How is a proton gradient generated across the thylakoid membrane? (when protons are pumped/accumulate in the lumen of the chloroplasts why are they trapped there?) What molecule donates electrons to the photosynthetic electron transport chain? What molecule is the final electron acceptor in the photosynthetic electron transport chain? Do plant cells contain mitochondria and chloroplasts? Explain. Why is excess light energy harmful to plant cells? The levels of NADPH and NADP+ in the Calvin cycle could impact the photosynthetic electron transport chain, leading to the production of reactive oxygen species that damage plant cells. Explain why certain changes in these levels can lead to oxidative stress. Compare and contrast the Calvin cycle with the light dependent reactions. Calvin cycle Light dependent reactions Location Starting substrates End products What are the end products used for?