MODEL OF A CELL MEMBRANE 207 



solution and more or less swollen by water. Overton suggested that 

 lipoids have a tendency to concentrate at interfaces, where the sub- 

 stances could participate in producing the membrane structure. He 

 then added that only those substances which are soluble in this lipoid 

 structure were able to penetrate the interior. Overton's experiments 

 support the view of the existence of such a structural framework. Later 

 work by Loeb and others indicated that lipoid-soluble fatty acids readily 

 penetrated the cell wall; on the other hand, acids which are practically 

 lipoid-insoluble did not penetrate. Except for its static structural point 

 of view a lipoid-solubility theory is as acceptable as any proposed to date. 



In general, it has been found that living cells are readily permeable to 

 lipoid-soluble non-electrolytes which possess an appreciable degree of 

 water solubility. The work of Collander and Barlund [1933] shows, 

 however, that the penetration is limited to molecular sizes which are 

 relatively small. It was found, for instance, that the relatively smaller 

 molecules like formamide, acetimide, and ethylene glycol penetrated 

 much more readily than much larger colloid molecules like albumin and 

 hemoglobin. Apparently the average lattice spaces are therefore about 

 5 X 10 — 8 cm in diameter. The existence of lattice spaces explains why 

 those gases that are readily soluble in plvysiological fluids can success- 

 fully pass through living membranes. 



It has been found that both red cells and paramecia are affected by 

 agents which penetrate or are adsorbed as lipoid and protein mono- 

 molecular layers; therefore, it can be concluded that their surface struc- 

 tures must contain lipoproteins or consist of a lipoid protein mosaic. 



A cell wall cannot be smooth but must have distributed over its sur- 

 face a multitude of irregular hills and valleys of atomic dimensions, an 

 excellent surface for the adsorption of atoms, ions, and ion clusters. 

 This structure can develop a difference in ionic concentration on the 

 opposite sides of its wall. The result is a polarized ionic barrier acting 

 as an effective control for ionic migrations. 



It has been found, however, that cells are more permeable, on the 

 average, to electrically neutral molecules than to ions. It is necessary 

 therefore to have a structure that is electrically polarizable. It must, 

 however, be able to change its degree and form of polarity in accordance 

 with the kind of electrolyte in which the cell may be immersed. 



The wall of the living cell has been found to be impermeable to polar 

 compounds but permeable to a weak polar group such as the hydroxyl. 

 The rate of entrance was found to be proportional to the ratio of the 

 number of non-polar groups to the polar groups. A static structure, 

 namely, a group of oriented molecules as found in the condensed stage 

 of a film, cannot change its polarity very readily. The previously pro- 



