Key Concepts of Section 12.1

Key Concepts of Section 12.1

First Step of Harvesting Energy from Glucose: Glycolysis

  • 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).