Question

In lactic acid fermentation, NADH is oxidized back to $\text{NAD}^+$ by ______.

• A. Pyruvate
• B. Lactate
• C. Glucose
• D. Acetyl Co-A

Hint:

The primary physiological goal of fermentation is not actually to produce lactic acid or ethanol, but to regenerate the cellular supply of $\text{NAD}^+$ so that glycolysis can continue generating ATP in the absence of oxygen.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question asks to identify the molecular species that acts as the electron acceptor to oxidize NADH back to $\text{NAD}^+$ during homolactic or heterolactic acid fermentation.
This biochemical step is essential for maintaining glycolysis in anaerobic conditions.

Step 2: Detailed Explanation:

• Glycolysis and NAD+ Depletion: During the pathway of glycolysis, glucose is broken down into two molecules of pyruvate. In this process, $\text{NAD}^+$ is reduced to NADH by the enzyme glyceraldehyde-3-phosphate dehydrogenase.

• The Regeneration Bottleneck: Because cells contain a limited pool of coenzyme $\text{NAD}^+$, glycolysis would quickly halt if NADH were not oxidized back to $\text{NAD}^+$. Under anaerobic conditions, oxidative phosphorylation in the mitochondria cannot occur to regenerate $\text{NAD}^+$.

• Lactic Acid Fermentation Pathway: To solve this bottleneck, lactic acid bacteria (such as Lactobacillus and Lactococcus) utilize the enzyme lactate dehydrogenase (LDH).

• Role of Pyruvate: The enzyme LDH transfers electrons directly from NADH to pyruvate (the end product of glycolysis). In this reaction:

• Pyruvate is reduced to lactate (lactic acid).

• NADH is oxidized back to $\text{NAD}^+$.

• Other Options: Lactate is the reduced product, not the oxidizing agent. Glucose is the starting material of glycolysis. Acetyl Co-A is a metabolic intermediate of aerobic respiration, not involved in anaerobic lactic acid fermentation.

Step 3: Final Answer:
During lactic acid fermentation, pyruvate acts as the terminal electron acceptor that oxidizes NADH back to $\text{NAD}^+$, corresponding to option (A).