CHAPTER X 

 PERMEABILITY 



According to the principles of diffusion, the molecules and ions of all sub- 

 stances in solution, and even colloidal micelles, tend to attain an equal dis- 

 tribution in terms of concentration through all parts of a system. The same 

 is true of gases, and the osmotic movement of water, as we have seen earlier, 

 is a response to the same general principle. In living organisms, however, 

 such an equality of distribution of the particles of a solute among the various 

 cells, or between cells and a liquid environment, is seldom attained. There 

 is a marked heterogeneity in the distribution of solutes among the different 

 cells in tissues, in entire organisms, and even among the different parts of the 

 same cell. The diffusion of substances in solution into or out of plant cells 

 is limited and modified in a number of ways. The diversity of the distribu- 

 tion of solutes within the plant body is a result of conditions which modify 

 or check their free diffusion. 



The perjneability of the cell, or some part thereof, is one of the most 

 important factors influencing the rate with which molecules and ions diffuse 

 into or out of plant cells. Plant cells and parts of plant cells such as nuclei 

 and plastids are bounded by membranes which are notable for their differences 

 in permeability to different substances. The permeability of a membrane to a 

 given substance is measured by the rate at which that substance will pass 

 through that membrane under a given "driving force." In the simplest cases, 

 this driving force depends upon the steepness of the concentration or diffusion 

 pressure gradient of the diffusing substance across the membrane. 



The concept of permeability is inseparable from the idea of a membrane. 

 Permeability, it should be clearly understood, is a property of the membrane 

 and not of the substance which diffuses through it. Common examples of 

 non-living membranes, some of which have already been described in previous 

 chapters, are thin sheets or layers of rubber, collodion, parchment paper, cello- 

 phane, gelatin, and copper ferrocyanide. Liquids may also serve as mem- 

 branes under certain conditions, as an example described later in this chapter 



shows. 



Most biologically important membranes are of the type characterized as 



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