138 



FINE-STRUCTURE OF PROTOPLASM 



II 



Fie. 



92. Molecular structure of cholesterol; 

 terminal group OH. 



contact with neighbouring hydrophilic groups. This applies in partic- 

 ular to the important group of the sterines (Windaus, 1923), from 

 among which the formula of the complicated cholesterol C27H45OH 

 is reproduced (Fig. 92). The molecule contains four rings and a double 



bond. According to X-ray ana- 

 lysis (Bernal, 1932) the length 

 of the molecule is 17-20 A and 

 its cross-section only 7.2 A. The 

 elongated form tallies well with 

 the optical finding that dissolved 

 cholesterol molecules can be 

 easily oriented in a field of flow 

 and like most rod-shaped mole- 

 cules show positive birefring- 

 ence of flow. 



Phosphatides. 

 Because of their 

 solubility in 

 ether, phosphat- 

 ides are usually also counted among the lipids, but 

 besides their lipid character they possess a marked 

 tendency toward hydrophily, which is shown by their 

 adsorption of water and the occurrence of myelin forms. 

 Thus, phosphatides represent compounds which are 

 intermediate between hydrophobic and hydrophilic 

 substances and for this reason belong to the most 

 important intermediates between the representatives 

 of these two extreme groups in the cytoplasm. By 

 way of example we may mention lecithin which, like o^ 

 the fats, consists partly of glycerol and fatty acids. ^- 

 In this case, however, only two OH-groups are occu- "^ 

 pied by fatty acids, the third being esterified by 

 phosphoric acid and the latter in its turn by the amino 

 alcohol choline (Fig. 93). 



Choline HOCH2-CH2-N(CH3)30H is a base whose 

 hydroxy 1 group is attached to a methylated ammonium 

 group. The three methyl groups might conceivably 

 give the end group -N(CH3)30H of the molecule a lipophilic character 



O 



HO-k=0 

 



CH2 CH2 



I I 







1 I 



CHy 



CH2 H^C 



CH2 



H2C 



CH2 H2C 



: j 



\h2 H2( 



CH2 



H2C 



CH2 H2C 



H.C 



CH2 O 



<\ 

 o 



CH2 



CH2 H2C 



HpC 



CH2 



CH2 H2C 



H,C 



CH, 



CH2 H2C 



H.C 



CH, 



\ 



Lecilhin 



Fig- 93- 

 Molecular struc- 

 ture of lecithin. 



