The article by Johnson et al. (1980) explores the use of permeant cationic fluorescent probes to monitor the mitochondrial membrane potential in living cells. The authors demonstrate that these probes selectively accumulate in mitochondria, which is dependent on the high trans-membrane potential maintained by functional mitochondria. The dissipation of this potential, through ionophores or inhibitors of electron transport, leads to the release of the probes from mitochondria. The application of these potential-dependent probes in fluorescence microscopy allows for the monitoring of mitochondrial membrane potential in individual living cells. The study observed marked elevations in mitochondria-associated probe fluorescence in cells engaged in active movement, suggesting a correlation between mitochondrial membrane potential and cellular energy requirements. The findings highlight the utility of this approach in future investigations of energy metabolism and specific biological functions at the cellular level.The article by Johnson et al. (1980) explores the use of permeant cationic fluorescent probes to monitor the mitochondrial membrane potential in living cells. The authors demonstrate that these probes selectively accumulate in mitochondria, which is dependent on the high trans-membrane potential maintained by functional mitochondria. The dissipation of this potential, through ionophores or inhibitors of electron transport, leads to the release of the probes from mitochondria. The application of these potential-dependent probes in fluorescence microscopy allows for the monitoring of mitochondrial membrane potential in individual living cells. The study observed marked elevations in mitochondria-associated probe fluorescence in cells engaged in active movement, suggesting a correlation between mitochondrial membrane potential and cellular energy requirements. The findings highlight the utility of this approach in future investigations of energy metabolism and specific biological functions at the cellular level.