B13: Respiration
Aerobic and anaerobic respiration, oxygen debt and metabolism
📋 Key Concepts
Respiration: An exothermic reaction in which glucose is broken down to release energy for living processes. Respiration happens in every living cell, all the time.
Key Terms
- aerobic respiration
- anaerobic respiration
- mitochondria
- lactic acid
- fermentation
- oxygen debt
- metabolism
- ethanol
- liver
📝 Aerobic Respiration
Aerobic respiration uses oxygen to break down glucose completely. It releases a large amount of energy and takes place in the mitochondria.
Word Equation:
glucose + oxygen → carbon dioxide + water
Symbol Equation:
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
glucose + oxygen → carbon dioxide + water
Symbol Equation:
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
Key points:
- Aerobic respiration releases much more energy per glucose molecule than anaerobic respiration
- It happens in the mitochondria (cells with high energy needs have many mitochondria, e.g. muscle cells, sperm cells)
- It is an exothermic reaction (releases energy to the surroundings)
- Energy released is used for: muscle contraction, protein synthesis, cell division, active transport, maintaining body temperature
Example 1
A muscle cell contains many mitochondria. A red blood cell contains no mitochondria. Explain these observations.
Solution:
Muscle cells need a lot of energy for contraction, so they require many mitochondria to carry out aerobic respiration at a high rate.
Red blood cells do not have mitochondria because they do not respire aerobically — they respire anaerobically so they do not use up the oxygen they are transporting.
📝 Anaerobic Respiration in Animals
Anaerobic respiration takes place without oxygen. In animal cells (including human muscle cells), glucose is broken down into lactic acid. It releases much less energy than aerobic respiration because the glucose is only partially broken down.
Word Equation (animals):
glucose → lactic acid
(Releases much less energy per glucose molecule than aerobic respiration)
glucose → lactic acid
(Releases much less energy per glucose molecule than aerobic respiration)
When does anaerobic respiration happen? During vigorous exercise, the body cannot supply enough oxygen to the muscles for aerobic respiration alone. The muscles switch to anaerobic respiration to provide extra energy, producing lactic acid.
Example 2
During a 100 m sprint, a runner's muscles produce lactic acid. Explain why.
Solution:
During a sprint, the muscles need energy very quickly. The heart and lungs cannot deliver oxygen fast enough for aerobic respiration alone. The muscles respire anaerobically as well, breaking glucose down into lactic acid, to provide the extra energy needed.
📝 Anaerobic Respiration in Yeast (Fermentation)
Fermentation is anaerobic respiration in yeast and some microorganisms. Glucose is broken down into ethanol and carbon dioxide. This process is used in brewing and baking.
Word Equation (yeast):
glucose → ethanol + carbon dioxide
glucose → ethanol + carbon dioxide
Applications of fermentation:
- Brewing: Yeast ferments sugar in barley to produce ethanol (alcohol) and CO₂
- Baking: Yeast ferments sugar in dough. The CO₂ produced makes the bread rise. The ethanol evaporates during baking.
Example 3
Explain why bread dough rises when yeast is added, and why the bread does not contain alcohol after baking.
Solution:
Yeast carries out fermentation, converting sugars in the dough into ethanol and CO₂. The CO₂ gas gets trapped in the dough, causing it to rise (the dough becomes larger and lighter). The ethanol evaporates during baking because the oven temperature is above the boiling point of ethanol (78°C).
📝 Oxygen Debt
Oxygen debt: The extra oxygen needed after exercise to break down the lactic acid that has built up in the muscles. The lactic acid is transported to the liver, where it is converted back to glucose using oxygen.
How oxygen debt is repaid:
- After exercise, breathing rate and heart rate remain high to deliver extra oxygen to the liver
- The liver uses this oxygen to convert lactic acid back to glucose
- Blood flow through the muscles is also increased to transport lactic acid to the liver
- This is why you continue to breathe heavily after stopping exercise
Example 4
After a 400 m race, an athlete continues to breathe heavily for several minutes. Explain why.
Solution:
During the race, the athlete's muscles built up lactic acid from anaerobic respiration. After the race, the athlete has an oxygen debt — they need extra oxygen to convert the lactic acid back to glucose in the liver. The heavy breathing supplies this extra oxygen, and the heart rate stays high to transport lactic acid to the liver and deliver oxygen.
📝 Aerobic vs Anaerobic Respiration
| Feature | Aerobic Respiration | Anaerobic Respiration (animals) | Anaerobic Respiration (yeast) |
|---|---|---|---|
| Oxygen required? | Yes | No | No |
| Products | CO₂ + water | Lactic acid | Ethanol + CO₂ |
| Energy released per glucose | Large amount | Much less | Much less |
| Location | Mitochondria | Cytoplasm | Cytoplasm |
| Glucose fully broken down? | Yes | No (partial breakdown) | No (partial breakdown) |
| Equation | C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O | C₆H₁₂O₆ → 2C₃H₆O₃ | C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ |
Example 5
Explain why aerobic respiration releases more energy per glucose molecule than anaerobic respiration.
Solution:
In aerobic respiration, glucose is completely broken down into CO₂ and water, releasing all the energy stored in the glucose molecule.
In anaerobic respiration, glucose is only partially broken down (into lactic acid, or ethanol and CO₂). Energy remains stored in the products, so less energy is released overall.
Example 6
A long-distance runner maintains a steady pace. A sprinter runs at maximum speed for 100 m. Compare the respiration in the muscles of each athlete.
Solution:
Long-distance runner: primarily aerobic respiration because the pace is steady enough for the blood to supply sufficient oxygen to the muscles. They respire anaerobically only briefly at the end in a sprint finish.
