In 1906, Harden and Young, in a series of classic studies on the fermentation of glucose to ethanol and CO 2 by extracts of brewer's yeast, made the observations inorganic phosphate was essential to fermentation; when the supply of phosphate was exhausted, fermentation ceased before all the glucose was used; during fermentation under these conditions, ethanol, CO 2 , and a sugar phosphate accumulated; when arsenate was substituted for phosphate, no sugar phosphate accumulated, but the fermentation proceeded until all the glucose was converted to ethanol and CO 2 . Which enzyme of glycolysis requires inorganic phosphate and, therefore, stops when no phosphate is available