Page "Lipid bilayer" Paragraph 53

In contrast to K < sub > a </ sub >, which is a measure of how much energy is needed to stretch the bilayer, K < sub > b </ sub > is a measure of how much energy is needed to bend or flex the bilayer.

Formally, bending modulus is defined as the energy required to deform a membrane from its intrinsic curvature to some other curvature.

Intrinsic curvature is defined by the ratio of the diameter of the head group to that of the tail group.

For two-tailed PC lipids, this ratio is nearly one so the intrinsic curvature is nearly zero.

If a particular lipid has too large a deviation from zero intrinsic curvature it will not form a bilayer and will instead form other phases such as micelles or inverted micelles.

Typically, K < sub > b </ sub > is not measured experimentally but rather is calculated from measurements of K < sub > a </ sub > and bilayer thickness, since the three parameters are related.