Gaseous Exchange in Humans and Plants
Senior 2 · Biology
Gaseous Exchange in Humans and Plants
Lesson 1 of 5
20 min
Let's begin
At rest you breathe in and out about 12 to 15 times a minute, without thinking about it. Every one of your cells needs a steady supply of oxygen to release energy from food, and must get rid of carbon dioxide. This unit follows those gases between the air and your cells, and between the air and a plant's leaves.
Worked example
Compare a small cube of side 1 cm with a larger cube of side 4 cm.
| Cube | Surface area | Volume | Surface area : volume |
|---|---|---|---|
| 1 cm | 6 × 1 × 1 = 6 cm² | 1 cm³ | 6 : 1 |
| 4 cm | 6 × 4 × 4 = 96 cm² | 64 cm³ | 1.5 : 1 |
As an organism gets bigger, its volume grows faster than its surface, so each cm³ of body has less outer surface to supply it. A flatworm is so small and flat that oxygen can diffuse straight through its skin to every cell. A large animal cannot rely on its outer skin, so it has a special respiratory surface with a very large area.
| Organism | Respiratory surface |
|---|---|
| Earthworm, frog (partly) | moist skin with capillaries just below it |
| Fish | gills: thin, highly folded filaments |
| Insect | tracheal system: tubes entering through pores called spiracles |
| Mammal, including humans | alveoli of the lungs |
The lungs sit in the thorax (chest cavity), protected by the ribs. Intercostal muscles run between the ribs, and a sheet of muscle called the diaphragm forms the floor of the thorax. Each lung is wrapped in pleural membranes with a thin layer of fluid between them, which reduces friction as the lungs move. The right lung has three lobes; the smaller left lung has two, leaving room for the heart.
| Part | Structure and job |
|---|---|
| Nose and nasal cavity | hairs and sticky mucus trap dust and microbes; the air is warmed and moistened |
| Pharynx and larynx | the throat, and the voice box; a flap called the epiglottis closes over the airway when you swallow, so food goes down the oesophagus |
| Trachea (windpipe) | held open by C-shaped rings of cartilage; lined with cells that make mucus and cells with tiny beating hairs (cilia) that sweep dirty mucus up to the throat |
| Bronchi (singular: bronchus) | two branches of the trachea, one to each lung, also supported by cartilage |
| Bronchioles | thousands of ever-smaller tubes without cartilage |
| Alveoli (singular: alveolus) | tiny air sacs in clusters like bunches of grapes, each covered by capillaries; this is where gas exchange happens |
Tip
Respiration releases energy (in cells). Breathing moves air (muscles). Gas exchange is diffusion (across a surface). Using the wrong word costs marks.
The airway, in order: Nose → Pharynx → Larynx → Trachea → Bronchus → Bronchiole → Alveolus.
Calculate the surface area to volume ratio of a cube of side 2 cm.
Solution: Surface area = 6 × 2 × 2 = 24 cm². Volume = 2 × 2 × 2 = 8 cm³. Ratio = 24 : 8 = 3 : 1.
Why must a respiratory surface be moist?
Solution: Oxygen and carbon dioxide must dissolve in water before they can diffuse through the cell membranes of the surface.
What would happen if the trachea had no cartilage rings?
Solution: It could collapse when the pressure inside it falls during breathing in, blocking the flow of air.
Explain how the lining of the trachea keeps the lungs clean.
Solution: Mucus traps dust and microbes. Cilia beat in waves, sweeping the mucus up to the throat, where it is swallowed or coughed out.
Remember
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