AQA A Level Biology

Revision Notes

5.2.8 Anaerobic Respiration

Anaerobic Respiration

  • Sometimes cells experience conditions with little or no oxygen
  • There are several consequences when there is not enough oxygen available for respiration:
    • There is no final acceptor of electrons from the electron transport chain
    • The electron transport chain stops functioning
    • No more ATP is produced via oxidative phosphorylation
    • Reduced NAD and FAD aren’t oxidised by an electron carrier
    • No oxidised NAD and FAD are available for dehydrogenation in the Krebs cycle
    • The Krebs cycle stops
  • However, there is still a way for cells to produce some ATP in low oxygen conditions through anaerobic respiration

Anaerobic pathways

  • Some cells are able to oxidise the reduced NAD produced during glycolysis so it can be used for further hydrogen transport
  • This means that glycolysis can continue and small amounts of ATP are still produced
  • Different cells use different pathways to achieve this
    • Yeast and microorganisms use ethanol fermentation
    • Other microorganisms and mammalian muscle cells use lactate fermentation

Ethanol fermentation

  • In this pathway reduced NAD transfers its hydrogens to ethanal to form ethanol
  • In the first step of the pathway pyruvate is decarboxylated to ethanal
    • Producing CO2
  • Then ethanal is reduced to ethanol by the enzyme alcohol dehydrogenase
  • Ethanal is the hydrogen acceptor
  • Ethanol cannot be further metabolised; it is a waste product

Ethanol Fermentation, downloadable AS & A Level Biology revision notes

The pathway of ethanol fermentation

Lactate fermentation

  • In this pathway reduced NAD transfers its hydrogens to pyruvate to form lactate
  • Pyruvate is reduced to lactate by enzyme lactate dehydrogenase
  • Pyruvate is the hydrogen acceptor
  • The final product lactate can be further metabolised

Lactate Fermentation, downloadable AS & A Level Biology revision notes

The pathway of lactate fermentation

Metabolization of lactate

  • After lactate is produced two things can happen:
    1. It can be oxidised back to pyruvate which is then channelled into the Krebs cycle for ATP production
    2. It can be converted into glycogen for storage in the liver
  • The oxidation of lactate back to pyruvate needs extra oxygen
    • This extra oxygen is referred to as an oxygen debt
    • It explains why animals breathe deeper and faster after exercise

Exam Tip

Note that ethanol fermentation is a two-step process (lactate fermentation is a one-step process). Carbon dioxide is also produced alongside the waste ethanol. This waste ethanol is what makes yeast vital in making alcoholic drinks like beer!


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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