What is respiration in the context of Edexcel IGCSE Biology?

In Edexcel IGCSE Biology, respiration refers to the biochemical process by which cells convert glucose and oxygen into energy, carbon dioxide, and water. This process is essential for providing the energy required for various cellular activities and is a key topic under the 'Structure and Functions in Living Organisms' section.

How does aerobic respiration differ from anaerobic respiration?

Aerobic respiration requires oxygen and produces a large amount of energy, along with carbon dioxide and water as byproducts. In contrast, anaerobic respiration occurs without oxygen, resulting in less energy production and different byproducts, such as lactic acid in animals or ethanol and carbon dioxide in plants and yeast. Understanding these differences is crucial for the Edexcel IGCSE Biology curriculum.

Why is respiration important for living organisms?

Respiration is vital for living organisms because it provides the energy necessary for all cellular processes, including growth, repair, and maintenance. This energy is stored in the form of ATP (adenosine triphosphate), which cells use to perform various functions. This concept is a fundamental part of the 'Structure and Functions in Living Organisms' topic in Edexcel IGCSE Biology.

What are the main stages of aerobic respiration?

Aerobic respiration consists of three main stages: glycolysis, the Krebs cycle, and the electron transport chain. Glycolysis occurs in the cytoplasm, breaking down glucose into pyruvate. The Krebs cycle takes place in the mitochondria, further oxidizing pyruvate to produce electron carriers. Finally, the electron transport chain, also in the mitochondria, uses these electrons to generate ATP. These stages are crucial for understanding cellular energy production in Edexcel IGCSE Biology.

How is respiration different from breathing?

Respiration is a cellular process that involves the breakdown of glucose to produce energy, while breathing is the physical act of inhaling oxygen and exhaling carbon dioxide. Breathing supports respiration by supplying oxygen needed for aerobic respiration and removing carbon dioxide, a byproduct. This distinction is important in the Edexcel IGCSE Biology syllabus under the 'Structure and Functions in Living Organisms' section.

Why is "Confusing respiration with breathing (ventilation / gas exchange)" a common mistake in Respiration?

Why it happens: Both involve oxygen and carbon dioxide, and 'respiration' is colloquially used to mean breathing. How to avoid it: **Respiration** = chemical reactions in CELLS that release ENERGY. **Breathing** = the muscular process of moving air in and out of the lungs (more correctly: ventilation). **Gas exchange** = the diffusion of O₂ and CO₂ at the alveoli. Never use 'respiration' for breathing in a biology exam. Source: 4BI1 Examiner Reports 2022-2024

Why is "Stating that yeast produces lactic acid in anaerobic respiration" a common mistake in Respiration?

Why it happens: Confusion with anaerobic respiration in human muscle. How to avoid it: **Humans (muscle):** glucose → **lactic acid**. **Yeast:** glucose → **ethanol + CO₂**. Two DIFFERENT pathways with DIFFERENT products. Source: 4BI1 Examiner Reports 2024

Why is "Writing 'energy' as a CHEMICAL substance on the right side of the respiration equation as if it were a molecule" a common mistake in Respiration?

Why it happens: Treating energy as if it had formula 'E'. How to avoid it: Energy is not a chemical substance — it is released by the reaction. Write it OUTSIDE the formal equation: C_6H_12O_6 + 6O_2 arrow 6CO_2 + 6H_2O + energy. The mark scheme accepts 'energy' or 'ATP'. Source: 4BI1 Examiner Reports 2023

Why is "Saying the body breathes hard after exercise 'to get more oxygen for the muscles'" a common mistake in Respiration?

Why it happens: Vague understanding of oxygen debt. How to avoid it: The muscles have STOPPED contracting — they don't need more O₂ for further work. The extra O₂ is needed to OXIDISE the LACTIC ACID that has accumulated, in the LIVER. Use the keywords 'oxygen debt', 'lactic acid', 'liver', 'oxidise'. Source: 4BI1 Examiner Reports 2023-2024

Why is "Saying respiration ONLY happens in mitochondria" a common mistake in Respiration?

Why it happens: Aerobic respiration is mostly in mitochondria, so students generalise. How to avoid it: AEROBIC respiration is MAINLY in mitochondria (the citric-acid cycle and oxidative phosphorylation). ANAEROBIC respiration (glycolysis in humans → lactic acid; alcoholic fermentation in yeast) happens in the CYTOPLASM. The first step of aerobic respiration (glycolysis) also happens in the cytoplasm.

Why is "Saying anaerobic respiration releases more energy because it's faster" a common mistake in Respiration?

Why it happens: Confusion of 'fast start' with 'high yield'. How to avoid it: Anaerobic respiration starts FASTER (doesn't need O₂ delivery) but releases FAR LESS energy per glucose — most of the glucose's energy stays trapped in the lactic acid (or ethanol) molecule. Aerobic releases ~38 ATP; anaerobic ~2 ATP per glucose. Source: 4BI1 Examiner Reports 2024

Structure and Functions in Living OrganismsEdexcel IGCSE Biology (4BI1)

Respiration

Work through the notes, try the practice questions, then take the quiz. The report tells you exactly what to revise next.

Previous Next

Learn this Topic

Start with the slides for the quick version, then go deeper with the full study notes.

Short Study Notes in the form of Slides

Read the notes first. If the method in a worked example clicks, you're ready for the questions.

Page 1 / 0

Detailed Study Notes

Full prose, callouts and a recap — built for A* mastery, not just a quick scan.

Take these study notes with you

Download a branded PDF — full prose, callouts, recap and memorise list for Respiration, ready to print or save offline.

Respiration — Pearson Edexcel International GCSE Biology 4BI1 Study Notes (2026 onwards, Spec Issue 4)

Aerobic and anaerobic respiration in humans and other organisms. Word and balanced equations, energy yields, mitochondria, oxygen debt during intense exercise, anaerobic respiration in yeast (fermentation) for brewing and baking, and the required practical investigating CO₂ release or O₂ uptake by germinating seeds. Covers spec 2.33-2.38.

