Sub-cellular fractions of photosynthetic organisms, such as purified thylakoid membranes, can also be used in biological photovoltaic systems.  A benefit of using material that contains both photosystem II and photosystem I is that electrons extracted from water by photosystem II can be donated to the anode at a more negative redox potential (from the reductive end of photosystem I).  A redox mediator (e.g. ferricyanide) is required to transfer electrons between the photosynthetic components and the anode.