Here's why:

* NADH (Nicotinamide adenine dinucleotide): NAD+ is reduced to NADH by accepting two electrons and one proton (H+) during the Krebs cycle.

* FADH2 (Flavin adenine dinucleotide): FAD is reduced to FADH2 by accepting two electrons and two protons (2H+) during the Krebs cycle.

Both NADH and FADH2 carry high-energy electrons to the electron transport chain, where they are used to generate ATP (adenosine triphosphate), the cell's primary energy currency.