Like other fuel cells, biological photovoltaic systems are divided into anodic and cathodic half-cells.
Oxygenic photosynthetic biological material, such as purified photosystems or whole algal or cyanobacterial cells, are employed in the anodic half-cell.  These organisms are able to use light energy to drive the oxidation of water, and a fraction of the electrons produced by this reaction are transferred to the extracellular environment, where they can be used to reduce an anode.  No heterotrophic organisms are included in the anodic chamber - electrode reduction is performed directly by the photosynthetic material.
The higher electrode potential of the cathodic reaction relative to the reduction of the anode drives current through an external circuit.  In the illustration, oxygen is being reduced to water at the cathode, though other electron acceptors can be used.  If water is regenerated there is a closed loop in terms of electron flow (similar to a conventional photovoltaic system), i.e. light energy is the only net input required for production of electrical power.  Alternatively, electrons can be used at the cathode for electrosynthetic reactions that produce useful compounds, such as the reduction of protons to hydrogen gas.