In which pathway is the most NADH generated?
The correct answer and explanation is:
The pathway in which the most NADH is generated is the citric acid cycle (also known as the Krebs cycle or TCA cycle).
The citric acid cycle occurs in the mitochondria and plays a central role in cellular respiration, which is the process by which cells extract energy from nutrients. The cycle begins with the combination of acetyl-CoA (derived from carbohydrates, fats, or proteins) and oxaloacetate to form citrate. Throughout the cycle, citrate undergoes a series of transformations, during which energy is harvested in the form of high-energy molecules, including NADH.
For each turn of the citric acid cycle, three molecules of NADH are produced. This is because, during the cycle:
- Isocitrate is oxidized to form α-ketoglutarate, producing NADH.
- α-Ketoglutarate is decarboxylated, producing NADH and releasing carbon dioxide.
- Malate is oxidized to form oxaloacetate, which regenerates the starting molecule and produces another NADH.
Each NADH molecule carries high-energy electrons that can be used in the electron transport chain (ETC) to generate ATP through oxidative phosphorylation. The NADH produced in the citric acid cycle is one of the most important contributors to cellular energy production, as it feeds directly into the ETC.
In total, for every glucose molecule, two acetyl-CoA molecules enter the citric acid cycle (one per cycle turn). This results in the production of six NADH molecules from one glucose molecule, as each glucose generates two acetyl-CoA molecules. Therefore, the citric acid cycle is the pathway that generates the most NADH in cellular respiration, surpassing glycolysis and the pyruvate decarboxylation step.