BIOLOGICAL TRANSPORT 



take of this cation, as well as an active extrusion of sodium ion, 

 appear inescapable for these cells. Evidence has accumulated that 

 sodium-ion extrusion and potassium-ion uptake are probably linked 

 into a single process. Sodium-ion efflux in many tissues has been 

 found to be sharply diminished in the absence of potassium ion in 

 the external medium. Consideration is often given to a carrier with 

 an affinity for potassium ion which may be converted to another 

 carrier that binds sodium ion (Shaw, 1954). Figure 14 shows Shaw's 

 model as redrawn by Glynn (1957a). We shall see below that both 

 ions must be present for the enzymatic catalysis of an ATP cleavage 

 believed to provide energy for their transport. 



A potential gradient, rather than producing a chemical gradient, 

 may arise from the active transport of ions across a membrane. 

 Ussing and Zerahn (1951) showed that sodium-ion transport across 

 frog skin continues unchecked when the potential difference across 

 the skin is cancelled out by experimentally opposing an equal poten- 

 tial. Indeed, the current flowing under these conditions could be 

 shown accurately equivalent to the net sodium-ion transport. Hence 

 the potential difference is produced by the sodium transport, rather 

 than sodium transport by the potential difference. 



Relationship between secretion and concentrative uptake 

 by cells 



The membrane-of-cells experiment of Oxender described on 

 page 31 also supports the view that secretion or transcellular trans- 

 port is produced by the ordinary concentrative activity of plasma 

 membranes, which has somehow been made to operate more vig- 

 orously at one portion of the boundary of the secretory cell than 

 at another. Figure 15 presents this concept. If the uphill transport 

 can be caused to be more vigorous from side A than from side B, 

 while the escape remains equal in both directions, uphill transport 

 from side A to side B will result. 



This view is an extension of the suggestion of Koefoed-Johnson 

 and Ussing (1953) that an uphill extrusion of sodium ion occurs 

 from the interior side, whereas only a mediated migration occurs 

 at the cornified exterior side of the germinal cell layer in frog skin 

 to produce the characteristic uphill transport of sodium ion across 

 this tissue. In the case of intestinal absorption, the well-documented 



36 



