The part I don’t get is how exactly is the movement directional. Wikipedia says that microtubules are polar, so I can see the asymmetry there. But when you look at the single step, what is there to prevent a step in a reverse direction? Gradient over a single step should be minuscule. Or there is nothing to prevent this, and we see a result of a random walk in electric gradient? Wikipedia links to https://www.ncbi.nlm.nih.gov/pubmed/10617199 but without access to the article, I’m not sure how much of the influence this effect would have, or how universal it is.