A slightly more detailed description is needed to account for one of the most well-known features of actin filament behavior, "treadmilling"; under certain conditions, monomer units are added to one end of a filament and lost from the other without any overall change in length, resulting in the filament appearing to move across a surface. We start by noting that the two ends of the filament are inequivalent due to the non-planar ATP-actin monomer. Thus, new monomer units can be added to either the "barbed" or "pointed" end of the filament ( Figure ). The distinguishable ends could lead to the simple but incorrect conclusion that ATP-actin monomers bind to the two ends with different association constants. However, as identified by adding the ATP-actin monomer to either end of the filament results in indistinguishable products -(ATP-actin)n→(ATP-actin)n+1 in both casesand so the association constants are thermodynamically required to be the same.