1. Polarity and Charge: The lipid bilayer is composed of phospholipids, which have a hydrophilic (water-loving) head and a hydrophobic (water-fearing) tail. This creates a barrier that is difficult for polar molecules and charged ions to penetrate. Polar molecules, like sugars and amino acids, have an uneven distribution of electrical charge, making them attracted to water and repelled by the hydrophobic interior of the membrane. Similarly, charged ions, like sodium and chloride, are also repelled by the hydrophobic tails.
2. Size: Large molecules, even if they are non-polar, may struggle to squeeze through the tight spaces between the phospholipid tails. The bilayer has a relatively small space for molecules to move through, acting as a size barrier.
In short, the lipid bilayer acts as a selective barrier, allowing small, non-polar molecules to pass through easily, while restricting the passage of larger, polar molecules and charged ions.
