Diffusion Factors
What makes diffusion fast or slow? Meet the three factors, use Fick's law to predict the rate, and see why a good exchange surface is built the way it is.
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Diffusion Factors 🌫️
Exchange surfaces all over the body rely on **diffusion**: the net movement of particles from where they are more concentrated to where they are less concentrated. But diffusion can be fast or slow. Three things set the **rate**, and knowing them explains why lungs, gills and leaves are all built the way they are.
What sets the rate 📊
Three factors change how fast diffusion happens: - **Concentration gradient**: a bigger difference between the two sides means faster diffusion. - **Surface area**: a larger area gives more room for particles to cross, so faster diffusion. - **Distance (thickness)**: a shorter distance to travel means faster diffusion. (A higher temperature also speeds diffusion up, by giving the particles more energy.)
Faster or slower?
Which of these changes would make diffusion FASTER?
- A steeper concentration gradient
- A thicker exchange membrane
- A smaller surface area
- A lower temperature
Fick's law 🧮
Fick's law puts the three factors into one relationship. The **rate of diffusion is proportional to**: **(surface area x concentration difference) / thickness** Surface area and concentration difference are on the **top**, so increasing either speeds diffusion up. Thickness is on the **bottom**, so a thicker barrier slows it down. Often you just need to say which factor changed and whether the rate goes up or down.
Work out the rate
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When exchange breaks down
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Match each adaptation to what it improves
- Millions of tiny alveoli
- Alveolar walls one cell thick
- A rich blood supply removing oxygen quickly
- Gives a large surface area
- Gives a short diffusion distance
- Keeps a steep concentration gradient
Thicker barrier
A cell's exchange surface becomes twice as thick. Using Fick's law, what happens to the rate of diffusion across it?
- It roughly halves, because thickness is on the bottom of the relationship
- It doubles
- It stays exactly the same
- It stops completely
Diffusion summary
Diffusion is faster when the concentration gradient is _____, when the surface area is _____, and when the distance is _____. Fick's law says the rate is proportional to surface area times concentration difference, divided by the _____.