Cells are remarkable machines capable of performing an exquisite range of functions, many of which depend crucially on the activity of molecular motors that generate forces. Recent experiments have shown that intracellular random movements are not solely thermal in nature but also arise from stochasticity in the forces from these molecular motors. Here we consider the effects of these non-thermal random forces. We show that stochastic motor force not only enhances diffusion but also leads to size-dependent transport of objects that depends on the local density of the cytoskeletal filaments on which motors operate. As a consequence, we find that objects that are larger than the mesh-size of the cytoskeleton should be attracted to regions of high cytoskeletal density, while objects that are smaller than the mesh-size will preferentially avoid these regions. These results suggest a mechanism for size-based organelle positioning and also suggests that motor-driven random forces can additionally enhance motor-driven transport.
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