Question

Which of the following represents a correct ordering of the events that occur in the catabolism of glucose in the absence of $O_2$?

• A. Glycolysis; oxidative phosphorylation
• B. Oxidative phosphorylation; TCA cycle
• C. TCA cycle; glycolysis
• D. Glycolysis; fermentation

Answer 1

The correct option is D Glycolysis; fermentation

The process of harvesting chemical energy for metabolic activities in form of ATP by oxidising the food molecule is called respiration. The chemical bond energy stored in various molecules is decomposed or oxidised in this process. The most common substrate used in this pathway is glucose.
Glucose is first converted to two molecules of pyruvate by a pathway known as glycolysis. It depends on the presence or absence of oxygen, that the pyruvate will enter mitochondria or will remain in the cytoplasm.

If oxygen is available in th cell, then pyruvate enters mitochondria, where Krebs cycle and oxidative phosphorylation with furthur release of ATP takes place.

But if the cell lacks oxygen, then anaerobic respiration (fermentation) occurs, resulting in the production of lactic acid or ethanol with a lesser synthesis of ATP molecules.
And as, in the given condition the cell lacks oxygen, so during catabolism of glucose, only gylcolysis and fermentation will occur.