Sprinter: both aerobic and anaerobic respiration. The muscles need energy very quickly and cannot get enough oxygen for aerobic respiration alone, so anaerobic respiration also occurs, producing lactic acid.
📝 Metabolism
Metabolism: The sum of all the chemical reactions in a cell or in the body. Metabolic reactions include both breakdown (catabolic) and building (anabolic) reactions. Many metabolic reactions are controlled by enzymes.
Examples of metabolic reactions:
- Conversion of glucose to starch, glycogen or cellulose
- Formation of lipids from glycerol and fatty acids
- Formation of amino acids from glucose and nitrates (in plants)
- Respiration
- Photosynthesis
- Breakdown of excess proteins to form urea (in the liver)
❓ Practice Questions
Q1: Write the word and symbol equations for aerobic respiration.
Q2: Write the word equations for anaerobic respiration in: (a) animal cells, (b) yeast cells.
Q3: Explain why aerobic respiration releases more energy per glucose molecule than anaerobic respiration.
Q4: After intense exercise, a person breathes heavily for several minutes. Explain what is happening in terms of oxygen debt and the role of the liver.
Q5: Explain how fermentation by yeast is used in both brewing and baking.
Q6: Define metabolism and give two examples of metabolic reactions.
✅ Answers
- Word equation: glucose + oxygen → carbon dioxide + water. Symbol equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O.
- (a) Animal cells: glucose → lactic acid. (b) Yeast cells: glucose → ethanol + carbon dioxide.
- In aerobic respiration, glucose is completely broken down to CO₂ and water, so all the energy stored in the glucose is released. In anaerobic respiration, glucose is only partially broken down (to lactic acid, or ethanol and CO₂), so energy remains stored in the products and less is released.
- During exercise, anaerobic respiration produced lactic acid in the muscles. This created an oxygen debt — the extra oxygen needed to convert lactic acid back to glucose. The heavy breathing supplies this extra oxygen. The lactic acid is transported to the liver, where it is converted back to glucose using the extra oxygen.
- Brewing: yeast ferments sugars in barley to produce ethanol (alcohol) and CO₂. Baking: yeast ferments sugars in bread dough; the CO₂ makes the dough rise, and the ethanol evaporates during baking.
- Metabolism is the sum of all chemical reactions in the body. Examples include: conversion of glucose to glycogen, respiration, formation of lipids, breakdown of proteins to urea, photosynthesis.
🎯 Exam Tips
- Respiration is NOT breathing — respiration is a chemical reaction in every cell
- Respiration is exothermic (releases energy); photosynthesis is endothermic (absorbs energy)
- Learn both word and symbol equations for aerobic respiration — it comes up every year
- Anaerobic in animals = lactic acid; Anaerobic in yeast = ethanol + CO₂ — don't mix these up
- When explaining oxygen debt, always mention the LIVER converting lactic acid back to glucose
- Aerobic respiration happens in mitochondria; anaerobic happens in the cytoplasm
- Metabolism includes ALL chemical reactions, not just respiration
- When comparing aerobic and anaerobic, always mention energy yield, products and whether oxygen is needed
🔢 Maths Skills
Mathematical Skills
Comparing energy yields: Aerobic respiration releases approximately 2870 kJ per mole of glucose, whereas anaerobic respiration in animals releases only about 150 kJ per mole. This means aerobic respiration releases roughly 19 times more energy per glucose molecule. Use ratio calculations: if anaerobic yields 2 ATP per glucose and aerobic yields 38 ATP, the ratio is 1:19.
⚠️ Common Misconceptions
Watch Out!
1. Respiration is the same as breathing. Wrong: respiration means breathing in and out. Correct: respiration is a chemical process in every cell that releases energy from glucose; breathing is the physical movement of air in and out of the lungs.
2. Anaerobic respiration produces no energy. Wrong: anaerobic respiration produces zero energy. Correct: anaerobic respiration produces less energy than aerobic respiration because glucose is only partially broken down, but it still releases some energy.
3. Only animals respire. Wrong: plants do not respire. Correct: all living organisms respire, including plants — respiration happens in every living cell all the time.
✍️ 6-Mark Question
Extended Answer
6 marks: Compare aerobic and anaerobic respiration.
Aerobic respiration requires oxygen and occurs in the mitochondria, completely breaking down glucose into carbon dioxide and water. It releases a large amount of energy per glucose molecule (approximately 38 ATP). Anaerobic respiration does not require oxygen and occurs in the cytoplasm, partially breaking down glucose — in animals producing lactic acid, and in yeast producing ethanol and carbon dioxide. Anaerobic respiration releases much less energy per glucose molecule (only 2 ATP) because the glucose is not fully broken down and energy remains stored in the products. Both processes release energy from glucose and are controlled by enzymes, but aerobic is more efficient for sustained activity while anaerobic provides energy quickly when oxygen supply is limited.
Mark scheme: 1 mark — aerobic needs oxygen, anaerobic does not; 1 mark — correct products for each; 1 mark — aerobic in mitochondria, anaerobic in cytoplasm; 1 mark — aerobic releases more energy; 1 mark — anaerobic is partial breakdown; 1 mark — valid comparison of when each is used.
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
The table shows data for an athlete before, during and after a 400 m sprint. Before the race, their breathing rate was 15 breaths/min and blood lactic acid was 2 mg/100 cm³. Immediately after the race, breathing rate was 40 breaths/min and lactic acid was 18 mg/100 cm³. Five minutes later, breathing rate was 22 breaths/min and lactic acid was 8 mg/100 cm³. Explain the changes in breathing rate and lactic acid levels, and suggest why both values had not returned to normal after five minutes.
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