Gluconeogenesis is the generation of glucose from non-sugar carbon substrates such as pyruvate, lactate, glycerol, and glucogenic amino acids (primarily alanine and glutamine). The process is essentially the reversal of the glycolysis pathway. The pathway is particularly important in animals, as the production of glucose from other metabolites is necessary for use as a fuel source by the brain, testes, erythrocytes and kidney medulla, all of which can use only glucose as the energy source. Gluconeogenesis in animals is perfomed mostly in the liver. The pathway is carried by cytoplasmic enzymes to the level of glucose-6-phosphate. Since free glucose tends to freely diffuse out of the cell, actual glucose formation is carried out in the lumen of the endoplasmic reticulum by gluocose-6-phosphatase . Glucose is then shuttled into the cytosol by glucose transporters. The main souces of carbon for gluconeogenesis in animals is lactate, formed in skeletal muscle cells during anaerobic glycolysis and carried to the liver in the blood stream. The conversion of glucose into lactate by glycolysis and back to glucose by gluconeogenesis is referred to as the Cori Cycle.
Pathway summary from [MetaCyc](http://biocyc.org/)