While we knew that mitochondria somehow generate heat, we didn’t exactly understand how. Researchers at the University of Illinois used a tiny thermometer to find out.

The team reports that mitochondria release heat in quick, powerful bursts using energy stored in internal proton batteries. The findings were made possible through the new tool the researchers built, as previous methods were too slow to pick up on the heat spikes.

“Producing heat is part of the mitochondria’s role in the center of metabolism activity,” said mechanical science and engineering professor Sanjiv Sinha. “It needs to produce the energy currency that’s used for the activities in the cell, and heat is one of the byproducts.”

Mitochondria also have a mechanism in place to increase heat output if needed, such as when the body’s overall temperature goes down. In order to get a better understanding of how this heat is generated, the team developed a fast-read thermometer probe measure the internal temperature of living cells. Tab of Rhanor Gillette, professor emeritus of molecular and integrative physiology at Illinois, helped test the probe in a mitochondria-rich strain of neurons.

The team then made the cells produce heat. They recorded very fast changes in temperature inside the neurons, “results that were completely different from what has been published before” according to first author Manjunath Rajagopal.

“We saw a sharp temperature spike that is significantly large and short-lived — around 5 degrees Celsius and less than one second,” he explains.

“The gold standard for measuring has been with fluorescence, but it is too slow to see this short, high burst of heat.”

The findings conflict with previous assumptions that mitochondria break down glucose to generate heat: the temperature spikes, Sinha says, are too large. In order to find the source of energy, the team turned to the mitochondria, and chemically induced them to open up protein channels on their membrane.

“In the mitochondria, one part of the glucose metabolism reaction stores some of the energy as a proton battery. It pushes all the protons to one side of a membrane, which creates an energy store,” Rajagopal said.

“We basically short-circuited the stored energy.”

In the future, the team wants to use their probe on other types of cells. One of their primary focus will be identifying therapeutic targets, they add. Better control over this energy sink could have applications against obesity and cancer.

The paper “Transient heat release during induced mitochondrial proton uncoupling” has been published in the journal Communications Biology.