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•  In 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

The first step in the controlled breakdown of carbohydrates is glycolysis, which occurs in the cytosol of the cell

  • In glycolysis, a hexose sugar (6C) is broken down into two molecules of pyruvate (3C)

The process of glycolysis involves many intermediate steps, but can be summarised by four key events:

1.  Phosphorylation

  • A hexose sugar (typically glucose) is phosphorylated by two molecules of ATP (to form a hexose biphosphate)
  • This phosphorylation makes the molecule less stable and more reactive, and also prevents diffusion out of the cell 

2.  Lysis

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

3.  Oxidation

  • Hydrogen atoms are removed from each of the 3C sugars (via oxidation) to reduce NAD+ to NADH (+ H+)
  • Two molecules of NADH are produced in total (one from each 3C sugar)

4.  ATP formation

  • Some of the energy released from the sugar intermediates is used to directly synthesise ATP
  • This direct synthesis of ATP is called 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, the following reactions have occurred:

  • Glucose (6C) has been broken down into two molecules of pyruvate (3C)
  • Two hydrogen carriers have been reduced via oxidation (2 × NADH + H+)
  • A net total of two ATP molecules have been produced (4 molecules were generated, but 2 were used)

Overview of Glycolysis


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•  Glycolysis gives a small net gain of ATP without the use of oxygen

Glycolysis involves the breakdown of glucose into pyruvate (
× 2), with a small net gain of ATP (two molecules)

  • Glycolysis occurs in the cytosol and does not require oxygen (it is an anaerobic process)

Depending on the availability of oxygen, the pyruvate may be subjected to one of two alternative processes:

  • Aerobic respiration occurs in the presence of oxygen and results in the further production of ATP (~ 34 molecules)
  • Anaerobic respiration (fermentation) occurs in the absence of oxygen and no further ATP is produced

Aerobic Respiration

  • If oxygen is present, the pyruvate is transported to the mitochondria for further breakdown (complete oxidation)
  • This further oxidation generates large numbers of reduced hydrogen carriers (NADH + H+ and FADH2)
  • In the presence of oxygen, the reduced hydrogen carriers can release their stored energy to synthesise more ATP
  • Aerobic respiration involves three additional processes – the link reaction, krebs cycle and the electron transport chain

Anaerobic Respiration (Fermentation)

  • If oxygen is not present, pyruvate is not broken down further and no more ATP is produced (incomplete oxidation)
  • The pyruvate remains in the cytosol and is converted into lactic acid (animals) or ethanol and CO2 (plants and yeast)
  • This conversion is reversible and is necessary to ensure that glycolysis can continue to produce small quantities of ATP
    • Glycolysis involves oxidation reactions that cause hydrogen carriers (NAD+) to be reduced (becomes NADH + H+)
    • Typically, the reduced hydrogen carriers are oxidised via aerobic respiration to restore available stocks of NAD+
    • 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)
    • Hence, anaerobic respiration allows small amounts of ATP to be produced (via glycolysis) in the absence of oxygen

Anaerobic Respiration (Fermentation)

anaerobic respiration