1. Overview
Aerobic respiration is a fundamental metabolic process that occurs in the cells of almost all living organisms. It is the chemical process by which cells break down food molecules (primarily glucose) in the presence of oxygen to provide the energy required for all life processes, such as muscle contraction, cell division, and protein synthesis.
Key Definitions
- Aerobic Respiration: The chemical reactions in cells that use oxygen to break down nutrient molecules to release energy.
- Glucose: A simple sugar (carbohydrate) that acts as the primary fuel source for respiration.
- Mitochondria: The specialized organelles within a cell where the most significant stages of aerobic respiration occur.
- Metabolism: The sum of all chemical reactions that happen within a cell or organism.
Core Content
Aerobic respiration is a multi-step chemical reaction that happens continuously in every living cell. If respiration stops, the cell dies because it no longer has energy for maintenance.
The Purpose of Aerobic Respiration: The main goal is to release energy. This energy is used for:
- Muscle contraction (movement).
- Maintaining a constant body temperature (in mammals and birds).
- Active transport (moving molecules against concentration gradients).
- Building large molecules (like proteins from amino acids).
The Process:
- Glucose enters the cell from the bloodstream (after digestion).
- Oxygen enters the cell from the bloodstream (after gas exchange in the lungs).
- These molecules react inside the cytoplasm and the mitochondria.
- The chemical bonds in glucose are broken, releasing energy.
- Carbon dioxide and water are produced as waste products and leave the cell.
Word Equation:
glucose + oxygen → carbon dioxide + water
Structure and Function:
- Mitochondria: These are often called the "powerhouses" of the cell. Cells that require a lot of energy (like muscle cells or sperm cells) contain many more mitochondria than less active cells (like skin cells).
- Internal Membrane: The inner membrane of the mitochondrion is highly folded to increase surface area, allowing more respiration reactions to happen simultaneously.
Extended Content (Extended curriculum only)
In the extended curriculum, you must be able to represent the chemical reaction using the correct molecular formulas and ensure the equation is balanced.
Balanced Chemical Equation: The breakdown of one molecule of glucose requires six molecules of oxygen to produce six molecules of carbon dioxide and six molecules of water.
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
Step-by-Step Balancing:
- Reactants: 6 Carbons, 12 Hydrogens, 18 Oxygens (6 from glucose + 12 from oxygen gas).
- Products: 6 Carbons, 12 Hydrogens (6x2), 18 Oxygens (12 from CO₂ + 6 from H₂O).
Key Equations
| Type | Equation |
|---|---|
| Word Equation | glucose + oxygen → carbon dioxide + water |
| Chemical Equation | C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O |
Symbols used:
- C₆H₁₂O₆: Glucose
- O₂: Oxygen gas
- CO₂: Carbon dioxide
- H₂O: Water
Common Mistakes to Avoid
- ❌ Wrong: "Respiration is the same as breathing."
- ✓ Right: Breathing (ventilation) is the physical movement of air; Respiration is the chemical reaction inside cells.
- ❌ Wrong: "Respiration produces energy."
- ✓ Right: Respiration releases energy (energy cannot be created or destroyed, only transferred from chemical stores).
- ❌ Wrong: "Plants only photosynthesize, they don't respire."
- ✓ Right: Plants respire 24 hours a day to stay alive; they only photosynthesize when light is available.
- ❌ Wrong: Writing the chemical equation without balancing it (e.g., C₆H₁₂O₆ + O₂ → CO₂ + H₂O).
- ✓ Right: Always include the "6" before O₂, CO₂, and H₂O.
Exam Tips
- Command Words: If the question asks you to "State" the equation, you can provide either the word or chemical equation unless the question specifies "balanced chemical equation."
- Question Types: Expect to see "fill in the blanks" for the equation or questions asking you to identify where in the cell respiration occurs.
- Real-world Context: Questions often link respiration to exercise. Remember that during exercise, the rate of aerobic respiration increases to provide more energy for muscle contraction, which is why your heart rate and breathing rate increase (to supply more O₂ and glucose).
- Typical Values: In paper 6 (Alternative to Practical), you might see data showing the uptake of oxygen by organisms. Remember that the more oxygen consumed, the higher the rate of aerobic respiration.
Exam-Style Questions
Practice these original exam-style questions to test your understanding. Each question mirrors the style, structure, and mark allocation of real Cambridge 0610 Theory papers.
Exam-Style Question 1 — Short Answer [5 marks]
Question:
(a) Define aerobic respiration. [2]
(b) State the balanced chemical equation for aerobic respiration. [3]
Worked Solution:
(a)
Aerobic respiration is the breakdown of nutrient molecules. Aerobic respiration is the breakdown of nutrient molecules...
...using oxygen... ...using oxygen...
...to release energy. ...to release energy.
How to earn full marks:
- Must mention the breakdown of nutrient molecules.
- Must mention the use of oxygen.
- Must mention the release of energy.
(b)
Correct formula for glucose. $C_6H_{12}O_6$
Correct formula for oxygen, carbon dioxide, and water. $6O_2 \rightarrow 6CO_2 + 6H_2O$
Balanced equation. $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O$
How to earn full marks:
- Must have the correct formula for glucose.
- Must have the correct formulas for oxygen, carbon dioxide, and water and the correct stoichiometric coefficients.
- Must have the whole equation balanced. $\boxed{C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O}$
Common Pitfall: Make sure you know the word equation and the balanced chemical equation for aerobic respiration. Don't forget to include the "energy" part in your definition, as that's the whole point of the process!
