Final answer:
The phospholipids hydrophilic head groups face the outside of the cellular membrane, interacting with the aqueous environment, while the hydrophobic tails face inward, away from water.
Step-by-step explanation:
When examining the structure of a phospholipid in the context of a cellular membrane, the parts that face the outside of the membrane are the hydrophilic heads. Each phospholipid has a hydrophilic head group that is attracted to water, and this part is what interfaces with the aqueous environment on both sides of the membrane. In contrast, the hydrophobic tails, which contain fatty acids, face the interior of the phospholipid bilayer, away from water, thus forming the hydrophobic core of the membrane.
The hydrophilic head groups of the phospholipids are essential for the formation of the bilayer as they interact with the polar molecules in the aqueous solutions that are external to and inside of the cell. The orientation of the phospholipids in the membrane ensures that the cellular membrane acts as a selective barrier, allowing the cell to maintain a distinct internal environment while being able to communicate and interact with its external environment.