Electrochemical potential

The electrochemical potential is a thermodynamic measure that reflects energy from entropy and electrostatics and is typically invoked in molecular processes that involve diffusion. It represents one of many interchangeable forms of potential energy through which energy may be conserved.

In electrochemistry, the term is typically applied in contexts where a chemical reaction is to take place, such as one involving the transfer of an electron at a battery electrode. In a battery, an electrochemical potential arising from the movement of ions balances the reaction energy of the electrodes. The maximum voltage that a battery reaction can produce is sometimes called the standard electrochemical potential of that reaction (see also electrode potential and Table of standard electrode potentials). In instances pertaining specifically to the movement of electrically charged solutes, the potential is often expressed in units of volts.

In biology too the term is sometimes used in the context of a chemical reaction, in particular to describe the energy source for the chemical synthesis of ATP. More generally, however it used to characterize the propensity of solutes to simply diffuse across a membrane (i.e., a process involving no chemical transformation).

With respect to a biological cell, organelle, or other subcellular compartment, the propensity of an electrically charged solute, such as a potassium ion, to move across the membrane is decided by the difference in its electrochemical potential on either side of the membrane, which arises from three factors:

• the difference in the concentration of the solute between the two sides of the membrane
• the charge or "valence" of the solute molecule
• the difference in voltage between the two sides of the membrane (i.e. the transmembrane potential).

A solute's electrochemical potential difference is zero at its "reversal potential", the transmembrane voltage at which the solute's net flow across the membrane is zero. This potential is predicted theoretically either by the Nernst equation (for systems of one permeant ion species) or the Goldman-Hodgkin-Katz equation (for more than one permeant ion species).

The electrochemical potential difference between the two sides of the membrane in mitochondria, chloroplasts, bacteria and other membranous compartments that engage in active transport involving proton pumps, is at times called a chemiosmotic potential (see chemiosmotic hypothesis). In this context protons are often considered separately, using units either of concentration or pH.

See also: electrodiffusion, cell potential, action potential,