Hans Adolf Krebs (1900 –1981) was awarded half the 1953 Nobel Prize in Physiology or Medicine for his discovery of the citric acid cycle, which is also known as the Krebs Cycle. Krebs's area of interest was intermediary metabolism, in which energy is extracted from cellular nutrients and used to build cellular components. Among the subjects he studied were the synthesis of urea in the mammalian liver, the synthesis of uric acid and purine bases in birds, the intermediary stages of the oxidation of foodstuffs within cells, the mechanism of active transport of electrolytes, and the relations between cell respiration and the generation of adenosine polyphosphates. His 1957 book, Energy Transformations in Living Matter, was the first major publication on the thermodynamics of biochemical reactions. Also, the appendix contained the first-ever published thermodynamic tables of equilibrium constants and Gibbs free energy of chemical species’ formation.