What molecule initiates the citric acid cycle?

The initiation of the citric acid cycle, also known as the Krebs cycle or the tricarboxylic acid (TCA) cycle, begins with acetyl-CoA. This molecule is crucial because it serves as the entry point for carbon atoms into the cycle. During cellular respiration, acetyl-CoA is produced from the breakdown of carbohydrates (like glucose), fats, and proteins. Once formed, it combines with oxaloacetate to form citrate, effectively kickstarting the cycle.

Acetyl-CoA is a central metabolic intermediate and plays a significant role in energy production. The citric acid cycle itself is a series of enzymatic reactions that ultimately lead to the production of energy-carrying molecules such as NADH and FADH2, which are then used in the electron transport chain to produce ATP. While glucose is metabolized to produce acetyl-CoA, it is not the direct initiator of the citric acid cycle. Similarly, NADH and FADH2 are products of the cycle rather than molecules that initiate it. Thus, the correct answer highlights the integral role of acetyl-CoA in the metabolic pathways of energy production.