Anaerobic Respiration

Where in the cell does anaerobic respiration take place?

• In the cytoplasm.

What is the net ATP yield of anaerobic respiration per glucose molecule?

• 2 ATP (produced solely during glycolysis).

Why must pyruvate be converted into lactate or ethanol during anaerobic respiration?

• To re-oxidise NAD (convert Reduced NAD back to NAD). This regenerates the NAD required for glycolysis to continue.

What is the end product of anaerobic respiration in mammals?

• Lactate (Lactic acid).

Which enzyme catalyses the conversion of pyruvate to lactate?

• Lactate Dehydrogenase.

What are the end products of anaerobic respiration in yeast (alcoholic fermentation)?

• Ethanol and Carbon Dioxide.

What is the intermediate compound in alcoholic fermentation?

• Ethanal.

Which enzyme removes Carbon Dioxide from pyruvate in yeast?

• Pyruvate Decarboxylase.

Is lactate fermentation reversible?

• Yes. Lactate is carried to the liver and converted back to pyruvate (or glucose) when oxygen is available.

Is ethanol fermentation reversible?

• Because a carbon atom is lost as CO₂, the reaction cannot be reversed to reform pyruvate.

Why can't the Krebs cycle and Link reaction occur without oxygen?

• Because they rely on NAD and FAD to accept hydrogens. If oxygen isn't there to accept electrons at the end of the chain, NAD/FAD remain "reduced" and cannot be recycled.

Can mammals survive on anaerobic respiration indefinitely?

• No. The ATP yield is too low, and lactate is toxic (lowers pH, affecting enzyme activity).

Compare anaerobic respiration in mammals and yeast. (4 marks)

• Product: Mammals produce lactate (1); Yeast produce ethanol and CO₂ (1).

• Number of Steps: Mammals is a single step reaction (1); Yeast is a two-step reaction involving ethanal (1).

• Reversibility: Mammals is reversible (1); Yeast is irreversible (1).

  (Any 4 points)

What are obligate aerobes?

• Can only synthesise ATP in the presence of oxygen.

What are obligate anaerobes?

• Organisms that can’t survive in the presence of Oxygen, and have to rely entirely on anaerobic respiration.

Explain the importance of regenerating NAD during anaerobic respiration. (3 marks)

• Glycolysis requires NAD to accept hydrogen (dehydrogenation of triose phosphate) (1).

• If NAD is not regenerated, the supply of oxidised NAD will run out (1).

• Glycolysis would stop, meaning no ATP would be produced and the cell would die (1).

Suggest why anaerobic respiration produces much less ATP than aerobic respiration.(2 marks)

• In anaerobic respiration, only glycolysis occurs (net 2 ATP) (1).

• The Krebs cycle and Electron Transport Chain (oxidative phosphorylation), which produce the vast majority of ATP (approx 28-32), cannot take place (1).

Describe the fate of lactate produced in muscle cells. (2 marks)

• It is transported by the blood to the liver (1).

• It is converted back into pyruvate (for aerobic respiration) or glucose (gluconeogenesis) when oxygen becomes available (1).

Yeast is a facultative anaerobe. Explain what this means and how it benefits the yeast. (3 marks)

• Facultative anaerobe: An organism that can respire aerobically when oxygen is present and anaerobically when it is absent (1).

• Benefit: This allows it to survive in low-oxygen environments (1) while maximizing ATP production (growth) when oxygen is plentiful (1).