Final answer:
The least frequent movement of phospholipids in bilayers is the flip-flop from one layer to another, as their hydrophilic heads and hydrophobic tails are positioned specifically to form a stable and selectively permeable lipid bilayer.
Step-by-step explanation:
The least frequent movement of phospholipid molecules in bilayers is when they flip-flop from one layer of the membrane to the other. This is because the hydrophilic heads of phospholipid molecules are not compatible with the hydrophobic interior of the lipid bilayer. In a typical arrangement, phospholipids orient their hydrophilic heads toward the polar molecules and their hydrophobic tails inward, away from the water, forming a lipid bilayer. Such bilayers are the structural basis of cell membranes, providing both stability and selective permeability, essential for various cellular functions.