Anaerobic Respiration

The reaction.

glucose → lactic acid (+ small amount of energy)

Symbol equation:

$\text{C}_{6}\text{H}_{12}\text{O}_{6} \to 2\text{C}_{3}\text{H}_{6}\text{O}_{3}$

Note: NO oxygen used. NO CO₂ produced. Just lactic acid.

When does this happen?

• During HEAVY EXERCISE — sprinting, weightlifting.
• Muscles demand more O₂ than the lungs can supply.
• They switch to anaerobic respiration to keep going.

Why limited?

• Releases only ~2 ATP per glucose — much less than aerobic.
• Lactic acid BUILDS UP in muscles → fatigue, cramp, pain.
• Lactic acid in blood lowers pH → uncomfortable.
• Can only sustain anaerobic for ~30-90 seconds.

Recovery: oxygen debt.

• After heavy exercise, you keep PANTING for minutes.
• Why? The body needs EXTRA OXYGEN to break down the lactic acid.
• Lactic acid + O₂ → CO₂ + water (or → glucose, in the liver).
• This extra O₂ requirement is the OXYGEN DEBT.
• Repaying the debt = panting until lactic acid is cleared.

Worked qualitative. Why do sprinters look exhausted at the END of a 100 m race, not during it?

• During: anaerobic respiration sustains the burst.
• At the end: lactic acid has accumulated → fatigue + 'burn' in muscles.
• After: panting repays the oxygen debt over minutes.
• A 1500 m runner uses more aerobic; doesn't accumulate as much lactic acid → recovers faster.

Cambridge tip. Always link OXYGEN DEBT to LACTIC ACID. Don't say 'breathing makes up for not breathing'.

The reaction.

glucose → ethanol + carbon dioxide (+ small amount of energy)

Symbol equation:

$\text{C}_{6}\text{H}_{12}\text{O}_{6} \to 2\text{C}_{2}\text{H}_{5}\text{OH} + 2\text{CO}_{2}$

(C₂H₅OH = ethanol)

Yeast. Single-celled fungus that can do BOTH aerobic and anaerobic respiration. When O₂ is present, it does aerobic. When O₂ is absent, it switches to fermentation.

Industrial uses:

1. Brewing (alcohol production).

• Sugars from grain (beer) or fruit (wine) + yeast in a sealed tank.
• Yeast ferments → produces ETHANOL.
• CO₂ escapes (or is captured for fizzy drinks).
• Stops when alcohol becomes too high (~12-15%) and kills yeast.
• Then the drink is bottled.

2. Bread making.

• Yeast added to dough.
• Sugars in flour fermented → CO₂ bubbles.
• Bubbles trapped in dough → it RISES.
• Baking kills yeast and evaporates ethanol → fluffy bread.

3. Biofuel.

• Sugars from sugar cane or maize fermented to ethanol.
• Ethanol burned as a fuel — alternative to fossil fuels.
• Brazilian cars run on a high-ethanol blend.

Worked qualitative. Why does yeast switch to fermentation when sealed in a brewing tank?

• Initially aerobic (uses O₂ to grow rapidly).
• O₂ runs out as yeast multiplies.
• Yeast switches to anaerobic — releases CO₂ (which builds up in the sealed tank) and ethanol (stays in the liquid).
• Brewer wants ethanol → seals the tank to force anaerobic conditions.

Cambridge tip. Memorise the YEAST equation separately from the ANIMAL one. Cambridge often asks for both.

Feature	Aerobic	Anaerobic (animal)	Anaerobic (yeast)
Oxygen	Required	Not used	Not used
Glucose breakdown	Complete	Partial	Partial
Products	CO₂ + H₂O	Lactic acid	Ethanol + CO₂
ATP per glucose	~30	~2	~2
Site	Mitochondria	Cytoplasm	Cytoplasm
Speed	Slower	Fast	Fast
When?	Normal	Heavy exercise	Brewing, breadmaking

Worked qualitative. Why can yeast survive in a sealed jar of fruit juice but a fish can't survive in stagnant water?

• Yeast can switch to anaerobic respiration (fermentation) — keeps growing without O₂.
• Fish (and most animals) can do anaerobic respiration only briefly — lactic acid builds up + fish can't get enough ATP from anaerobic alone.
• Microbes are far more flexible than animals.

Cambridge tip. Cambridge often gives a table for students to fill in. Memorise the row-by-row contrast.