Membrane Permeability Practice Test

A phospholipid bilayer has a hydrophobic interior. A solute’s permeability depends on how well it can enter this nonpolar region. Small, nonpolar molecules cross readily; polar molecules cross slowly; large polar molecules cross very slowly; ions cross least. Compare two uncharged molecules: ribose (a 5-carbon sugar with multiple hydroxyl groups) and isopropanol (a 3-carbon alcohol with one hydroxyl group). No transport proteins are present.

Which molecule would most likely be more permeable across the bilayer?

A model membrane is composed of a phospholipid bilayer with a hydrophobic interior. Molecules that are small and nonpolar tend to partition into the lipid core and diffuse across, whereas polar molecules interact strongly with water and are less soluble in the membrane interior. Charged molecules are surrounded by hydration shells and experience a large energetic barrier to entering the hydrophobic region. Consider two uncharged molecules of similar size: ethanol (contains a hydroxyl group) and propane (a hydrocarbon). No channels or carriers are present.

Which explanation best accounts for propane crossing the membrane more readily than ethanol?