Photosynthetic reactions in plants, algae and cyanobacteria are driven by photosystem I and II complexes, which specifically reduce or oxidize partner redox biomolecules. Photosynthetic complexes can also bind synthetic organic molecules, which inhibit their photoactivity and can be used both to study the electron transport chain, and as herbicides and algicides. Thus, their development, characterization and sensing bears fundamental and applied interest. Substantial efforts have been devoted to developing photosensors based on photosystem II to detect compounds that bind to the plastoquinone sites of the complex. In comparison, photosystem I based sensors have received less attention and could be used to identify novel substances displaying phytotoxic effects, including those obtained from natural product extracts. We have developed a robust procedure to functionalize gold electrodes with photo-and redox-active photosystem I complexes based on transparent gold and a thiolate self-assembled monolayer, and we have obtained reproducible electrochemical photoresponses. Chronoamperometric recordings with millisecond resolution have allowed to measure photocurrents in the presence of the viologen derivative paraquat. We have modeled their time course to identify the main electrochemical processes and limiting steps in the electron transport chain. Our results allow isolating the contributions from photosystem I and the redox mediator, and evaluating photocurrent features (spectral and power dependence, fast transient kinetics) that could be used as a sensing signal to detect other inhibitors and modulators of photosystem I activity.