Final answer:
The critical concentration of actin is lower at the plus end during steady state because of a higher affinity for ATP-bound actin monomers, leading to faster growth and a dynamic state known as treadmilling.
Step-by-step explanation:
The critical concentration of actin is lower at the plus end during steady state than at the minus end. Actin polymerization in cells is driven by ATP-bound G-actin monomers adding to the plus end, which grows faster than the minus end where ADP-bound monomers tend to dissociate.
This difference is due to the non-equilibrium nature of polymerization, where ATP is rapidly hydrolyzed to ADP after monomers join the polymer, thus causing the monomers to have a weaker binding at the minus end. This dynamic behavior leads to treadmilling, where there is concurrent growth at the plus end and shrinkage at the minus end, which keeps the total filament length constant while allowing movement.