The<unk> , also known as the tricarboxylic acid cycle (TCA cycle)  or the Krebs cycle, (On rare occasions the<unk>  is known by a fourth name, the Szent-Györgyi-Krebs cycle) is a series of enzyme-catalysed chemical reactions of central importance in all living cells that use oxygen as part of cellular respiration. In eukaryotes, the<unk>  is located in the matrix of the mitochondrion. The components and reactions of the<unk>  were established by seminal work from both Albert Szent-Györgyi and Hans Krebs.
In aerobic organisms, the<unk>  is part of a metabolic pathway involved in the chemical conversion of carbohydrates, fats and proteins into carbon dioxide and water to generate a form of usable energy. Other relevant reactions in the pathway include those in glycolysis and pyruvate oxidation before the<unk> , and oxidative phosphorylation after it. In addition, it provides precursors for many compounds including some amino acids and is therefore functional even in cells performing fermentation.
Two carbons are oxidized to CO2, and the energy from these reactions is transferred to other metabolic processes by GTP (or ATP), and as electrons in NADH and FADH2. The  NADH generated in the TCA cycle may later donate its electrons in oxidative phosphorylation to drive ATP synthesis; FADH2 is covalently attached to succinate dehydrogenase, an enzyme functioning both in the TCA cycle and the mitochondrial electron transfer chain in oxidative phosphorylation. FADH2 thereby facilitates transfer of electrons to coenzyme Q, an intermediate in the electron transfer chain.
The<unk>  is continuously supplied new carbons in the form of acetyl-CoA, entering at step 1 below.
Mitochondria in animals including humans possess two succinyl-CoA synthetases, one that produces GTP from GDP, and another that produces ATP from ADP. Plants have the type that produces ATP (ADP-forming succinyl-CoA synthetase).
The GTP that is formed by GDP-forming succinyl-CoA synthetase may be utilized by nucleoside-diphosphate kinase to form ATP (the catalyzed reaction is GTP + ADP -> GDP + ATP).
Citric acid cycle
Mevalonate pathway
Fatty acid metabolism
Malate-aspartate shuttle
Beta oxidation
Chiral pool synthesis
Arndt-Eistert synthesis
Rabi cycle