Optical tracking in vivo experiments reveal that diffusion of particles in
biological cells is strongly enhanced in the presence of ATP and the
experimental data for animal cells could previously be reproduced within a
phenomenological model of a gel with myosin motors acting within it [EPL 110,
48005 (2015)]. Here, the two-fluid model of a gel is considered where active
macromolecules, described as force dipoles, cyclically operate both in the
elastic and the fluid components. Through coarse-graining, effective equations
of motions for tracer particles displaying local deformations and local fluid
flows are derived. The equation for deformation tracers coincides with the
earlier phenomenological model and thus confirms it. For flow tracers,
diffusion enhancement caused by active force dipoles in the fluid component,
and thus due to metabolic activity, is found. The latter effect may explain why
ATP-dependent diffusion enhancement could also be observed in bacteria that
lack molecular motors in their skeleton or when the activity of myosin motors
was chemically inhibited
