Two acetyl CoA molecules are consumed to produce 4 CO2, (2ATP), 6 NADH and 2 FADH2. The ATP molecules are produced by substrate-level phosphorylation in glycolysis.
one NADH can make 3 ATP
one FADH2 can make 2 ATP
you go through krebs cycle twice because you have 2 pyruvates coming from glycolysis, so even though you make 3 NADH, you make 6 NADH in the end since you go through twice.
6 x 3 = 18 ATP
FADH2 is made once each go-around, so you have a total of 2 FADH2
2 x 2 = 4 ATP
18 + 4 = 22 ATP produced from JUST the krebs cycle
Two NADH molecules are produced in glycolysis for each glucose molecule that is catabolized. NADH is generated through the reduction of NAD+ during the conversion of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate as part of the glycolytic pathway.
NAD+ picks up the electrons that are removed during glycolysis --> makes a molecule of NADH. This NADH goes to the electron transport chain where it gives up the electron to the electron transport chain.