Membrane Potential

Abstract

A thin lipid bilayer membrane with embedded proteins surrounds all cells. The potential difference across this membrane depends on the ion concentrations on either side of this membrane and the permeability of the membrane to those ions. The permeability of the membrane is controlled by ion channel proteins that span the plasma membrane. These proteins form water‐filled pores that permit ions to cross the hydrophobic membrane. The ion concentration gradient is formed by ion pumps that use energy from adenosine triphosphate (ATP) hydrolysis to move ions against their concentration gradients. The ionic gradient forms a chemical force, while selective permeability of the membrane via ion channel regulation generates an electrical force. Together these forces govern the movement of ions across the plasma membrane to generate the membrane potential.

Key Concepts

  • All cells generate and maintain a membrane potential.
  • The membrane potential is essential for cell viability, since all cells use the energy of the membrane potential to transport essential molecules (e.g. amino acids) across the plasma membrane.
  • The membrane potential is generated by the combination of chemical and electrical forces.
  • Ion pumps use cellular energy to transport ions across the cell membrane to establish the ionic gradients that form the chemical force.
  • Ion channels form selective ion pathways across the plasma membrane to help form the electrical force.
  • Neurons and other excitable cells regulate ion channel activity to alter the membrane potential and produce electrical signals such as the action potential.

Keywords: resting potential; action potential; ion channels; potassium; sodium

Figure 1. The action potential. The depolarization phase of the action potential results from the movement of sodium ions into the cell (Na+ influx), which is driven by the sodium electrochemical gradient. The repolarization phase of the action potential (return to the resting potential) results in part from potassium movement out of the cells (K+ efflux). The action potential was simulated using the Hodgkin–Huxley model (Hodgkin and Huxley, ). The simulation programme (Axovacs) was written by Dr Stephen W Jones, Case Western Reserve University. Source: Dr Stephen W Jones, Case Western Reserve University.
Figure 2. Ion channels and pumps move ions across the plasma membrane. (a) Ion channels form water‐filled pore that permit the hydrophilic ions to pass through the hydrophobic lipid‐bilayer membrane. The direction of ion flux through the open channel is governed by electrochemical forces. (b) Ion pumps move ions by using energy from ATP hydrolysis. In this cartoon, the pump will simultaneously transport two different ions in opposite directions (analogous to the Na+/K+ ATPase). The pumps move the ions against their concentration gradients. In other words, the ions are moved from an area of low concentration to an area of high concentration. From left to right: Ions bind to sites in the pump (left). This binding triggers the hydrolysis of ATP (middle), which alters the conformation of the pump to release the ions on the opposite side of the membrane (right). Once the ions are released the pump returns to its resting position, ready to bind and transport more ions.
Figure 3. The forces that control the movement of molecules across the cell membrane. (a) Chemical forces control the movement of nonionic molecules across the membrane. At time zero, a cell is placed into a bath containing 1 mmol L−1 urea. The concentration gradient across the membrane is initially large, which creates a large chemical force to move urea into the cell. At equilibrium, the concentration of urea is equal across the membrane and the chemical force has dropped to zero. The two‐headed arrow is meant to indicate the urea still crosses the membrane, but at equilibrium efflux equals influx. (b) In this example, the cell membrane is permeable to K+, but not Cl. At time zero, the cell containing 100 mmol L−1 KCl is placed into a bath containing 10 mmol L−1 KCl. Initially, the concentration gradient generates a chemical force that drives K+ out of the cell. With time (a few milliseconds), a negative potential is created inside the cell by the Cl left behind by K+ efflux. This build‐up of negative charge creates an electrical force that pulls K+ back into the cell. At equilibrium, the negative membrane potential generates an electrical force that exactly balances the chemical force. Thus, the net K+ flux (summation of inward and outward flux) equals zero. Note that unlike urea, the concentration of K+ is virtually unchanged between the initial conditions and equilibrium. This is because only a small number of K+ ions need to cross the membrane to generate a voltage large enough to balance the chemical force.
Figure 4. Relative permeability of ions determines the membrane potential for a given set of ion concentrations. (a) Two examples where either K+ (left) or Na+ (right) is the only permeant ion. In each case, the membrane potential equals the equilibrium potential for the permeant ion. (b) In a real cell, the membrane is permeable to K+, Na+ and Cl. The Goldman equation is required to determine the membrane potential, since it considers both the ion concentrations across the membrane and the permeability of each ion.
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Ganong WF (2019) Review of Medical Physiology, 26th edn. McGraw‐Hill Education.

Levitan IB and Kaczmarek LK (2015) The Neuron: Cell and Molecular Biology, 4th edn. Oxford University Press: New York.

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Elmslie, Keith S(Oct 2020) Membrane Potential. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0029207]