What is the primary source of high energy electrons that enter the electron transport chain?

The primary source of high-energy electrons that enter the electron transport chain is NADH. During cellular respiration, particularly in the processes of glycolysis and the citric acid cycle, NADH is generated by the reduction of NAD+ as it accepts electrons. This reduction process is crucial as it captures high-energy electrons that are then utilized in the electron transport chain to drive the production of ATP through oxidative phosphorylation.

In the electron transport chain, NADH donates its electrons to Complex I, initiating a series of redox reactions that ultimately lead to the pumping of protons across the inner mitochondrial membrane. This creates a proton gradient that is used by ATP synthase to produce ATP. The electrons carried by NADH have a substantial amount of energy, making it a key contributor to cellular energy production.