Edexcel International GCSE Biology 4BI1: Gas exchange – Revision Notes

These notes condense sub-topic (g) Gas exchange of Topic 2 in the Pearson Edexcel International GCSE Biology (4BI1) specification, Issue 3: points 2.40B–2.50. The qualification is untiered. Points 2.40B–2.45B (flowering plants) carry a B reference and are Paper 2 only; points 2.46–2.50 (humans) can be tested on either paper. For full explanations and worked examples, use the gas exchange study guide.

Then test yourself with the gas exchange practice questions. Course links: Edexcel IGCSE Biology hub and printable checklist. Cell structure for the same topic is in the cell structure revision notes.

Key terms

Term	Meaning
Diffusion	Net movement of particles from a higher to a lower concentration
Gas exchange	Taking in one gas and giving out another across a surface, by diffusion
Net exchange	The overall change once respiration and photosynthesis are added together
Stoma (plural stomata)	Pore in the leaf epidermis between two guard cells
Ventilation	Moving air into and out of the lungs
Alveolus (plural alveoli)	Tiny air sac at the end of a bronchiole; site of gas exchange
Pleural membranes	Membranes around the lungs and lining the thorax; fluid between them reduces friction
Emphysema	Breakdown of alveolar walls, reducing surface area

Flowering plants (2.40B–2.45B, Paper 2 only)

2.40B–2.41B Diffusion, respiration and photosynthesis

• All gas movement into and out of a leaf is by diffusion through stomata and air spaces.
• Respiration: all the time, all living cells. Takes in oxygen, gives out carbon dioxide.
• Photosynthesis: in the light only. Takes in carbon dioxide, gives out oxygen.
• Cells using a gas keep its concentration low inside the leaf, so the gradient is maintained.

2.42B Leaf adaptations

Feature	Benefit
Thin	Short diffusion distance
Broad and flat	Large surface area
Air spaces in spongy mesophyll	Gases reach all cells; large internal surface area
Stomata, mostly on lower surface	Gases enter and leave
Moist cell walls	Gases dissolve, then diffuse into cells

2.43B Stomata

• Guard cells open and close the pore.
• Usually open in light (carbon dioxide in for photosynthesis, oxygen out).
• Usually closed in the dark or when water is short (reduces water loss, but limits gas exchange).

2.44B Net exchange and light intensity

Light	Comparison	Net result
Dark	Respiration only	Oxygen in, carbon dioxide out
Dim (balance point)	Photosynthesis rate = respiration rate	No net exchange
Bright	Photosynthesis faster	Carbon dioxide in, oxygen out

2.45B Hydrogen-carbonate indicator practical

Carbon dioxide level	Colour
Atmospheric	Red
Raised	Orange → yellow
Lowered	Magenta → purple

Method in steps

1. Same volume of red indicator in identical sealed tubes.
2. Leaf on gauze above the liquid (not touching it).
3. Vary light: foil (dark), gauze layers (dim), uncovered (bright).
4. No-leaf control tube.
5. Same temperature, same distance from lamp, same time; compare colours.

Humans (2.46–2.50)

2.46 The thorax

• Airway order: trachea → bronchi → bronchioles → alveoli.
• Ribs protect the lungs; intercostal muscles lie between the ribs.
• Diaphragm: muscle sheet below the lungs, separating thorax from abdomen.
• Pleural membranes with fluid between them reduce friction during breathing.

2.47 Ventilation in steps

Inhalation

1. External intercostal muscles contract; ribs move up and out.
2. Diaphragm contracts and flattens.
3. Thorax volume increases.
4. Pressure falls below atmospheric.
5. Air flows in.

Exhalation

1. External intercostal muscles relax; ribs move down and in (internal intercostals contract in forced exhalation).
2. Diaphragm relaxes and domes upwards.
3. Thorax volume decreases.
4. Pressure rises above atmospheric.
5. Air flows out.

2.48 Alveoli adaptations

Feature	Why it speeds diffusion
Huge number of alveoli	Large surface area
Alveolar and capillary walls one cell thick	Short diffusion distance
Dense capillary network, constant blood flow	Keeps concentration gradient steep
Ventilation	Replaces air, keeps gradient steep
Moist lining	Gases dissolve before diffusing

2.49 Smoking

Component	Effect
Tar	Carcinogens → lung cancer; irritates airways
Cilia damaged, more mucus	Mucus not cleared → smoker’s cough, bronchitis, chest infections
Emphysema	Alveolar walls break down → smaller surface area → breathlessness
Carbon monoxide	Binds to haemoglobin more readily than oxygen → blood carries less oxygen
Nicotine	Addictive; raises heart rate and blood pressure

Coronary heart disease chain: fatty deposits narrow coronary arteries → clot blocks artery → heart muscle short of oxygen → cannot respire aerobically → heart attack. Smoking raises the risk.

