What molecule determines whether pyruvic acid will undergo fermentation or be sent to the Krebs cycle?
The correct answer and explanation is :
The molecule that determines whether pyruvic acid will undergo fermentation or be sent to the Krebs cycle is oxygen (O₂).
Explanation (300 words)
Cellular respiration is a metabolic process that cells use to produce energy in the form of ATP. The fate of pyruvic acid (pyruvate) depends on the availability of oxygen in the cell.
In the presence of sufficient oxygen (aerobic conditions), pyruvic acid is transported into the mitochondria, where it is converted into acetyl-CoA by the enzyme pyruvate dehydrogenase. Acetyl-CoA then enters the Krebs cycle (citric acid cycle), where it undergoes a series of reactions that generate NADH and FADH₂. These electron carriers then donate electrons to the electron transport chain (ETC), leading to the production of large amounts of ATP through oxidative phosphorylation. This process is highly efficient and produces up to 38 ATP molecules per glucose molecule.
However, under anaerobic conditions (when oxygen is scarce or absent), the electron transport chain cannot function efficiently because oxygen is the final electron acceptor. This causes a backup in the system, preventing NADH from being oxidized back to NAD⁺. Since glycolysis requires NAD⁺ to continue, cells use an alternative pathway—fermentation—to regenerate NAD⁺. In lactic acid fermentation, pyruvate is converted into lactic acid (in animal cells), while in alcoholic fermentation, pyruvate is converted into ethanol and CO₂ (in yeast and some bacteria). Fermentation allows glycolysis to continue producing ATP, but it is much less efficient, yielding only 2 ATP molecules per glucose molecule.
Thus, the availability of oxygen is the key factor that determines whether pyruvic acid proceeds to the Krebs cycle (aerobic respiration) or undergoes fermentation (anaerobic respiration). Cells adapt their metabolic pathways based on oxygen availability to maintain energy production efficiently.