B20: Aerobic Respiration
Aerobic respiration equations, mitochondria, ATP, uses of energy from respiration, and comparison with photosynthesis for GCSE Biology.
Key Definitions
- Aerobic respiration – the chemical process by which glucose is broken down using oxygen to release energy for the cell; takes place in mitochondria.
- ATP (adenosine triphosphate) – the molecule that stores and releases energy for cellular processes.
- Mitochondria – organelles in cells where aerobic respiration takes place.
The Equations for Aerobic Respiration
Word Equation
glucose + oxygen → carbon dioxide + water
Symbol Equation
C6H12O6 + 6O2 → 6CO2 + 6H2O
Energy is released during this reaction but does not appear in the balanced chemical equation.
Where Does Aerobic Respiration Happen?
Aerobic respiration takes place in the mitochondria of cells. Cells that require a lot of energy (such as muscle cells and sperm cells) contain large numbers of mitochondria to meet their high energy demand.
Uses of Energy from Respiration
| Use of Energy | Explanation |
|---|---|
| Muscle contraction | Energy is needed for muscle fibres to contract during movement |
| Protein synthesis | Building proteins from amino acids on ribosomes requires energy |
| Cell division | Mitosis and the production of new cells require energy |
| Maintaining body temperature | In mammals and birds, respiration generates heat to maintain a constant internal body temperature |
| Active transport | Moving substances against a concentration gradient (e.g. mineral ions into root hair cells) requires energy |
| Transmission of nerve impulses | Pumping ions across neurone membranes to maintain resting potential requires energy |
Worked Example 1 – Mitochondria and Active Cells
Question: Muscle cells contain many more mitochondria than skin cells. Explain why.
Solution: Muscle cells require large amounts of energy for muscle contraction, which happens very frequently. Since aerobic respiration takes place in the mitochondria, having a large number of mitochondria allows the muscle cell to produce more ATP (energy) at a faster rate. Skin cells have a lower energy demand and therefore do not need as many mitochondria.
Worked Example 2 – Respiration and Body Temperature
Question: Explain how respiration helps mammals maintain a constant body temperature of about 37 °C.
Solution: During aerobic respiration, some of the energy released is in the form of heat. In mammals, this heat is used to maintain a constant internal body temperature of around 37 °C. When the external temperature drops, the body can increase the rate of respiration (e.g. by shivering, which involves rapid muscle contractions) to generate more heat and prevent the body from cooling down. This is an example of homeostasis.
Worked Example 3 – Respiration vs Photosynthesis Comparison
Question: Compare the equations for aerobic respiration and photosynthesis. What is the relationship between the two processes?
Solution:
Respiration: C6H12O6 + 6O2 → 6CO2 + 6H2O
Photosynthesis: 6CO2 + 6H2O → C6H12O6 + 6O2
The two equations are essentially the reverse of each other. The products of one process are the reactants of the other. This shows that respiration and photosynthesis are complementary processes. Plants carry out both. During the day, the rate of photosynthesis is typically higher than respiration (net gain of glucose and O2). At night, only respiration occurs (net release of CO2).
Worked Example 4 – Active Transport and Respiration
Question: Root hair cells absorb nitrates from the soil against a concentration gradient. Explain why these cells need to carry out aerobic respiration.
Solution: Absorbing nitrates against the concentration gradient requires active transport, which is an energy-requiring process. The energy for active transport comes from aerobic respiration, which produces ATP. Without respiration, the root hair cell would not have the energy to actively transport nitrates into the cell, and the plant would not be able to make amino acids for protein synthesis. This is why root hair cells also have many mitochondria.
ATP: The Energy Currency
Energy released during respiration is used to make ATP. ATP acts as an energy-carrying molecule. When a cell needs energy, ATP is broken down to ADP + phosphate, releasing energy. When energy is available from respiration, ADP + phosphate are combined to remake ATP. This is a continuous cycle.
Practice Questions
Foundation
Q1. Write the word equation and the balanced symbol equation for aerobic respiration.
Show Answer
Word equation: glucose + oxygen → carbon dioxide + water
Symbol equation: C6H12O6 + 6O2 → 6CO2 + 6H2O
Foundation
Q2. Name the organelle where aerobic respiration takes place and explain why muscle cells contain many of these organelles.
