I CYTOPLASM 139 



in spite of the hydrophilic OH-group. This, however, is not the case. 

 For, curiously enough, alkyl groups (-CH3, -C2H5) bound to am- 

 monium nitrogen are hydrophilic in behaviour (like methyl bound to 

 oxonium oxygen, which makes pectic acid and methyl cellulose 

 soluble in water, seep. 60). For this reason the ammonium end group 

 tends to escape from the neighbourhood of the lipophilic end groups 

 of the fatty acids. Consequently, the lecithin molecule resembles a 

 tuning fork (Fig. 94), in contrast to fats which can be represented 

 schematically by a three-pronged fork without a handle. The prongs 

 of the fork represent the lipophilic pole, the handle of the fork the 

 opposite hydrophilic pole of the lecithin molecule. 



The phosphatides react with the protein chains of the cytoplasm 

 by combining with either the lipophiHc or the hydrophilic end groups 

 of the side chains, as indicated in Fig. 94. This junction is not of a 



o ° o 

 ^oOH- 



o o 

 000 

 ■OH OH- 

 ■ CH- K 000 



'-"j > 00 



CHy 



-CH3 



Lecithin Fat 



Water Lecithin Polypeptide chain 



Fig. 94. Relation between polypeptide side chains and lecithin; o = water molecule. 



chemical nature, for the phosphatides can be extracted from the 

 cytoplasm with ether. Nevertheless the phosphatide molecules occupy 

 quite definite places, according to the character of the side groups in 

 the polypeptide molecules. Lipids without hydrophylic groups, such 

 as fats, can combine only with the lipophiHc side groups. For this 

 reason their possible combinations with protein chains are limited. As 

 shown in Fig. 94, they can only enter into relation with hydrophilic 

 side chains by interposition of phosphatides or other intermediates. 

 The sterines possess a polar structure similar to that of the phos- 

 phatides, but lecithin is more reactive: of its two hydroxyl groups at 

 the hydrophiUc tail one is acid (attached to phosphorus) and the other 

 basic (attached to nitrogen). For this reason it can form salts with 

 basic as well as with acid groups of the polypeptide chain. Phos- 

 phatides can therefore react with nearly all end groups occurring in 

 the side chains of proteins. Sterines, on the contrary, are only capable 

 of forming esters. Finally, for fats, all side chains of the polypeptides, 

 with the exception of the lipophilic end groups, are blocked. This 



