Microscopic ?bubbles? made from the fabric of cell walls can be trained to swim away from sugar, according to research reported in the 21 June issue of Physical Review Letters by physicists from Germany and the USA. The bubbles are like empty cells, and move despite the fact that they are devoid of life.

Single-celled organisms such as bacteria typically propel themselves around by waving their arms about. More accurately, they twirl long, floppy appendages called flagella, which act as a kind of corkscrew propeller to power the cells through water. Bacteria will often use their flagella to drive them towards food such as sugar or amino acids, the constituents of proteins. They can sniff out the direction in which the ?smell? gets stronger - that is, the direction in which the concentration of the attractant substance increases.

But Erich Sackmann from the Technical University of Munich and colleagues have now made lifeless, imitation cells that can sense a change in sugar concentration and move accordingly, even though these pseudo-cells have no flagella and no ?noses? - they are just empty bags.

These artificial cells are called vesicles, and they are made of the same sort of molecules that constitute the walls of cells: soap-like molecules called phospholipids. These molecules have a tadpole-like head-and-tail shape, and possess the useful property that, when in water, they will cluster together of their own accord into sheets. In such a sheet, the molecules sit side by side with all the heads in the same direction; and two such sheets stick back to back to form the fabric of cell membranes. Vesicles are formed when these double-layer sheets curl up into spheres - so they are rather like bubbles, except that they are filled with water and are only a few micrometres across, about the same size as living cells.

Cells themselves use even smaller vesicles as wrapping-paper for transporting around parcels of chemicals. In a living cell, these little parcels are carried along molecular tracks by tiny protein motors. But Sackmann and colleagues have shown how to send a vesicle-encased special delivery from one destination to another using only dissolved sugar to guide the movement.

It works like this. The double-layered phospholipid membranes are permeable to water but not to larger molecules like sugar. So if there is more dissolved sugar on one side of the membrane than the other, water will flow through it in the direction of the higher concentration to even out the difference - just as air will flow from a region of high to low pressure. This is called osmosis, and it is how flower stems stay rigid: the plant cells contain sugar, and so they suck up water and swell like balloons.

Sackmann and colleagues saw that vesicles would move away from a region of high sugar concentration. They reasoned that, because of osmosis, the vesicles would pump water towards the increasing sugar concentration - that is, from the end furthest from the sugar source to the end closest to it. This flow of liquid creates a recoil in the vesicle itself, just as the gas streaming from a jet engine drives it in the opposite direction. Like a tiny creature fleeing from some deadly influence, the vesicle retreats from the source of sugar. It is a very simple way of extracting mechanical work from a chemical system, and might turn out to be useful for marshalling chemicals in microscopic production plants.