Which characteristic of a phospholipid allows it to create a barrier for hydrophilic substances?

The correct choice highlights the amphipathic nature of phospholipids, which is critical for their role in forming cellular membranes. Phospholipids have a unique structure that consists of two distinct parts: a hydrophilic (water-attracting) head and two hydrophobic (water-repelling) tails. This dual characteristic allows them to spontaneously arrange themselves into a bilayer in an aqueous environment.

In the phospholipid bilayer, the hydrophilic heads face outward towards the water on both the extracellular and intracellular sides, while the hydrophobic tails are oriented inward, away from the water. This arrangement creates a semi-permeable barrier that is effective at restricting the passage of hydrophilic substances, such as ions and polar molecules, while allowing nonpolar molecules to pass more easily through the membrane.

This amphipathic feature is essential for maintaining the integrity of the cell membrane, allowing it to serve as a dynamic barrier that facilitates selective permeability, which is fundamental for cellular function and homeostasis.