Before revision:  To know more about the external force and GTP concentration dependent MT growth properties , and for the sake of  experimental verification of this two-state model, eight {\it critical forces} are defined and numerically discussed.
After revision:  To know more about the external force and GTP-tubulin concentration dependent properties of MT growth , and for the sake of experimental verification of this two-state model, eight {\it critical forces} are defined and numerically discussed.