Membrane potential
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Membrane potential (or transmembrane potential or transmembrane potential difference or transmembrane potential gradient), is the electrical potential difference across a cell's plasma membrane. In membrane biophysics it is sometimes used interchangeably with cell potential, but is applicable to any lipid bilayer or membrane. Hence every organelle and every membranous compartment (such as a synthetic vesicle) has a transmembrane potential (although the size of this potential may be zero).
Described in physical terms, it is the voltage "drop" or the difference in voltage between one face of a bilayer and its immediate opposite face. The property need not be uniform throughout the cell or compartment, but under some conditions may vary between one patch of membrane and another. A localized change in potential occurs at the synapse of nerve cell, for example, with the opening of ion channels by neurotransmitter. Likewise during an action potential, the magnitude of the membrane potential will vary in time and space along a nerve fiber. It is the membrane potential or membrane electric field that controls the membrane flow of charged solutes and the activity of voltage-gated ion channels.
A graded potential is a gradient of transmembrane potential difference.
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The Ionic Basis of the Membrane Potential
Cells are surrounded by a plasma membrane, which defines their extent and acts as a barrier between the cells and their external environment, for example interstitial fluid or blood plasma. The membrane, as a result of its lipid bilayer structure and specific membrane proteins, is selectively permeable (the hydrophobic interior prevents the passage of both large polar molecules and ions) and therefore will only allow certain species through. This selective permeability allows asymmetric concentrations of ions to exist between the intra- and extracellular fluids. These differences can be chemical or electrical (i.e. the difference in charge between the inside and outside). Most cells maintain a “membrane potential” of around –80mV relative to the surrounding fluid. The membrane potential is negative because usually cells have a net negative charge due to leakiness of potassium channels and the large size of negatively charged macromolecules such as proteins and RNA.
In animal cells, passive ion movement accounts for the majority of the electrical potential across the plasma membrane. This passive ion movement mostly consists of K+ ions. A sodium/potassium pump helps maintain an osmotic balance by keeping the concentration of intracellular Na+ low. Because the concentration of Na+ is so low inside the cell, other cations must be present to balance the negative charge carried by the cell's fixed protein anions. This balancing act is largely performed by K+ which is pumped in through the Na+/K+ pump and is also free to leave or enter the cell through the K+ leak channels. There is an electrostatic attraction for K+ due to the protein anions. This attraction balances against the tendency of K+ to diffuse out of the cell, down its concentration gradient, and it is these combined actions that create the membrane potential.
This can also be explained in the following way. Suppose that a cell initially has a membrane potential of zero – i.e. has no voltage gradient across the plasma membrane. However, the concentration of K+ inside the cell is higher than outside and so K+ will tend to leave the cell, driven by the concentration gradient. As it leaves the cell, the K+ leaves an unbalanced electrical charge. This creates a negative electrical charge, which is the membrane potential. The electrical field also opposes any further K+ leaving the cell. The membrane potential also tends to keep anions like Cl- out of the cell because their charge is also negative.
The cytosol, or interior, of a cell possesses a uniform electric potential or voltage compared to the extracellular solution. This voltage is the resting cell potential, also sometimes called the transmembrane potential of the resting cell. As an example, retinal ganglion cells have a resting cell potential of about -60 mV. Cells whose voltage is more negative than typical are said to be hyperpolarized, and those more positive are said to be depolarized. Healthy cells do not naturally hyperpolarize or depolarize except for brief intervals, for example during a nerve impulse or action potential. Among other roles, the cell potential acts as a reservoir for metabolic energy, which cells use to drive the transport of solute molecules across the membrane, to communicate with other cells and to trigger intracellular events.
Between the inside and outside of the cell (which is typically uniform electrically like the cytosol) the voltage rises very steeply just at the boundary created by the membrane. This create an electric field across the membrane, which exerts a force on ions and controls voltage-gated ion channels. Integral membrane proteins such as channels, pumps, and exchangers establish the membrane potential by transporting specific ions in or out. In essence, resting cells are negative because positively charged potassium ions, which are more concentrated inside than outside, are allowed to leak out. The resulting negative voltage difference between inside and out is therefore approximately equal to the reversal potential for potassium. Sodium-potassium exchangers maintain intracellular potassium at a high concentration while pumping sodium into the extracellular solution, where the concentration of sodium typically is high.
The Goldman equation can be used to calculate the membrane potential given the concentration of ions on either side of the membrane and their permeability.
Effects and Implications
While cells expend energy to transport ions and establish a transmembrane potential, they use this potential in turn to transport other ions and metabolites such as sugar. The transmembrane potential of the mitochondria drives the prodution of ATP, which is the common currency of biological energy.
Cells may draw on the energy they store in the resting potential to drive action potentials or other forms of excitation. These changes in the membrane potential enable communication with other cells (as with action potentials) or initiate changes inside the cell, which happens in an egg when it is fertilized by a sperm.
In neuronal cells, an action potential begins with a rush of sodium ions into the cell through sodium channels, resulting in depolarization, while recovery involves an outward rush of potassium through potassium channels. Both these fluxes occur by passive diffusion and tend to neutralize the concentration differences painstakingly established by the sodium-potassium exchanger and other pumps. As a result, although a cell can undergo rapidly recurrent excitation, it cannot do so indefinitely, because time and metabolic energy are required to restore the distribution of ions.
See also
References
- Alberts et al. Molecular Biology of the Cell. Garland Publishing; 4th Bk&Cdr edition (March, 2002). ISBN 0815332181. Undergraduate level.
- Guyton, Arthur C., John E. Hall. Textbook of medical physiology. W.B. Saunders Company; 10th edition (August 15, 2000). ISBN 072168677X. Undergraduate level.
- Ove-Sten Knudsen. Biological Membranes: Theory of Transport, Potentials and Electric Impulses. Cambridge University Press (September 26, 2002). ISBN 0521810183. Graduate level.de:Ruhemembranpotenzial