  Recent experiments reveal both passive subdiffusion of various nanoparticles
and anomalous active transport of such particles by molecular motors in the
molecularly crowded environment of living biological cells. Passive and active
microrheology reveals that the origin of this anomalous dynamics is due to the
viscoelasticity of the intracellular fluid. How do molecular motors perform in
such a highly viscous, dissipative environment? Can we explain the observed
co-existence of the anomalous transport of relatively large particles of 100 to
500 nm in size by kinesin motors with the normal transport of smaller particles
by the same molecular motors? What is the efficiency of molecular motors in the
anomalous transport regime? Here we answer these seemingly conflicting
questions and consistently explain experimental findings in a generalization of
the well-known continuous diffusion model for molecular motors with two
conformational states in which viscoelastic effects are included.
