The above 16-image sequence shows the expulsion of an
inner object (medium-sized circle) from an artificial version of a cell membrane (large
circle). The image should be viewed left to right, row by row. Scale bar is 10 microns, or 10
millionths of a meter. The small dark circle is simply another
object that has attached itself to the inner object, and is not believed to play any
role in the expulsion process. (Image courtesy Roy Bar-Ziv and Elisha Moses, Weizmann Institute of Science)
Cell membranes are the thin coverings which surround living cells and
help protect them from their surroundings. A cell membrane is made of lipid molecules that
arrange themselves into a closed, sac-like structure (called a vesicle)
to prevent energetically unfavorable contact between water and the water-repelling lipid
tails. For this reason, it's difficult to rip open a vesicle,
let alone expel interior objects.
Researchers at the Weizmann Institute of Science in Israel
and the University of Pennsylvania have discovered that lasers can cause artificial
versions of cell membranes to expel inner objects as large as 3/4 their diameter. The researchers focus a laser spot onto a simplified
artificial version of a vesicle (one made by using the majority lipid component found in natural membranes, but
leaving out some additional lipids and proteins).
In the experiment, the light's electric field pulls lipid into the spot and thereby places the vesicle under tension. To increase the entropy (amount of disorder) in the system,
water rushes into the vesicle to disperse the smaller structures, driving out an inner object.
A small pore opens in the membrane to allow the object to escape,
but this pore seals after the object has exited, with water replacing the volume vacated
by the object. In addition to providing insights into the complex forces within cells,
manipulating membranes with lasers may someday allow researchers to transform living cell membranes in desired ways.
This research is reported by J. David Moroz, Philip Nelson, Roy Bar-Ziv, and
Elisha Moses in the 13 January 1997 issue of Physical Review Letters.
To download a preprint of the paper (entitled "Spontaneous Expulsion of Giant Lipid Vesicles Induced by Laser Tweezers," in Postscript format) and
for other information and references go to http://dept.physics.upenn.edu/~nelson.
Link to related Physics News Update Item on the research
