Anaerobic Respiration (AQA A Level Biology): Revision Note

Anaerobic respiration

• When oxygen is absent, or in short supply, cells may switch to anaerobic respiration; this needs to happen because:

  • there is no final electron acceptor for the electron transport chain

  • the electron transport chain stops

  • oxidative phosphorylation stops producing ATP

• Examples of anaerobic respiration pathways are:

  • ethanol fermentation, e.g. in yeast cells

  • lactate fermentation, e.g. in mammalian cells

• Ethanol and lactate fermentation allow cells to produce ATP continuously via glycolysis

  • Without fermentation glycolysis cannot continue indefinitely because cells would run out of NAD

  • Ethanol and lactate fermentation allow NAD to be regenerated so that glycolysis can continue

Ethanol fermentation

• NAD is regenerated by ethanol fermentation as follows:

  1. pyruvate is decarboxylated to ethanal

     • CO₂ is given off as a waste product

  2. ethanal is reduced to ethanol

     • The hydrogen atoms that reduce ethanal come from NADH, so NADH is oxidised to regenerate NAD

     • This involves the enzyme alcohol dehydrogenase

• The products of ethanol fermentation are:

  • ethanol

  • carbon dioxide

  • NAD

Lactate fermentation

• NAD is regenerated by lactate fermentation as follows:

  • Pyruvate accepts hydrogen from NADH and is reduced to lactate

  • NADH is oxidised to regenerate NAD

• The products of lactate fermentation are

  • lactate

  • NAD