Modeling Membrane Permeability in a Coefficient Form PDE Model

Hello,

I am trying to model diffusion of water through a cell modeled by a circle placed in the surrounding environment modeled by a square. I first tried out the model using natural boundary conditions or "continuity" at the circle boundary (membrane boundary). I would like to now model the membrane to behave more naturally, i.e. have a permeability value. I tried using the Flux/Source Option under Coefficient Form PDE Model but does this place an additional source on the boundary? I am not quite sure how to go about this and would greatly appreciate any help.

Thanks in advance,

Caroline