PERMEABILITY AND THE PROTOPLASMIC MEMBRANE 289 



in the protoplasmic emulsion. It is very unlikely that proto- 

 plasm could exist as a living substance if fat were the continuous 

 phase. (Metabolic reactions take place in an aqueous medium.) 

 As the stability of an emulsion increases with decrease in size of 

 the dispersed particles, owing to a great increase in the surface 

 tension of the stabilizing membrane (see page 128), the ultra- 

 microscopic emulsion (if it exists) would be extremely difficult to 

 reverse. And, finally, the amount of fat in the dispersed globules 

 of an ultramicroscopic emulsion is probably insufficient to 

 enclose the aqueous medium. 



Role of Fats. — The permeability problem is intimately asso- 

 ciated with the role of fats in the life of the cell. It is generally 

 agreed that the outer surface of the plasma membrane is coated 

 with fatty material. This being true, we must grant the possi- 

 bility of its being emulsified, but that permeability control is 

 determined by the behavior of an emulsion is, we have seen, very 

 unlikely. There is another interpretation of the role of fats 

 at the surface of cells which in many respects is more in keeping 

 with the present-day theories of the behavior of oils at inter- 

 faces. This interpretation is in continuation of the idea already 

 expressed, that polar surface-molecules are comparable to logs 

 floating in a more or less upright position, thus forming a sievelike 

 structure. If these imaginary logs are fat molecules, then their 

 position will be determined, in large measure, by their polarity. 

 The polarity of fats, in the technical chemical sense, is not high, 

 but their molecules are linear, and they do form oriented mono- 

 or bimolecular films. The molecule of the fatty palmitic acid 

 is (after N. K. Adams) 24 A. U. in length and 4.7 A. U. in diam- 

 eter (21 sq. A. U. in cross section), five times as long as the mean 

 diameter. Tripalmitin occupies about the same area as three 

 molecules of palmitic acid {i.e., 63 sq. A. U., or about 8 A. U.-). 

 One dimension of the tripalmitin molecule is therefore three 

 times the mean of the other two. Lecithin does not appear 

 to give a compact "condensed" film. The area presumably 

 occupied by the molecule is consequently greater (13 A. U.^). 

 The long dimension is, however, at least twice the mean of the 

 other two in an "expanded" film. That these linear fatty 

 molecules are polar and therefore orient must be granted from the 

 fact that they form monomolecular films on water, because one 

 end of the molecule has an affinity for water, and the other not. 



