Edexcel International GCSE Biology 4BI1: Gas exchange – Study Guide

This guide teaches sub-topic (g) Gas exchange of Topic 2, Structures and functions in living organisms, in the Pearson Edexcel International GCSE Biology (4BI1) specification, Issue 3. It covers points 2.40B–2.50 in depth. The qualification is untiered. The flowering-plant points (2.40B–2.45B) are bold with a B reference in the specification, so they are assessed on Paper 2 only. The human points (2.46–2.50) can appear on either paper.

Use it with the gas exchange revision notes and the gas exchange practice questions. The full course is on the Edexcel IGCSE Biology hub, and you can tick off each point on the printable checklist. Gas exchange depends on diffusion and on cell structure, which are covered in the cell structure and biological molecules guide.

What this sub-topic covers

Spec point	What you must be able to do	Paper
2.40B	Understand the role of diffusion in gas exchange	Paper 2 only
2.41B	Understand gas exchange of carbon dioxide and oxygen in relation to respiration and photosynthesis	Paper 2 only
2.42B	Understand how leaf structure is adapted for gas exchange	Paper 2 only
2.43B	Describe the role of stomata in gas exchange	Paper 2 only
2.44B	Understand that respiration continues day and night, but net exchange depends on light intensity	Paper 2 only
2.45B	Practical: investigate the effect of light on net gas exchange from a leaf, using hydrogen-carbonate indicator	Paper 2 only
2.46	Describe the structure of the thorax	1 and 2
2.47	Understand the role of the intercostal muscles and diaphragm in ventilation	1 and 2
2.48	Explain how alveoli are adapted for gas exchange by diffusion	1 and 2
2.49	Understand the effects of smoking on the lungs and circulatory system, including coronary heart disease	1 and 2
2.50	Practical: investigate breathing, including release of carbon dioxide and the effect of exercise	1 and 2

Gas exchange in flowering plants (2.40B–2.45B, Paper 2 only)

Diffusion does all the work (2.40B)

Plants have no lungs and no ventilation. Every gas that enters or leaves a leaf moves by diffusion: the net movement of particles from a region of higher concentration to a region of lower concentration. Carbon dioxide and oxygen pass through the stomata, spread through the air spaces inside the leaf, dissolve in the thin film of water on the surface of the mesophyll cells and then diffuse across the cell wall and cell membrane.

Diffusion only works because cells keep the gradient going. A photosynthesising cell uses carbon dioxide, so the concentration inside the leaf stays lower than in the air outside, and carbon dioxide keeps diffusing in.

Two processes, opposite exchanges (2.41B)

Two processes in leaf cells exchange the same two gases in opposite directions:

Process	When it happens	Gas taken in	Gas given out
Respiration	All the time, in every living cell	Oxygen	Carbon dioxide
Photosynthesis	Only in the light, in cells with chloroplasts	Carbon dioxide	Oxygen

What you measure from a whole leaf is the net exchange: the result of both processes added together.

Leaf structure (2.42B) and stomata (2.43B)

Feature	How it helps gas exchange
Leaf is thin	Short diffusion distance from the air to every cell
Broad, flat shape	Large surface area for gases to enter
Spongy mesophyll with many air spaces	Gases spread quickly through the leaf; large internal surface area of moist cell walls
Many stomata, mostly in the lower epidermis	Openings that let gases in and out; fewer on the upper surface reduces water loss in direct sun
Moist cell surfaces	Gases dissolve before they diffuse into cells

Each stoma is a pore between two guard cells, which change shape to open or close it. Stomata usually open in the light, letting carbon dioxide in for photosynthesis, and close in the dark or when the plant is short of water. Closing reduces water loss but also cuts the carbon dioxide supply. In this sub-topic, describe their role as letting gases diffuse in and out.

Day, night and net exchange (2.44B)

Respiration never stops. Photosynthesis depends on light. So the net exchange changes with light intensity:

Light intensity	Rates compared	Net exchange
Dark	Respiration only	Oxygen taken in, carbon dioxide given out
Dim (one particular intensity)	Photosynthesis = respiration	No net exchange of either gas
Bright	Photosynthesis > respiration	Carbon dioxide taken in, oxygen given out

Where the two rates are equal, the carbon dioxide made by respiration is used at once by photosynthesis, and vice versa for oxygen. Both processes continue; the net exchange with the air is zero.

Practical: hydrogen-carbonate indicator (2.45B)

Hydrogen-carbonate indicator changes colour with the carbon dioxide concentration of the air above it:

Carbon dioxide compared with normal air	Indicator colour
Normal (atmospheric) level	Red
Higher	Orange, then yellow
Lower	Magenta, then purple

Method in outline

1. Put the same volume of red indicator into several identical boiling tubes.
2. Place a leaf (or leaf discs of the same area) in each tube, held on gauze so it does not touch the liquid.
3. Seal each tube with a bung.
4. Cover one tube with foil (dark), one with layers of gauze or tissue (dim), and leave one uncovered (bright).
5. Set up a control tube with no leaf.
6. Keep all tubes the same distance from the lamp and at the same temperature, then compare colours after the same time.

Worked example 1: interpreting the tubes

After two hours the tubes look like this: foil-covered tube yellow; gauze-covered tube red; uncovered tube purple; no-leaf control red.

• Foil: in the dark the leaf only respires. Carbon dioxide is added to the air in the tube, so the indicator turns yellow.
• Gauze: respiration and photosynthesis happen at the same rate. There is no net change in carbon dioxide, so the indicator stays red.
• Uncovered: photosynthesis is faster than respiration. Carbon dioxide is removed, so the indicator turns purple.
• Control: it stays red, which shows the colour changes were caused by the leaf and not by light or the tube itself.

