CHAPTER XXIII 

 FAT METABOLISM 



Fats and fat-like substances are present in every living cell. They are 

 produced as a result of certain chains of reactions which can occur in most 

 and probably all actively metabolizing plant cells. Fats and their chemical 

 relatives serve in a number of indispensable roles in plants as the succeeding 

 discussion will show. 



Esters. — Fats and most other lipids (see below) are compounds of the 

 type which are termed esters. The formation of a typical ester is represented 

 by the following equation : 



C2H5OH + CH3COOH ^ CH3COOC2H6 + H2O 



Ethyl Acetic Ethyl 



alcohol acid acetate 



As indicated in this equation the reaction between ethyl alcohol and acetic 

 acid results in the production of the ester, ethyl acetate, and water. All 

 esters may be regarded as resulting from similar reactions between an alcohol 

 and an acid. Fats and most other lipids are esters of fatty acids of relatively 

 high molecular weight with complex alcohols. 



The Lipids. — The fats and certain other compounds, more or less closely 

 related chemically, are often called the lipids. In general lipids may be 

 considered to be compounds which are insoluble in water, but soluble in fat 

 solvents (ether, chloroform, benzene, etc.) and which structurally are either 

 esters of fatty acids, or hydrolytic products of such esters. The following 

 classification (Bloor, 1925) is a useful one: 



I. Simple Lipids — esters of fatty acids with various alcohols. 



1. Fats — esters of fatty acids with glycerol, i.e. glycerides. 



2. Waxes — esters of fatty acids with alcohols other than glycerol. 

 II. Compound Lipids — esters of fatty acids containing groups in addi- 

 tion to alcohol and fatty acid radicals. 



1. Phospholipids (often called phosphatides) — substituted fats con- 

 taining phosphoric acid and nitrogen. Lecithin is the best known. 



2. Glycolipids — compounds of fatty acids with a carbohydrate, also 

 containing nitrogen. Compounds of this group are not definitely 

 known to occur in plants. 



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