Fig. 4. Dependence of MT behavior on the concentration of soluble tubulin ([Tu]_sol) available at steady-state. (A) Life-history plots of individual MTs at different values of [Tu]_soluble. * and **, average length of MTs at steady-state in 4.1 µM and 4.7 µM free tubulin, respectively. At 6.4 µM free tubulin, there is no steady-state length – the mass of polymer increases with time (MT growth is `unbound'; see also supplementary material Fig. S1); (B) relationship between [Tu]_soluble and the mean MT length at steady-state. (C) Relationship between persistent growth and [Tu]_soluble. The data plotted on the y-axis give average rates of increase in polymer length as a function of [Tu]_soluble. The concentration of free tubulin required for the transition to persistent growth is indicated by the x-axis intercept of the dashed line. In B and C, error bars give the standard deviation of values observed from 50 different simulations at the indicated [Tu]_soluble. Notice that, in all three panels, the MTs do not compete with each other for free tubulin (because free tubulin is held constant at the indicated value), and the cell size is made so large so that no MTs interact with the edge during the course of the simulation. This is similar to an experimental situation in vitro in which the pool of free tubulin is not depleted during the time course of the experiment.