Hot mitochondria and the second law of thermodynamics

Abstract Mitochondria are central hubs of cellular bioenergetics, converting chemical free energy into ATP while inevitably dissipating heat during respiration. Fluorescence-based thermometry has been interpreted as evidence for intracellular “hot spots” more than 10 °C above bulk physiological temperature, implying that mitochondria might operate far outside conventional thermal bounds. Yet such large gradients are difficult to reconcile with mitochondrial size, aqueous thermal conductivity, and limited metabolic power. Beyond passive heat diffusion, a remaining question is whether nonequilibrium biochemical processes, including respiration-driven proton pumping, could actively maintain local temperature gradients as nanoscale heat pumps. Here, we establish a model-independent thermodynamic bound based only on the Second Law. Applied to the inner mitochondrial membrane, this bound limits the temperature difference that any biochemically driven mechanism could sustain, even under perfect efficiency and maximal power. The resulting upper limit is a small fraction of a degree, ruling out multidegree mitochondrial overheating.