you may find some helpful information on this issue in the following publications addressing mathematical modeling of E Coli walks (http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0018623) and transport along roots and higher plant organs (Bohmann et al., 2013: Modelling transport processes in tissues and organs at a mesoscopic scale, Proceedings of the 7th International Conference on Functional-Structural Plant Models, Saariselkä, Finland).