3.1.3 Adaptation to Facilitate Exchange

Why Are Specialised Exchange Surfaces Needed?

1. Single-Celled Organisms:

• Have a high SA
  ratio, allowing substances to diffuse directly across their cell membrane.
• The diffusion distance is small, so no specialised surfaces are needed.

2. Multicellular Organisms:

• Have a low SA
  ratio, meaning diffusion alone cannot meet the needs of all cells.
• The distance substances must travel is greater, requiring specialised adaptations to facilitate exchange.

Features of Efficient Exchange Surfaces:

3. Large Surface Area:

• Maximises the amount of substance that can diffuse at any given time.
• Example:
• Root hair cells increase the surface area for water and mineral uptake.
• Alveoli in the lungs provide a large surface area for gas exchange.

4. Thin Barrier:

• Reduces the diffusion distance, making the exchange more efficient.
• Example:
• The walls of alveoli and capillaries are one cell thick, ensuring rapid gas exchange.

5. Steep Concentration Gradient:

• Maintained by a good blood supply or ventilation, ensuring a constant flow of substances.
• Example:
• Alveoli are surrounded by a network of capillaries that carry oxygen away and bring carbon dioxide to the lungs, maintaining a steep gradient.

6. Permeable Membranes:

• Allows the free passage of gases or other substances.
• Example:
• The membranes of alveoli are permeable to oxygen and carbon dioxide.

Examples of Specialised Exchange Surfaces:

7. Alveoli (Lungs):

• Large surface area: Millions of alveoli increase the area available for gas exchange.
• Thin walls: One cell thick for a short diffusion distance.
• Good blood supply: Capillaries surround alveoli to transport gases.
• Ventilation: Maintains a steep oxygen and carbon dioxide gradient.

8. Root Hair Cells (Plants):

• Large surface area: Increases water and nutrient absorption from the soil.
• Thin cell wall: Reduces the diffusion distance for water and minerals.

9. Fish Gills:

• Large surface area: Lamellae on gill filaments provide a large area for gas exchange.
• Counter-current flow: Blood and water flow in opposite directions, maintaining a steep gradient.
• Good blood supply: Delivers oxygenated blood to the body.

10. Leaves (Plants):

• Flat shape: Increases surface area for light absorption and gas exchange.
• Stomata: Allow gases like carbon dioxide and oxygen to diffuse in and out.
• Thin: Reduces the diffusion distance for gases.

Factors Affecting Exchange Efficiency:

11. Surface Area:

• Larger surfaces allow more substances to diffuse simultaneously.

12. Concentration Gradient:

• A steep gradient speeds up diffusion.

13. Thickness of the Barrier:

• Thin barriers reduce the time needed for diffusion.