ArticlesPhysiologyNervous systemAction potential and synapsesElectrochemical gradient

ArticlesPhysiologyNervous systemAction potential and synapsesElectrochemical gradient

Electrochemical gradient

Author: Konstantinos Kostakos • Reviewer: Alexandra Pliakopanou
Last reviewed: May 21, 2026
Reading time: 14 minutes

The movement of ions across cell membranes is key to many physiological processes, from neuron signaling to muscle contraction. Ion movement is controlled by two forces, i.e., the chemical gradient, representing the difference in ion concentration across the membrane, and the electrical gradient, representing the difference in electrical potential across the membrane. The two forces collectively constitute the electrochemical gradient that drives passive ion transport, but is also central to active transport mechanisms.

Ion dynamics are quantified and predicted by the Nernst and the Goldman equations, elucidating the equilibrium potential and the membrane potential, respectively.

Key facts about the electrochemical gradient
Electrochemical gradient	Key force for passive ion transport and active transport mechanisms resulting from: the chemical gradient i.e., the difference in ion concentration across the membrane, and the electrical gradient, i.e., the difference in electrical potential across the membrane.
Equilibrium potential	The potential at which there is no net ion movement for a single ion. Both the electrical and the chemical potential for the ion are balanced. Calculated by the Nernst equation (fixed value for given ion and temperature).
Membrane potential	The overall voltage difference across the cell membrane (multiple ions). Determined by: the ion distribution across the membrane the selective permeability of the membrane Calculated using the Goldman equation (dynamic in nature).
Electrophysical principles	Movement of ions in cellular systems apply to Ohm's law. Conductance (G) is the inverse of resistance and is related to the “leakiness” of the cell membrane to an ion, as a function of its permeability and the presence of channels. The current is proportional to the conductance of the membrane to the ion and the difference between the Vm and the Eion.

Contents

What is the electrochemical gradient?
Equilibrium potential
Membrane potential
Electrophysical principles
Sources

+ Show all

What is the electrochemical gradient?

An electrochemical gradient is the combined driving force for ion movement and results from two gradients:

the chemical gradient: Na⁺ concentration is higher outside the cell than inside, driving ions from high to low concentration.

the electrical gradient: the cell interior is more negative than the outside, drawing positive ions inward.

In passive transport, ions move down the electrochemical gradient passively. Na⁺, for example, enters driven by both the concentration gradient and the cell's negative resting potential.

Active transport requires energy; the Na⁺/K⁺ pump uses ATP to move 3 Na⁺ out and 2 K⁺ in, restoring the resting membrane potential after ion movement shifts it.

Equilibrium potential

The equilibrium potential (E_ion) refers to a single ion and its electrochemical balance. It is the potential at which there is no net ion movement across the membrane for a single ion. This means that the ion is in electrochemical equilibrium and both the electrical and the chemical potential for the ion are balanced.

The equilibrium potential for a given ion is calculated using the Nernst equation, which is based on thermodynamic principles.

Where:

R: the universal gas constant

T: the absolute temperature (in Kelvin)

z: the valence (charge) of the ion (e.g. +1 for K⁺)

F: the Faraday’s constant

In biological systems at 37 °C (or 310 K) the Nernst equation can be simplified to:

Potassium (K⁺)

For K⁺ ions, the typical concentrations in neurons are [K⁺]_inside= ~140 mM, [K⁺]_outside= ~4 mM, and z=+1. So, the equilibrium potential EK⁺ is:

K⁺ moves out along its concentration gradient, opposed by the negative intracellular potential. Cell membranes are most permeable to K⁺, making it the primary determinant of the resting membrane potential (~−70 mV).

Sodium (Na⁺)

For Na⁺ ions, the typical concentrations in neurons are [Na⁺]_inside= ~10-15 mM, [Na⁺]_outside= ~145 mM, and z=+1. So, the equilibrium potential ENa⁺ is:

Na⁺ enters the cell driven by both its concentration gradient and the negative membrane potential. It contributes less to the resting membrane potential compared to K⁺ due to the lower membrane permeability, but is greatly influential during depolarization.

Chloride (Cl^-)

For Cl^- ions, the typical concentrations are [Cl^-]_inside= ~4-30 mM, [Cl^-]_outside= ~110 mM, and z=-1. Assuming [Cl^-]_inside= 10 mM, the equilibrium potential ECl^- is:

At resting membrane potential, Cl⁻ moves inward along its concentration gradient. The electrical gradient opposes this: the negative interior repels the negatively charged ion. The concentration gradient is stronger, so net inward movement still occurs.

In many neurons, Cl⁻ is close to its equilibrium potential at rest, so little net flux occurs despite adequate permeability. Cl⁻ permeability is lower than K⁺ but comparable to or higher than Na⁺. The Goldman equation includes Cl⁻ as one of the three ions that set resting membrane potential.

