Questions: Glycolysis

Question 1

Glycolysis invests 2 ATP in the first phase and produces 4 ATP in the second phase. What is the net ATP yield per glucose molecule?

• A. 4 ATP
• B. 6 ATP
• C. 2 ATP
• D. 0 ATP

Answer: (C)

Net yield = ATP produced minus ATP invested = 4 − 2 = 2 ATP. The investment phase uses 2 ATP to phosphorylate glucose, making it more reactive. The payoff phase then produces 4 ATP by substrate-level phosphorylation. The gross yield of 4 is often confused with the net yield of 2 — always subtract the investment.

Question 2

Glycolysis requires oxygen to produce ATP from glucose.

• T. True
• F. False

Answer: False

Glycolysis is entirely anaerobic — it occurs in the cytoplasm without any oxygen. Oxygen is only required for oxidative phosphorylation in the mitochondria (the downstream stage). Glycolysis predates the evolution of mitochondria and is the universal, oxygen-independent ATP-generating pathway shared by nearly all living cells.

Question 3

Why does lactic acid fermentation regenerate NAD⁺, and why is this necessary for glycolysis to continue producing ATP in the absence of oxygen?

Model answer: During glycolysis, NAD⁺ accepts electrons to become NADH. When oxygen is unavailable, the electron transport chain cannot reoxidize NADH back to NAD⁺. Fermentation (e.g., reduction of pyruvate to lactate) serves as an alternative electron sink, regenerating NAD⁺ so that glycolysis can continue. Without NAD⁺ regeneration, glycolysis would halt and the cell would have no ATP source.

The key insight is that fermentation does not produce ATP — it is solely a recycling mechanism for NAD⁺. Glycolysis requires NAD⁺ at step 6 (glyceraldehyde-3-phosphate dehydrogenase) to proceed. NADH accumulates if not reoxidized, depleting the NAD⁺ pool and stalling glycolysis. Fermentation restores NAD⁺ by offloading electrons onto pyruvate, allowing glycolysis to continue as the cell's sole anaerobic ATP source.