Biopotentials - basics
Diffusion, equilibrium (Nernst) and Donnan potentials - conditions and mechanisms of generation, dependencies on concentration and time.
Diffusion is the movement of particles from a higher to a lower concentration. A diffusion potential is the potential difference generated across a membrane when an ion diffuses down its concentration gradient. It can only arise where the membrane is permeable to that ion, and its size depends on the size of the concentration gradient.
The equilibrium (Nernst) potential is reached when the electrical force on an ion exactly balances its concentration gradient. The ion's concentrations on the two sides stay different and the membrane keeps a potential difference, but there is no net flow.
At electrochemical equilibrium, the chemical and electrical driving forces acting on an ion are equal and opposite and no further net diffusion occurs.
The Nernst equation is used to calculate the equilibrium potential for an ion at a given concentration difference across a membrane the equation is:
Where:
E = Equilibrium potential (mV)
2.3RT/F = constant (about 60 mV at 37 °C)
Z = Charge on ion
F = Faraday's constant
Ci = Intra cellular concentration (mmol/l)
Ce = Extracellular concentration (mmol/l)
So Nernst equation is turning a concentration difference between two fluids into an electric voltage.
The Donnan potential arises when a charged molecule that cannot cross the membrane, such as an intracellular protein, sits on one side. Permeant ions then distribute unevenly, and the system reaches an equilibrium that keeps a potential difference across the membrane. This happens in many human cells.