In this report, we conceptually distinguish direct electron transfer (DET) from mediator-involved (mediated) electron transfer (MET) in a glucose/oxygen-based fuel cell (FC) using an electrode potential/Fermi energy diagram. The anodic and cathodic overvoltages deviating from the equilibrium potential (the Fermi energy of redox electrons) were taken into account for the organic/inorganic redox couple and the mental experiments were performed during the trip of redox electrons through the interface between the anodic/cathodic organic/inorganic active mass and electrodes to propose electron transfer pathway. The proposed schema (inequality (MET) and equality in Fermi energy (DET)) should be experimentally corroborated by measurement of the electromotive force (emf). The MET is of technological significance in the presence of an electron mediator of the redox couple, despite a slightly narrower emf estimated between two electrodes by roughly 1 to 2 mV at most than the DET, in view of the thermodynamic and electro-kinetic viewpoints.