Glycolysis

Glycolysis takes place in the cytoplasm of essentially every cell and consists of ten enzymatic steps split into two phases. The first is an investment phase that consumes 2 ATP to phosphorylate glucose and commit it to the pathway. The second is a payoff phase that generates 4 ATP by substrate-level phosphorylation and reduces 2 NAD⁺ to NADH. The net result per glucose is 2 ATP, 2 NADH, and 2 pyruvate.

Three steps are irreversible and therefore regulated. Hexokinase — or glucokinase in liver and pancreatic beta cells — traps glucose in the cell as glucose-6-phosphate. Phosphofructokinase-1 (PFK-1) is the rate-limiting enzyme and the main control point: it is inhibited by ATP and citrate, signals that the cell has ample energy, and activated by AMP and by fructose-2,6-bisphosphate. Pyruvate kinase catalyzes the final step. These three steps are bypassed by separate enzymes during gluconeogenesis, since the reactions cannot simply run backward.

What happens to pyruvate depends on oxygen and on the cell type. When oxygen is available, pyruvate enters the mitochondrion, is converted to acetyl-CoA, and feeds the citric acid cycle and electron transport chain, ultimately yielding far more ATP. Without adequate oxygen, pyruvate is reduced to lactate by lactate dehydrogenase, which regenerates the NAD⁺ that glycolysis needs to keep running. That regeneration is why anaerobic glycolysis can continue at high rates during intense exercise, and why cells lacking mitochondria — mature red blood cells, for instance — depend on glycolysis entirely.

Glycolysis is heavily tested. USMLE Step 1 covers its fundamentals and regulation in the biochemistry section, the MCAT includes it under bioenergetics and fuel molecule metabolism alongside gluconeogenesis, and the CSCS exam approaches it as the glycolytic energy system supplying activity lasting roughly 30 seconds to two minutes. Knowing the irreversible enzymes, the net yield, and the PFK-1 regulators covers most questions.