Regulation of Respiration

Principles Module 26

Principles Module 27


  • Describe the general principles of the regulation of metabolic pathways
  • Discuss how cells transform energy in the absence of oxygen
  • Provide examples of how glycolysis and the Krebs cycle are the ‘backbone’ of metabolism

Key points

  • Metabolism is regulated through feedback loops
    • products of the interconnected pathways act as signals to reduce flux through the pathway
    • examples include ATP, citrate, NADH
  • Metabolic pathways tend to be regulated at reactions that are highly spontaneous (large negative ΔG)
    • Phosphofructokinase catalyzes the rate-limiting step in glycolysis, and is inhibited by a high concentration of ATP (ΔG = –25.9 kJ/mol)
    • several enzymes of the Krebs Cycle are inhibited by NADH and/or their product
  • Cells can continue to transform high-energy compounds in the absence of oxygen, but at much lower efficiency
    • key concept of anaerobic respiration is to provide a ‘sink’ for electrons
    • there are a number of anaerobic respiration pathways
      • alcohol fermentation pathway
        • glycolysis produces pyruvate and yields a little ATP and NADH
        • pyruvate is oxidized to acetaldehyde, which is reduced to ethanol by NADH, thus regenerating NAD+ needed for glycolysis
      • lactic acid fermentation pathway
        • glycolysis produces pyruvate and yields a little ATP and NADH
        • pyruvate is directly reduced to lactate by NADH, thus regenerating NAD+
  • Glycolysis and the Krebs cycle form the backbone of cellular metabolism
    • the products of glycolysis (pyruvate, acetyl-CoA) are used by the cell in the production of sugars, fatty acids, and amino acids
    • the products of the Krebs cycle are used in the production of amino acids and nucleotides

In-class activities

  • Continue your metabolic map, adding in the general points of regulation discussed in class.

Questions for Practice

  • Name three products of respiration that regulate flux through the pathway. How is this adaptive to the cell?
  • Enzymes in respiration that are most subject to regulation tend to have a property in common. What is that property? Why are these steps key to regulation?
  • Why is anaerobic respiration useful to the cell, even though it is much less efficient than aerobic respiration?
  • List the four classes of biological macromolecules and indicate the metabolic source of each class of molecule.