Membrane, biological cell permeability coefficients

One of the earliest theoretical objections to the calculation of the water permeability coefficients of biological membranes was advanced by Dainty [22] in his excellent review on water relations in plant cells. He argues—and correctly so—that all the equations used in calculating permeability coefficients are based on the implicit assumption that the aqueous solutions on both sides of the membrane are so well stirred that the concentrations at the membrane faces are the same as the bulk concentrations. It is well recognized that such a situation is impossible to achieve... 

For most biological membranes the thickness of which is very small, the concentration gradient may be substituted by the concentration difference C —Q. where C. is the concentration on the external and C on the internal side of the membrane. Furthermore, since for the given cell-membrane the diffusion coefficient D, the effective permeation surface A and the thickness of the m brane d are constant, they are usually combined together to form permeability constant k the reciprocal value of which measures the resistance which the cell m brane imposes on the poietration of the substance studied. It is interesting to note in this connection that the permeability constant so d ned has the dimension of a clearance e.g. volume per unit of time. Pick s law may then be written in the form ... 

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