Cambridge IGCSE Biology 0610: Respiration – Study Guide

This study guide teaches Topic 12, Respiration, of the Cambridge IGCSE Biology 0610 syllabus for examination in 2026, 2027 and 2028. It covers sections 12.1 (Respiration), 12.2 (Aerobic respiration) and 12.3 (Anaerobic respiration). Core content is examined on Papers 1 and 3; Core plus Supplement is examined on Papers 2 and 4. Content marked Extended only is Supplement content: the balanced chemical equations and oxygen debt.

Useful links: the Cambridge IGCSE Biology hub, the printable Biology checklist, the respiration revision notes and the respiration practice questions. To check your gaps across the course, try the Core diagnostic or the Extended diagnostic.

What this topic covers

Syllabus ref	What you must be able to do	Tier
12.1.1	State the uses of energy: muscle contraction, protein synthesis, cell division, active transport, growth, passage of nerve impulses, maintaining a constant body temperature	Core
12.1.2	Investigate and describe the effect of temperature on respiration in yeast	Core
12.2.1	Describe aerobic respiration	Core
12.2.2	State the word equation for aerobic respiration	Core
12.2.3	State the balanced chemical equation for aerobic respiration	Extended only
12.3.1	Describe anaerobic respiration	Core
12.3.2	State that anaerobic respiration releases much less energy per glucose molecule than aerobic respiration	Core
12.3.3	State the word equation for anaerobic respiration in yeast	Core
12.3.4	State the word equation for anaerobic respiration in muscles during vigorous exercise	Core
12.3.5	State the balanced chemical equation for anaerobic respiration in yeast	Extended only
12.3.6	State that lactic acid builds up in muscles and blood during vigorous exercise, causing an oxygen debt	Extended only
12.3.7	Outline how the oxygen debt is removed after exercise	Extended only

What respiration is

Respiration is a set of chemical reactions in cells that break down nutrient molecules, such as glucose, to release energy. It happens in every living cell, all the time, in plants as well as animals. The reactions are controlled by enzymes, which is why temperature affects the rate.

Two words need care:

• Respiration releases energy. It does not “make”, “produce” or “create” energy.
• Respiration is not breathing. Breathing moves air in and out of the lungs; respiration is chemistry inside cells. Gas exchange supplies the oxygen and removes the carbon dioxide.

Uses of energy in living organisms (12.1.1)

The syllabus lists seven uses. Learn all seven by name, because a question may ask for two, three or more.

Use of energy	Example
Muscle contraction	Moving an arm; the heart pumping
Protein synthesis	Joining amino acids to make enzymes or antibodies
Cell division	Making new cells for repair
Active transport	Moving ions into root hair cells against a concentration gradient
Growth	Increase in size of a young organism
Passage of nerve impulses	Signals travelling along neurones
Maintenance of a constant body temperature	Keeping the human body close to 37 °C

Copy the syllabus wording where you can. “Movement” is weaker than “muscle contraction”, and “making proteins” is weaker than “protein synthesis”.

Aerobic respiration (12.2)

Aerobic respiration is the chemical reactions in cells that use oxygen to break down nutrient molecules to release energy. It releases much more energy per glucose molecule than anaerobic respiration.

Word equation (Core):

glucose + oxygen → carbon dioxide + water

Balanced chemical equation (Extended only):

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

Worked example 1: checking the balanced equation. Count each type of atom on both sides.

Atom	Left side	Right side
C	6 (in glucose)	6 × 1 = 6 (in 6CO₂)
H	12 (in glucose)	6 × 2 = 12 (in 6H₂O)
O	6 (glucose) + 6 × 2 (6O₂) = 18	6 × 2 (6CO₂) + 6 × 1 (6H₂O) = 18

Every atom balances. The pattern to remember is one glucose, six of everything else. So for 3 glucose molecules respired aerobically, 3 × 6 = 18 carbon dioxide molecules are released.

Mark schemes want correct formulae and correct numbers. Write subscripts as small numbers after the symbol (O₂, not O2 in large type, and never 2O as a formula for oxygen gas).

Anaerobic respiration (12.3)

Anaerobic respiration is the chemical reactions in cells that break down nutrient molecules to release energy without using oxygen. It releases much less energy per glucose molecule than aerobic respiration. The syllabus asks you to state this; it does not ask for figures.

The products depend on the organism.

In yeast (Core word equation):

glucose → alcohol + carbon dioxide

The alcohol is ethanol.

In yeast (Extended only, balanced equation):

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂

Worked example 2: checking the yeast equation.

Atom	Left side	Right side
C	6	2 × 2 (ethanol) + 2 × 1 (CO₂) = 6
H	12	2 × 6 (ethanol) = 12
O	6	2 × 1 (ethanol) + 2 × 2 (CO₂) = 6

The equation balances. Notice there is no oxygen on the left. For 5 glucose molecules, yeast releases 10 ethanol and 10 carbon dioxide molecules.

In muscles during vigorous exercise (Core word equation):

glucose → lactic acid

There is no carbon dioxide in the muscle equation, and no balanced equation for it on this syllabus. Carbon dioxide appears only in the yeast version of anaerobic respiration.

