In glycolysis and the Krebs cycle, NADH molecules are formed from NAD+. Meanwhile, in the electron transport chain, all of the NADH molecules are subsequently split into NAD+, producing H+ and a couple of electrons, too.

Where does the NADH come from?

The Electron Transport Chain

The ten NADH that enter the electron transport originate from each of the earlier processes of respiration: two from glycolysis, two from the transformation of pyruvate into acetyl-CoA, and six from the citric acid cycle. The two FADH2 originate in the citric acid cycle.

How does NAD+ become NADH?

NAD+ is an electron carrier which will pick up electrons during the course of cellular respiration. When NAD+ picks up an electron, it becomes reduced, and becomes NADH. NADH carries electrons all the way to the Electron Transport Chain, where it will then drop off the electrons.