At a glance

Respiration ≠ breathing. Respiration = chemical reactions in cells that release ENERGY.
Aerobic: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy}$ . In mitochondria. High yield (~38 ATP).
Anaerobic in humans: glucose → lactic acid. Small energy yield (~2 ATP). In muscle during sprints.
Anaerobic in yeast: glucose → ethanol + CO₂. Small energy. Used in brewing + baking.
Oxygen debt = extra O₂ needed after exercise to oxidise lactic acid in the liver.
Mitochondria = site of aerobic respiration. Cells with high energy demand have MANY.
Energy uses: muscle contraction, biosynthesis, active transport, body temperature, growth, cell division.
RP: germinating peas + soda lime + capillary drop = O₂ uptake. Boiled peas = control.

What you’ll learn

Mapped to the Cambridge IGCSE 4BI1 syllabus (2026 onwards).

2.33 — Understand that respiration is the chemical release of energy in cells. Recall the word and balanced symbol equations for aerobic respiration: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy}$ .
2.34 — Understand the differences between aerobic and anaerobic respiration in terms of need for oxygen, products and energy yield. Recall the word equations for anaerobic respiration in human muscle (→ lactic acid) and in yeast (→ ethanol + carbon dioxide).
2.35 — Practical: investigate the evolution of carbon dioxide and the uptake of oxygen by respiring organisms (e.g. germinating peas) using suitable apparatus.
2.36 — Understand the role of mitochondria as the site of aerobic respiration; understand that cells with high energy demand contain more mitochondria.
2.37 — Understand the use of energy released by respiration: muscle contraction, growth, cell division, active transport, biosynthesis, maintaining body temperature.
2.38 — Understand the concept of oxygen debt — extra oxygen consumed after intense exercise to oxidise the lactic acid that has accumulated, transported via the blood to the liver.

What is respiration? (spec 2.33)

Chemical reactions in cells that release energy from glucose.

Respiration is the set of chemical reactions in cells that break down nutrient molecules (mainly glucose) to release energy for metabolism. Every living cell respires continuously.

Distinguish from BREATHING.

Breathing (ventilation) = the physical act of moving air in and out of the lungs.
Gas exchange = the diffusion of O₂ into and CO₂ out of the blood at the alveoli.
Respiration = the CELLULAR process — the chemistry inside each cell that releases energy.

Using 'respiration' to mean 'breathing' is a common error that loses marks in an Edexcel paper.

Two types of respiration:

Feature	Aerobic	Anaerobic
Oxygen required?	YES	NO
Products (humans)	CO₂ + H₂O	Lactic acid
Products (yeast)	CO₂ + H₂O	Ethanol + CO₂
Energy yield per glucose	HIGH (~38 ATP)	LOW (~2 ATP)
Main site	Mitochondria	Cytoplasm
When used in humans	At rest and moderate exercise	Brief intense exercise

Energy currency: ATP. Respiration doesn't release energy as free heat — most is captured in ATP (adenosine triphosphate), the universal energy currency. When a cell needs energy, ATP is broken down to ADP + Pi, releasing energy on demand.

Respiration is the CELLULAR release of energy — NOT breathing.
Aerobic uses O₂; anaerobic doesn't.
Energy stored as ATP for muscle, transport, biosynthesis.

See the full worked example for respiration →

Aerobic respiration (spec 2.33)

Glucose + O₂ → CO₂ + H₂O + lots of energy.

Word equation: glucose + oxygen → carbon dioxide + water (+ energy)

Balanced symbol equation: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy (ATP)}$

Where: mainly in mitochondria — the double-membrane organelles found in eukaryotic cells. Glycolysis (the first step, glucose → pyruvate) happens in the cytoplasm; the rest of the cycle (citric-acid cycle, oxidative phosphorylation) happens inside mitochondria.

Why aerobic respiration releases so much energy. Glucose is fully oxidised — every C is converted to CO₂, every H to H₂O. The electrons released drive the production of ~38 ATP per glucose molecule.

Mitochondria and cell function (spec 2.36). The number of mitochondria in a cell depends on its energy demand:

High mitochondrial count: cardiac muscle, skeletal muscle, sperm tail mid-piece, liver cells, neurones.
Low mitochondrial count: mature red blood cells (NO mitochondria — they're stripped out so the cell can hold more haemoglobin), fat cells.

Uses of energy from respiration (spec 2.37):

Muscle contraction — the actin-myosin slide.
Active transport — pumping ions against a gradient (e.g. Na⁺/K⁺ pump).
Synthesising new molecules (biosynthesis) — amino acids → proteins, glucose → glycogen, etc.
Cell division — mitosis and meiosis.
Growth — building new cells.
Maintaining body temperature — endotherms (mammals, birds) generate heat as a by-product.
Nerve impulses — restoring resting potential after each impulse.

Aerobic: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy}$ .
Site: mitochondria.
~38 ATP per glucose.
Muscle, sperm, liver = many mitochondria.

See the full worked example for respiration →

Anaerobic respiration (spec 2.34)

Without O₂. Different products in humans vs yeast.

Anaerobic respiration is the partial breakdown of glucose without oxygen. The products and uses differ between organisms.

Anaerobic respiration in humans (muscle cells during intense exercise):

Word equation: glucose → lactic acid
Symbol: $C_6H_{12}O_6 \rightarrow 2C_3H_6O_3$
Energy yield: ~2 ATP per glucose (much less than aerobic).
Site: cytoplasm of muscle cells.
When: during brief intense exercise — sprinting, weightlifting, the final dash for the bus. When O₂ demand exceeds the rate of O₂ delivery, the muscle 'tops up' the aerobic ATP with anaerobic ATP.

