Cambridge O Level Biology 5090: Respiration (5090) – Revision Notes

For full explanations and worked examples, use the study guide for this topic. These notes condense Topic 10, Respiration, of the Cambridge O Level Biology 5090 syllabus for examination in 2026, 2027 and 2028 (version 4): every learning outcome in sections 10.1 Respiration, 10.2 Aerobic respiration and 10.3 Anaerobic respiration. The syllabus is not tiered, so all of it applies to every candidate.

When you are ready, move on to the practice questions. The course hub and the printable checklist cover the rest of the course, and the free 10-minute diagnostics show where to focus. The temperature effect depends on enzymes, revised in the Enzymes revision notes. Breathing during and after exercise is revised in Human gas exchange.

Key definitions

Term	Meaning (learn the wording)
Respiration (10.1.1)	The chemical reactions in all living cells that release energy from glucose
Aerobic respiration (10.2.1)	The release of a relatively large amount of energy by the breakdown of glucose in the presence of oxygen
Anaerobic respiration (10.3.1)	The release of a relatively small amount of energy by the breakdown of glucose without using oxygen
Oxygen debt / EPOC (10.3.4)	The extra oxygen needed after vigorous exercise to break down the lactic acid that built up; EPOC stands for Excess Post-exercise Oxygen Consumption

Three words to keep apart:

• Respiration: chemical reactions in cells.
• Breathing: muscle movements that move air in and out of the lungs.
• Gas exchange: diffusion of oxygen and carbon dioxide across the alveoli.

Equations to learn (10.2.2, 10.3.2, 10.3.3)

Type	Equation
Aerobic, word	glucose + oxygen → carbon dioxide + water
Aerobic, balanced	C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
Anaerobic in humans, word	glucose → lactic acid
Anaerobic in yeast, word	glucose → ethanol + carbon dioxide

Only aerobic respiration needs a balanced chemical equation in this syllabus. The anaerobic ones are word equations.

Memory check for the balanced equation. Count atoms. Left: C 6, H 12, O 6 + 12 = 18. Right: C 6, H 12, O 12 + 6 = 18. If your oxygen count does not reach 18 on both sides, a 6 is missing.

Ratio reminder. 1 glucose : 6 oxygen : 6 carbon dioxide : 6 water. So 4 glucose molecules need 24 oxygen molecules and give 24 carbon dioxide molecules.

Uses of energy (10.1.2)

The syllabus lists seven. Learn them as a list, then practise matching each one to a situation:

1. Muscle contraction
2. Protein synthesis
3. Cell division
4. Active transport
5. Growth
6. The passage of electrical impulses along neurones
7. The maintenance of a constant body temperature

If the question describes…	The use is…
Ions taken up against a concentration gradient	Active transport
Amino acids joined to make an enzyme	Protein synthesis
A nerve carrying a message	Passage of electrical impulses along neurones
A bird staying warm in winter	Maintenance of a constant body temperature
New cells forming at a root tip	Cell division (and growth)

Aerobic vs anaerobic: must-know distinctions

	Aerobic	Anaerobic (humans)	Anaerobic (yeast)
Oxygen	Used	Not used	Not used
Energy released per glucose	Relatively large	Relatively small	Relatively small
Products	Carbon dioxide + water	Lactic acid	Ethanol + carbon dioxide
Carbon dioxide made?	Yes	No	Yes

Both humans and yeast can respire either way. Your muscles switch to extra anaerobic respiration only when oxygen cannot reach them fast enough.

The yeast and temperature practical (10.1.3)

Method in steps

1. Yeast + glucose solution in tubes; same mass of yeast, same volume and concentration of glucose in each.
2. Each tube in a water-bath at a different temperature; check with a thermometer.
3. Leave time for the mixture to reach the water-bath temperature.
4. Measure carbon dioxide release for a fixed time: volume collected over water, bubbles per minute, or time for limewater to turn milky / hydrogencarbonate indicator to turn yellow.
5. Repeat and find a mean; add a control of boiled yeast.

Rate = volume (or number of bubbles) ÷ time. For a time-to-change method, a shorter time means a faster rate.

Expected pattern and explanation

• Rate rises as temperature rises: more kinetic energy, more collisions between enzymes and substrate.
• Rate peaks at the optimum.
• Rate falls above the optimum: enzymes denatured, active site changes shape, substrate no longer fits.

Worked reminder. 42 bubbles in 3 minutes = 42 ÷ 3 = 14 bubbles per minute.

