Scanning tunnelling spectroscopy of single intact Photosystem II molecules was used to directly show that the reaction centre behaves as an organic semiconductor and photovoltaic. The main electron conduction mechanisms observed were (a) semiconduction associated with the normal step-wise linear forward electron transfer pathway OEC-Y-z-P680-pheo-Q(A)-Q(B), (b) a photovoltaic effect, (c) tunnelling to surface vibronic and delocalized states, and (d) hopping conduction between polypeptide residues. Parameters for these conduction processes were obtained for both dark and illuminated conditions. The photovoltaic behaviour of the reaction centre contrasts with the photoconductive behaviour of the associated light-harvesting complex LHC-II.