What is the bilayer permeable to?

The bilayer of biological membranes is composed primarily of a phospholipid bilayer, which plays a critical role in determining what substances can pass through it. Gases like CO2 and O2 are small, nonpolar molecules that can easily dissolve in the lipid portion of the bilayer and move freely across it without the need for transport proteins or channels. This is due to their size and lack of charge, allowing them to diffuse through the hydrophobic core of the membrane.

In contrast, large polar molecules, ions, and charged polar molecules face significant challenges in crossing the bilayer. Large polar molecules struggle due to their size and polarity, which hinders their ability to traverse the hydrophobic environment of the lipid bilayer. Ions such as Na+ and Cl- cannot cross the bilayer freely because their charge creates strong interactions with water molecules, making them less likely to penetrate the hydrophobic interior. Charged polar molecules like ATP also face similar barriers, as their charge and polarity prevent them from diffusing through the membrane without specific transport mechanisms.

Thus, the correct option identifies that gases like CO2 and O2 are permeable through the bilayer, aligning with the known properties of membrane permeability.