Multi-modal stochastic transport in tubular cells

Eukaryotic cells utilize a variety of mechanisms to transport particles of different sizes throughout the cytoplasm. Commonly employed transport modes include diffusion driven by stochastic fluctuations in the medium, processive motor-driven transport along cytoskeletal tracks, and advection in a flowing cytoplasmic fluid. We use analytical theory and simulations to explore the efficiency and relative contributions of these different modes of transport. Focusing on tubular geometries such as those found in fungal hyphae and neuronal axons, we highlight the potential importance of hydrodynamic entrainment in the cytoplasm as well as the interplay between diffusive and processive motion in efficiently dispersing and delivering organelles within the cell. We quantify the consequences of tethering to microtubule tracks, deriving a simple expression for the parameter regime where tethering can aid or hinder overall transport efficiency. For the example system of peroxisome transport in hyphae, we show that both passive diffusion and directed "hitch-hiking" runs contribute substantially to the organelles' ability to efficiently find intracellular targets, while tethering aids in the initial establishment of a uniform organelle distribution.