The word glyconeogenesis literally means to form glucose again.
Animals (and humans) can make glucose from lactate, from some amino acids (see amino acid
metabolism) and from glycerol. Lactate is the end product of the glycolysis in muscles when high effort is made. The amino acids come from food (and when one is fasting from the proteins of your own muscles). Glycerol originates from fats (see fatty acid metabolism). The gluconeogenesis takes place mainly in the liver. The gluconeogenesis is important because the brain and the red blood cells strongly dependent on glucose. When someone is fasting for a period longer than one day and/or strong effort is made the gluconeogenesis will start to work.
The gluconeogenesis is no opposite glycolysis. The glycolysis has 3 irreversible reactions that in the gluconeogenesis are bypassed. All
three of these reactions are catalysed in the glycolysis by kinases. The irreversible reactions are represented below with four red arrows.
The start points for the precursors (lactate, amino acids and glycerol) are located in the gluconeogenesis at dihydroxyacetone phosphate, pyruvate and oxaloacetate.
The brain and the muscle tissue do not posses the gluconeogenesis. For the gluconeogenesis they are dependent on the liver and the kidneys that deliver the formed glucose to the blood.