23 



Diffusion, Permeability, and 

 Active Transport 



I. Introduction 



In the preceding chapter, the theory and terminology of thermodynamics 

 were used to discuss the rate constants of enzymatically catalyzed re- 

 actions. In the current chapter and in the following one, the motions 

 of molecules in diffusing through living systems and in permeating cell 

 walls are presented. The treatment of diffusion and permeability is 

 similar to the discussion of enzyme kinetics in that both are strongly 

 based on the ideas of energy and of thermodynamic equilibrium. 



Diffusion is a very rapid process when it occurs within a single bio- 

 logical cell. However, on a macroscopic scale it may be very slow if 

 unaided by stirring and convection. For example, if one puts several 

 spoonfuls of sugar into a cup of coffee, the sugar will sink to the bottom. 

 Soon, there will be a thin layer of coffee which is saturated with sugar. 

 In the absence of stirring, the sugar molecules will slowly spread, that 

 is, diffuse, throughout the coffee. On the gross scale of the coffee cup, 

 it may take days to approach equilibrium. (Usually we stir the sugar into 

 the coffee rather than waiting for diffusion.) On the microscopic scale 



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