Question

The end product of anaerobic respiration is

• A. Ethanol
• B. Pyruvate
• C. 3-Phosphoglyceric acid
• D. Glycerol

Hint:

Under anaerobic conditions, plants undergo ethanolic fermentation to prevent glycolytic arrest.
The oxidation of \( \text{NADH} \) to \( \text{NAD}^+ \) during the conversion of acetaldehyde to ethanol is essential to keep glycolysis running.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Respiration is the metabolic process by which cells break down organic nutrients to generate cellular energy (ATP).
In the absence of oxygen, cells undergo anaerobic respiration (fermentation).
This pathway allows the regeneration of the \( \text{NAD}^+ \) pool required to sustain the glycolytic pathway, enabling continued anaerobic ATP synthesis.

Step 2: Detailed Explanation:
Let us trace the biochemical steps of anaerobic respiration in plants and yeast (alcoholic fermentation):
1. Glycolysis: A single molecule of glucose is oxidized through ten enzymatic reactions to produce two molecules of pyruvate, two molecules of \( \text{NADH} \), and a net yield of two molecules of \( \text{ATP} \).
2. Decarboxylation of Pyruvate: In anaerobic plant tissues (such as flooded roots) and yeast cells, pyruvate is decarboxylated by the enzyme pyruvate decarboxylase to form acetaldehyde. This reaction releases carbon dioxide.
3. Reduction to Ethanol: Acetaldehyde is subsequently reduced to ethanol by the cytosolic enzyme alcohol dehydrogenase.
During this step, \( \text{NADH} \) is oxidized back to \( \text{NAD}^+ \).
This regeneration of \( \text{NAD}^+ \) allows glycolysis to continue in the absence of oxygen.
Therefore, under anaerobic conditions in plant systems, ethanol is the primary organic end product.
Pyruvate is the end product of aerobic glycolysis, while 3-phosphoglyceric acid is an intermediate of the Calvin-Benson cycle and glycolysis.

Step 3: Final Answer:
The organic end product of anaerobic respiration in plants is ethanol, which corresponds to option (A).