[5] Proton electrochemical potential gradient in vesicles, organelles, and prokaryotic cells

The measurement of a proton-electrochemical-potential gradient is central to the analysis of energy-conversion processes in mitochondria, chloroplasts, storage organelles, bacterial cells, algae, yeast, and other systems in which proton pumps play a primary role. The most direct measurement of the transmembrane-electrical-potential difference is obtained from the use of microelectrodes. The potential between two reference electrodes located at the bulk solutions bathing the two sides of a membrane is the membrane potential. However, the applications of microelectrodes to organelles, small cells, and vesicles are severely limited because of size limitation. Thus, the methods used in these systems are mostly indirect. This chapter discusses simple, two-compartment systems, such as vesicles, organelles, and prokaryotic cells. A transport mechanism that often ensures a true equilibration involves a partition of an ion into a membrane lipid phase, diffusion in the membrane, and dissociation.