Key Concepts of Section 12.3

• In stage II of glucose oxidation, the three-carbon pyruvate molecule is first oxidized to generate one molecule each of CO₂, NADH, and acetyl CoA. The acetyl group of acetyl CoA is then oxidized to CO₂ by the citric acid cycle (see Figure 12-14).

• Each turn of the citric acid cycle releases two molecules of CO₂ and generates three NADH molecules, one FADH₂ molecule, and one GTP (see Figure 12-16).

• Most of the energy released in stages I and II of glucose oxidation is temporarily stored in the reduced coenzymes NADH and FADH₂, which carry high-energy electrons that subsequently drive the electron-transport chain (stage III).

• Neither glycolysis nor the citric acid cycle directly uses molecular oxygen (O₂).

• The malate-aspartate shuttle regenerates the supply of cytosolic NAD⁺ necessary for continued glycolysis (see Figure 12-17).

• Like glucose oxidation, the oxidation of fatty acids takes place in four stages. In stage I, fatty acids are converted to fatty acyl CoA in the cytosol. In stage II, the fatty acyl CoA is first converted into multiple acetyl CoA molecules, with generation of NADH and FADH₂. Then, as in glucose oxidation, the acetyl CoA enters the citric acid cycle. Stages III and IV are identical for fatty acid and glucose oxidation (see Figure 12-14).

• In most eukaryotic cells, oxidation of short- to long-chain fatty acids occurs in mitochondria with production of ATP, whereas oxidation of very long chain fatty acids occurs primarily in peroxisomes and is not linked to ATP production (see Figure 12-18); the energy released during peroxisomal oxidation of fatty acids is converted to heat.