Synthetic systems that transport metal ions across membranes made of water-insoluble lipid molecules are well known. The ions are transported by a carrier molecule located in the membrane. The molecule binds a metal ion from an aqueous solution on one side of the membrane and releases it on the other. The driving force behind this transport is the difference in the ion concentration between the two sides of the membrane.

The artificial membrane developed by Gust and Moore uses a very different approach. In their membrane, the transport is driven not by concentration gradients but by light. Their system consists of a bilayer lipid membrane containing “shuttle” molecules that are soluble within the membrane but not in the aqueous solution on either side.

This shuttle molecule – through the addition and removal of electrons – binds calcium ions at the outer surface of the membrane. The molecule then takes the ions across the membrane and releases them at the inner surface of the membrane. The ions, which cannot remain in the lipid environment, then enter the aqueous solution inside the cell.

The transport process is controlled by a special molecule in the membrane known as an “artificial reaction centre”. Based on molecules used in biological photosynthesis, the reaction centre molecule is placed across the membrane and donates and reabsorbs electrons at opposite ends in response to light.

‘Cellular machines’ such as this could be used as ‘nanofactories’ that transport reactants in, and products out, of a biological membrane. There may also be applications for converting solar energy into electrical current.