Lecture #3 Notes

Nevertheless in the xed ensemble the total material

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Unformatted text preview: hich we will take as the bending energy since we are assuming inextensibility. Nevertheless, in the fixed γ ensemble, the total material area fluctuates as lipids are expelled and readsorbed. Thus the partition function is e−H/kB T , Z (γ, Ap ) = (2) C where the sum is performed over all possible configurations with fixed Ap , but any A. The free energy in this case is then Go (γ, Ap ) = −kB T ln Z (γ, Ap ), and the membrane mechanical tension is (3) 3 τ = lim Ap →∞ Go (γ, Ap ) . Ap (4) In the case of an isolated, unframed membrane, (A, τ )-ensemble, we can find the relevant thermodynamic potential by applying Legendre transforms stepwise, and transform the (γ, Ap )-ensemble first to the (A, Ap )-ensemble, then from the (A, Ap )-ensemble to the (A, τ )-ensemble. The first transform is effected by F (A, Ap ) = Go (γ, Ap ) − γA. (5) The value of γ that fixes the specific value of A is found implicitly by solving ∂F/∂γ = 0, or A = (∂G/∂γ )Ap . The next Legendre transformation is Gi (A, τ ) = F (A, Ap ) − τ Ap , (6) ∂F ∂Ap (7) where τ is found from ∂Gi /∂Ap , or τ= . A What is the meaning of tension τ for an isolated, unframed membrane patch? The mean projected area is can be found from ∂F/∂Ap = 0. If F (A, Ap ) has a minimum for 0 < Ap /A < 1, then τ = 0. Although the membrane is shrunk due to thermal fluctuations, it is nonetheless a flat, extended object and has τ = 0 since ∂F ∂Ap = 0. (8) Ap = Ap Only when Ap = 0 can τ = 0. B Energetics Now consider the bending part of the energy Hb . The bilayer material as a wide aspect ratio, that is, its thickness is about 4-5nm and much smaller than the lateral size of the surfaces of interest. Therefore, we can treat the bending deformations of the membrane with ideas from plate or thin shell theory. Figure 3: Typical constituents of lipid bilayers A standard model for this is one first written by Canham called the Helfrich free energy density: Hb [S] = κ(S) (C (S) − C0 (S))2 + κG (S)K (S), 2 (9) 4 which is a local energy density due to local bilayer bending at surface position S. In (9), C (S) − C0 (S) is the mean curvature (measured from the midplane of the lipid) at S, K (S) is the local...
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This note was uploaded on 11/14/2013 for the course MATH 201 taught by Professor Tomchou during the Fall '12 term at UCLA.

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