Glucose from noncarbohydrate precursors

Overview Gluconeogenesis synthesizes glucose from noncarbohydrate precursors,... 

The liver uses two mechanisms for endogenous glucose production, the mobilization of intracellular glycogen (glycogenolysis) and the synthesis of glucose from noncarbohydrate precursors (gluconeogenesis). 

Gluconeogenesis, the synthesis of new glucose molecules from noncarbohydrate precursors, occurs primarily in the liver. The reaction sequence is the reverse of glycolysis except for three reactions that bypass irreversible steps in glycolysis. 

Gluconeogenesis, the process by which glucose is synthesized from noncarbohydrate precursors, occurs mainly in the liver under fasting conditions. Under the more extreme conditions of starvation, the kidney cortex also may produce glucose. For the most part, the glucose produced by the kidney cortex is used by the kidney medulla, but some may enter the bloodstream. 

Gluconeogenesis The processes by which glucose is synthesized from noncarbohydrate precursors such as glycerol, lactate, some amino acids, and (in plants) acetyl-CoA. 

The carbon skeletons used for the synthesis of glucose are not of carbohydrate origin but are derived from particular amino acids. One exception to this is the carbon skeleton of lactate, itself a product of carbohydrate metabolism, which can be incorporated into a new glucose molecule. This process, the synthesis of new glucose from essentially noncarbohydrate precursors, is called gluconeogenesis. 

In summary, glucose can be synthesized in the liver and kidney from lactate and noncarbohydrate precursors (carbons from certain amino acids). The synthesis is accomplished essentially by a reversal of the glycolytic pathway, because the cells of these tissues possess the enzymes that are necessary to overcome the three irreversible glycolytic steps. 

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