SAPONIFIABLE UPIDS 79 



8 



H9COCR 



^' 



RCOCH 

 



t 



HoCOP— 



2 I 



OH 



phosphatidyl inositol 



The exact way in which these components are joined to form the lipid molecule is unknown. 

 The accepted structure for sphingomyelin from animal tissues is shown below, and plant 

 sphingolipids are presumed to bear some resemblance to it. However, animal sphingo- 

 myelin does not contain sugar units. Animal cerebrosides and gangliosides are nitrogen- 

 ous lipids containing sugar units but no phosphorus. 



OH 



I t ♦ 



CH3(CH2)|2CH=CHCHCHCH20P0CH2CH2N(CH3)3 



NH 0" 



I 



C=0 



(CH2)22 

 CH3 



sphingomyelin (animal) 



The lipositols and sphingolipids resemble the other phospholipids in many of their prop- 

 erties except for somewhat different solubilities. Since the solubilities are important for 

 purification procedures, they will be described below under "Isolation". 



The phospholipids are probably most important in cells because of their involvement 

 in membrane structures and as bridges or binding agents between polar and non-polar 

 cell constituents. This latter function is illustrated by the difficulty found in purifying 

 many plant phospholipids since they are often tightly bound in complexes with carbohydrate 

 and/or protein. The precise structure of such complexes is unknown. Proteolipids are 

 defined as lipid-protein complexes with solubility properties like the lipids, whereas 

 lipoproteins have solubility like the proteins. Bennet-Clark (13) has proposed a mech- 

 anism for salt uptake by cells which involves lecithin as a carrier of metal cations. 

 Phospholipids reach their highest concentration in seeds (up to 2% of dry weight). 



