Why is the resting membrane potential (Vm) approximately - 70 mV for most cells? Most cells contain a large concentration of Cl- making them negatively charged. The large concentration of proteins in the interstitial space draws Na+ out of the cell making it negatively charged. Most membranes are 10 times more permeable to Na+ than to K+. Most membranes are 40 times more permeable to K+ than to Na+.