Here we neglected the Gaussian bending energy term because we only considered a closed vesicle of spherical topology and the Gaussian bending energy is invariant under deformations of the membrane that retains the topology. The pressure difference between the interior and exterior of the vesicle was assumed to be a constant and denoted by p, and the volume enclosed by the vesicle was denoted by V . For an axisymmetric membrane, the shape is expressed as