Comparing the three equations

	Aerobic	Anaerobic, yeast	Anaerobic, muscles
Oxygen used?	Yes	No	No
Products	Carbon dioxide + water	Alcohol + carbon dioxide	Lactic acid
Energy per glucose	Much more	Much less	Much less

The uses of yeast respiration in making biofuels and bread belong to Topic 21 (Biotechnology), where you need these equations again.

Investigating temperature and respiration in yeast (12.1.2)

Yeast is a single-celled fungus. In a warm glucose solution it respires and releases carbon dioxide, and the rate of carbon dioxide release is a measure of the rate of respiration.

One method: counting bubbles

1. Mix a fixed volume of yeast suspension with a fixed volume and concentration of glucose solution in a test tube.
2. Fit a bung with a delivery tube that dips into a small tube of water, so released gas forms bubbles.
3. Stand the tube in a water bath at the first temperature and leave it for 5 minutes to reach that temperature.
4. Count the bubbles released in one minute. Repeat the count three times and calculate a mean.
5. Repeat at other temperatures with fresh yeast and glucose each time.

The variable you change is temperature. Keep the same: volume and concentration of glucose, mass or volume of yeast, time for counting, and the time allowed to warm up. Bubble counting is quick but bubbles vary in size, so collecting the gas and measuring its volume is more accurate. Another way to detect carbon dioxide is hydrogencarbonate indicator, which changes colour as carbon dioxide concentration rises.

Worked example 3: analysing results.

Temperature / °C	Count 1	Count 2	Count 3	Mean bubbles per minute
15	4	3	5	4
25	8	10	9	9
35	17	19	18	18
45	12	10	11	11
55	1	0	2	1

• Mean at 35 °C = (17 + 19 + 18) ÷ 3 = 18 bubbles per minute.
• From 25 °C to 35 °C, the rate rises from 9 to 18: an increase of (18 − 9) ÷ 9 × 100 = 100%. The rate doubles.
• Description: as temperature increases from 15 °C to 35 °C, the rate of respiration increases; the highest rate in this data is at 35 °C; above 35 °C the rate decreases, and it is almost zero at 55 °C.
• The true optimum lies somewhere between 25 °C and 45 °C. With readings 10 °C apart, you cannot say it is exactly 35 °C.

Why the shape? Respiration is controlled by enzymes. As temperature rises towards the optimum, molecules have more kinetic energy, so enzyme and substrate collide more often. Above the optimum, the enzymes are denatured: the active site changes shape and the substrate no longer fits. The kinetic-energy, collision and shape-and-fit explanation is Supplement content from Topic 5 (Enzymes), so use it in Extended answers; Core answers need only “the enzymes are denatured”.

Oxygen debt (12.3.6 and 12.3.7, Extended only)

During vigorous exercise, the heart and lungs cannot deliver oxygen to the muscles fast enough for all the energy needed to come from aerobic respiration. The muscles also respire anaerobically, producing lactic acid. Lactic acid builds up in the muscles and blood, and this causes an oxygen debt.

After exercise, the oxygen debt is removed in three ways, and the syllabus limits you to these three:

1. The heart rate stays fast, to transport lactic acid in the blood from the muscles to the liver.
2. Breathing stays deeper and faster, to supply oxygen for the aerobic respiration of lactic acid.
3. The lactic acid is broken down by aerobic respiration in the liver.

Worked example 4: interpreting recovery data. A cyclist sprints for one minute. Her readings:

Time	Heart rate / beats per minute	Breathing rate / breaths per minute
At rest, before sprint	64	14
End of sprint	172	40
5 minutes after	118	26
15 minutes after	66	15

• Both rates stay above resting for more than 5 minutes after the sprint has stopped. This is the oxygen debt being repaid.
• The high heart rate carries lactic acid from her leg muscles to her liver.
• The high breathing rate (and depth) supplies extra oxygen for aerobic respiration of the lactic acid in the liver.
• By 15 minutes both rates are close to resting values: most of the debt has been repaid.

A full answer links each observation to its own job. “She needs to get her breath back” earns nothing.

Common errors

• Saying respiration “produces energy”. It releases energy.
• Confusing respiration with breathing or with gas exchange.
• Giving “movement” or “keeping warm” instead of the syllabus terms “muscle contraction” and “maintenance of a constant body temperature”.
• Writing carbon dioxide as a product of anaerobic respiration in muscles. Muscles make only lactic acid.
• Writing lactic acid as a product of yeast. Yeast makes alcohol and carbon dioxide.
• Writing the yeast balanced equation with oxygen on the left.
• Saying anaerobic respiration releases “no energy”. It releases much less energy per glucose molecule.
• Saying the optimum temperature is exactly the reading with the highest rate, when readings are widely spaced.
• Saying the yeast “is denatured”. The enzymes in the yeast cells are denatured.
• Writing that lactic acid is “broken down in the muscles” as the way the oxygen debt is removed. The syllabus route is transport to the liver and aerobic respiration there.

Next steps

Condense this guide with the respiration revision notes, then try the respiration practice questions. Revisit gas exchange for how oxygen reaches the blood, and tick off each outcome on the Biology checklist.

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.