An MFC consists of an anode, a cathode, a proton or cation exchange membrane and an electrical circuit. In the left compartment, anode-respiring bacteria (like Geobacter) attach themselves to the anode, forming a sticky residue or biofilm.
In the course of their metabolic activity, these bacteria strip electrons from organic waste. The electrons then flow through a circuit to the cathode, producing electricity in the process, in addition to CO2 and water. Hydroxide or OH- ions are transported from the cathode into the surrounding electrolyte.
Note: in an experimental set-up lacking organic waste, an electron donor -- typically acetate -- is supplied in the growth medium, as a nutrient source for the anode respiring bacteria.