The Krebs cycle needs oxygen in order to proceed and it takes place in the inner matrix of the mitochondria, which are found inside cells. If oxygen is present, pyruvate that is formed in the first step of cellular respiration, is taken into the inner membrane and the Krebs cycle begins.
During the preparatory stage of the Krebs cycle, pyruvate molecules left over from glycolysis, give off a carbon atom in the form of CO2.
Next, the Krebs cycle proceeds the newly formed two carbon molecule combines with a molecule called Coenzyme A. This produces a new product that is important to the Krebs cycle, called Acetyl-CoA. The reactions required for producing Acetyl-CoA, store an electron in a molecule called NADH, that is used later during cellular respiration.
The Krebs cycle begins, when a four carbon molecule is combined with Acetyl-CoA, forming a six carbon molecule called citric acid.
Next, during a chain of reactions two carbons are given off from citric acid in the form of Co2, one ATP molecule is produced. And since high-energy electrons are produced during this chain of reactions, three NADH molecules are produced when NAD+ takes on an electron it becomes NADH. So three NADH molecules are produced.
When two electrons are stored in a molecule called FAD during the Krebs cycle, it becomes FADH2. This chain of reactions produces one FADH2 molecule.
We care about these stored electrons because they are used during the electron transport chain portion of cellular respiration, which occurs directly after the Krebs cycle. The Krebs cycle produces a lot of important molecules. When we started the Krebs cycle, we followed the story of one of the pyruvate molecules that was produced during glycolysis but glycolysis actually produces two pyruvate molecules, so we'll have to double the amount of products produced by the Krebs cycle in order to have an accurate estimate of how much energy cellular respiration can produce from just one glucose molecule.
The grand total for the Krebs cycle comes to 2 ATP, 6 NADH molecules and 2 FADH2 molecules. The electrons storing that has taken place during glycolysis and the Krebs cycle are used in the third and final stage of cellular respiration the electron transport chain.