Final answer:
Phospholipids in a membrane form a lipid bilayer, with hydrophilic heads facing the aqueous environments and hydrophobic tails facing inward. This arrangement creates a barrier that helps regulate the movement of molecules.
Step-by-step explanation:
A phospholipid is an amphipathic molecule, meaning it has a hydrophobic and a hydrophilic part. The "head" is composed of the hydrophilic phosphate group, and the “tail” contains the hydrophobic fatty acids. In a membrane, a lipid bilayer of phospholipids forms, with the hydrophilic heads facing the aqueous environment and the hydrophobic tails facing inward, creating a barrier.