Respiration — CIE IGCSE Biology Revision Notes

What you'll learn

Respiration is one of the seven characteristics of living organisms and appears frequently in CIE IGCSE Biology papers, particularly in questions worth 4-6 marks. This topic covers how cells release energy from glucose through aerobic and anaerobic pathways, the roles of mitochondria, and practical investigations into respiration rates. Understanding the precise equations and being able to compare the two types of respiration are essential skills for exam success.

Key terms and definitions

Respiration — the chemical reaction occurring in all living cells that releases energy from glucose; it is not the same as breathing (gas exchange).

Aerobic respiration — respiration that requires oxygen and produces carbon dioxide, water and a large amount of energy (ATP).

Anaerobic respiration — respiration without oxygen, producing lactic acid (in animals) or ethanol and carbon dioxide (in plants and yeast), releasing a small amount of energy.

Mitochondria — organelles where most of the reactions of aerobic respiration take place; cells with high energy requirements contain many mitochondria.

ATP (adenosine triphosphate) — the molecule that stores and transfers energy in cells for processes like muscle contraction, protein synthesis and active transport.

Oxygen debt — the amount of oxygen needed to break down accumulated lactic acid after anaerobic respiration in muscles.

Fermentation — anaerobic respiration in yeast, used commercially to produce alcoholic drinks and bread.

Respiratory substrate — the substance broken down during respiration, usually glucose but can include lipids or proteins.

Core concepts

The purpose of respiration

All living organisms respire continuously to release energy from glucose. This energy is not released as heat directly but stored in ATP molecules. Cells use ATP for:

Muscle contraction during movement and exercise
Protein synthesis for growth and repair
Cell division during growth and reproduction
Active transport of molecules against concentration gradients
Maintaining body temperature in mammals and birds
Transmission of nerve impulses in the nervous system
Building larger molecules from smaller ones (anabolism)

The energy released from respiration is transferred gradually through multiple enzyme-controlled steps. This prevents energy being wasted as heat and allows cells to capture more energy as ATP.

Aerobic respiration: the complete pathway

Aerobic respiration is the most efficient way to release energy from glucose. The word equation is:

glucose + oxygen → carbon dioxide + water

The balanced chemical equation that CIE IGCSE students must know is:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

This equation must be learned precisely, including the correct numbers of molecules. Exam questions regularly test whether students can recall and apply this equation.

The process occurs in two main locations within the cell:

Cytoplasm — glucose is initially broken down in a process called glycolysis, releasing a small amount of energy
Mitochondria — the breakdown products enter mitochondria where the main energy release occurs through a series of reactions

Aerobic respiration releases approximately 2,880 kJ per mole of glucose (or about 16 kJ per gram). This large energy yield explains why aerobic respiration is the preferred method when oxygen is available.

Anaerobic respiration in animals

When oxygen supply is insufficient for aerobic respiration (during vigorous exercise, for example), muscle cells switch to anaerobic respiration. The word equation is:

glucose → lactic acid

The chemical equation is:

C₆H₁₂O₆ → 2C₃H₆O₃

Key characteristics of anaerobic respiration in animals:

Releases only approximately 150 kJ per mole of glucose — much less than aerobic respiration
Produces lactic acid, which accumulates in muscles causing fatigue and pain
Glucose is only partially broken down, so much energy remains locked in lactic acid molecules
Occurs in the cytoplasm only (mitochondria are not involved)
Can only be sustained for short periods before lactic acid buildup becomes problematic

After exercise stops and breathing rate remains elevated, oxygen is used to break down lactic acid in the liver. This is called repaying the oxygen debt. The lactic acid is converted back to glucose or broken down completely to carbon dioxide and water.

