CIE iGCSE Biology-12.3 Anaerobic respiration- Study Notes- New Syllabus
CIE iGCSE Biology-12.3 Anaerobic respiration- Study Notes – New syllabus
CIE iGCSE Biology-12.3 Anaerobic respiration- Study Notes -CIE iGCSE Biology – per latest Syllabus.
Key Concepts:
Core
- Describe anaerobic respiration as the chemical reactions in cells that break down nutrient molecules to release energy without using oxygen
- State that anaerobic respiration releases much less energy per glucose molecule than aerobic respiration
- State the word equation for anaerobic respiration in yeast as:
glucose → alcohol + carbon dioxide - State the word equation for anaerobic respiration in muscles during vigorous exercise as:
glucose → lactic acid
Supplement
- State the balanced chemical equation for anaerobic respiration in yeast as:
C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ - State that lactic acid builds up in muscles and blood during vigorous exercise causing an oxygen debt
- Outline how the oxygen debt is removed after exercise, limited to:
(a) continuation of fast heart rate to transport lactic acid in the blood from the muscles to the liver
(b) continuation of deeper and faster breathing to supply oxygen for aerobic respiration of lactic acid
(c) aerobic respiration of lactic acid in the liver
Anaerobic Respiration (Without Oxygen)
📖 Definition:
Anaerobic respiration is a chemical process that takes place in cells, where nutrient molecules (like glucose) are broken down to release energy – without using oxygen.
⚡ Key Features:
- Happens when oxygen is absent or limited
- Releases less energy than aerobic respiration
- Occurs in muscle cells (during intense activity), and in some microorganisms like yeast and bacteria
🧪 Examples of Anaerobic Respiration
1. In Human Muscle Cells:
During vigorous exercise, oxygen may be unavailable:
Lactic acid builds up → causes muscle fatigue and cramps.
2. In Yeast (Fermentation):
Anaerobic respiration in yeast is used in baking and brewing:
This is known as alcoholic fermentation.
📌 Comparison with Aerobic Respiration
| Feature | Aerobic Respiration | Anaerobic Respiration |
|---|---|---|
| Oxygen used? | Yes | No |
| Energy released | More (high yield) | Less (low yield) |
| End products (humans) | CO₂ + H₂O | Lactic acid |
| End products (yeast) | CO₂ + H₂O | Ethanol + CO₂ |
Anaerobic respiration releases energy from food molecules without oxygen, producing less energy and different waste products than aerobic respiration.
Energy Release in Anaerobic vs Aerobic Respiration
Anaerobic respiration releases much less energy per glucose molecule than aerobic respiration.
🧠 Why?
- Aerobic respiration fully breaks down glucose using oxygen → high energy yield
- Anaerobic respiration only partially breaks down glucose → low energy yield
⚖️ Quick Comparison
| Type of Respiration | Oxygen Used? | Energy Released (ATP) | By-products |
|---|---|---|---|
| Aerobic | Yes | ~36–38 ATP per glucose | CO₂ and H₂O |
| Anaerobic (human) | No | ~2 ATP per glucose | Lactic acid |
| Anaerobic (yeast) | No | ~2 ATP per glucose | Ethanol + CO₂ |
Anaerobic respiration is useful when oxygen is limited, but it’s far less efficient.
It allows energy production to continue briefly, especially during intense exercise or in anaerobic environments.
Anaerobic Respiration in Yeast (Fermentation)
Glucose → Alcohol + Carbon dioxide
📌 Details:
| Substance | Role |
|---|---|
| Glucose | Sugar broken down for energy |
| Alcohol | Also called ethanol – a waste product |
| Carbon dioxide | Gas released – helps bread rise |
🧠 Key Points:
- Occurs in yeast cells when oxygen is absent
- Known as alcoholic fermentation
- Used in bread-making (CO₂ makes dough rise)
- Used in brewing (ethanol is the alcohol in drinks)
Releases a small amount of energy (about 2 ATP per glucose molecule)
In yeast, anaerobic respiration breaks down glucose into alcohol and carbon dioxide, releasing a small amount of energy without using oxygen.
Anaerobic Respiration in Muscles (During Vigorous Exercise)
Glucose → Lactic acid
📌 Details:
| Substance | Role |
|---|---|
| Glucose | Fuel broken down to release energy |
| Lactic acid | Waste product that builds up in muscles |
🧠 Key Points:
- Occurs when muscles don’t get enough oxygen during intense activity
- Allows energy to be produced quickly, but less efficiently
- Lactic acid causes muscle fatigue and cramps
- It is later broken down during recovery when oxygen becomes available
Produces much less energy than aerobic respiration (only ~2 ATP per glucose)
During vigorous exercise, muscles respire anaerobically, converting glucose into lactic acid without using oxygen and releasing a small amount of energy.
Balanced Chemical Equation for Anaerobic Respiration in Yeast
C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
📌 Explanation of Each Term:
| Formula | Substance | Role |
|---|---|---|
| C₆H₁₂O₆ | Glucose | The sugar used as fuel |
| C₂H₅OH | Ethanol (alcohol) | Waste product (used in brewing) |
| CO₂ | Carbon dioxide | Waste gas (used in bread rising) |
⚡ Key Points:
- Occurs in yeast and some microorganisms in anaerobic conditions (no oxygen)
- Also called alcoholic fermentation
- Produces a small amount of energy (~2 ATP per glucose)
- Important in bread-making and alcohol production
In yeast, anaerobic respiration breaks down glucose into ethanol and carbon dioxide without using oxygen, releasing a small amount of energy.
Lactic Acid and Oxygen Debt During Exercise
Key Statement:
Lactic acid builds up in the muscles and blood during vigorous exercise, causing an oxygen debt.
🧪 Why It Happens:
During intense activity, the muscles may not receive enough oxygen for aerobic respiration.
The body switches to anaerobic respiration to keep producing energy:
⚠️ Effects of Lactic Acid Build-Up:
- Causes muscle fatigue
- Leads to pain or cramps
- Lowers pH in the muscle tissue (making it more acidic)
🫁 Oxygen Debt:
After exercise, the body needs extra oxygen to:
- Break down lactic acid into carbon dioxide and water
- Restore normal conditions in cells
This need for extra oxygen is called oxygen debt.
Anaerobic respiration during vigorous activity produces lactic acid. The build-up of lactic acid causes muscle fatigue and results in oxygen debt – the extra oxygen needed to remove the lactic acid after exercise.
Removal of Oxygen Debt After Exercise
After vigorous exercise, the body needs to repay the oxygen debt – the extra oxygen required to:
- Break down lactic acid
- Restore normal body conditions
(a) Fast Heart Rate Continues
The heart rate remains high after exercise.
(b) Deeper and Faster Breathing Continues
(c) Aerobic Respiration of Lactic Acid in the Liver
The liver receives lactic acid and breaks it down using oxygen.
📌 Summary:
After exercise, oxygen debt is removed by:
- Fast heart rate transporting lactic acid to the liver
- Deep breathing supplying oxygen
- Liver cells using oxygen to aerobically break down lactic acid