The outside of most cell membranes is positively charged compared with the inside of the cell membrane the charge difference called the resting membrane potential develops for two reasons the concentration of K+ inside the cell membrane in higher than that outside the cell membrane and the cell membrane is more permeable to K+ than it is to other ions including negatively charged molecules such as proteins located inside the cell this occurs because some K+ channels are open whereas other ion channels such as those for Na+ are closed