CIE A Level Biology (9700) 2019-2021

Revision Notes

12.2.7 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

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

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!

Author: Lára

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.
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