= Citric acid cycle carbon and carrier balance
The <carbon> sequence is <oxaloacetate> $(4)$ plus an <acetyl group> $(2)$ to <citrate> $(6)$, then <isocitrate> $(6)$, <alpha-ketoglutarate> $(5)$, <succinyl-CoA> $(4)$, <succinate> $(4)$, <fumarate> $(4)$, <malate> $(4)$ and back to <oxaloacetate> $(4)$. The numbers count the cycling <carbon> skeleton, excluding <coenzyme A>. Two <carbon dioxide> releases accompany the changes $6\to5\to4$.<Biology 2e by Mary Ann Clark and others>[§7.3, Steps in the Citric Acid Cycle, steps 1–8 and Figure 7.11]{ref}
\Image[/citric-acid-cycle-carbon-and-carriers.png]
{title=<Carbon> counts and <electron carrier> outputs around the <citric acid cycle>}
{description=Original schematic of one forward turn. The <carbon> skeleton excludes <coenzyme A>. Labels show <NADH>, <carbon dioxide>, <GTP> and <ubiquinol> outputs. Positions and arrow lengths carry no structural or kinetic meaning.}
Per <acetyl group>: three <NADH>, one <FADH2> equivalent, one <GTP> or <ATP> by <substrate-level phosphorylation>, and two <carbon dioxide>. <Succinate dehydrogenase> retains <FADH2> and transfers its <Electrons> to <ubiquinone>, forming <ubiquinol>. The <FADH2> equivalent and <ubiquinol> count the same <Electron> pair.<Biology 2e by Mary Ann Clark and others>[§7.3, Citric Acid Cycle and Steps in the Citric Acid Cycle, steps 3–8, especially step 6, enzyme-bound reduced carrier; §7.4, Q and Complex II: ubiquinone reduction to QH2]{ref}
A useful check adds the stepwise changes. The <carbon> count is $4+2-1-1=4$; the <NADH> count is $1+1+1=3$. Each <NADH> carries two <Electrons>, and the <succinate dehydrogenase> route supplies another pair, so one turn exports $3\times2+2=8$ <Electrons>. This arithmetic is an accounting identity, not an <ATP> yield calculation. Converting it to <ATP> requires the separate <energy coupling> and transport assumptions discussed under <oxidative phosphorylation>.
For a chosen ideal conversion of $5\,\mathrm{\mu mol}$ of <acetyl groups>, multiplication gives $15\,\mathrm{\mu mol}$ of <NADH>, $5\,\mathrm{\mu mol}$ of <ubiquinol>, $5\,\mathrm{\mu mol}$ of <GTP> and $10\,\mathrm{\mu mol}$ of <carbon dioxide>. The <Electron> output is $40\,\mathrm{\mu mol}$. If all these <Electrons> subsequently reach <oxygen> with no competing sinks, four <Electrons> per <oxygen> <molecule> require $10\,\mathrm{\mu mol}$ of <oxygen>. That is consumption by the connected <electron transport chain>; it is not an <oxygen> term in the cycle's own <substrate> ledger.
No cycle step directly consumes molecular <oxygen>. In ordinary aerobic mitochondrial operation, however, the <mitochondrial respiratory chain> must reoxidize <NADH> and <ubiquinol> to restore the oxidized <electron carriers>. Without that outlet, a finite oxidized-carrier pool cannot sustain continued turnover. Other <electron acceptors> and other forms of <metabolism> require their own analysis.<Molecular Biology of the Cell, 4th edition by Bruce Alberts and others/How Cells Obtain Energy from Food>[Molecular Biology of the Cell, 4th edition by Bruce Alberts and others; Chapter 2, How Cells Obtain Energy from Food, The Citric Acid Cycle Generates NADH by Oxidizing Acetyl Groups to CO2: lack of direct O2 consumption and need to regenerate NAD+]{ref}
The <pyruvate dehydrogenase complex> acts upstream: one three-<carbon> <pyruvate> gives one two-<carbon> <acetyl group>, one <carbon dioxide> and one <NADH>. These are additional to the cycle ledger.<Biology 2e by Mary Ann Clark and others>[§7.3, Breakdown of Pyruvate, steps 1–3 and Figure 7.10]{ref} Consequently, processing one <pyruvate> through both stages gives three <carbon dioxide> and four <NADH>, while the <succinate dehydrogenase> contribution and the <substrate-level phosphorylation> contribution each remain one. Counting the upstream step inside the cycle would obscure where the extra <carbon dioxide> and <NADH> arise.
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