Exam-Style Question 2 — Short Answer [6 marks]
Question:
A student is investigating aerobic respiration in yeast. They place a suspension of yeast and glucose solution in a sealed flask connected to a manometer, as shown in
(a) State two observations that would indicate that aerobic respiration is taking place in the flask. [2]
(b) Explain why the level of the coloured liquid in the manometer changes during aerobic respiration. [4]
Worked Solution:
(a)
Oxygen is used up. Decrease in oxygen concentration.
Carbon dioxide is produced. Increase in carbon dioxide concentration.
How to earn full marks:
- Must mention both oxygen and carbon dioxide.
- Must indicate a change in concentration (increase or decrease).
(b)
Respiration produces carbon dioxide. Respiration produces carbon dioxide gas.
The number of gas molecules increases. The number of gas molecules in the flask increases.
Pressure in the flask increases. This increases the pressure inside the flask.
The coloured liquid in the manometer is pushed. The increased pressure pushes the coloured liquid in the manometer.
How to earn full marks:
- Must mention carbon dioxide production.
- Must link the gas production to an increase in pressure.
- Must explain how increased pressure affects the manometer.
Common Pitfall: Remember that aerobic respiration produces carbon dioxide. Some students incorrectly think that the overall gas pressure will stay the same because oxygen is used up. However, more CO2 is produced than O2 consumed, leading to a net increase in gas pressure.
Exam-Style Question 3 — Extended Response [8 marks]
Question:
An athlete is running a marathon. During the race, their muscle cells respire both aerobically and anaerobically.
(a) Describe the role of aerobic respiration in providing energy for the athlete's muscles. [4]
(b) Explain how the rate of aerobic respiration changes in the athlete's muscles during the marathon, from the start to the end of the race. [4]
Worked Solution:
(a)
Glucose is the substrate for respiration. Glucose is broken down.
Oxygen is required. Oxygen is used.
Energy is released. Energy is released.
ATP is produced. This energy is used to produce ATP (adenosine triphosphate).
ATP provides energy for muscle contraction. ATP is then used to provide energy for muscle contraction.
How to earn full marks:
- Must mention glucose as the substrate and oxygen being used.
- Must state that energy is released and ATP is produced.
- Must link ATP to muscle contraction.
(b)
Initially, the rate increases. At the start of the race, the rate of aerobic respiration increases.
More energy required. This is because the muscles need more energy for increased activity.
Oxygen supply increases. The body increases oxygen supply to the muscles.
Eventually the rate plateaus. Towards the end of the race, the rate may plateau or even decrease if the athlete is fatigued.
Oxygen demand exceeds supply. Because oxygen demand exceeds supply.
How to earn full marks:
- Must mention the initial increase in respiration rate.
- Must link this increase to increased energy demand and oxygen supply.
- Must state that the rate may plateau or decrease towards the end due to fatigue.
Common Pitfall: Don't confuse aerobic and anaerobic respiration. Aerobic respiration is the primary source of energy during sustained activity, but anaerobic respiration kicks in when oxygen supply is limited. Also, be sure to link the rate of respiration to the energy demands of the muscles.
Exam-Style Question 4 — Extended Response [9 marks]
Question:
An experiment is set up to investigate the effect of temperature on the rate of aerobic respiration in germinating pea seeds. The volume of oxygen consumed by the seeds is measured at different temperatures over a period of one hour. The results are shown in the table below:
| Temperature (°C) | Volume of Oxygen Consumed (cm³) |
|---|---|
| 10 | 2.5 |
| 20 | 7.0 |
| 30 | 14.0 |
| 40 | 10.5 |
| 50 | 3.0 |
(a) Describe the trend shown by the data in the table. [2]
(b) Calculate the rate of oxygen consumption at 20°C in $cm^3/minute$. [2]
(c) Explain the effect of temperature on the rate of aerobic respiration in the germinating pea seeds. [5]
Worked Solution:
(a)
As temperature increases, oxygen consumption increases... As the temperature increases from 10°C to 30°C, the volume of oxygen consumed increases.
...then decreases. Above 30°C, the volume of oxygen consumed decreases.
How to earn full marks:
- Must mention that the oxygen consumption increases with temperature initially.
- Must mention that the oxygen consumption decreases at higher temperatures.
(b)
Rate is volume/time. Rate = Volume / Time
Convert time to minutes. Time = 1 hour = 60 minutes
Rate calculation. Rate = 7.0 cm³ / 60 minutes = 0.11666666 cm³/minute
Correct answer. Rate = $\boxed{0.12 \text{ } cm^3/minute}$ (rounded to 2 s.f.)
How to earn full marks:
- Must show the correct formula (volume/time).
- Must convert the time to minutes.
- Must calculate the rate correctly and include the units.
(c)
Enzymes control respiration. Aerobic respiration is controlled by enzymes.
Rate increases with temperature (up to optimum). As temperature increases, the rate of enzyme activity increases, leading to a higher rate of respiration.
Optimum temperature. There is an optimum temperature for enzyme activity.
Above optimum, enzymes denature. Above the optimum temperature, the enzymes start to denature (change shape).
Denaturing reduces rate. This reduces the rate of reaction as the substrate no longer fits the active site.
How to earn full marks:
- Must mention that enzymes control respiration.
- Must explain that the rate increases with temperature due to increased enzyme activity.
- Must explain that the rate decreases above the optimum temperature due to enzyme denaturation.
Common Pitfall: Remember that enzymes are involved in respiration, and their activity is affected by temperature. Don't just say "temperature increases respiration"; explain why using enzyme concepts like optimum temperature and denaturation. Also, pay close attention to units when calculating rates.