A micropipette aspiration experiment on a giant unilamellar vesicle at is carried out…
PHYSICS · Multi-part · Fermi University Physics · Biophysics & Medical Physics · Macromolecules, membranes and cell architecture · Macromolecules, membranes and cell architecture
Forces, Machines and Signals, Ch The fluid membrane: bending, flicker and shape
Problem
A micropipette aspiration experiment on a giant unilamellar vesicle at 298K is carried out entirely in the low-tension, entropic-tension regime, where the area strain obeys α=8πκkBTln(σ0σ). Two points on the aspiration curve are recorded: at σ1=1.0μNm−1, α1=0.80%; at σ2=10.0μNm−1, α2=2.00%.
((a)) Use the two data points to extract the bending modulus κ of this membrane, in joules and in units of kBT.
((b)) A separate optical-tweezers pulling experiment on a single actin filament, a semiflexible polymer treated with the worm-like chain, finds a persistence length ξp=15μm. Using ξp=κf/kBT, where κf is the filament's bending stiffness, find κf in SI units.
((c)) Explain why κf from part (b) cannot be compared numerically to κ from part (a), even though both are called a bending modulus and both use the symbol κ. Name the dimensional difference and the geometric reason for it, bending a one-dimensional rod versus bending a two-dimensional sheet.
((d)) The pipette applies a real mechanical tension σ. Explain, using the idea of an entropic force, why the low-tension area response probed in part (a) is nevertheless called entropic, and why it depends on κ but not on KA.