Taking these points into account, the rich depolymerization behavior of actin filaments arises from its dependency not only on the monomer (ATP-actin) concentration, as for a simple supramolecular polymer, but also on the age of the filament (degree of hydrolysis), the concentration of Pi and the different rates of dissociation of Pi and ADP-actin at the barbed and pointed ends. Thus, once depolymerization starts, it can proceed extremely rapidly as the ADP-Pi-actin or ADP-actin units, which dissociate more rapidly than ATP-actin, 35 reach the filament ends. On the other hand, while the filament is growing, the polymer remains remarkably resistant to degradation, despite the hydrolysis of internal monomer units.