Final answer:
Hydrophilic heads face outwards because they are attracted to the water inside and outside of cells, forming the plasma membrane's phospholipid bilayer which is essential for cellular integrity.
Step-by-step explanation:
The hydrophilic heads of phospholipids must face outwards because they are attracted to water. These hydrophilic head groups face the aqueous solution on both sides of the cell membrane, whereas the hydrophobic tails, which are repelled by water, face each other, sequestered in the middle of the bilayer.
This structure forms the plasma membrane's phospholipid bilayer, which is critical for maintaining the cell's integrity in a watery environment. Phospholipids are amphipathic molecules, meaning they have both hydrophilic and hydrophobic regions, allowing them to form this barrier that is impermeable to many substances while still fluid enough to allow certain molecules to pass through or interact with the cell.