The researchers studied how light affects electron transport in bean chloroplasts. They found that the rate of electron transport decreases when chloroplasts are illuminated without certain molecules. This decrease is due to feedback inhibition controlled by the internal pH of the thylakoid. Adding certain molecules prevents this decrease. Uncouplers increase electron transport rate, while adding specific molecules causes light-driven inhibition. The regulation of electron transport occurs during plastoquinol oxidation by photosystem 1. Light also slows the reduction of a spin label, which interacts with semiquinone in photosystem 2. DCMU inhibits photosystem 2 at two points. The study discusses how chloroplasts regulate energy transmission through a proton gradient.