Cambridge O Level Biology 5090: Respiration (5090)

This study guide teaches Topic 10, Respiration, for Cambridge O Level Biology 5090. It is aligned to the Cambridge O Level Biology 5090 syllabus for examination in 2026, 2027 and 2028 (version 4), and covers every learning outcome in sections 10.1 Respiration, 10.2 Aerobic respiration and 10.3 Anaerobic respiration, including the yeast investigation the syllabus lists. The 5090 syllabus is not tiered, so every candidate studies all of it.

Use it with the revision notes for this topic and the practice questions. The course hub lists every topic, and the printable checklist lets you tick off each outcome. Respiration is a set of enzyme-controlled reactions, so the Enzymes study guide explains why temperature matters. To find your weak spots quickly, try one of the free 10-minute diagnostics.

What this topic covers

Syllabus ref	What you must be able to do
10.1.1	Describe respiration as the chemical reactions in all living cells that release energy from glucose
10.1.2	State the uses of energy in living organisms
10.1.3	Investigate and describe the effect of temperature on respiration in yeast
10.2.1	Describe aerobic respiration: a relatively large amount of energy released by breaking down glucose in the presence of oxygen
10.2.2	State the word equation and the balanced chemical equation for aerobic respiration
10.3.1	Describe anaerobic respiration: a relatively small amount of energy released by breaking down glucose without using oxygen
10.3.2	State the word equation for anaerobic respiration in humans
10.3.3	State the word equation for anaerobic respiration in yeast
10.3.4	Explain why lactic acid builds up during vigorous exercise, causing EPOC or an oxygen debt
10.3.5	Outline how the oxygen debt is removed after exercise (fast heart rate; deeper, faster breathing)

10.1.1 What respiration is

Learn the syllabus description word for word:

Respiration is the chemical reactions in all living cells that release energy from glucose.

• Chemical reactions. Respiration happens inside cells. It is not breathing. Breathing is the muscle movement that moves air in and out of the lungs; it is covered in Human gas exchange.
• All living cells. Plant cells, animal cells, yeast and bacteria all respire. A leaf cell respires all the time, in the light and in the dark, even while it is also photosynthesising.
• Release energy. Energy is never “made” or “produced”. It is stored in glucose and released when glucose is broken down.

Aerobic respiration happens in the mitochondria, which you identify in cells in Cell structure and organisation. Cells that use a lot of energy, such as muscle cells, have many mitochondria.

10.1.2 What the energy is used for

The syllabus names seven uses. Learn all seven, and be ready to match each one to a cell or situation.

Use of energy	Example
Muscle contraction	Leg muscles contracting while you run; heart muscle beating
Protein synthesis	Ribosomes joining amino acids to make enzymes and other proteins
Cell division	Cells dividing at a root tip or in a healing wound
Active transport	Root hair cells taking in mineral ions against a concentration gradient
Growth	A seedling producing new cells and tissues
Passage of electrical impulses along neurones	A nerve impulse travelling from the eye to the brain
Maintenance of a constant body temperature	A mammal or bird keeping warm in cold surroundings

When a question gives you a situation, name the use exactly: “active transport”, not “moving things”; “maintaining a constant body temperature”, not “keeping warm”.

10.2 Aerobic respiration

10.2.1 The description

Aerobic respiration is the release of a relatively large amount of energy by the breakdown of glucose in the presence of oxygen.

“Relatively large” means large compared with anaerobic respiration. Most of the energy your cells use comes from aerobic respiration.

10.2.2 The equations

Word equation

glucose + oxygen → carbon dioxide + water

Balanced chemical equation

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

The 6 in front of a formula counts molecules. The small number written low inside a formula counts atoms in one molecule. So 6O₂ means six oxygen molecules, each made of two oxygen atoms.

Worked example 1 (checking the balance). Show that the balanced equation has the same number of each type of atom on each side.

• Left side carbon: glucose C₆H₁₂O₆ has 6.
• Left side hydrogen: 12.
• Left side oxygen: 6 in glucose + (6 × 2) in oxygen = 6 + 12 = 18.
• Right side carbon: 6CO₂ has 6 × 1 = 6.
• Right side hydrogen: 6H₂O has 6 × 2 = 12.
• Right side oxygen: (6 × 2) in carbon dioxide + (6 × 1) in water = 12 + 6 = 18.

Every element balances.

Worked example 2 (using the ratio). One glucose molecule reacts with six oxygen molecules and gives six carbon dioxide and six water molecules. A muscle cell releases 12 molecules of carbon dioxide by aerobic respiration. How many glucose and oxygen molecules did it use?

• Carbon dioxide to glucose is 6 : 1, so glucose = 12 ÷ 6 = 2 molecules.
• Oxygen to carbon dioxide is 6 : 6, so oxygen = 12 molecules.

10.3 Anaerobic respiration

10.3.1 The description

Anaerobic respiration is the release of a relatively small amount of energy by the breakdown of glucose without using oxygen.

Anaerobic respiration releases much less energy from each glucose molecule than aerobic respiration, because the glucose is not fully broken down.

10.3.2 and 10.3.3 The word equations

In humans (for example, in muscle cells during vigorous exercise):

glucose → lactic acid

In yeast:

glucose → ethanol + carbon dioxide

Ethanol is the alcohol in alcoholic drinks. Note the two differences: humans make no carbon dioxide in anaerobic respiration, and yeast makes no lactic acid.

