  We consider a ubiquitous scenario where a fluctuating, semipermeable vesicle
is embedded in solution while enclosing a fixed number of solute particles. The
swelling with increasing number of particles or decreasing concentration of the
outer solution exhibits a continuous phase transition from a fluctuating state
to the maximum-volume configuration, whereupon appreciable pressure difference
and surface tension build up. This criticality is unique to
particle-encapsulating vesicles, whose volume and inner pressure both
fluctuate. It implies a universal swelling behavior of such vesicles as they
approach their limiting volume and osmotic lysis.
