Light energy can be harnessed using photosynthesis for either direct generation of electricity or production of energy rich fuels such as ethanol, propanol, butanol of hydrogen in photo-bioelectrochemical cells (1). A photo-bioelectrochemical cell (PBEC) based on natural photosynthesis is composed of an anode containing a photo-biocatalyst such as photosystem II, thylakoid membrane, or whole cell microorganism such as cyanobacteria coupled with an enzymatic cathode. With the help of light, the photo-biocatalyst on the anode oxidizes water to generate electrons and protons. On the cathode, enzymes such as laccase or bilirubin oxidase are employed to catalyze the reduction of oxygen and protons to water. When such anodes are coupled with cathodes containing enzymes such as hydrogenase or nitrogenase, the system generates hydrogen. In either case, water and light are the only raw materials used to generate electricity or hydrogen. The photo-biocatalysts on the anode utilize the photosynthetic light reactions to generate electrons and deliver them to the anode via either direct or mediated electron transfer. The electron transfer mechanisms of the light reactions determine the pathways for electron delivery to the anode, which in turn determines the photocurrent density, stability and overall conversion efficiency of the process. This talk will overview our research over the years on the understanding of the photosynthetic light reactions and the underlying pathways through which the electrons are channeled to the anode.