Topic 6: Anaerobic Respiration

Pyruvate

  • Three main ways pyruvate from glycolysis is used:
    • Pyruvate oxidation and Krebs cycle (aerobic conditions)
    • Alcoholic fermentation (anaerobic conditions)
    • Lactic acid fermentation (anaerobic conditions)

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Alcoholic Fermentation

  • Glycolysis does not require oxygen
    • Generates 2 ATP regardless
    • Does require NAD+
  • Yeast does not require a great amount of energy to survive
  • Yeast cells perform glycolysis but further break down pyruvate to ethanol in 2 steps:
    • Step 1: 2 pyruvate → 2 acetaldehyde + 2 CO2
    • Step 2: 2 acetaldehyde + 2 NADH + 2 H+  → 2 ethanol + 2 NAD+

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Lactic Acid Fermentation

  • During exercise additional ATP is supplied to muscle cells which use glucose faster than oxygen can be supplied
    • Aerobic respiration slows down and lactic acid fermentation begins
  • Pyruvate is reduced directly by NADH to form lactate (ionized form of lactic acid)
    • 2 pyruvate + 2 NADH + 2 H+ → 2 lactate + 2 NAD+
    • Lactate is acidic, which drops the pH of the blood leading to cramping and muscle fatigue
    • When at rest lactate is converted back to pyruvate in the liver

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Aerobic Alternatives

  • Fermentation and cellular respiration are anaerobic and aerobic alternatives for producing ATP from sugars
    • Both use glycolysis to oxidize sugars to pyruvate to form 2 ATP by substrate-level phosphorylation
    • Both use NAD+ as an electron acceptor
  • In fermentation, the electrons of NADH are passed to an organic molecule (ethanol or lactate/lactic acid), regenerating NAD+
  • Under aerobic respiration, a molecule of glucose yields 38 ATP, but the same molecule of glucose yields only 2 ATP under anaerobic respiration

Proteins and Fats as Energy Sources

  • Carbohydrates, fats, and proteins can all be catabolized through the same pathways in cellular respiration
    • They can enter glycolysis and the Krebs cycle at specific locations
  • Proteins must first be digested to individual amino acids and their amino groups removed via deamination
    • The carbon skeletons are modified by enzymes and enter as substrates in glycolysis or the Krebs cycle depending on their structure
  • Fats must be digested to glycerol and fatty acids
    • Glycerol can be converted to glyceraldehyde-3-phosphate, an intermediate of glycolysis
    • The rich energy of fatty acids is accessed when they are split into two-carbon fragments via oxidation
    • These fragments enter the Krebs cycle as acetyl CoA