Anaerobic respiration in plants and yeast

Plant cells and yeast can also respire anaerobically, but they produce different products. The word equation is:

glucose → ethanol + carbon dioxide

The chemical equation is:

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂

This process is called fermentation and has important commercial applications:

Brewing and wine-making — yeast ferments sugars in fruit juice or grain extracts, producing ethanol (alcohol); carbon dioxide is released as a by-product
Bread-making — yeast ferments sugars in dough, producing carbon dioxide gas that makes bread rise; the small amount of ethanol produced evaporates during baking
Bioethanol production — yeast ferments sugars from crops like sugar cane or corn to produce fuel

Plant root cells may respire anaerobically in waterlogged soil where oxygen cannot reach them. However, plants cannot tolerate anaerobic conditions for extended periods as ethanol is toxic to plant cells.

Comparing aerobic and anaerobic respiration

Feature	Aerobic	Anaerobic
Oxygen required?	Yes	No
Location	Cytoplasm and mitochondria	Cytoplasm only
Energy released	Large amount (~2,880 kJ/mol)	Small amount (~150 kJ/mol)
Products (animals)	CO₂ and H₂O	Lactic acid
Products (plants/yeast)	CO₂ and H₂O	Ethanol and CO₂
Glucose breakdown	Complete	Partial (incomplete)
Sustainability	Continuous	Short-term only

This comparison table frequently appears in CIE IGCSE exam questions, with students asked to complete missing information or explain differences.

Investigating respiration rates

CIE IGCSE Biology includes practical investigations into respiration. Common experiments include:

Investigating carbon dioxide production:

Place germinating seeds or small invertebrates (e.g., maggots) in a sealed container
Include a tube of limewater or hydrogencarbonate indicator solution
Limewater turns from colourless to cloudy white when CO₂ is produced
Hydrogencarbonate indicator changes from red to yellow in the presence of CO₂
Include a control experiment with boiled (dead) seeds to show living organisms are required

Investigating heat production:

Place germinating seeds in a vacuum flask (insulated container)
Measure temperature change over several days using a thermometer
Compare with a control flask containing boiled seeds
Temperature rise demonstrates that respiration releases energy as heat

Measuring oxygen uptake with a respirometer:

Living organisms (e.g., woodlice, maggots, germinating beans) are placed in a sealed tube
Soda lime absorbs the CO₂ produced
As oxygen is consumed, the volume of gas decreases
A drop of coloured liquid in a capillary tube moves toward the organism chamber
The distance moved indicates oxygen consumption rate

Variables to control in respiration experiments include temperature, mass/number of organisms, and volume of air in the apparatus.

Factors affecting respiration rate

Several factors influence how quickly organisms respire:

Temperature — increasing temperature increases respiration rate (up to a point) because enzyme activity increases; above approximately 45°C, enzymes denature and respiration stops.

Oxygen availability — when oxygen supply is limited, organisms must use less efficient anaerobic respiration; this is why breathing rate increases during exercise.

Activity level — more active organisms require more energy, so their respiration rate increases; muscle cells contain many mitochondria to meet energy demands.

Type of organism — smaller organisms and those with higher metabolic rates (like birds and mammals) respire faster than larger or less active organisms (like reptiles).

Worked examples

Example 1: Calculating energy efficiency

Question: A student investigated respiration in germinating peas. She found that 10 g of peas consumed 0.15 moles of oxygen during aerobic respiration. Calculate how many moles of glucose were respired. Show your working. [3 marks]

Answer: The equation for aerobic respiration is: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O [1 mark]

From the equation, 1 mole of glucose requires 6 moles of oxygen [1 mark]

Therefore, 0.15 moles of oxygen would respire: 0.15 ÷ 6 = 0.025 moles of glucose [1 mark]

Example 2: Explaining anaerobic respiration

Question: A sprinter runs a 100-metre race in 10 seconds. Explain why the sprinter continues to breathe heavily for several minutes after the race has finished. [4 marks]

Answer: During the race, the sprinter's muscles were working very hard and required lots of energy [1 mark]. Oxygen could not be supplied quickly enough to the muscles for aerobic respiration [1 mark]. The muscles therefore respired anaerobically, producing lactic acid [1 mark]. After the race, the sprinter continues breathing heavily to take in extra oxygen to break down the lactic acid in the liver / to repay the oxygen debt [1 mark].

