Dynamical crossover from motor-dominated to drag-dominated transport in a minimal active transport network

Abstract

Motor-driven intracellular transport is often described in terms of motor activity, but macroscopic transport also depends on how effectively motor-generated force is converted into coherent motion. Motivated by cytoplasmic streaming, a minimal active transport network is examined in which motor-driven transport competes with an effective slip-related dissipative resistance. The model is not intended as a quantitative reconstruction of Nitella cytoplasmic streaming, but as a minimal system for isolating the relation between motor activity, resistance, and transport output. A controlled scan over $\gamma_{\mathrm{Slip}}$ and $\alpha_m$, with three independent seeds per condition, shows that increasing $\gamma_{\mathrm{Slip}}$ strongly suppresses mean transport speed while leaving the motor-bound fraction nearly unchanged. The mean load and motor force remain finite in the high-$\gamma_{\mathrm{Slip}}$ regime, indicating that motors remain mechanically active even when transport is suppressed. The dependence of transport speed on $\alpha_m$ progressively disappears with increasing $\gamma_{\mathrm{Slip}}$: the motor dominance ratio decreases from $R\approx1.69$ to $R\approx1.01$, and the corresponding velocity difference decreases from $\sim1.9~\mu\mathrm{m/s}$ to $\sim0.003~\mu\mathrm{m/s}$. These results indicate a dynamical crossover from motor-dominated to drag-dominated transport. The minimal model provides a compact physical scenario in which active force generation persists while its contribution to net transport is suppressed by increased effective dissipative resistance.