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Pairs of laser light beams known as "optical tweezers" are being used to manipulate artificial bilayer lipid membranes--simplified artificial versions of the membranes which surround living cells and their subunits. The optical tweezers technique can create forces suddenly, at precise locations, offering the first opportunity to study the millisecond dynamics of micron-scale structures. Moreover the available range of forces (billionths of a Newton) and surface tensions (thousandths of a dyne/cm) is squarely in the range relevant for processes seen in living cells. The ability to induce by physical means sudden changes of topology and shape in membrane structures may help us to understand a host of processes in real cells, for example membrane fusion or the budding of vesicles from the Golgi apparatus, a membranous structure assisting chemical synthesis within cells.
Roy Bar-Ziv and Elisha Moses (Weizmann Institute, Israel) have seen a "pearling instability" in hollow tubular vesicles about a micron (the size of a typical bacterium) in radius. Producing a shape that resembles a long string of beads, the instability propagates outward from the point of laser illumination and persists after laser shutoff. The pictures above show the formation of the pearling instability (a) before, (b) during, and (c) after the laser beam is turned on.
Download an MPEG movie of the Pearling Instability (80 K)
Requires MPEG Viewer; Free Viewers available from MPEG.ORG Site
For more information and references see http://dept.physics.upenn.edu/~nelson. (Courtesy of Philip Nelson, University of Pennsylvania, nelson@hollebeek.hep.upenn.edu)