Permeability coefficient Nitella

Figure 3-20. Correlation between the reflection coefficients for a seines of nonelectrolytes (determined using rabbit gallbladder epithelium) and the permeability coefficients for the same compounds (measured by Runar Collander using Nitella mucromta). Most of the measurements are in the area indicated. [The curve is adapted from Diamond and Wright (1969) used by permission.]...

Fig. 1. Permeability of Nitella cell membranes to non-electrolytes in relation to the olive oil solubility of these solutes. On the ordinate the logarithm of the permeability (in units of 10 cm/sec) on the abscissa, the logarithm of the olive oil/water partition coefficient of the permeant. Data measured at 20°C. Crosses molecular weight up to 50 open circles molecular weights from 60 to 119 filled circles molecular weights above 120. Data taken from Collander [3]. The straight line is of unit slope.

Fig. 3. Calculated relative intramembrane diffusion coefficients across the Nitella cell membrane as a function of molecular weight of the permeant. Ordinate logarithm of the ratio of the permeability coefficient (in 10 cm/s) to the olive oil/water partition coefficient for the permeants of Fig. 1. Abscissa logarithm of their molecular weights. The solid straight line is the Unear regression of log(P/K) on log M with slope of — 1.22. The dashed lines are at a distance of one standard deviation away from the regression line.

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. 

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