Gluconeogenesis synthesizes glucose from noncarbohydrate precursors, involving pyruvate and lactate, citric acid cycle intermediates, the carbon skeletons of most glycerol and amino acids.  This is extremely significant since the brain and erythrocytes rely almost done on glucose as their energy source under normal conditions. The liver's accumulate of glycogen is enough to supply the brain with glucose for only about half a day during fasting. Thus gluconeogenesis is mainly significant in periods of vigorous or starvation exercise. In During of starvation the formation of glucose by gluconeogenesis particularly uses amino acids from protein breakdown and glycerol from fat breakdown.  During exercise, the blood glucose stages requires for skeletal and brain muscle function is maintained   by gluconeogenesis   in the liver using lactate produced   by the muscle.

The main site of gluconeogenesis is the liver, while it also happens to a far lesser extent in the kidneys.  Very small gluconeogenesis happens in brain or muscle. In the liver cells the 1st enzyme of pyruvate carboxylase, gluconeogenesis is situated in the mitochondrial matrix. The final enzyme glucose 6- phosphatase is bound to the smooth endoplasmic reticulum and the other enzymes of the pathway are located in the cytosol.