[64] The measurement of membrane potential and ΔpH in cells, organelles, and vesicles

The determination of both membrane potential and pH gradient (ΔpH) across a membrane allows the calculation of the proton electrochemical potential difference, or the proton-motive force. The value of the proton-motive force is important for an experimental evaluation of the chemiosmotic hypothesis. It is also an important indicator of the coupling in energy-conversion membrane systems. It is the most sensitive and quantitative indicator of coupling in these systems. As the proton-motive force can be generated by various means, the degree of coupling can be evaluated in systems that lack the complete machinery for oxidative phosphorylation or photophosphorylation. This is in contrast to the more conventional parameters of coupling, such as phosphate/oxygen (P/O), respiratory control, and phosphate potential. The determination of membrane potential or internal pH is not limited to systems of energy conversion; the chapter describes methods for determining the membrane potential in suspensions of cells, organelles, or vesicles.