Anaerobic Respiration (Cambridge International A-Level Biology)

Spec point — outline respiration in anaerobic conditions in mammals (lactate fermentation) and in yeast cells (ethanol fermentation)

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

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 the enzyme lactate dehydrogenase
• Pyruvate is the hydrogen acceptor
• The final product lactate can be further metabolised

The pathway of lactate fermentation

Metabolisation 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 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 making alcoholic drinks like beer!