  Electric pulses in biological cells (action potentials) have been reported to
be accompanied by a propagating cell-surface deformation with a nano-scale
amplitude. Typically, this cell surface is covered by external layers of
polymer material (extracellular matrix, cell wall material etc.). It was
recently demonstrated in excitable plant cells (Chara Braunii) that the rigid
external layer (cell wall) hinders the underlying deformation. When the cell
membrane was separated from the cell wall by osmosis, a mechanical deformation,
in the micrometer range, was observed upon excitation of the cell. The
underlying mechanism of this mechanical pulse has up to date remained elusive.
Herein we report that Chara cells can undergo a pearling instability, and when
the pearled fragments were excited even larger and more regular cell shape
changes were observed (about 10 to 100 um in amplitude). These transient
cellular deformations were captured by a curvature model that is based on three
parameters: surface tension, bending rigidity and pressure difference across
the surface. In this paper these parameters are extracted by curve-fitting to
the experimental cellular shapes at rest and during excitation. This is a
necessary step to identify the mechanical parameters that change during an
action potential.
