Citric acid cycle
Say it: SIT-rik AS-id SY-kul
The citric acid cycle is a loop of reactions in mitochondria that breaks down acetyl groups to carbon dioxide, capturing energy as NADH and FADH2.
Also: Krebs cycle, tricarboxylic acid cycle, TCA cycle
Definition
What is the citric acid cycle?
The citric acid cycle, also called the Krebs cycle or the tricarboxylic acid (TCA) cycle, is the central pathway of aerobic respiration. In eukaryotes it runs in the mitochondrial matrix. Fuel from carbohydrates, fats and proteins enters as acetyl CoA, which carries a two-carbon acetyl group. This group joins the four-carbon molecule oxaloacetate to make six-carbon citrate, and after eight steps oxaloacetate is regenerated, closing the loop.
Each turn releases two molecules of carbon dioxide and stores energy in three NADH, one FADH2 and one ATP or GTP. Because each glucose yields two acetyl CoA, the cycle turns twice per glucose. It makes little ATP directly; its main output is the NADH and FADH2, which hand their electrons to the electron transport chain to drive oxidative phosphorylation. The cycle does not use oxygen itself, but it stops without it, because only the oxygen-dependent electron transport chain turns NADH and FADH2 back into the NAD+ and FAD the cycle needs.
Its intermediates are also drawn off to build amino acids and other molecules. Hans Krebs worked out the sequence of reactions in the 1930s.
Word parts
Where does the term “citric acid cycle” come from?
- citr-
- citron, citrus fruitLatin citrus
- cycl-
- circle, wheelGreek kyklos
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Terms that go with it
- GlycolysisGlycolysis is the ten-step metabolic pathway that splits one glucose molecule into two pyruvate molecules, giving a net gain of two ATP.
- MitochondrionA mitochondrion is a double-membraned organelle that makes most of a eukaryotic cell's ATP by breaking down fuel molecules with oxygen.
- Electron transport chainThe electron transport chain is a series of membrane proteins that pass electrons to oxygen and use the energy released to pump protons.
- Oxidative phosphorylationOxidative phosphorylation is how cells make most of their ATP, using a proton gradient built by the electron transport chain to drive ATP synthase.
- ATPATP (adenosine triphosphate) is the molecule cells use as their main short-term energy carrier, releasing energy when a phosphate group is removed.
- MetabolismMetabolism is the full set of chemical reactions in a cell or organism, both those that break molecules down for energy and those that build them up.
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Checked against
Written for biofideX in plain words, after checking these references:
- OpenStax Biology 2e, 7.3 Oxidation of Pyruvate and the Citric Acid Cycle
- MeSH (NLM): Citric Acid Cycle
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