12.3 Anaerobic respiration
- Syllabus
- 0610–2026–2027
- Topic
- 12.3
- Level
- —
Learning objectives
12.3.1Anaerobic respiration as the chemical• Describe anaerobic respiration as the chemical reactions in cells that break down nutrient molecules to release energy without using oxygen12.3.2Anaerobic respiration releases much• State that anaerobic respiration releases much less energy per glucose molecule than aerobic respiration12.3.3Word equation for anaerobic• State the word equation for anaerobic respiration in yeast as: glucose → alcohol + carbon dioxide12.3.4Word equation for anaerobic• State the word equation for anaerobic respiration in muscles during vigorous exercise as: glucose → lactic acid12.3.5Anaerobic respiration equation• State the balanced chemical equation for anaerobic respiration in yeast as: C 6H12O6 → 2C2H5OH + 2CO212.3.6Lactic acid builds up in muscles• State that lactic acid builds up in muscles and blood during vigorous exercise causing an oxygen debt12.3.7How the oxygen debt is removed after• Outline how the oxygen debt is removed after exercise, limited to: (a) continuation of fast heart rate to transport lactic acid in the blood from the muscles to the liver (b) continuation of deeper and faster breathing to supply oxygen for aerobic respiration of lactic acid (c) aerobic respiration of lactic acid in the liver
Describe anaerobic respiration
Anaerobic respiration is the set of chemical reactions in cells that breaks down nutrient molecules to release energy without using oxygen.
| Feature | Anaerobic respiration |
|---|---|
| location | reactions occur in cells |
| oxygen | not used |
| nutrient | glucose is broken down |
| outcome | energy is released |
Anaerobic means without oxygen; it does not mean without respiration or without any energy release.
Compare energy release in anaerobic respiration
Anaerobic respiration releases much less energy per glucose molecule than aerobic respiration.
Glucose is only partly broken down anaerobically, so more chemical energy remains in lactic acid or alcohol than remains in the products of aerobic respiration.
The comparison is energy per glucose molecule, not necessarily the instantaneous rate of a whole organism's respiration.
State the word equation for anaerobic respiration in yeast
glucose → alcohol + carbon dioxide
| Role | Substance |
|---|---|
| reactant | glucose |
| products | alcohol (ethanol) and carbon dioxide |
Carbon dioxide makes bread dough rise; alcohol is the useful product in brewing.
Yeast produces alcohol and carbon dioxide anaerobically; it does not produce lactic acid in this syllabus equation.
State the word equation for anaerobic respiration in muscles
glucose → lactic acid
This pathway supplies some energy when vigorous exercise makes oxygen delivery insufficient for the muscle's demand.
The muscle equation does not include carbon dioxide, alcohol or oxygen.
State the balanced equation for anaerobic respiration in yeast
C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
| Element | Left side | Right side |
|---|---|---|
| carbon | 6 | 4 + 2 = 6 |
| hydrogen | 12 | 2 × 6 = 12 |
| oxygen | 6 | 2 × 1 + 2 × 2 = 6 |
One glucose molecule forms two ethanol molecules and two carbon-dioxide molecules.
The formula for ethanol is C₂H₅OH; coefficients are changed to balance the equation, not the subscripts in its formula.
Explain lactic acid build-up and oxygen debt
During vigorous exercise, oxygen delivery may not meet muscle demand, so muscle cells respire anaerobically and lactic acid accumulates in muscles and blood.
| Stage | Event |
|---|---|
| high demand | vigorous muscle contraction requires rapid energy release |
| limited oxygen | aerobic respiration cannot meet the full demand |
| anaerobic respiration | glucose is converted to lactic acid |
| accumulation | lactic acid builds up in muscles and enters the blood |
| consequence | an oxygen debt is created |
The oxygen debt is the extra oxygen needed after exercise to remove the accumulated lactic acid.
Lactic acid causes the oxygen debt; it is not produced by aerobic respiration.
Outline removal of the oxygen debt
Recovery continues oxygen delivery and transports lactic acid to the organ where it is removed.
| Recovery response | Function |
|---|---|
| heart rate remains fast | transports lactic acid in blood from muscles to the liver |
| breathing remains faster and deeper | supplies extra oxygen to the blood and liver |
| in the liver | lactic acid is respired aerobically |
| result | lactic acid concentration falls and the oxygen debt is removed |
The required destination is the liver, and removal uses aerobic respiration of lactic acid—not anaerobic respiration in the muscles.