The Krebs Cycle (Citric Acid Cycle): The Central Hub of Cellular Respiration
The Krebs cycle, also called the citric acid cycle or TCA cycle, is the central stage of aerobic cellular respiration in which acetyl-CoA is fully oxidized to carbon dioxide, harvesting energy as electron carriers. Discovered by Hans Krebs in 1937, it runs in the mitochondrial matrix and feeds the electron carriers that power most of the cell's ATP production.
The Krebs cycle (also known as the citric acid cycle or TCA cycle) is the central stage of aerobic cellular respiration, a closed loop of enzyme-catalyzed reactions that completely oxidizes a two-carbon acetyl group to carbon dioxide while capturing its chemical energy in electron-carrying molecules. It sits at the metabolic crossroads where the breakdown products of carbohydrates, fats, and proteins converge. In eukaryotic cells the cycle takes place inside the Mitochondria: The Powerhouse Organelles with Their Own DNA; in bacteria and other prokaryotes that lack mitochondria it runs in the cytosol. The fuel that enters the cycle is acetyl-CoA, derived from pyruvate produced during glycolysis as well as from fatty acid and amino acid breakdown. Acetyl-CoA combines with the four-carbon molecule oxaloacetate to form citrate, the six-carbon compound that gives the cycle its name. Across roughly eight to ten enzymatic steps, citrate is progressively rearranged and decarboxylated, eventually regenerating oxaloacetate so the loop can begin again. Each turn of the cycle (one acetyl-CoA) releases two molecules of carbon dioxide and produces three NADH, one FADH2, and one molecule of GTP (readily converted to ATP (Adenosine Triphosphate): The Universal Energy Currency of Living Cells). The cycle itself generates very little ATP directly; its real payoff is the reduced electron carriers NADH and FADH2, which deliver high-energy electrons to the The Electron Transport Chain: The Mitochondrial Assembly Line That Makes Most of Your ATP. There, through oxidative phosphorylation, those electrons drive the bulk of the cell's ATP synthesis. Because one glucose molecule yields two acetyl-CoA, the cycle turns twice per glucose. The pathway was worked out in 1937 by the German-born biochemist Hans Krebs (with William Johnson) at the University of Sheffield, work for which Krebs shared the 1953 Nobel Prize in Physiology or Medicine. Beyond energy harvesting, the cycle's intermediates serve as biosynthetic precursors for amino acids, heme, and other molecules, making it both a catabolic engine and an anabolic supply depot.