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Krebs cycle

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A cycle of reactions catalyzed by enzymes in which the pyruvate derived from nutrients and converted to Acetyl Coenzyme A is completely oxidized and broken down into carbon dioxide and water to produce high-energy phosphate compounds, which are the source of cellular energy


Cellular respiration is a series of metabolic processes wherein the biochemical energy is harvested from organic substance (e.g. glucose) and stored in energy-carriers (e.g. ATP) for use in energy-requiring activities of the cell. The major steps or processes of cellular respiration are (1) glycolysis, (2) Krebs cycle, and (3) oxidative phosphorylation.

The Krebs cycle is a stage of cellular respiration following glycolysis and is characterized by its decarboxylation of pyruvate. Krebs cycle is named after Hans Krebs, a British biochemist who identified it. It was also named Szent-Györgyi-Krebs cycle (although rare), after Albert Szent-Györgyi de Nagyrápolt who identified fumaric acid and other steps in what would become known as the Krebs cycle. The other names for the Krebs cycle are citric acid cycle (because the citric acid is both the first product and the final reactant) and tricarboxylic acid cycle (because of the involvement of the three carboxyl groups).

In particular, the Krebs cycle is one of the major metabolic pathways of cellular respiration. It involves a cyclic series of enzymatic reactions through which pyruvate -- converted into Acetyl Coenzyme A -- is completely oxidized to CO2. Along with this, hydrogen ion is removed from the carbon molecules, transferring the hydrogen atoms and electrons to electron-carrier molecules (e.g. NADH and FADH2), and metabolic energy to high energy bonds (e.g. ATP).

The carbon dioxide produced from the complete oxidation of pyruvate is removed from the cell into the blood. The electron and hydrogen carriers, NADH and FADH2, donate these electrons to the electron transport chain to generate ATP via oxidative phosphorylation, the final metabolic pathway of cellular respiration.

In eukaryotes the Krebs Cycle occurs in the matrix of the mitochondrion whereas in prokaryotes, it occurs in the cytoplasm.

Word origin: named after Hans Adolf Krebs, first described it in 1937.


  • citric acid cycle (CTC)
  • tricarboxylic acid cycle (TCA cycle)

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