A schematic representation of the system
From 2007.igem.org
1) The first state of the system, where there is a concentration of metal ions on one sideof the semi solid medium. The cells(grey-not bound to metal ions) are exposed to no particular wavelength at this time. They will bind metals through the metal binding domains on their surfaces. 
2) The pink cells are those that carry metal ions on their surfaces, they are moving towards lamp 2 by phototaxis.
3) As the cells reach the opposite end the third lamp will be switched on. It will activate the bacteriorhodopsin and cause release of metal ions, so the exposed cells are releasind the metal ions at the area illuminated by this lamp.
4) After metal ions are released, lamp 2 will be switched off and lamp 1 will be switched on, so the cells will begin to go back to the original location to collect the remaining metal ions.
5) 
6) Now the cycle will start again. Because the metal binding domains are extremely sensitive to metal ions, the cells will still be able to continue collecting them, when their concentration is decreased to very low levels (even at the level of single atoms). 
