Hydo-osmotic effects in cells
We are motivated by an organelle called the Contractile Vacuole Complex. This is an organelle that is based on membrane tubes that acts as a pump to regulate cell volume in certain small organisms. We build a theoretical model of a membrane tube with trans-membrane ion pumps and water channels. The ion pumps actively consume energy to transport ions against their concentration gradients, this leads to a swelling of the tube by osmosis.
As the tube swells beyond a critical radius, the geometry becomes unstable to fluctuations and an instability develops. The instability is somewhat similar in nature to a Rayleigh instability on a membrane tube, driven by a sudden increase in surface tension. However, unlike the Rayleigh instability, where the size of fastest growing mode is of the order of the tube radius, the wavelength of these hydro-osmotic instabilities is much longer and depends (weakly) on dynamical parameters - the product of the ion pumping rate to a timescale for viscous dissipation of fluid inside the tube. The wavelength that is selected, rather robustly, is in good agreement with the size of structures found in the contractile vacuole complex of Paramecium. Thus, we propose a new class of osmotic instabilities, with general applicability within Biology, in which the process is continuously driven, rather than suddenly imposed.
- Caption to figures: Fig 1 Showing the difference in wavelengths of the hydro-osmotic instability when compared to the classical Rayleigh instability.
- Publication: S. Al Izzi, G. Rowlands, P. Sens and M. S. Turner, Phys Rev Lett (in press).