To go with the flow: Molecular motors are a drag

In this thesis I study the influence of the cytosol on organelle transport in the eukaryotic cell. In particular, I am interested in the phenomena cytoplasmic and axoplasmic streaming. In both these phenomena, many organelles are transported in the same direction over relatively long distances. This molecular cargo transport is powered by molecular motors. These molecular motors are transport proteins that literally walk along the cytoskeleton while carrying a cargo such as an organelle. When a cargo is dragged through the cytosol it will experience resistance in the form of an opposing fluid friction force. The magnitude of this force depends upon the size and shape of the cargo as well as on the viscosity of the fluid. The latter is a physical material property that indicates how strongly a fluid resists deformation. In the cytosol the viscosity is a factor 1000 larger than in water. This means that, in the cell, molecular motors need to deliver a much larger force to obtain the same velocity as in water. However, the motor-cargo velocities measured in vivo (in a living cell) are similar to, or even larger than, the single motor velocities from in vitro (laboratory environment) experiments in water. The goal of this thesis is to explain the underlying mechanism that makes this possible and to provide a possible explanation for cytoplasmic and axoplasmic streaming.