432 Diffusion, Permeability, and Active Transport /23 : 5 



The values for P and D vary in very different fashions from one solute 

 to the next. For large molecules, for example, the diffusion constant 

 D varies roughly as the square root of the reciprocal of the molecular 

 weight. On the other hand, the permeability for urea is 



P 

 k = -j = 7,000 cm/hr 



whereas for glycerol it is 54 cm/hr and for sucrose cm/hr. Moreover, 

 the times for 90 per cent saturation for these three solutes would all be 

 less than 10 ~ 3 sec, if the limitation of the cell membrane could be 

 ignored, but range from 0.5 sec to oo, including the limitations at the 

 membranes. 



Certain general rules can be found for the relative values of P measured 

 for erythrocytes. First, the more soluble the solute is in lipids, the 

 greater is the value for P. For example, glycerol has a much lower 

 value for P than does its larger, lipid soluble ester, monacetin. There- 

 fore part, at least, of the red blood cell membranes appears to be of a 

 lipid nature. (This is also supported by other lines of evidence ; however, 

 it appears unlikely that the lipid forms a simple film or monolayer around 

 the cell.) 



The second general rule for the variations of P is that, given the same 

 lipid solubility, the smaller molecule goes through faster. For instance, 

 the rates for ethylene glycol, diethylene glycol, and triethylene glycol 

 decrease in the order of increasing molecular weights. This supports 

 a molecular-sieve picture of the cell membrane in which bigger mole- 

 cules, even though lipid soluble, have a hard time going through the 

 pores. 



In spite of these general rules, there exist other molecules such as 

 water, urea, and sodium ions, which appear to go through the cell 

 membrane at inordinately high rates. No simple picture of the cell 

 membrane can explain these very high rates. One must think of the 

 cell membrane as in some sense actively transporting certain molecular 

 species. 



5. Active Transport 



The extremely high permeability constants of erythrocyte membranes 

 for certain molecules suggest that in some fashion the cell membrane 

 actively moves these molecules rather than merely permitting them to 

 passively diffuse through the membrane, as described in Section 2 of 

 this chapter. Similar evidence for active transport comes from a variety 

 of other sources. It appears probable that all membranes actively 