Build-up of lactic acid in muscle lowers the pH → inhibits enzymes → muscle fatigue → cramp. The athlete must stop and rest while the lactic acid is cleared.

Anaerobic respiration in yeast — fermentation:

Word equation: glucose → ethanol + carbon dioxide
Symbol: $C_6H_{12}O_6 \rightarrow 2C_2H_5OH + 2CO_2$
Energy yield: ~2 ATP per glucose.
Different products from human anaerobic respiration — yeast makes ETHANOL + CO₂, not lactic acid.

Industrial uses of yeast fermentation:

Brewing. Yeast added to a sugar solution from malted barley (beer) or grape juice (wine). The vessel is closed to limit O₂. Yeast ferments sugars → ethanol (the alcohol in the drink) + CO₂. Brewing stops when alcohol reaches ~5-15 % (yeast killed by their own ethanol).
Bread-making. Yeast mixed with flour, sugar, water. Yeast ferments sugars → CO₂ + ethanol. CO₂ collects in bubbles → dough RISES. The ethanol evaporates during baking.

Which product matters?

Brewing → ethanol is the useful product.
Baking → CO₂ is the useful product (the ethanol evaporates).

Anaerobic in humans: glucose → lactic acid (in muscle cytoplasm).
Anaerobic in yeast: glucose → ethanol + CO₂ (fermentation).
Both yield ~2 ATP — much less than aerobic.
Brewing uses ethanol; baking uses CO₂.

See the full worked example for respiration →

Oxygen debt after intense exercise (spec 2.38)

Extra O₂ needed afterwards to break down accumulated lactic acid.

During intense exercise (e.g. a 400 m sprint), the body cannot deliver O₂ to the muscles fast enough to meet the energy demand. Muscle cells supplement aerobic respiration with anaerobic respiration, producing lactic acid.

The oxygen debt is the EXTRA oxygen the body needs AFTER exercise to oxidise this accumulated lactic acid.

The pathway:

During exercise → anaerobic respiration in muscle → lactic acid builds up.
Lactic acid is transported via the blood to the liver.
In the liver, the lactic acid is oxidised — reacted with oxygen to give CO₂ + H₂O (or converted back to glucose).
While there is still lactic acid to oxidise, the body keeps:
- Breathing deeply and rapidly — more O₂ in.
- Heart rate elevated — more O₂ delivered to the liver.
Once the lactic acid is cleared, breathing and heart rate return to normal.

The debt explained. The body 'borrowed' oxygen during exercise (used anaerobic respiration instead of aerobic) and now has to 'pay it back' after. The size of the debt depends on how much lactic acid built up.

Edexcel answer template (typical 4-mark question):

During exercise, O₂ demand > O₂ supply.
Muscles respire ANAEROBICALLY → produce LACTIC ACID.
After exercise, lactic acid transported in blood to LIVER.
Extra O₂ needed to OXIDISE the lactic acid → this is the OXYGEN DEBT.

Cause: anaerobic respiration during exercise → lactic acid build-up.
Repaid: extra O₂ delivered to LIVER to oxidise lactic acid.
Effect: deep, rapid breathing AND elevated heart rate continue after exercise.

See the full worked example for respiration →

Required practical — respiration in germinating peas (spec 2.35)

Soda lime + capillary drop = O₂ uptake; limewater = CO₂ release.

Edexcel names two go-to respiration experiments — one for O₂ uptake, one for CO₂ release.

A. Measuring O₂ uptake by germinating peas (respirometer method).

Boiling tube containing germinating peas resting on gauze.
Soda lime at the bottom — absorbs the CO₂ produced. Now any volume change in the tube reflects ONLY O₂ uptake.
Sealed with a rubber bung + capillary tube containing a coloured-water drop.
Place in a water bath at constant temperature (e.g. 25 °C).
As O₂ is taken up, pressure inside the tube falls → the drop moves INWARDS (towards the peas).
Measure distance moved in 5 or 10 minutes.
Rate of O₂ uptake = distance / time, OR volume = distance × π × r² (cross-sectional area of capillary).

B. Detecting CO₂ release using limewater (or hydrogencarbonate indicator).

Connect a pump or aspirator that draws air OVER the peas and through limewater.
Limewater (Ca(OH)₂ solution) turns from clear to MILKY/CLOUDY in the presence of CO₂.
An alternative is hydrogencarbonate indicator (red at atmospheric CO₂; yellow when CO₂ rises; purple when CO₂ falls).
Control: replace germinating peas with boiled (dead) peas — limewater should NOT turn milky.

Variables (both methods):

IV: temperature (water bath) OR type of organism / mass of organism.
DV: distance moved by drop (method A); time for limewater to turn milky (method B).
CVs: same mass / number of peas; same age; same volume of apparatus; same temperature (if not the IV).

Control: BOILED peas. Heat-killing the peas denatures their enzymes → no respiration. The coloured drop should NOT move and limewater should NOT turn milky. The control proves that any change in the live tube is due to LIVE peas respiring.

Reliability: repeat each measurement at least 3 times and calculate the mean.

Method A (O₂ uptake): soda lime absorbs CO₂; capillary drop tracks volume; measure drop movement per unit time.
Method B (CO₂ release): air drawn over peas through limewater (clear → milky) OR hydrogencarbonate indicator (red → yellow).
Control: BOILED peas (no respiration).
Repeat × 3 and take the mean.

See the full worked example for respiration →

How the body uses energy from respiration (spec 2.37)

Muscle, biosynthesis, active transport, body temperature, growth, cell division.

Cells use ATP from respiration to drive every active process:

1. Muscle contraction.

ATP powers the cross-bridge cycle between actin and myosin filaments.
Skeletal muscle (movement), cardiac muscle (heartbeat), smooth muscle (peristalsis, blood vessel diameter).

