A schematic of the electron transfer routes of Shewanella oneidensis MR-1. (i) MET, the electron transfer driven by a mediator-used process; red, reduced form; ox, oxidized form. (ii) DET, the electron transfer involving c-type cytochrome. (iii) Flavins, the electron transfer depends on the self-secreted flavins conveying electrons inside the metabolism to the anode. Microbes take up substrate generating carbon dioxide and protons. This process yields electrons for metabolic benefit, that is, growth, and reduces Medox in the cytosol into Medred. Medred diffuses into contact with the electrode, where Medred reduces the electrode, generating electrical current. The oxidized form, Medox, diffuses back through the anolyte for reuse by the microbes. Periplasm only exists in Gram-negative bacteria such as S. oneidensis MR-1.