How and Why Do We Measure Cellular Heat?–Significance and History—

Temperature is a fundamental physical quantity that governs chemical reaction rates, molecular diffusion, and protein stability, and is therefore intimately linked to biological function. In cells, heat is generated through a variety of nonequilibrium processes, including metabolism and mechanical activity. Cellular thermometry is increasingly recognized not only as a method for measuring temperature but also as a framework for investigating intracellular energy conversion, heat dissipation, and the spatial localization of cellular function. This review examines the significance of thermal measurement in cell-related research and traces the historical development of the field, from bulk calorimetry of cell populations to single-cell and subcellular measurements enabled by optical and microfabricated sensors. We summarize the major methodological platforms, including fluorescent polymeric thermometers, inorganic nanoparticles, fluorescent proteins, small-molecule probes, and contact-based microsensors, highlighting their respective strengths and limitations with regard to spatial and temporal resolution, targeting capability, and quantitative reliability. We also discuss the central controversy in the field, namely the “10 5 gap issue,” which refers to the discrepancy between experimentally reported intracellular temperature changes and theoretical predictions based on classical heat conduction. Finally, we consider future perspectives. Rather than remaining focused solely on temperature measurement, cellular thermometry is evolving toward a broader framework of cellular thermal analysis that encompasses heat production, heat transport, and cellular thermophysical properties.