2. Synthesis of new molecules (biosynthesis / anabolism).

Linking amino acids → proteins (ribosomes use ATP).
Linking glucose units → glycogen / starch.
Linking fatty acids + glycerol → lipids.
DNA replication.

3. Active transport.

Moving substances across membranes AGAINST a concentration gradient.
Examples: Na⁺/K⁺ pump in nerve cells; mineral ion uptake by root hair cells; glucose reabsorption in kidney tubules.

4. Cell division.

Mitosis and meiosis need ATP to build new cell components and separate chromosomes.

5. Growth.

New cells (mitosis) + new molecules (biosynthesis).

6. Maintaining body temperature.

In mammals and birds (endotherms), heat released by respiration keeps core body temperature constant (~37 °C in humans).
When cold, shivering (rapid muscle contraction) increases respiration rate and generates extra heat.
When hot, sweating dissipates heat.

7. Nerve impulses.

After each impulse, the Na⁺/K⁺ pump (active transport) resets the resting potential — requires ATP.

Why each one requires energy. Active processes go AGAINST a thermodynamic gradient — building order from disorder, pumping uphill, or pulling apart strong molecular bonds. Without ATP, the cell could not perform any of these.

Six main uses: muscle, biosynthesis, active transport, cell division, growth, temperature.
Endotherms (mammals/birds) use respiration to maintain body temperature; fish do NOT.
Cells without ATP cannot sustain any active process — they die.

See the full worked example for respiration →

How it’s examined

Respiration is a HIGH-yield topic — 10-15 marks per paper. Paper 1 (4BI1/1B): word + balanced equations, table comparing aerobic vs anaerobic, lactic-acid build-up + oxygen debt, mitochondria as the site, named uses of energy. Paper 2 (4BI1/2B): required-practical methods (germinating-pea respirometer), industrial use of yeast (brewing/baking), data analysis of breathing rate during/after exercise. Examiner reports stress that students MUST distinguish between 'respiration' (cellular) and 'breathing' (ventilation) — using one for the other is the single most common mark-loser.

Sources: Pearson Edexcel International GCSE Biology 4BI1 specification — Issue 4, September 2024 (valid for 2026 onwards); Pearson Edexcel 4BI1 Examiner Reports 2022-2024; 4BI1/1B May/Jun 2024 question paper and mark scheme; 4BI1/1B January 2024 question paper and mark scheme; 4BI1/2B May/Jun 2024 question paper and mark scheme; 4BI1/2B January 2024 question paper and mark scheme. Last reviewed 2026-05-12.

Master this Topic

Worked examples, formulae, definitions and the mistakes examiners flag — everything you need to push from a pass to an A*.

Take this whole topic with you

Download a branded revision sheet — worked examples, formulae, definitions and common mistakes for Respiration, ready to print or save as PDF.

Step-by-step worked examples — Respiration

Step-by-step solutions to past-paper-style questions on respiration, written exactly the way a tutor would explain them at the board.

