Elasticity of cell membranes

Elasticity of closed lipid vesicles

The simplest component of a membrane is the lipid bilayer which has a thickness that is much smaller than the length scale of the cell. Therefore the lipid bilayer can be represented by a two-dimensional mathematical surface. In 1973, based on similarities between lipid bilayers and nematic liquid crystals, Helfrich proposed the following expression for the curvature energy per unit area of the closed lipid bilayer

where k c , k ¯ are bending rigidities, c 0 is the spontaneous curvature of the membrane, and H and K are the mean and Gaussian curvature of the membrane surface, respectively.

The free energy of a closed bilayer under the osmotic pressure Δ p (the outer pressure minus the inner one) as:

where dA and dV are the area element of the membrane and the volume element enclosed by the closed bilayer, respectively, and λ is the surface tension of the bilayer. By taking the first order variation of above free energy, Ou-Yang and Helfrich derived an equation to describe the equilibrium shape of the bilayer as:

They also obtained that the threshold pressure for the instability of a spherical bilayer was

where R being the radius of the spherical bilayer.

Using the shape equation (3) of closed vesicles, Ou-Yang predicted that there was a lipid torus with the ratio of two generated radii being exactly 2 . His prediction was soon confirmed by the experiment Additionally, researchers obtained an analytical solution to (3) which explained the classical problem, the biconcave discoidal shape of normal red blood cells.

Elasticity of open lipid membranes

The opening-up process of lipid bilayers by talin was observed by Saitoh et al. arose the interest of studying the equilibrium shape equation and boundary conditions of lipid bilayers with free exposed edges. Capovilla et al., Tu and Ou-Yang carefully studied this problem. The free energy of a lipid membrane with an edge C is written as

where d s and γ represent the arclength element and the line tension of the edge, respectively. The first order variation gives the shape equation and boundary conditions of the lipid membrane:

where k n , k g , and τ g are normal curvature, geodesic curvature, and geodesic torsion of the boundary curve, respectively. e 2 is the unit vector perpendicular to the tangent vector of the curve and the normal vector of the membrane.

Elasticity of cell membranes

A cell membrane is simplified as lipid bilayer plus membrane skeleton. The skeleton is a cross-linking protein network and joints to the bilayer at some points. Assume that each proteins in the membrane skeleton have similar length which is much smaller than the whole size of the cell membrane, and that the membrane is locally 2-dimensional uniform and homogenous. Thus the free energy density can be expressed as the invariant form of 2 H , K , t r ( ε ) and det ( ε ) :

where ε is the in-plane strain of the membrane skeleton. Under the assumption of small deformations, and invariant between t r ε and − t r ε , (10) can be expanded up to second order terms as:

where k d and μ are two elastic constants. In fact, the first two terms in (11) are the bending energy of the cell membrane which contributes mainly from the lipid bilayer. The last two terms come from the entropic elasticity of the membrane skeleton.