AQA · A-Level · Biology
Course version: 7402 · Version from Sept 2016
If respiration is only anaerobic, pyruvate can be converted to ethanol or…
Full specification wording
Respiration (A-level only): If respiration is only anaerobic, pyruvate can be converted to ethanol or lactate using reduced NAD.
Without oxygen, the electron transfer chain stops and reduced NAD cannot be re-oxidised.
Checked against the AQA 7402 specification (opens in a new tab) on .
- Point 4 of 8
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Without oxygen, the electron transfer chain stops and reduced NAD cannot be re-oxidised. Glycolysis would halt once cellular NAD is depleted. Anaerobic respiration solves this by oxidising reduced NAD, allowing glycolysis to continue. In animals, pyruvate acts as the hydrogen acceptor from reduced NAD, forming lactate. In plants and yeast, pyruvate is decarboxylated to ethanal, releasing carbon dioxide. Ethanal then accepts hydrogen from reduced NAD, forming ethanol. Both pathways regenerate oxidised NAD. The only ATP yield is the net two molecules from glycolysis. While often taught that lactate lowers pH and causes fatigue, this is a simplified theoretical model.
What earns the marks
- Stating that in animals, pyruvate acts as the hydrogen acceptor from reduced NAD to form lactate.
- Stating that in plants and yeast, pyruvate is decarboxylated to form ethanal and carbon dioxide.
- Identifying that ethanal acts as the hydrogen acceptor from reduced NAD to form ethanol.
- Explaining that these reactions regenerate oxidised NAD, allowing glycolysis to continue.
- Stating that anaerobic respiration produces no further ATP beyond the net two molecules from glycolysis.
Common mistakes and how to put them right
Stating that pyruvate is the hydrogen acceptor in both pathways.
Put it right: Pyruvate is the acceptor in lactate fermentation, but ethanal is the acceptor in ethanol fermentation.
Conflating lactate with lactic acid.
Put it right: Lactate is the ionised form (C₃H₅O₃⁻), while lactic acid is the protonated form (C₃H₆O₃). Also, avoid stating that lactate directly causes muscle fatigue as an absolute fact, as this is a contested theoretical model.
Stating that anaerobic respiration produces ATP directly at the pyruvate conversion step.
Put it right: The only ATP produced anaerobically is the net gain from glycolysis.
Saying NAD is reduced during fermentation.
Put it right: Reduced NAD is being oxidised to regenerate NAD.
Exam tips
- Always specify the hydrogen acceptor accurately: pyruvate for animals, ethanal for plants and yeast.
- When explaining why anaerobic respiration is necessary, focus on the regeneration of oxidised NAD to keep glycolysis running, rather than just the end products.
Try the lesson
The start of the Respiration (A-level only): If respiration is only anaerobic, pyruvate can be converted to ethanol or lactate using reduced NAD. lesson, made from checked teaching notes for this specification point.
Have a go first
In anaerobic respiration, pyruvate is converted to ethanol or lactate. Which substance is used up in this conversion?
Listen
Without oxygen, the electron transfer chain stops and reduced NAD cannot be re-oxidised. Glycolysis would halt once cellular NAD is depleted. Anaerobic respiration solves this by oxidising reduced NAD, allowing glycolysis to continue. In animals, pyruvate acts as the hydrogen acceptor from reduced NAD, forming lactate. In plants and yeast, pyruvate is decarboxylated to ethanal, releasing carbon dioxide. Ethanal then accepts hydrogen from reduced NAD, forming ethanol. Both pathways regenerate oxidised NAD. The only ATP yield is the net two molecules from glycolysis. While often taught that lactate lowers pH and causes fatigue, this is a simplified theoretical model. A strong answer earns marks for these points. Stating that in animals, pyruvate acts as the hydrogen acceptor from reduced NAD to form lactate. Stating that in plants and yeast, pyruvate is decarboxylated to form ethanal and carbon dioxide. Identifying that ethanal acts as the hydrogen acceptor from reduced NAD to form ethanol. Explaining that these reactions regenerate oxidised NAD, allowing glycolysis to continue. A common slip: Stating that pyruvate is the hydrogen acceptor in both pathways. Instead, pyruvate is the acceptor in lactate fermentation, but ethanal is the acceptor in ethanol fermentation. Exam tip: Always specify the hydrogen acceptor accurately: pyruvate for animals, ethanal for plants and yeast.
Key words
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lactate fermentation
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ethanol and carbon dioxide
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hydrogen acceptors
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NAD regeneration
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See every definition
- lactate fermentation
- In animals, pyruvate acts as the hydrogen acceptor from reduced NAD, forming lactate.
- ethanol and carbon dioxide
- In plants and yeast, pyruvate is decarboxylated to ethanal, releasing carbon dioxide, and ethanal then accepts hydrogen from reduced NAD, forming ethanol.
- hydrogen acceptors
- Pyruvate is the hydrogen acceptor in animals, while ethanal is the hydrogen acceptor in plants and yeast.
- NAD regeneration
- Both pathways regenerate oxidised NAD, allowing glycolysis to continue.
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