74 UNITY AND DIVERSITY IN BIOCHEMISTRY 



(j) Complex Lipides 



Most of the complex lipides are diesters of orthophosphoric acid. Those 

 not belonging to this category will be described with those that do. The 

 fact that the complex lipides described here are extremely widely distributed 

 in the biosphere, confers upon them the status of fundamental cellular 

 constituents. 



1 . Glycerophosphatides 



(a) Lecithins or phosphatidylcholines — Lecithins are esters of phos- 

 phorylcholine with glycerol which is esterified in the remaining two posi- 

 tions by fatty acids. Two isomers are possible according to whether the 

 binding is with the a carbon or the ^ carbon of the glycerol. 



CH2— O— CO— Ri CH2— O— CO— R, 



I I 



CH — O-CO— R2 CH — O— P(0)— O— CH2-CH2— N(CH3)a 



I 

 CHa— O— P(0) — O— CH2— CH2— N (CH3)3 



I + 



Q- CH2— O— CO— R2 



I + 



o- 



a-lecithin j8-lecithin 



Living matter only synthesizes a-lecithins which are to be found in all 

 cells. The existence of so many a-lecithins is due to the diversity of the 

 groups Rj and Rg which may be saturated or unsaturated. Choline, the 

 base present in lecithin, is also widely distributed in the biosphere : 



(CH3)3N— CH2— CH2OH 



The lecithins are insoluble in water, in which they swell up, but they are 

 soluble in alcohol and ether although insoluble in acetone. 



(b) Cephalins or phosphatidylethanolamines — Cephalins are diacylgly- 

 cerylphosphorylethanolamines and they differ from the lecithins by the 

 substitution of choline by another base, aminoethanol. 



NH2 • CH2— CHoOH 



In the pure state, cephalins are soluble in methanol, ethanol, ether, 

 petroleum ether, chloroform, glacial acetic acid, and insoluble in acetone, 

 but when mixed with other glycerophosphatides (phosphatidylserine, 

 plasmalogens, etc.) they are insoluble in alcohol. It is likely that the 

 natural cephalins are of the a type, but this has not yet been completely 

 proved. 



{c) Phosphatidylserine — These glycerophosphatides are soluble in chloro- 

 form but are less soluble in ethanol or methanol and this allows their 



