  The glassy cytoplasm, crowded with bio-macromolecules, is fluidized in living
cells by mechanical energy derived from metabolism. Characterizing the living
cytoplasm as a non-equilibrium system is crucial in elucidating the intricate
mechanism that relates cell mechanics to metabolic activities. In this study,
we conducted active and passive microrheology in eukaryotic cells, and
quantified non-thermal fluctuations from the violation of the
fluctuation-dissipation theorem (FDT). The power spectral density corresponding
to active force generation was then estimated following the Langevin theory
extended to non-equilibrium systems. Experiments performed while regulating
cellular metabolic activity showed that the non-thermal displacement
fluctuation, rather than the active non-thermal force, directly correlates with
metabolism. We discuss how mechano-enzymes in living cells do not act as a
collection of microscopic objects; rather, they generate meso-scale collective
fluctuations that directly correlate with enzymatic activity. The correlation
is lost at long time scales because of the mesoscopic structural relaxations
induced by the metabolic activities. Since the efficiency with which energy is
converted to non-thermal fluctuations decreases as the cytoplasm becomes
fluidic, the fluidization of the cytoplasm stops at critical jamming.
Regardless of the presence or absence of structural relaxations in the
cytoplasm, we demonstrate that non-thermal fluctuations in a probe particle can
serve as a valuable indicator of those metabolic activities that typically
perturb the mechanical environment within cells.
