Nonequilibrium fluctuations, traveling waves, and instabilities in active membranes (original) (raw)
The stability of a flexible fluid membrane containing a distribution of mobile, active proteins (e.g., proton pumps) is shown to depend on the structure and functional asymmetry of the proteins. A stable active membrane is in a nonequilibrium steady state with height fluctuations whose statistical properties are governed by the protein activity. Disturbances are predicted to travel as waves at sufficiently long wavelength, with speed set by the normal velocity of the pumps. The unstable case involves a spontaneous, pump-driven undulation of the membrane, with clumping of the proteins in regions of high activity. 82.65.Dp The functioning of active proteins in energy-dissipating processes, such as ion transport, protein translocation, and biopolymer synthesis, generates forces on the membranes of the living cell and its organelles . As the active proteins diffuse around in the membrane, the resulting fluctuations in this force provide a nonthermal source of noise for shape fluctuations of the membrane. The membranes of a living cell are therefore nonequilibrium or active membranes. Although such active, nonequilibrium processes are abundant in biological membranes, physicists have focused mainly-with considerable success [3,4]-on the statistical mechanics of membranes at thermal equilibrium. There are however reasons [5,6] to suspect that nonequilibrium processes are at work even in red-bloodcell flicker, traditionally explained as thermal equilibrium shape fluctuations . The predictions of fluctuation enhancement in active membranes and the micropipette experiments [10] on membranes laden with the photoactive proton pump bacteriorhodopsin (bR) are further motivation for our studies.
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