PERMEABILITY OF THE CYTOPLASIVIIC MEMBRANES 121 



Electrolytes are all strongly polar compounds, hence it is not surprising 

 to find that the cytoplasmic membranes of many plant cells are not readily 

 permeable to them. However, as described later in this chapter, and still more 

 fully in Chap, XXIV, electrolytes often enter certain kinds of plant celb 

 readily. The actual mechanics of their penetration through the cytoplasmic 

 membranes is a more complex process than simple diffusion. Some authorities 

 contend that electrolytes penetrate as ions ; others that they penetrate only as 

 molecules. For the purpose of this discussion the first of these two view- 

 points will be adopted. 



Univalent cations such as Na+ and K+ usually penetrate through the 

 cytoplasmic membranes more readily than bi- and trivalent cations such as 

 Ca++ and A1+ + +. The same general relationship also seems to hold for 

 anions. 



Among non-electrolytes in general the less polar the molecules, the more 

 readily they can penetrate through the cytoplasmic membranes. Hydrocar- 

 bons such as benzene, being the least polar of all compounds, penetrate into 

 cells very rapidly, although in only very small quantities, because of their 

 low solubility in water. 



The cytoplasmic membranes are usually quite permeable to the gases 

 carbon dioxide and oxygen when in the dissolved state. This is in accord with 

 the general principle since the molecules of these gases are essentially non- 

 polar. When carbon dioxide dissolves in water a large proportion (more than 

 50 per cent) of the molecules present react with the water, forming carbonic 

 acid (H0CO3). This is a complicating factor which must be considered in 

 any evaluation of processes involving the diffusion of dissolved carbon dioxide. 



Most organic compounds containing only one polar group such as — OH, 

 ^COOH, or — NHo diffuse across the cytoplasmic membranes less rapidly 

 than those containing no polar groups, but more rapidly than compounds con- 

 taining two polar groups. Further increase in the number of polar groups 

 present in a molecule still further decreases the penetrability of the molecule. 

 Glycerol (C3H5(OH)3), for example, an alcohol containing three polar 

 — OH groups, penetrates relatively slowly, while monohydric alcohols such 

 as methyl (CH3OH) and ethyl (C2H5OH) penetrate through the cyto- 

 plasmic membranes very rapidly. 



Most studies indicate that the cytoplasmic membranes of most kinds of 

 cells are usually relatively impermeable to sugars. Impermeability to sucrose 

 is especially marked (Bonte, 1934, and others). Since molecules of sugars 

 contain a large number of polar groups (see the structural formulas for 

 hexoses and sucrose in Chap. XXII), this is in accord with the general con- 

 ception of the relation of the polarity of molecules to the permeability of the 



