2g Gas Exchange¶
Part of 2 Structure and Functions in Living Organisms.
Gas exchange links diffusion to whole-organism survival. Plants need carbon dioxide and oxygen moving in opposite directions at different times, while humans need a ventilated surface that keeps diffusion rapid.
Learning Objectives¶
| ID | Route | Official specification wording | Main teaching sections |
|---|---|---|---|
2g-lo-1 |
Biology-only | 2.40B understand the role of diffusion in gas exchange 2.41B understand gas exchange (of carbon dioxide and oxygen) in relation to respiration and photosynthesis 2.42B understand how the structure of the leaf is adapted for gas exchange 2.43B describe the role of stomata in gas exchange 2.44B understand how respiration continues during the day and night, but that the net exchange of carbon dioxide and oxygen depends on the intensity of light 2.45B practical: investigate the effect of light on net gas exchange from a leaf, using hydrogen-carbonate indicator Humans |
Gas Exchange in Leaves, Day and Night Gas Balance |
2g-lo-2 |
All students | 2.46 describe the structure of the thorax, including the ribs, intercostal muscles, diaphragm, trachea, bronchi, bronchioles, alveoli and pleural membranes 2.47 understand the role of the intercostal muscles and the diaphragm in ventilation 2.48 explain how alveoli are adapted for gas exchange by diffusion between air in the lungs and blood in capillaries |
Ventilation in Humans |
2g-lo-3 |
All students | 2.49 understand the biological consequences of smoking in relation to the lungs and the circulatory system, including coronary heart disease 2.50 practical: investigate breathing in humans, including the release of carbon dioxide and the effect of exercise |
Alveoli and Ventilation Mechanics |
Biology-Only Content: Gas Exchange in Leaves¶
This content is required for Biology-only students and is not required for Combined Science students.
In plants, gas exchange of oxygen and carbon dioxide is needed for both photosynthesis and respiration. All movement of gases is by diffusion.
Leaf adaptations for gas exchange:
| Structure | Adaptation |
|---|---|
| Spongy mesophyll | Air spaces increase the surface area to volume ratio, allowing faster diffusion of gases between cells and the stomata |
| Guard cells | Kidney-shaped cells that open and close the stomata by absorbing or losing water — when turgid they open the stomata |
| Stomata | Pores where gas exchange (CO₂ in, O₂ out during the day) and water vapour loss occur; open during the day and close at night |
| Thin leaves | Short diffusion distance from the stomata to the mesophyll cells |
| Flattened shape | Large surface area for absorption of light and CO₂ |
Gas Exchange in Leaves¶
Carbon dioxide diffuses into leaves for photosynthesis, while oxygen diffuses out as a product. Air spaces in the spongy mesophyll and the thin leaf structure help keep the diffusion distance short. Stomata are pores in the leaf surface; guard cells control whether they are open or closed, regulating both gas exchange and water loss.
Day and Night Gas Balance¶
- Plants respire all the time, releasing CO₂ and consuming O₂ in both daylight and darkness.
- Photosynthesis can happen only when light is available.
- During the day, photosynthesis generally runs at a rate equal to or greater than respiration, so there is net CO₂ uptake and net O₂ release from the plant.
- At night, photosynthesis stops but respiration continues, so there is net CO₂ release and net O₂ uptake from the plant.
Practical investigation of net gas exchange: A water plant is placed in a beaker of water containing hydrogen-carbonate indicator: - Hydrogen-carbonate indicator is red at normal CO₂ concentration, purple when CO₂ falls (photosynthesis dominant) and yellow when CO₂ rises (respiration dominant). - Moving a lamp closer increases light intensity, increasing the rate of photosynthesis and turning the indicator purple. - Moving the lamp further away reduces photosynthesis until respiration is dominant, turning the indicator yellow.
Ventilation in Humans¶
The thorax contains the main gas exchange structures:
| Structure | Role |
|---|---|
| Ribs | Bony cage that protects the lungs and assists in ventilation |
| Intercostal muscles | Located between ribs; contract or relax to move the ribcage during breathing |
| Diaphragm | Muscular dome at the base of the thorax; changes shape to alter thoracic volume and pressure |
| Trachea | Windpipe; conducts air from the mouth/nose into the thorax |
| Bronchi | Two branches from the trachea, one leading into each lung |
| Bronchioles | Smaller tubes branching from the bronchi, connecting to the alveoli |
| Alveoli | Tiny air sacs at the end of the bronchioles; the site of gas exchange |
| Pleural membranes | Membranes lining the outside of the lungs and inside of the chest wall; lubricate the lungs to reduce friction during breathing |
Alveoli and Ventilation Mechanics¶
Ventilation mechanism:
| Inhalation | Exhalation | |
|---|---|---|
| Intercostal muscles | Contract | Relax |
| Ribcage | Moves up and outwards | Moves down and inwards |
| Diaphragm | Contracts and flattens | Relaxes and domes upwards |
| Pressure in thorax | Decreases | Increases |
| Air movement | Moves in | Moves out |
Air moves from regions of high pressure to low pressure. When the thorax volume increases (ribcage up and out, diaphragm flat), pressure inside falls below atmospheric pressure, so air is drawn in.
Alveolar adaptations for efficient gas exchange: - Thin walls — each alveolus is just one cell thick, giving a very short diffusion pathway. - Folded structure — the many alveoli provide a very large total surface area. - Rich capillary supply — blood capillaries surround each alveolus, maintaining a steep concentration gradient by continuously removing O₂ and delivering CO₂.
Common Confusions¶
- Ventilation vs gas exchange: Ventilation moves air into and out of the lungs. Gas exchange is the diffusion of oxygen and carbon dioxide between the air in alveoli and the blood in capillaries.
- Day vs night gas balance: Students sometimes think plants only respire at night. Plants respire continuously; the difference is that photosynthesis also occurs in the day.
Key Terms¶
- Stoma: a pore in the leaf surface through which gases and water vapour move.
- Guard cell: a cell that controls the opening and closing of a stoma by changes in its water content.
- Ventilation: the movement of air into and out of the lungs by muscular action.
- Alveolus: a tiny air sac in the lungs where gas exchange occurs.
- Intercostal muscles: muscles between the ribs that help move the chest during breathing.
- Diaphragm: a dome-shaped muscle beneath the lungs whose contraction and relaxation drives ventilation.
- Bronchi: the two airways that branch from the trachea and enter the lungs.
- Bronchioles: small airways branching from the bronchi that lead to the alveoli.
- Pleural membranes: membranes that lubricate the surface of the lungs and the chest wall.