Anaerobic Respiration (Edexcel IAL Biology)

Spec point — 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

Exam Hint 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!