Outline of Campbell Biology Chapter 9

VI. FERMENTATION: THE ANAEROBIC ALTERNATIVE

  1. Introduction: Some definitions
    1. Aerobic = Existing in the presence of oxygen.
    2. Anaerobic = Existing in the absence of free oxygen.
    3. Fermentation = The anaerobic catabolism of organic nutrients.
  2. Glycolysis oxidizes glucose to two pyruvic acid molecules. The oxidizing agent for this process is NAD+, not oxygen.
    1. Some energy released from the exergonic process of glycolysis drives the production of 2 ATPs (net) by substrate-level phosphorylation.
    2. Glycolysis produces a net of 2 ATPs whether conditions are aerobic or anaerobic.
      1. Aerobic conditions: Pyruvic acid is oxidized further, and more ATP is made as NADH gasses electrons removed from glucose to the electron transport chain. NAD+ is regenerated in the process.
      2. Anaerobic conditions: Pyruvic acid is reduced, and NAD+ is regenerated. This prevents the cell from depleting the pool of NAD+ which is the oxidizing agent necessary for glycolysis to continue. No additional ATP is produced.
  3. Fermentation consists of anaerobic glycolysis plus subsequent reactions that regenerate NAD+ by reducing pyruvic acid. Two of the most common types of fermentation are:
    1. Alcohol Fermentation
      1. Pyruvic acid is converted to ethanol in two steps:
        1. Pyruvic acid loses carbon dioxide and is converted to the two- carbon compound acetaldehyde.
        2. NADH is oxidized to NAD+ and acetaldehyde is reduced to ethanol.
      2. Many bacteria and yeast carry out alcohol fermentation under anaerobic conditions.
    2. Lactic Acid Fermentation
      1. NADH is oxidized to NAD+ and pyruvic acid is reduced to lactic acid.
        1. Commercially important products of lactic acid fermentation include cheese, yogurt, acetone and methyl alcohol.
        2. When oxygen is scarce, human muscle cells switch from aerobic respiration to lactic acid fermentation. Lactic acid accumulates, but it is gradually carried to the liver where it is converted back to pyruvic acid when oxygen becomes available.