide is used by Bloor), it is best to restrict "lipoid" 

 to the phospholipides or phosphatides, thus excluding the sterols. 

 Among lipoids are such less well-known fatlike substances as the 

 carbohydrate containing glyco- and galactolipins (cerebrosides) 

 and sulpholipins. As all lipoids can be and are best grouped 

 under various other terms, it is well to consider them there. 



Phosphatides include lecithin, cephalin, and sphingomyelin. 

 The last mentioned is an unsaturated amino alcohol with the 

 probable empirical formula C17H35NO2. It occurs in brain, 

 liver, and egg yolk. Lecithin and cephalin have been known 

 longer. They are always found together and occur in all tissues. 

 Both contain two molecules of fatty acid {e.g., stearic, palmitic, 

 or oleic), one of phosphoric acid, one of glycerol, and one of a 

 base (which is choline in lecithin and amino ethyl alcohol in 

 cephalin). 



Lecithin is the most abundant of the phosphatides and is 

 widely distributed in the plant kingdom. It, like other lipoids, 

 is finely dispersed in protoplasm, probably in part as an emulsion. 



Sterols are solid alcohols and are distinguished by the hydroxide 

 group. They occur in all organisms (with the possible exception 

 of bacteria), being rather concentrated in the human brain. 



