7.5 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 

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

The pathway of ethanol fermentation