EXPERIMENTAL FIGURE 11-19 The electric potential across the plasma membrane of a live cell can be measured. A microelectrode, constructed by filling a glass tube of extremely small diameter with a conducting fluid such as a KCl solution, is inserted into a cell in such a way that the plasma membrane seals itself around the tip of the electrode. A reference electrode is placed in the extracellular medium. A potentiometer connecting the two electrodes registers the potential—in this case, –60 mV, with the cytosolic face negative with respect to the exoplasmic face of the membrane. A potential difference is registered only when the microelectrode is inserted into the cell; no potential is registered if the microelectrode is in the extracellular fluid.