Final answer:
Hydrophobic molecules, being lipid-soluble, can easily cross the plasma membrane through simple diffusion, whereas hydrophilic molecules require specialized transport proteins for facilitated diffusion.
Step-by-step explanation:
Molecules that are hydrophobic are lipid-soluble and can easily cross a plasma membrane, while hydrophilic molecules cannot. Hydrophobic molecules do not mix with water and can move through the hydrophobic interior of the lipid bilayer of the cell membrane. This characteristic allows them to diffuse directly across the membrane without requiring special transport mechanisms. On the other hand, hydrophilic molecules are attracted to water and cannot easily pass through the hydrophobic core of the plasma membrane. These molecules often require special transport proteins, such as channel or carrier proteins, to facilitate their movement across the membrane through a process called facilitated diffusion.