2.50 Breathing practical

• Limewater turns cloudy with carbon dioxide. Exhaled air turns it cloudy faster than inhaled air.
• Approximate composition: inhaled air about 21% oxygen, 0.04% carbon dioxide; exhaled air about 16% oxygen, 4% carbon dioxide, warmer and more water vapour.
• Exercise: count breaths per minute at rest, exercise for a fixed time, count again; repeat for a mean. Control type, length and intensity of exercise.
• Exercise → faster respiration in muscles → more oxygen needed and more carbon dioxide made → faster, deeper breathing.

Must-know distinctions

• Respiration vs gas exchange. Respiration is a chemical reaction in cells that releases energy. Gas exchange is the diffusion of gases across a surface. Breathing (ventilation) is a third thing: moving air.
• “No gas exchange” vs “no net gas exchange”. At the balance point both processes still run; only the net change is zero.
• Yellow vs purple. Yellow = more carbon dioxide (respiration winning). Purple = less carbon dioxide (photosynthesis winning).
• Contract vs relax. Breathing in: intercostals and diaphragm contract. Normal breathing out: they relax.
• Volume vs pressure. Volume goes up, pressure goes down. They always change in opposite directions.
• Tar vs carbon monoxide. Tar damages the lungs. Carbon monoxide affects the blood.

Small worked reminders

Reading a day-and-night net exchange curve (Paper 2 only). If a graph shows net carbon dioxide uptake by a plant over 24 hours, the curve is below zero at night (net release, respiration only), crosses zero twice (around dawn and dusk, when the two rates are equal) and peaks around midday, when light is brightest. The two crossing points are where the indicator would stay red.

Building a six-mark answer on breathing during exercise. Link each step: muscles contract more → faster respiration → more oxygen needed, more carbon dioxide produced → breathing rate and depth increase → more air ventilates the alveoli → concentration gradients stay steep → faster diffusion of oxygen into the blood and carbon dioxide out of it.

Converting counts. Breaths counted over part of a minute must be scaled: multiply a 30-second count by 2, a 15-second count by 4.

Quick self-test

1. Name the process by which gases enter a leaf.
2. Which gas does a leaf give out in bright light?
3. A sealed tube of hydrogen-carbonate indicator with a leaf is kept in the dark. What colour does it turn, and why?
4. Give two features of a leaf that help gas exchange.
5. What is the function of the fluid between the pleural membranes?
6. What happens to the diaphragm when you breathe in?
7. A student counts 6 breaths in 20 seconds. What is the breathing rate in breaths per minute?
8. Inhaled air is about 21% oxygen and exhaled air about 16%. What percentage of the inhaled oxygen is removed? Give your answer to 1 decimal place.
9. Exhaled air contains about 4% carbon dioxide and inhaled air about 0.04%. How many times higher is the exhaled concentration?
10. Explain why emphysema makes a person breathless.
11. Which substance in tobacco smoke reduces the oxygen-carrying capacity of the blood?
12. Why does a no-leaf tube need to be included in the indicator practical?

Answers

1. Diffusion (through the stomata).
2. Oxygen (net), because photosynthesis is faster than respiration.
3. Yellow. Only respiration happens, so carbon dioxide concentration rises.
4. Any two: thin; large surface area; air spaces in spongy mesophyll; many stomata; moist cell walls.
5. Reduces friction as the lungs move.
6. It contracts and flattens.
7. 6 × 3 = 18 breaths per minute.
8. (21 − 16) / 21 × 100 = 23.8%.
9. 4 / 0.04 = 100 times.
10. Alveolar walls break down, so the surface area for gas exchange falls; less oxygen diffuses into the blood.
11. Carbon monoxide.
12. It is a control: it shows any colour change is caused by the leaf, not by light or the apparatus.

Where marks are usually lost

• Stating that plants respire only at night, or photosynthesise “instead of” respiring during the day.
• Writing “no gas exchange” at the balance point instead of “no net gas exchange”.
• Reversing indicator colours: yellow is high carbon dioxide, purple is low.
• Saying the leaf in the indicator tube was in the liquid. It sits on gauze above it.
• Describing inhalation without the volume and pressure steps, or putting them in the wrong order.
• Saying the diaphragm relaxes when breathing in.
• Giving an alveolus feature (for example, “thin walls”) without the effect (“short diffusion distance”).
• Saying carbon monoxide damages the alveoli. It binds to haemoglobin.
• Linking smoking to heart disease without mentioning the coronary arteries or oxygen supply to heart muscle.
• In the exercise practical, forgetting to repeat readings or to keep the exercise the same.

Official syllabus

Pearson Edexcel International GCSE in Biology (4BI1) specification, Issue 3 (first teaching September 2017, first examination June 2019), published by Pearson Education Limited. Topic 2, sub-topic (g) Gas exchange, points 2.40B–2.50.