10 - Statistical mechanics

Summary

Nanoparticles can be used to probe the intracellular environment. By tracking their motion one can draw conclusions regarding transport inside a cell.

An investigator has placed a nanoparticle of diameter 100 nm inside of a Xenopus oocyte. The cytoplasm of this cell behaves like a viscous fluid with a viscosity 20 times that of water. Over a period of 20 s (at a temperature of 18 °C), the particle travels (on a somewhat erratic path) over a distance of approximately 3 μm from the periphery of the cell toward the nucleus in the center of the cell.

The investigator concludes that there is a preferential motion or “flow” from the periphery of the cell toward the nucleus. He would now like to plan a full study to examine what causes this “flow.” Does this seem like a reasonable next step? If so, justify why. If not, explain what next step you would suggest.

Fibrinogen has a diffusion coefficient in saline of approximately 2 × 10^–7 cm²/s at 25 ˚C. It is a rod-shaped molecule whose length is roughly 10 times its radius. Estimate the length of this molecule.