Gluconeogenesis is the process by which glucose is synthesized from non-carbohydrate precursors in the liver and kidneys. The major precursors include lactate, pyruvate, and glucogenic amino acids. This process is important for maintaining blood glucose levels during periods of fasting when glycogen stores have been depleted. Gluconeogenesis closely resembles glycolysis but bypasses its three irreversible steps through alternate enzymes. These include pyruvate carboxylase, phosphoenolpyruvate carboxykinase, and fructose-1,6-bisphosphatase. Gluconeogenesis is an energetically costly process that requires 6 molecules of ATP and 2 molecules of GTP.