Figure 14.

Fusion probability as a function of molecular area $\overline{A}$ for (a) the 14-nm and (b) the 28-nm vesicles. In both cases, the fusion probability, which represents the fraction of fusion attempts that lead to fusion within 20 μs, exhibits a maximum at ${\overline{A}}_{\text{max}}$ with 1.45 < ${\overline{A}}_{\text{max}}$ < 1.5 in panel a and ${\overline{A}}_{\text{max}}$ ≃ 1.5 in panel b corresponding to the tensions $\overline{\Sigma }$ ≃ 3.36 and $\overline{\Sigma }$ ≃ 4.25, respectively. At higher tensions, fusion becomes less likely because of membrane rupture; at lower tensions, fusion is more and more replaced by adhesion or hemifusion. A linear extrapolation of the data to smaller values of $\overline{A}$ indicates a molecular area threshold for fusion at ${\overline{A}}_{\text{th}}$ = 1.29 for the 14-nm and ${\overline{A}}_{\text{th}}$ = 1.36 for the 28-nm vesicle. This corresponds to a tension threshold ${\overline{\Sigma }}_{\text{th}}$ ≃ 0.56 for the 14-nm vesicle and ${\overline{\Sigma }}_{\text{th}}$ ≃ 1.79 for the 28-nm vesicle.