The oxidation of organic material produces both protons and electrons. The electrons are removed instantaneously via the conductive biofilm and the electrical circuit of the MFC. The larger protons have to migrate out of the biofilm to the cathode. This occurs at a much slower rate and may cause a bottleneck inhibiting power production. For every electron produced in the form of current, a proton is also produced within the biofilm (Figure 3). Using a modified MFC and a fluorometric pH sensitive dye, a proton gradient was observed across the biofilm between the anode surface and the bulk fluid .