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The following complexes are found in the photosynthesis electron transport chain:
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and the complex that makes ATP
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In addition to the complexes,
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three mobile carriers are also involved:
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Other key components include:
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and the electrons to form NADPH
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which combine to form ATP
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Photosynthesis occurs in the chloroplasts
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of plants and algae.
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The process is also found in single-cell organisms
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such as cyanobacteria
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that do not have chloroplasts.
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Like its mitochondrial counterpart,
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the chloroplast electron transport chain
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consists of several protein complexes and mobile electron carriers.
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First, a photon of light hits a chlorophyll molecule
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surrounding the Photosystem II complex.
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This creates resonance energy that is transferred through neighboring chlorophyll molecules.
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When this energy reaches the reaction center embedded in photosystem II,
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an electron is released.
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The reaction center chlorophyll contains electrons that can be transferred when excited.
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One photon is needed to excite each of the electrons in this chlorophyll.
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Once excited, two electrons are transferred to plastoquinone Qb, the first mobile carrier.
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In addition to the two electrons,
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Qb also picks up two protons from the stroma.
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The two electrons lost from photosystem II
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are replaced by the splitting of water molecules.
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Water splitting also releases hydrogen ions into the lumen.
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This contributes to a hydrogen ion gradient
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similar to the one created by mitochondrial electron transport.
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After two water molecules have been split, one molecule of molecular oxygen is created.
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Plastoquinone Qb then transfers the two electrons
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to the cytochrome b6-f complex.
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The two protons it picked up
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are released into the lumen.
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These transfers are coupled with the pumping of two more hydrogen ions into the lumen space
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by cytochrome b6-f.
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The electrons are next transferred to plastocyanin, another mobile carrier.
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Next,
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the electrons are transferred from plastocyanin
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to the Photosystem I complex.
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It is here that photons again energize each electron and propel their transfer to ferredoxin.
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Ferredoxin then transfers the electrons
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to the ferredoxin-NADP-reductase,
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also known as FNR.
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After two electrons are transfered to FNR,
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NADPH is made by adding the two electrons
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and a hydrogen ion to NADP+.
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The gradient created by the electron transport chain
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is utilized by ATP synthase
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to create ATP from ADP and Pi.
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This is similar to the way ATP is synthesized in the mitochondria.
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ATP, NADPH, and molecular oxygen are the final, vital, products of photosynthesis.