Each Factor Explained
| Factor | Effect on diffusion rate | Biological example |
|---|---|---|
| Concentration gradient | Steeper gradient → faster diffusion | Blood flow in capillaries around alveoli removes O₂ quickly, maintaining a steep gradient |
| Temperature | Higher temp → faster diffusion (more kinetic energy) | Warm-blooded animals maintain 37°C for optimal metabolic and diffusion rates |
| Surface area | Larger surface → more particles cross per unit time | Villi and microvilli in the small intestine increase surface area for absorption |
| Distance (membrane thickness) | Shorter distance → faster diffusion | Alveolar walls are one cell thick (short diffusion pathway) |
| Size of molecule | Smaller molecules → faster diffusion | O₂ (small) diffuses faster than glucose (larger) |
Exchange Surfaces Are Adapted for Fast Diffusion
Every efficient exchange surface in the body shares the same four adaptations: remember them as the "big four":
- Large surface area: folds, villi, alveoli all increase area.
- Thin walls: one or two cells thick for a short diffusion distance.
- Good blood supply: maintains a concentration gradient by constantly carrying substances away.
- Ventilation / movement: brings fresh supply of the diffusing substance (e.g. Breathing brings fresh air to alveoli).
When answering questions about any exchange surface, apply these four points.
Investigating Diffusion Rate (Paper 5/6)
A classic practical uses agar blocks containing an indicator:
- Cut agar blocks of different sizes containing phenolphthalein (pink in alkali).
- Place them in acid: the acid diffuses in and decolorises the indicator.
- Measure the time for the colour to completely disappear.
- Smaller blocks decolorise faster because the diffusion distance to the centre is shorter.
This links to surface area to volume ratio: smaller organisms have a larger SA:V ratio, so diffusion alone is sufficient for exchange. Larger organisms need specialised exchange surfaces and transport systems.