1953: Sir Hans Krebs
Krebs (MRC Cell Metabolism Research Unit) uncovered the citric acid cycle, also known as the tricarboxylic acid cycle, the TCA cycle, or the Krebs cycle – a series of chemical reactions that takes place in most plants, animals, fungi and many bacteria.
In organisms that have an oxygen-based metabolism, the citric acid cycle reactions reactions involve the breakdown of proteins, fats and carbohydrates into much smaller molecules. The products of this breakdown are then used as building materials for the cell. The cycle results in the liberation of carbon dioxide and electrons that are immediately used to form high-energy phosphate bonds in the form of adenosine triphosphate (ATP) - the chemical energy reservoirs of the cell.
Krebs discovered how certain individual reactions are linked to each other in a cyclic process and how energy is released by this process for use by the cell for all its activities. He proposed the key elements of this pathway in 1937. He was awarded the 1953 Nobel Prize in Physiology or Medicine for this work.