CIE A Level Biology (9700) exams from 2022

Revision Notes

12.2.3 Aerobic Respiration: Glycolysis

Aerobic Respiration: Glycolysis

  • Glycolysis is the first stage of respiration
  • It takes place in the cytoplasm of the cell and involves:
    • Trapping glucose in the cell by phosphorylating the molecule
    • Splitting the glucose molecule in two
  • It results in the production of
    • 2 Pyruvate (3C) molecules
    • Net gain 2 ATP
    • 2 reduced NAD

Steps of glycolysis

  • Phosphorylation: glucose (6C) is phosphorylated by 2 ATP to form fructose bisphosphate (6C)

Glucose + 2ATP → Fructose bisphosphate

  • Lysis: fructose bisphosphate (6C) splits into two molecules of triose phosphate (3C)

Fructose bisphosphate → 2 Triose phosphate

  • Oxidation: hydrogen is removed from each molecule of triose phosphate and transferred to coenzyme NAD to form 2 reduced NAD

4H + 2NAD → 2NADH + 2H+

  • Dephosphorylation: phosphates are transferred from the intermediate substrate molecules to form 4 ATP through substrate-linked phosphorylation

4Pi + 4ADP → 4ATP

  • Pyruvate is produced: the end product of glycolysis which can be used in the next stage of respiration

2 Triose phosphate → 2 Pyruvate

Exam Tip

It may seem strange that ATP is used and also produced during glycolysis. At the start ATP is used to make glucose more reactive (it is usually very stable) and to lower the activation energy of the reaction. Since 2 ATP are used and 4 are produced during the process, there is a net gain of 2 ATP per glucose molecule.

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