The stages, in order

  1. Preparatory step: pyruvate oxidation. Before the cycle, each pyruvate loses a carboxyl group as carbon dioxide, is oxidized to an acetyl group forming NADH, and is joined to coenzyme A as acetyl CoA.
  2. Step 1: condensation. The two-carbon acetyl group combines with four-carbon oxaloacetate to form six-carbon citrate. The step is irreversible and highly exergonic, and its rate falls as ATP levels rise.
  3. Step 2: isomerization. Citrate loses one water molecule and gains another, converting it into its isomer isocitrate. No carbon is lost and no carrier is reduced in this step.
  4. Step 3: first oxidative decarboxylation. Isocitrate is oxidized to the five-carbon molecule alpha-ketoglutarate, releasing one carbon dioxide and reducing NAD⁺ to NADH. Feedback from ATP and NADH regulates the step.
  5. Step 4: second oxidative decarboxylation. Alpha-ketoglutarate is oxidized and decarboxylated to a succinyl group, releasing a second carbon dioxide and forming another NADH. Coenzyme A binds it as succinyl CoA.
  6. Step 5: substrate-level phosphorylation. A carboxyl group replaces coenzyme A, forming succinate. The energy released drives substrate-level phosphorylation to make either GTP or ATP, depending on the tissue.
  7. Step 6: dehydrogenation. Succinate is converted to fumarate and two hydrogen atoms are transferred to FAD, reducing it to FADH₂. This carrier stays bound to its membrane-embedded enzyme.
  8. Steps 7 and 8: hydration and final oxidation. Water is added to fumarate to produce malate, then malate is oxidized to regenerate oxaloacetate, forming a third NADH and closing the loop for the next turn.

A closed loop, not a line

Unlike glycolysis, the citric acid cycle is a closed loop: the last part of the pathway regenerates the compound used in the first step, so the cycle runs continuously whenever reactants are available. It takes place in the mitochondrial matrix, and almost all of its enzymes are soluble there, with succinate dehydrogenase the single exception, embedded in the inner mitochondrial membrane. The eight steps are a series of redox, dehydration, hydration and decarboxylation reactions. The name varies by textbook: citric acid cycle for the first intermediate formed, TCA cycle because citrate and isocitrate are tricarboxylic acids, and Krebs cycle after Hans Krebs.

Where students go wrong

The biggest misreading is treating this as the cell's main ATP-producing step. It is not. The cycle produces very little ATP directly, just one GTP or ATP per turn, and does not consume oxygen itself. Its real output is reduced carriers, and OpenStax is explicit that the pathway only counts as aerobic because the NADH and FADH₂ it makes must hand their electrons to the next pathway, which does use oxygen. The second error is per-turn versus per-glucose accounting: one glucose yields two pyruvate, so all of these figures double when a question asks about a whole glucose molecule.

Where this leads next

The carriers made here are spent in oxidative phosphorylation, where the electron transport chain builds a proton gradient and ATP synthase converts it into ATP. That is also where the disputed ATP-per-glucose totals come from, which the cellular respiration equation page handles source by source. Going backwards, the acetyl CoA the cycle consumes comes from pyruvate, and pyruvate comes from glycolysis in the cytosol. The mitochondrial location also ties this page to the eukaryotic cell, since prokaryotes run the equivalent chemistry without a dedicated organelle.

Common mistakes

  • Calling the Krebs cycle the main source of ATP. It makes one GTP or ATP per turn; its important products are NADH and FADH₂.
  • Mixing per-turn and per-glucose figures. One glucose gives two pyruvate and therefore two turns, so the per-turn numbers double.
  • Saying the cycle uses oxygen. It does not consume oxygen directly, and is aerobic only because its carriers must unload onto a pathway that does.
  • Treating citric acid cycle, TCA cycle and Krebs cycle as three different pathways. They are three names for the same one.

Related concepts

Sources