In lactate fermentation, NAD+ is produced during:
the oxidation of glucose.
reduction of pyruvate to lactic acid.
hydrolysis of ATP to ADP.
reduction of acetaldehyde to ethanol.
oxidation of pyruvate to acetyl CoA.
The correct answer and explanation is :
The correct answer is: reduction of pyruvate to lactic acid.
Explanation:
Lactate fermentation is a type of anaerobic respiration that occurs when oxygen is scarce or unavailable, such as in muscle cells during intense exercise. The goal of fermentation is to regenerate NAD+, which is necessary for glycolysis to continue producing ATP.
Let’s walk through the process:
- Glycolysis first breaks down one molecule of glucose (a six-carbon sugar) into two molecules of pyruvate (a three-carbon compound). In this process, two molecules of NAD+ are reduced to NADH as they accept electrons. Glycolysis also generates a net gain of two ATP molecules.
- Under aerobic conditions (when oxygen is present), pyruvate enters the mitochondria and is fully oxidized through the citric acid cycle and electron transport chain, regenerating NAD+ in the process.
- However, under anaerobic conditions, the electron transport chain cannot operate because there is no final electron acceptor (oxygen). As a result, NADH builds up, and NAD+ becomes scarce. Without NAD+, glycolysis would halt because NAD+ is essential for the step in glycolysis where glyceraldehyde-3-phosphate is oxidized.
- To solve this problem, cells use lactate fermentation. In this pathway, the accumulated pyruvate is reduced to lactic acid (lactate) by the enzyme lactate dehydrogenase. During this reaction, NADH is oxidized back to NAD+.
- The reaction:
Pyruvate + NADH → Lactate + NAD⁺
- This regeneration of NAD+ allows glycolysis to continue producing ATP even when oxygen is unavailable.
In summary, NAD+ is regenerated during the reduction of pyruvate to lactic acid in lactate fermentation. This critical step ensures a continuous supply of ATP through glycolysis in anaerobic conditions. Without it, cells would quickly run out of energy when oxygen levels drop.