Biological photovoltaic systems that employ whole organisms are the most robust type, and lifetimes of multiple months have been observed.  The insulating outer membranes of whole cells impedes electron transfer from the sites of electron generation inside the cell to the anode.  As a result, conversion efficiencies are low unless lipid-soluble redox mediators are included in the system. Cyanobacteria are typically used in these systems because their relatively simple arrangement of intracellular membranes compared to eukaryotic algae facilitates electron export. Potential catalysts such as platinum can be used to increase permeability of the cellular membrane.