= Respiration and ATP coupling
{title2=$\mathrm{ADP}+P_i\longrightarrow\mathrm{ATP}$}
<Glycolysis> converts <glucose> into two <pyruvate> <molecules>, with a net two <ATP> and reduced <NADH>; it does not directly consume <molecular oxygen>. <Pyruvate> processing and the <citric acid cycle>, also called the <TCA cycle>, release <carbon dioxide> and supply reduced carriers. Thus releasing <carbon dioxide> and consuming <molecular oxygen> occur at different stages.<Biology 2e by Mary Ann Clark and others>[Biology 2e, §7.2, Outcomes of Glycolysis; §7.3, Breakdown of Pyruvate and Citric Acid Cycle]{ref}
\Image[/cellular-respiration-carbon-redox-energy-flow.png]
{title=<Cellular respiration>: separate <carbon>, <Electron> and <energy> accounts}
{description=Original pathway schematic. Blue arrows trace the fuel pathway and <carbon dioxide> release; purple arrows indicate reduced-carrier delivery and terminal <molecular oxygen> <reduction>; orange dashed arrows denote <energy coupling>. Carrier chemistry and intermediate steps are abbreviated. Arrow widths encode no rates or yields.}
An <electron transport chain> reoxidizes carriers such as <NADH>. Coupling some of its <redox> steps to <proton> transport establishes an <electrochemical gradient>; <ATP synthase> can use that gradient to form <ATP> from <adenosine diphosphate> and <inorganic phosphate>. This is <oxidative phosphorylation>. A separate route, <substrate-level phosphorylation>, transfers a <phosphate> group from a reacting <substrate> to <adenosine diphosphate>. In typical <eukaryotic> <biological cells>, the respiratory chain occupies the inner membrane of a <mitochondrion>.<Biology 2e by Mary Ann Clark and others>[Biology 2e, §7.1, Substrate Phosphorylation; §7.4, Electron Transport Chain and Chemiosmosis, Figures 7.12–7.14]{ref}
The useful achievement is controlled transfer: fuel <oxidation> can replenish an <ATP> supply that supports <active transport> and synthesis. <ATP> is a reusable chemical carrier, not newly created <energy>; <ATP> <hydrolysis> and the coupled cellular process have their own net <free energy> balance. Some fuel <energy> is dissipated as <heat>.<Biology 2e by Mary Ann Clark and others>[Biology 2e, §7.1, ATP in Living Systems and Energy from ATP; §7.4, ATP Yield]{ref}
There is no universal yield of $36$ or $38$ <ATP> per <glucose>. Actual <ATP> recovery depends on carrier transfer routes, coupling and diversion of intermediates to other <metabolism>. In particular, an <oxygen> uptake rate cannot be converted into an <ATP> production rate by assuming one fixed multiplier.<Biology 2e by Mary Ann Clark and others>[Biology 2e, §7.4, ATP Yield: shuttle differences and use of intermediates]{ref}
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