Glycolysis is an almost universal central pathway of glucose metabolism, the pathway with the largest flux of carbon in most cells, and occurs in the cytosol. The glycolytic breakdown of glucose is the sole source of metabolic energy in some mammalian tissues and cell type (erythrocytes, renal medulla, brain and sperm, for example). Glycolysis differs among species only in the details of its regulation and in the subsequent metabolic fate of the pyruvate formed

In glycolysis a molecule of glucose is degraded in a series of enzyme-catalyzed reactions to yield two molecules of the three-compound pyruvate. During the sequential reactions of glycolysis, some of the free energy released from glucose is conserved in the form of ATP and NADH.

The two phases of glycolysis:

-Preparatory phase: From glucose to glyceraldehyde 3-P. Phosphorylation of glucose at an expense of two ATPs and its conversion to glyceraldehyde 3-P;

-Payoff phase: From glyceraldehyde 3-P to pyruvate. Oxidative conversion of glyceraldehyde 3-phosphate to pyruvate and the coupled formation of ATP and NADH. For each molecule of glucose that passes through the preparatory phase, two molecules of glyceraldehyde 3-phosphate are formed, both pass through the payoff phase.

Energetics:

For each glucose molecule, two ATP are consumed in the preparatory phase and four ATP are produced in the payoff phase, giving a net yield of two ATP per molecule of glucose converted to pyruvate.

glucose + 2 NAD + 2 ADP + 2 Pi 2 pyruvate + 2 NADH + 2H+ + 2 ATP + 2 H2O

Remeber that each NADH produced is oxyzed in the electron transport chain coupled to the production of 3ATPs.

Glycolysis releases only a small fraction of the total available energy of the glucose molecule. It yields only 5,2% of the total energy that can be released from glucose by complete oxidation. The two molecules of pyruvate formed by glycolysis still contain most of the chemical potential energy of the glucose molecule. The major fates of the pyruvate formed are the Citric Acid Cycle (aerobic) and Fermentation (anaerobic).