A two-dimensional diffusion model quantifying intracellular transport with independent factors accounting for cytosol viscosity, binding, and steric hindrance

Transport of biomolecules within cells plays a critical role in the maintenance of the cellular functions. Compared to diffusion in aqueous phase, cytoplasmic diffusion is partly retarded by the elevated viscosity of cytosol. In addition, intracellular diffusion is affected by cytoplasmic physical properties such as tortuosity, binding, and molecule crowding. This work develops a two dimensional mathematical model describing the intracellular diffusion and yielding probe concentration distribution within the cell in time course. Initially, cytoplasmic mobility is quantified as apparent diffusion coefficient (ADC), with which all the diffusion resistances encountered are accounted for. Binding, however, as an important diffusion resistance, is then isolated from all others included in ADC. Using the theoretical analysis from the literature, furthermore we quantify the contribution of steric hindrance in retarded cytoplasmic mobility. In all, three major diffusion resistances including molecular binding to cytoarchitecture, molecule detouring caused by cytoskeleton obstacles, and elevated cytosol viscosity are elucidated and quantified in this work. All of these factors play significant roles in mobility retardness in intracellular transport.