What characteristic of the phospholipid bilayer allows it to be selectively permeable?

The characteristic of the phospholipid bilayer that allows it to be selectively permeable is that it consists of hydrophilic and hydrophobic regions. This unique structure is essential for the function of cell membranes. The bilayer is made up of two layers of phospholipids, where the hydrophilic (water-attracting) "heads" face outward toward the water in the intracellular and extracellular environments, and the hydrophobic (water-repellant) "tails" face inward, shielded from water.

This arrangement creates a semi-permeable barrier that allows certain substances to pass through while blocking others, depending on factors such as size, polarity, and charge. Small, nonpolar molecules can easily diffuse through the bilayer, while charged or large polar molecules typically cannot. This selective permeability is crucial for maintaining the internal environment of the cell and regulating the movement of substances in and out, which is vital for cellular functions.