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Phospholipids play a crucial role in forming the bilayer structure of cell membranes. This bilayer is fundamental to the membrane's function, as it creates a semi-permeable barrier that separates the internal contents of the cell from the external environment. The hydrophilic (water-attracting) heads of phospholipids face outward toward the aqueous environments, while the hydrophobic (water-repelling) tails are oriented inward, away from water. This unique arrangement allows the cell membrane to selectively regulate the passage of molecules in and out of the cell, thus maintaining homeostasis and facilitating interactions with the environment.

By controlling what enters and exits the cell, phospholipids contribute significantly to various cellular processes, including signaling, nutrient uptake, and waste removal. Their role is fundamental in maintaining the integrity and functionality of the cell membrane, which acts as a gatekeeper. This is why the answer highlighting their function in forming bilayer structures and regulating molecular passage is correct in identifying the primary function of phospholipids in cell membranes.