Describing a results table or graph, in steps

1. State the overall trend: the rate increases, then decreases.
2. Name the temperature with the highest rate, and quote that rate with its unit.
3. Quote at least one more pair of values, for example the lowest rate.
4. Say what happens at the highest temperature tested.

Describe first, then explain. “Describe” wants the pattern with data; “explain” wants the reasons (kinetic energy, collisions, denaturation).

Improving the investigation

• Use smaller temperature steps near the highest rate to find the optimum more precisely.
• Collect gas volume rather than counting bubbles, because bubbles vary in size.
• Repeat each temperature at least three times and find a mean; leave out any clear anomaly.
• Keep the tubes in the water-bath during collection so the temperature stays the same.

Where respiration fits in the course

• Cells: aerobic respiration happens in the mitochondria; muscle cells have many of them. See Cell structure and organisation.
• Enzymes: every step of respiration is enzyme-controlled, which is why temperature affects the rate.
• Gas exchange: the oxygen used in aerobic respiration enters the blood at the alveoli, and the carbon dioxide made leaves there. Expired air has more carbon dioxide because of respiration.
• Plants: plant cells respire all the time, in the light and the dark, while photosynthesis needs light. Never write that plants “photosynthesise instead of respiring”. See Plant nutrition.

Lactic acid and the oxygen debt (10.3.4, 10.3.5)

Why lactic acid builds up, in steps

1. Vigorous exercise: muscles need energy very fast.
2. Oxygen cannot be supplied fast enough for aerobic respiration alone.
3. Muscles also respire anaerobically.
4. Lactic acid is made and builds up in the muscles and blood.
5. Oxygen is needed later to break it down: the oxygen debt (EPOC).

How the debt is removed (the only two points the syllabus asks for)

What continues after exercise	Why
Fast heart rate	Transports lactic acid in the blood from the muscles to the liver
Deeper and faster breathing	Supplies oxygen for the breakdown of lactic acid in the liver

Measuring the debt. Subtract the resting oxygen uptake from each value after exercise, then add the differences. The total is the extra oxygen used, which is the size of the debt.

Quick self-test

1. Describe respiration.
2. State the word equation for aerobic respiration.
3. State the balanced chemical equation for aerobic respiration.
4. State the word equation for anaerobic respiration in yeast.
5. State the word equation for anaerobic respiration in humans.
6. Which type of respiration releases more energy from each glucose molecule?
7. State four uses of energy in living organisms.
8. How many oxygen molecules are needed to respire 3 glucose molecules aerobically?
9. Yeast released 42 bubbles in 3 minutes. Calculate the rate.
10. Resting oxygen uptake is 0.25 dm³ per minute. For four minutes after exercise, uptake is 1.25, 0.75, 0.45 and 0.25 dm³ per minute. Calculate the oxygen debt.
11. Why does the heart rate stay high after vigorous exercise?
12. Why does yeast release almost no gas at 60 °C?

Answers

1. The chemical reactions in all living cells that release energy from glucose.
2. glucose + oxygen → carbon dioxide + water
3. C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
4. glucose → ethanol + carbon dioxide
5. glucose → lactic acid
6. Aerobic respiration.
7. Any four: muscle contraction; protein synthesis; cell division; active transport; growth; passage of electrical impulses along neurones; maintenance of a constant body temperature.
8. 3 × 6 = 18 oxygen molecules.
9. 42 ÷ 3 = 14 bubbles per minute.
10. Differences: 1.00 + 0.50 + 0.20 + 0.00 = 1.70 dm³.
11. To transport lactic acid in the blood from the muscles to the liver.
12. The enzymes that control respiration are denatured, so the reactions stop.

Where marks are usually lost

• Writing that respiration “produces” or “makes” energy instead of releasing it.
• Using “respiration” to mean breathing.
• Adding carbon dioxide to the human anaerobic equation, or lactic acid to the yeast one.
• Writing “alcohol” with no gas, or “oxygen” as a product of anaerobic respiration in yeast.
• Missing the 6 in front of O₂, CO₂ or H₂O in the balanced equation.
• Giving a vague use of energy such as “moving” or “working” instead of a named use from the list.
• Saying lactic acid is broken down in the muscles rather than the liver.
• Saying breathing stays fast after exercise “for more energy” instead of “to supply oxygen to break down lactic acid in the liver”.
• Explaining the fall in yeast respiration above the optimum without saying enzymes are denatured.
• Forgetting that a shorter time for a colour change means a faster rate.

Official syllabus

Cambridge O Level Biology 5090 syllabus for examination in 2026, 2027 and 2028 (version 4), Topic 10: Respiration, published by Cambridge Assessment International Education.