Key Concepts of Section 12.1

• In a process known as aerobic oxidation, cells convert the energy released by the oxidation of glucose or fatty acids into the terminal phosphoanhydride bond of ATP.

• The complete aerobic oxidation of each molecule of glucose produces 6 molecules of CO₂ and approximately 30 ATP molecules. The entire process, which starts in the cytosol and is completed in the mitochondrion, can be divided into four stages: (I) degradation of glucose to pyruvate in the cytosol (glycolysis); (II) pyruvate oxidation to CO₂ in the mitochondrion coupled to generation of the high-energy electron carriers NADH and FADH₂ (via the citric acid cycle); (III) electron transport to generate a proton-motive force together with conversion of molecular oxygen to water; and (IV) ATP synthesis (see Figure 12-1). From each glucose molecule, two ATPs are generated by stage I and approximately 28 from stages II–IV.

• In glycolysis (stage I), cytosolic enzymes convert glucose to two molecules of pyruvate and generate two molecules each of NADH and ATP (see Figure 12-3).

• The rate of glucose oxidation via glycolysis is regulated by the inhibition or stimulation of several enzymes, depending on the cell’s need for ATP (see Figure 12-4). Glucose is stored, for example, as glycogen, when ATP is abundant.

• In the absence of oxygen (anaerobic conditions), cells can metabolize pyruvate to lactic acid or (in the case of yeast) to ethanol and CO₂, in the process converting NADH back to NAD⁺, which is necessary for continued glycolysis. In the presence of oxygen (aerobic conditions), pyruvate is transported into the mitochondrion, where it is metabolized to CO₂, in the process generating abundant ATP (see Figure 12-5).