Heat inside a cell is like traffic in a city: on some streets the lanes are wide and traffic flows fast, while in narrow alleys everything gets stuck. Scientists have found a way to map such 'traffic jams' for heat without opening the cell.
They took tiny diamonds, so small they are invisible to the naked eye, and placed them on a thin gold plate. The gold is heated with a light beam, and the diamonds start to glow differently when heated. From this glow, you can tell how hot it is around the diamond. By changing the frequency of heating, the researchers control how deep the heat penetrates into the cell: like adjusting the water pressure in a hose to water just one garden bed instead of the whole yard.
The method was tested on cells that experience stress from changes in salt concentration. It turned out that in response to such stress, heat flows inside the cell change, and this is linked to the formation of protein droplets, like oil gathering in soup. Such droplets are associated with diseases, for example amyotrophic lateral sclerosis.
Now biologists have a tool to see the thermal life of a cell in real time. This will help figure out how cells exchange energy, how diseases develop, and how drugs affect them.
The diamonds used as thermometers glow even at room temperature, and their glow changes so precisely that you can notice a difference of a fraction of a degree.