The light reactions of photosynthesis contain two photosystems, two electron transport chains, and ATP synthase. These parts are embedded in the thylakoid membranes of a chloroplast. The photosystems contain a collection of chlorophyll molecules. Chlorophyll, which is the green pigment of leaves, absorbs light energy that excites electrons to a higher energy level. Energized electrons from photosystem I are passed down an electron transport chain and added to NADP+ to form NADPH. Electrons from photosystem II are passed down another electron chain. Their energy is used to pump hydrogen ions (H+) from the stroma into the thylakoid compartment, creating a concentration gradient. Electrons leaving this electron transport chain enter photosystem 1, refilling its lost electrons. Photosystem II replenishes its electrons by splitting water. Hydrogen ions and oxygen are released into the thylakoid compartment. This is where the oxygen gas generated by photosynthesis comes from. The hydrogen......
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