Membrane Permeability

by Greg Crowther

Students often wonder why they need to know chemistry to do biology. Here is one compelling example: the rates and mechanisms by which substances cross biological membranes depends strongly on their chemical properties -- especially their polarity and size.

What can cross a lipid bilayer?
Lipids, gases, that's about all.
Large or charged or polar species
Cannot cross a lipid wall.

What can cross a lipid bilayer
If some proteins get installed?
Large or charged or polar species
Join or leave the cytosol.

• sheet music (with melody play-back)

Songs like this one can be used during class meetings and/or in homework assignments. Either way, the song will be most impactful if students DO something with it, as opposed to just listening.

An initial, simple follow-up activity could be to answer the study questions below. A more extensive interaction with the song might entail (A) learning to sing it, using an audio file and/or sheet music as a guide, and/or (B) illustrating it with pictures, bodily poses, and/or bodily movements. The latter activity could begin with students identifying the most important or most challenging content of the song, and deciding how to illustrate that particular content.

(1) Substances may be described as hydrophilic, hydrophobic, lipophilic, and/or lipophobic. In general, which of these types can pass through biological membranes without the aid of proteins?

(2) In the context of membranes, what is the difference between simple diffusion and facilitated diffusion?

(3) Explain how different types of proteins allow different types of molecules to pass through membranes.

(4) Explain how each of the following molecules gets across cell membranes: free fatty acids, Na⁺ ions, glucose, full-length proteins.