Mitochondria generate most of the heat in endotherms. Given some impedance of heat transfer across protein-rich bioenergetic membranes, mitochondria must operate at a higher temperature than body temperature in mammals and birds. But exactly how much hotter has been controversial, with physical calculations suggesting that maximal heat gradients across cells could not be greater than 10−5 K. Using the thermosensitive mitochondrial-tar- geted fluorescent dye Mito Thermo Yellow (MTY), Chre ́tien and colleagues suggest that mitochondria are optimised to nearly 50 ̊C, 10 ̊C hotter than body temperature. This extreme value questions what temperature really means in confined far-from-equilibrium systems but encourages a reconsideration of thermal biology.
ALDH2, aldehyde dehydrogenase 2; HEK293, human embryonic kidney cells 293; mtDNA, mitochondrial DNA; MTY, Mito Thermo Yellow