146 FINE-STRUCTURE OF PROTOPLASM II 



I. Homopolar cohesive bonds, i.e., mutual attraction of lipidic 



groups ; 

 11. Heteropolar cohesive bonds, i.e., attraction between groups of 

 pronounced dipole character; 



III. Heteropolar valency bonds, i.e., formation of salts and esters; 



IV. Homopolar valency bonds or bridge formation. 



We shall briefly discuss the characteristics of these types of bonds. 



I. Homopolar cohesive bonds are of the same kind as the forces which 

 keep a paraffin crystal together. Very little is known about the causes 

 of the attraction between lipophilic groups, for the electric charges 

 in these substances are distributed so regularly that the resulting field 

 of force is negligible, in contrast to dipole molecules. It has therefore 

 been suggested that weak dipole moments are induced in the neigh- 

 bouring molecules by periodic oscillations in the field of force, 

 brought about by vibrations within the electronic configurations 

 (Bartholome, 1956). We know more about the energy of these 

 bonds. As follows from Table IV (p. 32), the cohesion between 

 methyl and methylene groups is the weakest among the cohesive 

 forces. This kind of bond is loosened by small amounts of energy and 

 is therefore strongly sensitive to temperature changes. For this reason, 

 paraffins, fats and waxes melt at relatively low temperatures in spite 

 of their high molecular weight. 



A similar behaviour is shown by the homopolar cohesive bonds 

 between lipidic side groups of neighbouring polypeptide molecules. 

 By a rise in temperature, this kind of junction is easily loosened. 

 Similarly, lipids and phosphatides which are attached to these groups 

 become more mobile. This causes the living matter to liquefy to a 

 certain extent: the rapidly decreasing viscosity of the cytoplasm as a 

 function of the temperature is a well-known phenomenon (Heil- 

 BRUNN, 1930). Fig. 97 shows the rapid decrease in the viscosity of 

 amoeba cytoplasm between 10 and 20 degrees C, which is probably 

 due to the rupture of lipidic bonds in addition to the viscosity decrease 

 of the intermicellar water. At temperatures beyond 20° C. another 

 process sets in, vi2., a shrinkage at those spots where hydrophilic 

 chain ends come together, resulting in some kind of solidification. At 

 the same time, however, the rupture of lipidic junctions continues and 

 at 25° C. clearly surpasses the solidification brought about by de- 

 hydration. By raising the temperature still further, the curve should 