Comparing the three equations

	Aerobic	Anaerobic (humans)	Anaerobic (yeast)
Oxygen used?	Yes	No	No
Energy released	Relatively large	Relatively small	Relatively small
Products	Carbon dioxide and water	Lactic acid	Ethanol and carbon dioxide

10.1.3 Investigating temperature and respiration in yeast

Yeast is a single-celled fungus. When it respires it releases carbon dioxide, so the rate of carbon dioxide release is a measure of the rate of respiration. Carbon dioxide is released in both aerobic and anaerobic respiration in yeast.

A method

1. Mix a fixed mass of yeast with a fixed volume of glucose solution of fixed concentration.
2. Pour equal volumes into several boiling tubes and stand each tube in a water-bath at a different temperature, for example 10, 20, 30, 40, 50 and 60 °C. Check each temperature with a thermometer.
3. Leave the tubes for a few minutes so the mixture reaches the water-bath temperature.
4. Fit a bung and delivery tube. Collect the gas over water in an upturned measuring cylinder for a fixed time, or count the bubbles leaving the delivery tube per minute.
5. Repeat at each temperature and calculate a mean.

Other ways to show the carbon dioxide: bubble it through limewater (it turns milky) or hydrogencarbonate indicator (it turns from red/orange to yellow), and time the colour change.

Control variables: mass of yeast, volume and concentration of glucose solution, time allowed to reach temperature, collection time, size of tube. Control: a tube with boiled yeast shows that the gas comes from living yeast. Safety: take care with hot water; wear eye protection.

Results and explanation

Worked example 3. A student collected gas from yeast for 5 minutes at each temperature.

Temperature / °C	10	20	30	40	50	60
Volume of gas in 5 min / cm³	1.0	2.5	6.0	9.0	3.5	0.0

(a) Calculate the rate at each temperature. Rate = volume ÷ time.

• 10 °C: 1.0 ÷ 5 = 0.2 cm³ per min
• 20 °C: 2.5 ÷ 5 = 0.5 cm³ per min
• 30 °C: 6.0 ÷ 5 = 1.2 cm³ per min
• 40 °C: 9.0 ÷ 5 = 1.8 cm³ per min
• 50 °C: 3.5 ÷ 5 = 0.7 cm³ per min
• 60 °C: 0.0 ÷ 5 = 0 cm³ per min

(b) Describe the results. The rate increases from 10 °C to a maximum of 1.8 cm³ per min at 40 °C. It then falls, to zero at 60 °C. At 40 °C the rate is 9.0 ÷ 2.5 = 3.6 times the rate at 20 °C.

(c) Explain the results. Respiration is a series of reactions controlled by enzymes.

• As temperature rises towards the optimum, molecules gain kinetic energy, so enzyme and substrate molecules collide more often and the rate rises.
• Above the optimum, the enzymes are denatured: the active site changes shape, so the substrate no longer fits and the rate falls.
• At 60 °C the enzymes are denatured, so no gas is released.

The data shows the optimum is near 40 °C, but the true optimum could lie anywhere between 30 °C and 50 °C. To find it more precisely, repeat with smaller temperature steps in that range.

Worked example 4 (bubble counting). At 30 °C, 18 bubbles leave the delivery tube in 30 s. Rate = 18 × (60 ÷ 30) = 36 bubbles per minute. Bubble counts are less accurate than volumes, because bubbles may differ in size.

10.3.4 Why lactic acid builds up

During vigorous exercise, the muscles contract hard and fast, so they need energy very quickly.

1. The heart and lungs cannot supply oxygen to the muscles fast enough for aerobic respiration to release all the energy needed.
2. The muscles also respire anaerobically, which releases energy without oxygen.
3. Anaerobic respiration in muscle produces lactic acid.
4. Lactic acid builds up in the muscles and passes into the blood.
5. Oxygen is needed after exercise to break the lactic acid down. This extra oxygen is the oxygen debt, also called Excess Post-exercise Oxygen Consumption (EPOC).

10.3.5 How the oxygen debt is removed

The syllabus limits you to two points. Learn both, and the reason for each.

• Fast heart rate continues after exercise, to transport lactic acid in the blood from the muscles to the liver.
• Deeper and faster breathing continues after exercise, to supply oxygen for the breakdown of lactic acid in the liver.

When all the lactic acid has been broken down, the debt is repaid and heart rate and breathing return to resting values. The effect of exercise on breathing itself is covered in Human gas exchange.

Worked example 5 (measuring the debt). A person’s resting oxygen uptake is 0.30 dm³ per minute. After a sprint, oxygen uptake is measured each minute.

Minute after sprint	1	2	3	4	5	6
Oxygen uptake / dm³ per min	2.10	1.40	0.90	0.60	0.40	0.30

Calculate the oxygen debt (the extra oxygen used above resting).

• Subtract 0.30 from each value: 1.80, 1.10, 0.60, 0.30, 0.10, 0.00.
• Add them: 1.80 + 1.10 + 0.60 + 0.30 + 0.10 + 0.00 = 3.90 dm³.

By minute 6 uptake is back at the resting value, so the debt has been repaid.

Common errors

• Writing “respiration produces energy”. Energy is released, not produced or made.
• Confusing respiration with breathing. Respiration is chemical reactions in cells.
• Putting carbon dioxide in the human anaerobic equation. Humans make lactic acid only.
• Writing “glucose → alcohol + oxygen” for yeast. The gas is carbon dioxide.
• Writing 6O instead of 6O₂, or forgetting the 6 in front of H₂O.
• Saying lactic acid is broken down in the muscles. The syllabus says it is carried to the liver and broken down there.
• Explaining the fall in yeast respiration above the optimum as “the yeast gets too hot” without enzymes being denatured.
• Saying breathing stays fast after exercise “to get more energy”. It supplies oxygen to break down lactic acid in the liver.

Next steps

Condense this guide with the revision notes, then try the practice questions.

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.