Question

One way to measure membrane potentials in cells or organelles relies on the use of lipid-soluble ions

like TPP+, which distribute themselves passively across membranes and achieve transmembrane

concentration gradients which depend on the membrane potential. A suspension of mitochondria is exposed to 10 μM TPP+. At equilibrium, the intra-mitochondrial TPP+ concentration is measured as 3 mM. What is the membrane potential across the mitochondrial membrane?

Answer 1

Membrane potential fluorescent probes like TPP are positively charged compounds that distribute into negatively charged compartments in accordance with the Nernst equation:

dy = -60 log (F_in / F_out )

whrere,

dy is membrane potential in millivolts

F_in and F_out are fluorophore concentrations inside and outside a specific cellular compartment.

At equilibrium, the flux of fluorophore is equal in and out.

thus,

membrane potential = -60 log (3000 microM /10 microM)

= -60 log 300

= -60 * 2.47

= -148

Thus membrane potential of mitochondria is -148milli Volts