Which of the following is the bilayer not permeable to?

The bilayer's permeability is largely influenced by the size and charge of the molecules trying to cross it. Lipid bilayers are composed of hydrophobic (water-repelling) tails, making it more difficult for charged particles or larger polar molecules to pass through.

Ions such as potassium (K+), magnesium (Mg2+), and any charged polar molecules cannot easily cross the lipid bilayer due to their charge, which interacts unfavorably with the hydrophobic core of the membrane. This electrostatic repulsion prevents these ions from moving freely across the bilayer compared to uncharged molecules.

In contrast, gases like nitrogen (N2) and oxygen (O2) can readily diffuse through the cytoplasmic membrane because they are small and nonpolar. Small uncharged molecules are generally able to pass through as well, utilizing simple diffusion. Additionally, while water and urea can pass through the membrane to a certain extent, they are still more permeable than charged particles. Thus, charged ions remain the least likely to permeate the lipid bilayer, confirming that the bilayer is not permeable to them.