The correct option is 2 only
Explanation
Respiration involves the breakdown of glucose to release energy. In human muscle cells, when oxygen supply is insufficient (e.g., during vigorous exercise), the cells switch from aerobic respiration to anaerobic respiration to meet energy demands.
Statement-wise Analysis:
- Statement 1 is Incorrect: In the absence or lack of oxygen, pyruvate (the product of glycolysis) is converted into lactic acid in muscle cells. This process is known as lactic acid fermentation.
- Statement 2 is Correct: Lactic acid is a three-carbon molecule ($C_3H_6O_3$). The formation of a two-carbon molecule (ethanol) along with carbon dioxide occurs in yeast during alcoholic fermentation, not in human muscles.
- Statement 3 is Incorrect: Anaerobic respiration yields significantly less energy compared to aerobic respiration. The partial breakdown of glucose in anaerobic conditions releases only a small amount of energy (net 2 ATP), whereas aerobic respiration results in the complete breakdown of glucose, releasing a large amount of energy (up to 38 ATP).
Key Takeaway:
Anaerobic respiration in human muscles results in the formation of lactic acid (a 3-carbon molecule) from pyruvate and releases a very small amount of energy compared to the complete oxidation of glucose in aerobic respiration.