Carbon dioxide is not released in which of the following processes?

The correct option is Lactate fermentation.

To understand why carbon dioxide (CO₂) is not released during lactate fermentation, let us examine the biochemical pathways of each process listed in the options:

1. Aerobic respiration in animals and plants:
During aerobic respiration, glucose is completely oxidized in the presence of oxygen. The process involves glycolysis, the transition reaction, the Krebs cycle (Citric Acid Cycle), and the Electron Transport Chain. Carbon dioxide is released as a byproduct during the transition reaction (decarboxylation of pyruvate to acetyl-CoA) and during the Krebs cycle.
The overall balanced chemical equation for aerobic respiration is:
$$C_6{H}_{12}{O}_6+6O_2\to 6C{O}_2+6H_{2}O+\text{Energy (ATP)}$$ Since CO₂ is produced, aerobic respiration in both animals and plants releases carbon dioxide.

2. Alcoholic fermentation:
This is an anaerobic pathway commonly carried out by yeast. Glycolysis first converts glucose into pyruvate. Then, pyruvate is decarboxylated by the enzyme pyruvate decarboxylase to produce acetaldehyde, releasing a molecule of carbon dioxide (CO₂). Finally, acetaldehyde is reduced to ethanol by alcohol dehydrogenase. Because decarboxylation occurs, CO₂ is released during this process.

3. Lactate fermentation:
This anaerobic pathway occurs in certain bacteria (like lactic acid bacteria) and in animal muscle cells under anaerobic conditions (e.g., during strenuous exercise).
Here, glucose is broken down into two molecules of pyruvate during glycolysis. In the next step, pyruvate is directly reduced to lactate (lactic acid) by the enzyme lactate dehydrogenase, using the reducing power of NADH + H⁺:
$$\text{Pyruvate}+\text{NADH}+H^{+}\to \text{Lactate}+{\text{NAD}}^{+}$$ Since pyruvate (a 3-carbon compound) is converted directly into lactate (also a 3-carbon compound), there is no decarboxylation step, and no carbon dioxide (CO₂) is released.