Probes for energy transduction in membranes

Conclusions

It is possible to conclude, after the discussion on the application of different probes to energy-transducing membranes, that the efforts in the direction of the elucidation of the mechanism of energy conservation have not been wasted. Despite the lack of a final answer to the question whether the coupling device is chemiosmotic, chemical, or conformational in nature, a number of new important experiments have been performed which will finally contribute to the solution of the problem.

Apart from the important characterization of a number of physical parameters regarding the phospholipid environment and the interaction among proteins in the membrane, data concerning the onset of membrane potential(s) and pH gradients associated with energy conservation appear convincing. In fact, from different types of approach (lipophylic ions, charged fluorescent probes, carotenoids, and merocyanines) a common conclusion can be reached, namely that electrical potentials are set at the level of the mitochondrial and other energy-transducing membranes. There is still some controversy as to whether the potential is set across the entire thickness or only a small portion of the membrane.

It appears also well established that pH gradients are set across the coupling membrane during energy conservation and that pH gradients and membrane potentials have specific correlations. Whether the conclusions reached with the probe technique are perfectly in line with the chemiosmotic hypothesis, require some modification of it, or may also fit with a chemical hypothesis would go beyond the purpose of the present discussion. It seems, however, that some predictions of the chemiosmotic hypothesis have been, at least qualitatively, confirmed by the use of the probe approach described above.