Metabolic Pathways and Regulation · USMLE Step 1

Why is it important that glycolysis and gluconeogenesis are not active simultaneously in the same cell?

  1. Because both pathways share enzymes and would compete
  2. To prevent a 'futile cycle' that wastes ATP
  3. Because gluconeogenesis consumes oxygen while glycolysis produces it
  4. Because one occurs only in mitochondria and the other in cytosol
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Correct answer: To prevent a 'futile cycle' that wastes ATP

If glycolysis and gluconeogenesis occurred simultaneously, substrates would cycle without net gain and consume energy (ATP) — a wasteful futile cycle. Regulation ensures only one pathway predominates under given conditions.

Difficulty: Medium Question 8 of 19

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