Terminal electron acceptor in lactic acid fermentation: During homolactic fermentation of glucose by lactic acid bacteria, which compound serves as the final electron acceptor?

Introduction / Context:
Lactic acid fermentation is widely used in food biotechnology (yogurt, cheese, pickles) and is central to anaerobic energy metabolism in certain microbes and animal tissues. The key is understanding how cells reoxidize NADH produced in glycolysis when external electron acceptors are absent.

Given Data / Assumptions:

• Glycolysis converts glucose to pyruvate, generating ATP and NADH.
• Homolactic fermenters (e.g., Lactobacillus, Streptococcus) reduce pyruvate directly to lactate.
• NADH must be reoxidized to NAD+ to allow continued glycolytic ATP production.

Concept / Approach:
In homolactic fermentation, pyruvate accepts electrons from NADH, forming lactate and regenerating NAD+. Thus, pyruvate is the final electron acceptor. This internal redox balancing enables ATP yield via substrate-level phosphorylation independent of oxygen or respiratory chains.

Step-by-Step Solution:
Glycolysis: glucose → 2 pyruvate + 2 NADH + 2 ATP. Reduction step: 2 pyruvate + 2 NADH → 2 lactate + 2 NAD+. Identify the acceptor: pyruvate is reduced to lactate and accepts electrons from NADH. Select “Pyruvate.”

Verification / Alternative check:
Enzyme lactate dehydrogenase catalyzes the reaction, and measuring lactate accumulation with concurrent oxidation of NADH in vitro confirms the acceptor role of pyruvate.

Why Other Options Are Wrong:

• Fructose / Glucose: Substrates in glycolysis, not final electron acceptors in homolactic fermentation.
• Acetyl-CoA: Intermediate in respiration; not produced in homolactic fermentation.
• O2: Not used in fermentation; this is an anaerobic pathway.

Common Pitfalls:
Confusing heterolactic pathways (which produce multiple products) with homolactic ones; in both cases, an organic molecule derived from glucose, not oxygen, accepts electrons.

Final Answer:
Pyruvate is the final electron acceptor in lactic acid fermentation.