Final answer:
The slowest movement of phospholipids in a lipid bilayer is when large polar or ionic molecules try to cross the bilayer. Their movement is restricted to protein channels and specialized transport mechanisms in the cell membrane through facilitated diffusion.
Step-by-step explanation:
The slowest movement of phospholipids in a lipid bilayer is when large polar or ionic molecules, which are hydrophilic, try to cross the bilayer. These molecules cannot easily pass through the hydrophobic tails of the phospholipids. Instead, their movement is restricted to protein channels and specialized transport mechanisms in the cell membrane through a process called facilitated diffusion.
Example: Glucose, a large and polar molecule, cannot freely diffuse through the lipid bilayer of the cell membrane. Instead, it requires a specialized carrier protein called the glucose transporter to facilitate its movement into the cell.