Electrochemical potential

The electrochemical potential is a thermodynamic measure that reflects energy from entropy, enthalpy, and electrostatics.

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. With regard to the electrochemical cells of a battery (and in other instances pertaining specifically to the movement of electrically charged solutes), the potential may be expressed in units of volts (see electrode potential and Table of standard electrode potentials)

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 (cell potential) between the two sides of the membrane

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.

In the context of ATP synthesis, hydrogen ions (protons) represent an exception to the supposition that crossing the membrane involves no chemical transformation. Protons may chemically bind to any base or to molecules of water, which exist in abundance on either side of the membrane in any biological context. The energy gained or lost through such binding depends on the local concentration of protons (or pH), which may differ between the two sides of the membrane in mitochondria, chloroplasts, bacteria and other membranous compartments as well. Therefore a transmembrane difference in pH, in addition to the factors above, represents a distinct contributing component of the electrochemical potential in these contexts. This more complex potential might sometimes be called a chemiosmotic potential, after the chemiosmotic hypothesis of Peter Mitchell.