Figure 1. Schematic of PS-I in cellular membrane and in two types of biophotovoltaic cells.

(a) PS-I in ∼30Å thick cellular bilipid membrane (grey). Arrows indicate direction of electron travel with acceptor side facing down. The core subunits are shown in grey and the only prosthetic groups are the core electron transport associated cofactors including the P700 chlorophyll (Chl) dimer in the center, the four associated Chl a molecules (green), the two phyloquinone acceptors (orange), and the three FeS centers Fx, Fa and Fb (yellow) (sulfur) and brown (Fe). The ribbon diagram of stromal subunits PsaD, PsaC, and PsaE is shown protruding outside of the membrane and colored blue, red, and purple respectively. (b) The natural redox mediators cytochrome c and ferredoxin are absent, replaced by Z813 electrolyte and either a TiO₂ nanocrystalline sintered paste (left) or ZnO nanowires (right). Left: stabilized PS-I physisorbed to TiO₂ on fluorine-doped tin oxide (FTO) coated glass. Right: (bioengineered) PS-I self-assembled in the presence of an overabundance of PsaE-ZnO subunit on ZnO nanowires grown on ITO glass. In both cases, energy levels are matched to favor electron transfer from electrolyte to photoanode⁵,⁶,⁷,⁸.