Show Answer
Aerobic respiration takes place in the mitochondria. Muscle cells contain many mitochondria because they require large amounts of energy for muscle contraction. Having more mitochondria means the cell can carry out respiration at a faster rate and produce more ATP to meet its high energy demand.
Higher
Q3. Describe the role of ATP in cells and explain why cells need to produce ATP continuously.
Show Answer
ATP is the energy currency of the cell. It stores energy in a form that can be readily used by cellular processes. When a cell needs energy, ATP is broken down to ADP and phosphate, releasing energy for processes such as muscle contraction, protein synthesis, and active transport. Cells need to produce ATP continuously because each ATP molecule can only release a small amount of energy and is quickly used up, and cellular processes require a constant supply of energy.
Foundation
Q4. Give four uses of the energy released by aerobic respiration in the human body.
Show Answer
Any four from: muscle contraction, protein synthesis, cell division, maintaining body temperature, active transport, transmission of nerve impulses.
Higher
Q5. Explain the relationship between the equations for photosynthesis and aerobic respiration. Why is this relationship important for life on Earth?
Show Answer
The equations are the reverse of each other: photosynthesis uses CO2 and H2O to produce glucose and O2, while respiration uses glucose and O2 to produce CO2 and H2O. This complementary relationship is important because it means the outputs of one process are the inputs of the other, creating a natural balance of gases in the atmosphere. Plants produce the oxygen that animals need for respiration, and animals produce the CO2 that plants need for photosynthesis.
Exam Tips
- Make sure you can write both the word and symbol equations for aerobic respiration from memory.
- Always link “energy” to specific uses – don't just say “energy is released”, say what it is used for.
- When comparing respiration and photosynthesis, note they are the reverse of each other.
- Questions about mitochondria often link to active cells (muscle, sperm, root hair) – always explain the connection to energy demand.
- Remember: respiration is not breathing. Breathing is the physical process of ventilating the lungs; respiration is a chemical process in cells.
🔢 Maths Skills
Mathematical Skills
Calculating respiration rates: measure the volume of CO₂ produced or O₂ consumed per unit time (e.g. cm³/min). Compare respiration rates of different organisms or at different temperatures. Calculate mean rates from repeated measurements and identify anomalies.
⚠️ Common Misconceptions
Watch Out!
1. Wrong: Respiration is the same as breathing Correct: Respiration is a chemical process that occurs in cells (mitochondria), releasing energy from glucose. Breathing is the physical process of ventilating the lungs to exchange gases.
2. Wrong: Only animals respire Correct: All living organisms respire, including plants, fungi, and microorganisms. Plants respire continuously, even during the day when they also photosynthesise.
✍️ 6-Mark Question
Extended Answer
6 marks: Compare aerobic respiration with photosynthesis.
Aerobic respiration and photosynthesis are complementary processes. Respiration: glucose + oxygen yields carbon dioxide + water, releasing energy. Photosynthesis: carbon dioxide + water yields glucose + oxygen, requiring light energy. The products of one process are the reactants of the other. Respiration occurs in mitochondria of all living cells, continuously, day and night. Photosynthesis occurs in chloroplasts of plant cells, only in light. Respiration releases energy (exothermic); photosynthesis requires energy input (endothermic). Plants carry out both processes — during the day, photosynthesis rate is higher (net O₂ release); at night, only respiration occurs (net CO₂ release).
Mark scheme: 1 mark for equations being reverse of each other; 1 mark for respiration releases energy / photosynthesis requires energy; 1 mark for locations (mitochondria vs chloroplasts); 1 mark for when each occurs (continuous vs light only); 1 mark for both occur in plants with net gas exchange explanation; 1 mark for QWC
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
A student uses a respirometer to measure CO₂ production by germinating pea seeds at different temperatures: 5 °C = 0.8 cm³/min; 15 °C = 2.4 cm³/min; 25 °C = 4.6 cm³/min; 35 °C = 3.9 cm³/min; 45 °C = 0.5 cm³/min.
1. Describe the trend in respiration rate as temperature increases from 5 to 45 °C. 2. Explain why the rate decreases above 35 °C. 3. A scientist claims the optimum temperature for these enzymes is 25 °C. Evaluate this claim using the data.
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