AQA A Level Biology

Revision Notes

5.2.4 The Link Reaction

Entering the Link Reaction

  • The end product of glycolysis is pyruvate
  • Pyruvate contains a substantial amount of chemical energy that can be further utilised in respiration to produce more ATP
  • When oxygen is available pyruvate will enter the mitochondrial matrix and aerobic respiration will continue
  • Pyruvate moves across the double membrane of the mitochondria via active transport
    • It requires a transport protein and a small amount of ATP
  • Once in the mitochondrial matrix pyruvate takes part in the link reaction

Pyruvate enters the mitochondrial matrix from the cytosol (cytoplasm) by active transport

The Link Reaction

  • The link reaction takes place in the matrix of the mitochondria
  • It is referred to as the link reaction because it links glycolysis to the Krebs cycle
  • The steps are:
    • Pyruvate is oxidised by enzymes to produce acetate, CH3CO(O)-  and carbon doxide, requiring the reduction of NAD to NADH
    • Combination with coenzyme A to form acetyl coenzyme A (acetyl CoA)
  • It produces:
    • Acetyl coA
    • Carbon dioxide (CO2)
    • Reduced NAD (NADH)

pyruvate + NAD + CoA → acetyl CoA + carbon dioxide + reduced NAD

Link Reaction, downloadable AS & A Level Biology revision notes

The link reaction occurs in the mitochondrial matrix. It dehydrogenates and decarboxylates the three-carbon pyruvate to produce the two-carbon acetyl CoA that can enter the Krebs Cycle.

Role of coenzyme A

  • A coenzyme is a molecule that helps an enzyme carry out its function but is not used in the reaction itself
  • Coenzyme A consists of a nucleoside (ribose and adenine) and a vitamin
  • In the link reaction, CoA binds to the remainder of the pyruvate molecule (acetyl group 2C) to form acetyl CoA
  • It then supplies the acetyl group to the Krebs cycle where it is used to continue aerobic respiration
  • This is the stage that brings part of the carbohydrate (or lipid/amino acid) into the further stages of respiration and links the initial stage of respiration in the cytoplasm to the later stages in the mitochondria

Exam Tip

Remember that there are two pyruvate molecules produced per glucose molecule so you need to multiply everything by 2 when thinking about what happens to a single glucose molecule in aerobic respiration.


Alistair graduated from Oxford University in 2014 with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems and Societies.

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