What property of membranes allows them to act as barriers to polar molecules?

The correct choice highlights the importance of lipid bilayer formation in membrane structure. Membranes are primarily composed of phospholipids, which have hydrophilic (water-attracting) heads and hydrophobic (water-repellant) tails. When these phospholipids assemble into a bilayer, the hydrophobic tails face inward, shielded from the aqueous environment, while the hydrophilic heads interact with the water on either side of the membrane.

This unique arrangement results in a semi-permeable barrier that selectively allows certain substances to pass while blocking others. Polar molecules, which interact well with water due to their capacity for hydrogen bonding and their charge characteristics, cannot easily penetrate the hydrophobic core of the lipid bilayer. As a result, polar molecules find it difficult to cross the membrane, while non-polar molecules can more readily diffuse through the hydrophobic region of the bilayer.

Other properties like the presence of proteins or membrane asymmetry play roles in specific functions (like transport and signaling) but do not fundamentally explain the barrier property to polar molecules in the same way that lipid bilayer formation does. The hydrophobic effect is an underlying principle facilitating the spontaneous organization of the bilayer but is not in itself a structural characteristic of