Cambridge IGCSE Biology 0610: Respiration – Revision Notes

For full explanations and worked examples, use the respiration study guide. These notes condense Topic 12, Respiration (sections 12.1, 12.2 and 12.3), of the Cambridge IGCSE Biology 0610 syllabus for examination in 2026, 2027 and 2028. Core outcomes are for everyone. Supplement outcomes (12.2.3, 12.3.5, 12.3.6 and 12.3.7) are marked Extended only, for candidates taking Papers 2 and 4.

Links: Cambridge IGCSE Biology hub · Biology checklist · respiration practice questions · Core diagnostic · Extended diagnostic.

Definitions

Term	Syllabus meaning
Respiration	Chemical reactions in cells that break down nutrient molecules to release energy
Aerobic respiration	Chemical reactions in cells that use oxygen to break down nutrient molecules to release energy
Anaerobic respiration	Chemical reactions in cells that break down nutrient molecules to release energy without using oxygen
Oxygen debt (Extended only)	Caused by lactic acid building up in muscles and blood during vigorous exercise

Key verbs: respiration releases energy. It happens in cells. It is not breathing.

12.1.1 The seven uses of energy

Learn them in the syllabus wording:

1. Muscle contraction
2. Protein synthesis
3. Cell division
4. Active transport
5. Growth
6. Passage of nerve impulses
7. Maintenance of a constant body temperature

Memory hook, first letters: My Pet Cat Always Gets Nice Treats (muscle, protein, cell division, active transport, growth, nerve impulses, temperature).

Equations

Process	Word equation	Balanced equation
Aerobic	glucose + oxygen → carbon dioxide + water (Core)	C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (Extended only)
Anaerobic, yeast	glucose → alcohol + carbon dioxide (Core)	C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ (Extended only)
Anaerobic, muscles	glucose → lactic acid (Core)	not required

Checking a balanced equation, method in steps

1. Count C, H and O on the left.
2. Count C, H and O on the right, multiplying by each big number in front.
3. Every total must match.

Worked reminder, aerobic oxygen atoms: left 6 + (6 × 2) = 18; right (6 × 2) + (6 × 1) = 18.

Worked reminder, yeast carbon atoms: left 6; right (2 × 2) + (2 × 1) = 6.

Scaling: 1 glucose aerobically uses 6 O₂ and releases 6 CO₂. So 2 glucose use 12 O₂; 3 glucose release 18 CO₂.

12.3.2 Energy yield

Anaerobic respiration releases much less energy per glucose molecule than aerobic respiration. That exact comparison is what the syllabus asks for; you do not need numbers.

Must-know distinctions

	Aerobic	Anaerobic (yeast)	Anaerobic (muscles)
Oxygen needed	Yes	No	No
Carbon dioxide released	Yes	Yes	No
Alcohol made	No	Yes	No
Lactic acid made	No	No	Yes
Energy per glucose	Much more	Much less	Much less

Other distinctions:

• Respiration v breathing: chemical energy release in cells v air moving in and out of the lungs. See gas exchange.
• Release v produce: energy is released, never produced or made.
• Yeast v enzymes: yeast is a living fungus; the enzymes inside its cells are what denature.

12.1.2 Temperature and yeast respiration

Method in steps (bubble count)

1. Fixed volumes of yeast suspension and glucose solution in a tube.
2. Delivery tube into water; gas released forms bubbles.
3. Water bath at the chosen temperature; leave 5 minutes to equilibrate.
4. Count bubbles per minute; three repeats; calculate a mean.
5. Repeat at a range of temperatures, fresh yeast each time.

Variables

• Changed: temperature.
• Measured: rate of carbon dioxide release (bubbles per minute, or gas volume per minute).
• Kept the same: glucose volume and concentration, amount of yeast, counting time, warm-up time.

Typical pattern

• Rate rises as temperature rises towards the optimum.
• Rate is highest near the optimum.
• Rate falls above the optimum, and almost stops at high temperatures.

Explanation

• Core: respiration is controlled by enzymes; high temperatures denature them.
• Extended: more kinetic energy → more frequent effective collisions between enzyme and substrate; above the optimum the active site changes shape, so the substrate no longer fits.

Worked reminder: 40 bubbles in 5 minutes = 40 ÷ 5 = 8 bubbles per minute.

Worked reminder: a rise from 9 to 18 bubbles per minute is (18 − 9) ÷ 9 × 100 = 100%.

12.3.6 and 12.3.7 Oxygen debt (Extended only)

Cause: during vigorous exercise muscles respire anaerobically → lactic acid builds up in muscles and blood → oxygen debt.

