Science, Tech, Math › Science Why Is the Krebs Cycle Called a Cycle? Share Flipboard Email Print By Narayanese, WikiUserPedia, YassineMrabet, TotoBaggins/CC BY-SA 3.0/Wikimedia Commons Science Chemistry Biochemistry Basics Chemical Laws Molecules Periodic Table Projects & Experiments Scientific Method Physical Chemistry Medical Chemistry Chemistry In Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate By Anne Marie Helmenstine, Ph.D. Chemistry Expert Ph.D., Biomedical Sciences, University of Tennessee at Knoxville B.A., Physics and Mathematics, Hastings College Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels. our editorial process Facebook Facebook Twitter Twitter Anne Marie Helmenstine, Ph.D. Updated December 10, 2019 The Krebs cycle, also known as the citric acid cycle or tricarboxylic acid cycle, is part of a series of chemical reactions that organisms use to break down food into a form of energy that cells can use. The cycle occurs in the mitochondria of cells, using 2 molecules of pyruvic acid from glycolysis to produce the energy molecules. The Krebs cycle forms (per two molecules of pyruvic acid) 2 ATP molecules, 10 NADH molecules, and 2 FADH2 molecules. NADH and the FADH2 produced by the cycle are used in the electron transport system. Why It's a Cycle The final product of the Krebs cycle is oxaloacetic acid. It is a cycle because oxaloacetic acid (oxaloacetate) is the exact molecule needed to accept an acetyl-CoA molecule and start another turn of the cycle.