B21: Anaerobic Respiration
Anaerobic respiration in animals and yeast, fermentation, oxygen debt, and comparison with aerobic respiration for GCSE Biology.
Key Definitions
- Anaerobic respiration – the chemical process by which glucose is broken down without oxygen to release a small amount of energy.
- Fermentation – anaerobic respiration in microorganisms such as yeast.
- Oxygen debt – the extra oxygen needed after exercise to break down the lactic acid that has accumulated in the muscles.
- Lactic acid – the product of anaerobic respiration in animal cells, which causes muscle fatigue and cramp.
Anaerobic Respiration in Animals
During intense exercise, the body cannot supply oxygen to the muscles fast enough for aerobic respiration alone. The muscles switch to anaerobic respiration to provide additional energy.
Word Equation (Animals)
glucose → lactic acid
Symbol Equation (Animals)
C6H12O6 → 2C3H6O3
Key Points About Anaerobic Respiration in Animals
- It releases much less energy per glucose molecule than aerobic respiration (only about 5% of the energy).
- This is because glucose is only partially broken down – lactic acid still contains a lot of the stored chemical energy.
- The build-up of lactic acid causes muscle fatigue and cramp.
- It is useful for short bursts of intense exercise (e.g. sprinting).
Anaerobic Respiration in Yeast (Fermentation)
Yeast and some microorganisms carry out a different type of anaerobic respiration called fermentation.
Word Equation (Yeast)
glucose → ethanol + carbon dioxide
Symbol Equation (Yeast)
C6H12O6 → 2C2H5OH + 2CO2
Applications of Fermentation
- Brewing (beer and wine): Yeast ferments the sugars in grain or fruit juice, producing ethanol (alcohol) and CO2. The CO2 makes beer fizzy; in wine production, the CO2 is allowed to escape.
- Baking (bread): Yeast is added to dough. It ferments the sugars in the flour, producing CO2 gas, which gets trapped in the dough and makes it rise. The ethanol evaporates during baking.
Worked Example 1 – Brewing Beer
Question: Explain how yeast is used in the brewing of beer.
Solution: In brewing, yeast is added to a mixture containing sugars (from malted barley). The yeast carries out anaerobic respiration (fermentation), breaking down the glucose into ethanol and carbon dioxide. The ethanol gives the beer its alcoholic content. The CO2 may be retained to make the beer fizzy or allowed to escape depending on the type of beer. The process is carried out in the absence of oxygen to ensure the yeast ferments rather than respiring aerobically.
Worked Example 2 – Baking Bread
Question: Explain the role of yeast in bread-making and why the dough is left in a warm place to rise.
Solution: Yeast is mixed into the dough, where it ferments the sugars present in the flour. This produces carbon dioxide gas, which becomes trapped in the elastic dough and causes it to rise (increase in volume). The ethanol produced evaporates during baking. The dough is left in a warm place because yeast is a living organism and the enzymes controlling fermentation work best at warm temperatures (around 25–35 °C). If the temperature is too low, fermentation is very slow; if too high, the enzymes denature.
Oxygen Debt
During intense exercise, anaerobic respiration produces lactic acid, which builds up in the muscles. After exercise, the body needs extra oxygen to break down this lactic acid. This is called the oxygen debt.
How Lactic Acid Is Removed
- Lactic acid is transported via the blood to the liver.
- In the liver, lactic acid is converted back to glucose using oxygen – this is why you continue to breathe heavily after exercise.
- Some lactic acid may also be oxidised directly to CO2 and water.
- The extra oxygen required for this process is the oxygen debt.
Worked Example 3 – Oxygen Debt After a Sprint
Question: A student runs a 400 m sprint. After the race, they breathe heavily for several minutes. Explain why.
Solution: During the sprint, the muscles cannot get oxygen fast enough for aerobic respiration alone, so they also carry out anaerobic respiration, producing lactic acid. After the race, the body needs extra oxygen to repay the oxygen debt. This oxygen is used in the liver to convert the accumulated lactic acid back into glucose. The continued heavy breathing after exercise supplies the oxygen needed for this process. Once the lactic acid has been removed, breathing returns to normal.
Worked Example 4 – Comparing Aerobic and Anaerobic Respiration
Question: Compare aerobic and anaerobic respiration in terms of energy release, products, and when each is used.
Solution:
| Feature | Aerobic Respiration | Anaerobic (Animals) | Anaerobic (Yeast) |
|---|---|---|---|
| Oxygen required? | Yes | No | No |
| Reactant | Glucose + O2 | Glucose | Glucose |
| Products | CO2 + H2O | Lactic acid | Ethanol + CO2 |
| Energy released per glucose | Large amount | Small amount (~5%) | Small amount |
| When used | During rest and moderate exercise | During intense exercise when O2 supply is insufficient | In microorganisms in the absence of oxygen |
| Location | Mitochondria | Cytoplasm | Cytoplasm |
Worked Example 5 – Lactic Acid and Muscle Fatigue
Question: During a long-distance run, an athlete's leg muscles begin to cramp. Explain what causes this and how the body responds.
Solution: During the run, the leg muscles need large amounts of energy. If the oxygen supply cannot keep up with demand, the muscles begin to carry out anaerobic respiration, producing lactic acid. The build-up of lactic acid in the muscles causes muscle fatigue and cramp. Blood flow transports the lactic acid to the liver, where it is converted back to glucose using oxygen. However, if lactic acid builds up faster than it can be removed, muscle function is impaired and the athlete must slow down or stop to allow the oxygen debt to be repaid.
