Apentix/AP Biology/Unit 3: Cellular Energetics/Question
AP Biologymediummcq1 pt

Human muscle cells running low on oxygen rely on lactic acid fermentation. What problem does this pathway solve?

A.It regenerates NAD+ from NADH so glycolysis can keep producing ATP by substrate-level phosphorylation.
B.It extracts far more ATP per glucose than aerobic respiration does.
C.It supplies the oxygen needed for the final step of the electron transport chain.
D.It converts lactate directly into ATP within the muscle fiber.

Explanation

Core Concept

Glycolysis requires NAD+ at the glyceraldehyde 3-phosphate dehydrogenase step, and low oxygen blocks the electron transport chain that normally reoxidizes NADH. Lactate dehydrogenase transfers electrons from NADH to pyruvate, forming lactate and freeing NAD+ to serve again. Fermentation itself yields no ATP; its entire benefit is keeping glycolysis, the cell's remaining ATP source, running.

Correct Answer

AIt regenerates NAD+ from NADH so glycolysis can keep producing ATP by substrate-level phosphorylation.

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