122 PERMEABILITY 



cytoplasmic membranes to them. However, as in the case of electrolytes, there 

 are some kinds of cells which sugars enter relatively rapidly. 



Amino acids (Chap. XXVI), another extremely important group of 

 compounds physiologically, also penetrate through the cytoplasmic membranes 

 of most cells relatively slowly. Since each amino acid molecule contains one 

 or more — NH2 groups and one or more — COOH groups the behavior of 

 these compounds is in accord with the general principle. 



Water molecules, although decidedly polar, are an exception to the general 

 rule, usually penetrating through the cytoplasmic membranes with relative 

 ease. This is probably possible only because of the relatively small size of the 

 water molecule. That cytoplasmic membranes are often, at least, quite per- 

 meable to water is shown by the frequent rapid loss or gain of water by plant 

 cells. 



Another important difference between permeability to polar compounds 

 and to non-polar compounds is that ease of penetration of the former shows 

 a close correlation with particle size, while this is not true of the latter. As 

 among different ions of the same electrostatic charge, for example, the smaller 

 ions (due account being taken of hydration) seem to penetrate more rapidly. 



No such correlation is evident between particle size and permeability 

 among molecules of a predominantly non-polar structure. In fact, exactly the 

 reverse relation sometimes holds. For example, in the saturated series of fatty 

 acids the larger the number of non-polar groups present, i.e. the greater the 

 size of the molecule, at least up to a certain limit, the greater the perme- 

 ability of the cytoplasmic membranes to the acid. According to Loeb (1909) 

 the order of penetrability of the saturated fatty acids into sea urchin eggs is as 

 follows, the most rapidly penetrating acid being given first: nonylic (CH3 • 

 (CHo)7-COOH), caprylic (CH3 • (CH2)6 " COOH), caproic (CH3 • 

 (CHo)4'COOH), butyric (CH3 • (CH2)2 " COOH), proprionic (CH3 

 CHg-COOH), acetic (CH3COOH) and formic (H-COOH). 



Permeability of the cytoplasmic membranes to polar compounds differs 

 from permeability to non-polar compounds in one other respect. Permeability 

 to the former is markedly influenced by external factors and by internal 

 changes in the membrane. The same membrane apparently may vary all the 

 way from a condition of marked permeability to certain polar solutes to a 

 condition of almost complete impermeability to the same solutes. Permeability 

 to non-polar solutes, on the other hand, is not only relatively high, but much 

 more nearly constant, and much less influenced by environmental factors. 



Factors Affecting the Permeability of the Cytoplasmic Membranes. — • 

 A number of factors of external origin may influence the permeability of the 

 cytoplasmic membranes. Only the more important of these will be considered. 