Calcium (Ca²⁺)

For Ca²⁺, the typical concentrations in neurons are [Ca²⁺]inside= ~0.0001 mM, [Ca²⁺]outside= ~1-2 mM, and z=+2. So, the equilibrium potential ECa²⁺ is:

ECa²⁺ is approximately +130 mV. Working through the Nernst equation: (61/2) × log₁₀([Ca²⁺]out / [Ca²⁺]in) = 30.5 × log₁₀(2 mM / 0.0001 mM) ≈ 30.5 × 4.3. The 61 is halved because Ca²⁺ has a valence of 2. The ~20,000-fold concentration gradient (2 mM outside, 100 nM inside) gives a strongly positive equilibrium potential, so Ca²⁺ has a large drive to enter the cell at rest.
The table below compares the four ions by concentration, equilibrium potential, and direction of movement at rest.

	K⁺	Na⁺	Cl⁻	Ca²⁺
Intracellular concentration	~140 mM	~10 - 15 mM	~4 - 10 mM	~0.0001 mM
Extracellular concentration	~4 mM	~145 mM	~110 mM	~1 - 2 mM
Equilibrium potential	−90 mV	+60 mV	~−65 mV	~+125 mV
Net movement at rest	Out of cell	Into cell	Into cell	Into cell
Primary role	Sets resting Vm	Depolarisation	Near equilibrium at rest	Signal transduction

Membrane potential

The membrane potential (Vm) constitutes the overall voltage difference across the cell membrane, established by the voltage inside relative to the outside of the cell. It is dynamic in nature and changes during cell activities like action potentials or synaptic signaling. It is determined by:

the distribution of ions across the membrane

the selective permeability of the membrane to different ions via channels, i.e. how easily they cross the membrane.

The Goldman equation combines ion concentrations and permeabilities to calculate resting potential.

Where:

P_ion: the permeability of the membrane to the ion

[ion]_outside: extracellular ion concentration

[ion]_inside: intracellular ion concentration

Although cells contain many ions, the above simplified equation omits ions that are much less permeable to the cell membrane than Na⁺, K⁺ and Cl^-, because their contribution to resting membrane potential is negligible. For example, Ca²⁺ is omitted because its influence is negligible at rest, but plays a critical role in activation. PK⁺ dominates over PNa⁺ and PCl^- in terms of permeability; PK⁺ is ~100 times over PNa⁺. This results in leak across the membrane at different rates. The membrane is not permeable to proteins, which are negatively charged and remain intracellular. For positively charged ions (Na⁺, K⁺), the external concentration is in the numerator and internal in the denominator. For negatively charged ions (Cl⁻), the ratio is reversed. This reflects ions’ natural tendency to move down their electrochemical gradient and ensures the equation accurately embodies the electrochemical forces acting on ions of different charges.

The resting membrane potential of most neurons is approximately −70 mV. Large myelinated nerve fibres can reach approximately −90 mV, and skeletal muscle fibres typically sit around −85 mV. The value varies by cell type, but −70 mV is the figure most commonly used for neurons in the clinical and undergraduate literature. K⁺ is the major factor in setting the resting membrane potential, because its intracellular concentration is much higher than the extracellular, and permeability of the membrane is high relative to the other ions. Although resting membrane potential is similar to the equilibrium potential for K⁺, other ions contribute to the resting membrane potential. Leaking of Na⁺ through Na⁺ leak channels along its electrochemical gradient contributes to the fact that the resting membrane potential is more positive than the equilibrium potential for K⁺. The gradients of Na⁺ and K⁺ are maintained in cells by active transport via the Na⁺/K⁺ ATPase.

Electrophysical principles

Movement of ions in cellular systems follows some key electrophysical principles that also apply to conduction of electrical impulses. According to Ohm’s law:

V=IR

Where:

V= the voltage of potential difference (in volts (V))

I= the current (in amperes (A))

R= the resistance (in ohms)

Movement of ions across a permeable cell membrane constitutes an electrical system, where current will occur, driven by the potential difference in the system and opposed by the system’s resistance. Conductance (G) is the inverse of resistance, measured in Siemens:

G=1/R

Conductance for an ion in a cellular system is related to the “leakiness” of the cell membrane to the ion, as a function of permeability and the presence of channels. Therefore, the Ohm’s law can be restated to predict a specific ion current based on the membrane potential and the conductance of the ion as follows:

The current is proportional to the conductance of the membrane to the ion and the difference between the membrane potential and the equilibrium potential for the ion. The latter is the driving force for the ion’s movement:

If V_m ≠ E_ion: the ion will move in or out of the cell depending on the sign of the driving force. For example, if Vm > E_ion, the ion will move to make Vm more negative, for both positive and negative ions, driving Vm toward Eion..

If V_m = E_ion: the driving force is zero, so there is no net ionic current.

Thus, K⁺ ions continue to leak out of the cell at resting membrane potential, as the resting membrane potential (~-70 mV in most cells) is less negative than EK⁺ (-90 mV); Na⁺ will constantly leak into the cell, as its equilibrium potential (+60 mV) is more positive than the resting membrane potential of -70 mV.

Sources

All content published on Kenhub is reviewed by medical and anatomy experts. The information we provide is grounded on academic literature and peer-reviewed research. Kenhub does not provide medical advice. You can learn more about our content creation and review standards by reading our content quality guidelines.

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