The generation of a transmembrane electrical potential through ion movement across a cell membrane drives biological processes like nerve conduction, muscle contraction, hormone secretion, and sensation.  By convention, physiological voltages are measured relative to the extracellular region; a typical animal cell has an internal electrical potential of (&minus;70)&ndash;(&minus;50) mV.
An electrochemical gradient is essential to mitochondrial oxidative phosphorylation. The final step of cellular respiration is the electron transport chain, composed of four complexes embedded in the inner mitochondrial membrane. Complexes I, III, and IV pump protons from the matrix to the intermembrane space (IMS); for every electron pair entering the chain, ten protons translocate into the IMS.  The result is an electric potential of more than .  The resulting flux of protons back into the matrix powers the efforts of ATP synthase to combine inorganic phosphate and ADP.
Similar to the electron transport chain, the light-dependent reactions of photosynthesis pump protons into the thylakoid lumen of chloroplasts to drive the synthesis of ATP. The proton gradient can be generated through either noncyclic or cyclic photophosphorylation. Of the proteins that participate in noncyclic photophosphorylation, photosystem II (PSII), plastiquinone, and cytochrome bf complex directly contribute to generating the proton gradient. For each four photons absorbed by PSII, eight protons are pumped into the lumen.
Several other transporters and ion channels play a role in generating a proton electrochemical gradient. One is TPK, a potassium channel that is activated by Ca and conducts K from the thylakoid lumen to the stroma, which helps establish the electric field. On the other hand, the electro-neutral K efflux antiporter (KEA) transports K into the thylakoid lumen and H into the stroma, which helps establish the pH gradient.