Practice Questions
Foundation
Q1. Write the word equations for anaerobic respiration in animals and in yeast.
Show Answer
Animals: glucose → lactic acid
Yeast: glucose → ethanol + carbon dioxide
Foundation
Q2. Explain why anaerobic respiration releases much less energy than aerobic respiration.
Show Answer
In anaerobic respiration, glucose is only partially broken down. The products (lactic acid or ethanol) still contain much of the chemical energy originally stored in the glucose. In aerobic respiration, glucose is completely oxidised to CO2 and water, releasing all of the available energy.
Higher
Q3. Describe the role of the liver in repaying the oxygen debt after exercise.
Show Answer
After exercise, lactic acid that has built up in the muscles is transported in the blood to the liver. In the liver, the lactic acid is converted back to glucose. This process requires oxygen, which is why breathing remains heavy after exercise. The extra oxygen needed for this conversion is the oxygen debt. Some lactic acid may also be completely oxidised to carbon dioxide and water in the liver.
Foundation
Q4. Explain the role of yeast in both brewing and baking, referencing the relevant products of fermentation.
Show Answer
In both brewing and baking, yeast ferments glucose anaerobically to produce ethanol and carbon dioxide. In brewing, the ethanol is the desired product as it gives alcoholic drinks their alcohol content. In baking, the CO2 is the desired product as it gets trapped in the dough and makes it rise. The ethanol produced in bread-making evaporates during baking.
Higher
Q5. An athlete runs a race. During the last 100 m, their muscles produce lactic acid. After the race, their breathing rate remains high for several minutes. Explain this using the concept of oxygen debt.
Show Answer
In the final sprint, the muscles cannot receive oxygen fast enough for aerobic respiration alone, so they also carry out anaerobic respiration, producing lactic acid. This creates an oxygen debt – the body owes oxygen that was not supplied during the anaerobic phase. After the race, the athlete continues to breathe heavily to take in extra oxygen. This oxygen is used in the liver to convert the lactic acid back to glucose. Breathing remains elevated until all the lactic acid has been processed and the oxygen debt has been repaid.
Foundation
Q6. Complete the comparison: aerobic respiration releases [more/less] energy per glucose molecule than anaerobic respiration because the glucose is [completely/partially] broken down.
Show Answer
Aerobic respiration releases more energy per glucose molecule because the glucose is completely broken down to CO2 and water. Anaerobic respiration releases less energy because glucose is only partially broken down (to lactic acid in animals, or ethanol and CO2 in yeast).
Exam Tips
- Learn both anaerobic equations (animals: glucose → lactic acid; yeast: glucose → ethanol + CO2) – they are different and both commonly examined.
- Always explain oxygen debt in terms of the liver converting lactic acid back to glucose.
- When asked to compare aerobic and anaerobic respiration, use a table format and include: oxygen, products, energy released, and when each occurs.
- For brewing/baking questions, clearly state which product of fermentation is useful in each case (ethanol for brewing, CO2 for baking).
- Don't write “glucose → energy” – energy is released, it is not a product.
🔢 Maths Skills
Mathematical Skills
Comparing energy yields: aerobic respiration releases approximately 2870 kJ per mole of glucose, while anaerobic respiration in animals releases only about 150 kJ per mole. Calculate the percentage of energy released anaerobically compared to aerobically (approximately 5%). Compare CO₂ production rates in fermentation at different temperatures.
⚠️ Common Misconceptions
Watch Out!
1. Wrong: Anaerobic respiration produces no energy Correct: Anaerobic respiration produces less energy (about 5% of aerobic), because glucose is only partially broken down — the products still contain stored chemical energy
2. Wrong: Lactic acid directly causes muscle fatigue Correct: Lactic acid dissociates into lactate and H+ ions; the increased H+ lowers pH, which disrupts enzyme function and ion channels, interfering with muscle contraction
✍️ 6-Mark Question
Extended Answer
6 marks: Compare aerobic and anaerobic respiration in animals and yeast.
Both aerobic and anaerobic respiration break down glucose to release energy. Aerobic respiration requires oxygen, occurs in mitochondria, and completely breaks down glucose to CO₂ and water, releasing a large amount of energy. Anaerobic respiration in animals does not require oxygen, occurs in the cytoplasm, and partially breaks down glucose to lactic acid, releasing much less energy (about 5%). Anaerobic respiration in yeast (fermentation) also occurs without oxygen in the cytoplasm, but produces ethanol and CO₂ instead of lactic acid, also releasing little energy. Both anaerobic processes are useful when oxygen supply is insufficient — animals use it during intense exercise, and yeast uses it in brewing and baking. Oxygen debt occurs after anaerobic exercise in animals, requiring the liver to convert lactic acid back to glucose.
Mark scheme: 1 mark for aerobic (needs O₂, large energy, CO₂ + H₂O); 1 mark for anaerobic animals (no O₂, lactic acid, small energy); 1 mark for anaerobic yeast (ethanol + CO₂); 1 mark for comparing energy released; 1 mark for applications/context of each; 1 mark for QWC
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
Yeast fermentation rate was measured by CO₂ production at different temperatures: 10 °C = 0.5 cm³/10 min; 20 °C = 2.1 cm³/10 min; 30 °C = 4.8 cm³/10 min; 40 °C = 3.2 cm³/10 min; 50 °C = 0.2 cm³/10 min.
1. Plot the data and identify the optimum temperature for fermentation. 2. Explain the shape of the curve above 30 °C. 3. A brewer sets their fermenter to 40 °C to speed up production. Evaluate this decision using the data and your knowledge of enzyme function.
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