Photosystem I (PSI) is a key component of the photosynthetic electron transport system in plants. It functions to transfer electrons from chlorophyll to an electron acceptor in a reaction called photooxidation. Subsequent redox reactions culminate in the reduction of NADP+ to generate NADPH, which is required for CO2 fixation by the Calvin–Benson cycle. Paraquat is an herbicide that prevents reduction of NADP+ by accepting electrons from intermediate reductants in PSI. It also generates reactive oxygen species that are toxic to cells, thus killing the plant. Paraquat was used extensively in the 1980s as an herbicide in North and South America, primarily to destroy illegal marijuana crops by aerial spraying. In some cases, the herbicide-sprayed marijuana crops were harvested and sold in the United States, which led to a public outcry over fears that paraquat was causing human health problems as a result of people smoking paraquat-contaminated plant matter.