Membrane potential (also transmembrane potential or membrane voltage) is the difference in electrical potential between the interior and the exterior of a biological cell. All animal cells are surrounded by a plasma membrane composed of a lipid bilayer with a variety of types of proteins embedded in it. The membrane potential arises primarily from the interaction between the membrane and the actions of two types of transmembrane proteins embedded in the plasma membrane. The membrane serves as both an insulator and a diffusion barrier to the movement of ions. Ion transporter/pump proteins actively push ions across the membrane to establish concentration gradients across the membrane, and ion channels allow ions to move across the membrane down those concentration gradients, a process known as facilitated diffusion. In the most fundamental example of this, the ion transporter Na⁺/K⁺-ATPase pumps sodium ions from the inside to the outside, and potassium ions from the outside to the inside of the cell. This establishes two concentration gradients: a gradient for sodium where its concentration is much higher outside than inside the cell, and a gradient for potassium where its concentration is much higher inside the cell than outside. Transmembrane potassium-selective leak channels allow potassium ions to diffuse across the membrane, down the concentration gradient that was established by the ATPase, creating a charge separation, and thus a voltage, across the membrane. In almost all cases, the ion that determines the so-called "resting" membrane potential of a cell, is potassium, although other ions do contribute in more minor ways. By convention, the sign of the membrane potential is designated as the voltage inside relative to ground outside the cell. In the case of potassium, its diffusion down its concentration gradient, toward the outside of the cell, creates transmembrane voltage that is negative relative to the outside of the cell, and typically –60 to –80 millivolts (mV) in amplitude.