• 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

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

The pathway of lactate fermentation

Metabolization of lactate

• After lactate is produced two things can happen:

1. 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