Table of Contents

• Prequiz...
• Just Plain Movement
• 1: Measuring Movement
• 2: Directed vs. Undirected Movement
• 3: Measuring Movement Using Flux
• Introducing Gradients
• 4: What is a gradient?
• 5: Watching diffusion happen (applet)
• 6: Visualizing the Gradient
• Fick's Laws, Part I
• 7: Fick's First Law
• 8: The gradient in Fick's First Law
• 9: A familiar equation for Fick's first law
• 10: The diffusion coefficient is the slope
• 11: What are the units for D?
• 12: How does flux depend on distance?
• 13: A fragrant example
• 14: General Transport Equations and Fick's First Law
• Fick's Laws, Part II
• 15: Fick's Second Law ...
• 16: ... is about "time to diffuse"
• 17: A Very Useful Formula
• 18: Time-to-diffuse "explodes"
• 19: General Functions and Diffusion
• Biological Applications
• 20: Biological Applications
• 21: Why do rhinos have lungs and amoebas don't?
• 22: Diffusion is efficient in small organisms, but not big ones
• 23: Diffusion within lungs
• 24: How many macrophages does it take to kill a virus?
• 25: You need a lot of macrophages - or one smart one!
• Review
• 26: Final Review

A Very Useful Formula

 

On the last page, every time we multiplied distance by 10, the time to diffuse increased 100-fold!

10 x as far --> 100 x as much time

What’s going on here? If time to diffuse were directly proportional to distance, then 10 times the distance would require 10 times as long -- not 100. This overinflation is like if you went to McDonald’s to buy 10 hamburgers and they charged you 100 times (not 10 times) the price of a single hamburger. Not fair? Well, at least it's not directly proportional variation.