Active random force promotes diffusion in bacterial cytoplasm

TL;DR

This study models how active forces in bacterial cytoplasm enhance the diffusion of particles, especially larger ones, aligning with experimental observations and revealing a simple underlying stochastic mechanism.

Contribution

We introduce a white-noise active force model to explain size-dependent diffusion enhancement in bacterial cells, matching experimental data quantitatively.

Findings

Diffusion enhancement is more significant for larger particles.

The active force's autocorrelation scales with the cube of particle radius.

Active forces of about 0.57 pN can produce observed effects.

Abstract

Experiments have found that diffusion in metabolically active cells is much faster than in dormant cells, especially for large particles. However, the mechanism of this size-dependent diffusion enhancement in living cells is still unclear. In this work, we approximate the net effect of metabolic processes as a white-noise active force and simulate a model system of bacterial cytoplasm with a highly polydisperse particle size distribution. We find that diffusion enhancement in active cells relative to dormant cells can be more substantial for large particles. Our simulations agree quantitatively with the experimental data of Escherichia coli, suggesting an autocorrelation function of the active force proportional to the cube of particle radius. We demonstrate that such a white-noise active force is equivalent to an active force of about 0.57 pN with random orientation. Our work unveils an emergent simplicity of random processes inside living cells.