2.2Unit 1 · Nutrition, Respiration & Gas Exchange
Respiration — Unit 1 — Edexcel IGCSE Biology Modular - UNIT 1 Revision Notes & Topical Questions
10 min read 4 parts Lesson 3 of 4 1 quiz
Describe respiration as a process in all living cells releasing energy.Aerobicglucose + oxygen → carbon dioxide + water (+ energy).C6H12O6 + 6O2 → 6CO2 + 6H2OAnaerobic in animalsglucose → lactic acid.Anaerobic in yeast and plantsglucose → ethanol + carbon dioxide.Energy from resp…
- Respiration is a chemical reaction carried out in all living cells
- Aerobic respiration
- Why aerobic releases more energy
- Practical – CO2 production
- Practical – heat production
- Practical – rate and temperature
Section 1 of 4Respiration — Unit 1
Respiration is a chemical reaction carried out in all living cells
to release energy from food molecules such as glucose.- It is not the same as breathing.
- Energy is used to make ATP, which provides energy for processes such as muscle contraction, active transport, making large molecules, cell division and maintaining body temperature.
Aerobic respiration
- uses oxygen and mostly takes place in mitochondria.
- Glucose is completely broken down: C6H12O6 + 6O2 → 6CO2 + 6H2O.
- It releases a large amount of energy.
Anaerobic respiration in animals:
during vigorous exercise muscles may not get enough oxygen, so glucose is converted to lactic acid, releasing less energy.- Lactic acid builds up and causes muscle fatigue;
- after exercise extra oxygen is needed to break it down (oxygen debt), so breathing stays fast.
- Lactic acid builds up and causes muscle fatigue;
Anaerobic respiration in yeast and plants:
- glucose → ethanol + carbon dioxide.
- This is used in bread-making (CO2 makes dough rise) and brewing (ethanol).
- Plant roots in waterlogged soil may respire anaerobically.
Why aerobic releases more energy:
in anaerobic respiration glucose is only partly broken down, so much energy remains in lactic acid or ethanol.Practical – CO2 production:
air is drawn through limewater or hydrogencarbonate indicator past a small animal or germinating seeds;- the indicator changes (limewater cloudy;
- hydrogencarbonate indicator red → yellow).
- A control with no organisms shows no change.
Practical – heat production:
- place germinating peas in one vacuum flask and boiled, cooled peas (killed) in another, both washed in disinfectant to stop microbial respiration.
- Thermometers show a temperature rise only in the live peas.
Practical – rate and temperature:
a respirometer with soda lime measures oxygen uptake by the movement of a liquid bead.- Rate increases with temperature up to an optimum, as respiration is controlled by enzymes, and falls at high temperatures as enzymes denature.
Effects of lactic acid:
- Lactic acid lowers the pH in muscle tissue, and the more acidic conditions can reduce enzyme activity, contributing to fatigue.
- After exercise, oxygen is used to break the lactic acid down into carbon dioxide and water.
CO<sub>2</sub> practical – controls:
Connect three tubes to separate tubes of limewater: germinating seeds, the same number of boiled (dead) seeds, and glass beads of equal volume.- Only the live seeds turn limewater cloudy.
- Delivery tubes must end below the limewater surface so gas bubbles through it.
Heat practical – set-up details:
Plug each vacuum flask with cotton wool to hold the thermometer and cut heat loss, and invert the flasks.- Record room temperature too, since any change in the dead-seed flask shows a change in surroundings.
- Rinse seeds in dilute disinfectant so microbes cannot respire and add heat.
CORMS for respiration practicals:
Changelive seeds vs dead seeds or beads.- Organismsame type and mass or number of seeds.
- Repeat for reliability.
- Measurelimewater change or temperature after a stated time.
- Sameindicator volume, number of seeds, environmental temperature, flask size and type.
Section 2 of 4Process & sequence
- Set up a respirometer with germinating seeds (or small invertebrates) in a tube with soda lime.
- Set up a control tube with glass beads of equal volume instead of organisms.
- Place tubes in a water bath at a set temperature and allow to equilibrate.
- Soda lime absorbs CO2, so oxygen uptake reduces the gas volume.
- The coloured liquid in the capillary tube moves towards the organisms; measure distance in a set time.
- Repeat at different temperatures and calculate rate (distance ÷ time).
Measuring respirationwrite these steps in this order; each step is a marking point.
- Seeds in tube with soda lime
- Control with glass beads
- Equilibrate in water bath
- O2 absorbed, liquid moves
- Measure distance in time
- Calculate rate
Section 3 of 4Compare
| Feature | Aerobic | Anaerobic (animals) | Anaerobic (yeast/plants) |
|---|---|---|---|
| Oxygen needed | Yes | No | No |
| Products | CO2 + water | Lactic acid | Ethanol + CO2 |
| Energy released | Large amount | Small amount | Small amount |
| Location | Cytoplasm and mitochondria | Cytoplasm | Cytoplasm |
| Glucose breakdown | Complete | Incomplete | Incomplete |
Section 4 of 4Worked examples
In a respirometer, the liquid moved 24 mm in 8 minutes at 20 °C. Calculate the rate of oxygen uptake in mm per minute and explain why soda lime is needed.
- Rate = 24 ÷ 8 = 3 mm per minute.
- Soda lime absorbs the carbon dioxide released, so any change in volume is due only to oxygen being taken in by the organisms.
- Rate = 24 ÷ 8 = 3 mm per minute.
Explain why an athlete continues to breathe deeply after a sprint.
- During the sprint the muscles respired anaerobically, producing lactic acid.
- After exercise extra oxygen is needed to break down the lactic acid (repaying the oxygen debt), so breathing rate and depth remain high.
- During the sprint the muscles respired anaerobically, producing lactic acid.
Marked practiceQuizzes for this lesson
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🌳 Student Search Tree & Frequently Asked Exam Questions: respiration
Step-by-step problem-solving guide, mark scheme criteria, and experimental error analysis curated by Dr. Khaled Dabour (DABOURHUB).
1. Identify all given values and convert non-standard units, 2. State the primary formula or physiological/chemical principle, 3. Calculate to 3 significant figures and clearly state units.
Avoid confusing independent vs dependent variables in practical questions. In calculation questions, double-check percentage change formula: (change / original) × 100.
Where can I find respiration past paper questions by topic with the mark scheme?
On DABOURHUB every respiration question is grouped by syllabus point and shown with the mark-scheme wording, so you can see exactly which sentence earns the mark instead of scrolling through whole year papers.
What is the most common respiration exam question and why do students lose marks on it?
The repeated pattern on Edexcel papers is a short stem that asks you to apply respiration and then justify it with a command word (explain, suggest, compare). Marks are lost by stating the fact without linking it back to the wording of the question, so each respiration item here is written as a full-mark answer.
Is there an alternative to Save My Exams or Physics and Maths Tutor for respiration?
Yes: the respiration notes, topical questions and MCQ quizzes on this site follow the same syllabus headings as the official specification, and they open straight from the search result with no download step.
How do I revise respiration in two weeks before the exam?
Three passes: read the respiration notes once, then do the topical questions and mark them against the mark scheme, then redo only the ones you missed. The quiz on the page gives instant feedback so the third pass takes minutes rather than an evening.