The first step in photosynthesis is the absorption of photons from the sun by Photosystem II.
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The photons are then used to excite an electron to the next level where it is accepted by a primary electron acceptor.
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The primary electron acceptor does not know what to do with the electron so it sends it down an electron transport chain.
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In the electron transport chain the electron is passed from protein to protein. Some of the proteins can accept less energy than those before it, so energy is lost from the electron.
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The lost energy is used to form ATP from ADP by providing the energy needed to form a third phosphorus bond.
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The electron has now reached Photosystem I where it is once again excited to a higher level by photons from the sun.
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It is passed down another electron transport chain where the energy it loses is used to form NADPH from NADP. It obtains the hydrogen from water that Photosystem II has broken up in order to keep a supply of electrons for the continuous cycle.
 
 
 

CALVIN CYCLE
LIGHT REACTION
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