Mitochondria tightly coupled

Protoiis are extruded from the mitochondrion at the sites of NADH dehydrogenase, at the cytochrome b/c complex, and at cytochrome c oxidase. The exact number of protons driven out per electron at each of these steps has been difficult to determine. The ratio can vary under different experimental conditions. Extrusion of a proton at any particular step does not seem to be tightly coupled to the passage of an electron down the respiratory chain however, it is generally accepted that the passage of two electrons down the entire respiratory chain results in the translocation of 12 protons. 

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