A cell is deprived of oxygen and is forced to shift to anaerobic glycolysis. Which of the following is a byproduct of this process?

Core Concept

PILLAR 1 — MOLECULAR/CONCEPTUAL MECHANISM

Step-by-Step Analysis

When molecular oxygen (O₂) becomes unavailable to a eukaryotic cell, the electron transport chain (ETC) embedded in the inner mitochondrial membrane grinds to a halt. Complex IV (cytochrome c oxidase) can no longer transfer electrons to O₂, the terminal electron acceptor, which means the proton gradient across the inner membrane dissipates and ATP synthase ceases rotating. The immediate biochemical crisis, however, occurs upstream: NADH generated during glycolysis and the Krebs cycle cannot be oxidized back to NAD⁺ via oxidative phosphorylation. Glycolysis requires two molecules of NAD⁺ per glucose molecule to accept electrons at the glyceraldehyde-3-phosphate dehydrogenase (GAPDH) step, where glyceraldehyde-3-phosphate is phosphorylated and oxidized to 1,3-bisphosphoglycerate. Without NAD⁺ regeneration, glycolysis would arrest after just a few turnovers, depleting the cell's remaining ATP and threatening survival.

Why Other Options Are Wrong

Lactic acid fermentation solves this thermodynamic bottleneck by diverting pyruvate—the three-carbon end-product of glycolysis—away from the mitochondrial matrix and into the cytosol, where the enzyme lactate dehydrogenase (LDH) catalyzes its reduction. The hydride ion (H⁻) carried by NADH is transferred to the ketone carbonyl carbon (C₂) of pyruvate, converting it into the hydroxyl group of lactate. Concretely, NADH donates two electrons to the carbonyl carbon, and a proton from the surrounding aqueous solution protonates the oxygen, yielding lactic acid (or its deprotonated conjugate base, lactate, at physiological pH). This electron transfer oxidizes NADH back to NAD⁺, which re-enters glycolysis and sustains the modest yield of two net ATP per glucose via substrate-level phosphorylation at the phosphoglycerate kinase and pyruvate kinase steps. The entire fermentative bypass occurs in the cytosol, requires no membrane compartmentalization, and preserves the redox balance necessary for glycolytic flux under anaerobic conditions.

PILLAR 2 — STEP-BY-STEP LOGIC

The question stem identifies a specific physiological transition: the cell is deprived of oxygen and must shift to anaerobic glycolysis. This wording signals that glycolysis continues but that the pyruvate generated can no longer be oxidized via the pyruvate dehydrogenase complex and the Krebs cycle. Instead, pyruvate becomes the substrate for LDH. The question then asks for a byproduct—meaning a compound produced incidentally as a consequence of the anaerobic metabolic rerouting, rather than the primary energy-harvesting goal (ATP).

Tracing the carbon atoms: one glucose (six carbons) is split by hexokinase, phosphofructokinase-1, and aldolase into two molecules of glyceraldehyde-3-phosphate, each of which proceeds through the GAPDH reaction to produce pyruvate. Under aerobic conditions, pyruvate enters the mitochondrion. Under the anaerobic constraint described, LDH reduces both pyruvate molecules to lactate while oxidizing two NADH molecules back to NAD⁺. Lactic acid is therefore synthesized anew—it does not exist in the pathway prior to the anaerobic shift—and constitutes the metabolic byproduct the question demands. The correct choice is C.

PILLAR 3 — DISTRACTOR ANALYSIS

Option A (ATP) ensnares students who conflate the primary product of glycolysis with the byproduct of the anaerobic detour. ATP is indeed generated during glycolysis (two net molecules per glucose), but it is the purposeful energy currency output, not a byproduct arising specifically from the oxygen-deprived rerouting. The question asks what is produced because of the anaerobic shift, making ATP a misleading answer that reflects confusion between main products and incidental byproducts.

Option B (NADH) traps students who recall that NADH is central to metabolic redox chemistry but fail to track the direction of electron flow in fermentation. During lactic acid fermentation, NADH is consumed (oxidized to NAD⁺), not produced. This distractor exploits incomplete understanding of the stoichiometry: glycolysis does generate NADH at the GAPDH step, but the fermentative phase uses that NADH to reduce pyruvate, so NADH is a reactant, not a byproduct.

Option D (Pyruvate) attracts students who correctly associate pyruvate with glycolysis but misidentify its fate under anaerobic conditions. Pyruvate is the terminal intermediate of glycolysis itself, produced regardless of oxygen availability. In the anaerobic scenario described, pyruvate is the substrate that LDH converts into lactate; it is consumed, not excreted as a byproduct. Selecting this option reveals a failure to distinguish the starting material of fermentation from its output.