Anaerobic Respiration (HL) (HL IB Biology)

Anaerobic pathways

• Sometimes cells experience conditions with little or no oxygen, which prevents respiratory substrates such as glucose from being completely oxidised
  • This prevents most of the reactions that produce ATP from occurring
• However, there is still a way for cells to produce some ATP in low oxygen conditions through anaerobic respiration
• 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
  • There is a net yield of about two ATP molecules per glucose molecule
• Different cells use different pathways to achieve this
  • Yeast and microorganisms convert pyruvate to ethanol
  • Other microorganisms and mammalian muscle cells convert pyruvate to lactate

Converting pyruvate to lactate

• In this pathway reduced NAD transfers its hydrogens to pyruvate to form lactate
  • This allows NAD to be reoxidised in the absence of oxygen and pyruvate formation can continue
• Pyruvate is reduced to lactate by enzyme lactate dehydrogenase
• Pyruvate is the hydrogen acceptor
• The final product lactate can be further metabolised

Lactate Fermentation Diagram

The pathway of lactate fermentation

Metabolisation 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

• Alcoholic fermentation occurring in yeast cells have been useful to humans for thousands of years
  • Carbon dioxide causes bread dough to rise in bread making
  • Ethanol is the main ingredient in alcoholic beverages, such as beer and wine
• Bakers can make use of anaerobic cell respiration in yeasts to produce ethanol and carbon dioxide in baking
• Yeasts are single-celled fungi that live in areas where sugars are present e.g. on fruit or on leaves
  • They can respire aerobically or anaerobically
• Flour contains starch, and when mixed with water and yeast can form a bread dough
• The dough is kneaded to mix everything together
• The dough is then left in a warm place to encourage the yeast to respire
• Yeast cells grow rapidly in number while oxygen is still present in the dough
  • The yeast hydrolyses the starch into maltose and glucose and respires the sugars, aerobically at first
• The dough soon becomes anaerobic (all the oxygen within it is used up aerobically by the yeast)
• Anaerobic respiration takes over and CO₂ bubbles begin to form in the dough
• These bubbles allow the dough to rise (swell up)
• Baking the dough kills the yeast and the bubbles form the fluffy texture of the finished bread
• Ethanol, the other product of anaerobic respiration of yeast, is produced but evaporates during the final baking stage

Role of Yeast in Bread Making Diagram

The role of anaerobic respiration of yeast in bread making to cause bread dough to rise

• In the alcoholic fermentation 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

Alcohol Fermentation Diagram

The pathway of alcoholic fermentation