Which molecule donates electrons to the electron transport chain?

NADH is the molecule that donates electrons to the electron transport chain. During cellular respiration, particularly in the Krebs cycle, glucose is ultimately broken down through a series of reactions that lead to the formation of NADH. NADH carries high-energy electrons from the metabolic pathways in which it is generated.

When NADH is produced, it captures electrons that are released during the oxidation of substrates. It then donates these electrons to the electron transport chain, which is located in the inner mitochondrial membrane. The transfer of electrons through the chain leads to the establishment of a proton gradient that ultimately drives the production of ATP through oxidative phosphorylation.

While glucose is the initial substrate and is crucial for the whole process, it is not a direct electron donor to the electron transport chain. Acetyl-CoA is an important intermediate that feeds into the Krebs cycle but does not directly interact with the electron transport system. FADH2 also donates electrons to the chain, but NADH donates electrons at a point that results in a greater energy yield, making it a primary electron donor in the context of this question.