Removal, limited to three points

After exercise	Job
Fast heart rate continues	Transports lactic acid in the blood from the muscles to the liver
Deeper and faster breathing continues	Supplies oxygen for aerobic respiration of lactic acid
In the liver	Lactic acid is broken down by aerobic respiration

Method for data questions: describe the rates staying high after exercise, then give each its job from the table.

Phrases that earn marks

Instead of this	Write this
“Respiration makes energy”	“Respiration releases energy from glucose”
“Energy for moving”	“Energy for muscle contraction”
“Energy to keep warm”	“Energy to maintain a constant body temperature”
“Anaerobic gives no energy”	“Anaerobic releases much less energy per glucose molecule”
“The yeast dies at high temperature”	“The enzymes in the yeast are denatured”
“Breathing fast to get oxygen back”	“Deeper, faster breathing supplies oxygen for aerobic respiration of lactic acid”

Reading a yeast temperature graph

• The x-axis is temperature; the y-axis is rate (bubbles per minute or gas volume per minute).
• Describe in three parts: rise, peak, fall. Quote the temperature of the peak and at least one pair of values with units.
• Compare two temperatures with a calculation where you can, for example “the rate doubled from 9 to 18 bubbles per minute”.
• A rate that falls to almost zero at high temperature shows denaturation; a low rate at low temperature shows the enzymes have little kinetic energy, not that they are denatured.

Links to other topics

• Enzymes (Topic 5): temperature effects and denaturation.
• Gas exchange (Topic 11): supplies the oxygen and removes the carbon dioxide; explains faster breathing during exercise.
• Biotechnology (Topic 21): uses yeast respiration in bread-making and biofuel production.

Quick self-test

1. Define aerobic respiration.
2. State four uses of energy in the human body.
3. Write the word equation for aerobic respiration.
4. Write the word equation for anaerobic respiration in muscles during vigorous exercise.
5. Name the two products of anaerobic respiration in yeast.
6. Compare the energy released per glucose molecule by aerobic and anaerobic respiration.
7. A yeast mixture releases 40 bubbles in 5 minutes. Calculate the rate in bubbles per minute.
8. Explain why almost no bubbles are released by yeast at a very high temperature.
9. (Extended) Write the balanced chemical equation for aerobic respiration.
10. (Extended) How many molecules of oxygen are needed to respire 2 molecules of glucose aerobically?
11. (Extended) Write the balanced chemical equation for anaerobic respiration in yeast.
12. (Extended) State two ways the oxygen debt is removed after exercise.

Answers

1. The chemical reactions in cells that use oxygen to break down nutrient molecules to release energy.
2. Any four of: muscle contraction; protein synthesis; cell division; active transport; growth; passage of nerve impulses; maintenance of a constant body temperature.
3. glucose + oxygen → carbon dioxide + water
4. glucose → lactic acid
5. Alcohol (ethanol) and carbon dioxide.
6. Anaerobic respiration releases much less energy per glucose molecule than aerobic respiration.
7. 40 ÷ 5 = 8 bubbles per minute.
8. The enzymes that control respiration are denatured, so respiration almost stops and little carbon dioxide is released.
9. C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
10. 2 × 6 = 12 molecules of oxygen.
11. C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
12. Any two of: fast heart rate continues, carrying lactic acid from muscles to the liver; deeper and faster breathing continues, supplying oxygen for aerobic respiration of lactic acid; lactic acid is respired aerobically in the liver.

Where marks are usually lost

• Writing that respiration “makes” or “produces” energy instead of releases it.
• Using “movement” or “warmth” instead of the syllabus terms for uses of energy.
• Adding carbon dioxide to the muscle equation, or lactic acid to the yeast equation.
• Putting oxygen on the left of the yeast balanced equation, or misremembering the syllabus formula for ethanol, C₂H₅OH.
• Writing “anaerobic respiration releases no energy” rather than “much less energy per glucose molecule”.
• Giving temperature (the variable changed) when asked for a variable to keep constant in the yeast investigation.
• Calling the highest reading the exact optimum when readings are widely spaced.
• Saying the yeast is denatured, rather than its enzymes.
• Explaining oxygen debt removal without naming the liver.
• Giving “to get oxygen back” as the reason for fast breathing after exercise, instead of oxygen for aerobic respiration of lactic acid.

Next steps

Test yourself on the respiration practice questions, and review the gas exchange revision notes alongside these notes, since the two topics are often tested together.

Official syllabus

Cambridge IGCSE Biology 0610 syllabus for examination in 2026, 2027 and 2028, published by Cambridge Assessment International Education (Cambridge University Press & Assessment). Topic 12, Respiration, sections 12.1, 12.2 and 12.3.