Table of Contents

• Developing the Equations
• 1: Review of diffusion equations
• 2: The two-compartment model
• 3: What does continuous mean?
• 4: Examples of permeability
• 5: A formula for permeability
• 6: The area of the membrane
• 7: The final equations
• Examples
• 8: Calculating molecular weight
• 9: Molarity of sugar-goo
• 10: Practicing conversions
• 11: Rate of flux
• 12: Review
• Discrete vs. Continuous
• 13: A metaphor
• 14: Continuous curves
• 15: Discrete curves
• 16: Iterating the discrete curve

Our equations are ready

So let's add a linear term for area of the membrane. Hmm, let's call it A, for area. No one can accuse mathematicians of getting too fancy with their names. So here are our final (for now) equations:

Continuous version
Discrete version

In order to do any calculations, we will have to get a handle on the actual values of these variables.

That means we need to be able measure concentration. You might think this is pretty easy. For example, if you put 100 grams of sugar in 900 grams of water, you will have a solution that is 10% sugar by weight. And you probably don't want to try to drink it. (Honest, even Mountain Dew is only about 8% by weight).

However, unfortunately, diffusion doesn't care about weight, it cares (if a process can be said to care) about particles . How many particles of sugar were in that 100 grams?

Before you get out a magnifying glass and start counting sugar grains, let's be clear about what a particle is: as a first approximation, a particle is a molecule. So, you need to know the molecular weight of sugar C₆H₁₂O₆.

If you remember how to calculate molecular weight, go ahead and do it now. If not, the next page is just for you...