The role of transmembrane electric potential difference (delta psi) in mitochondria, chloroplasts, and bacteria has been considered. Since the electric capacitance of membranes is much lower than the pH buffer capacitance of water phases, delta psi proves to be the primary form of energy produced by generators of electrochemical H+ potential difference (delta mu-H). There are 11 distinct types of delta mu-H-generating systems in coupling membranes, involved in respiratory and light-dependent electron and proton transfer, as well as in ATP and PP1 hydrolysis and synthesis. Bacteriorhodopsin is the simplest delta mu-H generator. However, even in this case, the molecular mechanism of delta psi production remains obscure. Many types of work can be supported by delta mu-H with no ATP involved so that delta mu-H proves to be not only a transient intermediate of oxidative and photosynthetic phosphorylation but also a convertible energy currency for the cell. Among the delta mu-H-supported activities, mechanical work was recently demonstrated. It can be exemplified by the motility systems of (i) flagellar bacteria and (ii) blud--green algae. As was found in multicellular cyanobacteria, delta mu-H can be used for a power transmission over distances as long as 1 mm. It seems to be probable that in large cells of eukaryotes (e.g., in muscle fibers) giant mitochondria may serve as power-transmitting structures. Na+--K+ gradients can be used to stabilize delta mu-H in bacteria. It is suggested that the primary function of unequal distribution of these cations between the microbial cell and the medium is delta mu-H buffering.