Module 3: Exchange Surfaces
These free OCR A Level Biology Exchange Surfaces revision notes have been written for specification points 3.1.1(a), 3.1.1(b), 3.1.1(c) and 3.1.1(f).
Specialised Exchange Surfaces
Exchange surfaces are adaptations to overcome the limits of surface area.
Surface area to volume ratio
Surface area to volume ratio (SA:V) compares the surface area of an object to its internal volume.
It is calculated using the formula:
Surface area to volume ratio = Surface area ÷ Volume
The table below gives some examples:
| Cube Length (cm) | Surface Area (cm²) | Volume (cm³) | SA:V Ratio |
|---|---|---|---|
| 1 | 6 × 1² = 6 | 1³ = 1 | 6:1 |
| 2 | 6 × 2² = 24 | 2³ = 8 | 24:8 = 3:1 |
| 3 | 6 × 3² = 54 | 3³ = 27 | 54:27 = 2:1 |
| 4 | 6 × 4² = 96 | 4³ = 64 | 96:64 = 1.5:1 |
As size increases, the SA:V ratio decreases, so larger organisms (or cells) have less surface area per unit volume.
The illustration below demonstrates how the surface area to volume ratio (in micrometres) of organisms decreases with their size.
Exchange Surfaces
To maximise the rate of diffusion, osmosis or active transport, exchange surfaces are adapted to their function with a variety of different features.
In OCR A level Biology, the lungs, gills and root hair cells are the relevant exchange surfaces for transport systems that are studied.
The table below breaks down their specialised exchange surfaces by their features:
| Feature | Alveoli (Lungs) | Gills (Fish) | Root Hair Cells (Plants) |
|---|---|---|---|
| Large surface area | ~300 million alveoli provide ~70 m² surface area. | Gill filaments and lamellae create a large folded surface. | Long, thin extensions of root hair cells provide a vast surface area. |
| Thin barrier | Alveolar and capillary walls are one cell thick (~0.5 μm) and in close contact for a short diffusion distance. | Lamellae have thin epithelial layers for a short diffusion distance. | The cell membrane is thin to allow easy diffusion. |
| Good transport | An extensive capillary network for continuous blood flow to bring/remove substances. | Counter-current blood flow system maintains a steep gradient. | Active transport of ions maintains a steep water potential gradient. |
| Bulk movement | Ventilation by breathing refreshes air in the alveoli, maintaining concentration gradients. | Mouth and operculum create pressure that pushes water over the gills, maintaining concentration gradients. | Continuous uptake of minerals and water maintains flow into the root. |