Electron Transport Chain
Cellular respiration and the steps of the electron transport chain (ETC) allow animal and plant cells to produce usable energy.
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The electron transport chain is a series of protein complexes embedded in the mitochondrial membrane. Electrons captured from donor molecules are transferred through these complexes.
Coupled with this transfer is the pumping of hydrogen ions. This pumping generates the gradient used by the ATP synthase complex to synthesize ATP.
The following complexes are found in the electron transport chain: NADH dehydrogenase, cytochrome b-c1, cytochrome oxidase, and the complex that makes ATP, ATP synthase.
In addition to these complexes, two mobile carriers are also involved: ubiquinone, and cytochrome c.
Other key components in this process are NADH and the electrons from it, hydrogen ions, molecular oxygen, water, and ADP and Pi, which combine to form ATP.
At the start of the electron transport chain, two electrons are passed from NADH into the NADH dehydrogenase complex. Coupled with this transfer is the pumping of one hydrogen ion for each electron
Next, the two electrons are transfered to ubiquinone. Ubiquinone is called a mobile transfer molecule because it moves the electrons to the cytochrome b-c1 complex.
Each electron is then passed from the cytchrome b-c1 complex to cytochrome c. Cytochrome c accepts each electron one at a time. One hydrogen ion is pumped through the complex as each electron is transfered.
The next major step occurs in the cytochrome oxidase complex. This step requires four electrons. These four electrons interact with a molecular oxygen molecule and eight hydrogen ions. The four electrons, four of the hydrogen ions, and the molecular oxygen, are used to form two water molecules. The other four hydrogen ions are pumped across the membrane.
This series of hydrogen pumping steps creates a gradient. The potential energy in this gradient is used by ATP synthase to ATP from ADP and inorganic phosphate. The ATP synthesis steps you see here are discussed in greater detail in the ATP sythase gradients animation.
This animation illustrates two full cycles of electron donation. In biological systems, however, many electron transport cycles occur simultaneously–helping to ensure that the proton gradient is always maintained.
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