The citric acid cycle takes place inside mitochondria in eukaryotic cells. It results in the complete oxidation of the carbon atoms of the acetyl groups in acetyl CoA, converting them into CO2. But the acetyl group is not oxidized directly. Instead, this group is transferred from acetyl CoA to a larger, four-carbon molecule, oxaloacetate, to form the six-carbon tricarboxylic acid, citric acid, for which the subsequent cycle of reactions is named. The citric acid molecule is then gradually oxidized, allowing the energy of this oxidation to be harnessed to produce energy-rich activated carrier molecules. The chain of eight reactions forms a cycle because at the end the oxaloacetate is regenerated and enters a new turn of the cycle, as shown in outline in Figure 2–57.
We have thus far discussed only one of the three types of activated carrier molecules that are produced by the citric acid cycle; NADH, the reduced form of the NAD+/NADH electron carrier system (see Figure 2–36). In addition to three molecules of NADH, each turn of the cycle also produces one molecule of FADH2 (reduced flavin adenine dinucleotide) from FAD (see Figure 2–39), and one molecule of the ribonucleoside triphosphate GTP from GDP. The structure of GTP is illustrated in Figure 2–58. GTP is a close relative of ATP, and the transfer of its terminal phosphate group to ADP produces one ATP molecule in each cycle. As we discuss shortly, the energy that is stored in the readily transferred electrons of NADH and FADH2 will be utilized subsequently for ATP production through the rest of hydrocarbon tail fatty acyl CoA shortened by two carbons
Figure 2–56 The oxidation of fatty acids to acetyl Coa. (a) electron micrograph of a lipid droplet in the cytoplasm. (B) The structure of fats. fats are triacylglycerols. The glycerol portion, to which three fatty acids are linked through ester bonds, is shown in blue. fats are insoluble in water and form large lipid droplets in the specialized fat cells (adipocytes) in which they are stored. (C) The fatty acid oxidation cycle. The cycle is catalyzed by a series of four enzymes in mitochondria. each turn of the cycle shortens the fatty acid chain by two carbons (shown in red) and generates one molecule of acetyl Coa and one molecule each of naDh and faDh2. (a, courtesy of Daniel s. friend.)
NET RESULT: ONE TURN OF THE CYCLE PRODUCES THREE NADH, ONE GTP, AND                    ONE FADH2 MOLECULE, AND RELEASES TWO MOLECULES OF CO2 process of oxidative phosphorylation, the only step in the oxidative catabolism of foodstuffs that directly requires gaseous oxygen (O2) from the atmosphere.
Panel 2–9 (pp. 106–107) and Movie 2.6 present the complete citric acid cycle. Water, rather than molecular oxygen, supplies the extra oxygen atoms required to make CO2 from the acetyl groups entering the citric acid cycle. As illustrated in the panel, three molecules of water are split in each cycle, and the oxygen atoms of some of them are ultimately used to make CO2.
In addition to pyruvate and fatty acids, some amino acids pass from the cytosol into mitochondria, where they are also converted into acetyl CoA or one of the other intermediates of the citric acid cycle. Thus, in the eukaryotic cell, the mitochondrion is the center toward which all energy-yielding processes lead, whether they begin with sugars, fats, or proteins.