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Every developed and adapted biological system extracts useful energy from outside, converts, stores it, and uses for muscular contraction, substrate transport, protein synthesis, and other energy utilizing processes. This energy management in a living cell is called the bioenergetics, and the useful energy is the exergy, which is destroyed in every irreversible process because of the entropy production. The converted exergy is the adenosine triphosphate (ATP) produced through the oxidative phosphorylation coupled to respiration in which the exergy originates from oxidation of reducing equivalents of nutrients. A living cell uses the ATP for all the energy demanding activities; it has to maintain nonvanishing thermodynamic forces, such as electrochemical potential gradient, and hence is an open, nonequilibrium system, which manages the exergy destruction and power production to adapt the fluctuations in energy demand and production within mitochondria. A simplified example presented here shows that the use of exergy analysis is helpful for understanding and analysing oxidative phosphorylation in bioenergetics.