• Glycolysis is the first stage in aerobic respiration and the only stage in anaerobic respiration.
  • Involves converting one molecule of glucose into two molecules of pyruvate, forming 2ATP and 2 molecules of reduced NAD (NADH)
  • It occurs in the cytoplasm of cells.



  1. Glucose is phosphorylated to glucose phosphate using the phosphate group from the breakdown of ATP to ADP and Pi
  2. Another Pi group is added from the breakdown of another ATP, forming hexose bisphosphate
  3. Hexose bisphosphate is then split to form two molecules of triose phosphate
  4. Triose phosphate is oxidised (loses hydrogen), forming two molecules of pyruvate as well as two NADH and 4ATP
  • 4ATP are made, but 2ATP are used up in the process, so overall, 2ATP are produced
  • In aerobic respiration, the pyruvate moves onto the next stage (the link reaction)
  • In anaerobic respiration in animals, the pyruvate is converted to lactate, regenerating NAD and allowing glycolysis to continue
    • In plants and yeast, the pyruvate is converted to ethanal and then ethanol (alcohol)


Practice questions

Describe the process of glycolysis

  • (glucose) phosphorylated by ATP ;
  • raises energy level / overcomes activation energy ;
  • hexose bisphosphate ;
  • lysis / splitting, of, glucose / hexose ;
  • breaks down to two TP ;
  • 6C(2) x 3C ;
  • dehydrogenation / description ;
  • 2 NAD reduced formed (from each TP to pyruvate formed) ;
  • 4 ATP produced / net gain of 2 ATP ;
  • pyruvate produced ;
  • reduced NAD to oxidative phosphorylation / redox ;

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