cell membranes (the walled enclosure around a cell) are typically about 7.5 nm thick. they are partially permeable to allow charged material to pass in and out, as needed. equal but opposite charge densities build up on the inside and outside faces of such a membrane, and these charges prevent additional charges from passing through the cell wall. we can model a cell membrane as a parallel-plate capacitor, with the membrane itself containing proteins embedded in an organic material to give the membrane a dielectric constant of about 10.