• Cell Membrane
• very thin (7-15 nm) lipid-protein complex
• transmembrane ion channels (pores) allow flow of ions across the membrane
• like a leaky capacitor: a thin dielectric material acts as a charge separator
• impermeable to intracellular protein and other organic anions
• selectively permeable to sodium (Na+) potassium (K+) and chlorine (Cl-) ions
• ion concentration difference across membrane creates a diffusion gradient
• ions flow, creating an electric field that opposes flow, until an equilibrium is established
• similar to p-n junction, ions flow by diffusion and create a potential difference which inhibits further flow of charged ions