A surprising new study suggests that mitochondria, the 'powerhouse of the cell', actually run at a temperature that's far warmer than the human body.
Scientists have discovered they're running at a sizzling 50°C (122°F), surprisingly much hotter than our bodily average of 37°C ( 98.6°F), and it could force a rethink on how our energy generators actually work.
Our body's temperature is finely tuned to maintain a healthy balance that helps keep all our biochemical processes running smoothly. One of those processes is respiration, which takes place within tiny cell organelles called mitochondria.
These convert oxygen and nutrients into ATP (adenosine triphosphate) - the primary energy source for all our cells.
As an energy-creating process, respiration generates heat. And now a team of scientists led by Malgorzata Rak from Université Paris 7 have attempted to measure just how hot mitochondria run in warm-blooded species.
The scientists used one of a host of recently developed 'fluorescent thermometers', which are temperature-sensitive dyes that can bind to specific targets in a living cell, including mitochondria.
Their thermometer of choice was 'Mito thermo yellow', which a team led by researchers from the National University of Singapore first described in 2015. Its fluorescence gets dimmer as heat is detected.
Rak and his colleagues took human kidney and skin cells, as well as lung cancer cells, and applied the dye to them, and kept them toasty at 38°C (100.4°F). When the cells were exposed to an oxygen-rich environment that stimulated energy production, there was a surprising increase in temperature.
They found that mitochondria became 7 to 12°C (12 to 21°F) hotter than the rest of the cell, reaching a sizzling 50°C (122°F). For a person, that would be a life-threatening fever, and in fact exceeds the hottest body temperature ever recorded.
But the idea that the powerhouse of the cell may run extremely hot is not entirely preposterous.
"Mitochondria are the main sources of heat, and they have to be hotter than the rest of the body," University College London biochemist Nick Lane, who wasn't involved in the study, told Michael Le Page at New Scientist.
"I'd never really thought of that before."
It's important to note that the study hasn't yet been peer-reviewed, but if the results hold, they would call into question what we know about how our proverbial cellular powerhouses work.
"Our findings raise numerous questions concerning the biochemistry, physiology and pathology of mitochondria," the researchers write.
They note that most experiments involving mitochondria have been done assuming that they operate at body temperature, so what we know about respiration in hot-blooded animals may not hold ground any more.
"Amazing finding if true. Very significant implications for protein folding among other things," biochemist Darren Boehning from the University of Texas said in a tweet, adding that the results could be problematic, because several proteins found inside mitochondria are super-sensitive to temperature.
Scientists are also not entirely sure where precisely Mito thermo yellow sticks when it gets inside a mitochondrion, but it's postulated to be within the inner membrane.
These new results are particularly interesting, because just this February, a team of Japanese scientists published a study on a different fluorescent thermometer called gTEMP, which they tested on the mitochondria in a HeLa cell.
To see how well gTEMP worked, these researchers used a chemical agent called FCCP which is known to cause heat production inside mitochondria, affecting their normal functioning.
They measured a temperature increase of 6-9°C (10-16°F), overturning a previous result which showed that FCCP should only increase the temperature of a mitochondrion by 1°C (1.8°F).
But the medium temperature they detected was 37°C (98°F), which still doesn't reach the scorching results presented in this new study.
We will have to wait and see what peer review says, and if other researchers will be able to replicate these findings. But it's a tantalising result for sure, and could have a huge impact on how we understand our own cell function.
The paper has been published on the biology preprint server bioRxiv. The gTEMP study was published in PLOS One.