Final answer:
In a phospholipid bilayer, the hydrophilic heads face outward toward water, and the hydrophobic tails are arranged inward, away from water, creating a stable barrier that is critical for cellular function.
Step-by-step explanation:
In the phospholipid bilayer, the molecules are arranged in a unique orientation where the hydrophilic heads are exposed to the aqueous environments, that is, the cell's external surroundings and its internal cytoplasm. These hydrophilic heads have an affinity for water, effectively allowing them to interface with the watery conditions present on both sides of the bilayer. On the contrary, the hydrophobic tails, being repelled by water, face inward, shielded from water, and associate with each other in the center of the membrane. This arrangement forms a stable barrier that is essential for cellular function, encapsulating the internal components of the cell and regulating the movement of substances in and out of the cell. The bilayer's structure is crucial because it forms a selective barrier that is impermeable to many solutes while allowing the diffusion of specific molecules. The arrangement of the phospholipid bilayer is not static; it allows for fluidity and flexibility, enabling various proteins to function effectively within the membrane, and contributing to processes like signal transduction, transport, and cell communication.