• 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
• Certain types of micro-organisms 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

• 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