Final answer:
The two fatty acid chains of a phospholipid are hydrophobic tails, repelling water, and together with the hydrophilic head form an amphipathic molecule essential for creating cell membranes.
Step-by-step explanation:
The two fatty acid side chains of a phospholipid form two hydrophobic "tails" that lie parallel to each other. These tails are composed of hydrocarbon chains that are hydrophobic, meaning they repel and are repelled by water. A single phospholipid molecule has a polar and hydrophilic "head" that consists of a phosphate-containing group attached to a glycerol molecule, and this head is attracted to water. This makes phospholipids amphipathic molecules, containing both a hydrophilic and a hydrophobic region, allowing them to form the bilayer that is foundational to cell membranes.