1Word and balanced equation for aerobic respiration (3 marks, Paper 1)
Core• Adapted from 4BI1/1B January 2024 Q3(a)• aerobic respiration, spec-2.33
Question
(a) Write the word equation and balanced symbol equation for aerobic respiration. State the organelle in which aerobic respiration takes place. (3 marks)
Step-by-step solution
1. Step 1
  Word equation (B1). glucose + oxygen → carbon dioxide + water (+ ENERGY released as ATP).
2. Step 2
  Balanced symbol equation (B1). $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy}$ . Energy is written outside the equation because it is not a chemical substance — it is released as ATP for the cell to use.
3. Step 3
  Organelle (B1). Mitochondria — the 'powerhouse of the cell'. Cells with high energy demand (e.g. muscle, sperm, liver) contain MANY mitochondria.
Answer
Word: glucose + oxygen → carbon dioxide + water (+ energy / ATP). Symbol: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O$ + energy. Site: mitochondria.
Examiner tip
Mark scheme awards B1 word, B1 balanced symbol, B1 mitochondria. Examiner reports note that 'mitochondria' is required (NOT 'mitochondrion' if asked for the site — both are accepted but spelling errors like 'mitocondria' lose the mark). Energy must be written separately — not as 'C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy' as if energy were a molecule.
2Comparing anaerobic respiration in humans vs yeast (4 marks, Paper 1)
Core• Adapted from 4BI1/1B May/Jun 2024 Q4• anaerobic respiration, yeast, spec-2.34
Question
Write the word equation for anaerobic respiration in (a) human muscle cells during intense exercise, and (b) yeast cells. (c) Give ONE industrial use of anaerobic respiration in yeast. (4 marks)
Step-by-step solution
1. Step 1
  (a) Human muscle (B1). glucose → lactic acid (+ a small amount of energy).
2. Step 2
  (b) Yeast (B1). glucose → ethanol + carbon dioxide (+ a small amount of energy). This is the process called fermentation.
3. Step 3
  (c) Industrial use of yeast fermentation (B1 + B1, max 2 from 1 use). Brewing — the ethanol produced is the alcohol in beer, wine and spirits. Bread-making — the CO₂ released bubbles up through dough and makes it rise; the ethanol evaporates during baking.
Answer
(a) Human muscle: glucose → lactic acid. (b) Yeast: glucose → ethanol + carbon dioxide. (c) Brewing (alcohol) OR bread-making (CO₂ makes dough rise).
Examiner tip
Mark scheme requires PRECISE products: 'lactic acid' (not 'lactate' or 'acid'); 'ethanol' (not 'alcohol' alone — though 'ethanol = alcohol' is accepted in context); 'carbon dioxide' (not just 'gas'). Examiner reports flag candidates who write 'lactic acid' as a product of yeast respiration — yeast does NOT make lactic acid.
3Oxygen debt after intense exercise (5 marks, Paper 1)
Core• Adapted from 4BI1/1B May/Jun 2024 Q5(b)• oxygen debt, exercise, spec-2.34
Question
Explain why an athlete continues to breathe deeply and rapidly for several minutes AFTER finishing a 400 m sprint. (5 marks)
Step-by-step solution
1. Step 1
  During the sprint, oxygen demand exceeds supply (B1). The athlete cannot deliver enough O₂ to the muscles fast enough → muscle cells respire anaerobically in addition to aerobically.
2. Step 2
  Anaerobic respiration produces lactic acid (B1). glucose → lactic acid (+ small energy). Lactic acid builds up in the muscles → causes fatigue / pain / cramp.
3. Step 3
  After the sprint, the body has an OXYGEN DEBT (B1). The 'debt' is the extra oxygen needed to deal with the lactic acid that accumulated.
4. Step 4
  Lactic acid is transported in the blood to the liver (B1). In the liver, the oxygen reacts with lactic acid to convert it back to glucose / oxidise it to CO₂ + water.
5. Step 5
  Deep, rapid breathing increases O₂ uptake (B1). Until the lactic acid is removed, the body keeps breathing harder and the heart keeps beating fast to deliver extra O₂. Once the debt is repaid, breathing returns to normal.
Answer
During sprint: O₂ demand > supply → muscles respire anaerobically → lactic acid builds up. After sprint: the EXTRA O₂ needed to break down the accumulated lactic acid is the OXYGEN DEBT. Lactic acid is transported to the liver and oxidised. Continued deep + rapid breathing (and a high heart rate) deliver the extra O₂ to repay the debt.
Examiner tip
Mark scheme keywords: 'anaerobic respiration', 'lactic acid', 'oxygen debt', 'liver', 'oxidised'. Examiner reports stress that candidates should LINK the deep breathing to the NEED to oxidise the lactic acid — just saying 'to get more O₂' loses the final mark.
4Why muscle cells have many mitochondria (3 marks, Paper 1)
Core• Adapted from 4BI1/1B January 2024 Q2(d)• mitochondria, respiration, spec-2.33
Question
Cardiac muscle cells contain many mitochondria. Explain how this is related to their function. (3 marks)
Step-by-step solution
1. Step 1
  Cardiac muscle cells contract continuously (B1). Without rest — every ~1 s, for the entire lifetime.
2. Step 2
  Contraction requires a constant supply of energy (ATP) (B1). Continuous contraction = continuous demand for ATP from aerobic respiration.
3. Step 3
  Many mitochondria = lots of aerobic respiration = lots of ATP (B1). Aerobic respiration releases far more energy per glucose molecule than anaerobic respiration → using mitochondria allows the muscle to keep working without fatigue.
Answer
Cardiac muscle contracts continuously and so needs a constant supply of ATP. Many mitochondria = high rate of aerobic respiration = enough ATP for the continuous contraction. Aerobic respiration releases more energy per glucose than anaerobic, so the muscle avoids lactic-acid build-up.
Examiner tip
Mark scheme awards 1 mark per linked statement (function → energy demand → mitochondria provide ATP). Examiner reports flag answers that just say 'mitochondria release energy' without linking to the cell's specific function.
5Required practical — respiration in germinating peas (6 marks, Paper 2)
Extended• Adapted from 4BI1/2B January 2024 Q4• required practical, respiration, Paper 2, spec-2.35
Question
Describe an experiment to show that germinating peas respire and use up oxygen. State the variables and the expected results. (6 marks)
Step-by-step solution
1. Step 1
  Set up TWO boiling tubes with soda lime at the bottom (B1). Soda lime absorbs CO₂ produced by respiration — this allows any volume change to reflect O₂ uptake (not a net of O₂ in + CO₂ out).
2. Step 2
  Tube A — living, germinating peas. Tube B — peas that have been BOILED and cooled (control) (B1). Boiling kills the cells / denatures enzymes → no respiration possible in tube B.
3. Step 3
  Each tube has a capillary tube containing a coloured-water drop, sealed with a rubber bung (M1). As O₂ is used up by respiration, the volume of gas inside the tube falls → atmospheric pressure pushes the coloured drop INWARDS (towards the peas).
4. Step 4
  Place both tubes in a water bath at a constant temperature (e.g. 25 °C) (A1, depends on M1). Controls the rate of respiration (temperature is a control variable).
5. Step 5
  Measure the distance the coloured drop moves in 10 minutes (B1). This is the dependent variable. Distance moved (or volume = distance × cross-sectional area of capillary) is the rate of O₂ uptake by the peas.
6. Step 6
  Expected results (B1). In tube A the drop moves towards the peas (O₂ taken in by respiration). In tube B the drop does NOT move (boiled peas — no respiration). The difference confirms living peas are using O₂.
Answer
Two boiling tubes with soda lime (absorbs CO₂) + capillary with coloured drop, sealed. Tube A: living germinating peas. Tube B: BOILED peas (control). In a water bath at 25 °C. Measure how far the drop moves towards the peas in 10 min = O₂ uptake. Tube A drop moves; tube B drop does not → respiration uses O₂.
Examiner tip
Required-practical answers reward STRUCTURED design. Must include: (1) control = boiled peas; (2) soda lime to absorb CO₂ (so volume change = O₂ alone); (3) water bath to control temperature (CV); (4) capillary drop to detect volume change. Examiner reports flag candidates who forget the CONTROL or do not explain why soda lime is needed.

Key Formulae — Respiration

The formulae you need to memorise for respiration on the Pearson Edexcel IGCSE 4BI1 paper, with every variable defined in plain English and a note on when to use it.

