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Animation: The Light Reactions

by Pearson
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The Light Reactions Light reactions are the steps in photosynthesis that occur on the thylakoid membranes of the chloroplast and that convert solar energy to the chemical energy of ATP and NADPH, evolving oxygen in the process. The key components of the light reactions of photosynthesis are the two photosystems, two electron transport chains, and ATP synthase. These parts are embedded in the thylakoid membranes of a chloroplast. The photosystems consist of arrays of chlorophyll molecules. Chlorophyll -- the green pigment of leaves -- absorbs light energy. The absorbed energy 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. Meanwhile, energized electrons from photosystem II are passed through another electron transport 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 I, replenishing 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 buildup of hydrogen ions inside the thylakoid compartment stores potential energy. This energy is harvested by an enzyme called ATP synthase. As hydrogen ions diffuse through ATP synthase, down their concentration gradient, the enzyme uses the energy of the moving ions to make ATP. Next, ATP and NADPH are used in the sugar-making process of the Calvin cycle.
The Light Reactions Light reactions are the steps in photosynthesis that occur on the thylakoid membranes of the chloroplast and that convert solar energy to the chemical energy of ATP and NADPH, evolving oxygen in the process. The key components of the light reactions of photosynthesis are the two photosystems, two electron transport chains, and ATP synthase. These parts are embedded in the thylakoid membranes of a chloroplast. The photosystems consist of arrays of chlorophyll molecules. Chlorophyll -- the green pigment of leaves -- absorbs light energy. The absorbed energy 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. Meanwhile, energized electrons from photosystem II are passed through another electron transport 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 I, replenishing 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 buildup of hydrogen ions inside the thylakoid compartment stores potential energy. This energy is harvested by an enzyme called ATP synthase. As hydrogen ions diffuse through ATP synthase, down their concentration gradient, the enzyme uses the energy of the moving ions to make ATP. Next, ATP and NADPH are used in the sugar-making process of the Calvin cycle.