Electron transfer down the respiratory chain results in the translocation of protons from the mitochondrial matrix into the intermembrane space (which rapidly equilibrates with the bathing medium) so generating a pH gradient (DpH) across the inner mitochondrial membrane (outside more acidic than the inside). The tendency of protons to diffuse back into the matrix is the driving force for ATP synthesis by the mitochondrial ATP synthase. During steady state oxidative phosphorylation by a suspension of mitochondria in a medium of 7.4, the pH of the matrix has been estimated to be 7.7.
Calculate the number of protons in the matrix of a respiring liver mitochondrion, assuming that the inner membrane encloses a sphere of 1.5 µm diameter (a reasonable size estimate).
On the basis of your calculations, is the pH gradient sufficient to drive ATP synthesis? What assumptions do you have to make concerning the H+ -stoichiometry of the ATP synthase to draw these conclusions? Assume a temperature of 37o C.
If the pH gradient is insufficient, where does the energy necessary for the net synthesis of ATP come from?