Glycolysis Pathway

Glycolysis is a metabolic pathway that converts glucose into pyruvate, releasing free energy to form ATP and NADH. It occurs in the cytosol of all cells and can function both aerobically and anaerobically.

Glycolysis is divided into two phases: the Preparatory (investment) phase, where ATP is consumed, and the Payoff phase, where ATP and NADH are generated.

In the preparatory phase, glucose is converted to glucose 6-phosphate (using ATP and hexokinase/glucokinase), then to fructose 6-phosphate, and finally to fructose 1,6-bisphosphate (using ATP and phosphofructokinase). This molecule then splits into dihydroxyacetone 3-phosphate and glyceraldehyde 3-phosphate, with the former quickly isomerizing to the latter. Two molecules of glyceraldehyde 3-phosphate proceed to the next phase.

The payoff phase begins with glyceraldehyde 3-phosphate being oxidized to 1,3-bisphosphoglycerate, reducing NAD+ to NADH. Then, 1,3-bisphosphoglycerate transfers a phosphate to ADP, forming ATP and 3-phosphoglycerate. This is isomerized to 2-phosphoglycerate, which is then converted to phosphoenolpyruvate. Finally, phosphoenolpyruvate transfers a phosphate to ADP, forming ATP and pyruvate.

Pyruvate's fate depends on oxygen: anaerobic conditions lead to lactic acid fermentation, while aerobic conditions lead to the citric acid cycle. Glycolysis consumes 2 ATPs and forms 4 ATPs and 2 NADH molecules (equivalent to 5 ATPs), resulting in a net energy gain of 7 ATPs per glucose molecule.