Which of the following glycolytic reactions constitutes a major thermodynamic driving force for glycolysis in vivo

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The question asks which glycolytic reaction provides a major thermodynamic driving force in vivo. 
Option 1: Glucose 6-phosphate to fructose 6-phosphate is an isomerization step with a small, near-equilibrium ΔG, so it does not provide a strong drive to push glycolysis forward.
Option 2: Dihydroxyacetone phosphate to glyceraldehyde 3-phosphate is an interconversion that helps balance triose phosphate pool, but its ΔG is not highly exergonic; it mainly shuffles carbon between intermediates and does not serve as a large energy-driving step.
Option 3: Glyceraldehyde 3-phosphate + phosphate + NAD+ to 1,3-bisphosphoglycerate + NADH + H+ is the GAPDH reaction. While it is energetically favorable and important for generating NADH and a high-energy acyl phosphate, it is not the overwhelmingly large driving force that pushes flux through glycolysis the most; its ΔG, though negative, is moderated by cellular metabolite levels and enzyme coupling.
Option 4: 2-Phosphoglycerate to phosphoenolpyruvate + water is the enolase-catalyzed step, which is highly favorable but not as exergonic as the final step; its ΔG helps but does not constitute the dominant driving force when glycolysis proceeds under typical cellular conditions.
Option 5: Phosphoenolpyruvate + ADP to pyruvate + ATP is the pyruvate kinase step, a substrate-level phosphorylation that yields ATP directly and involves a highly negative ΔG in vivo. This large, exergonic move strongly propels glycolysis forward, providing the main thermodynamic pull through the pathway and helping to couple glycolytic flux to energy production.
In summary, while several steps contribute to the overall energetics of glycolysis, the final ATP-generating conversion of phosphoenolpyruvate to pyruvate (with ADP to ATP) is the dominant thermodynamic driving force that keeps glycolysis proceeding rapidly in living cells.

BIOL3611.13541.202610 In ClassQuizMet1

Which of the following glycolytic reactions constitutes a major thermodynamic driving force for glycolysis in vivo

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