during glycolysis glucose is broken down to pyruvate a two carbon fragment of pyruvate is used to form acetyl coa the acetyl coa enters the krebs cycle which occurs in the mitochondrion during the conversion of pyruvate to acetyl coa carbon dioxide co2 is produced and a molecule of nadh is formed the two carbon acetyl portion of the acetyl coa is transferred to a four carbon molecule producing a six carbon compound the coa carrier molecule is released carbon dioxide is then released from the six carbon molecule forming a five-carbon compound in this step hydrogen is removed and transferred to nad plus to form nadh next a second oxidation and decarboxylation occurs again nadh and carbon dioxide are produced in addition a molecule of atp is produced as a result of these reactions a four carbon molecule is formed in the krebs cycle finally the four carbon molecule is further oxidized and the hydrogens that are removed are used to form nadh and fadh2 these reactions regenerate the four carbon molecule that initially reacts with acetyl coa each glucose molecule is broken down into two pyruvate molecules during glycolysis each pyruvate is converted to acetyl coa which enters the krebs cycle thus for each glucose molecule the krebs cycle must complete two circuits to completely break down the two pyruvate molecules