RespirationOxford AQA IGCSE Biology: Revision notes
Section 1
What is aerobic respiration and how does it work?
Aerobic respiration is the breakdown of glucose using oxygen to release energy. This process occurs in the mitochondria of cells and is the main way organisms release energy for life processes.
Word equation for aerobic respiration: glucose + oxygen → carbon dioxide + water
Balanced chemical equation for aerobic respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
Aerobic respiration is highly efficient because oxygen is the final electron acceptor in the electron transport chain. The complete oxidation of glucose releases approximately 2,880 kJ of energy per mole, much of which is captured in ATP molecules for use by the cell.
Examiners expect you to write both the word equation and the balanced chemical equation. The balanced equation shows that 6 molecules of oxygen and 6 molecules of carbon dioxide are produced per glucose molecule.
Section 2
What is anaerobic respiration and when does it occur?
Anaerobic respiration occurs when oxygen is not available. It releases energy from glucose without using oxygen, but produces much less ATP than aerobic respiration.
In animals (including humans): Word equation: glucose → lactic acid
In plants and yeast: Word equation: glucose → ethanol + carbon dioxide
Conditions favouring anaerobic respiration:
- During intense exercise when oxygen supply cannot meet demand
- In anaerobic organisms or environments (e.g. deep soil, sediments)
- In cells with few mitochondria
Why anaerobic respiration is less efficient: Anaerobic respiration releases only about 2% of the energy that aerobic respiration does per glucose molecule. This is because glucose is only partially broken down; in animals, it is converted to lactic acid, and in plants and yeast, to ethanol and carbon dioxide. The glucose molecule still contains significant chemical energy that is not extracted.
Students often forget that anaerobic respiration in plants and yeast produces both ethanol AND carbon dioxide, not just one product. Ensure you write both products in the equation.
Think of aerobic respiration as fully burning fuel in a engine (complete combustion), while anaerobic respiration is like incomplete combustion – it releases some energy but leaves useful energy unused.
Section 3
What is oxygen debt and how is it repaid? (Higher Tier)
Oxygen debt is the volume of oxygen needed after exercise to break down the accumulated lactic acid and restore normal conditions in the body.
How oxygen debt occurs:
- During intense exercise, muscles work faster than aerobic respiration can supply oxygen
- Anaerobic respiration occurs, producing lactic acid which accumulates in muscle cells and blood
- Lactic acid causes muscle fatigue and cramping
How oxygen debt is repaid:
- After exercise stops, breathing remains elevated, delivering extra oxygen to the body
- The liver takes up lactic acid from the blood
- In the liver, lactic acid is converted back to glucose through gluconeogenesis
- This process requires ATP from aerobic respiration, which is why extra oxygen is needed
- Muscles gradually relax as lactate levels decrease and energy stores are replenished
- Normal resting breathing rate is restored
The amount of oxygen debt depends on exercise intensity and duration. More intense exercise creates greater oxygen debt because anaerobic respiration dominates for longer periods.
Higher Tier candidates must explain the full sequence: anaerobic respiration produces lactic acid during intense exercise, then oxygen is used to convert lactic acid back to glucose in the liver after exercise ends.
A runner completes a 400m sprint and continues breathing heavily for several minutes afterwards. The elevated breathing is repaying oxygen debt – delivering oxygen for the liver to break down the lactic acid accumulated during the sprint.
Section 4
How is energy from respiration used in the body?
Energy released from respiration is used for four main life processes:
| Use of Energy | Explanation |
|---|---|
| Muscle contraction | Energy (ATP) is needed to allow actin and myosin filaments to slide past each other, causing muscle contraction |
| Active transport | Energy is required to pump substances against the concentration gradient across cell membranes (e.g. sodium-potassium pump) |
| Maintaining body temperature | Heat energy from respiration maintains a constant internal body temperature in mammals; some energy is deliberately wasted as heat through shivering |
| Synthesising molecules | Energy is needed for anabolic processes such as protein synthesis, DNA replication, and formation of glycogen and fat stores |
Distribution of energy: Not all energy from respiration is captured in ATP. A significant proportion is lost as heat, which is why cells are typically only 25-40% efficient. The remainder maintains body temperature and is eventually lost to the environment.
Examiners want to see specific examples of each energy use. For muscle contraction, mention sliding filaments; for active transport, give examples like sodium-potassium pump or glucose uptake in the small intestine.
Section 5
How can the rate of respiration in yeast be investigated?
Respiration rate in yeast can be measured using various methods. The two most common approaches are:
Method 1: Measuring carbon dioxide production
- Yeast cells are placed in a glucose solution at a controlled temperature
- Gas is bubbled through limewater or collected in a gas syringe
- Carbon dioxide produced during respiration turns limewater cloudy
- The rate of gas production indicates respiration rate
- Variables can be changed: glucose concentration, temperature, pH
Method 2: Using a respirometer
- A sealed container holds yeast culture
- As oxygen is consumed, pressure decreases or volume changes
- Gas volume change is measured over time
- This directly measures oxygen uptake, indicating aerobic respiration rate
Variables that affect yeast respiration rate:
- Temperature: Enzyme activity increases with temperature up to an optimum (~40°C for yeast), then denatures
- Glucose concentration: Higher substrate concentration increases respiration rate until enzymes are saturated
- Oxygen availability: Anaerobic conditions reduce respiration rate significantly
- pH: Yeast has optimal pH around 4.5-6; extremes denature enzymes
Key experimental considerations:
- Keep variables controlled (maintain constant temperature, use same yeast mass)
- Measure volume/mass change at regular time intervals
- Use sterile conditions to prevent contamination
- Allow apparatus to equilibrate before recording measurements
In a respirometer experiment: 50g of yeast is added to 100cm³ of 5% glucose solution at 30°C. Gas volume is recorded every 5 minutes for 30 minutes. Results show volume increases at 2.5cm³/min, indicating rapid CO₂ production from aerobic respiration.
When describing investigations, state what you are measuring (e.g. 'volume of CO₂ produced' or 'rate of oxygen consumption'), the independent variable being tested, and how you keep other variables constant.
Must Know
- Aerobic respiration equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O; occurs in mitochondria and releases energy for life processes
- Anaerobic respiration in animals: glucose → lactic acid; in plants and yeast: glucose → ethanol + CO₂
- Anaerobic respiration is much less efficient than aerobic because glucose is only partially broken down; it occurs when oxygen is unavailable
- Oxygen debt: the extra oxygen needed after exercise to convert lactic acid back to glucose in the liver; explains why we continue breathing heavily after intense exercise
- Energy from respiration is used for: muscle contraction, active transport, maintaining body temperature, and synthesising molecules
- Respiration rate in yeast can be measured using CO₂ production (limewater cloudiness) or a respirometer (oxygen uptake); varies with temperature, glucose concentration, oxygen availability, and pH
That's the notes covered.
Carry on to the next subtopic.