Compare anaerobic respiration and energy yield
Outline lactate fermentation in mammals and ethanol fermentation in yeast, then explain why aerobic yield is greater.
Section 12.2 · Respiration
A Level; Cambridge International Biology 9700, 2028–2030. Written practice is self-checked; practical investigations need supervised work and human subject-teacher review remains pending.
Before you begin
- ATP transfers energy for cellular work, and enzymes control metabolic reactions.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which product forms during anaerobic respiration in yeast?
What you will learn
- Describe anaerobic pathways in mammals and yeast
- Compare energy yield per glucose in aerobic and anaerobic conditions
Define anaerobic respiration without oxygen use
| Feature | Meaning |
|---|---|
| Location | Cells |
| Process | Nutrient breakdown |
| Oxygen condition | Not used by this pathway |
| Outcome | Energy released |
Build the explanation
When oxygen is unavailable or limited, glycolysis can continue if reduced NAD is reoxidised. In mammalian cells, pyruvate is reduced to lactate. In yeast, pyruvate is decarboxylated and then reduced to ethanol, releasing carbon dioxide. These pathways regenerate NAD so glycolysis can continue, but they do not completely oxidise glucose, so their energy yield per glucose is much lower than in aerobic respiration. Aerobic respiration continues oxidation through the link reaction, Krebs cycle and oxidative phosphorylation; no detailed total ATP yield is required here.
Apply it carefully
Do not use 'lactic acid in yeast' or 'ethanol in mammalian muscle' for these syllabus examples. Anaerobic pathways are not simply oxygen-free versions of all aerobic stages: they allow glycolysis to continue by regenerating NAD. Compare energy per glucose molecule, not total energy without considering how much substrate is used.
Pause and explain
Why is energy yield per glucose much lower anaerobically?
Worked example
Name the main end product of anaerobic respiration in mammalian muscle cells in this syllabus model.
Show the worked solution
- Distinguish the mammalian pathway from yeast fermentation.
- Mammalian cells produce lactate.
Answer Lactate.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Compare anaerobic respiration in a mammalian muscle cell and in yeast, then explain the difference in energy yield.
Give me a hint
Name organism-specific products and connect yield to extent of oxidation.
Compare my reasoning
- Mammalian cells form lactate; yeast forms ethanol and carbon dioxide.
- Both regenerate NAD for glycolysis; anaerobic glucose is only partly oxidised, so yield per glucose is much lower than aerobic respiration.
Mammalian cells reduce pyruvate to lactate. Yeast decarboxylates pyruvate and forms ethanol and carbon dioxide. Both pathways regenerate NAD so glycolysis can continue. Anaerobic breakdown is incomplete and yields much less energy per glucose than aerobic respiration, which continues through mitochondrial stages.
Look for these in your work
- Mammal product correct
- Yeast products correct
- NAD regeneration linked
- Per-glucose yield explained
A student claims yeast fermentation has a higher energy yield because it releases carbon dioxide. Correct the reasoning.
Give me a hint
A gas product does not indicate how much glucose has been oxidised.
Compare my reasoning
- Carbon dioxide release is not the same as energy yield.
- Yeast fermentation only partly oxidises glucose; its energy yield per glucose is lower than aerobic respiration.
Releasing carbon dioxide does not show that more energy is transferred. Yeast fermentation only partly oxidises glucose and regenerates NAD for glycolysis, so its energy yield per glucose is much lower than aerobic respiration.
Look for these in your work
- Gas/energy distinction
- Partial oxidation stated
- Correct comparison
A muscle cell continues glycolysis during oxygen limitation. Explain how lactate formation supports this and state one limitation of the process.
Give me a hint
Connect NAD recycling to glycolysis; do not claim lactate releases the same energy as aerobic respiration.
Compare my reasoning
- Pyruvate is reduced to lactate, regenerating NAD.
- Glycolysis can continue but yields much less energy per glucose than aerobic respiration.
Reducing pyruvate to lactate reoxidises reduced NAD to NAD, allowing glycolysis to continue. This pathway yields much less energy per glucose because glucose is not fully oxidised through the aerobic mitochondrial stages.
Look for these in your work
- Lactate pathway correct
- NAD recycling explained
- Energy limitation stated
Common mistakes
- Swapping lactate and ethanol products
- Equating carbon dioxide release with high energy yield
- Giving a fixed ATP total not required by this syllabus
What can you explain now?
Which product forms during anaerobic respiration in yeast?
Compare with the explanation
Ethanol and carbon dioxide
Yeast pyruvate is decarboxylated and reduced to ethanol.
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Make it stick
How do lactate and ethanol fermentation differ, and why does aerobic respiration yield more energy per glucose?
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