The second stage in the breakdown of glucose (unprocessed sugar) into energy (ATP), carbon dioxide (as waste), and water Citric acid entering the Krebs cycle as a product of glycolysis (first stage) exits as NADH (reduced nicotinamide adenine dinucleotide), one molecule of GTP (guanosine triphosphate), and one molecule of FADH2 (reduced lavin adenine dinucleotide). NADH and FADH2 give their high-energy electrons to an electron transport system (third stage) that makes many ATP molecules. Named after German chemist Hans Krebs, who discovered the cycle in 1937. See Glycolysis, Carbohydrate Catabolism.