120 



PERMEABILITY 







G 



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c 



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polar" compounds and groups (Lewis, 191 6). Water, electrolytes, carbohy- 

 drates, and amino acids are typical strongly polar compounds. This "polarity" 

 is fundamentally an electrical phenomenon and is due to the grouping of the 

 atoms in a molecule, as well as to the actual structure of its constituent atoms 

 in terms of the spatial arrangement of protons and electrons. A polar mole- 

 cule can be likened to a tiny magnet since it possesses certain regions which 

 are electro-positive, while other regions are electro-negative (Fig. 26, A). 

 This property of polarity is more strongly marked in some species of mole- 

 cules than in others. Electrolytes possess the most strongly polarized of all 

 molecules; the production of ions by such compounds may be considered a 



manifestation of a marked polarity within the 

 molecules (Fig. 26, B). Polar compounds are, 

 in general, more soluble in polar solvents, of 

 which water is the most familiar example, than 

 in non-polar solvents. 



When the structural framework of molecules 

 and their constituent atoms is such that no 

 markedly contrasting electro-positive and electro- 

 negative regions are present they are classed as 

 non-polar (Fig. 26, C). Such molecules are 

 electrically inert. The most typical non-polar 

 compounds are the hydrocarbons of both the 

 chain and ring types, such as methane (CH4) 

 and benzene (CoHo). Non-polar compounds 

 are much more soluble in non-polar solvents such as benzene, chloroform, and 

 ether than in polar solvents such as water. 



The concept of polarity applies not only to molecules as a whole, but to 

 certain atomic groupings within molecules. Such groups as CH3 — , C2H5 — , 

 etc., are non-polar, while — COOH, —OH, — NHo, — CHO, — CN are 

 the more familiar of the polar groupings. The molecules of many types of 

 compounds, as for example the organic acids, contain both polar and non-polar 

 groups. One end of an acetic acid (CH3COOH) molecule, for example, is 

 decidedly polar, while the other is essentially non-polar (Fig. 26, D). In 

 general, the greater the number of non-polar groups which a molecule contains 

 in proportion to the number of polar groups present the more predominant 

 will be the non-polar properties of the molecule and vice versa. 



One of the few important generalizations which may be made regarding 

 the permeability of the cytoplasmic membranes is that they are usually much 

 more permeable to predominantly non-polar than to predominantly polar 

 molecules. 



Fig. 26. Diagrams illus- 

 trating: {A) polar molecule, 



(B) ionized polar molecule, 



(C) non-polar molecule, {D) 

 molecule which is polar at 

 one end; non-polar at the 

 other. 



