Which structure is characterized by a hydrophobic tail and a hydrophilic head?

Phospholipids are unique molecules that play a crucial role in the structure of cell membranes. They are characterized by their amphipathic nature, which means they have both a hydrophobic (water-repellent) tail and a hydrophilic (water-attracting) head. The hydrophilic head, which is typically composed of a phosphate group, interacts favorably with water, while the fatty acid tails repel water. This unique arrangement allows phospholipids to form bilayers in an aqueous environment, positioning the hydrophilic heads outward toward the water and the hydrophobic tails inward, away from the water. This bilayer structure is fundamental to the formation of cell membranes, providing a barrier that separates the internal cellular environment from the external surroundings, while still allowing certain substances to pass through.

In contrast, proteins, nucleic acids, and carbohydrates do not possess this distinct structural feature of having both a hydrophobic tail and a hydrophilic head, making phospholipids the correct choice in this context.