Thylakoids and photosystem I (PSI) reaction centers were imaged by scanning tunneling microscopy. The thylakoids were isolated from spinach chloroplasts, and PSI reaction centers were extracted from thylakoid membranes. Because thylakoids are relatively thick nonconductors, they were sputter-coated with Pd/Au before imaging. PSI photosynthetic centers and chemically platinized PSI were investigated without sputter-coating. They were mounted on flat gold substrates that had been treated with mercaptoacetic acid to help bind the proteins. With tunneling spectroscopy, the PSI centers displayed a semiconductor-like response with a band gap of 1.8 eV. Lightly platinized (platinized for 1 hr) centers displayed diode-like conduction that resulted in dramatic contrast changes between images taken with opposite bias voltages. The electronic properties of this system were stable under long-term storage.The photosynthetic apparatus in green plants and algae is contained in a unique cellular organelle, the chloroplast. The chloroplast is enclosed by a double membrane and contains thylakoids, which consist of stacked membrane disks (grana) and unstacked membrane disks (stromal region). Although scanning tunneling microscopy (STM) images of disrupted chloroplasts have been reported (1, 2), functional characterization of single isolated reaction centers has not yet been achieved. Thylakoids play an important role in electron transport during photosynthesis. With STM and scanning tunneling spectroscopy (STS), it is possible to investigate their operational electrooptical properties.Photosystem I (PSI) reaction centers are embedded in the thylakoid membrane. They drive the light-dependent transfer of electrons from plastocyanin (a copper-containing soluble protein located in the luminal space of chloroplast thylakoids) to ferredoxin (a [2Fe-2S]-containing soluble protein located in the chloroplast stroma). The PSI complex contains two high molecular mass subunits, the products of the psaA and psaB genes (the gene products for the chlorophyll a protein of PSI), and many low molecular mass subunits (3, 4). The PSI reaction center measured by electron microscopy is about 10 nm x 15 nm (5-7), or 7 nm x 12 nm after correction for attached detergent. Alekperov et al. (8) used STM to image photosynthetic reaction centers of purple bacteria and used the tip to transfer molecules or clusters of molecules from one area to another within the scanning range.The attachment of platinum on the reducing side of PSI has a significant effect on the electrical properties of PSI. This can be observed by transformed photobiocatalytic properties and a sustained steady-state vectorial flow of current (9, 10). In this paper, we describe the use of tunneling spectroscopy to characterize the electrical nature of bare and platinized PSI. For semiconductors, tunneling spectroscopy has been used to characterize surface states and to measure surface-state band gaps (11,12). We show that a band gap can be clearly seen in bare PSI, and platinized PSIs...