Respiration
Respiration is the process that makes every other process possible. It is also the topic where a single vocabulary error — confusing respiration with breathing — costs marks throughout the paper.
1. What respiration is
Respiration is the chemical reactions in cells that break down nutrient molecules and release energy for metabolism.
Three parts of that definition matter:
- It happens in cells
- It breaks down nutrient molecules — usually glucose
- It releases energy
Respiration releases energy — it does not “produce”, “make” or “create” it. Energy cannot be created; it is transferred from the chemical store in glucose. Mark schemes are strict about release, and this is one of the most reliably lost marks in the whole subject.
Respiration is not breathing
| Respiration | Breathing (ventilation) | |
|---|---|---|
| What it is | A chemical reaction | A physical process |
| Where | Inside cells | Lungs, ribs and diaphragm |
| Purpose | Releases energy | Moves air in and out of the lungs |
Breathing supplies the oxygen that respiration uses, but they are entirely different processes. If a question asks about respiration and your answer mentions the diaphragm, something has gone wrong.
What the energy is used for
- Muscle contraction, producing movement
- Protein synthesis from amino acids
- Cell division for growth and repair
- Active transport
- Growth
- Passage of nerve impulses
- Maintenance of a constant body temperature in mammals and birds
2. Aerobic respiration
Aerobic respiration is the chemical reactions in cells that use oxygen to break down nutrient molecules to release energy.
Word equation:
glucose + oxygen → carbon dioxide + water
Balanced chemical equation:
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
Aerobic respiration takes place in the mitochondria, which is why cells that need a lot of energy — muscle cells, sperm cells, root hair cells doing active transport — contain many mitochondria.
Do not write ”+ energy” as a product in the balanced chemical equation. Energy is not a chemical substance and does not balance. It is acceptable in the word equation, but the safest habit is to state separately that energy is released.
Photosynthesis is not respiration in reverse, even though the equations look mirrored. They happen in different organelles, at different times, for different purposes. Check carefully which way round a question is asking: making glucose is photosynthesis, breaking it down is respiration.
3. Anaerobic respiration
Anaerobic respiration is the chemical reactions in cells that break down nutrient molecules to release energy without using oxygen.
The products differ between organisms:
In muscle cells
glucose → lactic acid
This happens during vigorous exercise, when the heart and lungs cannot deliver oxygen fast enough for aerobic respiration alone. It allows muscles to keep contracting, but only for a short time.
In yeast
glucose → alcohol (ethanol) + carbon dioxide
This is the reaction behind bread making (the carbon dioxide makes dough rise), biofuel production and brewing.
Anaerobic respiration in muscle does not produce carbon dioxide. Only lactic acid. Carbon dioxide is produced by anaerobic respiration in yeast, and by aerobic respiration. Mixing these up is very common.
Energy comparison
Anaerobic respiration releases much less energy per glucose molecule than aerobic respiration, because the glucose is only partially broken down — the products (lactic acid, or alcohol) still contain a great deal of unreleased chemical energy.
| Aerobic | Anaerobic | |
|---|---|---|
| Oxygen | Required | Not required |
| Products (muscle) | Carbon dioxide + water | Lactic acid |
| Products (yeast) | Carbon dioxide + water | Alcohol + carbon dioxide |
| Energy released | Much more | Much less |
| Breakdown of glucose | Complete | Incomplete |
4. Oxygen debt
During vigorous exercise, lactic acid builds up in the muscles, causing fatigue and muscle cramp.
Oxygen debt is the extra oxygen the body needs after exercise to break down the lactic acid that accumulated during anaerobic respiration.
This is why breathing rate and heart rate stay high after you stop exercising — a favourite exam question. The reasoning:
- Lactic acid has built up in the muscles
- It is transported in the blood to the liver
- In the liver it is broken down, which requires oxygen
- So deep, rapid breathing continues until the debt is repaid
The lactic acid is broken down in the liver, not in the muscles where it was made. Naming the liver is often a mark on its own.
5. Investigating respiration
Respiration in yeast
Yeast is used because it respires anaerobically, producing carbon dioxide that is easy to measure — by counting bubbles, or collecting the gas in a gas syringe or an inverted measuring cylinder.
A typical investigation varies temperature and measures the volume of carbon dioxide produced in a set time.
Expected results: the rate increases with temperature up to an optimum, then falls sharply as the enzymes controlling respiration are denatured. At very high temperature there is no carbon dioxide produced at all, because the enzymes are denatured and the yeast is killed.
Controlled variables: volume and concentration of the glucose solution, mass of yeast, time, and pH.
Testing for the products
| Product | Test | Result |
|---|---|---|
| Carbon dioxide | Limewater | Turns milky / cloudy |
| Carbon dioxide | Hydrogen-carbonate indicator | Red → yellow |
Hydrogen-carbonate indicator is worth learning in both directions: yellow means a high carbon dioxide concentration, purple means a low one, and red is the middle. It appears in respiration, photosynthesis and gas exchange questions alike.
6. Mistakes that cost marks
Saying respiration “produces” or “makes” energy. It releases it.
Confusing respiration with breathing.
Writing ”+ energy” in the balanced chemical equation.
Saying anaerobic respiration in muscle produces carbon dioxide. It produces lactic acid only.
Saying anaerobic respiration produces no energy. It produces less, not none.
Saying lactic acid is broken down in the muscles. It goes to the liver.
Saying respiration happens in the lungs. It happens in all cells.
Saying mitochondria “produce energy” or are “the powerhouse of the cell.” They are the site of aerobic respiration, which releases energy.
Frequently asked questions
What is the difference between respiration and breathing? Respiration is a chemical reaction inside cells that releases energy. Breathing is the physical movement of air in and out of the lungs.
What is the word equation for aerobic respiration? Glucose + oxygen → carbon dioxide + water, releasing energy.
What does anaerobic respiration produce in humans? Lactic acid only.
What does anaerobic respiration produce in yeast? Alcohol (ethanol) and carbon dioxide.
Why does anaerobic respiration release less energy? The glucose is only partially broken down, so the products still contain unreleased chemical energy.
What is oxygen debt? The extra oxygen needed after exercise to break down the lactic acid that built up, which is why breathing stays deep and fast after you stop.
Where is lactic acid broken down? In the liver.
Where does aerobic respiration take place in the cell? In the mitochondria.
Quick revision checklist
- I can define respiration using the word release, not produce
- I can explain the difference between respiration and breathing
- I can list what the released energy is used for
- I can give the word and balanced equations for aerobic respiration
- I know aerobic respiration happens in the mitochondria
- I can define anaerobic respiration
- I know the products in muscle and in yeast, and don’t mix them up
- I can explain why anaerobic respiration releases less energy
- I can define oxygen debt and explain why breathing stays fast after exercise
- I know lactic acid is broken down in the liver
- I can describe an investigation of respiration in yeast and its variables
- I know the limewater and hydrogen-carbonate indicator results
These notes cover topic 12 of the Cambridge IGCSE Biology (0610) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across many one-to-one IGCSE Biology lessons, with particular attention to the errors students make most often and the wording examiners reward.
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