Explore ionic driving forces across selective biological membranes. Tune permeabilities, temperature, and concentrations for realism. See potentials instantly, then download clean reports anytime today.
| Parameter | Inside | Outside | Units |
|---|---|---|---|
| K+ | 140 | 5 | mM |
| Na+ | 12 | 145 | mM |
| Cl- | 4 | 120 | mM |
| Ca2+ | 0.0001 | 1.2 | mM |
Nernst potential for ion i:
Goldman-Hodgkin-Katz (monovalent K, Na, Cl):
The membrane potential is the voltage difference between intracellular and extracellular fluid created by selective ion movement across a thin membrane. In many neurons a resting value near -65 mV is typical, while skeletal muscle can sit closer to -80 mV, depending on potassium and chloride handling.
The Nernst equation predicts the equilibrium voltage for a single permeant ion. At 37 °C, RT/F is about 26.7 mV, and the base-10 form gives roughly 61.5 mV per tenfold concentration ratio for z=1. Comparing Vm to each Ei shows the direction of the ionic driving force.
Common intracellular/extracellular concentrations (mM) are: K+ 140/5, Na+ 12/145, Cl- 4/120, and free Ca2+ about 0.0001/1.2. These values are representative for mammalian cells and highlight why calcium equilibrium potentials are usually strongly positive.
Because voltage scales with absolute temperature, potentials shift with experimental conditions. RT/F is about 25.2 mV at 20 °C (293 K) and about 26.7 mV at 37 °C (310 K). For unchanged gradients, moving from room temperature to physiological temperature increases magnitudes by roughly 6%.
The Goldman-Hodgkin-Katz equation combines multiple monovalent ions under a constant-field assumption. A frequently used resting set is PK:PNa:PCl = 1:0.04:0.45. With the example concentrations, this often produces a Vm near -60 to -75 mV, consistent with many measured resting potentials.
Chloride is an anion, so its charge reverses the electrical contribution compared with cations. In the Nernst equation this is handled by using z = -1. In the GHK expression, chloride uses inside concentration in the numerator and outside in the denominator, matching the anion flux sign under the same field.
Membrane voltage can be highly sensitive to extracellular potassium. Doubling K+out from 5 to 10 mM shifts EK by about +18.5 mV at 37 °C because ln(2) multiplied by RT/F gives a sizable change. This is why hyperkalemia can depolarize cells and increase excitability.
These formulas assume ideal dilute solutions and do not explicitly include electrogenic pumps, buffering reactions, or activity coefficients. For higher accuracy, consider ion activities, liquid junction potentials, and explicit channel conductances. Calcium is included here via Nernst; multi-ion GHK extensions for divalents require additional assumptions.
Nernst gives the equilibrium voltage for one ion at a time. GHK estimates the overall membrane voltage when multiple monovalent ions contribute simultaneously, weighted by their relative permeabilities.
A positive Ei means outside is higher than inside for a cation, or the reverse for an anion. It is the voltage needed to balance diffusion so net flux is zero.
Use relative values that reflect the channel state you want to model. Resting membranes often have PK much larger than PNa. PCl varies with transporters and the cell type.
Chloride carries negative charge, so its flux direction under the same electric field is opposite to cations. The inside/outside placement in GHK accounts for this sign while keeping the logarithm structure consistent.
Yes. Enter all concentrations in the same unit system. If you choose micromolar, the calculator converts internally for consistency, so the computed voltage remains correct.
Intracellular free calcium is extremely low relative to outside, often by 10,000-fold or more. With z=2, the Nernst relationship yields a large positive equilibrium voltage, commonly above +100 mV.
Common mistakes include swapping inside and outside values, entering negative concentrations, setting a valence to zero, or setting all permeabilities to zero. Double-check units and ensure all concentrations are strictly positive.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.