2.2Unit 1 · Nutrition, Respiration & Gas Exchange

Respiration — 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

Part 1 of 7Key ideas

1Aerobic

Aerobic glucose + oxygen → carbon dioxide + water (+ energy).

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

3Anaerobic in animals

Anaerobic in animals glucose → lactic acid.

4Anaerobic in yeast and plants

Anaerobic in yeast and plants glucose → ethanol + carbon dioxide.

Energy from respiration is used for muscle contraction, active transport, protein synthesis, cell division and keeping warm.

Limewater turns cloudy (milky) with carbon dioxide.

Germinating seeds release heat; a vacuum flask with thermometer shows a temperature rise compared with boiled (dead) seeds.

Respiration is controlled by enzymes, so its rate is affected by temperature.

Part 2 of 7Learning objectives

Describe respiration as a process in all living cells releasing energy.

Write word and balanced symbol equations for aerobic respiration.

Compare aerobic and anaerobic respiration in animals and plants/yeast.

Describe experiments to investigate the production of CO₂ and heat by respiration.

Part 3 of 7Essential knowledge

1Respiration is a chemical reaction carried out in all living cells

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.

2Aerobic respiration

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.

3Anaerobic respiration in animals

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.

4Anaerobic respiration in yeast and plants

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.

5Why aerobic releases more energy

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

6Practical – CO₂ production

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.

7Practical – heat production

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.

8Practical – rate and temperature

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.

9Effects 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.

10CO<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.

11Heat 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.

12CORMS 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.

Part 4 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.

Seeds in tube with soda lime

Control with glass beads

Equilibrate in water bath

O₂ absorbed, liquid moves

Measure distance in time

Calculate rate

Part 5 of 7Worked examples

13 mm per minute

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.

2Explain 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.

Part 6 of 7Summary

Respiration releases energy from glucose in all cells.

Aerobic respiration uses oxygen and releases the most energy.

Anaerobic in muscles makes lactic acid; in yeast makes ethanol + CO₂.

Energy is transferred to ATP for cell processes.

Respiration rate can be measured with a respirometer.

Part 7 of 7Compare: Compare

Aerobic vs anaerobic respiration
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

Key terms

Command terms

Give / State / Name

Recall a fact; one idea per mark.

Define

Give the exact meaning using the specification keywords.

Describe

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

Explain

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

Suggest

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

Compare

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

Calculate

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

Discuss

Explore the issue from different angles and link the points.

Evaluate / Assess

Weigh strengths and weaknesses and reach a supported judgement.

Deduce

Draw a conclusion from the information given.

Justify

Support a statement/choice with evidence.

Exam technique

Keywords the mark scheme looks for in this lesson

Respiration · Aerobic respiration · Anaerobic respiration · ATP · Mitochondria · Lactic acid · Oxygen debt · Fermentation · Respirometer

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)

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

Link ideas

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

Use the information in the question

“discuss/evaluate” answers must use the data provided and match the number of marks.

Convert units carefully (mg → g, cm³ → dm³) and show working for calculations.

Avoid vague words

“it”, “stuff”, “things”, “goes up”, “energy is made”. Name the substance and the process.

Do not extrapolate a graph line beyond the data or to zero unless asked.

Six-mark extended responses are marked on content AND a logical, linked line of reasoning.

Last updated: 18 October 2026