The 2 carbons of acetyl-CoA enter the Krebs cycle as they combine with a 4-carbon molecule. The resulting 6-carbon compound is oxidized by NAD+, producing NADH and CO2. A second oxidation by NAD+ again yields NADH and CO2. The remaining 4-carbon compound undergoes reactions which yield ATP, FADH2, and NADH. At the end of the cycle, the original 4-carbon molecule has been produced and the cycle continues. CO2 is a waste product. NADH and FADH2 bring high-energy electrons to the electron transport system where many additional ATPs will be produced.