The Ionic Basis of the Membrane Potential
Cells are surrounded by a plasma membrane, which defines their extent and acts as a barrier between them and the external environment (the interstitial fluid and 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 allows asymmetry in concentration to exist between the intra and extra cellular fluids. These differences can be chemical or electrical (i.e. the difference in charge between the inside and outside). Most cells maintain a so-called “membrane potential” of around
–80mV relative to the surrounding fluid.
In animal cells, passive ion movement accounts for the majority of the electrical potential across the plasma membrane. It is also mainly reliant on K+ ions. An Na+/K+ 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 cells 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. K+ is attracted into the cell its own protein anions. This attraction works against the tendency of K+ to leak out of the cell and it is these combined actions that manifest themselves as the membrane potential.
This can be further explained in the following way. Say that a cell initially had 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.
Rederences: Alberts et al. Molecular Biology of the Cell, Anderson, J.
Ionic Basis of the Membrane Potential
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