The other widely noted feature of actin filaments is their unusual disassembly kinetics when compared with simple supramolecular polymers. The rate constants of association (kon) and dissociation (koff) of a simple supramolecular polymer are directly linked by the corresponding equilibrium constant but disassembly of aged actin filaments can occur more rapidly than would be predicted by consideration of koff of the ATP-actin monomer unit when the system is deprived of ATP. To qualitatively explain this observation, we must depart from our simple model in which the bulk filament is composed solely of ATPactin and note that ATP-actin hydrolysis is thought to take place with equal rate throughout the filament. Thus, an actin filament is actually composed of a random arrangement of ATPactin and hydrolyzed monomer units ( Figure ), the exact proportion of which will depend on the "age" of the filamentthe probability of a monomer unit being hydrolyzed depends on how long it has been embedded in the filament.