A device that exploits biological reactions for the conversion of biomass (chemical energy) to electricity (electrical energy). Microbial fuel cells harness the respiratory apparatus of microorganisms to generate electrons, whereas enzymatic fuel cells use enzymes as catalysts. The fuel is oxidized by oxygen from air by the living cells or enzymes, which are in close contact with the anode of the fuel cell. The anode conducts the electrons through a circuit to the cathode. One potential application is the generation of electricity from industrial waste and sewage. Microbial electrolysis cells can produce hydrogen, a useful fuel. All are classified as bioelectrochemical systems.