Abstract

Living systems operate far from equilibrium due to the continuous injection of energy provided by ATP supply. The dynamics of the intracellular components, such as proteins, organelles and cytoskeletal filaments, are driven by both thermal equilibrium fluctuations, and active stochastic forces generated by the molecular motors. Tracer particles are injected in living cells to study these fluctuations. To sort out genuine nonequilibrium fluctuations from purely thermal effects, measurements of spontaneous tracer fluctuations and of response are combined. We theoretically rationalize the observed fluctuations with a phenomenological model. This model, in turn, allows us to quantify the time and energy scales of active fluctuations in two different experimental systems—living melanoma cells and living mouse oocytes—and to estimate the energy transfers operating in such systems.