During Oxidative Phosphorylation electrons are transferred from NADH and FADH2 to oxygen. This transfer of electrons results in pumping protons across the inner mitochondrial membrane, from the matrix space to the intermembrane space. This generates a pH gradient with the pH higher in the matrix than in the cytosol, where the pH is generally close to 7. This difference in the proton concentration across the membrane gives rise to a membrane diffusion potential difference. Additionally, the transfer of protons is electrogenic due to the positive electrical charge that is transported by the protons across the membrane. This transfer of positive (proton) charges is only incompletely - and with a delay - compensated either by an opposite flow of cations (mainly K+) or by a parallel movement of anions (mainly Cl- and HCO3-): a membrane potential difference (deltaPSI) of about -200 mV results (negative inside the mitochondria).
Together, the diffusion potential difference (-2,3•RT•delta pH) and the membrane potential (deltaPSI) generate the so called protonmotive force (pmf):
Pmf = deltaPSI - 2,3•RT•delta pH
In metabolic active mitochondria, pmf remains nearly constant at about -230 mV.