In cytosolic glycolysis, what is the NET number of ATP molecules produced per molecule of glucose converted to two molecules of pyruvate (ignoring mitochondrial shuttle costs)?

Difficulty: Easy

Correct Answer: 2

Explanation:


Introduction / Context:
ATP accounting in glycolysis is essential for understanding cellular energy balance. This question asks for the net ATP yield per glucose in the cytosol before any mitochondrial oxidative phosphorylation or shuttle adjustments.


Given Data / Assumptions:

  • Pathway: glucose → 2 pyruvate.
  • No consideration of NADH oxidation in mitochondria (malate-aspartate or glycerol phosphate shuttles not counted).
  • Standard cytosolic reactions in eukaryotic cells.


Concept / Approach:
Two ATP are invested in the preparatory phase (hexokinase and phosphofructokinase-1). Four ATP are generated in the payoff phase (phosphoglycerate kinase and pyruvate kinase, each twice per glucose). Net ATP = 4 produced − 2 consumed = 2 ATP.


Step-by-Step Solution:
1) Investment: -1 ATP at hexokinase (glucose → G6P) and -1 ATP at PFK-1 (F6P → F1,6BP).2) Payoff: +2 ATP at phosphoglycerate kinase (1,3-BPG → 3-PG, twice).3) Payoff: +2 ATP at pyruvate kinase (PEP → pyruvate, twice).4) Net: 4 − 2 = 2 ATP per glucose.


Verification / Alternative check:
Two cytosolic NADH are also formed; their ATP equivalence depends on the shuttle used, but that does not change the glycolytic net ATP count of 2.


Why Other Options Are Wrong:
4: gross production, ignores the 2 ATP investment.1 or 3: do not match standard stoichiometry.


Common Pitfalls:
Confusing gross vs net ATP, or double-counting the investment phase.


Final Answer:
2

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