23 : 6/ Diffusion, Permeability, and Active Transport 



435 



external circuit. This demands energy, which in turn must be related 

 to some metabolic process. Inhibitors which block the active transport 

 of Na + do not uniformly block 2 consumption. In fact, one of the 

 most effective, dinitrophenol (DNP), stimulates 2 respiration while 



-+e 



Solution a 



f 

 k 



Variable 

 emf 



Frog Skin 



e+- 



f 



Solution b 



Lucite Container 



Vacuum 



Tube 

 Voltmeter 



Ammeter 



e= Electrode 



f= Nonpolarizing Electrode 



Figure 6. Diagram of apparatus for determining currents 

 through and potential across frog skin. Air is bubbled into 

 both solutions. The variable emf is adjustable over both 

 positive and negative values. For an open circuit potential, 

 the variable emf is adjusted to cause the ammeter to read zero, 

 whereas for short-circuit current determinations, the variable 

 emf is set to zero. 



decreasing the production of ATP. This latter suggests that ATP may 

 be the ultimate energy source for active transport in frog skin. As 

 noted earlier, the detailed molecular mechanism is not known for this or 

 any other of the numerous cases of active transport. 



6. Summary 



Mathematical theories describing the diffusion of molecules in solutions 

 and their penetration through membranes have been presented in this 

 chapter. These theories can be used to describe the uptake of oxygen 

 by cells of various shapes. This last description emphasizes the import- 

 ant role of the rate-limiting, enzymatically catalyzed steps. Attempts 

 to oversimplify lead to theoretical predictions that disagree with experi- 

 ments. However, the oversimplified theory is easier to carry through 

 and gives one a qualitative feeling for the types of phenomena which 

 occur. 



