Osmosis and Other Mechanisms - 117 



The factors that determine the capacity of 

 a given substance to penetrate the cell are not 

 completely understood, although the size of 

 the molecules is certainly important. In gen- 

 eral, larger molecules experience more diffi- 

 culty in traversing the membrane, and col- 

 loidal molecules generally display a negligible 

 capacity to penetrate. Such observations have 

 led biologists to postulate the existence of 

 pores in the structure of the plasma mem- 

 brane (Fig. 6-5), the dimensions of which 

 would prevent the passage of such larger 

 molecules into or out of the cell. 



Molecule size, however, is not the only fac- 

 tor; otherwise the inorganic electrolytes 

 (salts, etc.) would enter the cell more rapidly 

 than such substances as glucose and amino 

 acids. In the case of electrolytes, the interfer- 

 ing factor appears to be the electric charge 

 carried by the ions of these substances. The 



living membrane itself maintains an electric 

 charge (usually negative in sign, p. 191), and 

 this charge tends to repel ions of the same 

 charge. And since one type of ion in a solu- 

 tion cannot abandon the oppositely charged 

 ions, without setting up a counter electric 

 force, blocking one type of ion is almost 

 equivalent to blocking both. 



The great freedom afforded to the passage 

 of fat solvents, such as alcohol and ether, 

 represents another problem in cell permea- 

 bility. Fat solvents pass through the mem- 

 brane even more rapidly than water, and 

 this fact has led to a belief that fat solvents 

 enter the cell by a special route: via lipoid 

 phases in the membrane structure (Fig. 6-5). 

 Under normal circumstances, however, very 

 small quantities, if any, of fat solvents are 

 present in the protoplasmic system or its sur- 

 rounding media. 



PORE 



PLASMA 

 MEMBRANE 

 . 100 

 ANGSTROMS 



EXTRANEOUS COAT 

 (MUCOPROTEIN) 



OUTER PROTEIN 



FILM 



(PEPTIDE 



CHAINS) 



LIPID LAYER 

 (DOUBLE) 



— .Ji^KA'v- r/i^vi --,;»"" 



INNER 

 PROTEIN FILM 



•CYTOPLASM 



Fig. 6-5. Model of the molecular structure of a cell membrane, based on concepts 

 of J. F. Danielli, Kings College, London. In the plasma membrane proper, note the 

 peptide chains of the protein films, which consist of alternating C atoms (□) and 

 N atoms (0'< oriented tangentially; whereas the lipid molecules are arranged radi- 

 ally in a double layer, with the polar ends, or "heads" (A), directed toward the 

 membrane surfaces, and the nonpolar hydrocarbon "tails" directed away from the 

 membrane surfaces. The total thickness of such a molecular arrangement would be 

 80 to 100 Angstroms, which closely agrees with electronmicrographs of the cell 

 membrane (Fig. 6-1). 



