Chapter 3 

 SAPONIFIABLE LIPIDS 



The saponifiable lipids are operationally defined as those materials which are in- 

 soluble in water but soluble in organic solvents such as ether or chloroform and which on 

 heating with alkali form water-soluble soaps. The soaps are salts of long-chain fatty 

 acids, so that these fatty acids are a necessary component of any saponifiable lipid. In 

 this chapter a few compounds have been included which do not have long enough fatty acid 

 molecules to form real soaps since their salts in water form true solutions rather than 

 colloidal micelles. With this one exception the above definition will be strictly followed. 

 The saponifiable lipids are classified according to their structures into a few major cate- 

 gories: 



Fatty acids 



Simple lipids (fatty acid esters) 



Phospholipids or phosphatides 



Glycolipids 



These categories are broken down into subgroups which will be described in the following 

 sections. 



FATTY ACIDS 



All aliphatic carboyxlic acids may be described as "fatty acids," but the term is 

 usually restricted to the longer chain members of the series which are practically insolu- 

 ble in water but soluble in organic solvents. In this chapter the line will be arbitrarily 

 drawn below the Cg acids and the lower members included in Chapter 3. 



The free acids or their salts are of much less frequent occurrence in the plant king- 

 dom than are their esters which make up the other classes of saponifiable lipids. Never- 

 theless, occasional examples of unesterified acids are found, particularly in waxes. 

 Fatty acids found in nature almost always have an even number of carbon atoms, but this 

 generality is not followed without exception. All of the straight chain, odd-carbon acids 

 from C7 - Ci5 have been found free or as esters in higher plants. The vast majority of 

 natural fatty acids have an unbranched carbon chain and differ from one another in chain 

 length and degree of unsaturation. Oleic acid is the most widespread natural fatty acid, 

 occurring in practically every natural lipid mixture. Palmitic acid is nearly as ubiquitous, 

 and these two are then followed by the somewhat less common linoleic, palmitoleic, my- 

 ristic, and stearic acids. In the structures shown in Table 1 no attempt is made to indi- 

 cate cis-trans isomerism. 



Other fatty acids are peculiar to lipids of plants of particular taxonomic groups 

 rather than being widespread in the plant kingdom. They include acids with acetylenic un- 

 saturation, hydroxyl groups, carbocyclic rings, and branched chains. Sometimes the hy- 

 droxy acids are found as inner esters or lactones. Examples of some of these more un- 

 usual fatty acids are listed in Table 2 with their place of occurrence in the plant kingdom. 



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