Final answer:
The outside part of a phospholipid bilayer is made up of a hydrophilic head, which consists of a polar phosphate group. The inside part is made up of a hydrophobic tail, which consists of fatty acid chains.
Step-by-step explanation:
The outside and inside parts of a phospholipid bilayer are composed of different regions.
The outside part, or the hydrophilic head, is made up of a polar phosphate group that loves water. It faces either the outside environment or the inside of the cell, both of which are aqueous areas.
The inside part, or the hydrophobic tail, is made up of fatty acid chains that hate water. They face inside, away from water, and form the interior of the membrane.