What is the primary substrate for the Krebs cycle?

The primary substrate for the Krebs cycle is Acetyl CoA. Acetyl CoA is derived from the breakdown of glucose during glycolysis and from the oxidation of fatty acids and certain amino acids. Once formed, Acetyl CoA enters the Krebs cycle, where it combines with oxaloacetate to form citrate, initiating a series of enzymatic reactions that generate energy carriers such as NADH and FADH2, as well as ATP.

Glucose, although it is the starting point for cellular respiration and ultimately leads to the production of Acetyl CoA, does not directly enter the Krebs cycle. Instead, it is first metabolized through glycolysis into pyruvate, which is then converted into Acetyl CoA before entering the Krebs cycle. NAD+ is an important coenzyme involved in redox reactions, but it is not a substrate; rather, it accepts electrons during the cycle, facilitating the conversion of substrates into energy carriers. Pyruvate, while produced from glucose during glycolysis, is a precursor to Acetyl CoA but does not enter the Krebs cycle directly. Therefore, Acetyl CoA is the correct and primary substrate that drives the reactions of the Krebs cycle.