Hans Krebs, a German biochemist who fled to the UK after Nazi persecution for his Jewish origin, describes in 1937 at the University of Sheffield the complete sequence of chemical reactions by which cells oxidize nutrients to extract energy: the citric acid cycle, also known as the Krebs cycle. It explains how the breakdown products of carbohydrates, fats, and proteins converge into a common circular metabolic pathway inside mitochondria, releasing energy at each turn and regenerating the initial molecules. Fritz Lipmann, also a German exile, discovers in 1945 coenzyme A, essential for the products of fat and sugar digestion to enter the Krebs cycle, acting as a universal molecular carrier of acetyl groups in cellular metabolism. Together, the Krebs cycle and Lipmann's coenzyme A form the central core of cellular bioenergetics.