Edexcel A Level (IAL) Biology -2.1 Properties of Gas Exchange Surfaces- Study Notes- New Syllabus
Edexcel A Level (IAL) Biology -2.1 Properties of Gas Exchange Surfaces- Study Notes- New syllabus
Edexcel A Level (IAL) Biology -2.1 Properties of Gas Exchange Surfaces- Study Notes -Edexcel A level Biology – per latest Syllabus.
Key Concepts:
- 2.1 (i) know the properties of gas exchange surfaces in living organisms (large surface area to volume ratio, thickness of surface and difference in concentration)
(ii) understand how the rate of diffusion is dependent on these properties and can be calculated using Fick’s Law of Diffusion
(iii) understand how the structure of the mammalian lung is adapted for rapid gaseous exchange
Gas Exchange in Living Organisms
🌱 Introduction
All living organisms need to exchange gases mainly oxygen (O₂) for respiration and carbon dioxide (CO₂) as a waste product.
Gas exchange occurs by diffusion movement of gases from a region of high concentration → low concentration.
Efficiency of diffusion depends on the surface area, thickness, and concentration difference.
Small organisms (like amoeba) rely on simple diffusion, but larger ones (like humans) need specialized gas exchange surfaces (lungs, gills, etc.) because:
- Surface area to volume ratio decreases with size
- Diffusion distance increases
- Metabolic demands increase
🧩 (i) Properties of Efficient Gas Exchange Surfaces
- Large Surface Area to Volume Ratio (SA:V)
Allows more molecules to diffuse at a time.
Small organisms → large SA:V → diffusion sufficient.
Large organisms → small SA:V → require lungs/gills. - Thin Surface (Short Diffusion Path)
Gases travel a short distance → faster diffusion.
Example: alveolar and capillary walls are only one cell thick. - Steep Concentration Gradient
Maintained by continuous movement of gases or fluids (air, water, blood).
Example: ventilation and blood flow in mammals ensure high O₂ outside and low O₂ inside alveoli.
⚗️ (ii) Fick’s Law of Diffusion
👉 Rate of Diffusion ∝ (Surface Area × Concentration Difference) / Thickness
| Factor | Effect on Diffusion Rate |
|---|---|
| ↑ Surface Area | ↑ Rate (more area for gas exchange) |
| ↑ Concentration Gradient | ↑ Rate (steeper gradient) |
| ↑ Thickness | ↓ Rate (longer path slows diffusion) |
💡 So: Efficient gas exchange surfaces have large area, thin barrier, and steep concentration gradient.
🫁 (iii) Adaptations of the Mammalian Lung for Rapid Gas Exchange
- Large Surface Area → millions of alveoli (~700 million) provide ~70 m² area for diffusion.
- Thin Barrier → alveolar + capillary walls = one cell thick → short diffusion distance (~0.5 µm).
- Rich Blood Supply → network of capillaries maintains high O₂ and low CO₂ concentration.
- Ventilation Mechanism → breathing in and out refreshes air in alveoli to maintain the gradient.
- Moist Alveolar Lining → gases dissolve in moisture before diffusing, aiding exchange.
- Elastic Fibers in Alveoli → allow alveoli to stretch during inhalation and recoil during exhalation.
- Close Contact with Blood → capillaries closely surround alveoli, ensuring rapid diffusion between air and blood.
📘 Summary Table
| Feature | Adaptation | Function |
|---|---|---|
| Surface Area | Many alveoli | Allows more diffusion |
| Thickness | One cell thick | Short diffusion distance |
| Gradient | Maintained by ventilation & blood flow | Ensures fast diffusion |
| Moisture | Alveolar fluid lining | Dissolves gases |
| Elasticity | Elastic fibers in alveoli | Helps air movement |
🧠 Quick Recap
Gas exchange depends on: SA:V ratio, thin surface, and concentration gradient.
Fick’s Law: Rate ∝ (Surface Area × Concentration Difference) / Thickness.
Lungs: Alveoli → thin walls, moist lining, rich capillary network.
Ventilation & Circulation maintain the diffusion gradient.
Diffusion is the main mechanism for gas movement across alveolar surfaces.