Lipid bilayers are permeable to small, nonpolar molecules but not to ions

Question

BlackTom AI · Accepted Answer

The statement to evaluate is: 'Lipid bilayers are permeable to small, nonpolar molecules but not to ions.'

First, consider the general structure of a lipid bilayer. The core of the bilayer is hydrophobic, composed mainly of fatty acid tails, which forms a water‑repelling (hydrophobic) interior. This environment strongly disfavors the passage of charged or highly polar species, including ions, unless there are specific transport mechanisms present.

For small, nonpolar (lipophilic) molecules such as O2, CO2, and N2, the hydrophobic interior of the membrane presents relatively little energetic barrier. These molecules can dissolve in the lipid phase and diffuse passively across the membrane down their concentration gradient. Thus, small nonpolar molecules are typically permeable through the lipid bilayer by simple diffusion.

In contrast, ions (e.g., Na+, K+, Cl−, H+) are charged and highly polar. They interact strongly with the aqueous environment and face a large desolvation penalty when entering the hydrophobic core of the membrane. Without specific channels or transporters, ions do not readily cross the bilayer, making the membrane effectively impermeable to them under normal conditions.

There are nuances to mention. Water is small and polar, yet it can diffuse through the bilayer very slowly, and cells routinely regulate water movement via aquaporins; this demonstrates that not all polar molecules are strictly impermeable, but ions remain largely blocked without transport proteins. Also, the presence of membrane proteins, channels, or carriers can alter permeability for certain solutes, including ions, but in the absence of such structures, the lipid bilayer mostly prevents ion passage while allowing small nonpolar molecules to diffuse.

Therefore, the core claim aligns with standard membrane physiology: the lipid bilayer is permeable to small, nonpolar molecules but not to ions, due to the hydrophobic interior and the energetic costs associated with desolvation and charge.

If you consider potential caveats, you might acknowledge exceptions where specific transport mechanisms or channels exist to move ions across the membrane, or where exceptionally tiny nonpolar molecules might diffuse more slowly depending on membrane composition and temperature. However, those nuances do not contradict the general principle captured by the statement.

2261 BIOSC 1500 SEC1030 Cell Bio 1500'25 first midterm, 75 min version

Lipid bilayers are permeable to small, nonpolar molecules but not to ions

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