Abstract
An introductory analysis is provided for the 2-phase macroscopic kinetic model of the end of a microtubule. Some general relations are derived for 1 end of a very long microtubule in solution but the main results refer to the steady-state properties of microtubules grown on nucleated sites. The 2-phase model makes it possible to understand qualitatively how long microtubules can grow well below the citical concentration and also how grown microtubules can rapidly disappear from a nucleated site by shortening following a phase change.