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XVII 



PERMEABILITY AND ACTIVE 

 TRANSPORT: THE HAMSTER GUT 



(Readings: Weisz, pp. 277-285. Villee, pp. 44-^6; 299-303; 330-334. See also 

 H. W. Smith, "The Kidney," Sci. Am. 188, No. 1, 40-48, Jan. 1953, Reprint 

 No. 37; and further discussion of the kidney in S.P.T., pp. 156-158 and in 

 Weisz, pp. 459^62.) 



Living organisms, plants and animals alike, 

 are to a degree divided into compartments, 

 separated from one another and from the ex- 

 ternal environment by membranes. The com- 

 partments may be cells, cell organelles, tissues, 

 organs, or indeed entire multicellular organisms; 

 but they have in common the fact that they are 

 divided off from other compartments by mem- 

 branes. 



Each cell has its membrane. Each of such 

 intracellular structures as the nucleus and mito- 

 chondrion has its own membrane. An entire 

 tissue or group of tissues stretched between two 

 spaces or bounding the surface of an organ may 

 also function as a membrane. So, for example, 

 the multi-tissued animals may be thought of as 

 essentially saclike or tubular in construction, 

 with an outer surface facing the external environ- 

 ment, and an inner surface surrounding the di- 

 gestive cavity, both lined by membranes. Food- 

 stuffs, waste products, salts, water, oxygen, and 

 carbon dioxide — the continuous flow of mate- 

 rial into and out of the organism that is a large 

 part of its life — must all be transported through 

 membranes. 



This transfer takes place in various ways. 

 Even the simplest biological membranes are 

 semipermeable. They allow certain substances to 

 pass through the membrane, while blocking the 

 passage of others. In general, for water-soluble 

 substances, this choice depends mainly on 

 molecular size. The membrane acts as though it 

 possessed pores of a certain effective size, which 

 permit small enough molecules to go through 

 and block the passage of larger molecules. 



A second factor, added to semipermeability, is 

 selective permeability. So, for example, cell mem- 

 branes tend to pass fat-soluble molecules, almost 

 regardless of size. So also many cell membranes 

 tend to pass uncharged molecules much more 

 readily than charged molecules; and many exer- 

 cise a further selection by passing, for example, 

 negative ions more readily than positive ions. 



In the types of permeability so far mentioned, 

 the driving force is the difference in concentra- 

 tion of the permeating ion or molecule on both 

 sides of the membrane. Granted that the mem- 

 brane permits a molecule to pass through it, the 

 net diffusion is always from the more con- 

 centrated to the less concentrated side, and the 



86 



