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2.2Unit 1 · Nutrition, Respiration & Gas Exchange

Respiration — Unit 1 — Edexcel IGCSE Biology Modular - UNIT 1 Revision Notes & Topical Questions

10 min read 7 parts Lesson 3 of 4 1 quiz

📚 Edexcel IGCSE Biology Modular - UNIT 1 Hub / Topical Revision
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Lesson summary

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 7Respiration — Unit 1

  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.

  2. 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.

  3. 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.

  4. 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.

  5. Why aerobic releases more energy:

    in anaerobic respiration glucose is only partly broken down, so much energy remains in lactic acid or ethanol.

  6. 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.

  7. 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.

  8. 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.

  9. 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.

  10. 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.

  11. 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.

  12. CORMS for respiration practicals:

    Change
    • live seeds vs dead seeds or beads.
    • Organism: same type and mass or number of seeds.
    • Repeat for reliability.
    • Measure: limewater change or temperature after a stated time.
    • Same: indicator volume, number of seeds, environmental temperature, flask size and type.

Section 2 of 7Process & 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.

  1. Seeds in tube with soda lime
  2. Control with glass beads
  3. Equilibrate in water bath
  4. O2 absorbed, liquid moves
  5. Measure distance in time
  6. Calculate rate

Section 3 of 7Compare

Aerobic vs anaerobic respiration
FeatureAerobicAnaerobic (animals)Anaerobic (yeast/plants)
Oxygen neededYesNoNo
ProductsCO2 + waterLactic acidEthanol + CO2
Energy releasedLarge amountSmall amountSmall amount
LocationCytoplasm and mitochondriaCytoplasmCytoplasm
Glucose breakdownCompleteIncompleteIncomplete

Section 4 of 7Worked examples

  1. 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.

  2. Explain why an athlete continues to breathe deeply after a sprint.

Section 5 of 7Key terms

Section 6 of 7Command terms

  1. Give / State / Name

    Recall a fact; one idea per mark.

  2. Define

    Give the exact meaning using the specification keywords.

  3. Describe

    Give an account of what happens / the pattern in data, quoting values; no reasons needed.

  4. Explain

    Give reasons with linked statements (cause → effect). Each linked step is usually a separate marking point.

  5. Suggest

    Use knowledge in a new context; more than one answer may be accepted.

  6. Compare

    Similarities and differences; each point must refer to both items.

  7. Calculate

    • Show working;
      • include units;
      • check significant figures.
    • Method marks are given even if the final answer is wrong.

  8. Discuss

    Explore the issue from different angles and link the points.

  9. Evaluate / Assess

    Weigh strengths and weaknesses and reach a supported judgement.

  10. Deduce

    Draw a conclusion from the information given.

  11. Justify

    Support a statement/choice with evidence.

Section 7 of 7Exam technique

Keywords the mark scheme looks for in this lesson

  1. Respiration · Aerobic respiration · Anaerobic respiration · ATP · Mitochondria · Lactic acid · Oxygen debt · Fermentation · Respirometer
  2. Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.

How to get full marks (examiner guidance)

  1. Use specification keywords exactly (e.g. eukaryotic/prokaryotic, enzyme–substrate complex, active site, complementary).

  2. Link ideas

    for “explain” each marking point is usually a cause → consequence step; unlinked facts often score nothing.

  3. Use the information in the question

    “discuss/evaluate” answers must use the data provided and match the number of marks.
  4. Convert units carefully (mg → g, cm³ → dm³) and show working for calculations.

  5. Avoid vague words

    “it”, “stuff”, “things”, “goes up”, “energy is made”. Name the substance and the process.
  6. Do not extrapolate a graph line beyond the data or to zero unless asked.
  7. Six-mark extended responses are marked on content AND a logical, linked line of reasoning.

Last updated:

🌳 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).

Q1: How to solve respiration examination questions step-by-step?

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.

Q2: What are the common mistakes and experimental sources of error for respiration?

Avoid confusing independent vs dependent variables in practical questions. In calculation questions, double-check percentage change formula: (change / original) × 100.

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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.