BIOLOGICAL TRANSPORT 



consequence, these cells may be kept for some time in isotonic 

 glycerol, whereas the human cell is promptly lvsed, for reasons 

 that may be understood from Figure 6. An important spectrophoto- 

 metry method for observation of high rates of solute uptake 

 (0rskov, 1935) depends on the swelling of the cells occasioned by 

 solute entry or exit. The capacity of the human cell to admit 

 glycerol is very high, so that one can show more easily that media- 

 tion occurs by adding an analog, 1,3-propanediol, than by adding 

 excess glycerol. This analog has about 100 times as much affinity 

 for the transport site as glycerol does. As an informative point, 

 propanediol itself enters rapidly by other means, so that mediation 

 for its migration is not easily detected. This rapid entry is ascribed 

 to its more lipophilic nature, which presumably permits it to pene- 

 trate the lipid barrier easily. 



The behavior that has so far been shown for glycerol transport 

 could well arise simply from the presence of a fixed site on the red 



KC1, 0.165 M 

 KC1, 0A65M + Glycerol, 0.33M 



KC1, 0.06M 

 + Glycerol, 0.3 3 M 



(a) (b) (c) 



Figure 6 The swelling of a red cell in isotonic glycerol: (a) The 

 cell has been placed in 0.33 M glycerol; (b) hypothetical stage at which 

 glycerol has reached a uniform distribution, but no water has yet entered 

 the cell; (c) at this stage water has entered the cell until the internal salts 

 are diluted nearly three times; at this point the cell is ready to burst, but 

 there is still a difference in osmotic pressure, since more glycerol has 

 penetrated with the water. Therefore, water entry will continue. [From 

 Davson, H., and Danielli, J. F., The Permeability of Natural Mem- 

 branes, 2nd ed., Cambridge University Press, Cambridge, 1952, p. 24; 

 with permission.'] 



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