Membrane potential is the electrical voltage difference across a cell’s plasma membrane. This electrical charge separation is fundamental to all living cells. It is relevant for excitable cells, such as nerve and muscle cells, and central to various cellular processes.
The Foundation of Membrane Potential
Membrane potential arises from specific conditions maintained by the cell. A primary factor involves differing ion concentrations inside and outside the cell. For instance, sodium ions (Na+) and chloride ions (Cl-) are typically more concentrated outside, while potassium ions (K+) are more abundant inside.
These ion concentration gradients are actively maintained by specialized membrane proteins. The sodium-potassium pump expends energy to transport three sodium ions out for every two potassium ions in. This action helps establish and sustain the uneven distribution of charges.
Another factor is the selective permeability of the cell membrane. The membrane contains ion channels, protein structures that allow specific ions to pass through. Some channels are always open, while others open or close in response to signals, controlling ion flow across the membrane.
The combined effect of these concentration differences and selective permeability leads to an electrochemical gradient. This gradient encompasses both a chemical force, driven by the concentration difference, and an electrical force, influenced by the charge difference across the membrane. Ions move across the membrane in response to this combined gradient.
Calculating Equilibrium Potential: The Nernst Equation
Equilibrium potential is the membrane potential where the electrical force on a specific ion precisely counteracts its chemical force, resulting in no net movement. Each ion has a unique equilibrium potential, which depends on its concentration gradient across the membrane.
To calculate this theoretical potential for a single ion, scientists use the Nernst equation. The equation is E = (RT/zF)ln([ion]out/[ion]in), where ‘E’ is the equilibrium potential, ‘R’ the universal gas constant, ‘T’ the absolute temperature, and ‘z’ the ion’s charge.
‘F’ is Faraday’s constant. The terms ‘[ion]out’ and ‘[ion]in’ refer to the ion’s concentration outside and inside the cell. A simplified form at body temperature often uses approximately 61.5 mV for RT/F.
For example, if a membrane were permeable only to potassium, the potential would tend towards potassium’s equilibrium potential, typically around -90 mV. This calculation provides a theoretical value for a single ion, illustrating the electrical potential needed to prevent its net movement despite its concentration gradient.
Calculating Resting Membrane Potential: The GHK Equation
The Nernst equation is useful for understanding the equilibrium potential of a single ion, but it does not fully explain the actual resting membrane potential of a living cell. This is because a cell’s membrane is permeable to multiple types of ions simultaneously, and the degree of permeability varies for each ion. Thus, the real membrane potential is a composite of these individual ion influences.
To account for the contributions of multiple ions and their differing permeabilities, the Goldman-Hodgkin-Katz (GHK) equation is employed. This equation considers the concentration gradients of the primary ions, typically sodium (Na+), potassium (K+), and chloride (Cl-), as well as the membrane’s relative permeability to each of these ions.
The GHK equation provides a more accurate representation of the resting membrane potential, reflecting the dynamic interplay of these factors. While the equation itself is mathematically complex, its core principle is that the resting membrane potential is a weighted average of the equilibrium potentials of the contributing ions, with the weighting determined by each ion’s permeability.
This equation is the standard method for calculating the resting membrane potential in physiology. It demonstrates that the ion with the highest membrane permeability will exert the greatest influence on the overall membrane potential, drawing it closer to that ion’s equilibrium potential. For many cells, potassium has the highest resting permeability, making the resting membrane potential often closer to potassium’s equilibrium potential.
Factors Influencing Membrane Potential
Membrane potential is not static; it can change from its calculated resting state due to various internal and external factors. One significant influence is alterations in the concentrations of key ions across the membrane. For instance, an increase in extracellular potassium concentration can reduce the concentration gradient for potassium, making the membrane potential less negative.
Changes in the membrane’s permeability to specific ions also dramatically affect membrane potential. This often occurs through the opening or closing of ion channels. These channels can be regulated by various stimuli, such as neurotransmitters or changes in voltage across the membrane.
When ion channels open or close, they alter the flow of ions, thereby shifting the membrane potential. For example, an increase in sodium permeability will cause sodium ions to rush into the cell, making the inside of the cell more positive. These changes in ion concentrations and membrane permeability are fundamental to how cells respond to their environment and transmit signals.