Aerobic respiration (spec 2.33)
$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy (ATP)}$
$C_6H_{12}O_6$
glucose — the respiratory substrate
$O_2$
oxygen — final electron acceptor; comes from breathing / from solution in aquatic organisms
$CO_2$
carbon dioxide — waste product; removed via lungs / gills
$H_2O$
water — metabolic water added to body fluids or removed in breath / urine / sweat
$\text{energy}$
released as ATP — drives muscle contraction, active transport, biosynthesis, growth, cell division, maintaining body temperature in mammals/birds
When to use
Both papers. The defining equation of aerobic respiration. Takes place in mitochondria. Must be balanced — 6 carbons, 12 hydrogens, 18 oxygens each side. 'Energy' is written outside the chemical equation (it is not a molecule) — the mark scheme accepts 'energy' or 'ATP'.
Example
Compare the energy yield of aerobic vs anaerobic respiration: aerobic releases ~38 ATP per glucose; anaerobic in humans (→ lactic acid) releases only ~2 ATP; anaerobic in yeast (→ ethanol + CO₂) also only ~2 ATP. Why aerobic respiration produces more energy: glucose is FULLY oxidised to CO₂ and H₂O.
Anaerobic respiration in humans (spec 2.34)
$C_6H_{12}O_6 \rightarrow 2C_3H_6O_3 + \text{(small energy)}$
$C_6H_{12}O_6$
glucose
$C_3H_6O_3$
lactic acid (lactate) — accumulates in muscle, causes fatigue / cramp
$\text{small energy}$
~2 ATP per glucose — much less than aerobic respiration
When to use
Anaerobic respiration in muscle cells during INTENSE exercise (when O₂ demand exceeds supply). Word equation: glucose → lactic acid. No oxygen is used; no carbon dioxide or water is produced. The lactic acid build-up creates an OXYGEN DEBT that is repaid by continued deep breathing after exercise.
Example
Explain why a sprinter's muscles cramp at the end of a 400 m race: insufficient O₂ delivery during sprint → anaerobic respiration → lactic acid builds up in muscle → low pH inhibits enzymes → fatigue / cramp.
Anaerobic respiration in yeast — fermentation (spec 2.34)
$C_6H_{12}O_6 \rightarrow 2C_2H_5OH + 2CO_2 + \text{(small energy)}$
$C_6H_{12}O_6$
glucose
$C_2H_5OH$
ethanol (alcohol) — used in brewing of beer, wine, spirits
$CO_2$
carbon dioxide — causes bread dough to rise
$\text{small energy}$
~2 ATP per glucose
When to use
Anaerobic respiration in yeast (a single-celled fungus). Word: glucose → ethanol + carbon dioxide. Different products from anaerobic respiration in humans (which gives lactic acid). Industrial uses: brewing (alcohol from yeast in malted barley + hops; from grape sugars in wine) AND bread-making (CO₂ raises dough; ethanol evaporates during baking).
Example
In a brewery, yeast is added to a sugar solution and the air supply is restricted so the yeast respires anaerobically. After several days, the ethanol concentration reaches ~12-15 %, after which the yeast die (alcohol toxicity).

Model Answers — Respiration

High-scoring sample answers for respiration on the Pearson Edexcel IGCSE 4BI1 paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.

Question
Adapted from 4BI1/2B May/Jun 2024 Q48 marks
Compare aerobic and anaerobic respiration in humans, referring to substrates, products, energy yield and conditions. (8 marks)
Model answer
Both aerobic and anaerobic respiration release energy from glucose, but they differ in their requirements and products.

Aerobic respiration.

Substrate: glucose.

Other reactant: oxygen.

Products: carbon dioxide + water + ENERGY (as ATP).

Equation: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy}$ .

Site: mainly mitochondria.

Energy yield: ~38 ATP per glucose — HIGH.

Conditions: oxygen present; happening continuously in all body cells at rest and moderate exercise.

Anaerobic respiration in human muscle.

Substrate: glucose.

Other reactant: none — no oxygen required.

Product: lactic acid (a 3-carbon molecule).

Equation: glucose → lactic acid.

Site: cytoplasm of muscle cells.

Energy yield: ~2 ATP per glucose — LOW. Most of the glucose's chemical energy remains locked in the lactic acid molecule.

Conditions: during intense exercise when oxygen supply cannot meet demand (e.g. 400 m sprint).

Comparison points to make in the answer (B1 each):

Both start with glucose as substrate.

Both release energy / ATP.

Aerobic needs O₂; anaerobic does not.

Aerobic produces CO₂ + H₂O; anaerobic produces lactic acid in humans.

Aerobic releases MUCH MORE energy per glucose than anaerobic.

Aerobic happens mainly in mitochondria; anaerobic in cytoplasm.

Anaerobic produces an OXYGEN DEBT in humans that must be repaid by deep breathing afterwards.

Anaerobic is faster (in terms of starting up) but unsustainable due to lactic acid build-up.
Why this scores
Mark scheme (8-mark comparison): B1 per correctly contrasted pair, max 8. The most-credited pairs are: oxygen needed vs not (1), CO₂ + H₂O vs lactic acid (1), more energy vs less (1), mitochondria vs cytoplasm (1), continuous vs limited (oxygen debt) (1). Examiner reports note that candidates who write a list of facts without making the comparison explicit ('aerobic releases more energy THAN anaerobic') lose half the marks.
Question
Adapted from 4BI1/2B January 2024 Q3(b)6 marks
Yeast is used in bread-making and brewing. Explain the role of yeast in each, referring to the products of anaerobic respiration. (6 marks)
Model answer
Yeast is a single-celled fungus that can respire anaerobically (no oxygen). The word equation is: glucose → ethanol + carbon dioxide (+ small amount of energy). This process is called fermentation.

Bread-making:

Yeast is mixed with flour, sugar, water and salt to form dough.

The yeast ferments the sugar, producing ethanol and carbon dioxide.

The CO₂ collects in bubbles within the dough → the dough RISES (becomes light and aerated).

The dough is kneaded and left in a warm place (~37 °C) to give the yeast its optimum temperature.

