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The electron transport chain is a series of protein complexes embedded in the mitochondrial

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membrane. Electrons captured from donor molecules are transferred through these complexes.

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Coupled with this transfer is the pumping of hydrogen ions. This pumping generates the

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gradient used by the ATP synthase complex to synthesize ATP. The following complexes are

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found in the electron transport chain, NADH dehydrogenase, cytochrome BC1, cytochrome oxidase,

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and the complex that makes ATP, ATP synthase. In addition to these complexes, two mobile carriers

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are also involved, ubiquinone and cytochrome C. Other key components in this process are

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NADH and the electrons from it, hydrogen ions, molecular oxygen, water, and ADP and PI, which

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combine to form ATP. At the start of the electron transport chain,

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Two electrons are passed from NADH into the NADH dehydrogenase complex.

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Coupled with this transfer is the pumping of one hydrogen ion for each electron.

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Next, the two electrons are transferred to ubiquinone.

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Ubiquinone is called a mobile transfer molecule because it moves the electrons to the cytochrome

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BC1 complex.

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Each electron is then passed from the cytochrome BC1 complex to cytochrome C.

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Cytochrome C accepts each electron one at a time.

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One hydrogen ion is pumped through the complex as each electron is transferred.

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The next major step occurs in the cytochrome oxidase complex.

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This step requires four electrons.

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These four electrons interact with a molecular oxygen molecule and eight hydrogen ions.

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The four electrons, four of the hydrogen ions, and the molecular oxygen

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are used to form two water molecules.

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The other four hydrogen ions are pumped across the membrane.

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This series of hydrogen pumping steps creates a gradient.

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The potential energy in this gradient is used by ATP synthase

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to make ATP from ADP and inorganic phosphate.

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The ATP synthesis steps you see here are discussed in greater detail

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in the ATP synthase gradients animation.

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This animation illustrates two full cycles of electron donation.

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In biological systems, however, many electron transport cycles occur simultaneously,

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helping to ensure that the proton gradient is always maintained.