Biology · Lesson 9

Gaseous exchange & breathing

JAMBWAECNECO

What the syllabus asks

JAMB: Gaseous exchange: respiratory organs and surfaces of amoeba, insects, fish, toad and mammals; mechanism of breathing in mammals; gaseous exchange in plants

WAEC: Respiratory system: respiratory surfaces and their characteristics, gaseous exchange in unicellular organisms, insects (tracheal system), fish (gills), amphibians, mammals (lungs) and flowering plants (stomata, lenticels, pneumatophores), mechanism of breathing, composition of inhaled and exhaled air, lung capacities, respiratory disorders

NECO: Respiratory system: respiratory surfaces and their characteristics, gaseous exchange in unicellular organisms, insects (tracheal system), fish (gills), amphibians, mammals (lungs) and flowering plants (stomata, lenticels, pneumatophores), mechanism of breathing, composition of inhaled and exhaled air, lung capacities, respiratory disorders

What you will learn

Distinguish breathing, gaseous exchange and cellular respiration.
List and explain the characteristics of efficient respiratory surfaces.
Describe gaseous exchange in Amoeba, insects, fish and amphibians.
Describe the structure of the human respiratory system and relate each part to its function.
Explain the mechanism of inspiration and expiration.
Compare inhaled and exhaled air and interpret lung volumes.
Describe gaseous exchange in flowering plants and the opening and closing of stomata.
Describe common respiratory disorders and the effects of smoking.

Key points

Respiration releases energy in cells; gaseous exchange is diffusion of O₂ and CO₂; breathing is ventilation.
Respiratory surfaces: large, thin, moist, permeable, blood-rich, ventilated.
Amoeba: whole cell membrane; earthworm and toad skin: moist and blood-rich.
Insects: spiracles → tracheae (chitin rings) → tracheoles → cells.
Fish: gills with many filaments, operculum, one-way water flow, counter-current exchange.
Human air path: nose → pharynx → larynx → trachea → bronchi → bronchioles → alveoli.
Inspiration: external intercostals and diaphragm contract; volume up, pressure down, air in.
Exhaled air: about 16% O₂, 4% CO₂, saturated with water vapour.
Tidal volume ≈ 500 cm³; breathing controlled by the medulla responding to CO₂.
Plants: stomata, lenticels, pneumatophores; guard cells open when turgid; smoking damages cilia and alveoli.

Start here: Respiration, breathing and gaseous exchange

Three words in this topic are often mixed up, and WAEC examiners deduct marks when they are. Get them clear now.

Cellular (tissue) respiration is the chemical breakdown of food, usually glucose, inside cells to release energy. It happens in the mitochondria of every living cell (the lesson “Respiration”).
Gaseous exchange is the diffusion of oxygen into the body (or blood) and carbon dioxide out, across a respiratory surface.
Breathing (ventilation) is the muscular pumping of air or water over the respiratory surface. It is a physical process, not a chemical one.

So when you run for the school bus, your muscle cells respire faster, more carbon dioxide is produced, and you breathe faster and deeper to speed up gaseous exchange in your lungs. All three are linked, but they are not the same thing.

Why do organisms need gaseous exchange at all? Aerobic respiration uses oxygen and produces carbon dioxide. Oxygen must be supplied continuously, and carbon dioxide must be removed, because in solution it forms an acid that can harm cells.

⚠️ Common mistake: writing “respiration is breathing in and out”. Breathing moves air; respiration releases energy in cells.

💡 Exam tip: plants carry out gaseous exchange and respiration all the time, but they do not “breathe”, because they have no muscular ventilation movements.

Try it yourself. Classify each as breathing, gaseous exchange or respiration: (a) glucose broken down in a mitochondrion; (b) oxygen diffusing into a capillary; (c) the diaphragm contracting.

Show solution

(a) respiration; (b) gaseous exchange; (c) breathing.

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Practice questions

1. Gaseous exchange in humans takes place in the

alveoli
trachea
bronchi
larynx

Show answer

A: alveoli
Thin walls and a rich blood supply.

2. Which feature does NOT help alveoli in gaseous exchange?

A large surface area
A moist lining
A rich supply of capillaries
Thick walls

Show answer

D: Thick walls
Walls must be thin.

3. During inspiration, the diaphragm

does not move
moves the ribs down
contracts and flattens
relaxes and domes upward

Show answer

C: contracts and flattens
This increases the chest volume.

4. During inspiration, the external intercostal muscles

contract, lowering the ribs
contract, raising the ribs
relax, lowering the ribs
do not move

Show answer

B: contract, raising the ribs
The chest expands.

5. Inhaled air compared with exhaled air contains

more oxygen and less carbon dioxide
less oxygen
more carbon dioxide
more water vapour

Show answer

A: more oxygen and less carbon dioxide
About 21% O₂ in, about 16% out.

Key words

Gaseous exchange: The diffusion of O₂ into and CO₂ out of an organism across a respiratory surface.
Breathing (ventilation): The muscular movement of air (or water) in and out of the respiratory organs.
Respiratory surface: The surface across which gaseous exchange occurs.
Spiracle: A pore on an insect’s body that opens into the tracheal system.
Trachea (insect): An air tube, strengthened by chitin rings, branching through an insect’s body.
Tracheole: The finest branch of an insect trachea, ending at the cells.
Gill filament: A thin, blood-rich projection on a fish gill arch where exchange occurs.
Operculum: The bony flap covering a bony fish’s gills.
Alveolus: A tiny air sac in the lung where gaseous exchange takes place.
Diaphragm: A dome-shaped sheet of muscle separating the thorax from the abdomen.
Intercostal muscles: Muscles between the ribs that move the rib cage.
Tidal volume: The volume of air breathed in or out in a normal breath (about 500 cm³).

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