Anaerobic respiration is respiration WITHOUT OXYGEN.
It occurs when oxygen is in short supply — for example, during very intense exercise when muscles cannot get enough oxygen fast enough.
Anaerobic respiration:
Does NOT require oxygen.
Starts with glucose, like aerobic respiration.
Produces MUCH LESS ATP than aerobic respiration (only about 2 ATP per glucose compared to ~36 for aerobic).
Produces different waste products depending on the organism.
It is a short-term emergency measure — useful for bursts of intense activity but not sustainable for long.
Anaerobic Respiration in Animals (including Humans)
In animal muscles (and some bacteria):
Word equation:
glucose → lactic acid
No oxygen needed. Only a small amount of ATP is produced.
Lactic acid is produced because glucose cannot be fully broken down without oxygen — lactic acid is the incomplete breakdown product.
Effects of lactic acid:
Lactic acid LOWERS the pH inside muscle cells.
This disrupts enzyme activity → muscles stop contracting properly.
This causes the BURNING SENSATION in muscles during intense exercise.
Leads to MUSCLE FATIGUE — the muscles cannot maintain maximum effort.
Lactic acid is NOT toxic long-term — after exercise, it is transported to the LIVER where it is converted back to glucose (or broken down further in aerobic respiration).
Anaerobic Respiration in Yeast (Fermentation)
In YEAST (and also in plants under waterlogged conditions):
Word equation:
glucose → ethanol + carbon dioxide
This process is called FERMENTATION.
Fermentation is enormously important in food and drink production:
BREAD MAKING:
Yeast is added to dough.
Yeast ferments sugars in the dough → produces CO₂.
CO₂ bubbles make the dough RISE (become light and airy).
Ethanol produced evaporates during baking.
ALCOHOLIC DRINKS (beer, wine, cider):
Yeast ferments sugars in grain (beer), grapes (wine) or fruit (cider).
Ethanol accumulates → forms the alcohol in the drink.
CO₂ also produced → gives fizzy drinks their bubbles.
BIOFUELS:
Ethanol from fermentation can be used as a biofuel — a renewable energy source.
Comparing Aerobic and Anaerobic Respiration
Here is a clear side-by-side comparison:
AEROBIC RESPIRATION:
Needs oxygen? YES.
Products: CO₂ + water.
ATP produced: ~36–38 per glucose.
Where: Mitochondria.
When used: Normal conditions.
ANAEROBIC RESPIRATION (animals):
Needs oxygen? NO.
Products: Lactic acid.
ATP produced: ~2 per glucose.
Where: Cytoplasm.
When used: Intense exercise, emergency.
ANAEROBIC RESPIRATION (yeast):
Needs oxygen? NO.
Products: Ethanol + CO₂.
ATP produced: ~2 per glucose.
Where: Cytoplasm.
When used: Absence of oxygen (e.g. in dough, in fermenting liquid).
⚠️ Common Mistake
Anaerobic respiration in ANIMALS produces LACTIC ACID. Anaerobic respiration in YEAST produces ETHANOL + CO₂. These are different products — this distinction comes up repeatedly in exams. Do NOT say 'lactic acid' when asked about yeast fermentation.
Animals: glucose → lactic acid (no O₂, little ATP, causes muscle fatigue). Yeast: glucose → ethanol + CO₂ (fermentation — used in bread, beer, wine). Both produce far less ATP than aerobic respiration.
🎯 Matching Activity — Aerobic or Anaerobic — and Which Organism?
Sort each statement into the correct type of respiration. — drag the symbols on the right to match the component names on the left.
Aerobic
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Anaerobic — animals
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Anaerobic — yeast
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Anaerobic — animals
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Anaerobic — yeast
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CO₂ produced makes bread dough rise
Causes muscle fatigue during intense exercise
No oxygen — produces lactic acid, causes burning feeling in muscles
No oxygen — produces ethanol and CO₂, used in bread and alcohol production
Requires oxygen — produces CO₂, water and ~36 ATP
🎯 Test Yourself
Question 1 of 4
1. What is produced by anaerobic respiration in yeast?
2. Why does bread dough rise when yeast is added?
3. During a sprint race, an athlete's muscles switch to anaerobic respiration. Why?
4. Which process produces more ATP per glucose molecule?
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