Membrane Bioenergetics as Viewed from Reconstitution Experiments

Abstract

There is a plethora of literature on mitochondria and chloroplasts whose main task is energy transduction. That is, most of the energy for cellular processes in the form of ATP (Adenosine triphosphate) is produced by the organelle membranes of mitrochondria and chloroplasts1–3. The mechanism of energy transduction has been elegantly explained by Mitchell’s chemiosmotic hypothesis4. Briefly, mitochondria contain the enzymes and coenzymes that mediate the electron transport from NADH (a reduced nucleotide) down the chain of redox proteins to the ultimate electron acceptor, oxygen. As electrons move down the redox protein chain, an electrochemical gradient of protons is generated. Free energy is conserved in the synthesis of ATP as a result of the discharge of the proton gradient across the cristae membrane of mitochondria. The essential feature of the Mitchell theory is “energy coupling” among the various membrane-bound processes: electron transfer, redox reaction, ion and proton gradient, membrane potential and ATP synthesis. Obviously, these membrane-based activities, many of which are electrical in nature, should be probed and investigated by electrochemical methods, as have been carried out on whole cells and nerve axons by electrophysiologists. Unfortunately, mitochondria, along with other organelles such as chloroplasts, are too tiny to stick microelectrodes into them for reliable experiments, although there have been several isolated reports5,6.