Unknown author · Public domain

A German-British biochemist who worked out the citric acid cycle — the sequence of chemical reactions by which nearly every living cell converts food and oxygen into usable energy — a discovery still taught in every biology class as the Krebs cycle. It won him the 1953 Nobel Prize in Physiology or Medicine. He later identified the related urea cycle and, with Hans Kornberg, the glyoxylate cycle found in plants and bacteria, and spent his final years at Oxford.