Morphological hallmarks of eukaryotic cells characterized by micron-scale membrane curvature and septin assembly. Septins have been implicated in membrane organization events where micron-scale curvature takes place. (A) A septin ring acts as a scaffold for cytokinesis proteins and forms a diffusion barrier at the cytokinetic furrow of a dividing cell. (B) A septin ring forms a diffusion barrier at the annulus of a mammalian spermatozoon, which separates the anterior and posterior tail. (C) A septin ring forms a diffusion barrier at the base of a cilium to separate the ciliary membrane from the plasma membrane. (D) In neurons, a septin-dependent diffusion barrier can localize at the base of dendritic spine necks. (E) During phagocytosis, a cup is formed at the plama membrane; septin rings assemble at the base of the phagocytic cup to regulate entry.