Physics-Biology interface seminar: Étienne Fodor


11:00 - 12:00

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Tracking nonequilibrium physics in living systems

Étienne Fodor (Université Paris-Diderot)

Living systems operate far from equilibrium due to the continuous injection of energy provided by ATP supply. The dynamics of the intracellular components is 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. Alternatively, vesicles which are already present in the cytoplasm serve as probes of the intracellular dynamics.

To sort out genuine nonequilibrium fluctuations from purely thermal effects, we combine passive and active microrheology methods. They consist in measuring the spontaneous tracer fluctuations and extracting the response from an external oscillatory perturbation. By testing the fluctuation-dissipation theorem, we quantify the deviation from equilibrium appearing at low frequency. Removing the thermal contribution in the tracer fluctuations, we estimate the spectrum of the active forces. Eventually, we report non-Gaussian tails in the tracer displacement distribution as a result of directed motion events.

We recapitulate theoretically the observed fluctuations by modeling the dynamics with a confining harmonic potential which experiences random bursts as a result of motor activity [1]. This minimal model allows us to quantify the time and length scales of the active forces, along with the energy scale injected by the ensuing fluctuations [2, 3]. Finally, we estimate the energy dissipated by the tracers in the surrounding environment, leading us to define an efficiency of the energy conversion driving the tracer dynamics [4].


  • [1] É. Fodor et al., Phys. Rev. E 90, 042724 (2014)
  • [2] É. Fodor et al., EPL 110, 48005 (2015)
  • [3] W. W. Ahmed et al., arXiv:1510.08299
  • [4] É. Fodor et al., arXiv:1511.00921
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