= Mitochondrial respiratory chain
<NADH> supplies two <Electrons> to <respiratory complex I>, then <ubiquinone>, <respiratory complex III>, <cytochrome c> and <cytochrome c oxidase>; terminal <oxygen> <reduction> forms <water>. <Succinate dehydrogenase>, or <respiratory complex II>, supplies another entry to <ubiquinone>. <Respiratory complex I>, <respiratory complex III> and <cytochrome c oxidase> move <protons> outward; <respiratory complex II> does not.<Biology 2e by Mary Ann Clark and others>[§7.3, Citric Acid Cycle, succinate oxidation; §7.4, Figure 7.12, Complex I, Q and Complex II, Complex III and Complex IV: routes and proton transport]{ref}
\Image[/mitochondrial-electron-and-proton-routes.png]
{title=<Electron> and <proton> routes in the <mitochondrial respiratory chain>}
{description=Original qualitative schematic. Blue arrows follow reducing equivalents; orange arrows denote outward <proton> transfer and green arrows denote return through <ATP synthase>. The <inner mitochondrial membrane> separates the <mitochondrial matrix> from the <intermembrane space>. Arrow widths encode no <reaction rates> or <stoichiometry>. The chemical <protons> consumed in forming <water> are distinct from pumped <protons>; the sketch omits transport of <ADP> and <inorganic phosphate>.}
For an original <Electron> balance, supply exactly $6\,\mathrm{\mu mol}$ of <NADH> to an ideal preparation with no other <electron donor> or competing <electron acceptor>. The donor supplies $12\,\mathrm{\mu mol}$ of <Electrons>. Complete <reduction> of one <oxygen> <molecule> needs four <Electrons>, so the corresponding <oxygen> consumption is $3\,\mathrm{\mu mol}$. Adding the two formal <half-reactions> makes the result explicit:
$$
\begin{aligned}
2\,\mathrm{NADH}&\longrightarrow2\,\mathrm{NAD}^{+}+2\,\mathrm H^{+}+4e^-,\\
\mathrm O_2+4\,\mathrm H^{+}+4e^-&\longrightarrow2\,\mathrm{H_2O},\\
2\,\mathrm{NADH}+\mathrm O_2+2\,\mathrm H^{+}&\longrightarrow2\,\mathrm{NAD}^{+}+2\,\mathrm{H_2O}.
\end{aligned}
$$
The cancelling <Electrons> determine the donor-to-<oxygen> ratio. These chemical equations do not count transmembrane <protons> or determine an <ATP> yield. Chemical <protons> consumed in <water> formation and transported <protons> have different roles in the ledger.
Back to article page
