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Our body temperature might not ever get much hotter than 37°C. But it turns out that the insides of our cells can reach a scorching 50°C.

Our cells effectively burn food in oxygen to produce energy. Unlike a fire, this is a controlled process involving several steps, but it still generates a lot of heat.

But because respiration, as this process is known, happens inside tiny structures inside cells called mitochondria, measuring just how hot they get has not been possible. However, in the past year or so, several research teams around the world have developed dyes that fluoresce in different ways as temperatures change.

Pierre Rustin of INSERM in France and colleagues have now used a dye developed by a group in Singapore to measure the temperature inside the mitochondria of human kidney and skin cells kept at 38°C. They found that mitochondria operate at temperatures at least 6 to 10°C higher than the rest of the cell.

Hot hot heat

While Rustin’s study is the first to look specifically at the temperature of mitochondria, another group might just have beaten them to the punch. A paper published in February by a team in Japan that briefly mentions that mitochondria in human cancer cells appear to be 6 to 9°C hotter than the rest of the cell.

The finding makes sense when you think about it, says at University College London, author of . “Mitochondria are the main sources of heat, and they have to be hotter than the rest of the body,” he says. “I’d never really thought of that before.”

If the mitochondria in mammals – and presumably birds – have indeed evolved to operate at higher temperatures than we realised, biologists may have to recheck the many previous experiments that assume they operate at body temperature, Rustin’s team writes.

The mitochondria in cold-blooded plants and animals presumably operate at far lower temperatures, but this is something else that now needs to be checked.

Because mitochondria power cells, people can suffer from serious diseases or even die young if they inherit faulty mitochondria. This can now prevented by replacing the mitochondria in an embryo with ones from a donor.