Gas exchange in humans (2.46–2.50)

Structure of the thorax (2.46)

Air enters through the nose and mouth and passes down the trachea. The trachea divides into two bronchi (one to each lung). Each bronchus divides into many smaller bronchioles, which end in clusters of air sacs called alveoli, where gas exchange happens.

The lungs sit inside the ribcage. The intercostal muscles run between the ribs. The diaphragm is a sheet of muscle that separates the thorax from the abdomen. Two pleural membranes line the outside of the lungs and the inside of the thorax, with a thin layer of fluid between them. The fluid reduces friction as the lungs move during breathing.

Ventilation (2.47)

Ventilation moves air in and out of the lungs. Air always moves from higher pressure to lower pressure.

Step	Breathing in (inhalation)	Breathing out (exhalation)
External intercostal muscles	Contract	Relax
Ribcage	Moves up and out	Moves down and in
Diaphragm	Contracts and flattens	Relaxes and domes upwards
Volume of thorax	Increases	Decreases
Pressure inside thorax	Falls below atmospheric pressure	Rises above atmospheric pressure
Air movement	Air is drawn in	Air is forced out

When you breathe out hard (during exercise, for example) the internal intercostal muscles also contract to pull the ribs down faster.

Write the chain in order: muscles, volume, pressure, air movement.

Alveoli (2.48)

Oxygen diffuses from the air in the alveoli into the blood in the capillaries. Carbon dioxide diffuses the other way. Each feature below increases the rate of that diffusion:

• Very many alveoli, giving a very large surface area.
• Walls one cell thick, and capillary walls one cell thick, so the diffusion distance is very short.
• A dense network of capillaries around each alveolus. Blood flow carries oxygen away and brings carbon dioxide, keeping a steep concentration gradient.
• Ventilation brings fresh air in and removes stale air, which also keeps the gradient steep.
• A moist lining, so gases dissolve before diffusing across.

Always link the feature to the effect: “thin walls give a short diffusion distance, so diffusion is faster”.

Smoking (2.49)

Tobacco smoke harms both the lungs and the circulatory system.

Substance or effect	What it does
Tar	Contains carcinogens, which increase the risk of lung cancer. Irritates the airways.
Damage to cilia and extra mucus	Mucus with trapped dirt and bacteria is not swept away, so it builds up. This causes a smoker’s cough and more chest infections, including bronchitis.
Emphysema	Walls of alveoli break down, forming fewer, larger air spaces. The surface area for gas exchange falls, so less oxygen enters the blood and the person becomes breathless.
Carbon monoxide	Combines with haemoglobin in red blood cells more readily than oxygen does, so the blood carries less oxygen.
Nicotine	Addictive. Raises heart rate and narrows blood vessels, increasing blood pressure.

Coronary heart disease. The coronary arteries supply the heart muscle with oxygen and glucose. Smoking increases the risk that fatty deposits build up in their walls, narrowing them. A blood clot can then block a narrowed artery. The heart muscle beyond the blockage gets no oxygen, cannot respire aerobically, and may die: a heart attack. Carbon monoxide makes things worse because the blood that does reach the heart carries less oxygen.

Practical: investigating breathing (2.50)

Carbon dioxide release. Air is breathed in through one tube of limewater and out through another, using a two-tube apparatus. The limewater that exhaled air bubbles through turns cloudy (milky) much sooner, which shows exhaled air contains more carbon dioxide. As a rough guide, inhaled air is about 21% oxygen and 0.04% carbon dioxide; exhaled air is about 16% oxygen and 4% carbon dioxide, and it is warmer and holds more water vapour.

Effect of exercise. Count breaths per minute at rest, do a set exercise (for example, step-ups for two minutes), then count again straight away. Keep the type, length and intensity of exercise the same, and repeat to get a mean.

Worked example 2: breathing-rate data

A student counts breaths for one minute at rest three times: 14, 15 and 16. After two minutes of skipping the counts are 26, 29 and 29. Calculate the percentage increase in mean breathing rate.

Mean at rest        = (14 + 15 + 16) / 3 = 15 breaths per minute
Mean after exercise = (26 + 29 + 29) / 3 = 28 breaths per minute
Increase            = 28 - 15 = 13
Percentage increase = 13 / 15 × 100 = 86.7%

Explanation. During exercise muscles respire faster, so they need more oxygen and produce more carbon dioxide. Breathing faster and deeper increases ventilation, which keeps the concentration gradients at the alveoli steep, so more oxygen diffuses into the blood and more carbon dioxide is removed.

If you count for 30 seconds, double the count to get breaths per minute: 8 breaths in 30 s is 16 per minute.

Common errors

• Saying plants respire only at night. Respiration happens all the time; at night it is the only process.
• Saying “no gas exchange” at the compensation intensity. Write “no net gas exchange”.
• Stating hydrogen-carbonate colours the wrong way round. Yellow means more carbon dioxide; purple means less.
• Leaving out a no-leaf control or forgetting to keep temperature constant in the indicator practical.
• Describing inhalation as “lungs expand, so air rushes in” without the muscles, volume and pressure steps.
• Writing “the diaphragm relaxes and flattens” during inhalation. It contracts and flattens.
• Listing alveolus features without saying how each one speeds up diffusion.
• Saying carbon monoxide “blocks the lungs”. It reduces the oxygen-carrying capacity of the blood.
• Forgetting that emphysema reduces surface area, not just “damages the lungs”.

Next steps

• Condensed recall and a quick self-test: gas exchange revision notes.
• Original exam-style questions with mark-by-mark answers: gas exchange practice questions.
• Find your weak areas with the free diagnostics.

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.