Table of Contents

• Prequiz...
• Fick's Laws, Part II
• 1: Fick's Second Law ...
• 2: ... is about "time to diffuse"
• 3: A Very Useful Formula
• 4: Time-to-diffuse increases dramatically
• 5: General Functions and Diffusion
• Biological Applications
• 6: Biological Applications
• 7: Why do rhinos have lungs and amoebas don't?
• 8: Diffusion is efficient in small organisms, but not big ones
• 9: Why we have lungs
• 10: How many macrophages does it take to kill a virus?
• 11: You need a lot of macrophages - or one smart one!
• Review
• 12: Summary

It's about "time to diffuse"

Fick’s first law tells us HOW MUCH flux to expect. Fick’s second law gets into more detail, telling us how fast the concentration is changing at any given point in space. This law involves both rates of change with respect to space (position) and time, and takes the form of something called a “partial differential equation”, but we won’t need to go into that much detail here.

If this sounds complicated … it is. We won’t go into how this law works, except to say that when you integrate over time, you get a very useful (and easy to understand) result: a formula for how much time diffusion takes over a specified distance:

 

 

Assuming you know a diffusion coefficient, this formula allows you to determine how much time is necessary (on average) for particles moving by diffusion to traverse any given distance.

Try it out:

Any fish hanging out at 0.1 cm below the water surface would die from lack of oxygen if they had to rely on diffusion to bring them oxygen – – luckily there are also water currents.