FIGURE 17-8 Determination of filament formation by actin concentration. The critical concentration (C_c) is the concentration at which G-actin monomers are in equilibrium with actin filaments. At monomer concentrations below the C_c, no polymerization takes place. When polymerization is induced at monomer concentrations above the C_c, filaments assemble until steady state is reached and the monomer concentration falls to C_c.