membrane potential

When an electric field exists across a cell membrane, we describe that electrical imbalance (E) as a membrane potential, measured in millivolts (mV). For cells, we always specify the charge imbalance with respect to the inside of a cell: "negative inside" or "positive inside".

Conceptual example: using electrodes to measure an electrical imbalance between two solutions in a laboratory experiment (see the figure below). A thin membrane (dashed line) separates a potassium-rich solution on the left from a solution on the right side that has a low potassium concentration. A potassium diffusion potential (E_K, see diagram below) will be present across the membrane after some positively charged potassium has diffused through the membrane from the left compartment to the right compartment.