12. Respiration
- Syllabus
- 0610–2026–2027
- Section
- 12
- Level
- —
Exam analysis
Most tested topics
12.1 Respiration
- Syllabus
- 0610–2026–2027
- Topic
- 12.1
- Level
- —
State how living organisms use energy
Respiration releases energy that cells transfer to processes that cannot proceed by passive movement alone.
| Required use | What the energy enables |
|---|---|
| muscle contraction | movement, posture, heartbeat and ventilation |
| protein synthesis | joining amino acids to make proteins |
| cell division | copying and separating cell contents to form new cells |
| active transport | moving substances against a concentration gradient |
| growth | making new cell material and increasing cell number or size |
| passage of nerve impulses | maintaining ion gradients and transmitting signals |
| constant body temperature | replacing heat lost to the surroundings |
Diffusion, osmosis and evaporation are passive processes; they do not directly use energy released by respiration.
Investigate temperature and respiration in yeast
Yeast respiration is enzyme-controlled, so its rate changes with temperature.
| Stage | Fair-test decision |
|---|---|
| prepare | use equal volumes and concentrations of yeast and glucose solution |
| vary temperature | place identical mixtures in water baths at a suitable range of temperatures and allow them to equilibrate |
| measure rate | collect carbon dioxide in a gas syringe, or count bubbles over the same timed interval |
| control | keep pH, yeast amount, glucose concentration, total volume and measurement time constant |
| reliability | repeat each temperature and calculate a mean rate |
| Temperature range | Expected respiration rate | Explanation |
|---|---|---|
| low to warmer | increases | particles have more kinetic energy, causing more successful enzyme–substrate collisions |
| optimum | highest | respiratory enzymes work at their fastest rate |
| above optimum | decreases | enzymes denature and active sites lose their complementary shape |
Temperature is the independent variable and carbon-dioxide production per unit time is the rate measure; comparing only final gas volume after unequal times is not a fair rate comparison.
12.2 Aerobic respiration
- Syllabus
- 0610–2026–2027
- Topic
- 12.2
- Level
- —
Describe aerobic respiration
Aerobic respiration is the set of chemical reactions in cells that uses oxygen to break down nutrient molecules and release energy.
| Part of the definition | Meaning |
|---|---|
| chemical reactions in cells | respiration is a cellular process, not simply breathing |
| uses oxygen | oxygen is a reactant |
| breaks down nutrient molecules | glucose is the required example |
| releases energy | energy is transferred for cellular and organism processes |
Energy is released by respiration; cells do not ‘make energy’, and ventilation alone is not respiration.
State the word equation for aerobic respiration
glucose + oxygen → carbon dioxide + water
| Side | Substances |
|---|---|
| reactants | glucose and oxygen are used |
| products | carbon dioxide and water are formed |
The arrow means ‘react to form’; it does not mean the equation can be reversed for respiration.
Energy is released during the reactions, but the required syllabus word equation lists the four substances shown above.
State the balanced equation for aerobic respiration
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
| Element | Left side | Right side |
|---|---|---|
| carbon | 6 | 6 |
| hydrogen | 12 | 12 |
| oxygen | 18 | 18 |
The ratio is 1 glucose : 6 oxygen : 6 carbon dioxide : 6 water. For three glucose molecules, multiply every coefficient by three.
Balance an equation by changing coefficients in front of formulae, never by changing subscripts inside a chemical formula.
12.3 Anaerobic respiration
- Syllabus
- 0610–2026–2027
- Topic
- 12.3
- Level
- —
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.