Example 3: Practical investigation

Question: A student set up an experiment with two flasks. Flask A contained 20 germinating bean seeds. Flask B contained 20 boiled bean seeds. Both flasks were sealed and left for 24 hours. The student then tested the air in each flask with limewater.

(a) Predict the result with limewater in Flask A and explain your answer. [2 marks]

(b) Explain why Flask B is needed in this experiment. [1 mark]

Answer:

(a) Limewater would turn cloudy/milky white [1 mark] because the germinating seeds respired and produced carbon dioxide [1 mark].

(b) Flask B is a control to show that living seeds are needed / to show that carbon dioxide is not produced by dead seeds [1 mark].

Common mistakes and how to avoid them

Mistake: Confusing respiration with breathing/gas exchange. Writing "respiration is when you breathe in oxygen." Correction: Respiration is a chemical reaction in cells that releases energy from glucose. Breathing is the physical movement of air in and out of lungs. Gas exchange is the diffusion of oxygen and carbon dioxide across surfaces.
Mistake: Stating that anaerobic respiration produces "no energy" or that it "doesn't release ATP." Correction: Anaerobic respiration does release energy (about 150 kJ/mol), just much less than aerobic respiration (about 2,880 kJ/mol). Always write "releases less energy" not "no energy."
Mistake: Writing that lactic acid is "broken down by oxygen" immediately after exercise. Correction: Lactic acid is transported in the blood to the liver where it is converted back to glucose or broken down to carbon dioxide and water using oxygen. Oxygen doesn't directly break down lactic acid in muscles.
Mistake: Mixing up products: writing that animals produce ethanol or that yeast produces lactic acid. Correction: Animals produce lactic acid only. Plants and yeast produce ethanol and carbon dioxide. Learn the two different anaerobic equations separately.
Mistake: Forgetting to balance the aerobic respiration equation or writing the coefficients incorrectly (e.g., writing 3O₂ instead of 6O₂). Correction: Memorize the balanced equation exactly: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O. Practice writing it from memory regularly.
Mistake: Stating that mitochondria "produce energy" or "make ATP from nothing." Correction: Mitochondria are the site where aerobic respiration reactions occur. They don't create energy; they convert chemical energy stored in glucose into a more usable form (ATP). Energy cannot be created or destroyed.

Exam technique for Respiration

Learn both equations precisely — CIE examiners expect you to recall the balanced chemical equation for aerobic respiration (C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O) and the word equations for both types of anaerobic respiration. Questions asking you to "state the equation" will award marks only if coefficients are correct.
"Explain" questions need cause and effect — when asked to explain why breathing rate increases during exercise, you must link oxygen demand → aerobic respiration → energy release. Simply stating facts without connecting them loses marks. Typically award 3-4 marks, so provide 3-4 linked points.
Distinguish between similar terms — examiners test whether you understand the difference between respiration and breathing, and between aerobic and anaerobic. Use precise terminology: "respiration releases energy" not "respiration gives you energy."
Practical questions require controls — when describing respiration experiments, always identify the control (usually boiled/dead organisms) and explain why it's needed. Questions often allocate 1-2 marks for discussing controls and variables.

Quick revision summary

Respiration releases energy from glucose in all living cells. Aerobic respiration requires oxygen and produces CO₂ and H₂O, releasing large amounts of energy (equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O). Anaerobic respiration occurs without oxygen, producing lactic acid in animals or ethanol and CO₂ in plants/yeast, releasing less energy. Most aerobic respiration occurs in mitochondria. During vigorous exercise, muscles respire anaerobically, building up an oxygen debt. ATP stores energy for cell processes including muscle contraction, protein synthesis and active transport.