When the dough is baked at high temperature in the oven, the ethanol EVAPORATES (loaf doesn't taste alcoholic) and the yeast cells are killed.

Brewing (beer):

Yeast is added to a sugar solution (extracted from malted barley + hops, called 'wort').

The brewing vessel is closed to limit O₂ → yeast is forced to respire anaerobically.

Yeast ferments the sugar → ethanol (the alcohol in the beer) + carbon dioxide.

After several days the alcohol concentration reaches ~5 % (beer) or ~12 % (wine), at which point most of the yeast die (alcohol toxicity).

The brew is filtered, sometimes pasteurised, bottled and sometimes given a second fermentation in the bottle (to add fizz).

Comparison. In bread-making the useful product is CO₂ (makes the loaf rise); the ethanol is a wasted by-product. In brewing the useful product is ethanol (the alcohol); the CO₂ is allowed to escape OR retained for fizz.
Why this scores
Mark scheme: B1 word equation; B1 bread = CO₂ raises dough; B1 ethanol evaporates during baking; B1 brewing = ethanol is the alcohol; B1 yeast killed by high alcohol; B1 fermentation (anaerobic). Max 6. Examiner reports note that candidates often confuse the useful product in the two processes — say WHICH product matters in each.
Question
Adapted from 4BI1/2B May/Jun 2024 Q68 marks
Describe an experiment to investigate the effect of temperature on the rate of respiration in germinating peas. State the variables, the method and the expected pattern of results. (8 marks)
Model answer
Aim: measure the rate of O₂ uptake by germinating peas at different temperatures.

Apparatus. Boiling tube containing germinating peas, gauze platform supporting them, soda lime at the bottom (absorbs CO₂ produced — so any change in tube volume reflects O₂ uptake alone), rubber bung with capillary tube containing a coloured-water drop, water bath, stopwatch, ruler.

Variables.

IV: temperature of the water bath: 10, 20, 30, 40 °C (range, ≥4 values).

DV: distance moved by the coloured drop in 5 minutes → rate of O₂ uptake = distance / time (mm min⁻¹).

CVs: same MASS of germinating peas in each run; same age / species of pea; same volume of soda lime; same capillary diameter; equilibration time before measuring.

Method:

Weigh out 10 g of germinating peas. Place them on the gauze above the soda lime.

Seal the tube with the bung and capillary; place a coloured drop in the open end of the capillary.

Place the tube in a water bath at 10 °C; leave for 5 minutes to equilibrate.

Mark the position of the drop on the capillary. Start the stopwatch.

After 5 min, mark the new position of the drop. Measure distance moved (mm).

Reset the drop. Repeat at 20, 30 and 40 °C.

Repeat each temperature THREE TIMES and calculate the mean rate.

Control. Repeat the whole experiment using BOILED (dead) peas. The coloured drop should NOT move (no respiration) — confirming that any movement in the live tube is due to respiration.

Expected results:

Rate of O₂ uptake increases with temperature from 10 °C → 40 °C (more kinetic energy → enzymes work faster).

Rate may peak around the optimum temperature for the respiration enzymes (~35-40 °C).

Above the optimum, enzymes denature → rate FALLS sharply.

Reliability: repeats and means reduce random error. Accuracy: equilibration time, narrow-bore capillary for sensitivity, water bath maintained to ±1 °C.
Why this scores
Mark scheme: IV + values (1); DV (1); 3 CVs (3); structured method (2); control = boiled peas (1); expected pattern with optimum (1); reliability — repeats + mean (1). Max 8. AVP credits: equilibration before reading; preparing soda lime; measuring O₂ volume rather than distance. Examiner reports flag candidates who omit the CONTROL (boiled peas) — without it, you can't show that movement is caused by respiration.
Question
Adapted from 4BI1/1B January 2024 Q3(c)4 marks
List FOUR processes in the human body that require energy from respiration. For each, briefly explain why energy is needed. (4 marks)
Model answer
Energy released by respiration (in the form of ATP) is used for every active process in the body.

1. Muscle contraction. Actin and myosin filaments slide past each other — this sliding requires energy. Used for movement (skeletal muscle), the heartbeat (cardiac muscle), and peristalsis (smooth muscle in the gut).

2. Synthesis of new molecules (anabolism / biosynthesis). Building large molecules from small ones — for example, amino acids → proteins, glucose → glycogen, fatty acids + glycerol → lipids. All these reactions require ATP input.

3. Active transport. Moving ions and molecules across cell membranes AGAINST their concentration gradient. Examples: sodium-potassium pump in nerve cells; absorption of mineral ions by root hair cells; reabsorption of glucose in kidney tubules.

4. Maintaining body temperature (thermoregulation). Mammals and birds are endothermic — heat released as a by-product of respiration keeps body temperature at ~37 °C in mammals. Extra respiration (shivering) generates extra heat when cold.

Other uses (AVP):

Cell division (mitosis / meiosis) — building new cell components, separating chromosomes.

Growth — making new cells.

Nerve impulses — restoring resting potential after each impulse (Na⁺/K⁺ pump).
Why this scores
Mark scheme: B1 per use + explanation; max 4. Examiner reports note that 'moving' and 'growing' alone (without specifying muscle contraction or biosynthesis) score half — be specific. 'Heat / temperature' must be linked to mammals/birds (endotherms) — fish do not use respiration to maintain temperature.

Key Definitions and Keywords — Respiration

Definitions to memorise and the exact keywords mark schemes credit for respiration answers — sharpened from recent examiner reports for the 2026 Pearson Edexcel IGCSE 4BI1 sitting.

