A series of key cellular chemical reactions starting and ending with oxaloacetic acid. Also called the citric acid – or tricarboxylic acid – cycle, it produces energy in the form of ADENOSINE TRIPHOSPHATE (ATP) and is the last stage in the biological oxidation of fats, proteins, and carbohydrates. Named after Sir Hans Krebs, a German biochemist working in England in 1900, who won the Nobel Prize for his discovery.