Assertion (A): Fermentation is a less energy efficient process.

Reason(R): The absence of oxygen does not allow complete oxidation of glucose.

Both (A) and (R) are true and (R) is the correct explanation of (A)

Assertion is true because during fermentation, the complete oxidation of glucose does not occur. The process involves the partial breakdown of glucose into various products, such as lactic acid (in lactic acid fermentation) or ethanol and carbon dioxide (in alcoholic fermentation). Since the complete oxidation of glucose to carbon dioxide and water (as in aerobic respiration) does not happen in fermentation, much less energy is extracted from each glucose molecule. As a result, the yield of ATP in fermentation is significantly lower compared to aerobic respiration.

Reason is also true. In the absence of oxygen, the electron transport chain (ETC) and oxidative phosphorylation, which are the final stages of aerobic respiration, cannot occur. In aerobic respiration, oxygen serves as the final electron acceptor, allowing the efficient extraction of energy from glucose. Without oxygen, cells resort to fermentation as an alternative means to generate ATP, but it is less efficient because only a small amount of energy is obtained through substrate-level phosphorylation.