Now notice we have a semipermandal membrane. So we have hydrophilic heads and hydrophobic tails. These guys are transport membranes or transport proteins. And these proteins allow ions to flow in or as a cell, the active transport or passive transport. Now, so today we're only going to look at this guy here, potassium ion. But notice that our resting electrical potential, the cell or our cell voltage, is generated by simply adding up the cell voltages of all these four ions.