The net energy gain in fermentation is 2 ATP molecules/glucose molecule. In both lactic acid and alcoholic fermentation, all the NADH produced in glycolysis is consumed in fermentation, so there is no net NADH production, and no NADH to enter the ETC and form more ATP.

Fermentation of Glucose to Alcohol or Lactic Acid: Net ATP Synthesis

Introduction:
Fermentation is an anaerobic metabolic process that allows the production of energy in the absence of oxygen. It occurs in microorganisms such as yeast and some bacteria. When glucose is fermented, it can be converted into either alcohol (ethanol) or lactic acid, depending on the organism involved.

1. Glycolysis:
The first step in the fermentation process is glycolysis, a common pathway for both alcohol and lactic acid fermentation. During glycolysis, one molecule of glucose is broken down into two molecules of pyruvate. This process occurs in the cytoplasm and does not require oxygen.

2. ATP Production in Glycolysis:
Glycolysis produces a small amount of ATP through substrate-level phosphorylation. For each molecule of glucose, two molecules of ATP are consumed in the initial steps of glycolysis, but four molecules of ATP are generated later. Thus, a net gain of two ATPs is produced during glycolysis.

3. Alcohol Fermentation:
In alcohol fermentation, pyruvate is converted into ethanol. This process occurs in yeast and some bacteria.

3.1 Conversion of Pyruvate to Acetaldehyde:
In the first step of alcohol fermentation, pyruvate is converted into acetaldehyde. This process releases one molecule of carbon dioxide per pyruvate molecule.

3.2 Conversion of Acetaldehyde to Ethanol:
In the second step, acetaldehyde is reduced to ethanol. This step is coupled with the oxidation of NADH to NAD+. This reaction regenerates NAD+, which is crucial for the continuation of glycolysis.

4. Lactic Acid Fermentation:
In lactic acid fermentation, pyruvate is converted into lactic acid. This process occurs in some bacteria and our muscle cells when oxygen is limited.

4.1 Conversion of Pyruvate to Lactic Acid:
During lactic acid fermentation, pyruvate is directly reduced to lactic acid. This reaction also regenerates NAD+ from NADH, allowing glycolysis to continue.

Net ATP Synthesis:
In both alcohol and lactic acid fermentation, the net ATP synthesis is two molecules of ATP per molecule of glucose. This is because during glycolysis, four molecules of ATP are generated, but two molecules of ATP are consumed initially. The final ATP production during fermentation compensates for the initial investment, resulting in a net gain of two ATPs.

Conclusion:
When glucose is fermented to alcohol or lactic acid, the net ATP synthesis is two molecules of ATP. This limited ATP production is due to the absence of the final electron acceptor, which is usually oxygen in aerobic respiration. Despite the lower energy yield compared to aerobic metabolism, fermentation allows organisms to generate ATP in the absence of oxygen, providing a crucial energy source in anaerobic conditions.

2 ATP