Figure 4.

The comparison of the entropic (bending) and the tensile elasticity effects in the effective stretching modulus of a filament. Logarithmic force-extension curves show the response of extensible filaments using Equation (16). Parameters correspond to those of an actin filament of the persistence length $l_p=17\mathsf{\mu }$m, Young modulus $Y=2$ GPa, filament diameter 7 nm, and several values of natural length L labelled in the plot. The crossover regime starts at $f_c$, when the ’linear-entropic’ response becomes ’nonlinear-entropic’ (as in Figure 3). The width of this crossover regime is determined by the ratio of $k_m/k_e$ (see text), after which the ’linear-mechanical’ (enthalpic) regime dominates, with the elastic modulus $k_m$.