266 FINE-STRUCTURE OF PROTOPLASM II 



lipids is at p^ 2.7. They are thought to play a decisive part in the 

 permeability phenomena of the red blood corpuscles. 



Cholesterol. Approximately one molecule of cholesterol is found 

 for every four phosphatide molecules in the stroma (exact ratio 3.5:1, 

 Winkler and Bungenberg de Jong, 1941). As may be seen in 

 Fig. 92 (p. 138), cholesterol, unlike the phosphatides, possesses 

 no ionogenic groups. Bungenberg de Jong therefore assigns to it 

 an important part in the formation and build-up of lipid structures, 

 for,- in a lecithin solution, the individual lipid molecules remain 

 separated from each other as the result of their negative charge. 

 Although the fatty acid chains have a tendency to agglomerate, the 

 repellent effect of the ionized phosphoric acid groups predominates 

 and the molecules are therefore kept at a distance from each other. 

 If cholesterol is added to a solution of this kind, these neutral mole- 

 cules are able to penetrate in between the lecithin molecules and 

 association follows as the result of Van der Waals cohesive forces, 

 as the repelling action of the charges does not span the width of the 

 cholesterol molecule. Cholesterol therefore acts as a sensitizer in the 

 precipitation of lipid solutions with ionogenic groups. Conversely, in 

 lipid films of phosphatides, cholesterol acts as a stabiliser, as it counter- 

 acts solution of the film by ionogenic influences. 



Nucleic acids are only present during the development of the ery- 

 throcytes in the bone marrow. The stem cells contain 5 % cytoplasmic 

 nucleic acid, but during differentiation and maturation of the red cell, 

 its concentration drops to below 0.5% (Thorell, 1948). 



c. Submicroscopic Structure of Erythrocytes 



Stromatin as tricompkx system. Winkler and Bungenberg de Jong 

 (1941) have pubhshed an instructive design of the structure of the 

 erythrocyte sheath (Fig. 133). By exact measurement of the electric 

 migration velocity of the red blood corpuscles in the most various 

 salt solutions, these investigators find quantitatively the same be- 

 haviour as in phosphatides, from which they conclude that the surface 

 of the erythrocytes is covered by a phosphatide film (layer I in Fig. 

 133), which is stabilized by cholesterol. The I.E. P. of the stroma with 

 Ph 5.2 being between that of the phospholipids (2.7) and of the 

 stromatin (5.8), it is assumed that the phosphoHpids form a complex 

 system with the stromatin (layer IV), their positive choline groups 



