RespirationEdexcel IGCSE Biology: Revision notes
Section 1
What is respiration and why do cells need it?
Respiration is the process by which living cells release energy from glucose. This energy is transferred to ATP, which the cell can then use to power processes such as movement, active transport, and building large molecules.
Respiration takes place continuously in every living cell — it is not the same as breathing (ventilation), which is simply the physical movement of air in and out of the lungs.
Never confuse respiration (a chemical process in cells) with breathing (the physical movement of air) — they are different processes.
Section 2
Aerobic respiration
Aerobic respiration requires oxygen and releases a large amount of energy.
Word equation: glucose + oxygen → carbon dioxide + water
Balanced symbol equation: C6H12O6 + 6O2 → 6CO2 + 6H2O
Most respiration in animals, plants and many microorganisms is aerobic, as it releases far more energy per glucose molecule than anaerobic respiration.
Give both the word equation and the balanced symbol equation when asked to 'describe' or 'write the equation for' aerobic respiration.
Section 3
Anaerobic respiration
Anaerobic respiration occurs without oxygen and releases much less energy than aerobic respiration, because glucose is only partially broken down.
- In animals (e.g. human muscle during vigorous exercise): glucose → lactic acid
- In plants and yeast: glucose → ethanol + carbon dioxide
Anaerobic respiration in muscles allows exercise to continue when oxygen supply cannot keep up with demand, but lactic acid build-up causes muscle fatigue and must later be broken down (requiring oxygen — the 'oxygen debt').
During a sprint, muscles respire anaerobically once oxygen supply cannot match demand, producing lactic acid and causing fatigue.
Section 4
Comparing aerobic and anaerobic respiration
| Feature | Aerobic | Anaerobic |
|---|---|---|
| Oxygen needed? | Yes | No |
| Energy released | Large amount | Small amount |
| Products (animals) | CO2 + water | Lactic acid |
| Products (plants/yeast) | CO2 + water | Ethanol + CO2 |
| Glucose breakdown | Complete | Incomplete |
Section 5
Investigating respiration in seeds
The evolution of carbon dioxide and heat from respiring seeds (or other organisms) can be investigated by placing soaked, germinating seeds in an insulated flask with a thermometer, and comparing to a control flask of dead (boiled) seeds. A rise in temperature over time shows heat released by respiration. Carbon dioxide production can be shown by passing air from the flask through limewater, which turns cloudy (milky), or using hydrogen-carbonate indicator, which turns yellow.
Always include a control (dead/boiled seeds) in this investigation to show the temperature/CO2 change is due to respiration, not the environment.
Must Know
- Respiration releases energy (as ATP) from glucose in all living cells, continuously
- Aerobic respiration: glucose + oxygen → carbon dioxide + water; releases a large amount of energy
- Anaerobic respiration in animals: glucose → lactic acid (no oxygen, small energy release)
- Anaerobic respiration in plants/yeast: glucose → ethanol + carbon dioxide
- Anaerobic respiration releases much less energy than aerobic because glucose breakdown is incomplete
- Respiring seeds release heat and CO2, shown using a thermometer and limewater/hydrogen-carbonate indicator, with a dead-seed control
That's the notes covered.
Carry on to the next subtopic.