Murakami, A.; Sato, T.; Suzuki, H.; Funatsu, T.; Okabe, K.

Intracellular thermometry has revealed temperature gradients within living cells, indicating the presence of localized energy flow. Moreover, increasing evidence suggests that cells can autonomously regulate heat production and use this heat to support cellular functions. Although heat is generally assumed to dissipate rapidly, recent studies suggest that slow, non-diffusive heat dissipation can generate high local temperatures, implying that heat may be retained within cells. However, such intracellular heat retention at the single-cell level has not been experimentally examined. Here, we measured the dissipation of metabolically generated heat using calorimetry and evaluated cellular heat balance. We further separated the dissipated heat into heat transfer and heat production components using an intracellular temperature imaging-based method. Through this approach, we found that perturbation of the cell membrane accelerated intracellular heat transfer rates and increased heat dissipation. These findings provide the first experimental evidence of intracellular heat retention, showing that heat generated by catabolic metabolism is not fully dissipated but is partly retained within cells by the cell membrane, which separates the intracellular space from the external environment. Furthermore, we identified the mechanism underlying this heat retention, demonstrating that membrane lipid dynamics are a key molecular determinant and that the motions of membrane lipids form an energetic barrier to heat dissipation. Together, our findings redefine metabolic heat, long regarded as mere waste, as a functional resource that supports efficient energy use, and they establish membrane-controlled heat retention as a fundamental principle of bioenergetic optimization in living systems.