The filled symbols show the model values of P_H/2 for the permeability of both headgroups and the open symbols show the theoretical values of the hydrocarbon core permeability P_C versus the measured P for two parameter choices from Fig. 2. The parameter δ in the legend gives an effective thickness of the core region for different bilayers as d_C − δ where d_C is the structurally determined thickness that includes the aliphatic chains but not the carbonyls.