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Casimir effect

Across a crowded membrane


Hendrik Casimir's 1948 prediction of fluctuation-induced forces is famously associated with the classical example of two charge-neutral conducting plates in a vacuum, mutually attracted due to the constraints they impose on the quantum fluctuations of the electromagnetic field. But Casimir's interest in the problem originally stemmed from his work on colloidal solutions — and its most recent application doesn't stray far from this soft-matter theme. Benjamin Machta and colleagues have identified a role for Casimir forces in the plasma membrane of the mammalian cell (illustrated in the computer-generated image below), in which the relevant fluctuations are not quantum mechanical, but compositional, in origin (Phys. Rev. Lett. (in the press); preprint at; 2012).

Cell membranes are two-dimensional liquids comprising thousands of proteins that form structures varying dramatically in size — some up to a hundred times the size of the proteins themselves. Energy estimates suggest that maintaining this heterogeneity comes at a hefty cost, so one might well wonder what the membrane gains in return. Some indication came with the results of experiments suggesting that membranes exist in close proximity to a miscibility critical point in the two-dimensional universality class — but the payoff was still unclear. Now, Machta et al. have shown, using conformal field theory and Monte Carlo simulations, that the fluctuations in membrane composition associated with this criticality are capable of mediating long-range Casimir forces between membrane-bound proteins.

The idea is that near a miscibility critical point, small free-energy differences between clustered and unclustered states might allow the cell to control the spatial organization of the membrane more easily. And this gives the membrane good reason to retain its costly heterogeneity. Key binding events during the process of signal transduction within the membrane often involve large-scale spatial reorganization. Perturbations to composition that disrupt this reorganization are known to affect signalling. The results of the study by Machta et al. indicate that such disruptions might act to distance the membrane from its critical point, thus interfering with the associated Casimir forces.


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Klopper, A. Across a crowded membrane. Nature Phys 8, 641 (2012).

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