Anaerobic Respiration (Edexcel International A Level Biology)

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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 (oxygen) 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
    • The link reaction also 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

Lactate fermentation

  • In this pathway reduced NAD transfers hydrogen to pyruvate to form lactate
  • NAD can now be reused in glycolysis
  • Pyruvate is reduced to lactate by enzyme lactate dehydrogenase
  • Pyruvate is the hydrogen acceptor
  • The final product lactate can be further metabolised
  • A small amount of ATP is produced 

Lactate Fermentation

The pathway of lactate fermentation

Processing Lactate

  • Lactate (lactic acid) can build up in the cells after a period of time
  • After lactate is produced two things can happen:
    • It can be oxidised back to pyruvate which is then channelled into the Krebs cycle for ATP production
    • It can be converted into glucose by the liver cells for use during respiration or for storage (in the form of glycogen)
  • 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

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 occurs in yeast and plant cells

Ethanol Fermentation

The pathway of ethanol fermentation

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 producing alcoholic drinks like beer!

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Author: Marlene

Marlene graduated from Stellenbosch University, South Africa, in 2002 with a degree in Biodiversity and Ecology. After completing a PGCE (Postgraduate certificate in education) in 2003 she taught high school Biology for over 10 years at various schools across South Africa before returning to Stellenbosch University in 2014 to obtain an Honours degree in Biological Sciences. With over 16 years of teaching experience, of which the past 3 years were spent teaching IGCSE and A level Biology, Marlene is passionate about Biology and making it more approachable to her students.

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