Gross effects of lipid solubility and molecular size

Gross effects of lipid solubility and molecular size 

Clearly, from Fig. 1, the solubility of a solute in an organic solvent correlates very well with the permeability of the Nitella membrane for that solute. But it is also clear that the correlation is only partial. Thus, of two solutes with the same partition coefficient the one with smaller molecular weight would seem to permeate faster. Solute size as well as hpid solubility are both important determinants of permeation rate. The particular solvent chosen, olive oil, seems however to be a very good model for the ability of the membrane barrier to discriminate between the various permeants, since the overall increase in permeability as the structure of the permeant is varied correlates closely with the increase in partition coefficient. Were the two parameters to be strictly linked all the data would fall on the line of unit slope in the figure, the line of identity. Later we shall see cases where the data do not support such a close similarity between certain membranes and model solvents. 

Why should the permeabilities correlate so well with the Upid solubihties Fig. 1 

How would a 40 A thick layer of lipid molecules comprise a barrier to solute diffusion Consider now Fig. 2, which represents a lipid phase separating two aqueous phases and acting as a permeability barrier between those two phases. The net flux of molecules from side 1 to side 2 of this barrier is v and, from the definition of permeability coefficient given above, is given by 

Once solute does not acccumulate within the membrane, i.e. at the steady state, the net flux of solute across the membrane ( , in Eqn. 3), must be equal to the net flux within the membrane given by 

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