Explanation

During citric acid cycle, 3 molecules of $NAD^{+}$ and one molecule of FAD (Flavin Adenine Dinucleotide) are reduced to produce NADH and $FADH_{2}$ respectively. These reduced electron carriers pass on the hydrogen atoms to oxygen through electron transport system, yielding II more ATP molecules for each molecule of pyruvic acid. In addition one ATP molecules is generated directly during the cycle to give a total of 12 ATP molecule per pyruvic acid molecules. As two molecules of pyruvic acid are produced from each molecule of glucose a total of 24 molecules of ATP are formed during the citric acid cycle