Respiration
Examiner keyword
The chemical reactions in cells that break down nutrient molecules (mainly glucose) to release energy for metabolism. Takes place in ALL living cells, ALL the time. Two types: aerobic (with O₂, in mitochondria) and anaerobic (without O₂, in cytoplasm).
Aerobic respiration
Examiner keyword
Respiration using oxygen: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy}$ . Takes place mainly in mitochondria. Releases the maximum energy per glucose (~38 ATP).
Anaerobic respiration
Examiner keyword
Respiration WITHOUT oxygen. In humans: glucose → lactic acid (+ small energy). In yeast: glucose → ethanol + carbon dioxide (+ small energy). Releases much less energy than aerobic.
Mitochondria
Examiner keyword
Double-membrane organelles in eukaryotic cells where AEROBIC respiration takes place. Cells with high energy demand (muscle, sperm, liver, neurones) contain many mitochondria.
ATP
Examiner keyword
Adenosine triphosphate — the energy currency of cells. Made by respiration; broken down to release energy for muscle contraction, active transport, biosynthesis and maintaining body temperature.
Oxygen debt
Examiner keyword
The extra oxygen required AFTER intense exercise to oxidise the lactic acid that has accumulated during anaerobic respiration. The lactic acid is transported to the liver, where it is oxidised to CO₂ + H₂O OR converted back to glucose.
Fermentation
Examiner keyword
Anaerobic respiration in yeast: glucose → ethanol + carbon dioxide. Industrial uses include brewing (alcohol from yeast in malted barley / grape juice) and bread-making (CO₂ makes the dough rise).
Lactic acid
Examiner keyword
A 3-carbon waste product of anaerobic respiration in human muscle cells. Build-up lowers muscle pH, causing fatigue and cramp. Cleared by transport to the liver and oxidation, requiring oxygen (the 'oxygen debt').

Common Mistakes and Misconceptions — Respiration

The traps other students keep falling into on respiration questions — taken from recent Pearson Edexcel IGCSE 4BI1 examiner reports and mark schemes — and how to avoid them.

✕Confusing respiration with breathing (ventilation / gas exchange)
4BI1 Examiner Reports 2022-2024
Why it happens
Both involve oxygen and carbon dioxide, and 'respiration' is colloquially used to mean breathing.
How to avoid it
Respiration = chemical reactions in CELLS that release ENERGY. Breathing = the muscular process of moving air in and out of the lungs (more correctly: ventilation). Gas exchange = the diffusion of O₂ and CO₂ at the alveoli. Never use 'respiration' for breathing in a biology exam.
✕Stating that yeast produces lactic acid in anaerobic respiration
4BI1 Examiner Reports 2024
Why it happens
Confusion with anaerobic respiration in human muscle.
How to avoid it
Humans (muscle): glucose → lactic acid. Yeast: glucose → ethanol + CO₂. Two DIFFERENT pathways with DIFFERENT products.
✕Writing 'energy' as a CHEMICAL substance on the right side of the respiration equation as if it were a molecule
4BI1 Examiner Reports 2023
Why it happens
Treating energy as if it had formula 'E'.
How to avoid it
Energy is not a chemical substance — it is released by the reaction. Write it OUTSIDE the formal equation: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O$ + energy. The mark scheme accepts 'energy' or 'ATP'.
✕Saying the body breathes hard after exercise 'to get more oxygen for the muscles'
4BI1 Examiner Reports 2023-2024
Why it happens
Vague understanding of oxygen debt.
How to avoid it
The muscles have STOPPED contracting — they don't need more O₂ for further work. The extra O₂ is needed to OXIDISE the LACTIC ACID that has accumulated, in the LIVER. Use the keywords 'oxygen debt', 'lactic acid', 'liver', 'oxidise'.
✕Saying respiration ONLY happens in mitochondria
Why it happens
Aerobic respiration is mostly in mitochondria, so students generalise.
How to avoid it
AEROBIC respiration is MAINLY in mitochondria (the citric-acid cycle and oxidative phosphorylation). ANAEROBIC respiration (glycolysis in humans → lactic acid; alcoholic fermentation in yeast) happens in the CYTOPLASM. The first step of aerobic respiration (glycolysis) also happens in the cytoplasm.
✕Saying anaerobic respiration releases more energy because it's faster
4BI1 Examiner Reports 2024
Why it happens
Confusion of 'fast start' with 'high yield'.
How to avoid it
Anaerobic respiration starts FASTER (doesn't need O₂ delivery) but releases FAR LESS energy per glucose — most of the glucose's energy stays trapped in the lactic acid (or ethanol) molecule. Aerobic releases ~38 ATP; anaerobic ~2 ATP per glucose.

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

Past paper style quiz

Get a report showing which sub-topics you've nailed and which ones still need work.

Video lesson

Short walkthrough of the concepts students most often get stuck on.

Respiration — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Respiration

Short Study Notes in the form of Slides

Detailed Study Notes

What you’ll learn

How it’s examined

1Word and balanced equation for aerobic respiration (3 marks, Paper 1)

2Comparing anaerobic respiration in humans vs yeast (4 marks, Paper 1)

3Oxygen debt after intense exercise (5 marks, Paper 1)

4Why muscle cells have many mitochondria (3 marks, Paper 1)

5Required practical — respiration in germinating peas (6 marks, Paper 2)

Aerobic respiration (spec 2.33)

Anaerobic respiration in humans (spec 2.34)

Anaerobic respiration in yeast — fermentation (spec 2.34)

Respiration

Aerobic respiration

Anaerobic respiration

Mitochondria

ATP

Oxygen debt

Fermentation

Lactic acid

✕Confusing respiration with breathing (ventilation / gas exchange)

✕Stating that yeast produces lactic acid in anaerobic respiration

✕Writing 'energy' as a CHEMICAL substance on the right side of the respiration equation as if it were a molecule

✕Saying the body breathes hard after exercise 'to get more oxygen for the muscles'

✕Saying respiration ONLY happens in mitochondria

✕Saying anaerobic respiration releases more energy because it's faster

Practice questions

Past paper style quiz

Assess your understanding

Video lesson

Respiration — frequently asked questions