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Time Irreversibility, Entropy Production, and Effective Temperature Are Independently Regulated in the Actin Cortex of Living Cells

N Narinder1,2 and Elisabeth Fischer-Friedrich1,2,*

  • *Contact author: elisabeth.fischer-friedrich@tu-dresden.de
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Phys. Rev. X 16, 011007 – Published 8 January, 2026

DOI: https://doi.org/10.1103/5zyn-kgs3

Abstract

Living cells exhibit nonequilibrium dynamics emergent from the intricate interplay between molecular motor activity and the viscoelastic cytoskeletal matrix. The deviation from thermal equilibrium can be quantified through frequency-dependent effective temperature or time-reversal symmetry breaking quantified, e.g., through the Kullback-Leibler divergence. Here, we investigate the fluctuations of an AFM tip embedded within the active cortex of mitotic human cells with and without perturbations that reduce cortex activity through inhibition of material turnover or motor proteins. While inhibition of motor activity significantly reduces both effective temperature and time irreversibility, inhibited material turnover leaves the effective temperature largely unchanged but lowers the time irreversibility and entropy production rate associated with the fluctuation-dissipation theorem violation of tip dynamics. Our experimental findings in combination with a minimal model highlight that time irreversibility, effective temperature, and entropy production rate can follow opposite trends in active living systems, challenging, in particular, the validity of effective temperature as a proxy for the distance from thermal equilibrium, particularly in the presence of mechanical changes. Furthermore, we propose that biological activity regulates the occurrence of time-asymmetric deflection spikes in the dynamics of observables, providing a previously unrecognized link between entropy production and time irreversibility.

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Even when a cell is stationary, its interior is busy: Proteins build up and break down in never-ending cycles, and molecular motors pull on internal structures. This ceaseless activity keeps cells far from physical equilibrium, yet measuring just how far has remained a challenge. In this study, we quantify the degree of nonequilibrium behavior in dividing human cells by tracking their minute surface movements using an atomic force microscope. From these fluctuations, we extract three key indicators: entropy production rate (the energy dissipated by the system), effective temperature (the intensity of fluctuations), and time irreversibility (the extent to which behavior differs when time is reversed).

By systematically altering cellular activity, we find that these measures do not always change in unison. When we inhibit molecular motors, all three indicators—entropy production, effective temperature, and time irreversibility—decline together. But when we block the ongoing renewal of cellular components without disrupting motor activity, only entropy production and time irreversibility decrease, while the effective temperature remains high. This discrepancy shows that a system can appear equally noisy in its fluctuations even as its internal dynamics become more or less directional over time. Our analysis further reveals that sharp, spikelike deflections in the motion data play a major role in breaking time symmetry.

These findings suggest that effective temperature alone cannot reliably gauge how far a living system is from equilibrium, especially when its mechanical properties vary. Instead, we propose that time irreversibility provides a more robust indicator of nonequilibrium behavior, particularly in systems with inherently asymmetric dynamics such as biological pulses, repair cycles, or other excitable events.

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Phys. Rev. X 16, 011007– Published 8 January, 2026
  • Received 21 March 2025
  • Accepted 24 October 2025
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

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