Lipids contain electrical dipoles in their headgroups. In the figure below, the dipoles are further apart in the upper monolayer than the lower monolayer when the membrane is curved. The differential dipole density induces a net polarization and consequently a transmembrane electrical potential. This phenomenon is formally defined as the flexo-electric effect, first envisioned for liquid crystals. The reverse effect is equally compelling: a membrane will bend in response to an applied transmembrane electrical potential. Thus, curvature and electrical potential are strongly coupled in lipid systems.