Isolated photosystems offer the most direct connection between water photolysis and anode reduction.  Typically, photosystems are isolated and adsorbed to a conductive surface.  A soluble redox mediator (a small molecule capable of accepting and donating electrons) may be required to improve the electrical communication between photosystem and anode.  Because other cellular components required for repair are absent, biological photovoltaic systems based on isolated photosystems have relatively short lifetimes (a few hours) and often require low temperatures to improve stability.