Glycolysis

• Glycolysis


  Glucose is converted into pyruvate in the cytoplasm
• The main organic compound used in cell respiration is carbohydrates (glucose) – although lipids and proteins can be used
• Lipids are not preferentially used as they are harder to transport and digest (although will yield more energy per gram)
• Proteins are not preferentially used as they release potentially toxic nitrogenous compounds when broken down
• Glycolysis


  1. Phosphorylation

  2. Lysis

  3. Oxidation

  4. ATP formation
• Phosphorylation


  A hexose sugar (typically glucose) is phosphorylated by two molecules of ATP (to form a hexose bisphosphate)
• Lysis


  The hexose biphosphate (6C sugar) is split into two triose phosphates (3C sugars)
• Oxidation


  Hydrogen atoms are removed from each of the 3C sugars (via oxidation) to reduce NAD+ to NADH (+ H+)
• ATP formation


  Some of the energy released from the sugar intermediates is used to directly synthesise ATP (substrate level phosphorylation)
• In total, 4 molecules of ATP are generated during glycolysis by substrate level phosphorylation (2 ATP per 3C sugar)
• At the end of glycolysis, glucose (6C) has been broken down into two molecules of pyruvate (3C), two hydrogen carriers have been reduced via oxidation (2 × NADH + H+), and a net total of two ATP molecules have been produced (4 molecules were generated, but 2 were used)
• Glycolysis


  • Gives a small net gain of ATP without the use of oxygen
  • Occurs in the cytosol and does not require oxygen (it is an anaerobic process)
• Aerobic respiration


  1. Pyruvate is transported to the mitochondria for further breakdown (complete oxidation)

  2. Generates large numbers of reduced hydrogen carriers (NADH + H+ and FADH2)

  3. Releases stored energy from reduced hydrogen carriers to synthesise more ATP
• Anaerobic respiration (fermentation)


  1. Pyruvate is not broken down further and no more ATP is produced (incomplete oxidation)

  2. Pyruvate is converted into lactic acid (animals) or ethanol and CO2 (plants and yeast)

  3. Necessary to ensure glycolysis can continue to produce small quantities of ATP
• In the absence of oxygen


  Glycolysis will quickly deplete available stocks of NAD+, preventing further glycolysis
• Fermentation of pyruvate


  Involves a reduction reaction that oxidises NADH (releasing NAD+ to restore available stocks)
• Anaerobic respiration allows small amounts of ATP to be produced (via glycolysis) in the absence of oxygen