436 Diffusion, Permeability, and Active Transport /23 : 6 



Red blood cells can be used to measure diffusion through the cell 

 membrane. The permeability of the membrane has been studied for 

 a wide variety of molecules. These emphasize the lipoid nature of the 

 membrane, the pore-size properties of the membrane, and the existence 

 of a few molecules which, in spite of size or solubility, pass through the 

 membrane at comparatively rapid rates. 



The last phenomena may well be the result of active transport which 

 uses metabolic energy to pump certain molecules in a preferred direction 

 through the membrane. Diffusion and permeability theory allows one 

 to predict the relative rates of flux of molecules through a membrane 

 if active transport does not occur. Measurements using tracer tech- 

 niques have shown that many membranes, including nerve axons, 

 muscle fibers, frog skin, gastric mucosa, kidney-tubule epithelium, and 

 red blood cells, do actively transport molecules and ions. Although 

 the molecular mechanisms are not known, it has been demonstrated 

 that active transport depends eventually on metabolic energy. 



REFERENCES 



1. Rashevsky, Nicolas, Mathematical Biophysics (Chicago, Illinois: University 

 of Chicago Press, 1948). 



2. Jacobs, M. H., "The Measurement of Cell Permeability With Particular 

 Reference to the Erythrocyte," Barron, E. S. G., ed., Modern Trends in 

 Physiology and Biochemistry (New York: Academic Press, Inc., 1952) pp. 

 149-172. 



3. Ussing, H., "Active Transport of Inorganic Ions," Brown, R., and J. F. 

 Danielli, eds., Active Transport and Secretion [Symposia No. 8, Society for 

 Experimental Biology] (New York: Academic Press, Inc., 1954) pp. 

 407-422: 



Although there are numerous articles and references on the topics discussed 

 in this chapter, the author feels that the above-mentioned three are especially 

 worth the detailed study of anyone wishing to pursue further the topics 

 discussed here. 



