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Glycolysis consists of an energy-requiring phase and an energy-releasing phase. In the energy-requiring phase, two ATP molecules are consumed to split glucose into two glyceraldehyde-3-phosphate molecules. The energy-releasing phase then catabolizes these intermediates to produce four ATP, two NADH, and two pyruvate molecules, yielding a net gain of two ATP.

The net yield of glycolysis is two ATP molecules, two pyruvate molecules, and two NADH molecules. Although four ATP are produced during the energy-releasing phase, two ATP are consumed in the energy-requiring phase, resulting in a net gain of only two ATP per glucose molecule.

Glycolysis extracts less than a quarter of the energy stored in glucose. In the presence of oxygen, pyruvate enters mitochondria for further oxidation to maximize energy extraction. Inside the mitochondria, pyruvate is converted to acetyl-coenzyme-A through oxidation production acetyl coa mitochondria, which then enters the citric acid cycle to generate additional NADH molecules.

In anaerobic conditions, cells rely on glycolysis as their primary ATP source. These cells use fermentation to convert NADH back into NAD+, which is required to continue glycolysis. Mature mammalian red blood cells, which lack mitochondria, depend entirely on anaerobic respiration lactic acid fermentation pathways for ATP production.

NADH molecules produced during glycolysis donate their electrons to the electron transport chain during oxidative phosphorylation. This process regenerates NAD+ for continued glycolysis while producing ATP. The electron transport chain uses these electrons to drive chemiosmotic ATP synthesis, completing the oxidation of glucose.

Cancer cells and stem cells rely on aerobic glycolysis for rapid ATP generation despite oxygen availability. Additionally, mature mammalian red blood cells, which lack mitochondria, depend entirely on glycolysis for ATP production. These cells prioritize rapid ATP synthesis over maximum energy extraction from glucose.

After glycolysis, pyruvate enters mitochondria where it undergoes further breakdown through pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation. These subsequent stages extract the remaining energy from glucose, producing additional NADH and ATP molecules that complete the full oxidation of the original glucose molecule.