What characteristic of lipid bilayers prevents permeability to polar or charged molecules?

The amphipathic nature of lipids is key to understanding why lipid bilayers are impermeable to polar or charged molecules. Amphipathic lipids consist of a hydrophilic (water-attracting) "head" and hydrophobic (water-repelling) "tails." When these lipids organize into a bilayer, the hydrophobic tails face inward, shielded from water, while the hydrophilic heads face outward, interacting with the aqueous environment.

This arrangement creates a barrier that is highly effective at preventing the passage of polar or charged substances, which cannot easily cross the hydrophobic interior of the bilayer. The lipid bilayer acts as a selective barrier—nonpolar molecules can diffuse through more readily, but polar or charged molecules struggle to penetrate due to the unfavorable interactions with the hydrophobic region.

While carbohydrates may play roles in cell recognition and signaling, they do not contribute directly to the permeability characteristics of the bilayer itself. The fluid mosaic model describes the structure and dynamic nature of cell membranes but does not explain the specific impermeability to these types of molecules. Hydrogen bonding with water pertains to interactions with aqueous environments but doesn't directly contribute to the bilayer's barrier function against polar or charged molecules. Thus, the amph